WO1997011389A1

WO1997011389A1 - Gamma ray detector

Info

Publication number: WO1997011389A1
Application number: PCT/US1996/015129
Authority: WO
Inventors: Steven G. Walters; Ducan W. Macarthur
Original assignee: Koster, James, E.
Priority date: 1995-09-21
Filing date: 1996-09-20
Publication date: 1997-03-27
Also published as: AU7366996A

Abstract

A gamma detector (10) comprises a sealed electrically conductive enclosure (11) defining a cavity filled with a gas, and with signal collection means (12) insulatively mounted in the cavity and positioned to substantially span the cavity so that gas ions generated in the cavity as a result of gamma ray interaction are electrostatically captured by the signal collection means (12) when an electric potential is applied between the electrically conductive enclosure (11) and the signal collection means (12). Indicator means (19) are attached to the signal collection means (12) for indicating an electrical current produced by collection of gas ions generated in the cavity by gamma rays in the cavity. A voltage source (18) is connected between the indicator means (19) and the electrically conductive enclosure (11) to provide the electrical potential for collection of the gas ions.

Description

GAMMA RAY DETECTOR

FIELD OF THE INVENTION The present invention generally relates to radiation detection, and, more specifically, to the detection of gamma rays. This invention was made with Government support under

Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

The detection of gamma rays is of extreme importance to those who are required to work with and around radioactive materials, because of the extreme health hazards they pose. Basically, gamma rays are electromagnetic radiation emitted by radioactive decay and having energies in the range extending up to several million electron volts. Ofthe emitted radiation, gamma rays are arguably the most dangerous because of their ability to penetrate most materials.

Many gamma ray detectors are difficult to employ, especially in field applications. Many others are extremely complex and difficult to operate with accuracy. There are several basic gamma detector designs currently in use: among these are the scintillation detectors, the semiconductor detectors, and gas filled detectors.

The scintillation detectors work by way of a scintillator producing light when impacted by gamma radiation, the light being detected by a photomultiplier tube. These type detectors are generally relatively bulky and heavy, and are very sensitive to shock. They are quite useful in a laboratory or other protected environment, but often are not sufficiently rugged for field use.

The semiconductor detectors usually employ a semiconductor diode, that, in the presence of gamma radiation, creates electron-hole pairs at the junction which can be easily detected, ln order to operate in this manner, the semiconductor diode often must be cooled to liquid nitrogen temperatures. This type of detector is more accurate than the scintillation detectors, but is even more delicate and cumbersome.

The gas filled detectors operate in a manner similar to the semiconductor detectors except that an electron-ion pair is created by the gamma radiation interacting with a specialized gas which often is at a high pressure. The signal collector in many of these detectors is a very fine wire so that the electric field near the wire is very strong. This strong field is able to accelerate the nearby electrons and cause the liberation of additional electrons which can be collected.

While it is true that these detectors are excellent laboratory devices, they each lack the necessary ruggedness to provide reliable service in field applications. It is here that the present invention surpasses the prior art in providing a simple, easily operated gamma detector which provides accurate output either in the laboratory or in the field.

The present invention provides a gamma detector based on technology which is contained in several U.S. Patents which disclose various devices for the long range detection of alpha particles. The first is U.S. Patent No. 5,184,019, issued February 2, 1993, for a Long Range Alpha Particle Detector. The second is U.S. Patent No. 5,194,737, issued March 16, 1993, for Single and Double Grid Long Range Alpha Detectors. The third is U.S. Patent No. 5,187,370, issued February 16, 1993, for Alternating Current Long Range Alpha Particle Detectors. The fourth is U.S. Patent No. 5,281,824, issued January 25, 1994, for Radioactive Detection. The fifth is U.S. Patent No. 5,31 1,025, issued May 10, 1994, for Fan-less Long Range Alpha Detector. Another recently filed application bears serial number 08/833020, filed November 1, 1994, and is entitled "Event Counting Alpha Detector." Still other recently filed applications bear serial number 08/382,333, filed February 1 , 1995, entitled "Background Canceling Surface Alpha Detector," serial number 08/395,934, filed February 27, 1995, entitled "High Air Flow Alpha Detector," and serial number 08/456,272, filed May 31 , 1995, entitled "Segmented Surface Alpha Detector." As previously described, the principle underlying each of these issued patents and patent applications is that alpha particles, although themselves of very short range in air. ionize various of the molecular species in air. The present invention modifies and improves on these apparati to provide for fast and reliable detection of gamma radiation. It is therefore an object of the present invention to provide apparatus for the detection of gamma radiation.

It is another object of the present invention to provide apparatus which allows for the detection of gamma radiation in a field environment without the use of specialized gases, crystals, cryogenic cooling, fine wires, or complex electronics. Additional objects, advantages and novel features ofthe invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination ofthe following or may be learned by practice of the invention. The objects and advantages ofthe invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a gamma detector comprising a sealed electrically conductive enclosure defining a cavity containing a gas with signal collection means insulatively mounted in the cavity and positioned to substantially span the cavity so that gas ions generated in the cavity as a result of gamma ray interaction are electrostatically captured by the signal collection means when an electric potential is applied between the electrically conductive enclosure and the signal collection means. Indicator means are attached to the signal plane means for indicating an electrical current produced by collection of gas ions generated in the cavity by gamma rays in the cavity. A voltage source is connected between the indicator means and the electrically conductive enclosure to provide the electrical potential for collection ofthe gas ions.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incoφorated in and form a part o the specification, illustrate the embodiments ofthe present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 is a schematical cross-sectional view of one embodiment ofthe present invention in which all components ofthe invention are located inside an openable sealed electrically conductive enclosure.

FIGURE 2 is a plot of the response of one embodiment of the present invention and a Nal detector as a function ofthe distance to a gamma-ray source ("FLATTOP")

DETAILED DESCRIPTION The present invention provides apparatus which is capable of detecting gamma radiation while at the same time being equally capable both in the laboratory and in the field. It employs technology which originated with the detection of alpha particles, but improves on this technology to provide accurate and sensitive detection of gamma radiation.

The invention can be most easily understood through reference to the drawings. In Figure 1 , a schematical cross sectional view of one embodiment of the invention reveals the components of detector 10. As shown, electrically conductive enclosure H defines cavity 1 l a and contains signal collector J_2, insulatively mounted to electrically conductive enclosure JJ_ through insulative stand-offs J_2. Electrically conductive enclosure JJ_. is a sealed enclosure, openable for maintenance and repair, which can be made of any desirable electrically conductive material, such as aluminum, and in most any desired shape.

Although Figure 1 schematically illustrates the walls of electrically conductive enclosure JJ_. as being relatively thick, this is not to indicate that such thickness is a requirement of the present invention. In fact, the walls of electrically conductive enclosure 11 may be as thin as desired as long as it is capable of containing a gas such as air. An interesting aspect of electrically conductive enclosure JJ. as used in the present invention is that it could even take the form of an electrically conductive flexible enclosure, perhaps comprising a material such as aluminized mylar. This would allow electrically conductive enclosure JJ. to be folded into a small size for transport, and erected at a particular field site. If air is the gas of choice, there would be no need to bring along heavy cannisters of gas. Using an electrically conductive flexible enclosure as electrically conductive enclosure JJ. also allows detector JO to be as large as desired for a particular application.

It should be understood that whatever electrically conductive material is used to construct electrically conductive enclosure JJ., the gas contained in electrically conductive enclosure JJ. may be, does not have to be pressurized. Ambient pressure is totally satisfactory for proper operation of detector J_0.

Signal collector J_2 is made of an electrically conductive material, and may be made in any desired shape, as long as signal collector 12 spans substantially the width and/or length of electrically conductive enclosure H depending on the configuration of signal collector 12. Signal collector 12 may be solid, although it is often preferable that the gas in electrically conductive enclosure JJ. be free to circulate through it. For this reason, it may be preferable that signal collector \2 be constructed as a wire mesh, or as a solid sheet with perforations. Also, signal collector J_2 may simply be a wire electrode spanning the width of electrically conductive enclosure JJ., or any other electrically conductive mass.

Signal collector 12 must be insulated from electrically conductive enclosure JJ.. This may be accomplished in any manner suitable for a particular electrically conductive enclosure JJ.. ln some embodiments ofthe present invention, either LEXAN® or TEFLON® stand-offs can be used. Voltage source Jj? need supply typically 300 V or less for proper operation of detector

10. When detector JJ) is used in field operations, it will be most convenient if voltage source 18 is a battery. In many other applications, use of a battery or other direct current source will be preferred. However, an alternating current source could also be used. In this event, gas ions of both polarities will be detected by signal collector JJ2.

Also shown connected to signal collector JJ2, in line with voltage source 18 is electrometer 19, representing the means for displaying the detection ofthe gamma rays by detector Jj). ln some applications, the output of electrometer 19 could be digitized and displayed on a computer monitor (not shown).

In the embodiment illustrated in Figure 1, voltage source 18 and electrometer J9 are placed inside electrically conductive enclosure H. In other embodiments, it may be more desirable for voltage source J_8 and electrometer J9 to be located outside of electrically conductive enclosure 1 1.

In operation, detector JjO, connected to voltage source J_8 and electrometer 19, is placed into a position subject to penetration by gamma rays 20. Depending on the volume of electrically conductive enclosure JJ., gamma rays 20 encountering detector 1_0 will interact with either electrically conductive enclosure JJ_., in which metallic electrons 2J_ will be liberated and create gas ions in cavity 1 la, or will pass through electrically conductive enclosure JJ_. and directly interact with gas molecules in cavity 1 la, creating gas ions. The greatest effect ofthe metallic ions liberated from electrically conductive enclosures JJ_. will occur near the inner surface of electrically conductive enclosures JJ.. Therefore, in smaller volume electrically conductive enclosures JJ., these metallic ions will contribute to the signal collected by signal collector 12. In larger electrically conductive enclosures JJ_., the metallic ions will have less effect.

The gas ions created by the gamma interaction will be attracted to signal collector 12. where they will create a current in electrometer 19. In most embodiments, the gas inside electrically conductive enclosure JJ_. will be air. EXAMPLE

A test of the present invention was conducted over a two month period at Los Alamos National Laboratory. Detector J O was placed at known distances, ranging from 1000 to 5400 feet from a critical assembly running at different power levels. A Sodium Iodide (Nal)detector was placed near detector JO so that a comparison with the well established detection capabilities of the Nal detector could be made.

Locations for the detectors were chosen for the distance from the critical assembly as well as whether there was a direct line of sight to the critical assembly, or whether the line of sight was blocked by earth features. With the detectors at a non-line of sight position, the received signal was due to "skyshine," the redirection of ionizing and non-ionizing radiation by scattering events in the air.

The data from each detector were normalized to an ionization chamber positioned next to the critical assembly ("FLATTOP") which is used to measure the critical assembly's power level. The direct line of sight results for the Nal detector and for detector 10 are tabulated in Table I, while the skyshine results are tabulated in Table II. The output of detector 10 is in femtoamperes (fA) so the output listed in Tables 1 and II represent the normalized fA reading. The output ofthe Nal detector is in cps, and the output listed in Tables 1 and II represent the normalized cps reading.

TABLE I Line qf Sight Locations

DISTANCE (ft) DETECTOR 10 Nal 1000 9074 no data taken 1687 1420 2525 3508 208 242

TABLE II

DISTANCE (ft) DETECTOR 10 Nal 1300 1 1273 9250 1600 151 1 1930 1750 605 1 151 2100 225 no data taken 2500 178 312 5375 5.8 15

In Figure 2, these data are plotted for both detector JJ) and a Nal detector as a function of distance from a gamma ray source known locally as "FLATTOP." This plot clearly shows that the response of detector JJ), based on ion collection, is very similar to the response of the well established capabilities of the Nal detectors. The plot also illustrates the sensitivity of detector JJ) when located more than 5000 feet from a source. Distances even greater than this will be possible with the present invention.

These data illustrate that detector K) is extremely useful in detecting gamma radiation, even at distances greater than 5000 ft. The output obtained from detector JO is comparable to the Nal detector for both the line of sight and skyshine locations. This is very important, since detector JJ) needs neither a photo tube nor a sensitive and expensive crystal is necessary for its operation. It should also be realized that detector JO is extremely rugged and durable, and can be left in operation, unattended, for years at a time. The foregoing description ofthe embodiments ofthe invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light ofthe above teaching. The embodiments were chosen and described in order to best explain the principles ofthe invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope ofthe invention be defined by the claims appended hereto.

The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope ofthe invention be defined by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A gamma detector comprising: a sealed electrically conductive enclosure defining a cavity containing a gas; signal collection means insulatively mounted in said cavity and positioned to substantially span the cavity so that gas ions generated in the cavity as a result of gamma ray interaction are electrostatically captured by the signal collection means when an electric potential is applied between the electrically conductive enclosure and the signal collection means; indicator means attached to the signal plane means for indicating an electrical current produced by collection of gas ions generated in said cavity by gamma rays in said cavity; a voltage source connected between said indicator means and said electrically conductive enclosure to provide said electrical potential for collection of said gas ions.

2. The gamma detector as described in Claim 1 wherein said sealed electrically conductive enclosure is comprised of aluminum.

3. The gamma detector as described in Claim 1 wherein said sealed electrically conductive enclosure comprises an electrically conductive flexible enclosure.

4. The gamma detector as described in Claim 3 wherein said electrically conductive flexible enclosure is comprised of aluminized mylar.

5. The gamma detector as described in Claim 1 , wherein said signal collection means comprises solid sheets of aluminum.

6. The gamma detector as described in Claim 1 , wherein said signal collection means comprises perforated sheets of electrically conductive material.

7. The gamma detector as described in Claim 1, wherein said signal collection means comprises planar electrically conductive grids.

8. The gamma detector as described in Claim 1 , wherein said signal collection means is insulatively mounted inside said electrically conductive enclosure using LEXAN® stand¬ offs.

9. The gamma detector as described in Claim 1 , wherein said signal collection means is insulatively mounted inside said electrically conductive enclosure using TEFLON® standard-offs.

10. The gamma detector as described in Claim 1, wherein said voltage source comprises a battery having a voltage of approximately 300 VDC.

1 1. The gamma detector as described in Claim 1, wherein said voltage source comprises a source of approximately 300 VAC.

12. The gamma detector as described in Claim 1 , wherein said indicator means comprises an electrometer.

13. The gamma detector as described in Claim 1 , wherein said gas comprises air.

14. The gamma detector as described in Claim 1, wherein said sealed electrically conductive enclosure is openable.