Activity measurement method based on adaptive coincidence window and dead time correction

Abstract

The invention discloses an activity measurement method based on an adaptive coincidence window and dead time correction. The method comprises the following steps: (1) arranging three photomultipliers around a sample to be detected, monitoring three paths of signals generated by the three photomultipliers, and when any path of signal has a rising edge, triggering a self-adaptive coincidence window of which the width is determined by a counting rate R; when R < =, = to ensure that a real coincidence event is captured; at gt; rgt; when R is greater than R, decreasing gradually along with the increase of R so as to reduce accidental coincidence counts; when R > =, = to maximize coincidence efficiency; 2) performing coincidence event judgment in the adaptive coincidence window; the width of the dead time window is adaptively adjusted according to the counting rate R, when R is larger than or equal to R, the width of the dead time window is shorter, and when R is smaller than or equal to R, the width of the dead time window is longer, (4) after the dead time window is finished, normal monitoring is recovered, and (5) when measurement is terminated, the activity of the sample to be measured is reversely deduced through coincidence counting.

Description

Technical Field

[0001] This invention belongs to the field of nuclear radiation measurement and signal processing technology, and in particular relates to a triple coincidence counting method for radioactivity measurement. Background Technology

[0002] The triple coincidence ratio (TDCR) method is a standard measurement method for determining the absolute activity of radioactive samples, particularly suitable for liquid scintillation counting of low-energy beta nuclides (such as tritium and carbon-14). Its core lies in using coincidence counting techniques to estimate and correct detection efficiency, thus allowing the calculation of the sample's true decay rate without the need for external standards. Existing technical solutions are as follows.

[0003] 1. Basic Hardware Configuration Three photomultiplier tubes (PMTs) with identical performance parameters, denoted as A, B, and C, are placed symmetrically at 120° angles in the same plane around a transparent test bottle containing a liquid scintillator and the sample to be tested. This geometric arrangement aims to maximize light collection efficiency while ensuring symmetry.

[0004] 2. Signal Processing Flow Signal output: Each PMT converts the detected flash light into a weak current pulse signal.

[0005] Signal conditioning: The three signals are amplified by a preamplifier and usually processed by a discriminator to be converted into digital pulses of logic level so that they can be processed by subsequent conformal logic circuits.

[0006] 3. The core logic of conformal counting (using fixed parameters) This is a hallmark of traditional methods, whose processing patterns are pre-defined and fixed: a. Fixed-width conformal window When a real flicker event occurs, the photons it produces are detected by three PMTs almost simultaneously.

[0007] The coincidence circuit continuously monitors the three pulses. When any one of the signals first reaches the discrimination threshold, it immediately triggers the opening of a coincidence window with a fixed width. The width of this window (e.g., 20-200 nanoseconds) is manually set in advance based on the scintillator's emission decay time and remains constant throughout the measurement process.

[0008] During the window's open period, other PMT signals from the same event are considered to be "coincident" in time.

[0009] b. Counting triple and double coincidences Triple compliance (T or If signals A, B, and C all appear within the same fixed coincidence window, it is determined to be a triple coincidence event, and the corresponding counter is incremented by one.

[0010] Double conformity (D or , , Within the same fixed coincidence window, if only any two signals appear (such as A and B), while the third signal (C) is absent, it is determined to be a double coincidence event, and the corresponding double coincidence counter is incremented by one.

[0011] c. Fixed dead time After each coincidence event (whether double or triple coincidence) occurs, the system enters a preset, fixed dead time (e.g., a few microseconds).

[0012] During the dead time, the system stops processing any new pulses to prevent the tail of a previous event's pulse from being misinterpreted as a new independent event, thus avoiding accumulated errors. The system only resumes monitoring the next event after the dead time has ended.

[0013] 4. Principle of Activity Calculation Calculate the TDCR value: After the measurement, calculate the ratio of the sum of the triple coincidence count rate (T) and the double coincidence count rate (D), i.e.: .

[0014] Efficiency Calculation and Activity Determination: There is a theoretical functional relationship between the TDCR value and the detection efficiency of a single photon (and consequently the detection efficiency of the entire system). The detection efficiency (ε) can be deduced from the measured TDCR value using a theoretical model or an experimentally calibrated quenching correction curve.

[0015] Finally, the absolute activity (A) of the sample is calculated using the following formula: ;in, and These are the detection efficiencies for triple and double coincidence events, respectively.

[0016] The traditional TDCR method uses a fixed-width conformance window and a fixed dead-time processing mode, which has the following inherent defects.

[0017] 1. Limitations of a fixed coincidence window: An excessively wide window introduces too many accidental coincidence counts, leading to a higher background and decreased measurement accuracy; an excessively narrow window will miss true coincidence events, especially at high count rates, where pulse accumulation widens the pulse time distribution, exacerbating the missed count rate. A single set of fixed parameters cannot adapt to a wide range of measurements from low to high activity.

[0018] 2. The dangers of lacking proactive dead-time management: After processing an event, the system experiences a "dead time" during which it cannot respond to new events. Traditional methods either ignore this dead time or use a simple fixed dead-time model for correction. This can lead to severe count loss at high count rates, and the correction is inaccurate, introducing significant errors and affecting the linearity of activity measurements. Summary of the Invention

[0019] To address the problems existing in the prior art, the present invention aims to provide an activity measurement method based on adaptive coincidence window and dead time correction, so as to achieve optimal coincidence capture and count correction at different activity levels, thereby improving the accuracy, linearity and dynamic range of the measurement.

[0020] The core innovation of this invention lies in upgrading the core parameters of the triple-complementary activity measurement system from a "fixed mode" to an "adaptive mode," thereby improving measurement performance across all scenarios through dynamic optimization. The key points are listed below in order of importance: 1. Adaptive window adjustment: The core lies in breaking away from the traditional fixed window mode and introducing a dynamic window mechanism that is linked to the real-time counting rate R. ). Calculated using Formula 1, a piecewise dynamic function is employed to adapt to different count rate scenarios: in: : The width (ns) of the adaptive conformal window, which is the target value adjusted in real time.

[0021] R: Real-time count rate (cpm), the statistical value of nuclide decay counts collected by the system in real time.

[0022] : Wide window baseline value at low count rate (value: 200ns~400ns).

[0023] : Narrow window reference value at high count rate (50-100ns, based on measurement resolution requirements).

[0024] , : These are the set low count rate threshold and high count rate threshold (set to 5000 cpm and 50000 cpm respectively, which can be calibrated experimentally).

[0025] : Mid-count rate interval reference window (value: 150ns).

[0026] : Adjustment coefficient (value: 10-20, to suit different nuclide measurement scenarios).

[0027] By monitoring the count rate in real time through an FPGA or microprocessor and dynamically adjusting the window width according to a preset function, a balance between capturing the real signal and suppressing interference is ensured.

[0028] 2. Adaptive Dead Time Management: Abandoning the traditional fixed dead time design, a dynamic dead time strategy based on count rate correlation is adopted. ). Equation 2 shows that, based on the logarithmic correlation model of count rate, the counting loss at high count rates is optimized as follows: in: Width of the dead time window (unit: μs), the detector recovery time adjusted in real time by the system.

[0029] R: Real-time counting rate (unit: cpm), sharing the same feedback signal with conformal window functions.

[0030] a: Dead time reference coefficient (value: 2.0-2.5 μs), determined through detector calibration experiments.

[0031] b: Count rate correlation coefficient, adapted to the detector response characteristics, ensuring the monotonicity of the function; obtained experimentally under different count rate conditions.

[0032] At high count rates, shorten the dead time to reduce count loss, and at low count rates, extend the dead time to ensure signal processing integrity, thereby optimizing system throughput and measurement linearity.

[0033] 3. Dual-Parameter Collaborative Optimization: Instead of adjusting a single parameter in isolation, the system collaboratively manages two core related parameters: the window size and the dead time. Through the combined functions f(R) and g(R), a linkage logic is constructed between the parameters. The system uses the real-time counting rate R as the core feedback signal, calculating the counting rate R every 1 second and comparing it with a preset threshold. (e.g., 5000 cpm) (e.g., 50,000 cpm) Compare this to the previous data, dividing the work into three intervals: low, medium, and high. Low count rate interval ( Prioritize detection efficiency. Fixed This is a baseline value (e.g., 1.8 μs). Set as To avoid missing nuclide decay events. Medium count rate range ( Balancing efficiency and resolution. Simultaneous calls to two dynamic functions. The narrowing occurs linearly as R increases. The nonlinearity decreases as R increases; the adjustment step size is set to R * 5% / time to avoid sudden parameter changes. High count rate range ( ): Prioritize reducing count loss. First adjust to the minimum value (0.5μs), then... Narrow to This prevents detector saturation and creates a combination scheme with optimal measurement performance.

[0034] The activity measurement method based on adaptive coincidence window and dead time correction of the present invention includes the following steps: monitoring the rising edge of the three photomultiplier tube signals to trigger a coincidence event; dynamically determining the width of the adaptive coincidence window for the current coincidence event based on the real-time measured count rate R. Within the adaptive conformance window, conformance events are determined; after a conformance event ends, a dead-time window with a width of [value missing] is dynamically determined and started based on the real-time counting rate R. .

[0035] The dynamic determination of the window width Functional relationship Satisfy: When (At low count rate threshold) Use a wider default value ;when >R> (At a medium count rate) it conforms to the window width Calculated from Formula 1; when (At high count rates) conforms to the window width Use the narrowest value To maximize compliance efficiency.

[0036] The dynamic determination of the dead time window width Functional relationship Satisfy: When R is high, use a shorter [length / length] For example, if the count per second (cps) is >= 10000, a shorter Tdead value is used, ranging from 400ns to 1us; when R is low, a longer Tdead value is used. The specific details can be derived from Formula 2.

[0037] The real-time counting rate R is selected from one of the following: the single-channel counting rate of any one of the three signals, the sum of the double coincidence counting rates of the three signals, or the triple coincidence counting rate.

[0038] The step of dynamically determining the window width and / or dead-time window width is completed in real time using hardware logic via FPGA (Field Programmable Gate Array).

[0039] The function This is achieved through a lookup table pre-installed in memory, where the current count rate R is queried to obtain the corresponding value. value.

[0040] A radioactivity measurement system for implementing the above method is characterized by comprising three photomultiplier tubes, a signal conditioning circuit, and an acquisition and processing unit containing an FPGA, wherein the FPGA is configured to execute control logic with adaptive coincidence window and adaptive dead time.

[0041] The advantages of this invention are as follows: 1. Dual error suppression significantly improves measurement accuracy. Traditional fixed coincidence windows suffer from the problem of "introducing interference when too wide and missing signals when too narrow." This invention addresses this issue by adaptively matching the coincidence window to the count rate. At low count rates, a wide window captures the full true signal, while at medium to high count rates, the window shrinks with the count rate to suppress accidental coincidence interference. This dynamic adjustment significantly reduces measurement errors and dramatically improves the accuracy of activity measurements. This improvement is primarily due to the precise matching of the window to the count rate and the rapid response capability of the hardware.

[0042] 2. Controlled count loss, improved linearity and dynamic range. Traditional fixed dead time methods are prone to significant count loss at high count rates. This invention addresses this by intelligently linking dead time and count rate. At high count rates, the dead time is shortened to reduce count loss, while at low count rates, the dead time is extended to avoid signal misinterpretation. This design effectively broadens the linear range and dynamic interval of activity measurement, adapting to samples with different activity levels. It solves the problem of repeatedly adjusting parameters required by traditional methods, and its advantages stem from the flexible control of the dead time and the precise control of the hardware.

[0043] 3. Highly adaptable, enabling "intervention-free" operation across all scenarios. Traditional fixed parameters are susceptible to interference from factors such as sample quenching and ambient temperature. This invention indirectly senses these interferences through the count rate and automatically adjusts the parameters, ensuring count stability without manual intervention. The system can adapt to the measurement needs of different quenching degrees and different nuclides, completely solving the limitation of traditional methods where "one parameter only applies to one scenario," and achieving adaptive optimization for all measurement scenarios.

[0044] 4. Low hardware modification cost, feasible upgrade of old systems. This invention is fully compatible with the existing TDCR system hardware architecture, requiring no replacement of core hardware such as the PMT and amplifier circuits. Adaptive logic can be integrated simply through firmware upgrades or software updates. This feature significantly reduces the cost of technology implementation, enabling rapid upgrades for numerous existing systems and substantially enhancing the engineering practicality of the solution. Attached Figure Description

[0045] Figure 1This is a flowchart of the method of the present invention. Detailed Implementation

[0046] 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.

[0047] The method flow of the present invention is as follows: Figure 1 As shown, the following technical solution is adopted: Step S1: Obtain the signals output by the three photomultiplier tubes.

[0048] Step S2: Continuously monitor the three signals. When any signal has a rising edge, trigger an adaptive coincidence window.

[0049] Step S3: Within the adaptive coincidence window, record the coincidence status of the three signals and perform a coincidence event decision; wherein, the width of the adaptive coincidence window... The count rate R is dynamically determined by real-time measurement, and its functional relationship is as follows: And the function satisfies: at low count rates ( )hour, Use a wider default value (400ns) to ensure capture of true coincidence events; at medium to high count rates ( >R> )hour, It decreases as R increases, and can be calculated using Formula 1 to reduce random coincidence counts; at high count rates ( When ), conforms to the window width Use the narrowest value To maximize compliance efficiency.

[0050] Step S4: After the adaptive coincidence window ends, a configurable dead-time window is started; within the dead-time window, the system ignores all newly arriving pulse signals and does not perform any coincidence judgment or counting; the width of the dead-time window... The adaptive adjustment is based on the count rate R, and its functional relationship is as follows: Furthermore, this function satisfies the following condition: at high count rates, a shorter [length / length] is used. To maximize system efficiency; at low count rates, a longer [time period] is used. To ensure that the previous event is fully processed, the specific details can be derived from Formula 2.

[0051] Step S5: After the dead time window ends, the system resumes normal monitoring.

[0052] Step S6: Terminate the measurement according to the predetermined measurement time and obtain the conformity count ( , Using coincidence counting, the activity of the sample to be tested can be deduced.

[0053] The present invention also provides a radioactivity measurement system, comprising three photomultiplier tubes, a signal conditioning circuit, and a data acquisition and processing unit; The photomultiplier tubes are evenly arranged around the sample to be tested, and are used to detect the signal of the sample. The signal conditioning circuit is used to condition the signals output by the three photomultiplier tubes to form three signals, which are then sent to the acquisition and processing unit. The acquisition and processing unit is used to monitor the three signals. When a rising edge appears in any of the signals, an adaptive matching window is triggered; the width of the adaptive matching window... The count rate R is dynamically determined by real-time measurement; where, in hour, To ensure that true matching events are captured; in hour, It decreases as R increases, in order to reduce the number of coincident coincidences; hour, To maximize compliance efficiency; The set wide window baseline value, The set narrow window reference value, To set a low count rate threshold, A high count rate threshold is set; then, within the adaptive coincidence window, the coincidence status of the three signals is recorded, and a coincidence event decision is made; after the adaptive coincidence window ends, a configurable dead time window is started; the width of the dead time window is... Adaptive adjustment based on the current count rate R; where, when At that time, use a shorter To maximize system efficiency; when At that time, a longer duration was used. To ensure that the previous event is fully processed; after the dead time window ends, normal monitoring resumes; when the measurement is terminated, the coincidence count is obtained; and then the activity of the sample to be tested is deduced from the coincidence count.

[0054] 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. An activity measurement method based on adaptive coincidence window and dead time correction, comprising the following steps: 1) Three photomultiplier tubes with identical performance parameters are evenly arranged around the sample to be tested. The three signals generated by the three photomultiplier tubes are monitored. When a rising edge appears in any of the signals, an adaptive coincidence window is triggered; the width of the adaptive coincidence window is... The count rate R is dynamically determined by real-time measurement; where, in hour, To ensure that true matching events are captured; in hour, It decreases as R increases, in order to reduce the number of coincident coincidences; hour, To maximize compliance efficiency; The set wide window baseline value, The set narrow window reference value, To set a low count rate threshold, The set high count rate threshold; 2) Within the adaptive compliance window, record the compliance status of the three signals and make a compliance event decision; 3) After the adaptive conformance window ends, a configurable dead-time window is started; the width of the dead-time window is... Adaptive adjustment is made based on the current count rate R; where, when At that time, use a shorter To maximize system efficiency; when At that time, a longer duration was used. To ensure that the previous event is fully processed; 4) After the dead time window ends, resume normal monitoring; 5) When the measurement is terminated, obtain the coincidence count; then use the coincidence count to deduce the activity of the sample to be tested.

2. The method according to claim 1, characterized in that, The counting rate R is selected from one of the following: the single-channel counting rate of any one of the three signals, the sum of the double coincidence counting rates of the three signals, or the triple coincidence counting rate.

3. The method according to claim 1 or 2, characterized in that, when >R > hour, ; For the set reference window of the medium count rate range, This is the adjustment coefficient.

4. The method according to claim 3, characterized in that, Medium count rate interval benchmark window =150ns; Adjustment coefficient The value ranges from 10 to 20.

5. The method according to claim 1, characterized in that, when hour, A set baseline value is used to maximize system efficiency; when hour, a is the dead time baseline coefficient, and b is the count rate correlation coefficient.

6. The method according to claim 1, characterized in that, when Adjustment The narrowing occurs linearly as R increases, and the adjustment... The nonlinearity decreases as R increases; the adjustment step size is set to R * 5% / time; R ≥ hour, First adjust to the set minimum value, then adjust... Narrow to This is to prevent detector saturation.

7. A system for measuring radioactivity, characterized in that, It includes three photomultiplier tubes, a signal conditioning circuit, and a data acquisition and processing unit; The photomultiplier tubes are evenly arranged around the sample to be tested and are used to detect the signal of the sample. The signal conditioning circuit is used to condition the signals output by the three photomultiplier tubes to form three signals, which are then sent to the acquisition and processing unit. The acquisition and processing unit is used to monitor the three signals. When a rising edge appears in any of the signals, an adaptive matching window is triggered; the width of the adaptive matching window... The count rate R is dynamically determined by real-time measurement; where, in hour, To ensure that true matching events are captured; in hour, It decreases as R increases, in order to reduce the number of coincident coincidences; hour, To maximize compliance efficiency; The set wide window baseline value, The set narrow window reference value, To set a low count rate threshold, A high count rate threshold is set; then, within the adaptive coincidence window, the coincidence status of the three signals is recorded, and a coincidence event decision is made; after the adaptive coincidence window ends, a configurable dead time window is started; the width of the dead time window is... Adaptive adjustment based on the current count rate R; where, when At that time, use a shorter To maximize system efficiency; when R≤ At that time, a longer duration was used. To ensure that the previous event is fully processed; after the dead time window ends, normal monitoring resumes; when the measurement is terminated, the coincidence count is obtained; and then the activity of the sample to be tested is deduced from the coincidence count.

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