JP2004061492A - Scintillator for x-ray detector, its manufacturing method, and x-ray detector and x-ray ct system using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a scintillator of high precision and high output free from dispersion in characteristics such as detection precision between elements by a simple process. <P>SOLUTION: A grid-like cut-in groove 20 having a prescribed pitch, a prescribed width and a prescribed depth along an x-direction and a y-direction is formed in a rectangular-paralleloid-shaped scintillator material 10. A separator of three-layer structure comprising a white diffusion reflection layer/a metal layer/a white diffusion reflection layer is inserted into the groove 20, and a resin containing reflecting material powder is injected/cured using a centrifuge in the groove inserted with the separator to form a reflecting layer. Then, cutting is carried out along a z-direction to obtain the scintillator arrayed lattice-likely with columnar scintillator elements formed with the reflecting layer in its four peripheries. In the scintillator, a detector of which the characteristics of the respective elements are uniform and has no dispersion is constituted, since dispersion in a width of the reflecting layer is ±0.010mm or less, and since a bubble contained in the reflecting layer is made substantially zero. <P>COPYRIGHT: (C)2004,JPO

Description

【数２】

【００４０】
一方、気泡の移動速度ｕ_０は、式（３）となる。故に、有機樹脂６中に分散する無機化合物粉末粒子７と気泡８の移動速度比は、式（４）で表される。
【００４１】
【数３】

【数４】

【００４２】
例えば、ρ_０＝０、ρ_ｓ＝４．２ｇ／ｃｍ^３、ρ＝１．４ｇ／ｃｍ^３、ｄ_０＝１０μｍ、ｄ_ｓ＝１μｍとすると、ｕ_０／ｕ_ｓ＝−５０となる。即ち、気泡８は無機化合物粉末７と反対方向に、５０倍の速度で遠心力とは逆方向に移動する。このことから、遠心注型によって狭くて深い切り込み溝に、気泡がなくなるような状態で効率よく注入できることがわかる。
【００４３】
上述した遠心注型においては、前述の気泡の除去と同時に、無機化合物粉末７の有機樹脂６に対する相対変位が生じる（図４）。その結果、図５に示すように、反射層は注入方向（溝の深さ方向）に無機化合物含有量が段階的に異なる３つの領域Ａ、Ｃ、Ｂに分かれる。上側の領域Ａは無機化合物の含有量が少なく、下側の領域Ｂは無機化合物の含有量が多い。これら領域は、後続するスライス工程で切断されシンチレータとしては利用しない領域であり、できるだけ狭いことが望ましい。これら領域Ａ、Ｂを狭くするためには、式（４）において粒子の密度ρ_ｓと有機樹脂の密度ρとの差をできるだけ小さくするか、粒子の粒子径ができるだけ小さくする必要がある。具体的には、無機化合物粉末の平均粒径を３μｍ以下とすることにより、領域Ａ、Ｂを全体の厚みの約５％以下に抑え、シンチレータとして利用できる領域Ｃを広くすることができる。
【００４４】
一方、反射層として有効な領域Ｃは、無機化合物粉末の粒子径分布に起因して注入方向に無機化合物含有量の傾斜αを生じる。本発明者らの研究によれば、シンチレータの反射率の変化は、厚さ方向について４％／ｃｍ以内であれば許容することができ、反射率の変化４％／ｃｍは無機化合物含有量の傾斜２０ｗｔ％／ｃｍに対応する。無機化合物粉末の粒子径分布の指標である標準偏差を２．０μｍ以下にすることにより、無機化合物含有量の傾斜２０ｗｔ％／ｃｍ以下に抑えることができ、感度むらのない検出器を構成することができる。
【００４５】
従って、本発明のシンチレータの製造方法においては、シンチレータとして利用できる領域Ｃをできるだけ広くし且つその厚さ方向の反射率の変化を４％／ｃｍ以内に抑えるために、無機化合物粒子として平均粒子径３μｍ以下、標準偏差２．０μｍ以下のものを用いることが望ましい。
【００４６】
こうして反射層形成材料を切り込み溝２０に注入した後、有機樹脂を硬化させて（ステップ２０６）、反射層１３を形成する。しかる後に、溝の深さ方向と直交する方向（即ち、ｘ、ｙ平面に平行）に所望の厚さにシンチレータをスライスする（ステップ２０７）。この場合、図５に示す領域Ａ、Ｂを除去し、領域Ｃからシンチレータとなる部分をスライスする。最後に、必要に応じてスライス面を研磨し（ステップ２０８）、放射線検出器用シンチレータ１００を得る。
【００４７】
尚、反射層は、可視光を遮蔽するためにモリブデン、タングステン、ステンレス等からなる金属遮蔽板やセパレータ板を挿入することができる。反射層に金属遮蔽板等を挿入する場合には、遠心分離注入する前に（ステップ２０３の前に）、切り込み溝に金属遮蔽板を挿入し、その後反射層形成材料を注入する。これにより例えばＸ線検出器のチャンネル方向に金属遮蔽板が入ったシンチレータを得ることができる。
【００４８】
本発明のシンチレータを放射線検出器として利用するためには、さらにシンチレータ１００の周囲及び光検出器が配置される面の反対側に、反射層を形成する。この反射層は、上述した無機化合物粉末を含む有機樹脂を硬化させたものを通常用いるが、不都合がなければ異なる素材であってもよい。
【００４９】
上記製造方法によって製造されたシンチレータは、切り込み溝を形成する加工精度と同程度の寸法精度の反射層を有し、且つ反射層に気泡が含まれることがないので、シンチレータ素子毎の特性のばらつきがなく、検出精度の優れた放射線検出器を提供することができる。
【００５０】
次に本発明の第２の実施形態によるシンチレータ及びそれを用いた放射線検出器を説明する。本実施形態によるシンチレータ２００の構成を図６に、それを用いた放射線検出器の構成を図７にそれぞれ示す。図６及び図７において、図１と同じ要素は同じ符号で示している。
【００５１】
本実施形態においても、シンチレータ要素１２及び反射層１７は、所定の大きさのシンチレータブロックに所定のピッチで所定幅の溝を形成し、この溝に反射層を構成する材料を充填することにより形成されている点は第１の実施形態と同様である。またシンチレータ素材としても第１の実施形態と同様に、例えば発光中心元素がＣｅで、Ｇｄ、Ａｌ、Ｇａ、Ｏを主要元素とするガーネット構造の酸化物蛍光体を好適に用いることができる。
【００５２】
但し、本実施形態のシンチレータ及び検出器は、反射層１７の構造に特徴を有し、反射層１７を白色の無機化合物粉末を含有する樹脂（以下、白色樹脂という）からなる反射拡散層１３とセパレータ板１６とから構成している。このようなセパレータ板を挿入することにより、シンチレータ要素間の光の漏洩を防止し、チャンネル間のクロストークをなくすことができる。
【００５３】
セパレータ板１６としては、図示するように白色拡散反射層１４／金属膜１５／白色拡散反射層１４の３層構造のセパレータ板１６を用いる。このような３層構造のセパレータ板を用いることにより、金属板のみを用いる場合に比べ、反射層の反射率を向上することができ、且つセパレータ板１６を反射層１７のほぼ中央に位置させることができ、シンチレータ要素に隣接する反射層１３及び白色拡散反射層１４の厚みをシンチレータ要素毎にほぼ均一にすることができる。
【００５４】
本実施形態のシンチレータ２００において、白色拡散反射層１３は、その十分厚い試料での拡散反射率Ｒ∞が、セパレータ板１６（金属膜１５／白色拡散反射層１４）の拡散反射率Ｒｂよりも大きいことが好ましい。具体的には白色拡散反射層１３の拡散反射率Ｒ∞は９７％以上、セパレータ１６の拡散反射率Ｒｂは８５％以上であることが好ましい。これによりシンチレータ要素界面での拡散反射率を９０％とすることができ、シンチレータの光出力を向上することができる。
【００５５】
上述したような拡散反射率を実現するために、白色拡散反射層１３は、有機樹脂中に白色の無機化合物を分散させた白色樹脂からなり、無機化合物／（無機化合物＋有機樹脂）の重量比（％）で３０〜８０ｗｔ％であることが好ましい。但し、８０ｗｔ％を越えると白色樹脂の粘度が極めて高くなり、遠心注入法で注入しても気泡を完全に除去することが困難になるので、重量比は８０ｗｔ％以下であることが好ましい。無機化合物は、酸化チタン、アルミナ、硫酸バリウム等を使用することができるが、特に酸化チタン（ＴｉＯ_２）が好適であり、また無機化合物粉末は粒子径が小さく且つ粒子径分布が狭いものが好ましい。有機樹脂としては、エポキシ樹脂、ポリエステル樹脂、アクリル樹脂、フェノール樹脂などの無色透明樹脂のほか、５００〜６００ｎｍ付近の可視光に強い吸収のない白色の樹脂を用いることができる。
【００５６】
また白色反射拡散層１４／金属膜１５／白色反射拡散層１４からなる３層構造のセパレータ板１６は、例えば、金属膜の両面に白色顔料を樹脂に分散してなる白色塗料を塗工することにより製造することができる。この場合、金属膜１５の材質としては、モリブデン、タングステン、鉛、アルミニウム、ステンレス及びリン青銅などの銅合金等を用いることができる。金属膜の厚さは、実質的に可視光を遮光できる厚さであればよいが、塗工層を形成する作業性等を考慮し、厚さ１０〜５０μｍの範囲とする。白色拡散反射層１４を構成する材料としては、白色拡散反射層１３を構成する無機化合物粉末及び有機樹脂を用いることができるが、無機化合物粉末として特に粒子径０．２〜２μｍ程度の酸化チタンが好適である。有機樹脂としては、エポキシ樹脂、ポリエステル樹脂、アクリル樹脂、フェノール樹脂などの無色透明樹脂のほか、５００〜６００ｎｍ付近の可視光に強い吸収のない白色の樹脂を用いることができる。このような無機化合物を塗工することによって白色拡散反射層１４を形成する場合、スクリーン印刷法、ドクターブレード法、刷毛塗り、スプレー塗りなど公知の塗装法を採用することができる。白色拡散層１４の厚さは、セパレータ板の厚さにより異なるが、例えば１０〜６０μｍ、好ましくは２０〜５０μｍとする。
【００５７】
３層構造のセパレータ板は、上述したもののほか、白色ポリマーフィルムに片面にアルミニウム等の金属膜を蒸着等により形成し、その蒸着面に白色ポリマーフィルムを接着することによって製造してもよい。金属蒸着膜の厚さは可視光の遮光が可能な厚さとし、具体的には０．０５〜２μｍ程度とする。白色ポリマーフィルムとしては、例えば、白色ポリエステルフィルム、白色ポリエチレンフィルム、白色ポリ塩化ビニルフィルム等を用いることができる。
【００５８】
セパレータ板の厚さは、反射層として形成された溝幅に依存し、溝幅に対し３０％以上、９０％未満であることが望ましい。セパレータの厚さが３０％に満たない場合には、溝幅に対して白色拡散反射層１３の占める割合が大きくなるため、白色拡散反射層１３の厚さに偏りが生じやすい。一方、セパレータの厚さが９０％以上であると、溝にセパレータを挿入し難く、また白色拡散反射層１３を形成する白色樹脂を注入する隙間が小さいため白色樹脂の注入が困難になる。これにより気泡の残留などにより拡散反射層１３が不均質になり、各シンチレータ要素の特性がばらつきやすい。
【００５９】
次に第２の実施形態によるシンチレータの製造方法を説明する。図８に、製造方法の概要を示す。
【００６０】
まず直方体形状のシンチレータ素材１０に、その厚さ方向（ｚ方向）と直交する二方向（図ではｘ方向及びｙ方向として示す）に切り込み溝２０を形成する。切り込み溝の深さは、シンチレータ素材の高さよりも浅く、シンチレータ素材は切断されず数ｍｍ程度残した状態とする。溝の深さ、幅、ｘ方向及びｙ方向のピッチは、放射線検出器の用途によって異なるが、Ｘ線ＣＴ用のＸ線検出器の場合、例えば溝の深さ１０〜３０ｍｍ、幅０．１〜０．３ｍｍ、ピッチ１．０〜２．５ｍｍ程度である（ステップ８０１）。この溝入れも、第１の実施形態と同様に、ワイヤーを往復運動させながら加工するマルチワイヤーソーや、ダイヤモンドで被覆された内周切刃を有するインナーソー等の高精度の加工機械を用いて形成することができ、これにより±０．００５ｍｍ以下の高精度の加工が可能になる。
【００６１】
このように溝入れした後のシンチレータ素材にアニール処理を行なう（ステップ８０２）。アニール処理は、シンチレータ素材が酸化物蛍光体である場合に、そのシンチレータ特性を付与するための処理であり、例えば温度１１００〜１６００℃の高温で数１０分〜数時間、酸素中で熱処理する。
【００６２】
一方、シンチレータ素材に形成された溝に挿入するためのセパレータ板を作製する（ステップ８０３）。セパレータ板は、上述したように金属膜の両面に白色拡散反射層を形成した３層構造のものであり、金属膜の厚さが溝幅の３０〜９０％のものを用意する。次いでセパレータ板をシンチレータ素材に形成された各溝に配置する（ステップ８０４）。セパレータ板は、標準的には、ＣＴ画像の分解能に重要な、チャンネル間を分離する溝（例えばｙ方向溝）のみに配置する。この場合にはシンチレータ素材のｙ方向の長さとほぼ同じ長さのセパレータ板をｙ方向溝と同数用意して、ｙ方向溝に挟み込む。またスライス間を分離する溝（例えばｘ方向溝）にもセパレータ板を入れる場合には、例えばシンチレータ素材のｘ方向の長さとほぼ同じ長さのセパレータ板をｘ方向の溝と同数用意し（ｘグループとする）、それぞれにｙ方向の溝のピッチと同じピッチで厚さ方向に１／２の深さの切り込みを形成するとともに、ｙ方向の長さと同じ長さのセパレータ板をｙ方向の溝と同数用意し（ｙグループとする）、これらについてもｘ方向の溝のピッチと同じピッチで厚さ方向に１／２の深さの切り込みを形成し、ｘグループのセパレータ板とｙグループのセパレータ板が直交するように互いの切り込みを上下から噛合わせて、格子状の切り込み溝に対応した格子状のセパレータ板の組み立て体を作製しておくことが好ましい。
【００６３】
このようにセパレータ板を切り込み溝に配置した後、溝内に白色樹脂を注入する（ステップ８０５）。白色樹脂の注入は第１の実施形態と同様に遠心注入法で行なうことが好ましい。遠心注入法を採用することにより、白色樹脂の粘度が１０万ｃＰｓ程度の高粘度であっても、気泡を生じることなく確実に深い切り込み溝に注入することができる。
【００６４】
尚、白色樹脂の注入に先立って、切り込み溝の表面に上記白色樹脂を塗布しておいてもよい（ステップ８０３’）。予め溝表面に白色拡散反射層の一部をなす白色樹脂の塗布層を形成しておくことにより、シンチレータと白色拡散反射層との間に空気の層ができるのを確実に防止することができ、高い拡散反射率が達成できる。溝表面に白色樹脂を塗布する方法としては、例えば、遠心注入法により白色樹脂を溝内に充填した後、逆向きに遠心をかけて、溝内の白色樹脂を排出させる方法を採用することができる。
【００６５】
注入した白色樹脂を硬化させた後、切り込み溝に垂直にシンチレータ素材をスライスする（ステップ８０６、８０７）。スライスには、例えばマルチワイヤーソー、内周刃スライサー、外周刃スライサー等の機械加工を用いることができる。この場合にも、白色樹脂の注入法として遠心注入法を採用した場合には、白色無機化合物の含有量に傾斜を生じるので、図５に示す傾斜２０ｗｔ％／ｃｍ以下の領域を切り出し、シンチレータとする。最後にスライス面をラップ研磨等することにより目的厚さのシンチレータを得る（ステップ８０８）。
【００６６】
このように製造したシンチレータを放射線検出器として利用するためには、図７に示したように、放射線の入射面を反射材１８で覆うとともに、その反対面を、シンチレータ要素と同一ピッチで光検出器（フォトダイオード）１１が形成された基板２１に透明接着剤１９等により接着する。
このような放射線検出器の入射面に放射線が入射すると、入射放射線の線量に応じてシンチレータが発光し、その光をフォトダイオードが光電変換する。ここで各シンチレータ要素が、白色拡散反射層１３／白色拡散反射層１４／金属膜１５／白色拡散反射層１４／白色拡散反射層１３からなる反射層１７で分離されているので、要素間のクロストークがなく、シンチレータ要素で発生した光が反射層１７で高反射率で反射されるため高い検出出力を得ることができる。
【００６７】
次に本発明のＸ線ＣＴ装置について説明する。図９は、Ｘ線ＣＴ装置の概略を示す図で、この装置はガントリ部６０と画像再構成部７０とを備え、ガントリ部６０には、被検体が搬入される開口部６１ａを備えた回転円板６１と、この回転円板６１に搭載されたＸ線管６２と、Ｘ線管６２に取りつけられＸ線の放射方向を制御するコリメータ６３と、Ｘ線管６２に対向して回転円板６１に搭載されたＸ線検出器６４と、Ｘ線検出器６４で検出されたＸ線を特定の信号に変換する検出器回路６５と、回転円板６１の回転及びＸ線束の幅を制御するスキャン制御回路６６とを備えている。画像再構成部７０は、検査条件などを入力するための入力部７１と、画像を表示するモニター７２とを備えている。ここでＸ線検出器６４として、本発明のシンチレータを用いたＸ線検出器を用いる。
【００６８】
このような構成において、開口部に設置された寝台に被検者を寝かせた状態で、Ｘ線管６２からＸ線が照射される。このＸ線はコリメータ６３によって指向性を得て、Ｘ線検出器によって検出される。回転円板を被検者の周りを回転させることによって、Ｘ線の照射方向を変えながらＸ線を検出し、画像再構成部７０で断層像を作成し、モニター７２に表示される。
【００６９】
このＸ線ＣＴ装置は、本発明のシンチレータを用いることによって、Ｘ線感度、線質特性の各素子間、各チャンネル間でのばらつきが小さく、高精度でＸ線を検出できるため、身体頭頂部の断面を画像処理する場合にもアーチファクトを生じず、高画質、高分解能の画像を得ることができる。
【００７０】
【実施例】
以下、本発明の実施例を説明する。
【００７１】
［実施例１］
組成式が（Ｇｄ，Ｃｅ）_{８（１−ｘ）}（Ａｌ，Ｇａ）_８ｘＯ_１２、Ｘ＝０．６２０、Ｃｅ／Ｇｄ＝０．００８、Ｇａ／（Ｇａ＋Ａｌ）＝０．４４である粉末を用いてホットプレス焼結を行い直径１６０ｍｍ、厚さ約１５ｍｍの焼結体を得た。この焼結体の相対密度は９９．９％以上であった。この焼結体から、マルチブレードソーを用いてｘ方向、ｙ方向及びｚ方向のサイズが４０ｍｍ×２５ｍｍ×約１５ｍｍであるシンチレータ素材を８個切り出した。
【００７２】
切り出したブロックに、線径０．１６ｍｍのマルチワイヤーソーを用いて、ｘ−ｙ面に対して垂直な格子状の切り込み溝を形成した。まずｘ方向に２．２５ｍｍのピッチで溝入れを行い、次にシンチレータ素材を９０°回転させ、ｙ方向に１．０ｍｍのピッチで同様に溝入れを行った。溝の深さはいずれも約１３ｍｍとした。こうして形成した格子状の切り込み溝は、ｘ方向、ｙ方向ともに何れも溝幅が０．２００±０．００５ｍｍの精度を有していた。
【００７３】
次に、このシンチレータ素材にシンチレータ特性を付与するための酸素中でアニール処理を行った。熱処理条件は、温度１３００℃、４時間とした。上記組成の希土類酸化物蛍光体は、このようなアニール処理によって、Ｘ線感度、残光の減衰率等のＸ線ＣＴ用検出器に最適な特性を付与することができることが本発明者らにより確認されている（特願２０００−３８９３４３号）。
【００７４】
一方、エポキシ樹脂に、粉末平均粒径０．３μｍのＴｉＯ_２粉末を混合割合（ＴｉＯ_２／（ＴｉＯ_２＋エポキシ樹脂））が６５ｗｔ％となるように添加し、これらを均一になるように混合した。このＴｉＯ_２含有エポキシ樹脂の粘度は９８，０００ｃＰｓであった。
【００７５】
アニール処理を行ったシンチレータ素材に有機樹脂との密着性を向上させるための表面処理を施した後、有機樹脂を注型するための注型枠の中に入れ、切り込み溝が形成された表面に、上述のＴｉＯ_２含有エポキシ樹脂を配置した。次いで、このシンチレータ素材を注型枠ごと遠心機のバケットにセットして、３５００ｒｐｍにて５分間回転させ、ＴｉＯ_２含有エポキシ樹脂を切り込み溝に注型した。注型後のシンチレータ素材を、６０℃で３時間加熱し、エポキシ樹脂を硬化させて反射材を形成した。この反射材内部には気泡は見られず、表面から脱離除去されていた。
【００７６】
このように格子状の反射材を形成したシンチレータ素材に対し、ｘ−ｙ面に平行に、即ちｚ軸に垂直に厚さ２．０ｍｍになるようにマルチワイヤーソーを用いてスライス加工して、シンチレータ要素の周囲４面が反射材で囲まれた５枚のシンチレータ板を作製した。これらシンチレータ板をラッピングマシーンを用いて、シンチレータ板の厚さが１．８００±０．００５ｍｍとなるように研磨した。以上の工程によって、２．０５ｍｍ×０．８ｍｍ×１．８００ｍｍのシンチレータ要素からなる５枚のシンチレータ板が得られた。
【００７７】
シンチレータ板の高さ方向（ｚ軸方向）の反射材のＴｉＯ_２含有量を調べるため、ＥＤＳ（Ｅｎｅｒｇｙ　Ｄｉｓｐｅｒｓｉｖｅ　Ｓｐｅｃｔｏｒｓｃｏｐｙ）法を用いてＴｉ元素の含有量を計測した。この結果、１．８００ｍｍの高さ方向に対して、０．８ｗｔ％の違いが認められた。なお、この測定時の標準偏差は２σで０．３１ｗｔ％であった。
【００７８】
［実施例２］
実施例１で製造したシンチレータにフォトダイオードを組み合わせて図１に示すような検出器を作り、Ｘ線源（１２０ｋＶ、０．５ｍＡ）から１５ｃｍ離れたところに検出器を置きＸ線感度および残光特性を評価した。その結果、Ｘ線感度は典型的なＸ線ＣＴシンチレータであるＣｄＷＯ_４の約２倍とすることができ、３００ｍｓでの残光の減衰率は約１０^−５とすることができた。
【００７９】
［実施例３］
実施例１と同一組成の粉末を用いてホットプレス焼結を行い直径１６０ｍｍ、厚さ約１５ｍｍの焼結体を得た。この焼結体から、マルチブレードソーを用いてｘ方向、ｙ方向及びｚ方向のサイズが３３ｍｍ×２５ｍｍ×約１５ｍｍであるシンチレータ素材を１１個切り出した。
切り出したブロックに、線径０．１１ｍｍのマルチワイヤーソーを用いて、ｘ−ｙ面に対して垂直な格子状の切り込み溝を形成した。まずｘ方向に１．０ｍｍのピッチで深さ約１３ｍｍに溝入れを行い、次にシンチレータ素材を９０°回転させ、ｙ方向にも１．０ｍｍのピッチで深さ約１３ｍｍに溝入れを行った。こうして形成した格子状の切り込み溝は、ｘ方向、ｙ方向ともに何れも溝幅が０．１３０±０．００５ｍｍの精度を有していた。
【００８０】
次に、このシンチレータ素材にシンチレータ特性を付与するための酸素中でアニール処理を行った。熱処理条件は、温度１３００℃、４時間とした。上記組成の希土類酸化物蛍光体は、このようなアニール処理によって、光出力は典型的なＸ線ＣＴ用シンチレータであるＣｄＷＯ_４の約２倍と大きく、３００ｍｓでの残光の減衰率は約１×１０^−５といった特性が得られた。
【００８１】
一方、厚さ３８μｍの白色ポリエステルフィルムの片面に、厚さ０．１μｍのＡｌ膜を蒸着によって形成し、次いでこのＡｌ蒸着面に厚さ３８μｍの白色ポリエステルフィルムを接着して、白色ポリエステルフィルム／Ａｌ蒸着膜／白色ポリエステルフィルムの３層構造からなるセパレータ板を製造した。このセパレータ板の拡散反射率は、５５０ｎｍの波長で８６％であった。
【００８２】
またシンチレータの溝に注入する白色樹脂として、エポキシ樹脂とＴｉＯ_２粉末を重量比で１：１で均一に混合した。この白色樹脂の拡散反射率Ｒ∞は５５０ｎｍの波長で９８％であった。溝入れ後のシンチレータ素材の溝に上記のように製造したセパレータ板を挿入した後、注型枠の中にセットし、この白色樹脂をシンチレータ素材表面に配置した。その後、注型枠を遠心機にセットして、３０００ｒｐｍで１０分間遠心注入を行なった。
【００８３】
注入後、白色樹脂を室温で１２時間硬化させた後、さらに６０℃で３時間硬化させた。次いでシンチレータ素材の上面に平行に、即ちｚ軸と垂直にマルチワイヤソーを用いてスライス加工し、厚さ方向中央部から厚さ１．９５ｍｍのシンチレータ板を５枚切り出した。これらシンチレータ板の両面を、ラッピングマシーンで厚さ１．８００±０．００５ｍｍとなるように研磨した。以上の加工により、０．８７ｍｍ×０．８７ｍｍ×１．８００ｍｍのシンチレータ要素からなる、２４チャンネル３２マルチスライス用シンチレータが５枚製造できた。
【００８４】
［比較例１］
実施例３と同様に切り込み溝を有するシンチレータ素材を作製した。この溝に実施例３と同様のセパレータ板を挿入し、溝とセパレータ板との間の隙間にエポキシ系透明接着剤を注入し硬化させた。その後、実施例３と同様にスライス加工、ラッピングを行い、シンチレータ板を製造した。
【００８５】
［実施例４、比較例２］
実施例３及び比較例１で製造したシンチレータに、それぞれフォトダイオードを組み合わせて図７に示すような検出器を作り、１５ｃｍ離れたところからＸ線（１２０ｋＶ、０．５ｍＡ）を照射し、Ｘ線感度を測定した。その結果、Ｘ線感度は実施例２のシンチレータで製造した検出器（実施例４）は、比較例１のシンチレータで製造した検出器（比較例２）の１．１〜１．２倍のＸ線感度が得られた。また実施例４の検出器は各シンチレータ要素の感度分布のばらつきは５．０％以下であり、均一性にも優れていた。
【００８６】
［実施例５］
実施例３と全く同様に格子状の切り込み溝を有するシンチレータ素材を製造した。一方、セパレータ板として、厚さ１０μｍのモリブデン板の両面に白色樹脂をスクリーン印刷によって塗布、硬化し、厚さ３５±１０μｍの白色拡散反射層を形成し、白色拡散反射層／Ｍｏ板／白色拡散反射層の３層構造のセパレータ板を製造した。ここで白色拡散反射層を形成する白色樹脂は、エポキシ樹脂に対し粒径０．２５μｍのＴｉＯ_２粉末を重量比１：１で均一に混合したものを用いた。このセパレータ板の拡散反射率は、５５０ｎｍの波長で９２〜９３％であった。
【００８７】
このセパレータ板をシンチレータ板の溝に挿入した後、実施例３と同じ白色樹脂（エポキシ樹脂：ＴｉＯ_２粉末＝１：１、拡散反射率Ｒ∞＝９８％）を用いて実施例と同様にシンチレータ板の溝に注入、硬化し、スライス加工及びラッピングを行い、０．８７ｍｍ×０．８７ｍｍ×１．８００ｍｍのシンチレータ板５枚を製造した。
このシンチレータを用いて、検出器を作り、１５ｃｍ離れたところからＸ線（１２０ｋＶ、０．５ｍＡ）を照射し、Ｘ線感度を測定した。その結果、比較例２の検出器の１．１５〜１．２５倍のＸ線感度が得られた。また各シンチレータ要素の感度分布のばらつきは５．０％以下であり、均一性にも優れていた。
【００８８】
［実施例６］
実施例３と全く同様に格子状の切り込み溝を有するシンチレータ素材を製造した後、このシンチレータ素材の切り込み溝の表面に白色樹脂の塗布層を形成した。白色樹脂としては、実施例３で溝に注入した白色樹脂と同じものを用いた。塗布層の形成のために、まず白色樹脂を遠心注入法により溝に注入し溝内に充填した後、溝の解放部を遠心注入のときと逆向きにセットして、即ち溝の底部が遠心機の中心方向を向くようにして、遠心機を稼動し溝内の樹脂を除去した。遠心機は樹脂の注入時、樹脂の排出時ともに１０００ｒｐｍ、５分間行なった。これにより溝表面に厚さ約３０μｍの白色樹脂の塗布層を形成した。この塗布層を室温で１２時間硬化させることにより、１５μｍ±１０μｍの白色拡散反射層を形成した。
【００８９】
こうして表面に白色拡散反射層を形成したシンチレータ素材の溝に、実施例３と同様の白色ポリエステルフィルム／Ａｌ蒸着膜／白色ポリエステルフィルムの３層構造からなるセパレータ板を挿入し、さらに切り込み溝とセパレータ板との間に白色樹脂を遠心注入して白色拡散反射層を形成した。白色樹脂としては、エポキシ樹脂とＴｉＯ_２粉末を重量比１：１で混合したものを用い、遠心条件は３０００ｒｐｍ、１０分間とした。注入後室温で１２時間硬化させた後、さらに６０℃で３時間硬化した。次いで実施例３と同様にスライス加工及びラッピングを行い、０．８７ｍｍ×０．８７ｍｍ×１．８００ｍｍのシンチレータ板５枚を製造した。
【００９０】
このシンチレータを用いて、検出器を作り、１５ｃｍ離れたところからＸ線（１２０ｋＶ、２００ｍＡ）を照射し、Ｘ線感度を測定した。その結果、比較例２の検出器の１．１〜１．２倍のＸ線感度が得られた。また各シンチレータ要素の感度分布のばらつきは５．０％以下であり、均一性にも優れていた。
【００９１】
［実施例７］
実施例２で作成したＸ線検出器を、図９のＸ線ＣＴ装置として用い、Ｘ線管電圧１２０ｋＶ、Ｘ線管電流１７５ｍＡ、スライス幅１０ｍｍ、スキャン時間２ｓの条件で、頭頂部近似ファントムを用いてＸ線ＣＴ像の撮影を行った。アーチファクトは見られず高品質の断層像が得られた。
【００９２】
［実施例８］
実施例４で作成したＸ線検出器を、図９のＸ線ＣＴ装置として用い、Ｘ線管電圧１２０ｋＶ、Ｘ線管電流１７５ｍＡ、スキャン時間０．８ｓの条件で、頭頂部近似ファントムを用いてＸ線ＣＴ像の撮影を行った。アーチファクトは見られず高品質の断層像が得られた。
【００９３】
【発明の効果】
本発明によれば、４周面を反射層で囲んだ柱状シンチレータを多数配列した検出器において、反射層の厚さばらつき、並びにピッチ精度を、反射層用切り込み溝を形成するために用いた加工機の加工精度と実質的に同程度にまで高めることができると同時に、気泡等の反射層の欠陥が無く、最小限のシンチレータ製造工程で高精度のシンチレータを得ることができる。また本発明によれば、各シンチレータ要素間に設けられる反射層を、第１反射層／第２の反射層／金属膜／第２の反射層／第１の反射層で構成することにより、シンチレータ要素界面において高い拡散反射率を達成することができるとともにシンチレータ要素間のクロストークを確実に防止できるので、検出感度及び検出出力の高い検出器を提供することができる。従って、このシンチレータをＸ線ＣＴ装置用Ｘ線検出器に適用することによって、アーチファクトのない高品質の断層像が得られるＸ線ＣＴ装置を提供できる。
【図面の簡単な説明】
【図１】本発明が適用される放射線検出器の概略を示す図
【図２】本発明のシンチレータ製造工程の一例を説明するための図
【図３】本発明の遠心注型法のメカニズムを説明するための図
【図４】シンチレータ素材の一部分を説明するための図
【図５】本発明のシンチレータの反射材の無機化合物粉末粒子含有割合を説明するための図
【図６】本発明の第２の実施形態によるシンチレータの構成を示す図
【図７】図６のシンチレータを用いた放射線検出器の構成を示す図
【図８】図６のシンチレータの製造工程の一例を説明するための図
【図９】本発明が適用されるＸ線ＣＴ装置の概略を示す図
【符号の説明】
６・・・有機樹脂
７・・・無機化合物粉末
１０・・・シンチレータ素材
１３・・・反射層（第１の反射層）
１４・・・第２の反射層
１５・・・金属膜
１６・・・セパレータ板
２０・・・切り込み溝
１００、２００・・・シンチレータ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a radiation detector for detecting X-rays, γ-rays and the like, and in particular, a scintillator for a radiation detector suitable for a radiation detector such as an X-ray CT apparatus or a positron camera, a method for manufacturing the same, and an X-ray using the same. The present invention relates to a line CT apparatus.
[0002]
[Prior art]
Conventionally, a radiation detector used for an X-ray CT apparatus or the like includes a gas chamber of xenon, a combination of bismuth germanate (BGO crystal) and a photomultiplier, and cadmium tungstate (CdWO). ₄ ) And photodiodes have been used, but in recent years, rare-earth phosphors with high conversion efficiency from radiation to light have been developed, and radiation detectors combining such phosphors and photodiodes have been developed. Has been put to practical use.
[0003]
Such a radiation detector has, for example, a structure in which elements provided with a reflective material on four surfaces around a columnar scintillator element are arranged in a linear or lattice shape. Is required to have uniform arrangement pitch of elements and uniformity of performance of each element.
[0004]
As a method of manufacturing a radiation detector having the above-described structure, for example, in Japanese Patent Application Laid-Open No. H11-163, (1), a plurality of plate-shaped scintillator materials are laminated at predetermined intervals via a reflecting material to form a primary laminated block, and (2) a lamination thereof. The block is cut to a predetermined thickness to produce an intermediate plate composed of a large number of prismatic scintillator materials and a reflective material. 3) The intermediate plate is laminated at a predetermined interval via the reflective material to form a secondary. 4) A method of cutting the secondary laminated block to a predetermined thickness is disclosed (referred to as prior art 1). Japanese Patent Application Laid-Open No. H11-157, 1) describes 1) a cutout groove having a width of about 0.5 mm and a depth of about 12 mm is formed in a plate-like BGO crystal at a predetermined pitch in a lattice shape, and 2) a reflector powder is contained in the cutout groove. A method is described in which a resin solution is filled, and 3) the resin solution is placed in a decompression device, and the resin solution is vacuum-degassed, desolventized, and dried (referred to as Conventional Technique 2).
[Patent Document 1] Japanese Patent Application Laid-Open No. H11-231061
[Patent Document 2] JP-A-5-19060
[0005]
[Problems to be solved by the invention]
In recent years, with the increase in the accuracy of radiation detectors, scintillators constituting detectors have a size of about 1 x 2 x 2 mm. ³ As a result, the thickness of the surrounding reflectors has been reduced to about several hundred μm. In a detector having such dimensions, even if the dimensional accuracy of the scintillator element or the reflecting material varies by about several tens of μm, it affects the detection accuracy of the detector and causes ring-shaped artifacts. Also, if there are bubbles or defects inside the reflecting material or at the interface between the reflecting material and the scintillator, the sensitivity distribution of the scintillator element will vary greatly, which also causes artifacts.
[0006]
In such a situation, in the above-mentioned prior art 1, it is necessary to perform high-precision lamination twice in order to form the reflective material around the scintillator element, and there is a problem that a great deal of time and labor are required. . Further, in the prior art 2, the precision of the scintillator interval (thickness of the reflective material) can be increased to the precision of the machining by forming the vertical and horizontal notches by the same machining, but when the width of the notches becomes narrow, the resin solution becomes thinner. It is difficult to fill the groove in the groove, and even if it can be filled, there is a limit to vacuum defoaming by the decompression device, and residual bubbles vary for each reflective layer, and a high-precision detector cannot be obtained. There was a problem. Further, in the prior art 2, there was a region that was not separated by the scintillator element in the final form.
[0007]
On the other hand, as a material inserted between each scintillator element, a reflective material for reflecting light emitted by the scintillator to increase the output of the scintillator and a material for suppressing light transmission and preventing crosstalk between the scintillator elements are used. There is a shielding material. As the reflective material, a mirror-finished metal plate, a white paint containing a reflective material made of a white pigment such as titanium dioxide or barium sulfate, or the like is used. As a material for suppressing light transmission, a heavy metal thin plate such as lead or molybdenum is used.
[0008]
In the case where the material of the reflection layer is filled in the groove formed between the scintillator elements as in the above-described prior art 2, a material in which a reflection material is dispersed in a resin is used. However, in the conventional manufacturing method, since a high-viscosity material cannot be used to fill the narrow groove with the material, the amount of the reflective material that can be contained in the reflective layer is limited, and sufficient reflective characteristics can be obtained. There were problems such as no crosstalk.
[0009]
On the other hand, when a scintillator block is manufactured by laminating plate-shaped scintillator elements, a metal thin plate provided with a coating layer of white paint on both surfaces, a white polymer film, and a separator combining these are used. Used. Patent Literature 3 describes that a laminate of two white films on which a metal film is formed such that the metal film is on the inside is used as a separator.
[Patent Document 3] Japanese Patent Publication No. 2930823
[0010]
When a separator provided with a coating layer of white paint on both sides of a metal plate is used as a separator, the effect of preventing crosstalk can be improved, but due to the uneven thickness of the coating layer, the scintillator element However, there is a problem that the reflection characteristics vary. On the other hand, a film using a white film has a lower reflection characteristic than a coating layer of a white pigment, and cannot achieve sufficient performance in applications requiring extremely high detection sensitivity.
[0011]
An object of the present invention is to solve various problems relating to a scintillator and a detector manufactured using the above-described conventional manufacturing methods and materials. That is, one object of the present invention is to provide a manufacturing method capable of manufacturing a scintillator for a high-precision radiation detector in which a scintillator element is separated in a relatively simple process, whereby a variation in sensitivity of the scintillator element is provided. It is an object of the present invention to provide a high-precision scintillator for radiation detectors. It is another object of the present invention to provide a scintillator for a radiation detector in which a reflective layer provided between scintillator elements is uniform, has excellent reflection characteristics, and has high detection sensitivity. It is a further object of the present invention to provide an X-ray CT apparatus having high detection accuracy and suppressing artifacts by using such a scintillator.
[0012]
[Means for Solving the Problems]
The scintillator for a radiation detector of the present invention that achieves the first object has a plurality of columnar scintillator elements arranged in a first direction and a second direction orthogonal to the first direction, respectively. In a scintillator for a radiation detector in which reflection layers are provided on four peripheral surfaces in the arrangement direction, at least the reflection layer provided between the scintillator elements is made of a resin containing a reflection material, and the content of the reflection material is orthogonal to the arrangement direction. It changes monotonically in the direction of the movement. The rate of change of the content of the reflecting material is preferably more than 0 wt% / cm per unit length and 20 wt% / cm or less.
[0013]
Further, the method for manufacturing a scintillator for a radiation detector according to the present invention is characterized in that a rectangular parallelepiped scintillator material is cut at a predetermined pitch, a predetermined width, and a predetermined depth along a first direction and a second direction orthogonal to the first direction. A step (1) of forming a groove, a step (2) of injecting a resin containing a reflector powder into the cut groove using a centrifuge, a step (3) of curing the resin, and after the resin is cured. The method includes a step (4) of cutting along a third direction orthogonal to the first and second directions.
[0014]
When the scintillator material is made of a rare earth oxide phosphor, it is preferable to include a step of annealing to enhance the scintillator characteristics. In this case, the step of annealing is performed before the step (1) and / or It is preferable to carry out between step (1) and step (3). Annealing performed prior to the step (1) can reduce thermal strain and the like generated during the production of the scintillator material, and annealing can be performed efficiently by performing annealing after forming the cut grooves in the step (1). A scintillator having excellent scintillator characteristics can be obtained.
[0015]
The scintillator formed by the manufacturing method of the present invention is capable of reliably injecting the material forming the reflective layer into the cut grooves by centrifugal method without air being mixed even in a narrow groove or a deep groove. Material can be injected into the interior. The scintillator thus formed has a gradient in the concentration of the reflector powder in the depth direction of the groove, but by appropriately selecting the particle size and distribution range of the reflector powder, the change in the reflectance in the groove depth direction can be reduced. It can be kept within the allowable range.
[0016]
Further, in the scintillator of the present invention formed by this manufacturing method, the thickness dimensional variation of the reflective layer can be increased to the same level as the processing accuracy when forming the cut grooves. Specifically, the distance can be set to ± 0.010 mm or less in each of the first and second directions. As a result, a scintillator for a radiation detector without variation in characteristics of each scintillator element can be obtained.
[0017]
In the above-described scintillator for a radiation detector of the present invention, preferably, the reflection layer further includes a separator plate at a central portion thereof. The separator plate has, for example, a three-layer structure in which a white diffuse reflection layer is laminated on both surfaces of a metal film.
By inserting a separator plate into a reflection layer made of a resin containing a reflection material, the reflection characteristics and light transmission blocking characteristics of the reflection layer can be further enhanced. In particular, by adopting a three-layer structure having a white diffuse reflection layer on both sides of a metal film as a separator plate, each scintillator element has two reflection layers on both sides, thereby preventing crosstalk. Moreover, the reflection characteristics can be greatly improved, and the spatial resolution and the detection sensitivity can be increased.
[0018]
A scintillator for a radiation detector that achieves the second object of the present invention has a plurality of columnar scintillator elements arranged in a first direction and a second direction orthogonal to the first direction, and an arrangement of each scintillator element. In a scintillator for a radiation detector comprising reflection layers on four sides in the direction, the reflection layer is inserted into a first reflection layer made of a resin containing a reflection material, and a central part of the first reflection layer, A separator having a second reflective layer in contact with the first reflective layer.
[0019]
In this scintillator for a radiation detector, since the reflection layer is composed of three layers of the first reflection layer / the second reflection layer composed of the white separator / the first reflection layer, high detection sensitivity is realized for each scintillator element. Is done. In particular, when the white separator has a metal film between the two white reflective diffusion layers, a high crosstalk prevention effect of the reflective layer can be obtained.
[0020]
Furthermore, in the scintillator for a radiation detector of the present invention, it is desirable that the diffuse reflectance R∞ of the first reflective layer of a sufficiently thick sample is larger than the diffuse reflectance Rb of the second reflective layer. Here, the diffuse reflectance of a sufficiently thick sample refers to the diffuse reflectance at a thickness at which the diffuse reflectance does not change even if the sample is further thickened.
By making the diffuse reflectance R∞ of the first reflective layer larger than the diffuse reflectance Rb of the second reflective layer, the reflectance of the entire reflective layer can be made 90% or more, and the Light output can be improved.
[0021]
The radiation detector of the present invention uses the above-described scintillator of the present invention. Specifically, a scintillator in which a plurality of scintillator elements in which a reflective layer is arranged on four pillar-shaped peripheral surfaces is arranged, and a scintillator of this scintillator is used. It has a photodetector for detecting light emission. In the radiation detector, since the scintillator of the present invention is used as the scintillator, there is no variation in characteristics between elements, and high detection accuracy can be realized.
[0022]
Since this X-ray CT apparatus includes an X-ray detector having high detection accuracy without variation in the characteristics between the elements, ring artifacts can be suppressed and a high-quality CT image can be obtained.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a scintillator for a radiation detector of the present invention, a radiation detector using the scintillator, and a method for manufacturing the same will be described.
[0024]
FIG. 1 is a diagram illustrating a radiation detector according to a first embodiment of the present invention. The radiation detector includes a plurality of columnar scintillator elements 12 arranged in a grid and a periphery 4 of the columnar scintillator elements 12. A reflection layer 13 provided on the surface, a photodetector 11 provided on one surface orthogonal to the peripheral surface of the scintillator element 12, and a reflection provided on a surface parallel to the surface on which the photodetector 11 is provided. And a layer 13 '. Although the figure shows a part of the radiation detector and shows only three scintillator elements 12 arranged in one direction (paper surface, lateral direction), a direction perpendicular to this direction (a direction perpendicular to the paper surface) is shown. ), A plurality of scintillator elements are arranged in a similar structure.
[0025]
As the photodetector 11, a known photodetector such as a photomultiplier tube, a photodiode, and a PIN photodiode can be used.
The scintillator element 12 generates light by receiving X-rays, γ-rays, and the like, and is made of a known material such as a rare-earth phosphor. As a preferable rare earth phosphor, for example, an oxide phosphor having a garnet structure in which the emission center element is Ce and Gd, Al, Ga, and O are the main elements proposed by the present inventors (Japanese Patent Application No. 2000-389343). ) Can be used.
[0026]
The reflection layer 13 is made of an organic resin containing an inorganic compound powder as a reflection material. As the organic resin, a resin which is less likely to be deteriorated or colored by radiation and has a high adhesive strength is preferable. Specifically, an epoxy resin, an acrylic resin, a phenol resin, or the like can be used.
[0027]
As the inorganic compound powder, a material that can secure a light reflectance of 90% or more at a wavelength of about 535 nm or more and is hardly deteriorated by irradiation with radiation is preferable. Specifically, an inorganic compound powder such as titanium oxide, alumina, and barium sulfate is used. can do. The inorganic compound powder preferably has a small particle size and a narrow particle size distribution. Specifically, those having an average particle diameter of 3 μm or less, a particle diameter distribution (standard deviation) of 2 μm or less, more preferably an average particle diameter of 1 μm or less, and a particle diameter distribution of 1 μm or less are preferable.
[0028]
In order to reflect as much light as possible from the scintillator element and to shield the adjacent scintillator element from light, the content of the reflector is 30 to 80 wt% in terms of the weight ratio (%) of reflector / (reflector + resin). Is preferable. If the content of the reflecting material is less than 30% by weight, sufficient reflectivity and shielding property cannot be obtained, and the detection ability is reduced and crosstalk between scintillator elements occurs. On the other hand, when the content exceeds 80 wt%, the viscosity before curing of the resin becomes high, and it becomes difficult to inject the material for forming the reflection layer between the scintillator elements when forming the reflection layer by the manufacturing method described later. It is preferable that the viscosity of the reflection layer forming material in which the reflection material is added to the organic resin before curing is about several thousands to several hundred thousand cPs. The viscosity of such a reflective layer forming material can be adjusted by selecting the organic resin, adjusting the content of the inorganic compound powder, and the like.
[0029]
Further, the scintillator of the present invention is characterized in that the content of the reflecting material changes in the height direction of the scintillator. This is attributable to the manufacturing method of the scintillator of the present invention, and the scintillator of the present invention reflects bubbles harmful to crosstalk by adopting a manufacturing method that causes a gradient in the concentration of the reflecting material. A scintillator having excellent reflection characteristics without being included in any of the layers and having uniform characteristics in all the reflection layers can be obtained. The concentration gradient is more than 0 wt% / cm per unit length, 20 wt% / cm or less, preferably 10 wt% / cm or less, more preferably 5 wt% / cm or less.
[0030]
When the concentration gradient is 20 wt% / cm or more, the light reflectance of the reflective layer changes by about 4% or more in the thickness direction. Assuming that the thickness of the scintillator is about 2.0 mm, the light reflectivity differs by 0.8% in the thickness of the scintillator, which causes uneven sensitivity when used as a detector and causes artifacts. By setting the concentration gradient to 20 wt% / cm or less, such sensitivity unevenness can be eliminated.
[0031]
The thickness of the reflective layer varies depending on the radiation detector, but can be, for example, in the range of 0.1 to 0.3 mm. In the radiation detector according to the present invention, the accuracy of the width of the reflective layer can be suppressed to ± 0.010 mm or less by manufacturing using the manufacturing method described below. Further, the radiation detector of the present invention may include a metal shielding plate made of molybdenum, tungsten, stainless steel, or the like in the reflection layer in order to reduce crosstalk between elements.
[0032]
Next, a method for manufacturing the scintillator of the present invention configured as described above will be described.
FIG. 2 is a view showing an example of a manufacturing process of the scintillator of the present invention, and shows a case where an oxide phosphor is used as a scintillator material. As shown in the drawing, a rectangular parallelepiped scintillator material 10 is prepared, and cut grooves 20 are formed in two directions perpendicular to each other and perpendicular to one surface of the scintillator material 10 (step 201). In the figure, when the vertical direction of the scintillator material is y, the horizontal direction is x, and the height direction is z, the groove is formed in the x direction and the y direction so that the depth direction is z. The depth H of the cut groove is shallower than the height Z of the scintillator material, and the scintillator material is left uncut and remains about several mm.
[0033]
The depth and width of the groove, and the pitches in the x and y directions differ depending on the application of the radiation detector. For example, the depth of the groove is 10 to 30 mm. In the case of an X-ray detector for X-ray CT, for example, the width is about 0.1 to 0.3 mm and the pitch is about 1.0 to 2.5 mm. In the conventional method of injecting into the groove by the weight of the reflective layer forming material by its own weight, mechanical pressing method such as press, and impregnation method in vacuum, the reflective layer forming material is injected into the narrow groove without generating bubbles. Although it is difficult to perform the method, the method of the present invention can reliably inject the material for forming the reflective layer to the bottom without generating bubbles even in the narrow groove and the deep groove described above.
[0034]
Such a cut groove can be formed using a high-precision processing machine such as a multi-wire saw that performs reciprocating motion while supplying a wire or a slicer having a cutting edge coated with diamond. Thereby, the pitch and width of the groove can be increased to the same level as the precision of the processing machine. Specifically, the groove width variation and the pitch variation of the cut groove can be made ± 0.010 mm or less. If the variation accuracy is 0.01 mm or more, the accuracy of the detector decreases, and ring-shaped artifacts are likely to occur.
[0035]
After forming the cut grooves, the scintillator material is annealed (step 202). The annealing treatment is generally performed to impart scintillator characteristics to the oxide phosphor. For example, heat treatment is performed at a high temperature of 1100 to 1600 ° C. for several tens minutes to several hours in oxygen. Such an annealing process can be performed before forming the cut grooves in the scintillator material, but in the manufacturing method of the present embodiment, since the cut grooves are formed before the annealing process, it is possible to increase the annealing efficiency. it can.
[0036]
Next, a material for forming the reflective layer is injected into the cut groove (steps 203 to 205). Therefore, first, the scintillator material is placed in the molding frame such that the cut groove is on the upper surface (step 203), and an uncured organic resin containing a reflecting material is placed on the cut groove upper surface (step 204). Then, it is centrifugally cast (step 205). As described above, the material forming the reflective layer is made of an inorganic resin powder dispersed in an organic resin, and is adjusted to have a viscosity of about several thousands to several hundred thousand cps. By using the reflective layer forming material whose viscosity is adjusted, the material can be injected to the bottom of the groove by centrifugal casting, and air bubbles in the resin can be substantially eliminated.
[0037]
The mechanism for removing air bubbles by centrifugal casting will be described with reference to FIG. The density of the organic resin 6 is ρ, the viscosity is μ, and the density of the inorganic compound powder 7 is ρ _s , Diameter d _s , The density of bubbles 8 is ρ ₀ And the diameter d ₀ And In general, particles in liquid (diameter d _s , Density ρ _s The movement of ()) reduces the velocity of the particles by u _s Then, the Reynolds number Re (= d _s u _s ρ _s / Μ), Stokes' law, Allen's law, and Newton's law are applied. In the above-mentioned material for forming a reflective layer, the viscosity μ is large, the resistance to the movement of the particles is generated solely by friction, the Reynolds number is 5.8 or less, and Stokes' law is applied.
[0038]
According to Stokes' law, the resistance R acting on a particle is R = 3πd _s u _s It is represented by μ. On the other hand, the effective centrifugal force for the relative displacement between the organic resin and the particles is given by the following equation (1) when the particles having the mass m rotate on the circumference having the radius r at the angular velocity ω. Therefore, from the balance between the two, the moving speed of the particles u _s Is represented by equation (2).
[0039]
(Equation 1)

(Equation 2)

[0040]
On the other hand, the bubble moving speed u ₀ Becomes the equation (3). Therefore, the moving speed ratio between the inorganic compound powder particles 7 dispersed in the organic resin 6 and the bubbles 8 is represented by the formula (4).
[0041]
[Equation 3]

(Equation 4)

[0042]
For example, ρ ₀ = 0, ρ _s = 4.2 g / cm ³ , Ρ = 1.4 g / cm ³ , D ₀ = 10 μm, d _s = 1 μm, u ₀ / U _s = −50. In other words, the bubbles 8 move in the direction opposite to the direction of the centrifugal force at a speed 50 times faster than the direction of the inorganic compound powder 7. From this, it can be seen that centrifugal casting can efficiently inject into narrow and deep cut grooves with no bubbles.
[0043]
In the above-described centrifugal casting, a relative displacement of the inorganic compound powder 7 with respect to the organic resin 6 occurs at the same time as the removal of the air bubbles (FIG. 4). As a result, as shown in FIG. 5, the reflective layer is divided into three regions A, C, and B in which the content of the inorganic compound varies stepwise in the injection direction (the depth direction of the groove). The upper region A has a lower content of the inorganic compound, and the lower region B has a high content of the inorganic compound. These regions are cut in the subsequent slicing step and are not used as scintillators, and are desirably as narrow as possible. In order to make these areas A and B narrow, the particle density ρ _s And the density ρ of the organic resin should be as small as possible, or the particle diameter of the particles should be as small as possible. Specifically, by setting the average particle size of the inorganic compound powder to 3 μm or less, the areas A and B can be suppressed to about 5% or less of the entire thickness, and the area C that can be used as a scintillator can be widened.
[0044]
On the other hand, in the region C effective as the reflection layer, the inclination α of the inorganic compound content occurs in the injection direction due to the particle size distribution of the inorganic compound powder. According to the study of the present inventors, the change in the reflectance of the scintillator can be tolerated if it is within 4% / cm in the thickness direction, and the change in the reflectance of 4% / cm is the content of the inorganic compound. This corresponds to a slope of 20 wt% / cm. By setting the standard deviation, which is an index of the particle diameter distribution of the inorganic compound powder, to 2.0 μm or less, the slope of the inorganic compound content can be suppressed to 20 wt% / cm or less, and a detector having no sensitivity unevenness can be configured. Can be.
[0045]
Therefore, in the method for manufacturing a scintillator of the present invention, the average particle size of the inorganic compound particles is set so that the region C usable as a scintillator is made as large as possible and the change in the reflectance in the thickness direction is kept within 4% / cm. It is desirable to use one having a size of 3 μm or less and a standard deviation of 2.0 μm or less.
[0046]
After the reflection layer forming material is injected into the cut grooves 20, the organic resin is cured (step 206), and the reflection layer 13 is formed. Thereafter, the scintillator is sliced to a desired thickness in a direction perpendicular to the depth direction of the groove (that is, parallel to the x, y plane) (step 207). In this case, regions A and B shown in FIG. 5 are removed, and a portion serving as a scintillator is sliced from region C. Finally, the slice surface is polished as necessary (step 208), and the scintillator 100 for a radiation detector is obtained.
[0047]
In addition, a metal shielding plate or a separator plate made of molybdenum, tungsten, stainless steel, or the like can be inserted into the reflection layer to shield visible light. When a metal shielding plate or the like is inserted into the reflection layer, the metal shielding plate is inserted into the cut groove before centrifugal separation and injection (before step 203), and then the reflection layer forming material is injected. Thereby, for example, a scintillator having a metal shielding plate in the channel direction of the X-ray detector can be obtained.
[0048]
In order to use the scintillator of the present invention as a radiation detector, a reflection layer is further formed around the scintillator 100 and on the side opposite to the surface on which the light detector is arranged. As the reflective layer, a material obtained by curing an organic resin containing the above-mentioned inorganic compound powder is usually used, but a different material may be used as long as there is no inconvenience.
[0049]
The scintillator manufactured by the above-described manufacturing method has a reflective layer having the same dimensional accuracy as the processing accuracy for forming the notch groove, and since the reflective layer does not contain bubbles, variation in characteristics of each scintillator element Therefore, a radiation detector with excellent detection accuracy can be provided.
[0050]
Next, a scintillator according to a second embodiment of the present invention and a radiation detector using the same will be described. FIG. 6 shows a configuration of the scintillator 200 according to the present embodiment, and FIG. 7 shows a configuration of a radiation detector using the scintillator 200. 6 and 7, the same elements as those in FIG. 1 are denoted by the same reference numerals.
[0051]
Also in the present embodiment, the scintillator element 12 and the reflective layer 17 are formed by forming grooves having a predetermined width at a predetermined pitch in a scintillator block having a predetermined size, and filling the grooves with a material forming the reflective layer. This is the same as in the first embodiment. Further, as the scintillator material, similarly to the first embodiment, for example, an oxide phosphor having a garnet structure whose main element is Gd, Al, Ga, and O, for example, can be preferably used as the luminescent center element.
[0052]
However, the scintillator and the detector according to the present embodiment are characterized by the structure of the reflective layer 17, and the reflective layer 17 includes a reflective diffusion layer 13 made of a resin containing a white inorganic compound powder (hereinafter, referred to as a white resin). And a separator plate 16. By inserting such a separator plate, light leakage between scintillator elements can be prevented, and crosstalk between channels can be eliminated.
[0053]
As the separator plate 16, a separator plate 16 having a three-layer structure of a white diffuse reflection layer 14, a metal film 15, and a white diffuse reflection layer 14 is used as shown. By using such a three-layered separator plate, the reflectance of the reflective layer can be improved as compared with the case where only a metal plate is used, and the separator plate 16 can be positioned substantially at the center of the reflective layer 17. Thus, the thickness of the reflection layer 13 and the white diffuse reflection layer 14 adjacent to the scintillator element can be made substantially uniform for each scintillator element.
[0054]
In the scintillator 200 of the present embodiment, the diffuse reflectance R 層 of the white diffuse reflection layer 13 in a sufficiently thick sample is larger than the diffuse reflectance Rb of the separator plate 16 (metal film 15 / white diffuse reflection layer 14). Is preferred. Specifically, the diffuse reflectance R∞ of the white diffuse reflection layer 13 is preferably 97% or more, and the diffuse reflectance Rb of the separator 16 is preferably 85% or more. Thereby, the diffuse reflectance at the scintillator element interface can be made 90%, and the light output of the scintillator can be improved.
[0055]
In order to realize the above-described diffuse reflectance, the white diffuse reflection layer 13 is made of a white resin in which a white inorganic compound is dispersed in an organic resin, and has a weight ratio of inorganic compound / (inorganic compound + organic resin). (%) Is preferably 30 to 80 wt%. However, if it exceeds 80% by weight, the viscosity of the white resin becomes extremely high, and it becomes difficult to completely remove bubbles even when the resin is injected by a centrifugal injection method. Therefore, the weight ratio is preferably 80% by weight or less. As the inorganic compound, titanium oxide, alumina, barium sulfate and the like can be used. In particular, titanium oxide (TiO 2) ₂ ) Is preferable, and the inorganic compound powder preferably has a small particle size and a narrow particle size distribution. As the organic resin, a colorless transparent resin such as an epoxy resin, a polyester resin, an acrylic resin, and a phenol resin, and a white resin that does not strongly absorb visible light of about 500 to 600 nm can be used.
[0056]
The separator plate 16 having a three-layer structure including the white reflection / diffusion layer 14 / the metal film 15 / the white reflection / diffusion layer 14 is formed by, for example, applying a white paint in which a white pigment is dispersed in a resin on both surfaces of the metal film. Can be manufactured. In this case, as a material of the metal film 15, a copper alloy such as molybdenum, tungsten, lead, aluminum, stainless steel, and phosphor bronze can be used. The thickness of the metal film may be any thickness as long as it can substantially block visible light, but is in the range of 10 to 50 μm in consideration of workability of forming a coating layer and the like. As a material constituting the white diffuse reflection layer 14, an inorganic compound powder and an organic resin constituting the white diffuse reflection layer 13 can be used. In particular, titanium oxide having a particle diameter of about 0.2 to 2 μm is used as the inorganic compound powder. It is suitable. As the organic resin, a colorless transparent resin such as an epoxy resin, a polyester resin, an acrylic resin, and a phenol resin, and a white resin that does not strongly absorb visible light of about 500 to 600 nm can be used. When the white diffuse reflection layer 14 is formed by applying such an inorganic compound, a known coating method such as a screen printing method, a doctor blade method, a brush coating, and a spray coating can be employed. The thickness of the white diffusion layer 14 varies depending on the thickness of the separator plate, but is, for example, 10 to 60 μm, and preferably 20 to 50 μm.
[0057]
In addition to the above, the three-layer separator plate may be manufactured by forming a metal film such as aluminum on one surface of a white polymer film by vapor deposition and bonding the white polymer film to the vapor deposition surface. The thickness of the metal deposition film is set to a thickness capable of blocking visible light, specifically, about 0.05 to 2 μm. As the white polymer film, for example, a white polyester film, a white polyethylene film, a white polyvinyl chloride film and the like can be used.
[0058]
The thickness of the separator plate depends on the width of the groove formed as the reflection layer, and is preferably 30% or more and less than 90% of the groove width. When the thickness of the separator is less than 30%, the ratio of the white diffuse reflection layer 13 to the groove width becomes large, so that the thickness of the white diffuse reflection layer 13 tends to be uneven. On the other hand, when the thickness of the separator is 90% or more, it is difficult to insert the separator into the groove, and it is difficult to inject the white resin because the gap for injecting the white resin forming the white diffuse reflection layer 13 is small. As a result, the diffuse reflection layer 13 becomes inhomogeneous due to residual bubbles and the like, and the characteristics of each scintillator element tend to vary.
[0059]
Next, a method for manufacturing the scintillator according to the second embodiment will be described. FIG. 8 shows an outline of the manufacturing method.
[0060]
First, cut grooves 20 are formed in a rectangular parallelepiped scintillator material 10 in two directions (shown as x and y directions in the figure) orthogonal to the thickness direction (z direction). The depth of the cut groove is shallower than the height of the scintillator material, and the scintillator material is left uncut and about several mm is left. The depth and width of the groove and the pitch in the x and y directions vary depending on the use of the radiation detector. In the case of an X-ray detector for X-ray CT, for example, the groove depth is 10 to 30 mm and the width is 0.1. The pitch is about 0.3 mm and the pitch is about 1.0 to 2.5 mm (step 801). Similar to the first embodiment, this grooving is performed by using a high-precision processing machine such as a multi-wire saw for processing while reciprocating the wire or an inner saw having an inner peripheral cutting edge coated with diamond. This enables high-precision processing of ± 0.005 mm or less.
[0061]
An annealing process is performed on the scintillator material after the grooving (step 802). The annealing treatment is a treatment for imparting scintillator characteristics when the scintillator material is an oxide phosphor. For example, a heat treatment is performed at a high temperature of 1100 to 1600 ° C. in oxygen for several tens of minutes to several hours.
[0062]
On the other hand, a separator plate to be inserted into the groove formed in the scintillator material is manufactured (Step 803). As described above, the separator plate has a three-layer structure in which the white diffuse reflection layer is formed on both surfaces of the metal film, and a metal film having a thickness of 30 to 90% of the groove width is prepared. Next, a separator plate is arranged in each groove formed in the scintillator material (step 804). The separator plate is typically placed only in a groove (for example, a y-direction groove) that separates channels, which is important for the resolution of a CT image. In this case, the same number of separator plates as the y-direction grooves are prepared in the same number as the length of the scintillator material in the y-direction, and are sandwiched between the y-direction grooves. In the case where separator plates are also inserted in grooves (for example, grooves in the x direction) separating the slices, for example, the same number of separator plates as the grooves in the x direction are prepared (x Each of them is formed with a notch having a depth of 1/2 in the thickness direction at the same pitch as the pitch of the groove in the y direction, and a separator plate having the same length as the length in the y direction is formed in the groove in the y direction. The same number as that of the y-groups is prepared, and the cuts having the same pitch as the groove pitch in the x-direction are formed at a depth of 1/2 in the thickness direction, and the x-group separator plate and the y-group separator are formed. It is preferable that the cuts are engaged with each other from above and below so that the plates are orthogonal to each other to produce a grid-like separator plate assembly corresponding to the grid-like cut grooves.
[0063]
After arranging the separator plate in the cut groove, white resin is injected into the groove (step 805). The injection of the white resin is preferably performed by the centrifugal injection method as in the first embodiment. By employing the centrifugal injection method, even if the viscosity of the white resin is as high as about 100,000 cPs, it can be reliably injected into the deep cut groove without generating bubbles.
[0064]
Prior to the injection of the white resin, the white resin may be applied to the surface of the cut groove (step 803 ′). By previously forming a coating layer of a white resin that forms a part of the white diffuse reflection layer on the groove surface, it is possible to reliably prevent an air layer from being formed between the scintillator and the white diffuse reflection layer. , A high diffuse reflectance can be achieved. As a method of applying the white resin to the groove surface, for example, a method of filling the groove with the white resin by a centrifugal injection method, and then centrifuging in the opposite direction to discharge the white resin in the groove may be adopted. it can.
[0065]
After curing the injected white resin, the scintillator material is sliced perpendicular to the cut grooves (steps 806 and 807). For slicing, for example, mechanical processing such as a multi-wire saw, an inner peripheral slicer, and an outer peripheral slicer can be used. Also in this case, when the centrifugal injection method is employed as the white resin injection method, a gradient occurs in the content of the white inorganic compound. Therefore, a region having a tilt of 20 wt% / cm or less shown in FIG. I do. Finally, the slice surface is lap-polished or the like to obtain a scintillator having a target thickness (step 808).
[0066]
In order to use the scintillator manufactured in this way as a radiation detector, as shown in FIG. 7, a radiation incident surface is covered with a reflector 18 and the opposite surface is photodetected at the same pitch as the scintillator element. The substrate (photodiode) 11 is formed on the substrate 21 with a transparent adhesive 19 or the like.
When radiation enters the incident surface of such a radiation detector, the scintillator emits light according to the dose of the incident radiation, and the light is photoelectrically converted by the photodiode. Here, since each scintillator element is separated by the reflective layer 17 composed of the white diffuse reflective layer 13 / white diffuse reflective layer 14 / metal film 15 / white diffuse reflective layer 14 / white diffuse reflective layer 13, a cross between the elements is provided. There is no talk, and light generated by the scintillator element is reflected by the reflection layer 17 with high reflectance, so that a high detection output can be obtained.
[0067]
Next, the X-ray CT apparatus of the present invention will be described. FIG. 9 is a diagram schematically showing an X-ray CT apparatus. This apparatus includes a gantry unit 60 and an image reconstructing unit 70. The gantry unit 60 has a rotary unit having an opening 61a into which a subject is carried. A disk 61, an X-ray tube 62 mounted on the rotating disk 61, a collimator 63 attached to the X-ray tube 62 for controlling the radiation direction of X-rays, and a rotating disk facing the X-ray tube 62 An X-ray detector 64 mounted on 61, a detector circuit 65 for converting the X-rays detected by the X-ray detector 64 into a specific signal, and controlling the rotation of the rotating disk 61 and the width of the X-ray flux. And a scan control circuit 66. The image reconstruction unit 70 includes an input unit 71 for inputting inspection conditions and the like, and a monitor 72 for displaying an image. Here, an X-ray detector using the scintillator of the present invention is used as the X-ray detector 64.
[0068]
In such a configuration, X-rays are emitted from the X-ray tube 62 in a state where the subject is laid on a bed installed in the opening. This X-ray obtains directivity by the collimator 63 and is detected by the X-ray detector. By rotating the rotating disk around the subject, X-rays are detected while changing the irradiation direction of the X-rays, and a tomographic image is created by the image reconstruction unit 70 and displayed on the monitor 72.
[0069]
Since the X-ray CT apparatus uses the scintillator of the present invention, the X-ray sensitivity and the quality characteristic of each element and each channel are small, and X-rays can be detected with high accuracy. Even when image processing is performed on the cross section, an image with high image quality and high resolution can be obtained without causing artifacts.
[0070]
【Example】
Hereinafter, examples of the present invention will be described.
[0071]
[Example 1]
The composition formula is (Gd, Ce) _{8 (1-x)} (Al, Ga) _8x O ₁₂ , X = 0.620, Ce / Gd = 0.008, and Ga / (Ga + Al) = 0.44 were hot-pressed to obtain a sintered body having a diameter of 160 mm and a thickness of about 15 mm. . The relative density of this sintered body was 99.9% or more. Using a multi-blade saw, eight scintillator materials each having a size of 40 mm × 25 mm × about 15 mm in the x, y, and z directions were cut out from the sintered body.
[0072]
Using a multi-wire saw having a wire diameter of 0.16 mm, a lattice-shaped cut groove perpendicular to the xy plane was formed in the cut-out block. First, grooving was performed at a pitch of 2.25 mm in the x direction, then the scintillator material was rotated by 90 °, and grooving was performed at a pitch of 1.0 mm in the y direction. The depth of each groove was about 13 mm. The lattice-shaped cut grooves thus formed had an accuracy of a groove width of 0.200 ± 0.005 mm in both the x and y directions.
[0073]
Next, the scintillator material was annealed in oxygen for imparting scintillator characteristics. The heat treatment was performed at a temperature of 1300 ° C. for 4 hours. The inventors of the present invention have found that the rare earth oxide phosphor having the above composition can provide the X-ray CT detector with optimal characteristics such as X-ray sensitivity and afterglow decay rate by such annealing treatment. It has been confirmed (Japanese Patent Application No. 2000-389343).
[0074]
On the other hand, TiO having an average powder particle diameter of 0.3 μm ₂ Mix powder (TiO 2) ₂ / (TiO ₂ + Epoxy resin)) to be 65 wt%, and mixed them uniformly. This TiO ₂ The viscosity of the contained epoxy resin was 98,000 cPs.
[0075]
After performing a surface treatment on the scintillator material that has been annealed to improve the adhesiveness with the organic resin, put it in a casting frame for casting the organic resin, and place it on the surface where the cut grooves are formed. TiO ₂ The contained epoxy resin was placed. Next, this scintillator material was set together with the casting frame in a bucket of a centrifuge, and rotated at 3500 rpm for 5 minutes to obtain TiO 2. ₂ The contained epoxy resin was cast into the cut grooves. The scintillator material after casting was heated at 60 ° C. for 3 hours, and the epoxy resin was cured to form a reflecting material. No air bubbles were observed inside the reflector, and the reflector was desorbed and removed from the surface.
[0076]
The scintillator material thus formed with a grid-like reflector is sliced using a multi-wire saw so as to have a thickness of 2.0 mm parallel to the xy plane, that is, perpendicular to the z-axis. Five scintillator plates were produced in which four peripheral surfaces of the scintillator element were surrounded by a reflective material. These scintillator plates were polished using a lapping machine so that the thickness of the scintillator plates was 1.800 ± 0.005 mm. Through the above steps, five scintillator plates each composed of scintillator elements of 2.05 mm × 0.8 mm × 1.800 mm were obtained.
[0077]
TiO as a reflector in the height direction (z-axis direction) of the scintillator plate ₂ In order to examine the content, the content of the Ti element was measured by using an EDS (Energy Dispersive Sectorscopy) method. As a result, a difference of 0.8 wt% was observed in the height direction of 1.800 mm. The standard deviation at the time of this measurement was 0.31 wt% in 2σ.
[0078]
[Example 2]
A detector as shown in FIG. 1 is made by combining the scintillator manufactured in Example 1 with a photodiode, and the detector is placed at a distance of 15 cm from an X-ray source (120 kV, 0.5 mA), and X-ray sensitivity and afterglow. The properties were evaluated. As a result, the X-ray sensitivity is CdWO which is a typical X-ray CT scintillator. ₄ And the decay rate of afterglow in 300 ms is about 10 ^-5 And could be.
[0079]
[Example 3]
Hot press sintering was performed using powder having the same composition as in Example 1 to obtain a sintered body having a diameter of 160 mm and a thickness of about 15 mm. Using a multi-blade saw, eleven scintillator materials having a size of 33 mm × 25 mm × about 15 mm in the x, y, and z directions were cut out from the sintered body.
Using a multi-wire saw having a wire diameter of 0.11 mm, a lattice-shaped cut groove perpendicular to the xy plane was formed in the cut-out block. First, a groove was formed in the x direction at a pitch of 1.0 mm to a depth of about 13 mm, and then the scintillator material was rotated by 90 °, and a groove was formed in the y direction to a depth of about 13 mm at a pitch of 1.0 mm. . The lattice-shaped cut grooves formed in this manner had a groove width of 0.130 ± 0.005 mm in both the x and y directions.
[0080]
Next, the scintillator material was annealed in oxygen for imparting scintillator characteristics. The heat treatment was performed at a temperature of 1300 ° C. for 4 hours. The light output of the rare earth oxide phosphor of the above composition is CdWO which is a typical X-ray CT scintillator by such annealing treatment. ₄ Approximately twice as large, and the decay rate of afterglow in 300 ms is about 1 × 10 ^-5 Such characteristics were obtained.
[0081]
On the other hand, an Al film having a thickness of 0.1 μm was formed on one surface of a white polyester film having a thickness of 38 μm by vapor deposition, and a white polyester film having a thickness of 38 μm was adhered to the Al vapor-deposited surface to obtain a white polyester film / Al A separator plate having a three-layer structure of a vapor-deposited film / white polyester film was manufactured. The diffuse reflectance of this separator plate was 86% at a wavelength of 550 nm.
[0082]
Epoxy resin and TiO2 are used as white resin to be injected into the scintillator groove. ₂ The powder was uniformly mixed at a weight ratio of 1: 1. The diffuse reflectance R∞ of this white resin was 98% at a wavelength of 550 nm. After inserting the separator plate manufactured as described above into the groove of the scintillator material after grooving, it was set in a casting frame, and this white resin was placed on the surface of the scintillator material. Thereafter, the casting frame was set in a centrifuge, and centrifugal injection was performed at 3000 rpm for 10 minutes.
[0083]
After the injection, the white resin was cured at room temperature for 12 hours, and further cured at 60 ° C. for 3 hours. Next, slice processing was performed using a multi-wire saw parallel to the upper surface of the scintillator material, that is, perpendicular to the z-axis, and five scintillator plates having a thickness of 1.95 mm were cut out from the center in the thickness direction. Both surfaces of these scintillator plates were polished with a lapping machine to a thickness of 1.800 ± 0.005 mm. By the above processing, five scintillators for 24 channels and 32 multi-slices each consisting of a scintillator element of 0.87 mm × 0.87 mm × 1.800 mm were manufactured.
[0084]
[Comparative Example 1]
In the same manner as in Example 3, a scintillator material having a cut groove was produced. The same separator plate as in Example 3 was inserted into this groove, and an epoxy-based transparent adhesive was injected into the gap between the groove and the separator plate and cured. Thereafter, slicing and lapping were performed in the same manner as in Example 3 to produce a scintillator plate.
[0085]
[Example 4, Comparative Example 2]
Each of the scintillators manufactured in Example 3 and Comparative Example 1 was combined with a photodiode to form a detector as shown in FIG. 7, and irradiated with X-rays (120 kV, 0.5 mA) from a distance of 15 cm. The sensitivity was measured. As a result, the detector manufactured by the scintillator of Example 2 (Example 4) has an X-ray sensitivity that is 1.1 to 1.2 times that of the detector manufactured by the scintillator of Comparative Example 1 (Comparative Example 2). Linear sensitivity was obtained. In the detector of Example 4, the variation in the sensitivity distribution of each scintillator element was 5.0% or less, and the detector was excellent in uniformity.
[0086]
[Example 5]
A scintillator material having a lattice-shaped cut groove was manufactured in exactly the same manner as in Example 3. On the other hand, as a separator plate, a white resin is applied on both sides of a 10 μm-thick molybdenum plate by screen printing and cured to form a 35 ± 10 μm-thick white diffuse reflection layer, and a white diffuse reflection layer / Mo plate / white diffuse reflection layer is formed. A three-layer separator plate having a reflective layer was manufactured. Here, the white resin forming the white diffuse reflection layer is made of TiO 2 having a particle size of 0.25 μm with respect to the epoxy resin. ₂ A powder obtained by uniformly mixing powders at a weight ratio of 1: 1 was used. The diffuse reflectance of this separator plate was 92 to 93% at a wavelength of 550 nm.
[0087]
After inserting this separator plate into the groove of the scintillator plate, the same white resin (epoxy resin: TiO ₂ (Powder = 1: 1, diffuse reflectance R∞ = 98%), injected into the groove of the scintillator plate, cured, sliced and wrapped in the same manner as in Example, and 0.87 mm × 0.87 mm × 1. Five 800 mm scintillator plates were manufactured.
Using this scintillator, a detector was made, and X-rays (120 kV, 0.5 mA) were irradiated from a distance of 15 cm to measure the X-ray sensitivity. As a result, an X-ray sensitivity 1.15 to 1.25 times that of the detector of Comparative Example 2 was obtained. The variation in the sensitivity distribution of each scintillator element was 5.0% or less, and the uniformity was excellent.
[0088]
[Example 6]
After producing a scintillator material having a lattice-shaped cut groove exactly in the same manner as in Example 3, a coating layer of a white resin was formed on the surface of the cut groove of the scintillator material. The same white resin as that injected into the groove in Example 3 was used as the white resin. In order to form a coating layer, first, white resin is injected into the groove by centrifugal injection and filled into the groove, and then the opening of the groove is set in the opposite direction to that at the time of centrifugal injection, that is, the bottom of the groove is centrifuged. The resin in the groove was removed by operating the centrifuge so as to face the center of the machine. The centrifugation was performed at 1,000 rpm for 5 minutes both when the resin was injected and when the resin was discharged. Thus, a coating layer of a white resin having a thickness of about 30 μm was formed on the groove surface. This coating layer was cured at room temperature for 12 hours to form a white diffuse reflection layer of 15 μm ± 10 μm.
[0089]
In this manner, a separator plate having a three-layer structure of a white polyester film / Al vapor-deposited film / white polyester film similar to that of Example 3 was inserted into the groove of the scintillator material having the white diffuse reflection layer formed on the surface. A white resin was centrifugally injected between the plates and a white diffuse reflection layer was formed. Epoxy resin and TiO as white resin ₂ A mixture of powders mixed at a weight ratio of 1: 1 was used, and the centrifugation conditions were 3000 rpm for 10 minutes. After the injection, the mixture was cured at room temperature for 12 hours, and further cured at 60 ° C. for 3 hours. Next, slicing and lapping were performed in the same manner as in Example 3 to produce five 0.87 mm × 0.87 mm × 1.800 mm scintillator plates.
[0090]
Using this scintillator, a detector was made, and X-rays (120 kV, 200 mA) were irradiated from a distance of 15 cm to measure the X-ray sensitivity. As a result, an X-ray sensitivity 1.1 to 1.2 times that of the detector of Comparative Example 2 was obtained. The variation in the sensitivity distribution of each scintillator element was 5.0% or less, and the uniformity was excellent.
[0091]
[Example 7]
The X-ray detector created in Example 2 was used as the X-ray CT apparatus shown in FIG. An X-ray CT image was taken using the apparatus. No artifacts were seen and high quality tomographic images were obtained.
[0092]
Example 8
The X-ray detector created in Example 4 was used as the X-ray CT apparatus in FIG. 9 under the conditions of an X-ray tube voltage of 120 kV, an X-ray tube current of 175 mA, and a scan time of 0.8 s, using a head approximation phantom. An X-ray CT image was taken. No artifacts were seen and high quality tomographic images were obtained.
[0093]
【The invention's effect】
According to the present invention, in a detector in which a large number of columnar scintillators whose four peripheral surfaces are surrounded by a reflective layer are arranged, a process in which the thickness variation of the reflective layer and the pitch accuracy are used to form the notch for the reflective layer. It is possible to increase the processing accuracy of the machine to substantially the same level, and at the same time, it is possible to obtain a high-precision scintillator with no defects in the reflective layer such as air bubbles and a minimum number of scintillator manufacturing steps. Further, according to the present invention, the scintillator is configured such that the reflection layer provided between each scintillator element is composed of the first reflection layer / second reflection layer / metal film / second reflection layer / first reflection layer. Since a high diffuse reflectance can be achieved at the element interface and crosstalk between the scintillator elements can be reliably prevented, a detector with high detection sensitivity and detection output can be provided. Therefore, by applying this scintillator to an X-ray detector for an X-ray CT apparatus, it is possible to provide an X-ray CT apparatus capable of obtaining a high-quality tomographic image without artifacts.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a radiation detector to which the present invention is applied.
FIG. 2 is a view for explaining an example of a scintillator manufacturing process of the present invention.
FIG. 3 is a view for explaining the mechanism of the centrifugal casting method of the present invention.
FIG. 4 is a view for explaining a part of a scintillator material.
FIG. 5 is a view for explaining the content ratio of inorganic compound powder particles in the reflecting material of the scintillator of the present invention.
FIG. 6 is a diagram showing a configuration of a scintillator according to a second embodiment of the present invention.
FIG. 7 is a diagram showing a configuration of a radiation detector using the scintillator of FIG. 6;
FIG. 8 is a view for explaining an example of a manufacturing process of the scintillator of FIG. 6;
FIG. 9 is a view schematically showing an X-ray CT apparatus to which the present invention is applied.
[Explanation of symbols]
6 ... Organic resin
7 ... Inorganic compound powder
10 ... Scintillator material
13. Reflective layer (first reflective layer)
14... Second reflective layer
15 ... Metal film
16 ・ ・ ・ Separator plate
20 ... notch groove
100, 200 ... scintillator

Claims (11)

A plurality of columnar scintillator elements are arranged in a first direction and a second direction orthogonal to the first direction, respectively, and a scintillator for a radiation detector is provided with reflection layers provided on four peripheral surfaces in the arrangement direction of each scintillator element. Wherein at least a reflection layer provided between the scintillator elements is made of a resin containing a reflection material, and the content of the reflection material monotonically changes in a direction orthogonal to the arrangement direction. Dexterous scintillator. The scintillator for a radiation detector according to claim 1, wherein a separator plate is inserted at a central portion of at least a reflection layer provided between each of the scintillator elements. The scintillator for a radiation detector according to claim 2, wherein the separator plate has a three-layer structure in which a white diffuse reflection layer is laminated on both surfaces of a metal film. A step of forming a cut groove having a predetermined pitch, a predetermined width, and a predetermined depth along a first direction and a second direction orthogonal to the first direction on a rectangular parallelepiped scintillator material, and using a centrifuge, A step of injecting a resin containing a reflective material powder into the cut grooves, a step of curing the resin, and a step of cutting the resin along a third direction orthogonal to the first and second directions after the resin is cured. A method for producing a scintillator for a radiation detector, comprising: Forming, in a rectangular parallelepiped scintillator material, cut grooves having a predetermined pitch, a predetermined width, and a predetermined depth along a first direction and a second direction orthogonal to the first direction; A step of inserting a separator having a thickness smaller than the width, a step of injecting a resin containing a reflective material powder into a cut groove into which the separator is inserted by using a centrifuge, a step of curing the resin, and a step of curing the resin And subsequently cutting along a third direction orthogonal to the first and second directions. A scintillator for a radiation detector manufactured by the manufacturing method according to claim 4. A plurality of columnar scintillator elements are arranged in a first direction and a second direction orthogonal to the first direction, respectively, and a scintillator for a radiation detector is provided with reflection layers provided on four peripheral surfaces in the arrangement direction of each scintillator element. , Wherein the reflective layer has a first reflective layer made of a resin containing a reflective material, and a separator having a second reflective layer inserted at the center of the first reflective layer and in contact with the first reflective layer And a scintillator for a radiation detector. The scintillator for a radiation detector according to claim 7, wherein the separator has a three-layer structure in which a white diffuse reflection layer is laminated on both surfaces of a metal film. 8. The scintillator for a radiation detector according to claim 7, wherein a diffuse reflectance R∞ of a sufficiently thick sample of the first reflective layer is larger than a diffuse reflectance Rb of a second reflective layer of the separator. A scintillator for a radiation detector, comprising: A radiation detector comprising a scintillator and a photodetector for detecting light emission of the scintillator, wherein the scintillator according to any one of claims 1 to 3 and 6 to 9 is used as the scintillator. Detector. An X-ray source, an X-ray detector arranged opposite to the X-ray source, a rotating disk holding the X-ray source and the X-ray detector and driven to rotate around a subject, 11. An X-ray CT apparatus comprising: an image reconstruction unit configured to reconstruct a tomographic image of the subject based on the intensity of X-rays detected by the X-ray detector. An X-ray CT apparatus using a detector.

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