JPH068862B2 - Radioactivity measurement method by liquid scintillation counter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for measuring radioactivity using a liquid scintillation counter, and in particular, a liquid for discriminating a nuclide of a sample to be measured and selecting an appropriate quenching correction method. Radioactivity measurement method using scintillation counter.

[Prior Art] Radioisotopes are widely used safely in various fields including medical treatment, and for example, in the medical field, they are used for diagnosis and treatment of diseases. Under such circumstances, there is a demand for an apparatus and method for easily and accurately determining the radioactivity of radioisotopes.

By the way, as a device for obtaining the radioactivity of the sample to be measured,
Liquid scintillation counters are known. This liquid scintillation counter mixes the sample to be measured with a liquid scintillator (phosphor), the radiation from the sample excites the scintillator, and the light generated by this excitation is detected by, for example, a photomultiplier tube, and the detection result The amount of radioactivity is obtained from this.

And according to this liquid scintillation counter,
In general, various types of radiation such as β rays, β-γ rays, and α rays can be detected, and in addition, for example, low energy β rays can be detected.

However, this liquid scintillation counter has a known problem of quenching caused by mixing the sample to be measured with the liquid scintillator.

This quenching, when the sample to be measured is mixed with the liquid scintillator, the liquid scintillator is colored, or the fluorescence efficiency is lowered due to a change in chemical composition, and as a result,
This is a phenomenon in which the total count value becomes low and an accurate radioactivity amount cannot be obtained.

FIG. 5 shows a change in energy spectrum due to the effect of quenching when radiation is detected by the liquid scintillation counter. Here, the horizontal axis indicates the energy E of the radiation, and the vertical axis indicates the count value (count value) corresponding to each energy.

As is clear from comparison between (A) and (B), as the quenching becomes stronger, the energy spectrum shifts to the lower energy side depending on the nuclide. As a result, those below the minimum detected energy also increase, and the total count value also decreases, so the error included in the amount of radioactivity obtained as a measurement result increases.

As described above, the radioactivity measuring method using the liquid scintillation counter has a problem of quenching. Conventionally, in order to solve the problem of quenching, the following method has been adopted.

As a method for correcting the decrease in the count value due to quenching, various methods have been conventionally used, but here, the external standard radiation source channel ratio method and the efficiency tracer method will be described (for the efficiency tracer method, for example, JP-A-6
0-173489).

The external standard source channel ratio method will be described below.

This external standard source channel ratio method creates a correction curve from samples with known radioactivity and different quenching (mixed in a liquid scintillator of the sample to be measured),
The quenching is corrected based on this correction curve. This correction curve is created as follows.

As shown in FIG. 6, first, a sample is irradiated with γ-rays from an external standard radiation source to obtain a Compton spectrum. here,
As is well known, the spectrum of Compton spectrum changes depending on quenching.

Then, as shown in FIG. 7, a threshold value (Rγ) for dividing the Compton spectrum at a predetermined ratio is obtained. In FIG. 7, this threshold value (Rγ) is defined as a value that divides the Compton spectrum into 3: 1. Here, LL is a cut energy value (ESCR value = K · Rγ).

Then, this Rγ value is multiplied by a predetermined multiplier K to obtain an ESCR value (External Standard Channel Ratio value).

This ESCR value is obtained for various samples with different quenching and is further graphed, which is the correction curve shown in FIG.

Therefore, for a sample whose quenching is unknown and whose nuclide is known, the ESC can be obtained by irradiating the sample with γ-rays from an external standard radiation source to obtain a Compton spectrum.
The R value is obtained, and the obtained ESCR value is associated with the correction curve shown in FIG. 8 to obtain the counting efficiency (%) which is the vertical axis thereof.

This counting efficiency indicates the amount of change in the count value that has been reduced by quenching, and the total count value when there is no quenching by correcting the measurement result based on this counting efficiency, that is, the radiation The ability can be calculated.

Next, the efficiency tracer method will be described.

This efficiency tracer method can correct quenching even if the nuclide is unknown, and an efficiency tracer curve is created from the energy spectrum obtained by measuring the sample to be measured, and this efficiency tracer curve is extrapolated. By doing so, the total amount of radioactivity is obtained. The details will be described below.

First, as shown in FIG. 9, an energy spectrum obtained by measuring a sample to be measured is analyzed by a plurality of windows, for example, W.
1 to W6 are divided, and the sum L of the count values of each window is plotted as shown in FIG. 10 to obtain an efficiency tracer curve. Here, the shaded area in FIG. 9 indicates the radiation in the energy range that cannot be detected by the photomultiplier tube. In order to correct this area, the efficiency tracer curve in FIG. It is extended and extrapolated to obtain a count rate when the energy level is zero.

Here, the position where the energy level is 0 cannot be specified as it is because it is below the detection limit value. However, for example, a standard sample is preliminarily measured, and an efficiency tracer curve similar to the above is created for this standard sample to set the 0 level. It is possible to ask.

Therefore, for samples whose nuclide is unknown and whose degree of quenching is unknown, create an efficiency tracer curve based on this efficiency tracer method, and then extrapolate and correct it to obtain the total radioactivity. be able to.

As described above, according to this efficiency tracer method, even if the nuclide of the sample is not known, its radiation dose can be corrected.

On the other hand, the external standard source channel ratio method requires that the nuclide be known as described above, but generally has an advantage that the correction accuracy is better than the efficiency tracer method.

[Problems to be Solved by the Invention] However, when the radioactivity is measured using the liquid scintillation counter as described above, the above-described quenching correction is necessary, and in the past,
Although the quenching correction method was appropriately selected according to the nuclide to be measured, there was a problem that the selection generally required a high degree of knowledge and was complicated.

Accordingly, there has been a demand for an automated device for obtaining a radioactivity amount using a liquid scintillation counter and a method for realizing the device.

The present invention has been made in view of the above conventional problems,
An object of the present invention is to provide a radioactivity measuring method using a liquid scintillation counter, which can automatically determine quenching correction without relying on artificial judgment and determine the radioactivity using an optimum correction method. .

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method for measuring radioactivity by a liquid scintillation counter, which mixes a sample to be measured with a liquid scintillator to measure the amount of radioactivity of the sample. An ESCR value calculating step of calculating an ESCR value indicating a value that divides a Compton spectrum obtained by irradiating the sample with γ-rays from a radiation source, and a predetermined energy spectrum of radiation obtained by the measurement of the sample. SCCR value (Self Con
The SCCR value calculation step of calculating the stant channel ratio value) and the ESCR value in the preset nuclide determination table.
A nuclide determination step of determining the nuclide of the sample in association with the value and the SCCR value, and an appropriate correction selection step of selecting a quenching correction method according to the nuclide determined in the nuclide determination step, A correction curve correction step of performing quenching correction according to a quenching correction curve previously created for each nuclide when the external standard source channel ratio method is selected in the appropriate correction selection step, and the appropriate correction selection step. In the case where the efficiency tracer method is selected in, an efficiency tracer curve is created based on the energy spectrum obtained in the measurement of the sample, and an extrapolation correction step of extrapolating the efficiency tracer curve to perform quenching correction, And determining the nuclide of the sample, and selecting a quenching correction method according to the determination result.

[Operation] According to the above configuration, the SCCR value calculation step and the ESCR
By correlating the two values obtained in the value calculation step with the nuclide determination table, it is possible to determine what nuclide the sample to be measured is.

Then, an appropriate quenching correction method can be selected according to the nuclide determined in the appropriate correction selection step.

Here, for example, the external standard source channel ratio method is applied to the nuclide for which the correction curve is created in advance, while the efficiency tracer method is applied to the nuclide for which the correction curve is not created.

Therefore, the operator does not need to artificially select a correction method, and can always automatically apply an appropriate correction to obtain the amount of radioactivity.

[Embodiment] A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a liquid scintillation counter to which the method for measuring radioactivity by the liquid scintillation counter according to the present invention is applied.

In FIG. 1, the sample to be measured is mixed with a liquid scintillator and placed as a sample 10 inside a lead shield 12. Here, the lead shield 12 is for shielding cosmic rays and radiation from the outside. Sample 1
Two photomultiplier tubes A and B are arranged close to the sample 10 with their light-receiving surfaces facing each other with 0 interposed therebetween.

In the figure, 14 is an external standard radiation source when the external standard radiation source channel ratio method is applied, and in this embodiment,
¹³⁷ Cs is used, and γ rays are emitted from here. The external standard line 14 is driven by a driving means (not shown), and when the external standard line source channel ratio method is applied, the external standard line 14 is inserted inside through an opening formed in the lead shield.

A high voltage is supplied from the high voltage power supply 16 to the photomultiplier tubes A and B.

The outputs from the photomultiplier tubes A and B are input to the pulse addition circuit 18 and the high speed coincidence counting circuit 20, respectively.

The pulse adder circuit 18 adds the outputs from the photomultiplier tubes A and B and outputs the result to a multi-channel analyzer described later, in order to effectively detect the fluorescence regardless of the fluorescence position in the sample 10. , The outputs from the two photomultiplier tubes A and B are added.

The high-speed coincidence counting circuit 20 sends a detection signal to the multi-channel analyzer 22 only when the outputs from the two photomultiplier tubes are simultaneously input, and has the same function as the AND gate.

That is, in the detection of radiation by a liquid scintillation counter, generally, the radiation from the sample is weak, so that the fluorescence is extremely small, and the noise level of the photomultiplier tube may be equal to or lower than the noise level. This high-speed coincidence counting circuit 20 is provided in order to detect radiation while eliminating the contribution of noise.

The signal output from the pulse adder circuit 18 is input to the multi-channel analyzer 22, and the multi-channel analyzer 22 outputs the pulse only when the coincidence counting confirmation signal from the high-speed coincidence counting circuit 20 is obtained. The pulse signal from the adder circuit 18 is accepted.

Therefore, in this multi-channel analyzer 22,
Pulse from high speed coincidence counting circuit 20 and pulse addition circuit 1
A pulse having a wave height corresponding to the energy of radiation from 8 and a pulse are obtained only when they are simultaneously obtained, and the result is integrated for each energy value to obtain an energy spectrum.

Then, the obtained energy spectrum information is sent to the microcomputer 24, and the microcomputer 24 corrects the above-mentioned quenching. Further, the measurement result is recorded in the printer 26.

Next, FIG. 2 shows each step of the method for measuring radioactivity by the liquid scintillation counter according to the present invention.

What is characteristic here is that a nuclide determination step (S105) is provided in order to perform appropriate quenching correction according to the nuclide to be measured. In the nuclide determination, the nuclide is specified for the one having the above-mentioned correction curve, and the nuclide determination from the low energy nuclide to the high energy nuclide is performed for the one having no correction curve. .

That is, as described above, since the external standard radiation source channel ratio method has better correction accuracy than the efficiency tracer method, it is suitable to be applied to all nuclides. A great deal of time and effort is required, and on the basis of such a premise, the nuclide for which the correction curve is created is determined. In this embodiment, correction curves for ³ H and ¹⁶ C, which are low energy nuclides, are prepared. Also,
Since the energy spectrum of high-energy nuclides is mostly distributed on the high energy side, the efficiency tracer curve is almost flat, and the error is small even if extrapolation correction is performed, so the efficiency tracer method is applied.

Here, first, a nuclide determination method will be described with reference to FIGS. 3 and 4.

FIG. 3 shows a nuclide determination table.

This nuclide determination table shows the correlation between the ESCR value obtained from the Compton spectrum described above and the SCCR value described later, and a specific nuclide is determined from the intersection of these two values.

As shown in FIG. 4, the SCCR value is a threshold value (R which divides the measured energy spectrum at a predetermined ratio).
β) multiplied by a predetermined multiplier 1 (SCCR value = 1 · Rβ).

Here, as shown in (A) and (B), the Rβ value fluctuates depending on the spectrum of the nuclide.

Therefore, in FIG. 3, the ESCR value shows the change in the Compton spectrum due to quenching in the nuclide, while the SCCR value is a value that varies depending on the quenching of the nuclide and the nuclide itself.

For the nuclide for which the correction curve has been created, the nuclide can be specified from the two values of the ESCR value and the SCCR value by simultaneously creating the judgment curve 200 in this nuclide judgment table. For example, the third
In the figure, when the ESCR value is a and the ECCR value is b, ³ H is judged from the judgment point P.

Note that nuclides that are not specifically specified are classified into high-energy nuclides and low-energy nuclides.
That is, as will be described later, this is to apply the efficient tracer method applicable to high-energy nuclides even if the nuclide is unknown.

As described above, in this example, the nuclide determination table is divided into four groups for determination. Of course, it is preferable to create a nuclide determination curve for nuclides other than ¹⁴ C and ³ H shown in FIG. ³ as the nuclide determination curve, and it is possible to further specify a specific nuclide.

Next, the radioactivity measuring method according to the present invention will be described with reference to FIG. 2 again.

In step 101, the sample 10 is irradiated with γ-rays from the external standard radiation source 14 to obtain a Compton spectrum.

In step 102, the ESCR value is calculated from the Compton spectrum obtained in step 101.

Next, in step 103, the radiation energy spectrum measurement of the sample 10 is performed for a predetermined time.

In step 104, the above-mentioned SCCR value is calculated from the energy spectrum obtained in step 103.

Next, in step 105, the SCCR obtained above is calculated.
The nuclide is specified by associating the value and the ESCR value with the nuclide determination table shown in FIG. As described above, the specification of this nuclide is ¹⁴ C, ³ H in this embodiment.
Or high energy nuclides or low energy nuclides.

In step 106, it is determined whether or not the identified nuclide has a correction curve. Then, if there is a correction curve, that is, ³ H and ¹⁴ C in this embodiment,
Moving to 07, quenching is corrected from the correction curve of the external standard source channel ratio method.

On the other hand, if it is determined in step 106 that there is no correction curve, the process proceeds to step 108, where it is determined whether the energy is a low energy nuclide or a high energy nuclide.

That is, high-energy nuclides are effective because the error due to the correction is small even when correction is performed based on the efficiency tracer method.On the other hand, for low-energy nuclides, the energy spectrum is mostly distributed on the low-energy side. Therefore, the unknown amount that has not been measured is large,
As a result, the correction error becomes large, and if the determination is made for the low energy nuclide, step 110 is performed.
Then, the counting result without correction is printed out. However, low-energy nuclides other than ³ H and ¹⁴ C are extremely special and rarely actually corrected.

If it is judged as a high energy nuclide in step 108,
Control goes to step 109. In this step 109,
An efficiency tracer curve based on the efficiency tracer method is created,
Further, the extrapolation of the curve corrects the quenching.

Then, the process proceeds from step 107 and step 109 to step 110, and the corrected radioactivity is printed out.

As described above, according to this method, for the method to which the external standard source channel ratio method can be applied, the method is preferentially applied, while the external standard source channel ratio method cannot be applied, that is, For nuclides that do not have a correction curve, the low energy or high energy nuclide is judged, and the efficiency tracer method is applied for high energy nuclides, so reliable and accurate radioactivity Can be measured.

Even for a low-energy nuclide that does not have a correction curve, a highly accurate external standard source channel ratio method can be applied by creating a nuclide determination curve in the nuclide determination table and creating a correction curve. As described above, the radioactivity of the nuclide can be easily measured without requiring artificial judgment while maintaining the accuracy of a predetermined level or higher. Therefore, the correction process, which has been complicated in the past, is automated, and the efficiency in diagnosis and research is improved.
In addition, according to this method, it is possible to realize an automated radioactivity measuring device using a liquid scintillation counter, and in the field where the measurement of radioactivity is desired,
A useful device can be provided.

[Effects of the Invention] As described above, according to the radioactivity measuring method by the liquid scintillation counter of the present invention, an appropriate quenching correction method can be selected and applied according to the nuclide of the sample to be measured. The quenching correction can always be performed with high accuracy and can be performed easily.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a liquid scintillation counter to which the method according to the present invention is applied, FIG. 2 is a flowchart diagram of the method according to the present invention, FIG. 3 is an explanatory diagram showing a nuclide determination table, and FIG. Is an explanatory diagram showing the relationship between the energy spectrum and a threshold value for dividing the spectrum at a predetermined ratio, FIG. 5 is an explanatory diagram showing the effect of quenching, FIG. 6 is an explanatory diagram showing a Compton spectrum, and FIG. 7 is FIG. 8 is an explanatory view showing a threshold value for dividing the Compton spectrum at a predetermined ratio, FIG. 8 is a graph showing a correction curve, FIG. 9 is an explanatory view showing each window when the efficiency tracer method is applied, and FIG. 10 is an efficiency tracer. It is explanatory drawing which shows the case where a curve is extrapolated and the radioactivity is calculated. 10 ... Sample 14 ... External standard radiation source 18 ... Pulse addition circuit 20 ... High-speed coincidence counting circuit 22 ... Multi-channel analyzer 24 ... Microcomputer

Claims (1)

[Claims] 1. A radioactivity measurement method using a liquid scintillation counter in which a sample to be measured is mixed with a liquid scintillator to measure the radioactivity of the sample, and a Compton spectrum obtained by irradiating the sample with γ-rays from an external standard radiation source. E that indicates the value that divides the
An ESCR value calculating step of calculating an SCR value, an SCCR value calculating step of calculating an SCCR value indicating a value that divides the energy spectrum of the radiation obtained by the measurement of the sample at a predetermined ratio, and a preset nuclide determination table Corresponding the ESCR value and the SCCR value to the nuclide determination step of determining the nuclide of the sample, and a proper correction selection for selecting a quenching correction method according to the nuclide determined in the nuclide determination step. A correction curve correction step for performing quenching correction according to a quenching correction curve created in advance for each nuclide when the external standard source channel ratio method is selected in the appropriate correction selection step; When the efficiency tracer method is selected in the correction selection process, the efficiency tracer method is based on the energy spectrum obtained from the measurement of the sample. An extrapolation correction step of creating a line and extrapolating this efficiency tracer curve to perform quenching correction, and determining the nuclide of the sample, and selecting the quenching correction method according to the determination result. A method for measuring radioactivity using a liquid scintillation counter.