JP2019152436A - Counting method and radiation detection device - Google Patents

Abstract

To provide a counting method for accurately counting radiation and the like over a wide range from low to high count rates.SOLUTION: The present invention relates to: a counting method of setting a pulse height discrimination threshold and counting the frequency of signals exceeding the pulse height discrimination threshold, and for the target component of the count showing a peak in the wave height distribution spectrum where an arbitrary value selected from a range of (μ-0.4σ) to (μ + 0.6σ) obtained by fitting a Gaussian function for the peak is applied as the pulse height discrimination threshold; and a radiation detection device which processes the signal according to the counting method. According to the counting method and the radiation detection device of the present invention, it is possible to reduce the error that pileup gives to the count value, and is possible to accurately count radiation and the like over a wide range from low to high radiation dose rates.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a radiation counting method and a radiation detection apparatus. Specifically, the present invention relates to a method for counting radiation and the like with high accuracy even under high count rate conditions and a radiation detection apparatus.

  In a radiation detection device that combines a radiation detection element such as a scintillator or a semiconductor with a pulse height discrimination circuit, a pulse signal corresponding to the energy of the radiation detected by the radiation detection element is obtained, the peak value is read, and the peak value is a peak detection threshold value. A method for obtaining a dose rate by counting pulse signals exceeding the range is widely used.

  For example, in the case of a neutron detection apparatus that obtains the dose rate of neutron beams, it is necessary to set a peak value that does not count γ rays that become background noise as the pulse height discrimination threshold. As a method for optimizing the peak discrimination threshold, as described in Patent Document 1, μ (average value of peak values) and σ (waves) obtained by fitting a Gaussian function to the peak indicated by the peak distribution spectrum. There is a method of setting based on the standard deviation of the high value.

Previous WO2014-192321 Japanese Patent No. 561357 JP2015-227854-A JP2012-136667A JP2013-167467A Previous WO2015-128905 Special table 2001-524217

  For example, an environment to which a radiation detection element is exposed in cancer radiotherapy may have a very high radiation dose rate. In such a high dose rate environment, as described in Patent Document 2 and Patent Document 3, the signal waveform overlaps due to the incidence of the next pulse immediately after measurement of a certain pulse (so-called pile-up phenomenon). There is a problem that counting loss occurs and counting accuracy is lowered.

  Patent Documents 2 and 3 disclose a method in which pulses are digitized by an AD converter, a plurality of piled-up pulses are separated and counted, and a change in peak value due to baseline fluctuation is corrected. . However, depending on the embodiment, a plurality of piled-up pulses cannot be separated, and there is a case where counting loss occurs. The wave height distribution spectrum when there is a radiation detection event in which two pulses (two radiation detection events) are processed as one pulse (one radiation detection event) is, for example, as shown in FIG. A second peak is observed at a peak value that is approximately twice as high. Note that the second peak is not observed in the wave height distribution spectrum in the absence of pileup as shown in FIG.

  And when the frequency of pile-up increases further (when the dose rate is higher or the sensitivity of the detection element is higher), the presence of a radiation detection event that treats three pulses as one pulse. As shown in the pulse height distribution spectrum of FIG. 3, the third peak may be observed at a peak value that is about three times the first peak.

  Thus, when a plurality of pulses are processed as one pulse, a counting loss occurs and the counting accuracy decreases. For this reason, it is necessary to reduce the frequency of pile-up by reducing the radiation detection sensitivity by reducing the size of the radiation detection element. However, lowering the radiation detection sensitivity may cause other problems, so a multifaceted approach is required.

  One is to use a radiation detection element having a short pulse decay time (fluorescence lifetime) as described in Patent Document 4.

  One is to devise a circuit configuration so that the pulse decay time is shortened (for example, an amplifier having a short time constant is used, or a differentiating circuit is used).

  One is to increase the sampling point of digital data by increasing the speed of the AD converter.

  However, even with such an approach, there is a case where the problem that the counting accuracy is lowered due to pile-up cannot be solved.

  In view of the above problems, the present inventors have intensively studied and found that the decrease in counting accuracy due to pile-up can be suppressed by a method completely different from the above approach of optimizing the pulse height discrimination threshold. It came to be completed.

  That is, the present invention is a method of setting a pulse height discrimination threshold and counting the frequency of signals exceeding the pulse height discrimination threshold, and when the target component of the count shows a peak in the pulse height distribution spectrum, a Gaussian function is calculated for the peak. The counting method and the radiation detection apparatus are characterized in that an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting is used as the pulse height discrimination threshold.

  According to the present invention, it is possible to suppress a decrease in counting accuracy due to pileup, and it is possible to accurately count radiation over a wide range from a low radiation counting rate to a high situation.

The pulse height distribution spectrum obtained when the accelerator neutrons were counted by a neutron detector using Eu: LiCaAlF ₆ crystal. The pulse height distribution spectrum obtained when the accelerator neutrons were counted by a neutron detector using Eu: LiCaAlF ₆ crystal. The pulse height distribution spectrum obtained when the accelerator neutrons were counted by a neutron detector using Eu: LiCaAlF ₆ crystal. The schematic of the radiation detection apparatus of this invention. The signal waveform diagram for demonstrating the influence which pileup gives. The figure (simulation) explaining the production | generation method of a pulse height distribution spectrum. Wave height distribution spectrum (simulation) shown when piled up. A count value (simulation) greater than or equal to the pulse height discrimination threshold analyzed for the pulse height distribution spectrum shown when piled up. Simulation result of error that pileup gives to the count value. Comparison of the measured value and the simulation value of the counting error in the most piled-up situation in Example 1. The figure explaining that the error which pileup gives to a count value can be made small by setting a crest discrimination threshold value in mu vicinity. The schematic of the neutron detector used in the Example. The time series graph of the neutron count rate and accelerator electric current value which were obtained in Example 1. FIG. The graph which shows the relationship between the accelerator electric current value in Example 1, and a neutron count rate. The time series graph of the neutron count rate and accelerator current value obtained in Comparative Example 1. The graph which shows the relationship between the accelerator electric current value in the comparative example 1, and a neutron count rate. The time series graph of the neutron count rate and accelerator current value obtained in Comparative Example 2. The graph which shows the relationship between the accelerator electric current value in the comparative example 2, and a neutron count rate.

  FIG. 4 shows a schematic diagram of a radiation detection apparatus which is an example of an apparatus for carrying out the present invention. The object of the radiation detection apparatus can be achieved by a combination of the radiation detection element 1 that outputs a pulsed electric signal upon incidence of radiation and the wave height discrimination circuit 2. When using a medium that does not directly output an electrical signal and emits light upon incidence of radiation, such as a scintillator, radiation detection is performed by combining the scintillator 3 and a device 4 that converts the light emitted by the scintillator 3 into an electrical signal. Element 1 can be obtained. As the device 4 for converting the light emitted from the scintillator into an electric signal, a photodetector such as a photomultiplier tube, a photodiode, an avalanche photodiode, or a Geiger mode avalanche photodiode is generally used.

  The pulse height discriminating circuit 2 only needs to be able to analyze the peak value of the pulse output from the radiation detection element 1 and to count pulses exceeding the pulse height discrimination threshold. The present invention can be applied to both reading by digital data processing.

  The pulse height discriminating circuit 2 used in the embodiment of the present invention is configured to read a pulse by digital data processing, which includes an amplifier (analog amplifier), an AD converter, a digital data processing device, a display device, and the like. Hereinafter, an apparatus for reading pulses by digital data processing will be described as an example.

  According to the above-described embodiment, the pulse generated when the radiation detection element 1 detects radiation is amplified and amplified along the time axis by the analog amplifier, and the amplified pulse signal is fixed to a certain level by the AD converter. It is converted into digital data at a sampling interval, and the digital data is read by a digital data processor. A wave height distribution spectrum, a count value exceeding the wave height discrimination threshold, and the like are displayed on a display device including a computer, software, a display monitor, and the like.

  FIG. 5 is a signal waveform diagram of a pulse amplified by an analog amplifier for conceptually explaining the influence of pileup. Each point in the diagram is a digital signal at a constant time interval on the analog signal indicated by a solid line. The discrete values obtained in the case of the conversion are shown.

  FIG. 5A shows a case where pile-up is not performed. In this case, the digital data processing apparatus counts as “the event corresponding to the pulse height (Pulse Height) H is twice”.

  On the other hand, FIG.5 (b) is a case where it piles up. Originally, it should be counted as “three events corresponding to the peak value H”, but another radiation is incident immediately after the first pulse, and the second pulse rises. Since the signal value continues to rise, the first pulse and the second pulse are not recognized separately, and an event corresponding to “H” (≈2H) is counted once.

  The third pulse is counted as “one event corresponding to H ′ (<H)” due to the shift of the baseline. The problem to be solved by the present invention is the counting loss that occurs for the first and second pulses. In the following description, for the sake of convenience, a case where a plurality of pulses such as the first pulse and the second pulse could not be separated and counted is treated as being piled up, and the number is separated from the previous pulse and counted as the third pulse. The completed one is treated as not piled up. The problem that the peak value is reduced for the third pulse is a count loss when the pulse to be processed as the peak value exceeding the peak discrimination threshold is reduced to a peak value less than the peak discrimination threshold. Regarding this count loss, the count loss can also be reduced by using Gaussian fitting, which will be described later, and adjusting the pulse height discrimination threshold in accordance with the reduction of the peak value.

  In order to simulate the error given to the count value by the pile-up described above, first, the wave height distribution spectra respectively shown in the case where the pile-up is not performed and the case where the pile-up is performed are represented by the Gauss of the following formula (1). Generated using a function. The maximum count N in the Gaussian function of Expression (1) is a count indicated by the average peak value μ. FIG. 6 is a diagram for explaining a method for generating a pulse height distribution spectrum.

図２で示したパイルアップが無い状態の波高分布ピークのμおよびσを参考に、パイルアップしていない第１ピークのμおよびσをそれぞれμ_１＝２００ｃｈ、σ_１＝２６ｃｈとした。２個のパルスがパイルアップして現れる第２ピークはμ_２＝４００ｃｈ、σ_２＝５２ｃｈ、３個のパルスがパイルアップして現れる第３ピークはμ_３＝６００ｃｈ、σ_３＝７８ｃｈとした。式（１）を用いた独立したデータとして第１〜第３ピークを生成した後、波高値毎に計数を足し合わせて１つの波高分布スペクトルとしている。なお、図１〜図３の波高分布スペクトルのように、計数の目的成分とは異なる放射線種によって低波高値側にノイズ成分が観測され目的成分のピークと一部重なる場合もあるが、当該シミュレーションでこのようなノイズ成分を考慮する必要はないため無視した。

With reference to μ and σ of the peak distribution peak without pile-up shown in FIG. 2, μ and σ of the first peak not piled up were set to μ ₁ = 200 ch and σ ₁ = 26 ch, respectively. The second peak that appears when two pulses pile up is μ ₂ = 400 ch and σ ₂ = 52 ch, and the third peak that appears when three pulses pile up is μ ₃ = 600 ch and σ ₃ = 78 ch. After the first to third peaks are generated as independent data using Expression (1), the counts are added for each peak value to form one peak distribution spectrum. Although the noise component is observed on the low peak value side due to a radiation type different from the target component of counting as in the pulse height distribution spectrum of FIGS. 1 to 3, it may partially overlap with the peak of the target component. Therefore, it was ignored because it was not necessary to consider such noise components.

  FIG. 7 shows the pulse height distribution spectrum generated by the above-mentioned method with the pile-up rate being 0%, 10%, 20%, and 30%. The wave height distribution spectrum with a pile-up rate of 0% is data having only the first peak. The total count of each wave height distribution spectrum from 0 to 1023ch is made to coincide if the count of the second peak is doubled and the count of the third peak is tripled.

  Next, the pulse height discrimination threshold (Threshold) was set to μ + Xσ, and a count equal to or higher than the pulse height discrimination threshold when X was changed in the range of −3 to +3 was obtained from the pulse height distribution spectrum. The result is shown in FIG. Further, from the results shown in FIG. 8, count errors of 10%, 20%, and 30% of pileup rates with respect to the count exceeding the pulse height discrimination threshold when the pileup is 0% were obtained. The result is shown in FIG. 8 and 9, the smaller the X, the larger the negative error due to the counting loss, and the larger the X, the larger the positive error due to the excessive counting, but the wave height discrimination threshold (ie, X = It can be seen that it is difficult to generate counting errors due to pileup in the vicinity of 0).

  Further, in order to set the optimum wave height discrimination threshold based on the actual measurement value, the pile height is highest in the first embodiment in the situation where the pile up is the lowest (accelerator current is 5.7 μA, wave height distribution spectrum is FIG. 2). The counting error was analyzed under the circumstances (accelerator current is 379.6 μA, wave height distribution spectrum is FIG. 3). For each first peak in FIG. 2 and FIG. 3, μ and σ are obtained by fitting a Gaussian function, a pulse height discrimination threshold μ + Xσ is set based on the obtained μ and σ, and counting is performed in the same manner as in the simulation. FIG. 10 shows the results of error determination (however, the dose rate difference shows the same tendency as the accelerator current difference because it is necessary to consider the dose rate difference when obtaining the pulse height distribution spectra of FIGS. 2 and 3). The counts in FIGS. 2 and 3 were normalized by each accelerator current value). Moreover, it was found from the analysis of the wave height distribution spectrum in FIG. 3 that the pile-up rate of the wave height distribution spectrum is about 25%. FIG. 10 also shows the simulation value of the counting error when the pile-up rate is 25%.

  The counting error due to pileup shown in FIG. 10 is slightly different between simulation and actual measurement. This is probably because the simulation does not consider the reduction of the peak value due to pile-up, whereas the actual measurement causes the reduction of the peak value due to pile-up. As described above, although there is a slight difference between the simulation and the actual measurement, it can be understood that the pulse height discrimination threshold (that is, near X = 0) close to μ is hardly affected by the count loss due to the pile-up even in the actual measurement. In addition, when the peak value is reduced in actual measurement, it is understood that it is optimal to set the peak discrimination threshold slightly higher than μ.

  Here, the principle that the counting error due to pileup can be reduced by setting the wave height discrimination threshold in the vicinity of μ will be described with reference to FIG.

First, using FIG. 11A, only the pile-up of two pulses is considered. When the total count of the first peak not piled up is N _s and the total count of the second peak piled up by two pulses is N _d , the number of radiation detected by the radiation detection element is N _s + 2N _d It can be said that there is theoretically.

Here, generally, is to set the threshold value around for example T ₁ can count all events, the threshold value or more counts obtained when the T ₁ and wave height discrimination threshold N _{s +} N _d, and the Compared with the theoretical value N _s + 2N _d , a negative error of N _d is generated. In particular, when N _d increases due to significant pile-up, the counting error also increases. In contrast, the counting method of the present invention is characterized in that the counting error is reduced by setting a pulse height discrimination threshold in the vicinity of μ of the first peak. For example, the count exceeding the threshold obtained when μ (T ₂ ) of the first peak is set as the pulse height discrimination threshold is 1 / 2N _s + N _d , and exactly half is counted when compared with the theoretical value N _s + 2N _d. Become. In other words, if the count obtained by using μ of the first peak as the wave height discrimination threshold is doubled, a count as the theoretical value can be obtained. At this time, the count obtained using μ of the first peak as the pulse height discrimination threshold is always half of the theoretical value regardless of the pileup level. Therefore, according to the counting method of the present invention, the counting error due to pileup is calculated. Does not occur.

Next, with reference to FIG. 11B, a description will be given in consideration of a pileup of two pulses and a pileup of three pulses. The total count of the first peak not piled up is N _s , the total count of the second peak piled up by two pulses is N _d , and the total count of the third peak piled up by three pulses is N _t In this case, it can theoretically be said that the number of radiation detected by the radiation detection element is N _s + 2N _d + 3N _t . When T ₁ capable of counting all events is set as the pulse height discrimination threshold, the count exceeding the threshold obtained is N _s + N _d + N _t , and a negative error of N _d + 2N _t is compared with the theoretical value N _s + 2N _d + 3N _t. It will occur. On the other hand, a count equal to or greater than the threshold obtained when μ (T ₂ ) of the first peak is the pulse height discrimination threshold is ½ N _s + N _d + N _t . Compared with ½N _s + N _d + 3 / 2N _t which is half the theoretical value, a minus error of ½N _t occurs, but the error can be made much smaller than when the threshold is set to T ₁ .

  The counting method of the present invention that reduces the counting error due to pileup by setting a pulse height discrimination threshold close to μ is based on the theory described above, and the simulation results shown in FIG. It shows the validity of the invention.

Note that the above-mentioned simulation takes into account the pile-up of two pulses and the pile-up of three pulses, and it is consistent with this theory that a slight negative error occurs at the threshold value μ + 0σ. Note that the third peak count N _t that is the cause of this minus error is sufficiently smaller than the first peak count N _s , resulting in a small contribution to the count error caused by pileup. Furthermore, there is a possibility that the fourth, fifth, sixth,... Peak in which four or more pulses have piled up may be observed, but the contribution to the counting error caused by the pileup is even smaller than the third peak, so It is not a problem for most applications.

  According to the present invention, there is provided a method for setting a pulse height discrimination threshold and counting the frequency of signals exceeding the pulse height discrimination threshold, wherein when the target component of the count shows a peak in the pulse height distribution spectrum, a Gaussian function is used for the peak. By adopting a counting method in which an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting is used as the wave height discrimination threshold, the counting accuracy is reduced due to pileup. Can be suppressed.

The target component for counting is an event to be counted, and is not particularly limited. For example, in the case of a neutron detector that can provide a neutron dose rate, an accumulated dose, and the like, a neutron incident event is a target component for counting. There are various types of radiation such as X-rays, α-rays, γ-rays, and neutrons, but a radiation detection device that detects two or more types of radiation may be used, and in that case, the pulse height discrimination threshold of the present invention is applied.
Any one type of radiation to be detected may be used, or two or more types of radiation may be used.

  Furthermore, the target component for counting is not limited to the incident event of radiation, but can be applied to the detection of electromagnetic waves such as ultraviolet rays, visible rays, infrared rays, and microwaves, and can be applied to various fields. It is.

  The counting method of the present invention can be applied when the target component of counting shows a peak in the wave height distribution spectrum. For example, for a neutron detection device, if the horizontal axis indicates the peak value in the pulse height distribution spectrum and the vertical axis indicates the frequency of the event indicating each peak value, the event in which the neutron is detected shows a peak in the peak height distribution spectrum. An arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting a Gaussian function for the peak can be used as the threshold value, and can be applied by counting events exceeding the threshold value. When the event in which neutron is detected does not show a peak in the wave height distribution spectrum, the counting method of the present invention is not applicable.

As an example of a form in which a peak can be formed by detecting neutrons, there is a method of combining an Eu: LiCaAlF ₆ crystal (neutron scintillator), a photodetector, and a pulse height discriminating circuit described in Patent Document 5.

  The shape of the peak distribution peak is not particularly limited, and the effect of the present invention can be obtained without any problem as long as the shape approximately follows a Gaussian distribution. It is statistically possible that a higher counting accuracy can be expected when μ obtained by fitting a Gaussian function by the method described later and the peak value P indicating the maximum count match in the peak indicated by the target component of the count in the pulse height distribution spectrum. As a matter of course, it is possible to perform counting with particularly high accuracy if the values coincide within a range of 0.9 μ ≦ P ≦ 1.1 μm.

  Note that before fitting a Gaussian function for the peak, processing such as baseline correction, peak separation, and smoothing may be performed on the wave height distribution spectrum in advance.

  As the reference for determining the pulse height discrimination threshold of the present invention, μ and σ are fitted to the Gaussian function of Equation (1) for the first peak that is not piled up among the peak distribution peaks indicated by the target component of the count. And is irrelevant to the values obtained for the second peak piled up by two pulses and the third peak piled up by three pulses. The μ and σ described below are μ and σ obtained for the first peak.

  The data range of the pulse height distribution spectrum used when fitting the Gaussian function is based on the peak value indicating the maximum count (hereinafter, the peak value is represented by P) at the peak indicated by the target component of the count in the pulse height distribution spectrum. 0.75P to 1.25P is preferable, but if the noise count by a component different from the target component of the count affects the analysis values of μ and σ, the lower limit value and / or the upper limit value may be narrowed. good. On the contrary, if there is no noise affecting the analysis values of μ and σ, the data range used when fitting the Gaussian function may be expanded. For example, the data range of the pulse height distribution spectrum used when fitting a Gaussian function may be set to 0.75P to 3P. Here, when the second peak appears, the count of the second peak becomes noise when performing fitting, but the count of the second peak that appears due to pileup is sufficiently smaller than the count of the first peak. The influence of the noise on the analysis values of μ and σ is limited. Note that the least square method is preferably used for fitting the Gaussian function, and it is more preferable to fit the maximum count N together with μ and σ.

  As understood from the above description, the essence of the counting method of the present invention is to reduce the counting error due to pileup by setting the pulse height discrimination threshold in the vicinity of μ. Then, the method of using an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting the above-mentioned Gaussian function as an appropriate value in the vicinity of μ An index for setting the discrimination threshold is given.

  In the present invention, for the purpose of simplifying the circuit for setting the pulse height discrimination threshold, shortening the analysis time, etc., the present invention can also be performed by the method described below, for example, without performing analysis by a method of fitting a Gaussian function. The wave height discrimination threshold according to the above can be determined.

  Note that any wave height discrimination threshold determined by the method described below is included in the range of (μ−0.4σ) to (μ + 0.6σ) obtained from the fitting if a Gaussian function is fitted.

  The determination method of the pulse height discrimination threshold that does not require analysis by the method of fitting the first Gaussian function is the highest in the count n obtained by the following formula (2) for the peak distribution peak indicating the maximum count N ′ at the peak value P. In this method, a peak value indicating a close count is used as a peak discrimination threshold.

式（２）は、式（１）のガウス関数における任意の波高値χに波高弁別閾値μ＋Ｘσを代入して、計数ｎをＸの関数として表したものである。すなわち、波高弁別閾値をＰ以下とする場合には、−０．４〜０．０の範囲から選ばれる任意の値をＸとし、式（２）によって求められる計数ｎに最も近い計数を示しているＰ以下の波高値を波高弁別閾値とすれば良く、波高弁別閾値をＰ以上とする場合には、０．０〜０．６の範囲から選ばれる任意の値をＸとし、式（２）によって求められる計数ｎに最も近い計数を示しているＰ以上の波高値を波高弁別閾値とすれば良い。

Equation (2) represents the count n as a function of X by substituting the pulse height discrimination threshold μ + Xσ into an arbitrary wave height value χ in the Gaussian function of Equation (1). That is, when the wave height discrimination threshold is set to P or less, an arbitrary value selected from the range of −0.4 to 0.0 is set to X, and the count closest to the count n obtained by the equation (2) is indicated. The peak value below P may be used as the peak height discrimination threshold. When the peak height discrimination threshold is set to P or higher, an arbitrary value selected from the range of 0.0 to 0.6 is set as X, and the formula (2) The peak value equal to or higher than P indicating the closest count to the count n obtained by the above may be used as the pulse height discrimination threshold.

  A method for determining the pulse height discrimination threshold that does not require analysis by the method of fitting the second Gaussian function is a method for determining the pulse height discrimination threshold based on the half-value width FWHM instead of the standard deviation σ. In the Gaussian distribution, μ and the crest value P are the same, and the relationship of the following formula (3) exists between the standard deviation σ and the half-value width FWHM.

したがって、（μ−０．４σ）〜（μ＋０．６σ）の範囲は、（Ｐ−０．２８ＦＷＨＭ）〜（Ｐ＋０．４２ＦＷＨＭ）と同等の範囲であるため、当該波高値Ｐと半値幅ＦＷＨＭを解析することによっても本発明に準ずる波高弁別閾値を決定できる。

Therefore, since the range of (μ−0.4σ) to (μ + 0.6σ) is the same as (P−0.28 FWHM) to (P + 0.42 FWHM), the peak value P and the half width FWHM are analyzed. By doing so, the pulse height discrimination threshold according to the present invention can be determined.

  The determination method of the pulse height discrimination threshold that does not require analysis by the third Gaussian fitting is to determine the peak value P indicating the maximum count of the peak distribution peak from the appearance of the peak distribution spectrum, and to determine the peak value near the peak value P. This is a method for setting a pulse height discrimination threshold.

  As mentioned above, although the determination method of the wave height discrimination threshold which does not require the analysis by the 1st-3rd Gaussian fitting was illustrated, the wave height discrimination threshold may be determined by a method different from these, and the wave height discrimination threshold determined by some method Is included in the range of (μ−0.4σ) to (μ + 0.6σ) analyzed by Gaussian fitting, the effect of the present invention is exhibited.

In the counting method of the present invention, for the purpose of removing noise on the higher peak side than the target component of counting, in addition to the peak discrimination threshold (T _A ) of the present invention, another peak discrimination threshold (T _B ) may be provided to count the following pulse T _a and not more than T _B. In that case, the threshold value T _B for preventing the loss of counts of the second peak and the third peak is preferably not less than 3.mu., more preferably not less than 5 [mu]. In addition, a pulse height discrimination threshold for counting the first target component may be set, and a pulse height discrimination threshold for counting the second target component may be set separately.

  According to the present invention, by setting an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) as the wave height discrimination threshold, it is possible to suppress a decrease in counting accuracy due to pileup. According to the measured value of the counting error due to pileup shown in FIG. 10, when counting pulse signals that exceed the pulse height discrimination threshold, the counting error caused by pileup can be suppressed to about ± 10% or less. With such counting errors, highly accurate measurement can be realized in most applications.

  However, there is a possibility that an error exceeding ± 10% may occur depending on the frequency of pile-up and the degree of peak value reduction by the pile-up, but according to the theory described with reference to FIG. Compared with the case where the pulse height discriminating threshold deviates from the range, it is possible to realize much superior counting accuracy. According to the measured value of the counting error due to pileup shown in FIG. 10, when the wave height discrimination threshold is set to a value lower than (μ + 0.3σ), the counting loss due to pileup has an effect as the wave height discrimination threshold is lowered. If the counting error is easy to expand on the minus side and the wave height discrimination threshold is higher than (μ + 0.3σ), the higher the wave height discriminating threshold, the more the second peak and the third peak increase, and the positive side. The counting error tends to increase.

  The more preferable pulse height discrimination threshold is an arbitrary value selected from the range of (μ + 0.0σ) to (μ + 0.5σ), and the more preferable pulse height discrimination threshold is selected from the range of (μ + 0.2σ) to (μ + 0.4σ). It is an arbitrary value and can be suppressed to a count error of about ± 5% and ± 3%, respectively.

  The counting method of the present invention can be suitably employed in a radiation detection apparatus in which the scintillator 3, the photodetector 4, and the wave height discrimination circuit 2 are combined.

  The present invention is a radiation detection apparatus using a combination of a scintillator 3, a photodetector 4, and a wave height discrimination circuit 2, wherein the wave height discrimination circuit 2 includes an amplifier that amplifies the signal of the photodetector 4, and the amplifier A wave height discrimination circuit comprising: an AD converter that converts an analog signal output into a digital signal; and a digital data processing device that performs signal processing on the digital signal output of the AD converter, wherein the wave height discrimination circuit uses the counting method. A radiation detection apparatus for signal processing is provided.

  As the wave height discriminating circuit 2, as described in Patent Document 2 and Patent Document 3, an amplifier that amplifies the signal of the photodetector, an AD converter that converts an analog signal output of the amplifier into a digital signal, and And a digital data processing device that performs signal processing on the digital signal output of the AD converter. If the pulse height discriminating circuit that reads pulse signals by digital data processing is used, the circuit configuration for reducing the peak value due to the pile-up is less expensive than the pulse height discriminating circuit that reads pulse signals by analog data processing. In addition, it is possible to easily correct the peak value. By reducing the peak value due to pile-up, the effect of the present invention can be obtained with higher accuracy.

  In the present invention, the digital data processing device creates a wave height distribution spectrum at a predetermined time interval, fits a Gaussian function to a peak indicated by a target component of counting in the created wave height distribution spectrum, and obtains μ and σ It is preferable to implement a function for sequentially setting a pulse height discrimination threshold with reference to. By implementing such a function, the peak discrimination threshold can be automatically updated following the reduction of the peak value by the pile-up, and an increase in counting error due to the reduction of the peak value can be prevented.

  In addition, when using a pulse height discriminating circuit in which the peak value is reduced by pileup, a function for fitting a Gaussian function and a function for sequentially setting a pulse height discrimination threshold are not implemented, and a pulse height discrimination threshold according to the present invention is not implemented. May be analyzed in advance and counted based on the pulse height discrimination threshold from the start to the end of counting.

  As the amplifier, a preamplifier (preamplifier), a waveform shaping amplifier (shaping amplifier), or the like can be applied, and a combination thereof may be used. Further, a circuit configuration in which a differential circuit is combined may be used. In addition to the circuit configurations described in Patent Document 2 and Patent Document 3, the circuit configurations described in Patent Document 6 and Patent Document 7 are also suitable.

The scintillator 3 is composed of LiCaAlF ₆ crystal, LiSrAlF ₆ crystal, LiF / CaF ₂ eutectic, LiF / SaF ₂ eutectic, NaI crystal, Bi ₄ Ge ₃ O ₁₂ crystal containing an activator such as Eu, Ce, Tl, and Na. Inorganic scintillators such as Gd ₃ Al ₂ Ga ₃ O ₁₂ crystal and CeF ₃ crystal, organic scintillators composed of organic phosphors such as anthracene, stilbene and diphenyloxazole, polystyrene containing the organic phosphor, polyvinyl toluene, Plastic scintillators such as polyethylene terephthalate, liquid scintillators such as toluene and xylene containing the organic phosphor, and gas scintillators such as helium, argon, xenon, and krypton can be used without any limitation depending on the application. .

In the present invention, the scintillator 3 is preferably a LiCa _x Sr _1-x AlF ₆ crystal (x is 0 to 1). Specifically, LiCaAlF ₆ crystals and LiSrAlF ₆ crystals containing additives such as Eu and Ce as activators can be suitably used. These scintillator crystals are neutron scintillators, and Li-6 contained in the scintillator crystals reacts with neutrons to generate α rays and tritium, and the α rays and tritium give the energy of 4.8 MeV to the scintillator crystals. Then, light corresponding to the applied energy is emitted by the activator contained in the scintillator crystal. The LiCaAlF ₆ crystal and the LiSrAlF ₆ crystal can exhibit a peak that matches the Gaussian distribution with an accuracy of 0.95 μ ≦ P ≦ 1.05 μ, depending on the event of detecting neutrons. It is particularly suitable as a scintillator to be used.

The LiCa _x Sr _1-x AlF ₆ crystal containing Eu as an activator, such as the Eu: LiCaAlF ₆ crystal used in the examples of the present invention, has a particularly high light emission amount and can be measured with a high signal / noise ratio. It can use suitably for this invention. In addition, LiCa _x Sr _1-x AlF ₆ crystal containing Eu as an activator has a fluorescence lifetime of 1.6 μsec, and in a general scintillator material, it has a long fluorescence lifetime, so that pileup is likely to occur. By applying the counting method of the present invention, it is possible to accurately count even if pileup occurs.

On the other hand, a LiCa _x Sr _1-x AlF ₆ crystal containing Ce as an activator can be suitably used in the present invention because of its particularly short fluorescence lifetime of several tens of nsec. Since the LiCa _x Sr _1-x AlF ₆ crystal containing Ce as an activator, such as Ce: LiCaAlF ₆ crystal and Ce: LiSrAlF ₆ crystal, originally has a short fluorescence lifetime and hardly causes pile-up, the counting method of the present invention is used. If applied, measurement at an extremely high counting rate can be realized.

  The neutron sensitivity of the neutron scintillator containing Li-6 can be adjusted by adjusting the scintillator size and adjusting the Li-6 isotope ratio. Li-6, Li-7, which is a stable isotope of Li, has a ratio adjusted, Li-6 is concentrated, natural ratio, Li-7 is concentrated, Li-6 The higher the isotope ratio, the higher the neutron sensitivity. Moreover, you may adjust neutron sensitivity by mixing these. By adjusting the neutron sensitivity as described above, it is possible to provide a radiation detection apparatus corresponding to the dose rate to be counted.

  In the present invention, the means for propagating the light of the scintillator 3 to the photodetector 4 is not particularly limited, and the scintillator 3 and the photodetector 4 may be directly connected as shown in FIG. As shown in FIG. 12, it may be indirectly connected using a light guide such as the optical fiber 5. When the scintillator 3 and the light detector 4 are directly connected, the light detector 4 is exposed to the radiation field together with the scintillator 3, so that noise observed when radiation is incident on the light detector 4. There is a possibility that the counting accuracy is lowered due to the influence, or the photodetector 4 is broken. Therefore, when counting radiation in a high dose rate environment, the scintillator 3 is installed in the high dose rate environment to be counted, and the photodetector 4 is installed far away with a sufficiently low dose rate. It is preferable to indirectly connect the scintillator 3 and the photodetector 4 using a light guide such as an optical fiber 5.

Examples of radiation fields with high dose rates include cancer radiotherapy devices. Among them, neutron dose rates are about 1 × 10 ⁹ n / cm ² / sec in a radiotherapy device for boron neutron capture therapy (BNCT). And is known to be very expensive. Conventionally, BNCT treatment using nuclear reactor neutrons has been performed, but in recent years, high dose rate neutrons can be obtained by cyclotron accelerators and electrostatic accelerators, and accelerators that can be installed in hospitals. BNCT treatment devices have been developed.

  The counting method and radiation detection apparatus of the present invention can be suitably used in various radiation fields, and are particularly suitable for the detection of the accelerator neutrons. In particular, it is suitable for the detection of neutrons for boron neutron capture therapy with a very high neutron dose rate.

In the case of detecting neutrons for boron neutron capture therapy, the best mode for carrying out the present invention is a radiation detection apparatus that combines a neutron scintillator 3, a photodetector 4, and a pulse height discriminating circuit 2. One of Eu: LiCaAlF ₆ crystal, Ce: LiCaAlF ₆ crystal, Eu: LiSrAlF ₆ crystal, and Ce: LiSrAlF ₆ crystal, and the neutron scintillator 3 and the photodetector 4 are connected by an optical fiber 5 It is.

  In such an aspect, it is preferable to optically bond the neutron scintillator 3 and the tip of the optical fiber 5 in order to increase the light propagation efficiency from the neutron scintillator 3 to the optical fiber 5. The method for optical bonding is not particularly limited, but a method using an organic adhesive or an inorganic adhesive having good transmittance at the emission wavelength of the scintillator 3 and good radiation resistance, or the tip of the optical fiber 5 is temporarily attached. A method in which the scintillator 3 is melted and fused is preferable. Moreover, in order to improve the light collection efficiency from the neutron scintillator 3 to the optical fiber 5, it is preferable to cover the periphery of the neutron scintillator 3 with a reflecting material. The reflective material is not particularly limited, but white materials such as tetrafluoroethylene, polyethylene, titanium oxide, and barium sulfate are preferable. The optical fiber 5 can be made of quartz fiber, plastic fiber or the like, and preferably has a large numerical aperture (NA) and a small transmission loss. Specifically, NA of 0.45 or more and transmission loss of 200 dB / km or less are preferable. If a sufficient amount of light can be propagated to the photodetector 4 by using one having a large NA and a small transmission loss, a Gaussian distribution with an accuracy of 0.95 μ ≦ P ≦ 1.05 μ depending on the event of detecting a neutron. Is particularly preferred since a peak corresponding to

  If it is the said form, it is not necessary to arrange | position the photodetector 4 in a high dose rate environment, and it is possible to form the peak recognized as the event which the scintillator 3 detected the neutron in the wave height distribution spectrum. For the peak, an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting a Gaussian function is used as a wave height discrimination threshold, and an event exceeding the wave height discrimination threshold is determined. By counting, it is possible to provide a radiation detection apparatus capable of accurately counting neutrons over a wide range from a low dose rate to a high dose rate with little decrease in counting accuracy due to pileup.

  Hereinafter, embodiments of the present invention will be described in more detail with specific experimental examples, but the present invention is not limited thereto.

Example 1
First, a neutron detection device combining the neutron scintillator 3, the photomultiplier tube 4, and the wave height discrimination circuit 2 was manufactured as follows. The manufactured neutron detector is shown schematically in FIG.

As the neutron scintillator 3, Eu: LiCaAlF ₆ crystal produced using a Li raw material having a Li-6 isotope ratio concentrated to 95% was used. An epoxy adhesive is applied to the tip of the quartz optical fiber 5 having a core diameter of 1 mm and a length of 10 m, with the reverse end as the back end, and Eu: LiCaAlF ₆ crystal processed to 0.6 mm × 0.6 mm × 0.6 mm And optically bonded. The quartz optical fiber 5 has an NA of 0.48 and a transmission loss of 100 dB / km at an emission wavelength (370 nm) of Eu: LiCaAlF ₆ crystal. The optical bonding surfaces of the Eu: LiCaAlF ₆ crystal 3 and the quartz optical fiber 5 were each subjected to optical polishing. Furthermore, Eu: LiCaAlF ₆ crystal 3 was covered with a reflective material made of barium sulfate to improve the light collection efficiency on the optical fiber, and the optical fiber 5 was covered with a light shielding material to prevent room light from entering.

The rear end of the quartz optical fiber 5 to which the Eu: LiCaAlF ₆ crystal 3 was bonded was optically connected to the photomultiplier tube 4. The photomultiplier tube 4 was electrically connected to the wave height discrimination circuit 2.

  The wave height discrimination circuit 2 includes an amplifier that amplifies the signal of the photomultiplier tube, an AD converter that converts an analog signal output of the amplifier into a digital signal, and a digital data processing device that performs signal processing on the digital signal output of the AD converter; And a display device for displaying the result of the digital data processing.

  A display device including a computer, software, and a monitor was used. The monitor can display the wave height distribution spectrum and the count value of pulses exceeding the wave height discrimination threshold.

  The digital data processing device creates a wave height distribution spectrum every 10 seconds, fits a Gaussian function to the peak indicated by the target component of the count in the created wave height distribution spectrum, and uses the obtained μ and σ as references. Implemented the function to set the peak height discrimination threshold sequentially. In this example, as the pile-up frequency increases, the peak height distribution peak gradually shifts to the low peak value side, and in the situation where the pile-up frequency is the highest, μ is smaller than the situation where the pile-up frequency is the lowest. Although it decreased by about 14%, the wave height discrimination threshold is automatically updated according to the peak position at that time, so the count loss due to the decrease in the wave height value was slight.

  Next, using the manufactured neutron detector, accelerator neutrons were counted according to the method described below, and the error that pileup gave to the count value was analyzed.

A tip of a quartz optical fiber 5 in which a polyethylene block is arranged as a neutron moderator near a neutron irradiation port of a neutron generator using an electrostatic accelerator, and Eu: LiCaAlF ₆ crystal 3 is bonded to an insertion hole provided in the polyethylene block. Was inserted and fixed. The photomultiplier tube 4 and the wave height discriminating circuit 2 were placed away from the neutron irradiation port by extending the quartz optical fiber 5.

Neutrons were generated by an electrostatic accelerator, and the neutrons were detected by a neutron detector. The detected pulses of neutrons were analyzed, and pulses exceeding the pulse height discrimination threshold were counted every second to obtain a neutron count rate [cps] per unit time. The pulse height discrimination threshold in Example 1 was (μ + 0.3σ). The neutron dose rate [n / cm ² / sec] can be obtained by multiplying the obtained neutron count rate [cps] by a coefficient for conversion to a neutron dose rate. The neutron dose rate in the environment in which the detection element was arranged at the maximum output of the accelerator was about 1 × 10 ⁷ n / cm ² / sec.

  FIG. 13 shows the change over time in the count rate [cps] obtained in Example 1 (left vertical axis). FIG. 13 also shows the current value [μA] of the accelerator (right vertical axis), and the current value represents the history of the accelerator output in the first embodiment. The right vertical scale in FIG. 13 is adjusted so that the accelerator current value overlaps the neutron count rate on the graph for the neutron count rate of the portion showing 6 kcps or less. The pulse height distribution spectrum when the accelerator current value is about 5 μA is the lowest, as shown in FIG. 2, and the pulse height distribution spectra when the accelerator current value is about 45 μA and about 380 μA are shown in FIGS. 1 and 3, respectively. It can be seen that the higher the accelerator current value, that is, the higher the neutron dose rate, the higher the pileup frequency.

  FIG. 14 shows a graph in which the average values of the accelerator current value and the neutron count rate are calculated and compared for each period in which the accelerator current value is stable for the data in FIG. The regression line in FIG. 14 is obtained from a portion (four points of about 1 kcps to 5 kcps) where the neutron count rate is low and the influence of pileup is small. The error of the neutron count rate actually obtained with respect to the neutron count rate expected from the regression line is as shown in Table 1, and it was a good result with an error of -2.1% at the maximum.

比較例１
実施例１と同一の中性子検出装置を用いて、波高弁別閾値を（μ−１．０σ）とした以外は、実施例１と同様にして加速器中性子を計数した。

Comparative Example 1
Using the same neutron detector as in Example 1, accelerator neutrons were counted in the same manner as in Example 1 except that the pulse height discrimination threshold was (μ-1.0σ).

  15 and 16 show the results of analyzing the neutron count rate and the accelerator current value obtained in Comparative Example 1 by the same method as in Example 1. FIG. In FIG. 15, in the part where the neutron count rate exceeds 40 kcps, the neutron count rate is significantly lower than the accelerator current value. This shows the result of counting loss caused by pileup. The difference between the neutron count rate and the regression line at each accelerator current value is as shown in Table 1 and produced a counting error exceeding -10%.

Comparative Example 2
Using the same neutron detector as in Example 1, accelerator neutrons were counted in the same manner as in Example 1 except that the pulse height discrimination threshold was (μ + 0.7σ).

  FIG. 17 and FIG. 18 show the results of analyzing the neutron count rate and the accelerator current value obtained in Comparative Example 2 by the same method as in Example 1. FIG. In FIG. 17, in the part where the neutron count rate exceeds 15 kcps, the neutron count rate is remarkably higher than the accelerator current value. This is affected by the increase in the counts of the second peak and the third peak due to pileup. The results are shown. The difference between the neutron count rate and the regression line at each accelerator current value is as shown in Table 1 and caused a counting error exceeding + 10%.

In the above examples, it was shown that it was possible to suppress a decrease in counting accuracy due to pile-up by adopting the counting method of the present invention even under high count rate conditions where the frequency of pile-up was high. . If a Eu: LiCaAlF ₆ crystal having a Li-6 isotope ratio of natural ratio and reduced in size to 0.3 mm × 0.3 mm × 0.2 mm is used as the neutron scintillator 3, the neutron detector manufactured in this example Therefore, if the neutron detection device uses the Eu: LiCaAlF ₆ crystal, the neutron sensitivity can be accurately measured even at a neutron dose rate of 1 × 10 ⁹ n / cm ² / sec in BNCT treatment. Counting is possible.

1: Radiation detection element 2: Wave height discrimination circuit 3: Scintillator 4: Photodetector 5: Optical fiber

Claims (5)

A method of setting a pulse height discrimination threshold and counting the frequency of signals exceeding the pulse height discrimination threshold,
When the target component of the count shows a peak in the wave height distribution spectrum,
A counting method in which an arbitrary value selected from the range of (μ−0.4σ) to (μ + 0.6σ) obtained by fitting a Gaussian function for the peak is used as the pulse height discrimination threshold. A radiation detection device using a combination of a scintillator, a photodetector, and a wave height discrimination circuit,
The wave height discrimination circuit is:
An amplifier for amplifying the signal of the photodetector;
An AD converter for converting the analog signal output of the amplifier into a digital signal;
A digital data processing device that performs signal processing on the digital signal output of the AD converter;
A wave height discrimination circuit comprising:
The radiation detection apparatus which processes a signal by the counting method of Claim 1 in the said wave height discrimination circuit. The radiation detector according to claim 2, wherein the scintillator is a LiCa _x Sr _1-x AlF ₆ crystal (x is 0 to 1).   The radiation detection apparatus according to claim 2, wherein the radiation detected by the scintillator is an accelerator neutron.   The radiation detection apparatus according to any one of claims 2 to 4, wherein the radiation detected by the scintillator is a neutron for boron neutron capture therapy.

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