CN110082368B - Positron annihilation life spectrometer based on silicon photomultiplier (SiPM) - Google Patents
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Abstract
The invention discloses a positron annihilation life spectrometer based on a silicon photomultiplier (SiPM), which comprises a SiPM detector module, an analog signal processing module, a timing and discriminating circuit module, a digital acquisition circuit module, a coincidence unit and a low-voltage power supply module. The SiPM detector receives gamma photons and generates scintillation light, and the scintillation light is converted into a photoelectric signal by the SiPM; the photoelectric signal is further amplified, subjected to noise reduction and the like through the analog signal processing module and is processed into a pulse signal; the pulse signals are timed and discriminated by the timing and discrimination circuit module to obtain the position information, the energy information and the time information of gamma photons; the information is collected by a digital collecting circuit module and is screened into coincidence events by a coincidence unit; all the modules are powered by a low-voltage power supply module. This positron annihilation life-span spectrometer can improve measurement accuracy, reduces measuring time, reduce cost by a wide margin, reduces the power supply demand, and the spectrometer structure obtains simplifying.
Description
Technical Field
The invention relates to the technical field of positron annihilation life spectrometers, in particular to a positron annihilation life spectrometer based on a silicon photomultiplier (SiPM).
Background
Positron annihilation spectroscopy is widely used for solid physics and material science research. After entering a material sample, positrons lose energy in various forms, defects such as vacancies, dislocations and the like exist in the sample, the thermalized positrons are likely to stay at the positions to stop diffusion under the action of coulomb force, and finally electrons around the defects are annihilated. The lifetime of a positron may reflect the electron concentration at its annihilation site. Positron annihilation lifetime spectrometers are also one of several methods by which single-atom-scale defects can be detected.
With the development of semiconductor technology, silicon photomultipliers now provide an attractive alternative to PMTs. A silicon photomultiplier (SiPM) is a photosensor device that solves the problem of detecting, timing, and quantifying low optical signals to single photon levels. It combines the low light detection capabilities of a PMT, while providing all the advantages of a solid state sensor, as compared to conventional photomultiplier tubes (PMTs). SiPM features low voltage operation, insensitivity to magnetic field and high response uniformity. It also has the advantages of good energy linear response, fast pulse rise time, good position resolution, etc. SiPM is easy to package and suitable for being made into a multi-channel SiPM array, and in order to better match the light emission spectrum range of the SiPM, LYSO, LSO and LFS-3 scintillation crystals are selected to be coupled with the SiPM, LYSO, LSO and LFS-3 crystals, so that the high light yield and the short time resolution are achieved, and the excellent time performance is provided.
For radioactive elements22As the Na source, when22Positron is emitted when Na undergoes β + decay,22transition of Na to22Ne, and the excitation energy of the excited state is 1.28 MeV. Excited state is deactivatable by emitting 1.28MeV gamma photons22The ground state of Ne. Resolution time at time spectrometer 10-10In the second order, it can be considered that e is emitted during the nuclear transition process+And γ are performed simultaneously. In measuring positron lifetime, a gamma photon of 1.28MeV can serve as the initiation signal for positron generation. The gamma photons emitted by positron annihilation can be used as time signals for positron annihilation. The time difference between these two time signals is measured and the positron lifetime is obtained.
Positron Annihilation Lifetime Spectroscopy (PALS) is one of the most basic applications of Positron Annihilation techniques. The conventional positron life spectrometer consists of a scintillation crystal, a photomultiplier, a high-voltage constant-ratio timing discriminator, a fast coincidence circuit, a delayer, a time-amplitude converter, an NIM case, a computer multichannel analyzer and a computer. As shown in fig. 1, two scintillation detectors, one as a start detector detects 1.28MeV gamma photons and one as a stop detector detects 511keV gamma photons. After the scintillation crystal detects gamma rays, the gamma rays are converted into scintillation light which passes through a photomultiplier to generate an over-current signal, a pulse electrical signal enters a constant ratio timing discriminator, the constant ratio timing discriminator discriminates the amplitude of an input signal on one hand, and the constant ratio timing discriminator discriminates the amplitude of the input signal on the other hand, and outputs a standard signal determined by a constant ratio timing method, wherein the amplitude of the input signal meets upper and lower thresholds. Then, dividing the signals into two paths of standard output signals in a constant ratio timing discriminator: one path of output positive signal is used for fast coincidence to trigger a door opening signal, and the other path of output signal enters a time-amplitude converter after time delay. After the fast coincidence output gate signal is sent to the time-amplitude converter, the time-amplitude converters of the two paths of standard signals convert the time difference into a signal in direct proportion to the amplitude, the amplitude is discriminated by the analog-to-digital converter or the multiple paths, the amplitude signal is converted into a numerical value file to be stored, and finally, a spectrum obtained by accumulating a plurality of events is a positron life spectrum.
A conventional positron life spectrometer usually comprises a scintillation detector consisting of a plastic scintillator or a barium fluoride crystal or a barium bromide crystal coupled with a photomultiplier, and the scintillation detector serves as a starting detector and a stopping detector of the life spectrometer, but the detectors have certain defects. Although the plastic scintillator emits light quickly and has a small attenuation constant, the plastic scintillator has low detection efficiency for gamma photons. The barium fluoride scintillator has short decay time and larger density, but still has luminescent components with long decay time, and is easy to accumulate under the condition of high counting rate; the Brolan crystals are easy to deliquesce, have requirements on the temperature and humidity of the environment, and are not suitable for long-term use.
The high voltage of a photomultiplier tube required by a scintillation detector in a life spectrometer is different from several kilovolts, although the response time is very fast, the single photon detection capability is weak, and the excellent time resolution capability cannot be realized. Second, the external environment also affects the performance of the PMT. The changes of temperature and humidity and the existence of vibration have influence on the operation of the PMT; in addition, the external magnetic field has a large influence on the PMT, and the gain can be greatly reduced by the magnetic field of a few gauss.
Disclosure of Invention
The invention aims to provide a novel positron annihilation life spectrometer, which can improve the performance of the spectrometer, namely time resolution and counting rate, thereby improving the measurement precision and reducing the measurement time; the cost is greatly reduced; the power supply requirement of a power supply is reduced, and the spectrometer structure is simplified.
The technical scheme adopted by the invention is as follows: the positron annihilation life spectrometer comprises a silicon photomultiplier (SiPM) detector module, an analog signal processing module, a timing and discriminating circuit module, a digital acquisition circuit module, a coincidence unit and a low-voltage power supply module.
The SiPM detector modules are made up of one or more silicon photomultipliers (sipms) and one or more scintillation crystals, which receive gamma photons produced by the decay of radioactive sources and positron annihilation and deposit energy to produce scintillation light and convert it to an electrical signal. The number of SiPM detector modules is two or more.
The analog signal processing module: and relevant filtering, shaping, amplifying and signal matching processing are carried out on signals of the detector module according to needs, so that fast reading of output fast signals of the SiPM is realized, and the requirements of high bandwidth and low noise are met.
Timing and screening circuit module: the gamma photon sedimentation detection device comprises a timing module and a discrimination module, wherein the timing module realizes high-precision signal triggering timing, and the triggering timing is used as the moment of gamma photon sedimentation generation. The discrimination module realizes the judgment of the energy of gamma photon deposition, and discriminates the gamma photons of 1.28MeV and 511keV through the judgment of the amplitude or the charge quantity.
The SiPM detector module, the analog signal processing module and the timing and discrimination circuit module are respectively provided with an initial unit and a stopping unit, wherein the initial unit detects 1.28MeV gamma photons and the stopping unit detects 511keV gamma photons.
The digital acquisition circuit module: pulse signals generated by the timing and discrimination signal module are collected, and position information, energy information and time information of photons are analyzed and processed; screening the position, energy and time of the photon by using a coincidence unit to obtain a coincidence event, and sending data to a computer terminal;
a coincidence unit: and providing a time window to complete screening of coincidence events, wherein the coincidence events are events of which the time intervals of successive occurrence are smaller than coincidence distinguishing time, so that the true coincidence efficiency is improved, and the accidental coincidence counting rate is reduced.
A computer terminal: and receiving the data by upper computer software in the terminal, processing the data to obtain a time spectrum, and performing spectrum decomposition by software to obtain the service life of the sample.
The SiPM detector module comprises a silicon photomultiplier and a scintillation crystal, wherein the silicon photomultiplier comprises a silicon photomultiplier (SiPM) or an SiPM array or an APD array; the scintillation crystal comprises LYSO, LSO, LFS-3 and other scintillation crystals, and the scintillation crystal is coupled on a silicon photomultiplier through optical silicone oil to form an SiPM detector module for gamma photon detection.
The analog signal processing module comprises a reading circuit of an analog signal, an amplifying circuit, a filtering circuit, a pole-zero cancellation circuit, a matching circuit and the like, and is used for processing of rapid amplification, forming and the like of a pulse signal.
The timing and discrimination circuit module comprises a leading edge timing circuit or a constant ratio timing circuit and a single-threshold or multi-threshold discrimination circuit, and is used for timing and amplitude (electric charge quantity) discrimination of pulse signals and the like.
The digital acquisition circuit module comprises a programmable logic device (FPGA) and a related peripheral circuit thereof. The method is used for data acquisition of position information, energy information and time information of the whole system.
The computer terminal comprises a computer and software, the software comprises upper computer software and spectrum analysis software for analyzing a time spectrum, the upper computer software is used for parameter control and data storage control of the whole digital acquisition circuit module, and the spectrum analysis software comprises software for spectrum analysis of a life spectrum, including PATFIT or LT 9.0.
Compared with the prior art, the invention has the advantages that:
(1) the service life spectrum is obtained by utilizing a novel SiPM silicon photomultiplier, which has good single photon response capability and time resolution capability reaching below hundred picoseconds level, and matching with a scintillation crystal with high stopping power and fast decay time and performing online processing through a series of electronic systems or offline processing through an oscilloscope. The photomultiplier comprises a silicon photomultiplier (SiPM) or a SiPM array or an APD array, and has the advantages of small volume, high sensitivity, high quantum efficiency, low required bias voltage, easy packaging, suitability for being made into a multichannel array and the like.
(2) Scintillators include crystals of LYSO, LSO, LFS-3, and the like, and arrays thereof. Has the advantages of high density, high stopping power, fast attenuation time, high light yield, etc. The effective atomic number (Z) of the scintillation crystal is high, the photoelectric effect cross section is in direct proportion to the 5 th power of Z, and the Compton scattering cross section is in direct proportion to Z, so that high detection efficiency and low scattering proportion can be achieved; in addition, the scintillation crystal has extremely high luminescence front and high light yield, the luminescence rise time is about 30ps, the light yield is about 30000photons/MeV, and excellent time performance is provided.
(3) The electronic module has a simple structure, adopts a high-performance integrated chip, has high integration level, fully considers signal integrity design, ensures the realization of system indexes, and ensures that the system can update logic on line by virtue of a modularized design concept and the application of a programmable device, thereby being convenient to maintain and upgrade.
Drawings
FIG. 1 is a conventional positron annihilation lifetime spectrometer;
FIG. 2 is a positron annihilation lifetime spectrometer based on SiPM;
fig. 3 is a positron annihilation lifetime spectrometer embodiment based on SiPM implemented in the present invention.
Detailed Description
The invention is further described with reference to the following figures and detailed description.
As shown in fig. 2, the positron annihilation lifetime spectrometer based on the silicon photomultiplier (SiPM) of the present invention includes a SiPM detector module, an analog signal processing module, a timing and discriminating circuit module, a digital acquisition circuit module, a coincidence unit, a low voltage power supply module and a computer terminal.
The SiPM detector modules are made up of one or more silicon photomultipliers (sipms) and one or more scintillation crystals, which receive gamma photons produced by the decay of radioactive sources and positron annihilation and deposit energy to produce scintillation light and convert it to an electrical signal. The number of SiPM detector modules is two or more.
The SiPM detector module has the advantages of small volume, high sensitivity, high quantum efficiency, low required bias voltage, easy packaging, suitability for being made into a multi-channel array and the like.
Scintillation crystals include LYSO, LSO, LFS-3 crystals and arrays thereof. Has the advantages of high density, high stopping power, fast attenuation time, high light yield, etc. The effective atomic number (Z) of the scintillation crystal is high, the photoelectric effect cross section is in direct proportion to the 5 th power of Z, and the Compton scattering cross section is in direct proportion to Z, so that high detection efficiency and low scattering proportion can be achieved; in addition, the scintillation crystal has extremely high luminescence front and high light yield, the luminescence rise time is about 30ps, the light yield is about 30000photons/MeV, and excellent time performance is provided.
The analog signal processing module: and carrying out relevant filtering, forming, amplifying, zero cancellation, signal matching and other processing on the signal of the detector module as required, so as to realize the quick reading of the output quick signal of the SiPM and meet the requirements of high bandwidth and low noise.
Timing and screening circuit module: the gamma photon sedimentation detection device comprises a timing module and a discrimination module, wherein the timing module realizes high-precision signal triggering timing, and the triggering timing is used as the moment of gamma photon sedimentation generation. The discrimination module realizes the judgment of the energy of gamma photon deposition, and discriminates the gamma photons of 1.28MeV and 511keV through the judgment of the amplitude or the charge quantity.
The SiPM detector module, the analog signal processing module and the timing and discrimination circuit module are respectively provided with an initial unit and a stopping unit, wherein the initial unit detects 1.28MeV gamma photons and the stopping unit detects 511keV gamma photons.
The digital acquisition circuit module: and collecting digital pulse signals generated by the timing and discriminating signal module, and analyzing and processing the position information, the energy information and the time information of photons. Screening the position, energy and time of the photon by using a coincidence unit to obtain a coincidence event, and sending data to a computer terminal;
a coincidence unit: and providing a time window to complete screening of coincidence events, wherein the coincidence events are events of which the time intervals of successive occurrence are smaller than coincidence distinguishing time, so that the true coincidence efficiency is improved, and the accidental coincidence counting rate is reduced.
A computer terminal: and receiving the data by upper computer software in the terminal, processing the data to obtain a time spectrum, and performing spectrum decomposition by software to obtain the service life of the sample.
Examples
The positron annihilation lifetime spectrometer of the invention utilizes two pieces of 3 × 3mm2SiPM and two LYSO or two LFS-3 crystals, using an oscilloscope to collect event signals received by a detector module, and processing the signals off line through a computer terminalThe correct lifetime spectrum is obtained, as shown in fig. 3. The service life spectrum is obtained by utilizing a novel SiPM silicon photomultiplier, which has good single photon response capability and time resolution capability reaching below hundred picoseconds level, and matching with a scintillation crystal with high stopping power and fast decay time and performing online processing through a series of electronic systems or offline processing through an oscilloscope.
First, a source of positive electrons (typically22Na) undergoes β + decay to produce a positron which is incident on the sample and annihilates with an electron in the sample to produce a pair of 511keV gamma photons, which then deposit energy in the crystal and produce scintillation light which is received by the coupled sipms to produce a photoelectric signal.
The radioactive source and the sample form a sandwich structure, the radioactive source and the sample are close to the end face of the scintillator of the detector as far as possible, the two detector modules are placed at a right angle, and the SiPM inside the detector modules and the scintillator are in coupling contact through silicon oil.
One detector detects gamma photons of 1.28MeV as a start detector and the other detector detects gamma photons of 511keV as a stop detector, while the signal of the stop detector is delayed by a delay line.
Two paths of signals are input into a high-bandwidth high-sampling-rate digital oscilloscope through a coaxial cable, appropriate oscilloscope parameters are set, a coincidence time window is set by utilizing trigger logic inside the oscilloscope, a time window of 20ns is selected, the trigger logic is that when a channel 1 detects 1.28MeV gamma photons, and when a channel 3 detects 511keV gamma photons within the time less than 20ns, a coincidence event is obtained, and the event is recorded according to a certain data format.
The recorded coincidence events, namely waveform files, are used for timing the leading edge of the signal through fitting or interpolation to determine the time difference, so that the timing precision is improved. The accumulated time difference is formed into a histogram which is the measured life spectrum.
In the embodiment of the invention, a spectrometer design with high time resolution is realized by adopting a novel solid photomultiplier SiPM and adopting LYSO crystals and LFS-3 crystals which have high density, fast decay time and high light yield.
Claims (3)
1. A positron annihilation lifetime spectrometer based on a silicon photomultiplier (SiPM), characterized by: the system comprises an SiPM detector module, an analog signal processing module, a timing and discriminating circuit module, a digital acquisition circuit module, a coincidence unit and a low-voltage power supply module; wherein:
the SiPM detector module is composed of one or more silicon photomultipliers (sipms) and one or more scintillation crystals, receives gamma photons generated by radioactive source decay and positron annihilation, deposits energy to generate scintillation light, and converts the scintillation light into an electrical signal; the number of the SiPM detector modules is two or more;
the analog signal processing module: relevant filtering, forming, amplifying, zero cancellation and signal matching processing are carried out on signals of the detector module according to requirements, fast reading of output fast signals of the SiPM is achieved, and the requirements of high bandwidth and low noise are met;
timing and screening circuit module: the system comprises a timing module and a discrimination module, wherein the timing module realizes high-precision signal triggering timing, and the triggering timing is used as the moment when gamma photon deposition is generated; the discrimination module judges the energy of gamma photon deposition and discriminates the gamma photons of 1.28MeV and 511keV through the judgment of amplitude or electric charge quantity;
the system comprises an SiPM detector module, an analog signal processing module and a timing and discriminating circuit module, wherein the SiPM detector module, the analog signal processing module and the timing and discriminating circuit module are respectively provided with an initial unit and a stopping unit, the initial unit is used for detecting gamma photons of 1.28MeV, and the stopping unit is used for detecting gamma photons of 511 keV;
the digital acquisition circuit module: collecting digital pulse signals generated by a timing and discriminating signal module, and analyzing and processing position information, energy information and time information of photons; screening the position, energy and time of the photon by using a coincidence unit to obtain a coincidence event, and sending data to a computer terminal;
a coincidence unit: providing a time window to complete screening of coincidence events, wherein the coincidence events are events of which the time intervals of successive occurrence are smaller than coincidence resolution time, so that the true coincidence efficiency is improved, and the accidental coincidence counting rate is reduced;
a computer terminal: receiving data by upper computer software in the terminal, processing the data to obtain a time spectrum, and performing spectrum decomposition by software to obtain the service life of the sample;
the SiPM detector module comprises a silicon photomultiplier device and a scintillation crystal, wherein the silicon photomultiplier device comprises a silicon photomultiplier (SiPM) or an SiPM array or an APD array; the scintillation crystal comprises LYSO, LSO, LFS-3 scintillation crystal, the scintillation crystal is coupled on the silicon photomultiplier through optical silicone oil to form SiPM detector module, used for gamma photon detection;
the analog signal processing module comprises a reading circuit of an analog signal, an amplifying circuit, a filter circuit, a zero cancellation circuit and a matching circuit, and is used for quickly amplifying and forming a pulse signal;
the timing and discrimination circuit module comprises a leading edge timing circuit or a constant ratio timing circuit and a single-threshold or multi-threshold discrimination circuit, and is used for discriminating the timing and the amplitude (namely the charge quantity) of a pulse signal.
2. A positron annihilation lifetime spectrometer based on a silicon photomultiplier (SiPM) as claimed in claim 1, characterized in that: the digital acquisition circuit module comprises a programmable logic device (FPGA) and a related peripheral circuit thereof and is used for acquiring the position information, the energy information and the time information of the whole system.
3. A positron annihilation lifetime spectrometer based on a silicon photomultiplier (SiPM) as claimed in claim 1, characterized in that: the computer terminal comprises a computer and software, the software comprises upper computer software and spectrum analysis software for analyzing a time spectrum, the upper computer software is used for parameter control and data storage control of the whole digital acquisition circuit module, and the spectrum analysis software comprises some software for spectrum analysis of a life spectrum, including PATFIT or LT 9.0.
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| CN111007558A (en) * | 2019-11-21 | 2020-04-14 | 中国辐射防护研究院 | SiPM-based small-sized LaBr3Gamma spectrometer |
| CN111024743A (en) * | 2019-12-19 | 2020-04-17 | 南昌大学 | Device and method for waveform sampling of positron life spectrum |
| CN111025412B (en) * | 2019-12-19 | 2021-01-19 | 南昌大学 | Stratum tomography system and method based on gamma rays |
| CN111221027A (en) * | 2020-03-24 | 2020-06-02 | 明峰医疗***股份有限公司 | Circuit and method for TOF-PET leading edge discrimination to reduce time walk |
| CN111487664B (en) * | 2020-05-09 | 2022-04-12 | 中国科学院高能物理研究所 | Positron annihilation angle correlation measurement device and method based on positioning algorithm |
| CN111596335A (en) * | 2020-05-26 | 2020-08-28 | 上海仁机仪器仪表有限公司 | Airborne pod radiation environment monitoring device |
| CN111650490B (en) * | 2020-06-01 | 2022-09-06 | 北京遥测技术研究所 | Accelerated life test system suitable for photomultiplier |
| CN113655513A (en) * | 2021-08-16 | 2021-11-16 | 中国科学技术大学 | Digitized anti-coincidence multi-path interaction-starting positron annihilation life spectrometer |
| CN113721286B (en) * | 2021-08-16 | 2023-07-14 | 中国科学技术大学 | Positron annihilation life spectrometer for pulse discrimination by using support vector machine |
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