Multi-channel coincidence activity accurate measurement method based on self-calibration and energy spectrum analysis

Abstract

The invention discloses a multi-channel coincidence activity accurate measurement method based on self-calibration and energy spectrum analysis, which comprises the following steps: 1) placing a standard radioactive source with known activity in a tube to be tested, and uniformly arranging three photomultipliers with consistent performance parameters around the standard radioactive source; obtaining double coincidence counts in a period of time; 2) calculating the relative deviation of any two counts of the three double-coincidence counts, and if any relative deviation exceeds a preset threshold value, starting balance adjustment; 3) repeating the steps 1)-2) until the relative deviation of any two counts is lower than a preset threshold value; (4) uniformly arranging three photomultipliers with consistent performance parameters around the sample to be measured, and obtaining double coincidence counts within preset measurement time; and 5) calculating a three-pair coincidence ratio TDCR, and reversely deducing the activity of the to-be-detected sample through a preset quenching correction curve according to the TDCR. According to the method, the noise pulse of which the amplitude is not within an expected energy range can be effectively eliminated, and the signal-to-noise ratio is fundamentally improved.

Description

Technical Field

[0001] This invention belongs to the field of nuclear radiation measurement technology, and in particular relates to a coincidence counting method and system for measuring low-level radioactivity. Background Technology

[0002] In radioactivity measurements, especially liquid scintillation measurements of low-energy β or α nuclides, the measurement accuracy of traditional single-probe detectors is easily affected by photomultiplier tube dark noise, ambient background radiation, and sample quenching effects, leading to large fluctuations and poor stability in measurement data. The triple coincidence ratio (TDCR) method is an authoritative standard method for radioactivity measurements, particularly in liquid scintillation counting. Its core lies in using three independent photomultiplier tubes (PMTs) to detect the same scintillation liquid sample vial, and determining the detection efficiency by analyzing the ratio of the triple coincidence count (T) to the double coincidence count (D) among the three signals. This allows for precise correction of the quenching effect, ultimately calculating the absolute activity of the sample. However, the theoretical accuracy of this method is highly dependent on an ideal premise: the responses of the three detection channels must be completely consistent. Any slight mismatch in gain, quantum efficiency, time response, or electronic delay will lead to a decrease in the coincidence count rate and a systematic deviation in the TDCR value, directly introducing significant measurement errors and becoming an inherent bottleneck limiting its ultimate accuracy.

[0003] The existing technology has the following drawbacks.

[0004] 1. The complexity and instability of hardware adjustments: Cumbersome operation: Traditional methods rely on manually adjusting the gain of each channel's preamplifier or using a precise adjustable high-voltage power supply to provide independent bias voltage for each PMT. This process requires repeated measurements, comparisons, and adjustments, and is highly dependent on the operator's experience. The calibration process is cumbersome, time-consuming, and difficult to automate.

[0005] Easy to drift: The adjustment elements based on analog circuits (such as potentiometers) and the high-voltage power supply itself are susceptible to temperature fluctuations and equipment aging, causing the balanced operating point to drift slowly. This makes the long-term stability of the system poor, makes it difficult to maintain the initial optimal calibration state, requires frequent recalibration, and results in high maintenance costs.

[0006] 2. Delays and information loss in software post-processing: Poor real-time performance: Some improvement methods attempt to perform weighted compensation on the count rate or energy spectrum using offline software algorithms after data acquisition. However, this "hindsight" approach cannot be applied to online measurement or control systems with high real-time requirements due to inherent processing delays.

[0007] The use of information is limited: More importantly, whether it is hardware or existing software methods, the core balancing basis is mostly limited to the time conformity information of the signal (i.e. the presence or absence of a pulse), while generally ignoring the rich physical information contained in the pulse waveform itself, especially the pulse amplitude (energy information), which makes it impossible to effectively distinguish noise from real events with different energies. Summary of the Invention

[0008] To address the problems existing in the prior art, the present invention aims to provide a method for accurate measurement of multichannel coincidence activity based on self-calibration and energy dispersive spectroscopy analysis. This invention utilizes a classic three-channel photomultiplier tube (PMT) coincidence counting architecture, and through automatic calibration technology integrated within the FPGA, monitors key parameters such as gain drift and baseline offset of each channel in real time. It automatically triggers calibration algorithms for dynamic compensation and parameter correction, effectively avoiding inconsistencies in channel response caused by environmental temperature changes and device aging. This significantly improves the long-term stability of the system and greatly reduces the impact of noise on the measurement results.

[0009] The technical solution of this invention is as follows: A method for accurate measurement of multichannel coincidence activity based on self-calibration and energy dispersive spectroscopy analysis includes the following steps: 1) Place a standard radioactive source with known activity in the test tube and execute steps S1~S4 to obtain the double coincidence counts C_AB', C_AC', and C_BC' over a period of time; Step S1: Arrange three photomultiplier tubes with identical performance parameters evenly around the tube to be tested, and denoted as photomultiplier tube A, photomultiplier tube B, and photomultiplier tube C; Step S2: Convert the signals output by the three photomultiplier tubes into pulse signals with constant amplitude via a peak hold circuit, and convert the signals output by the photomultiplier tubes into analog signals that conform to the range of the analog-to-digital converter (ADC). Step S3: Input the three pulse signals and three analog signals to the acquisition board; Step S4: The FPGA in the acquisition board performs a triple coincidence algorithm on the three pulse signals. Specifically, it continuously monitors the rising edge of the three pulse signals. When one of the pulse signals has a rising edge, it immediately triggers and opens a coincidence window of a preset width. During the opening of the specified window, perform the following operations: a) Time compliance judgment: Real-time monitoring of whether each pulse signal has a rising edge. For the i-th pulse signal that has a rising edge, the temporary counter corresponding to the i-th pulse signal is set to 1, and the temporary counter corresponding to the pulse signal that has not a rising edge is kept at 0. b) Amplitude compliance judgment: The i-th pulse signal is sampled by the analog-to-digital converter (ADC) and the amplitude value of the i-th pulse signal that has a rising edge within the compliance window is obtained; if the amplitude value of the i-th pulse signal is within the preset energy window range, the amplitude valid flag of the i-th pulse signal is set to 1, otherwise it is set to 0. After the compliance window ends, compliance event judgment and counting are performed: when the temporary counter corresponding to the i-th pulse signal is 1 and the amplitude valid flag is 1, the i-th pulse signal is determined to be valid, and the corresponding photomultiplier tube state is valid; then, counting is performed based on the final valid states of photomultiplier tube A, photomultiplier tube B, and photomultiplier tube C: If the states of photomultiplier tubes A, B, and C are all valid, the triple coincidence counter C_ABC' increments by 1; if the states of photomultiplier tubes A and B are valid, the double coincidence counter C_AB' increments by 1; if the states of photomultiplier tubes A and C are valid, the double coincidence counter C_AC' increments by 1; if the states of photomultiplier tubes B and C are valid, the double coincidence counter C_BC' increments by 1. 2) Calculate the relative deviation between any two counts of C_AB', C_AC', and C_BC'. If any relative deviation exceeds a preset threshold, initiate a balance adjustment. The balance adjustment includes: applying a programmable digital delay to the channel corresponding to each pulse signal within the FPGA to compensate for the time difference generated during the transmission and processing of the corresponding pulse signal; and introducing a correction coefficient greater than 1 for channels with counts below a set threshold L to increase the corresponding count value, and introducing a correction coefficient less than 1 for channels with counts above a set threshold H to decrease the corresponding count value. 3) Repeat steps 1) to 2) until the relative deviation of any two counts among C_AB', C_AC', and C_BC' is lower than the preset threshold; 4) Arrange three photomultiplier tubes with identical performance parameters evenly around the sample to be tested, and obtain the double coincidence counts C_AB, C_AC, and C_BC at the predetermined measurement time or total count according to the method of steps S2 to S4. 5) Calculate the triple coincidence ratio TDCR = (C_AB + C_AC + C_BC) / (2 × C_ABC); based on the triple coincidence ratio TDCR, the activity of the sample to be tested is deduced by using the preset quenching correction curve.

[0010] Preferably, the method for generating the quenching correction curve is as follows: C14 / H3 with known activity is gradually added to a standard scintillation fluid, and the relationship curve between TDCR and activity is measured sequentially to generate the quenching correction curve.

[0011] Preferably, three photomultiplier tubes with identical performance parameters are placed symmetrically at 120° around the sample to be tested in the same plane.

[0012] The key technical point of this invention lies in the deep integration of "real-time amplitude discrimination" and "automatic channel balancing" into a single FPGA-based hardware processing flow, thereby systematically solving the two inherent bottlenecks of traditional TDCR methods, specifically in the following ways: 1. Dual-dimensional compliance judgment mechanism: Within the FPGA, the judgment of each event no longer relies solely on the "presence" or "absence" of the pulse (time compliance), but adds the crucial dimension of "amplitude compliance." This essentially involves real-time, synchronous processing of energy window setting and compliance judgment at the hardware level, effectively eliminating noise pulses whose amplitude is outside the expected energy range, fundamentally improving the signal-to-noise ratio.

[0013] 2. Real-time and integrated processing based on FPGA: All core algorithms (constant window control, time judgment, ADC triggering and amplitude judgment, counter management) are implemented within the FPGA. This results in nanosecond-level response speed and deterministic timing logic, ensuring the accuracy of conformal event processing and avoiding delays in later software processing.

[0014] 3. Intelligent automatic channel balance calibration: Innovation in calibration criteria: The calibration trigger and judgment criteria are the relative consistency of three sets of double coincidence counts (C_AB', C_AC', C_BC'), which directly reflects the matching degree of the three-way detection efficiency, and the goal is very clear.

[0015] Composite correction method: The balance adjustment is not a single method, but combines "digital delay adjustment" (fine-tuning in time to resolve inconsistencies in signal transmission delay) and "weight correction" (software compensation in counting to resolve minor differences in channel gain / efficiency). This is a hybrid strategy of "hardware logic adjustment + software algorithm compensation", which is more flexible and accurate.

[0016] 4. Long-term self-stabilization capability of the system: By introducing an automatic calibration process, the system has the ability to self-diagnose and adjust, which can compensate for performance changes caused by temperature drift and device aging, significantly reducing the reliance on manual intervention and maintenance costs.

[0017] This invention performs time compliance judgment and amplitude compliance judgment for three pulse signals simultaneously within the same compliance window, and only counts valid compliance events for signals that simultaneously meet the time and amplitude compliance conditions.

[0018] The present invention provides a radioactivity measurement system for implementing the above method, comprising three photomultiplier tubes, a signal conditioning circuit, and a data acquisition board. The system is characterized in that the data acquisition board includes an FPGA, which is configured to implement the synchronization time and amplitude conformity judgment logic and execute the automatic channel balance calibration process.

[0019] Conformance window management: The conformance window is triggered to open by the first rising edge of any one of the three signals.

[0020] The specific implementation of amplitude conformity: The amplitude conformity judgment is achieved by comparing the amplitude of the analog signal that has a rising edge within the ADC sampling conformity window with a preset energy window.

[0021] Automatic channel balance calibration process: steps including measuring double coincidence counts using a standard source, calculating relative deviation, and triggering digital delay adjustment and / or weight correction when the deviation exceeds a threshold.

[0022] Digital delay adjustment: This technique involves using programmable digital delay lines within an FPGA to compensate for the delay in the pulse signal channel.

[0023] Application of weight correction: When calculating the TDCR value, a specific algorithm is used to introduce a correction coefficient for unbalanced channels.

[0024] The advantages of this invention are as follows: 1. An automatic channel balancing calibration mechanism has been introduced, which can automatically compensate for channel efficiency differences caused by factors such as component aging and temperature drift, significantly improving the long-term stability of the system and the reliability of data, and reducing maintenance costs and requirements.

[0025] 2. By combining pulse amplitude (energy) information for compliance judgment, it can effectively suppress noise interference from non-target energies (such as dark noise from photomultiplier tubes, environmental gamma rays, etc.) and has the potential to distinguish different energy nuclides in the measured sample, greatly improving the signal-to-noise ratio and application range of the measurement. Attached Figure Description

[0026] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0028] This solution deeply integrates pulse amplitude (energy information) into the real-time judgment logic. While performing coincidence counting, it accurately identifies the energy characteristics of the pulse signal, efficiently eliminating false pulses caused by external electromagnetic interference, dark current, and other irrelevant noise. The method flow of this invention is as follows: Figure 1 As shown, the specific implementation steps are as follows: Step S1: Place three photomultiplier tubes with identical performance parameters, labeled A, B, and C, symmetrically at 120° angles around the scintillation liquid sample bottle containing the sample, in the same plane.

[0029] Step S2: The signals output from the three photomultiplier tubes are converted into pulse signals with constant amplitude via a peak hold circuit and into analog signals that conform to the ADC range via an amplifier circuit.

[0030] Step S3: Input the three pulse signals and three analog signals to the acquisition board.

[0031] Step S4: Within the FPGA, perform a triple-double coincidence algorithm on the three pulse signals, specifically as follows: S41: By default, the window is closed; S42: Continuously monitor the rising edge of the three pulse signals. When any one of the three signals first shows a rising edge, immediately trigger and open a matching window of a preset width. S43: During the opening of the specified window, perform the following operations: a. Timing compliance judgment: Real-time monitoring of whether each signal has a rising edge. For each signal that has a rising edge, the temporary counter of the corresponding signal is set to 1; otherwise, it remains at 0. b. Amplitude compliance judgment: The amplitude value of the pulse signal that appears with a rising edge within the compliance window is sampled and obtained by analog-to-digital converter (ADC); it is determined whether the amplitude value is within the preset energy window range; if the pulse amplitude of a certain signal is within the energy window range, the amplitude validity flag of the signal is set to 1, otherwise it is set to 0; S44: After the compliance window ends, compliance event judgment and counting are performed: a signal is considered valid in this event only if it simultaneously satisfies "time compliance" (temporary counter is 1) and "amplitude compliance" (amplitude valid flag is 1). Counting is performed based on the final valid states of channels A, B, and C. If all three paths A, B, and C are valid, then the triple counter C_ABC increments by 1; If both paths A and B are valid, the dual coincidence counter C_AB increments by 1; If both paths A and C are valid, the dual coincidence counter C_AC increments by 1; If both B and C are valid, the dual coincidence counter C_BC increments by 1.

[0032] Step S5: Perform the automatic channel balancing calibration procedure before performing the activity measurement: S51: Place a standard radioactive source with known activity in the test tube and perform the measurement process of steps S1 to S4 above to obtain the double coincidence counts C_AB', C_AC', and C_BC' within a specific time period; S52: Calculate the relative deviation of the three-way dual coincidence count. If any relative deviation exceeds a preset threshold, initiate a balance adjustment. The balance adjustment includes: - Digital delay adjustment: Inside the FPGA, a programmable digital delay is applied to the pulse signal of the corresponding channel to compensate for the time differences generated during the transmission and processing of each signal; - Weighting correction: Introduce a correction factor greater than 1 for channels with low counts, and a correction factor less than 1 for channels with high counts; S53: Repeat steps S51 and S52 until the three-way double coincidence counts reach a balanced state (i.e., the relative deviations are all below the preset threshold).

[0033] Step S6: Terminate the measurement according to the predetermined measurement time or total count, and calculate the TDCR value, wherein the TDCR value = (C_AB + C_AC + C_BC) / (2 × C_ABC).

[0034] Step S7: Based on the TDCR value, the activity of the sample to be tested is deduced using a preset quenching correction curve or algorithm. Method for generating the quenching correction curve: C14 / H3 with known activity is gradually added to a standard scintillation solution, and the relationship curve between TDCR and activity is measured sequentially to generate the quenching correction curve.

[0035] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A method for accurate measurement of multichannel coincidence activity based on self-calibration and energy dispersive spectroscopy analysis, comprising the following steps: 1) Place a standard radioactive source with known activity in the test tube and execute steps S1~S4 to obtain the double coincidence counts C_AB', C_AC', and C_BC' over a period of time; Step S1: Arrange three photomultiplier tubes with identical performance parameters evenly around the tube to be tested, and denoted as photomultiplier tube A, photomultiplier tube B, and photomultiplier tube C; Step S2: Convert the signals output by the three photomultiplier tubes into pulse signals with constant amplitude via a peak hold circuit, and convert the signals output by the photomultiplier tubes into analog signals that conform to the range of the analog-to-digital converter (ADC). Step S3: Input the three pulse signals and three analog signals to the acquisition board; Step S4: The FPGA in the acquisition board performs a triple coincidence algorithm on the three pulse signals. Specifically, it continuously monitors the rising edge of the three pulse signals. When one of the pulse signals has a rising edge, it immediately triggers and opens a coincidence window of a preset width. During the opening of the specified window, perform the following operations: a) Time compliance judgment: Real-time monitoring of whether each pulse signal has a rising edge. For the i-th pulse signal that has a rising edge, the temporary counter corresponding to the i-th pulse signal is set to 1, and the temporary counter corresponding to the pulse signal that has not a rising edge is kept at 0. b) Amplitude compliance judgment: The i-th pulse signal is sampled by the analog-to-digital converter (ADC) and the amplitude value of the i-th pulse signal that has a rising edge within the compliance window is obtained; If the amplitude value of the i-th pulse signal is within the preset energy window range, then the effective amplitude flag of the i-th pulse signal is set to 1; otherwise, it is set to 0. After the compliance window ends, compliance event judgment and counting are performed: when the temporary counter corresponding to the i-th pulse signal is 1 and the amplitude valid flag is 1, the i-th pulse signal is determined to be valid, and the corresponding photomultiplier tube state is valid; then, counting is performed based on the final valid states of photomultiplier tube A, photomultiplier tube B, and photomultiplier tube C: If the states of photomultiplier tubes A, B, and C are all valid, the triple coincidence counter C_ABC' increments by 1; if the states of photomultiplier tubes A and B are valid, the double coincidence counter C_AB' increments by 1; if the states of photomultiplier tubes A and C are valid, the double coincidence counter C_AC' increments by 1; if the states of photomultiplier tubes B and C are valid, the double coincidence counter C_BC' increments by 1. 2) Calculate the relative deviation between any two counts of C_AB', C_AC', and C_BC'. If any relative deviation exceeds a preset threshold, initiate a balance adjustment. The balance adjustment includes: applying a programmable digital delay to the channel corresponding to each pulse signal within the FPGA to compensate for the time difference generated during the transmission and processing of the corresponding pulse signal; and introducing a correction coefficient greater than 1 for channels with counts below a set threshold L to increase the corresponding count value, and introducing a correction coefficient less than 1 for channels with counts above a set threshold H to decrease the corresponding count value. 3) Repeat steps 1) to 2) until the relative deviation of any two counts among C_AB', C_AC', and C_BC' is lower than the preset threshold; 4) Arrange three photomultiplier tubes with identical performance parameters evenly around the sample to be tested, and obtain the double coincidence counts C_AB, C_AC, and C_BC at the predetermined measurement time or total count according to the method of steps S2 to S4. 5) Calculate the triple coincidence ratio TDCR = (C_AB + C_AC + C_BC) / (2 × C_ABC); based on the triple coincidence ratio TDCR, the activity of the sample to be tested is deduced by using the preset quenching correction curve.

2. The method according to claim 1, characterized in that, The method for generating the quenching correction curve is as follows: C14 / H3 with known activity is gradually added to a standard scintillation fluid, and the relationship curve between TDCR and activity is measured sequentially to generate the quenching correction curve.

3. The method according to claim 1 or 2, characterized in that, Three photomultiplier tubes with identical performance parameters are placed symmetrically at 120° around the sample under test in the same plane.

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