A stable spectrum method for underwater gamma spectrum measurement

Abstract

The application relates to a stable spectrum method for underwater gamma spectrum measurement, belonging to the technical field of gamma spectrum measurement. The stable spectrum method is characterized in that the stable spectrum step is as follows: a channel address of a compound peak is set, a full spectrum is saved as a first spectrum at an interval of 0.5min to 2min; whether there is a peak is determined at 200 channels on the left and right of the preset compound peak channel address; when there is a peak, whether the preset compound peak count rate range is met is determined; the channel address of a La-138 compound peak peak position is compared with the preset compound peak channel address, and a channel address difference is calculated; the working voltage is adjusted by using a channel address and working voltage relationship function, and a spectrum in a second time period is recorded; the working voltage is adjusted by repeatedly using the channel address and working voltage relationship function until the channel address of the compound peak peak position coincides with the preset compound peak channel address; and thus, the automatic stable spectrum of the start-up measurement is completed. The application can adapt to the stable spectrum of most ordinary detectors, and can better adapt to the temperature effect problem caused by the underwater gamma spectrum detector after entering water.

Description

Technical Field

[0001] A stable spectrum method for underwater gamma-ray spectrum measurement belongs to the field of gamma-ray spectrum measurement technology. Background Technology

[0002] Since gamma spectroscopy measurements are typically performed over extended periods, changes in ambient temperature, magnetic field, and voltage are highly likely to occur during the measurement process. These variations can lead to transverse shifts in the spectrum, resulting in decreased resolution of the full-energy peaks of the measured radionuclide, or even the inability to form Gaussian peaks. Therefore, spectral stability is crucial in gamma spectroscopy measurements, directly impacting the measurement quality.

[0003] The magnetic field changes in the seawater environment are extremely small, and voltage changes mainly depend on the hardware system of the gamma spectrometer. However, due to the great depth of seawater, the significant temperature changes between the upper and lower layers are the main factor causing spectral drift in underwater gamma spectroscopy measurements. Unlike the operating conditions of land-based spectrometers, the ambient temperature of the spectrometer changes drastically during the process from deployment at the sea surface to a depth of 300m, which continuously causes changes in the temperature of the detection system.

[0004] Lanthanum bromide crystals are inorganic scintillators. The luminescence properties of inorganic scintillators are generally affected by temperature, resulting in a temperature effect. The general rule is that the pulse height gradually decreases as the temperature increases. Figure 1 The image shows the temperature-relative pulse height relationship for 662 keV gamma rays in four main inorganic scintillators. Since the multichannel spectrometer outputs a differential spectrum based on the pulse height (i.e., the higher the pulse height, the higher the count address), as the temperature increases, all gamma rays of the same energy are lost within the crystal. Therefore, with the photomultiplier tube performance remaining constant, the count address will correspondingly drift to a lower address. Lanthanum bromide crystal (Saint-Gobain B380) shows no significant temperature effect compared to sodium iodide crystal (NaI), especially below 50°C, where it is significantly superior to sodium iodide crystal.

[0005] The temperature of a photomultiplier tube can range from below 0°C to above 40°C. Within this temperature range, its output can vary by more than 5%. As the temperature of the photomultiplier tube increases, its gain decreases. That is, as the temperature rises, the electrical signal output from the photomultiplier tube to the same crystal will weaken, and the channel address counted by the multichannel spectrometer will correspondingly drift to a lower channel address.

[0006] Therefore, both the temperature effect of the detector crystal and the temperature effect of the photomultiplier tube are negative effects, and the temperature effect of the detector is a combined result of the temperature effects of both. For lanthanum bromide detectors, the temperature effect of the photomultiplier tube is more significant. The temperature effect of lanthanum bromide detectors can reach more than 5%, causing significant drift in the spectral output of the downstream multichannel spectrometer. For the same energy γ full-energy peak, this will lead to a decrease in resolution, or even the inability to form a full-energy Gaussian peak, causing great difficulties for subsequent analysis. Therefore, it is necessary to solve the peak drift temperature caused by the temperature effect.

[0007] Currently, there are four main methods for stabilizing the spectrum of inorganic scintillator gamma spectrometers: reference source stabilization, LED stabilization, software stabilization, and adding a temperature control device. Each method has its own advantages and disadvantages. Among them, LED stabilization is relatively low-cost and reliable; the energy of the LED reference peak can be adjusted according to the measurement needs to stabilize it at a suitable position, which can be used not only for low and medium energies but also for high-energy (>3MeV) measurements, thus broadening the application range of the spectrometer; and the LED reference peak is relatively clear, unlike the gamma reference source stabilization method where Compton scattering affects the measured information in the low-energy range. However, the stable lifespan of existing LED light sources is limited (generally only 2-5 years), and once the intensity and spectrum of the LED light output change, it needs to be readjusted; furthermore, its structure is more complex than that of ordinary detectors, requiring the design and embedding of the LED light source during the crystal packaging of the detector.

[0008] The reference source stabilization mainly includes γ reference source stabilization and α reference source stabilization. The γ reference source method uses the characteristic peaks of embedded radioactive sources such as Cs-137 and Co-60, while the α reference source method uses the characteristic peaks of the Am-241 α source (equivalent γ energy of about 2.8 MeV). Its advantages are: (1) The energy of the radiation emitted by the radioactive source remains unchanged, avoiding the problems caused by the change of the light source spectrum in LED stabilization; (2) The reference radioactive source generally has a long half-life, such as the half-life of Am-241 reaching 400 years. Even though the half-life of Co-60 is shorter, the half-life has been accurately measured, and its radiation intensity can be accurately obtained during the lifespan of the detector. Its disadvantages are: (1) Due to the Compton scattering emitted by the reference radioactive source and the detector crystal, the background count in the low-energy band will be significantly increased, affecting the measured information in the low-energy band; (2) Since the reference radioactive source used may also be the radioactive nuclide of interest in the measurement, the measurement of the same nuclide as the reference radioactive source will produce a large error; (3) The reference source is usually made into a surface source and placed in front of the detector crystal, especially the α reference source, which will put high requirements on the packaging of the detector crystal.

[0009] Analysis of existing spectrum stabilization methods reveals that LED spectrum stabilization, similar to that of gamma and alpha reference sources, is a "set reference peak" method, which is hardware-based. This means that the reference source and the light source are compared during the design and production of the detector. However, ordinary detectors cannot achieve spectrum stabilization and cannot adapt to the temperature effects caused by underwater gamma spectrum detectors after entering the water. Summary of the Invention

[0010] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a stable spectrum method for underwater gamma-ray spectrum measurement that can adapt to the seawater detection environment.

[0011] The technical solution adopted by this invention to solve its technical problem is: a spectral stabilization method for underwater gamma-ray energy spectrum measurement, which uses a lanthanum bromide detector for gamma-ray detection, characterized in that the spectral stabilization step is as follows:

[0012] 1) Set the channel address of the composite peak pseudo-stabilized spectrum. After the measurement starts, the background establishes a cache and saves a full spectrum as the first spectrum at intervals of 0.5 min to 2 min; and identifies the peak positions of all peaks.

[0013] 2) Determine if there is a peak within 200 channels to the left and right of the preset composite peak address; if there is a peak, determine if the preset composite peak count rate range is met; if it is met, determine if the preset energy resolution range is met; if it is met, the La-138 composite peak has been found.

[0014] 3) Compare the peak position address of the La-138 composite peak with the preset composite peak address and calculate the address difference;

[0015] 4) Using the relationship function between channel address and working voltage, adjust the working voltage and start recording the spectrum of the second time period. Then compare the channel address of the composite peak with the preset composite peak channel address and calculate the channel address difference.

[0016] 5) Repeatedly use the function relating channel address and operating voltage to adjust the operating voltage until the composite peak position channel address coincides with the preset composite peak channel address; this completes the automatic spectrum stabilization of the power-on measurement.

[0017] This invention establishes a composite peak stabilization technique based on the La-138's own background peak by utilizing rapid peak identification, reference channel peak position determination, count rate determination, resolution determination, operating voltage adjustment, and channel address matching determination. This invention achieves spectral stabilization for underwater gamma-ray spectrometry measurements through composite peak and spectral stabilization methods, solving the problem that hardware stabilization cannot adapt to ordinary detectors, and can better adapt to the temperature effects brought about by underwater gamma-ray spectrometers after immersion in water.

[0018] A preferred method for stabilizing the underwater gamma-ray spectrum, in step 1), involves identifying all peak positions using a nuclide identification algorithm. During stabilization, accurate identification of various peaks in the spectrum within the shortest possible time is crucial for peak parameter determination. The composite peak count rate is a key parameter for determining the 1435 keV and 1435+32 keV composite peaks. The composite peak count rate can be derived by calculating the detection efficiency at 1435 keV. Using MCNP modeling and derivation, the detection efficiency of the lanthanum bromide detector for the 1435 keV emitted by the crystal itself is 0.1777, and the composite peak count rate is 62.64 s. -1 .

[0019] In actual measurement applications, errors are introduced due to statistical fluctuations and peak area calculations. Therefore, the following judgment range is set for the composite peak count rate: the composite peak count rate judgment range mentioned in step 2) is 3000 min. -1 ~4200min -1 .

[0020] A preferred method for stabilizing underwater gamma spectroscopy measurements is described, wherein the energy resolution of the lanthanum bromide detector is within 3%. The lanthanum bromide detector has high crystal density and high detection efficiency, and exhibits excellent peak shift due to temperature influences. The energy resolution of the lanthanum bromide detector can reach within 3%, which has significant advantages for identifying and quantitatively measuring radionuclides in water using gamma characteristic rays.

[0021] The lanthanum bromide detector itself has a high background, providing a stable reference peak position and peak count for gamma-ray spectroscopy measurements, and providing conditions for spectral stabilization based on the La-138 background composite peak. A preferred spectral stabilization method for underwater gamma-ray spectroscopy measurements involves a La-138 composite peak at 1435 keV and 1435+32 keV. Because La-138 is uniformly distributed in the detector crystal, its rays interact directly with the crystal in large quantities, especially since almost all low-energy photons are deposited in the detector crystal, resulting in a very significant cascade recombination effect. The 1435 keV and 1435+32 keV composite peak is the most significant, and under most measurement conditions, it recombines with the K-40@1460 keV peak in the environment, forming a significant peak. Using this composite peak for spectrum stabilization has the following advantages: First, the gamma spectrum measurement range is generally 0 keV-3 MeV, and this peak is located near the center of this range, which is beneficial for controlling the full spectrum drift after stabilization; second, the peak has a stable count rate and fixed peak energy, which is beneficial for distinguishing it from other peaks with different count rates; third, the peak has a special shape and fixed resolution, which is beneficial for distinguishing it from other periodic full-energy peaks with Gaussian distributions.

[0022] Determining the time interval requires considering both the accurate identification and judgment of the composite peak count rate and full width at half maximum (FWHM) within that interval. The time interval for adjusting the operating voltage mentioned in steps 4) and 5) is 1 minute, during which the channel drift is less than 2.5 channels, and the composite peak count rate is approximately 3600 / s. This adequately meets the requirements for use of a 3-inch lanthanum bromide detector in seawater.

[0023] In a preferred method for stabilizing the underwater gamma spectrum, the voltage control accuracy of adjusting the working voltage in steps 4) and 5) is 0.1V. By changing the voltage of the last multiplication stage, the voltage changes with the peak drift, thereby altering the gain of the photomultiplier tube and enabling temperature compensation, thus improving its stability.

[0024] The full width at half maximum (FWHM) of the composite peak is an important parameter for determining the composite peak of 1435 keV and 1435+32 keV. This parameter depends on the inherent parameter of detector energy resolution. Different types of detectors have significant differences in their energy resolution. In this invention, the FWHM of the composite peak of 1435 keV and 1435+32 keV is 3.0% to 4.2%.

[0025] Compared with existing technologies, the spectral stabilization method for underwater gamma-ray spectrum measurement of the present invention has the following beneficial effects: the present invention solves the following problems: first, rapid peak finding; second, accurate determination of the net count rate of composite peaks; third, accurate determination of the full width at half maximum (FWHM) of composite peaks; fourth, scientific determination of the voltage adjustment time interval; and fifth, precise voltage adjustment, improving stability. The present invention achieves spectral stabilization for underwater gamma-ray spectrum measurement through composite peaks and a spectral stabilization method, solving the problem that hardware stabilization cannot adapt to ordinary detectors, and can better adapt to the temperature effect caused by underwater gamma-ray spectrum detectors after immersion in water. Attached Figure Description

[0026] Figure 1 The temperature-relative pulse height relationship of an inorganic scintillator for 662 keV gamma rays.

[0027] Figure 2 The full spectrum of the composite peak at 1435 keV and 1435+32 keV is shown.

[0028] Figure 3 This is an enlarged view of the composite peak of 1435 keV and 1435+32 keV of the present invention.

[0029] Figure 4 This is the logical roadmap for the composite peak stable spectrum based on the La-138 background of the present invention.

[0030] Figure 5 This is a screenshot of the count rate of the composite peaks at 1435 keV and 1435+32 keV of the 3-inch lanthanum bromide detector of this invention. Detailed Implementation

[0031] The following is in conjunction with the appendix Figures 2-5 The present invention will be further described below.

[0032] Example 1

[0033] The underwater gamma-ray spectrum stabilization method of the present invention utilizes rapid peak identification, reference channel peak position determination, count rate determination, resolution determination, working voltage adjustment, channel address matching determination, etc., to establish a spectrum stabilization technology based on the La-138 background composite peak.

[0034] See attached document Figure 4 The process involves: setting the channel address for the composite peak's pseudo-stabilized spectrum; establishing a cache in the background after measurement begins; saving the first spectrum at regular intervals (e.g., saving a full spectrum every 1 minute); and quickly identifying all peak positions using a "nuclide identification algorithm." The system checks for peaks within 200 channels to the left and right of the preset composite peak address. If a peak is found, it checks if the preset count rate range is met. If so, it checks if the preset energy resolution range is met. If so, the La-138 composite peak has been found. The composite peak's position channel address is compared with the preset composite peak channel address, the channel difference is calculated, and the operating voltage is adjusted using the channel address-operating voltage relationship function. The spectrum is then recorded for the second minute. This process is repeated until the composite peak's position channel address coincides with the preset composite peak channel address. This completes the automatic spectrum stabilization during startup measurement.

[0035] Lanthanum bromide detectors are excellent gamma-ray detectors. Their crystal density is higher than that of NaI crystals, resulting in higher detection efficiency. Furthermore, NaI exhibits superior peak shift due to temperature influences. Compared to the 7% energy resolution of NaI detectors, lanthanum bromide detectors can achieve an energy resolution of less than 3%. This provides a significant advantage for identifying and quantitatively measuring radionuclides in water using gamma-ray characteristic rays.

[0036] However, compared to NaI detectors, lanthanum bromide detectors exhibit a significantly higher background. This is primarily due to the fact that NaI's background mainly originates from the naturally occurring radioactive isotope K-40 in K, which belongs to the same group as Na. During NaI crystal manufacturing, K impurities are introduced, thus introducing trace amounts of K-40. In contrast, the main elements in lanthanum bromide crystals are Br and La. The naturally occurring radioactive isotope La-138 in La is significantly introduced, resulting in a much higher La-138 content in LaBr3 crystals compared to the K-40 content in NaI crystals. Calculations show that the activity of La-138 in a 3-inch lanthanum bromide crystal is 542.18 Bq. While this issue leads to a higher background for lanthanum bromide detectors, theoretically lowering their detection limit, it provides a stable reference peak position and peak count for gamma-ray spectroscopy measurements, enabling stable spectra based on the La-138 background composite peak.

[0037] The spectral stabilization principle of La-138 based on its own background recombination peaks is similar to that of the reference radiation source, but there are subtle differences. Because La-138 is uniformly distributed in the detector crystal, its radiation interacts with the crystal extensively and directly, especially since almost all low-energy photons are deposited in the detector crystal, resulting in a very significant cascade recombination effect. The most prominent are the recombination peaks at 1435 keV and 1435+32 keV, which, under most measurement conditions, recombine with the K-40@1460 keV peak in the environment, forming a significant peak (see...). Figure 2 Its basic peak shape is shown Figure 3 .

[0038] Using this composite peak for spectrum stabilization has the following advantages: First, the gamma spectrum measurement range is generally 0 keV-3 MeV, and this peak is located near the center of this range, which is beneficial for controlling full-spectrum drift after stabilization; second, the peak has a stable count rate and fixed peak energy, which is beneficial for distinguishing it from other peaks with similar count rates; third, the peak has a unique shape and fixed resolution, which is beneficial for distinguishing it from other periodic full-energy peaks with Gaussian distributions. Further development of corresponding techniques can be carried out to use peak shape functions to distinguish it from other composite peaks.

[0039] During the spectral stabilization process, it is necessary to accurately identify various peaks in the spectrum within the shortest possible time to provide conditions for peak parameter determination. This invention uses the "Nuctoplasmic Recognition Algorithm Based on Sequential Bayesian Method" developed by the Army Chemical Defense Academy and the Ninth Research Institute of the China Academy of Engineering Physics to achieve rapid peak finding.

[0040] The composite peak count rate is an important parameter for determining the composite peaks at 1435 keV and 1435+32 keV. This invention employs two methods.

[0041] (1) Measurement Method: A digital multichannel detector from Beijing Zhongzhi Nuclear Safety and a domestically produced Huakailong 3-inch lanthanum bromide detector were connected to form a gamma-ray spectrometer measurement system. With proper waterproofing, the detector's background was measured in a 30-meter deep freshwater pool within a 500-meter saturated submersible system. The measurement depth was approximately 8 meters to shield against interference from cosmic rays. Freshwater was used to avoid the cumulative counting of the composite peak by the naturally occurring radioactive nuclide K-40@1460keV in seawater. The measurement yielded a count rate of 62.60 s for the 1435keV and 1435+32keV composite peaks. -1 .

[0042] (2) Theoretical calculations: From Figure 5 Spectral analysis of the 1435 keV and 1435+32 keV composite peaks revealed that this composite peak was mainly composed of the 1435 keV full-energy peak and the 1435+32 keV cascade peak of La-138. While emitting 1435 keV γ-rays, La-138 also emitted 31.819 keV and 32.206 keV X-rays. Since the X-rays were emitted from inside the detector, almost all of them were recorded. Due to time resolution issues, it was impossible to distinguish between the simultaneous generation of 1435 keV γ-rays and 31.819 keV and 32.206 keV X-rays, so they were combined and recorded, forming the 1435+32 keV cascade peak. The full-energy peak at 1435 keV and the cascaded peak at 1435+32 keV were calculated separately. Analysis showed that both peaks are based on the recorded 1435 keV. Therefore, the count rate of the composite peak can be derived by calculating the detection efficiency at 1435 keV. Using MCNP modeling and derivation, the detection efficiency of the LaBr3 detector for the 1435 keV emitted by the crystal itself is 0.1777, and the count rate of the composite peak is 62.64 s. -1 .

[0043] In actual measurement applications, errors are introduced due to statistical fluctuations and peak area calculations. Therefore, a judgment range of 3000-4200 min is set for the composite peak count rate. -1 .

[0044] The full width at half maximum (FWHM) of the composite peak is a crucial parameter for identifying the 1435 keV and 1435+32 keV composite peak. This parameter depends on the inherent energy resolution of the detector, and different types of detectors exhibit significant differences in their energy resolution. Table 1 shows the typical energy resolution of a 3-inch lanthanum bromide detector. As shown in Table 1, the energy resolution varies with the gamma-ray energy. To distinguish the 1435 keV and 1435+32 keV composite peak from the surrounding full-energy peaks, this invention focuses on the resolution of the full-energy peaks within a certain energy range around 1467 keV, with a typical energy resolution of approximately 1.9% to 2.1%. Although there are typical recommended values, due to manufacturing processes, even detectors of the same brand and model can have different energy resolutions. This invention utilizes a domestically produced Huakailong 3-inch lanthanum bromide detector from our department for measurement, obtaining a FWHM of 3.61% for the 1435 keV and 1435+32 keV composite peak. This is significantly different from the typical energy resolution of 1.9% to 2.1%, demonstrating high identification and discrimination capabilities.

[0045] Table 1 compares the energy resolution and peak efficiency of Saint-Gobain's recommended LaBr3 for NaI detectors at different energy levels.

[0046]

[0047] Furthermore, since the adjustment time interval is short and the composite peak count rate is not high, the peak shape of the composite peak measured in each time interval may have some differences, resulting in different calculated composite peak half-widths. Therefore, this invention sets the composite peak half-width to be 3.0% to 4.2%.

[0048] During spectral stabilization, to obtain superior full-energy peak resolution, adjustments must be made as soon as possible after peak position drift. Therefore, the adjustment time interval should be set to the minimum reasonable value. Since the initial detector temperature varies significantly with the ambient temperature, the temperature effect is significant during initial measurements, approximately 1.9 channels / min to 2.5 channels / min. Determining the time interval also requires considering the accurate identification and determination of the composite peak count rate and full width at half maximum (FWHM) within that time interval. In this invention, the time interval is set to 1 minute, during which the channel drift is less than 2.5 channels, and the composite peak count rate is approximately 3600 / s, which can well meet the requirements of a 3-inch lanthanum bromide detector used in seawater. Changing the voltage of the final multiplication stage, so that the voltage changes with peak drift, thereby altering the gain of the photomultiplier tube, allows for temperature compensation of the photomultiplier tube, improving its stability. The voltage control accuracy is 0.1V.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A stabilization method for underwater gamma-ray energy spectrum measurement, utilizing a lanthanum bromide detector for gamma-ray detection, characterized in that... The spectral stabilization steps are as follows: 1) Set the channel address of the composite peak pseudo-stabilized spectrum. After the measurement starts, the background establishes a cache and saves a full spectrum as the first spectrum at intervals of 0.5 min to 2 min; and identifies the peak positions of all peaks. 2) Determine if there is a peak within 200 channels to the left and right of the preset composite peak address; if a peak is found, determine if the preset composite peak count rate range is met; if so, determine if the preset energy resolution range is met. The composite peak count rate determination range is 3000 min. -1 ~4200min -1 After the conditions were met, the La-138 composite peak was found. 3) Compare the peak position address of the La-138 composite peak with the preset composite peak address and calculate the address difference; 4) Using the relationship function between channel address and working voltage, adjust the working voltage and start recording the spectrum of the second time period. Then compare the channel address of the composite peak with the preset composite peak channel address and calculate the channel address difference. 5) Repeatedly use the function relating channel address and operating voltage to adjust the operating voltage until the composite peak position channel address coincides with the preset composite peak channel address; this completes the automatic spectrum stabilization of the power-on measurement. The voltage control accuracy for adjusting the working voltage as described in steps 4) and 5) is 0.1V.

2. The stabilization method for underwater gamma-ray spectrum measurement according to claim 1, characterized in that: The lanthanum bromide detector described above has a detection efficiency of 0.1777 for the 1435 keV emitted by the crystal itself, and a composite peak count rate of 62.64 s. -1 .

3. The stabilization method for underwater gamma-ray energy spectrum measurement according to claim 1, characterized in that: The energy resolution of the lanthanum bromide detector is within 3%.

4. The stabilization method for underwater gamma-ray spectrum measurement according to claim 1, characterized in that: The La-138 composite peak is a composite peak of 1435 keV and 1435+32 keV, and the full width at half maximum (FWHM) of the composite peak of 1435 keV and 1435+32 keV is 3.0%~4.2%.

5. The stabilization method for underwater gamma-ray spectrum measurement according to claim 1, characterized in that: The time interval for adjusting the working voltage as described in steps 4) and 5) is 1 minute, during which the channel address drift is less than 2.5 channels and the composite peak count rate is 3600 / s.

Images