JP4131538B2 - Method for quickly and easily measuring the concentration of Cd contained in a small amount of food by prompt gamma ray analysis - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention combines the multiple gamma ray analysis technology used in the field of nuclear science and the prompt gamma ray analysis technology used in the field of microanalysis, thereby reducing the poor quantitative accuracy that has become a serious defect in prompt gamma ray analysis. Overcoming this, it is possible to obtain a resolution 1000 times higher than that of conventional prompt gamma ray analysis. As a result, it is expected that the Cd concentration in foods, which has become a problem in recent years, can be measured quickly and easily, and can greatly contribute to the field of food hygiene.
[0002]
[Prior art]
Currently, inductively coupled plasma mass spectrometry (ICP-MS) is generally used to detect trace amounts of Cd contained in agricultural products and soil. However, ICP-MS generally handles liquid or gas samples, and requires an operation of making food into a solution as a pretreatment. This operation has a problem that it requires skill and time. Further, since ICP-MS is based on particle detection, it cannot be confirmed whether the measured value is really due to Cd, and there is a problem that the reliability is required when performing microanalysis.
[0003]
Prompt gamma analysis, on the other hand, is a method of analyzing elements or isotopes by measuring the energy of prompt gamma rays that are released immediately when a sample is irradiated with neutrons, other particle beams, and gamma rays. The nature is high.
[0004]
This analysis method has greatly advanced as a highly sensitive nondestructive inspection analysis method since the Ge (Li) semiconductor detector was developed in the 1960s and the low energy neutron guide beam was developed in the 1990s. In fact, prompt gamma ray analysis is very effective for the analysis of elements that are difficult to analyze by ICP-MS, and the ability to analyze many elements in a non-destructive manner because of the low neutron irradiation and negligible activation of the sample. It is widely used for analysis of precious materials such as archaeological samples and difficult samples such as meteorites and rocks.
[0005]
In conventional prompt gamma ray analysis (for example, see Non-Patent Documents 1, 2 and 3 below), prompt gamma rays emitted from excited nuclei are measured with a Ge measuring device, the horizontal axis is energy, and the vertical axis is the gamma ray. A one-dimensional spectrum with the number of detections (count) is made and measured. Since the energy of prompt gamma rays is determined by the nucleus, the relevant elements and isotopes are quantified based on the count of the prompt gamma rays of the relevant energy.
[0006]
[Non-Patent Document 1]
IAEA International Nuclear Data Committee Report INDC (NDS) -411 (2000) and INDC (NDS) -424 (2001)
[Non-Patent Document 2]
J. et al. K. Tuli: “Prompt gamma neutron activation analysis”, Edit by Z. B. Alfassi, C Chuna. pp. 177 (1995)
[Non-Patent Document 3]
Yonezawa Nakashiro "Prompt gamma-ray analysis with reactor neutrons", Analytical Chemistry Vol. 51, no. 2, pp. 61-96 (2002)
Here, when considering the analysis of Cd in food, ¹¹³ Cd emits two gamma rays (multiple) mainly at 558.5 keV and 651.3 keV. On the other hand, hydrogen, which is one of the main components of food, emits gamma rays with higher energy than 2223.3 keV and ¹¹³ Cd (C has no significant effect because of its small cross-sectional area). Therefore, as shown in FIG. 1, the Cd photoelectric peak (the peak of the energy of the prompt gamma ray that is emitted when neutron hits) comes in the region where the Compton component of the prompt gamma ray from H appears. As a noise, Cd detection sensitivity deteriorates.
[0007]
Even now, in order to remove such Compton components, a scintillator detector or the like is installed on the outer periphery of the detector so that the Compton components are not detected as much as possible. However, this is not enough, and the Compton component of H is the detection limit of Cd. Is determined. For this reason, there was a significant limitation on the applicable area when prompt gamma analysis of food was performed without any countermeasures.
[0008]
[Problems to be solved by the invention]
As described above, when a small amount of Cd in food is to be quantified, the photoelectric peak of Cd is hidden in the Compton component of prompt gamma rays from hydrogen, which is one of the main components of food, and the detection sensitivity deteriorates. Although a method of removing H that interferes with measurement by chemical treatment is also conceivable, the advantage of easy gamma ray analysis is lost by performing chemical separation, and further, the yield etc. are obtained through the operation of chemical separation. There is a possibility that an error will be introduced and the analysis system will be affected.
[0009]
Therefore, in the present invention, by using a Gd foil, a chopper or the like to cut a neutron in a portion lower than the peak value of the absorption cross section of Cd, in the prompt gamma ray analysis, the influence of the Compton component of H that becomes noise is reduced. Focusing on the technique and the fact that ¹¹³ Cd emits two prompt gamma rays of strong intensity at the same time, combining the multiple gamma ray detection method and the prompt gamma ray analysis method to prevent gamma rays from H from being measured. suggest.
[0010]
[Means for Solving the Problems]
The following two are proposed as means for reducing the Compton component of H prompt gamma rays.
[0011]
One is that ¹¹³ Cd has an absorption cross section peak (0.2 eV) in the epithermal region unlike other general nuclei, and the Cd absorption cross section is measured using a Gd foil or chopper. This is a technique for reducing the influence of the Compton component of H that becomes noise in prompt gamma ray analysis by cutting neutrons in a portion lower than the peak value.
[0012]
The other is that ¹¹³ Cd emits two strong prompt gamma rays at the same time (H emits only one gamma ray) and combines the multiple gamma ray detection method with the prompt gamma ray analysis method. The prompt gamma rays emitted at that time are measured by a multiple gamma ray detector, and the gamma rays from H are not measured.
[0013]
As shown in FIG. 2, the former is ¹¹³ Cd, which is one of the stable isotopes of Cd, and has a wide absorption cross section in the epithermal region (0.025 to 1 eV) unlike other general nuclei such as H. Focus on having a peak (the ease of emitting prompt gamma rays is proportional to the absorption cross section, and the larger the absorption cross section, the easier it is to generate prompt gamma rays).
[0014]
In general, neutrons derived from nuclear reactors have a wide range of peaks from the outside to the epithermal region. As shown in FIG. 3, many elements such as H have a tendency of an absorption cross section called 1 / v rule, and the absorption cross section becomes larger as the energy of neutron becomes lower. In contrast, ¹¹³ Cd has a thin peak (0.2 eV) with an absorption cross section in the epithermal region. When neutrons from a nuclear reactor are used as they are, neutrons having a large H absorption cross section are also irradiated to the sample as they are, and a large amount of prompt gamma rays of H are generated, which lowers the detection limit.
[0015]
Here, if a chopper that cuts neutrons of 0.1 eV or less or a Gd foil of about 1 um is used, the neutrons hitting the sample are almost 0.1 eV or more. In this region, the neutron absorption cross section of Cd is relatively larger than the neutron absorption cross section of H compared to the epithermal region. That is, the amount of prompt gamma rays generated in both H and Cd is reduced, but the prompt gamma rays of H are significantly reduced, and as a result, the detection limit is improved.
[0016]
The latter, focusing on the fact that ¹¹³ Cd emits two prompt gamma rays of strong intensity at the same time, while H emits only one gamma ray, and as shown in FIG. The simultaneous gamma rays are measured simultaneously, and a two-dimensional matrix is formed for each of the coincidence counts with each energy as an axis. In this way, not only the influence of H can be reduced, but also those other than H that happen to be coincidentally counted at a certain probability are uniformly distributed to all energies, so the noise value is counted several times. It is reduced to the following, and even a weak signal can be detected.
[0017]
However, creating a two-dimensional matrix requires a special measurement device and places a heavy burden on the measurement system. Therefore, the two gamma rays for simultaneous counting are limited to the specific neutron energy of the target prompt gamma ray, and the energy of the other gamma ray is recorded only when the gamma ray of that energy comes, and a one-dimensional matrix is recorded. Create With this method, the nuclei that can be detected are limited to those in the limited energy range (energy of prompt gamma rays measured by the detector), but high resolution can be easily achieved without placing a heavy burden on the measurement system. be able to.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
In the detection method of the present invention, as shown in FIG. 5, a Gd foil or chopper 2 is installed on a neutron beam 1 from a nuclear reactor so that only the optimum neutron energy for Cd measurement is applied to the sample. A sample 3 is installed on the rear side, and a plurality of gamma ray detectors 4 are installed so as to surround it. If the gamma ray energy measured at that time is one of the prompt gamma rays from Cd, the gate is created for a certain period of time, and the gamma ray energy measured while the gate is open is used for one dimension. Create a spectrum of. An example of a flowchart of a specific method is shown in FIG. By doing this, H, which emits only one prompt gamma ray, is not measured unless a gamma ray having the same energy as Cd is detected coincidentally and enters the detector when the gate is created.
[0019]
That is, in FIG. 6, each of the following operations is performed, but this is not a flowchart of the detection method but a flowchart for each detector, and therefore a phase for confirming whether or not radiation is detected by other detectors. Is included.
[0020]
(1) A signal enters the Ge detector.
(2) Obtain the energy of the gamma ray.
(3) It is confirmed whether or not a gate signal is output from another detector (since it is a flowchart for each detector, it is necessary to confirm the presence or absence of a signal from another detector).
[0021]
(4) If a gate signal is output, the coincidence count is established for the second signal, so the detected energy is recorded in a one-dimensional spectrum (contingent events other than Cd energy are also used for confirmation of resolution, etc.) Record it as it will be needed.)
(5) If no gate signal is output, since this signal is the first gamma ray, it is checked whether it is the energy of the prompt gamma ray of Cd.
[0022]
(6) Since the coincidence gamma ray energy of Cd needs to be measured, gates for a certain period of time are created for all gamma ray detectors (if gamma rays are detected during this time, the above (4) Energy is recorded in the process.)
[0023]
(7) If it is not Cd energy, it originates from other nuclei such as H and is discarded because it is a noise event.
[0024]
【The invention's effect】
Since the measuring method of the present invention has advantages such as no need for chemical pretreatment, no special technique is required, and the Cd concentration in foods, which has recently become a problem, can be measured quickly and easily. It is expected to contribute greatly to the field of food hygiene.
[0025]
Further, the present invention can achieve a resolution of 1000 times or more compared with the conventional method. Furthermore, since this measurement method has the advantage that no chemical pretreatment is required, no special technique is required, and the Cd concentration in foods, which has recently become a problem, can be measured quickly and easily. It can greatly contribute to the field of food hygiene.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between Compton scattering and detection accuracy.
FIG. 2 is a diagram showing a neutron energy absorption cross section of ¹¹³ Cd.
FIG. 3 is a diagram showing a neutron energy absorption cross section of H.
FIG. 4 is a diagram showing the principle of multiple gamma ray detection.
FIG. 5 is a diagram showing a state in which a neutron beam irradiated to a sample is partially cut using the Gd foil or chopper of the present invention.
FIG. 6 is a diagram showing a flowchart for creating a one-dimensional spectrum of prompt gamma rays by the method of the present invention.

Claims (1)

In a method of measuring the concentration of Cd contained in a trace amount in food by prompt gamma ray analysis, a Gd foil or chopper having a predetermined thickness is placed on a neutron beam before being irradiated on a sample collected from the food , how the neutron beam cutting at least a portion of the neutron is irradiated to the sample, reducing the influence of Compton component of H as noise with energy less than 0.1 eV.

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