Method for forming right triangle of digital nuclear pulse

Abstract

The invention discloses a nuclear pulse right triangle forming method of a digital nuclear spectrum measurement system. Aiming at a digital nuclear energy spectrum measurement system using a scintillator detector, the method does not adopt nuclear pulse amplitude to represent incident particle energy, but solves the incident particle energy according to an energy formula of discrete time signals, and then the energy value is used as right triangle amplitude in a time domain. A right triangle pulse with an adjustable bottom width and an adjustable slope can be formed through simple operation, and original nuclear pulse data are converted into sawtooth waves formed by right triangles. Compared with a classical nuclear pulse forming method, the right triangle forming method does not need complex operation such as a z transformation method, a convolution method or a recursion method, is small in design difficulty, small in calculation amount, low in cost and easy to implement in a digital chip, and can exert the performance of the scintillator detector to the maximum extent.

Description

Technical Field

[0001] The present invention relates to nuclear technology and its application field, and in particular to a key technology of a digital nuclear energy spectrum measurement system: a nuclear pulse shaping method. Background Art

[0002] The digital nuclear energy spectrum measurement system primarily consists of a nuclear signal detector, a preamplifier, a main amplifier, an analog-to-digital converter (ADC), a digital signal processing module, and host computer spectrum acquisition and processing software. The digital signal processing module primarily performs functions such as baseline estimation and subtraction, noise filtering, digital shaping, pile-up rejection, and pulse amplitude analysis for nuclear pulse data.

[0003] Digital shaping of nuclear pulses is a key technology in digital nuclear energy spectrum measurement systems. To improve the system's energy resolution or pulse throughput, the fully digitized nuclear pulse must be properly shaped to maximize the signal-to-noise ratio. This optimizes the pulse shape to accurately measure radiation energy based on signal amplitude, minimize signal accumulation at high count rates, and ensure measurement accuracy and reliability.

[0004] The resulting digitized nuclear pulse waveforms are generally categorized into several digital shaping methods (hereinafter referred to as "classic nuclear pulse shaping methods"): Gaussian shaping, trapezoidal (triangular) shaping, linear shaping, and rectangular shaping. These methods typically employ digital signal processing techniques such as discrete convolution in the time domain, system function methods in the z-transform domain, and time-domain recursion.

[0005] Although classical nuclear pulse shaping methods are an effective way to improve the energy resolution and pulse pass rate of digital nuclear energy spectrum measurement systems, and improving these nuclear pulse shaping methods is a research direction for high-precision, high-performance digital nuclear energy spectrum measurement systems, these classical nuclear pulse shaping methods are difficult to design, require large amounts of calculations, and are expensive, and are generally difficult to apply to commercial nuclear energy spectrum measurement systems.

[0006] Compared to semiconductor detectors, scintillator detectors have lower energy resolution. Semiconductor detectors, such as high-purity germanium detectors, can accurately distinguish between rays of different energies, with an energy resolution of 0.1%-0.2%. However, among scintillator detectors, even the relatively high-performance lanthanum bromide detectors have an energy resolution of only around 3% for gamma rays. The commonly used NaI(Tl) detector has an energy resolution of 5%-10%.

[0007] While these aforementioned classical nuclear pulse shaping methods, characterized by high design complexity, computational complexity, and high cost, can theoretically improve the energy resolution of digital nuclear energy spectrum measurement systems, due to the inherently poor energy resolution of scintillator detectors, even with these classical nuclear pulse shaping methods, digital nuclear energy spectrum measurement systems based on scintillator detectors only achieve limited improvements in system energy resolution and, in turn, reduce the system's real-time response speed. Digital nuclear energy spectrum measurement systems based on nuclear signal detectors with better energy resolution (such as semiconductor detectors) cannot maximize detector performance using the right-angled triangle shaping method of the present invention. For digital nuclear energy spectrum measurement systems using scintillator detectors, the right-angled triangle shaping method provided by the present invention offers minimal design complexity, minimal computational complexity, and low cost. It is easily implemented in digital chips such as FPGAs and can maximize the performance of scintillator detectors. All raw nuclear pulse data is ultimately shaped into a sawtooth wave composed of right-angled triangles, which is intuitive and convenient, and facilitates subsequent energy spectrum analysis and processing by the system. Summary of the Invention

[0008] From the circuit principle of a typical scintillator detector + RC feedback type charge sensitive preamplifier, the nuclear pulse amplitude output by the circuit does reflect the energy of the incident particle. However, according to the energy formula of discrete time signal , is the starting time of the original nuclear pulse, is the termination time of the original nuclear pulse, By using the value of each sampling point of the original nuclear pulse, the energy of the incident particle can be more accurately determined, and then shaped into a simple right-angled triangle pulse in the time domain. This shaping method is computationally simple and does not require transformation to the z-domain, complex convolution operations, or recursive solutions. Determining the energy of the incident particle in this way can reduce the impact of high-frequency noise and more accurately represent the energy of the incident particle than the nuclear pulse amplitude. However, if classical nuclear pulse shaping methods do not use the nuclear pulse amplitude to represent the energy of the incident particle, but instead calculate the energy of the incident particle based on the energy formula of the discrete-time signal, the design of classical nuclear pulse shaping methods based on convolution, recursion, or z-transformation will be more complex, computationally intensive, and more difficult to implement.

[0009] The technical solution of the present invention shapes each original nuclear pulse into a right triangle pulse, which is different from the trapezoidal forming method that shapes each original nuclear pulse into an isosceles trapezoid or an isosceles triangle. The right triangle forming method is not as complicated as the trapezoidal forming method.

[0010] The digital nuclear energy spectrum measurement system using a scintillator detector is compatible with the right-angle triangle forming method of the present invention.

[0011] Regardless of whether MATLAB simulation or Verilog HDL hardware description language programming is used, the following steps can be followed to implement the right triangle forming method of the present invention.

[0012] Step 1: All the original nuclear pulse data are processed by digital signal processing methods (such as sliding average filtering, Savitzky-Golay smoothing filtering, etc.) After performing noise reduction, baseline estimation and subtraction, the right triangle forming method of the present invention can be implemented.

[0013] The second step is to set the nuclear pulse starting threshold, Value_threshold_start, based on the typical nuclear pulse output by the scintillator detector and the RC feedback charge-sensitive preamplifier. If the nuclear signal sample value at a certain moment exceeds this threshold, a nuclear pulse begins. Because the rising edge of the nuclear pulse is relatively steep, it is better to set the threshold slightly higher at the start time.

[0014] The third step is to set the nuclear pulse termination threshold, Value_threshold_end, based on the typical nuclear pulse output from the scintillator detector and the RC feedback charge-sensitive preamplifier. If the nuclear signal sample value at a certain moment falls below this threshold, the nuclear pulse is terminated. Because the falling edge of the nuclear pulse is relatively slow, a slightly lower threshold at the termination time is preferred.

[0015] Step 4: Traverse all the original nuclear pulse data , add two arrays to record the information of each nuclear pulse: still use array Record the value of each sampling point of the nuclear signal and increase the array Record the start time of each nuclear pulse and increase the array The termination moment of each nuclear pulse was recorded. and are of equal length, and their length is the number of nuclear pulses. The end time minus the The starting time, get The width of a nuclear pulse.

[0016] Step 5: Based on the typical nuclear pulse output from the scintillator detector and the RC feedback charge-sensitive preamplifier, set the maximum pulse width threshold (Width_max_threshold) and the typical pulse width (Width_typical). The maximum pulse width threshold is greater than the typical pulse width. Pulses exceeding the maximum pulse width threshold, which are excessively wide, are typically tail pileup pulses. If the pulse width exceeds the maximum pulse width threshold but is less than twice the typical pulse width, the number of pileup pulses is considered to be two. If the pulse width exceeds twice the typical pulse width, the number of pileup pulses is considered to be three (there may actually be more than three pileup pulses, but since accurate determination is difficult, only three is considered). All pileup pulses are eliminated, and the number of pileup pulses is accumulated in the variable elimination_counter.

[0017] The sixth step is to use a loop statement to calculate the energy of each nuclear pulse according to the energy formula of the discrete time signal and store it in the array Based on the typical nuclear pulse output from a scintillator detector and a RC-feedback charge-sensitive preamplifier, the maximum energy threshold (energy_max_threshold) and typical energy value (energy_typical) are set. Pulses exceeding the maximum energy threshold are generally considered peak pileup pulses. If the pulse energy exceeds the maximum energy threshold and is less than twice the typical energy value, the number of pileup pulses is considered to be two. If the pulse energy exceeds twice the typical energy value, the number of pileup pulses is considered to be three (the actual number of pileup pulses may be more than three, but accurate determination is difficult, so only three is considered). All pileup pulses are eliminated, and the number of pileup pulses is accumulated in the variable elimination_counter.

[0018] The fifth and sixth steps above also eliminate those pulses that are both peak pile-up and tail pile-up.

[0019] In MATLAB, for the pile-up pulses to be eliminated, it is only necessary to change the corresponding and The elements are assigned to the empty matrix [], but not to 0. According to the statistical characteristics of the nuclear pulse signal, as long as the measurement time is long enough and the number of detected nuclear pulses is large enough, these eliminated accumulation pulses will not affect the final shape of the nuclear energy spectrum. The number of eliminated accumulation pulses is accumulated in the variable elimination_counter. If the nuclear energy spectrum measurement system only performs qualitative analysis, then the number of eliminated accumulation pulses is not needed. If the nuclear energy spectrum measurement system needs to perform quantitative analysis, after calculating the energy of all nuclear pulses, the energy array of the nuclear pulses can be used. Each element value is multiplied by this scaling factor . Proportional coefficient =(array length of the array + the number of accumulated pulses (elimination_counter) ÷ (array length).

[0020] Step 7: Based on the energy array of the nuclear pulse , start time array Shaped sawtooth wave.

[0021] In order to save storage space, in-situ calculation is adopted here. First, all the original nuclear pulse data Set each element of to zero, and then store the formed sawtooth wave data into the array middle.

[0022] Let the base width of the right triangle be (suggestion It is one third of the typical value of pulse width Width_typical, and preferably not more than two thirds. If it is too small, it will be difficult to observe. Too large will cause accumulation). The subroutine flow for shaping a nuclear pulse into a right triangle is as follows: The slope of a right triangle is: ; The time coordinate of the first discrete point of the right triangle is: ; Give the right triangle the following in chronological order: Assign discrete points: ; ;……; .

[0023] The width of the base of a right triangle Under fixed conditions, the amplitude of the right triangle represents the energy of the nuclear pulse. , the slope of the right triangle is also adjusted.

[0024] According to the energy array of nuclear pulse With the formed sawtooth wave, we can do nuclear energy spectrum analysis and draw the corresponding energy spectrum diagram. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 , basic composition of the digital nuclear energy spectrum measurement system of embodiment 1 of the present invention.

[0026] Figure 2 , Schematic diagram of the pulse waveform before and after forming of Example 1 of the present invention.

[0027] Figure 3, schematic diagram of setting thresholds for the start and end times of a nuclear pulse according to embodiment 1 of the present invention; Figure 3 Chinese: 1. Starting point coordinates (32, 1.0837), 2. End point coordinates (120, 0.2007).

[0028] Figure 4 , schematic diagram of pile-up pulses according to embodiment 1 of the present invention; Figure 4 Middle: 3. Typical pulse, 4. Tail pile-up pulse, 5. Peak pile-up pulse, 6. Pulse with both peak pile-up and tail pile-up. DETAILED DESCRIPTION

[0029] In order to better understand the present invention, the present invention is further described below in conjunction with Example 1 and the accompanying drawings. However, Example 1 is only used to explain the present invention and does not constitute a limitation on the scope of protection of the present invention.

[0030] Embodiment 1:

[0031] The digital nuclear energy spectrum measurement system is mainly composed of nuclear signal detector (scintillator detector), preamplifier, main amplifier, analog-to-digital converter ADC, digital signal processing module and host computer energy spectrum acquisition and processing software. The basic components are as follows: Figure 1 The digital signal processing module primarily performs functions such as baseline estimation and subtraction, noise filtering, digital shaping, pile-up rejection, and pulse amplitude analysis for nuclear pulse data. Digital shaping of nuclear pulses is one of the key technologies in digital nuclear energy spectrum measurement systems.

[0032] For the digital nuclear energy spectrum measurement system using scintillator detectors, the present invention adopts such a forming method: instead of characterizing the energy of the incident particle by the nuclear pulse amplitude, the energy formula of the discrete time signal is used. The energy of the incident particle is obtained, and then this energy value is used as the amplitude of the right triangle to form a right triangle pulse with adjustable base width and slope. In this way, all the original nuclear pulse data It is formed into a sawtooth wave composed of right triangles, such as Figure 2 .

[0033] Based on the circuit principle of a typical scintillator detector combined with a charge-sensitive preamplifier with RC feedback, the nuclear pulse amplitude output by this circuit does reflect the energy of the incident particle. However, the energy formula for discrete-time signals can be used to more accurately determine the energy of the incident particle. This pulse can then be shaped into a simple right-angled triangle pulse in the time domain. This shaping method is computationally simple and does not require the z-transform, complex convolution operations, or recursive solutions. Classical nuclear pulse shaping methods (Gaussian, trapezoidal, linear, and rectangular) do not use nuclear pulse amplitude to represent the energy of the incident particle, but instead use the energy formula for discrete-time signals to determine the energy of the incident particle. This makes the design of these classical nuclear pulse shaping methods based on convolution, recursion, or the z-transform more complex, computationally intensive, and difficult to implement.

[0034] The digital nuclear energy spectrum measurement system using a scintillator detector is compatible with the right-angle triangle forming method of the present invention.

[0035] Regardless of whether MATLAB simulation or Verilog HDL hardware description language programming is used, the following steps can be followed to implement the right triangle forming method of the present invention.

[0036] The first step is to reduce the noise of all the original nuclear pulse data through digital signal processing methods (such as sliding average filtering, Savitzky-Golay smoothing filtering, etc.), and then perform baseline estimation and subtraction, and then the right triangle forming method of the present invention can be implemented.

[0037] The second step is to set the nuclear pulse starting threshold Value_threshold_start according to the typical nuclear pulse output by the scintillator detector + RC feedback charge sensitive preamplifier. If the sampling value of the nuclear signal at a certain moment exceeds the threshold, a nuclear pulse starts. Figure 3 From the marker 1 "starting point coordinates (32, 1.0837)", we can see that because the rising edge of the nuclear pulse is relatively steep, it is better to set the threshold at the starting moment slightly larger. Figure 3 The example in the example can set the starting threshold at For the convenience and intuitiveness of the description of the present invention, Figure 3 The starting point coordinates are for reference only.

[0038] The third step is to set the nuclear pulse termination threshold Value_threshold_end according to the typical nuclear pulse output by the scintillator detector + RC feedback charge sensitive preamplifier. If the sampling value of the nuclear signal at a certain moment is lower than the threshold, the nuclear pulse is terminated. Figure 3Mark 2 "End point coordinates (120, 0.2007)" shows that because the falling edge of the nuclear pulse is relatively slow, it is better to have a slightly smaller threshold value at the end time. Figure 3 The example in

[15] sets the termination threshold at For the convenience and intuitiveness of the description of the present invention, Figure 3 The ending point coordinates are for reference only.

[0039] Step 4: Traverse all the original nuclear pulse data , add two arrays to record the information of each nuclear pulse: still use array Record the value of each sampling point of the nuclear signal and increase the array The group records the starting time of each nuclear pulse and increases the array The termination moment of each nuclear pulse was recorded. and are of equal length, and their length is the number of nuclear pulses. The end time minus the The starting time, get The width of a nuclear pulse.

[0040] Step 5: Set the maximum pulse width threshold (Width_max_threshold) and the typical pulse width (Width_typical) based on the typical nuclear pulse output from the scintillator detector and the RC feedback charge-sensitive preamplifier. The maximum pulse width threshold is greater than the typical pulse width. Pulses exceeding the maximum pulse width threshold are typically overly wide pulses with tail pile-up. Figure 4 Mark 3 is "typical pulse", Figure 4 Mark 4 is a "tail pileup pulse." If the pulse width exceeds the maximum width threshold but is less than twice the typical pulse width, the number of pileup pulses is considered two. If the pulse width exceeds twice the typical pulse width, the number of pileup pulses is considered three. (The actual number of pileup pulses may be more than three, but accurate determination is difficult, so only three is considered.) All pileup pulses are eliminated, and the number of pileup pulses is accumulated in the variable elimination_counter.

[0041] The sixth step is to use a loop statement to calculate the energy of each nuclear pulse according to the energy formula of the discrete time signal and store it in the array Based on the typical nuclear pulse output from a scintillator detector and a RC feedback charge-sensitive preamplifier, set the maximum energy threshold energy_max_threshold and the typical energy value energy_typical. Pulses exceeding the maximum energy threshold are usually peak pileup pulses. Figure 4Mark 5 is a "peak pileup pulse." If the pulse energy exceeds the maximum energy threshold and is less than twice the typical energy value, the number of pileup pulses is considered to be two. If the pulse energy exceeds twice the typical energy value, the number of pileup pulses is considered to be three. (The actual number of pileup pulses may be more than three, but since accurate determination is difficult, only three is considered.) All pileup pulses are eliminated, and the number of pileup pulses is accumulated in the variable elimination_counter.

[0042] The fifth and sixth steps above also eliminate those pulses that are both peak pile-up and tail pile-up. Figure 4 The mark 6 is "a pulse with both peak accumulation and tail accumulation".

[0043] In MATLAB, for the pile-up pulses to be eliminated, it is only necessary to change the corresponding and The elements are assigned to the empty matrix [], but not to 0. According to the statistical characteristics of the nuclear pulse signal, as long as the measurement time is long enough and the number of detected nuclear pulses is large enough, these eliminated accumulation pulses will not affect the final shape of the nuclear energy spectrum. The number of eliminated accumulation pulses is accumulated in the variable elimination_counter. If the nuclear energy spectrum measurement system only performs qualitative analysis, then the number of eliminated accumulation pulses is not needed. If the nuclear energy spectrum measurement system needs to perform quantitative analysis, after calculating the energy of all nuclear pulses, the energy array of the nuclear pulses can be used. Each element value of is multiplied by the scaling factor . Proportional coefficient =(array length of the array + the number of accumulated pulses (elimination_counter) ÷ (array length).

[0044] Step 7: Based on the energy array of the nuclear pulse , start time array Shaped sawtooth wave.

[0045] In order to save storage space, in-situ calculation is adopted here. First, all the original nuclear pulse data Set each element of to zero, and then store the formed sawtooth wave data into the array middle.

[0046] Let the base width of the right triangle be (suggestion It is one third of the typical value of pulse width Width_typical, and preferably not more than two thirds. If it is too small, it will be difficult to observe. Too large will cause accumulation). The subroutine flow for shaping a nuclear pulse into a right triangle is as follows: The slope of a right triangle is: ; The time coordinate of the first discrete point of the right triangle is: ; Give the right triangle the following in chronological order: Assign discrete points: ; ;……; .

[0047] The width of the base of a right triangle Under fixed conditions, the amplitude of the right triangle represents the energy of the nuclear pulse. , the slope of the right triangle is also adjusted.

[0048] According to the energy array of nuclear pulse With the formed sawtooth wave, we can do nuclear energy spectrum analysis and draw the corresponding energy spectrum diagram.

Claims

1. A right triangle shaping method for digital nuclear pulses, the principle of which is: in the nuclear pulse shaping process of a digital nuclear energy spectrum measurement system using a scintillator detector, the energy of the incident particle is not represented by the nuclear pulse amplitude, but by the energy formula of the discrete time signal. ∑ n = n s n e x n 2 Find the energy of the incident particle, is the starting time of the original nuclear pulse, is the termination time of the original nuclear pulse, is the value of each sampling point of the original nuclear pulse, and then in the time domain, this energy value is used as the amplitude of the right triangle to form a right triangle pulse with adjustable base width and adjustable slope. In this way, all the original nuclear pulse data are formed into a sawtooth wave composed of right triangles.

2. The right-angled triangle forming method of a digital nuclear pulse according to claim 1, characterized in that: This method is limited to digital nuclear energy spectrum measurement systems using scintillator detectors.

3. The right-angled triangle forming method of a digital nuclear pulse according to claim 1, characterized in that: Before forming, the energy of the incident particle is not represented by the nuclear pulse amplitude, but by the energy formula of the discrete time signal ∑ n = n s n e x n 2 Find the energy of the incident particle.

4. The right-angled triangle forming method of a digital nuclear pulse according to claim 1, characterized in that: The forming process does not use convolution method, recursion method or z-transform method, but takes the energy value of the incident particle as the amplitude of the right triangle in the time domain to form a right triangle pulse with adjustable base width and adjustable slope.

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