Radionuclide activity measuring method based on delay coincidence

Abstract

The invention relates to a radionuclide activity measuring method based on delay coincidence, which comprises the following steps of: determining an over-threshold time difference when a beta signal and a gamma signal begin to coincide through an integral method or a differential method, and obtaining an optimal coincidence time window according to the over-threshold time difference when a true coincidence count reaches a maximum value; and then calculating beta-ray detection efficiency, gamma-ray detection efficiency and true coincidence count of energy peaks of interest, and calculating the radionuclide activity according to the results. According to the method, the problem of low efficiency of activity coincidence measurement of radionuclides with transient-steady state decay daughter nuclei is solved, and the coincidence efficiency and the detection sensitivity are improved by calculating the optimal coincidence time window.

Description

Technical Field

[0001] The invention belongs to the technical field of radioactive sample activity measurement, and in particular relates to a radionuclide activity measurement method based on delayed coincidence. Background Art

[0002] The β-γ coincidence method is a method for measuring gas radioactive nuclides with high detection sensitivity. The Swedish SAUNA, American RASA and Russian ARIX systems all use the β-γ (NaI) coincidence method to measure the activity of xenon isotopes. However, for nuclides with transient stable states in decay daughter nuclei, such as 85 When measuring the activity of Kr using the β-γ coincidence method, the inability to accurately coincide causes the problem of reduced coincidence efficiency, which in turn affects the detection sensitivity. According to the characteristics of radionuclide decay, the delayed coincidence method can be used to improve the coincidence efficiency and solve the problem of low coincidence measurement efficiency of radionuclide activity with transient steady-state decay daughter nuclei. Summary of the invention

[0003] Aiming at the characteristic of decay daughter nuclei existing in transient stable state, the present invention provides a radionuclide activity measurement method based on delayed coincidence, which solves the problem of low coincidence measurement efficiency of radionuclide activity of decay daughter nuclei existing in transient stable state. By setting the optimal delayed coincidence time window, the coincidence efficiency can be increased and the detection sensitivity can be improved.

[0004] In order to solve the above technical problems, the present invention provides a method for measuring radionuclide activity based on delayed coincidence, which is characterized by comprising the following steps:

[0005] Step 1: Determine the best matching time window WT;

[0006] Step 2: Calculate the β-ray detection efficiency;

[0007] Step 3: Calculate the gamma ray detection efficiency;

[0008] Step 4: Calculate the true coincidence count of the energy peak of interest;

[0009] Step 5: Calculate the activity of the radionuclide based on the results obtained in steps 1 to 4 above.

[0010] Furthermore, in step 1, the threshold crossing time difference when the β signal and the γ signal begin to coincide is determined by integration or differentiation method as t0, t1 is the threshold crossing time difference when the true coincidence count reaches the maximum value, and the optimal coincidence time window WT = [t0, t1].

[0011] Furthermore, the integration method is as follows: for radionuclides with transient stable decay daughter nuclei, when using a β-γ coincidence device for measurement, the true coincidence counts within a unit time interval are integrated in the interval between the threshold time difference ts and t to obtain the true coincidence count integral value. When the integral starting point ts At \(t = t_0\), the fitted \(t\) s拟合 = \(t_0\); when the integration starting point \(t\) s \(> t_0\), the fitted \(t\) s拟合 = \(t\) s .

[0012] Furthermore, the true coincidence count integral value is obtained by the integration method as:

[0013]

[0014] where \(N\) γC (\(t\)) is the true coincidence integral count within the time range from \(t\) s to \(t\); \(\lambda\) is the half-life of the metastable daughter nucleus; \(n_0\) is the coincidence \(\gamma\)-ray peak count rate at \(t=\infty\); \(t_0\) is the time difference between the threshold crossing of the \(\beta\) signal and the \(\gamma\) signal when coincidence starts.

[0015] Furthermore, the differential method is as follows: for a radionuclide with a metastable decay daughter nucleus, when measured by a \(\beta\)-\(\gamma\) coincidence device, the variation law of the true coincidence differential count \(P\) of \(\beta\)-\(\gamma\) rays with the time difference \(t\) between the threshold crossings within a unit time interval is considered.

[0016] Furthermore, in the differential method, the variation law of the true coincidence differential count \(P\) of \(\beta\)-\(\gamma\) rays with the time difference \(t\) between the threshold crossings within a unit time interval is:

[0017] \(P(t)=P_0e\) -λ(t-t0) \((2)\)

[0018] where \(\lambda\) is the half-life of the metastable decay daughter nucleus, \(t_0\) is the time difference between the threshold crossings of the \(\beta\) signal and the \(\gamma\) signal when coincidence starts, and \(P_0\) is the true coincidence count when the time difference between the threshold crossings is \(t_0\).

[0019] Furthermore, in step two, the \(\beta\)-ray detection efficiency \(\varepsilon\) β is calibrated as:

[0020]

[0021] where \(E\) β and \(E\) γ are a pair of \(\beta\) and \(\gamma\) ray energies with a coincidence relationship; \(N\) γC is the \(\gamma\) spectrum peak count after coincidence; \(N\) γ is the original \(\gamma\) spectrum peak count; \(W\) β is the \(\beta\)-ray energy window range; \(W\) T is the coincidence time window range determined in step one, and \(K\) s is the \(\beta\)-ray self-absorption correction factor.

[0022] ​​​​​Furthermore, in step 3, a mixed standard γ calibration source with the same material and geometry as the sample to be tested is placed on the detector to obtain the relationship curve between the γ-ray detection efficiency and energy, and the energy is calculated as E γ The gamma-ray detection efficiency ε γ ;

[0023] In step 4, according to the best matching time window W determined in step 1 T , with W T , W β Open the door to get the energy spectrum, and calculate the energy to be E γ The true coincidence count N of the γ-ray region of interest γC (E γ ,W T ,W β ).

[0024] Furthermore, in step 5, the radionuclide activity is calculated as follows:

[0025]

[0026] Where A is the activity of the radionuclide, Bq; λ0 is the nuclide decay constant, s -1 ;P βγ is the emission probability of β-γ; t l ,t r and t are the sample measurement live time, clock time and the time interval from the start of sample measurement to the reference time, s, respectively; for long-lived nuclides, equation (4) is simplified to equation (5).

[0027] Furthermore, the β-γ coincidence system is used to measure 85 Kr gas sample, the coincidence measurement system includes a lead shielding room and a high-purity germanium detector 2 placed in the lead shielding room, a plastic scintillator detector 1 with an air filling pipeline 6 and a valve 5, and the high-purity germanium detector is cooled by a liquid nitrogen cooling device 4; two high-pressure modules provide high voltage for the high-purity germanium detector and the plastic scintillator detector respectively; the plastic scintillator detector signal is input into a digital coincidence instrument 10 via a preamplifier and a main amplifier; the high-purity germanium detector signal is input into the digital coincidence instrument via the main amplifier; the digital coincidence instrument is connected to a computer, and data processing is performed by the computer.

[0028] Beneficial effects: The present invention solves the problem of low efficiency in coincidence measurement of radionuclide activity with transient steady-state decay daughter nuclei, and increases coincidence efficiency and improves detection sensitivity by calculating the optimal coincidence time window. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the β-γ coincidence device;

[0030] Figure 2 is 133 Xe coincidence count vs. delay time curve;

[0031] Figure 3 is 85 Kr coincidence count vs. delay time curve;

[0032] Figure 4 is the original γ background energy spectrum and coincidence γ background energy spectrum obtained by β-γ delayed coincidence;

[0033] Where: 1 - plastic scintillator detector equipped with PMT; 2 - HPGe detector; 3 - lead shielding chamber; 4 - liquid nitrogen cooling device; 5 - valve; 6 - gas filling pipeline; 7 - high voltage; 8 - preamplifier; 9 - amplifier; 10 - digital coincidence meter; 11 - computer. Detailed implementation mode

[0034] To make the objectives, content and advantages of the present invention clearer, the following further describes in detail the specific implementation mode of the present invention.

[0035] A method for measuring the activity of radionuclides based on delayed coincidence proposed by the present invention includes the following steps:

[0036] Step 1. Determine the optimal coincidence time window W T : Determine the threshold crossing time difference t0 when the β signal and the γ signal start to coincide by the integral method or the differential method. t1 is the threshold crossing time difference when the true coincidence count reaches 98% of the maximum value. Then the optimal coincidence time window WT = [t0, t1];

[0037] The specific method is as follows:

[0038] 1.1) Integral method. For radionuclides with metastable decay daughter nuclei, when measured by a β-γ coincidence device, integrate the true coincidence count within the unit time interval from the threshold crossing time difference ts to t to obtain the integral value of the true coincidence count:

[0039]

[0040] where, N γC (t) is the true coincidence integral count within the time from t s to t; λ is the half-life of the metastable daughter nucleus; n0 is the coincidence γ-ray peak count rate at t = ∞; t0 is the threshold crossing time difference when the β signal and the γ signal start to coincide. When the integration starting point t s < t0, the fitted t s拟合 = t0; when the integration starting point t s > t0, the fitted t s拟合 = t s .

[0041] 1.2) Differential method: For radionuclides with transient stable decay daughter nuclei, when using a β-γ coincidence device for measurement, the true coincidence differential count P of β-γ rays in a unit time interval changes with the threshold time difference t as follows: (2)

[0043] Among them, λ is the transient steady-state half-life of the decay daughter nucleus, t0 is the threshold-crossing time difference when the β signal and the γ signal begin to coincide, and P0 is the true coincidence count when the threshold-crossing time difference is t0.

[0044] Step 2: Calculate the β-ray detection efficiency ε β Scale: (3)

[0046] Among them, E β With E γ is a pair of β and γ ray energies with a coincident relationship; N γC N is the peak count of the γ spectrum after the match; γ is the original γ spectrum peak count; W β W is the β-ray energy window range; T is the time window range determined in step 1, K s is the β-ray self-absorption correction factor.

[0047] Step 3: Calculate the gamma-ray detection efficiency ε γ scale;

[0048] Place a mixed standard γ calibration source that is consistent with the material and geometry of the sample to be tested on the detector to obtain the relationship curve between the γ-ray detection efficiency and energy, and calculate the energy as E γ The gamma-ray detection efficiency ε γ ;

[0049] Step 4: Calculate the true coincidence count N of the energy peak of interest γC ;

[0050] According to the best matching time window W determined in step 1 T , with W T , W β Open the door to get the energy spectrum, and calculate the energy to be E γ The true coincidence count N of the γ-ray region of interest γC (E γ ,W T ,W β ).

[0051] Step 5: Calculate the activity of radionuclides:

[0052] According to the best matching time window W determined in step 1 T , the β detection efficiency ε obtained in step 2 β(E β ), the γ detection efficiency ε obtained in step 3 γ (E γ ) and the energy calculated in step 4 is E γ The true coincidence count N of the γ-ray region of interest γC (E γ ,W T ,W β ), we can get the radionuclide activity: (4) (5)

[0055] Where A is the activity of the radionuclide, Bq; λ0 is the nuclide decay constant, s -1 ;P βγ is the emission probability of β-γ; t l ,t r and t are respectively the sample measurement live time, clock time and the time interval from the start of sample measurement to the reference time, s. For long-lived nuclides, equation (4) can be simplified to equation (5).

[0056] The β-γ coincidence method of the present invention is a delayed coincidence, which uses the integral method and the differential method to accurately determine the time window, increase the coincidence efficiency, and improve the detection sensitivity.

[0057] The best fitting time window was obtained by exponential fitting.

[0058] The present invention utilizes the established β-γ coincidence system ( Figure 1 ) measured 85 Kr gas sample, in the coincidence measurement system, a high-purity germanium detector 2 and a plastic scintillator detector 1 with an air filling pipeline 6 and a valve 5 are placed in a lead shielding room 3, and a liquid nitrogen cooling device 4 is filled with liquid nitrogen for cooling the high-purity germanium detector; high-pressure modules 7-1 and 7-2 provide high voltage for the high-purity germanium detector and the plastic scintillator detector respectively; the plastic scintillator detector signal is input into a digital coincidence instrument 10 via a preamplifier 8 and a main amplifier 9-1; the high-purity germanium detector signal is input into a digital coincidence instrument 10 via a main amplifier 9-2; the digital coincidence instrument 10 is connected to a computer 11, and data processing can be performed through the computer 11.

[0059] Figure 2-Figure 3 They are 133 Xe and 85 Kr coincides with the curve of the count changing with the time difference of crossing the threshold, reflecting 133 Xe coincidence is instantaneous (or the half-life of the transient steady state is much shorter than the coincidence resolution time), and its coincidence count is Gaussian distributed along with the time difference of crossing the threshold. 85 Kr coincidence is a delayed coincidence, and its coincidence count is exponentially decayed and convolved with the threshold time difference.

[0060] Using integration and differentiation methods 85 Kr measured data is processed to determine whether the delay meets the best time window W of the measurement system T The measurement efficiency of radioactive nuclides with transient stable decay daughter nuclei is low. 85 Kr true coincidence count, substitute into the activity calculation formula, we can get 85 Kr activity.

[0061] The 7-day coincidence background spectrum and the original background spectrum of the delayed coincidence measurement system are shown in Figure 4 ,The results show that compared with the original background, the delayed coincidence measurement method ,can reduce the background by about 96%, which can greatly improve the ,system detection sensitivity.

[0062] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for measuring radionuclide activity based on delayed coincidence, characterized in that: The following steps are involved: Step 1: Determine the best matching time window WT; Step 2: Calculate the β-ray detection efficiency; Step 3: Calculate the gamma ray detection efficiency; Step 4: Calculate the true coincidence count of the energy peak of interest; Step 5: Calculate the activity of the radionuclide based on the results obtained in steps 1 to 4 above.

2. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: In step 1, the threshold crossing time difference when the β signal and the γ signal begin to coincide is determined by integration or differentiation method as t0, t1 is the threshold crossing time difference when the true coincidence count reaches the maximum value, and the optimal coincidence time window WT = [t0, t1].

3. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: The integration method is as follows: for a radionuclide with a metastable decay daughter nucleus, when measured using a β-γ coincidence device, the true coincidence counts within a unit time interval are integrated from the threshold crossing time difference ts to the time t to obtain the integrated value of the true coincidence counts. When the integration starting point t< t0, the t s拟合 obtained by fitting = t0; when the integration starting point t s > t0, the t s拟合 obtained by fitting = t s .​ 4. The method for measuring radionuclide activity based on delayed coincidence according to claim 3, characterized in that: The integral value of true coincidence count obtained by the integration method is: Among them, N γC (t) is t s The true coincidence integral count within time t ~ t; λ is the half-life of the transient steady-state daughter nucleus; n0 is the coincidence gamma-ray peak count rate at time t = ∞; t0 is the threshold crossing time difference when the β signal and the γ signal begin to coincide.

5. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: The differential method is as follows: for radioactive nuclides with transient steady-state decay daughter nuclei, when a β-γ coincidence device is used for measurement, the true coincidence differential count P of β-γ rays in a unit time interval varies with the threshold-crossing time difference t.

6. The method for measuring radionuclide activity based on delayed coincidence according to claim 5, characterized in that: In the differential method, the variation law of the true coincident differential count P of β-γ rays in a unit time interval with the threshold time difference t is: Among them, λ is the transient steady-state half-life of the decay daughter nucleus, t0 is the threshold-crossing time difference when the β signal and the γ signal begin to coincide, and P0 is the true coincidence count when the threshold-crossing time difference is t0.

7. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: In step 2, the β-ray detection efficiency ε β The scale is: Among them, E β With E γ is a pair of β and γ ray energies with a coincident relationship; N γC N is the peak count of the γ spectrum after the match; γ is the original γ spectrum peak count; W β W is the β-ray energy window range; T is the time window range determined in step 1, K s is the β-ray self-absorption correction factor.

8. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: In step 3, a mixed standard γ calibration source with the same material and geometry as the sample to be tested is placed on the detector to obtain the relationship curve between the γ-ray detection efficiency and energy, and the energy is calculated as E γ The gamma-ray detection efficiency ε γ ; In step 4, according to the best matching time window W determined in step 1 T , with W T , W β Open the door to get the energy spectrum, and calculate the energy to be E γ The true coincidence count N of the γ-ray region of interest γC (E γ ,W T ,W β ).

9. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: In step 5, the radionuclide activity is calculated as follows: Where A is the activity of the radionuclide, Bq; λ0 is the nuclide decay constant, s -1 ;P βγ is the emission probability of β-γ; t l ,t r and t are respectively the sample measurement live time, clock time and the time interval from the start of sample measurement to the reference time, s; for long-lived nuclides, equation (4) is simplified to equation (5).

10. The method for measuring radionuclide activity based on delayed coincidence according to claim 1, characterized in that: Measured using the β-γ coincidence system 85 Kr gas sample, the coincidence measurement system includes a lead shielding room and a high-purity germanium detector 2 placed in the lead shielding room, a plastic scintillator detector 1 with an air filling pipeline 6 and a valve 5, and the high-purity germanium detector is cooled by a liquid nitrogen cooling device 4; two high-pressure modules provide high voltage for the high-purity germanium detector and the plastic scintillator detector respectively; the plastic scintillator detector signal is input into a digital coincidence instrument 10 via a preamplifier and a main amplifier; the high-purity germanium detector signal is input into the digital coincidence instrument via the main amplifier; the digital coincidence instrument is connected to a computer, and data processing is performed by the computer.

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