CN101013095A - Positron annihilation lifetime spectrometer - Google Patents

Abstract

A spectrometer for positron annihilation lifetime spectrum measurement is characterized in that the positron annihilation lifetime spectrometer adopts a detector using lanthanum chloride crystal as a scintillator. Lanthanum chloride crystal not only has fast time response, but also has much superior energy resolution than BaF ₂ crystal. Therefore, the energy window selection circuit can be used to limit the signals received by the start track detector and stop track detector to a very narrow energy range, which can effectively exclude gamma radiation from other sources (sample material, environment, etc.) The impact on the measurement process greatly reduces the background of the lifetime spectrum, so that the detector of the spectrometer has better energy resolution while maintaining a fast time response, so that the positron annihilation lifetime can be performed under strong γ-ray interference Spectrum measurement, to achieve the purpose of extracting effective information of positron annihilation lifetime spectrum.

Description

A Positron Annihilation Lifetime Spectrometer

technical field

The present invention relates to a positron annihilation lifetime spectrometer (Positron Annihilation Lifetime Spectrometer), in particular to a positron annihilation lifetime spectrometer which adopts a novel lanthanum chloride crystal as a scintillator and can resist strong gamma radiation interference.

Background technique

After years of rapid development, positron annihilation technology has become a mature technology widely used in the fields of solid state physics, metal physics and material science. The positron annihilation technique has many advantages. It is a non-destructive research method in itself and does not require specially prepared samples. And because the information obtained from the experiment is carried by the annihilation gamma radiation with strong penetrating ability, it is very easy to be used for on-site and in-situ research.

The time elapsed from when a positron is injected into a material to when it annihilates is generally called the positron lifetime. Dislocations, vacancies, and cavities generated by solid defects such as deformation and radiation damage can all lead to changes in local electron density, thereby changing the positron annihilation lifetime τ. Therefore, the change of material properties can be studied through the change of τ, which is one of the main applications of positron annihilation technology. The core of the positron annihilation lifetime spectroscopy measurement device is a fast coincidence time measurement system, which is mainly divided into three parts: a scintillator photomultiplier tube detector, used to detect gamma photons; a fast coincidence time spectrometer, used to measure two events The time difference; the microcomputer multi-channel pulse amplitude analyzer is used to display and record the time spectrum (reference: He Yuanjin, Ma Xingkun editor-in-chief "Modern Physics Experiment" (Tsinghua University Press, 2003), experiment 13 Positron annihilation lifetime spectrum measurement, pp171 -184).

In the traditional positron annihilation lifetime spectrometer, the start track and stop track detectors for measuring the time signal are generally composed of a fast scintillator coupled with a fast photomultiplier tube. This fast scintillator is generally a plastic scintillator or BaF ₂ (barium fluoride) crystal. In order to limit the start-track detector and the stop-track detector to receive a start signal and a stop signal of an annihilation event respectively, it is generally necessary to use a coincidence circuit to select the amplitude of the signal (ie, the energy of the gamma radiation). For the instrument using ²² Na as the positron source, the energy range of the start track signal should be centered on 1.28MeV, and the energy range of the stop track signal should be centered on 0.511MeV. Choosing an appropriate energy window can reduce the occasional coincidence caused by scattering and radioactive background, so that the measured positron annihilation lifetime spectrum can more truly reflect the physical state of the sample.

That is to say, in the positron annihilation lifetime spectrometer, we not only require the detector to give a time signal, but also require it to give an energy signal. Although the resolution of the energy signal given by the fast scintillator is not high, it can generally meet the above requirements for signal energy selection.

But when the sample measured by positron lifetime spectrometry is itself gamma radioactive, the situation is completely different. If the radioactivity of the sample itself is quite strong, and its energy is quite close to 0.511MeV or 1.28MeV, the traditional BaF ₂ crystal detector cannot distinguish the γ-activity of the sample itself from the γ-activity of the positron source, resulting in extremely high cost. In severe cases, the positron annihilation lifetime spectrum will be distorted and the instrument cannot work normally.

In addition, when there is a strong background gamma radiation around the working environment of the positron annihilation lifetime spectrometer, for example, when the radiation effect of a certain sample is measured in situ, a similar situation as above may also occur.

The above problems boil down to one point, that is, a spectrometer capable of performing positron annihilation lifetime spectrum measurement under strong γ-ray interference is required. The point is that the detector of the spectrometer should have better energy resolution while maintaining a fast time response.

Taking BaF ₂ scintillator detector, the most commonly used detector in positron annihilation lifetime spectrometer, as an example, the energy resolution of BaF ₂ detector for 0.511MeV gamma rays is about 14%, that is, about 72keV. If the radioactive sample itself If the γ-ray source with energy between 475KeV and 547KeV is included, the signal will be seriously mixed into the signal stop channel, causing the lifetime spectrometer to fail to work normally.

Contents of the invention

Aiming at the deficiencies and defects of the prior art, the object of the present invention is to provide a positron annihilation lifetime spectrometer, which enables the detector of the spectrometer to have better energy resolution while maintaining fast time response, and can be used in strong Positron annihilation lifetime spectroscopy measurements under γ-ray interference.

Technical scheme of the present invention is as follows:

A positron annihilation lifetime spectrometer, including two detectors of a start track and a stop track, a fast coincidence time spectrometer and a microcomputer multi-channel pulse amplitude analyzer, each detector is coupled by a scintillator and a photomultiplier tube, The fast coincidence time spectrometer contains a fast coincidence circuit, a constant-ratio timing discriminator with an energy window of the start track, a constant-ratio timing discriminator and a time-amplitude converter with an energy window of the stop track, and is characterized in that: The scintillator described above is a lanthanum chloride scintillator.

The technical feature of the present invention is that: the start track energy window is an energy window with an energy range of 1.2MeV-1.30MeV; the stop track energy window is an energy window with an energy range of 0.49MeV-0.53MeV.

Compared with the prior art, the present invention has the following advantages and outstanding effects. The positron annihilation lifetime spectrometer using the lanthanum chloride crystal as the scintillator detector not only has a fast time response, but also has a much superior energy resolution than the BaF ₂ crystal. Therefore, the energy window selection circuit can be used to limit the signals received by the start track detector and stop track detector to a very narrow energy range, which can effectively exclude gamma radiation from other sources (sample material, environment, etc.) The impact on the measurement process greatly reduces the background of the lifetime spectrum, so that the detector of the spectrometer has better energy resolution while maintaining a fast time response, so that the positron annihilation lifetime can be performed under strong γ-ray interference Spectrum measurement, to achieve the purpose of extracting effective information of positron annihilation lifetime spectrum.

Description of drawings

Fig. 1 is a schematic diagram of the overall structure of the positron annihilation lifetime spectrometer provided by the present invention.

Fig. 2 is a schematic structural diagram of a detector using lanthanum chloride crystal as a scintillator.

Figure 3 shows the full energy spectrum of the initial channel and the energy spectrum after setting the energy window.

Figure 4 shows the full energy spectrum of the stop track and the energy spectrum after setting the energy window. Among them, curve A is the full energy spectrum, and curve B is the energy spectrum after setting the energy window.

Detailed ways

Fig. 1 is a schematic diagram of the overall structure of the positron annihilation lifetime spectrometer provided by the present invention. The spectrometer includes two detectors of start track and stop track, fast coincidence time spectrometer and multi-channel pulse amplitude analyzer of microcomputer. Each detector is coupled by lanthanum chloride scintillator 1 and photomultiplier tube 2. The power supplies the high voltage required for the photomultiplier tube to work. The fast coincidence time spectrometer comprises a fast coincidence circuit, a constant-ratio timing discriminator with an energy window of a start track, a constant-ratio timing discriminator with an energy window of a stop track, and a time-amplitude converter. Lanthanum chloride crystal is a new type of scintillation crystal discovered in the world around 2000. It not only has a fast time response, but also has relatively high energy resolution. According to international reports, Φ25×25mm lanthanum chloride crystal is sensitive to 662keV The energy resolution is 4.2% (references: Marcin Balcerzyk, Marek Moszyn'ski, Maciej KapustaComparison of LaCl ₃ : Ce and NaI(Tl) scintillators in g-ray spectrometry Nuclear Instruments and Methods in Physics Research A 537(2005) 50- 56). Therefore, this crystal can be completely used to construct the above-mentioned positron annihilation lifetime spectrometer that can work under the interference of strong gamma radiation. The principle block diagram of this positron annihilation lifetime spectrometer is shown in Fig. 1 . A detector composed of two lanthanum chloride crystals coupled with a fast photomultiplier tube is used. This is the main difference between the spectrometer and the traditional positron annihilation lifetime spectrometer; in addition, there is a big difference in the selection of the energy window from the traditional lifetime spectrometer. The energy window of the spectrometer is set very narrow. The energy window of the start track is set to 1.2MeV-1.30MeV; the energy window of the stop track is set to 0.49MeV-0.53MeV, which ensures that the system can discriminate gamma rays with energies other than 1.28MeV and 0.511MeV, thus achieving a good Anti-interference effect of strong gamma background radiation.

When the positron annihilation lifetime spectrometer is in the lifetime measurement working mode, the anode signal of the photomultiplier tube of the initial detector passes through the constant-ratio timing discriminator with an energy window and is delayed and sent to the time-amplitude converter as the initial signal. The anode signal of the photomultiplier tube of the stop track detector is sent to the time-amplitude converter as a stop signal after being delayed by a constant-ratio timing discriminator with an energy window, and the time-amplitude converter converts the time difference between the start signal and the stop signal The pulse signal converted into a certain amplitude is sent to the microcomputer multi-channel analyzer for amplitude analysis through the signal multiplexer, and the obtained time spectrum is the positron annihilation lifetime spectrum. When the positron annihilation lifetime spectrometer is in the energy window adjustment mode, the dynode signal of the photomultiplier tube is amplified by the amplifier, and then sent to the input terminal of the microcomputer multi-channel analyzer through the signal multiplexer, and the initial channel constant ratio timing discriminator The energy signal output by the constant-ratio timing discriminator and the stop channel is sent to the gate control terminal of the microcomputer multi-channel analyzer as a gate control signal, so that the multi-channel analyzer can display the energy spectrum with adjustable energy windows.

Fig. 2 is a schematic structural diagram of a detector using a lanthanum chloride crystal as a scintillator, wherein the lanthanum chloride scintillator 1 and the photomultiplier tube 2 are coated with silicon grease for optical coupling, and the magnetic shield 3 made of permalloy is used for Shield stray electromagnetic fields from interfering with photomultiplier tubes. The outer casing 4 is made of steel plate, which fixes the lanthanum chloride scintillator and the photomultiplier tube to form a whole detector, and the high-voltage voltage divider circuit 5 provides the voltage of each electrode required for the photomultiplier tube to work.

Fig. 3 and Fig. 4 are respectively the full energy spectrum diagram of the start track and the stop track and the energy spectrum diagram after setting the energy window, wherein, curve A is the full energy spectrum diagram, and curve B is the energy spectrum diagram after setting the energy window . The energy resolution of the lanthanum chloride scintillator detector can be obtained by fitting the photoelectric curves of 0.511MeV and 1.28MeV in the figure. In this embodiment, the energy resolution of the detector to the gamma rays of 0.511MeV is about 6%, while The initial gamma energy resolution is 4% for 1.28 MeV. The energy range of the start track energy window is 1.2MeV-1.30MeV; the energy range of the stop track energy window is 0.49MeV-0.53MeV;

Through the selection of the narrow energy window, the interference of other background gamma radiation can be effectively eliminated, and the normal measurement of the positron annihilation lifetime spectrum can be guaranteed.

Claims (2)

1. A positron annihilation lifetime spectrometer, including two detectors of the start track and the stop track, a fast coincidence time spectrometer and a microcomputer multi-channel pulse amplitude analyzer, each detector is coupled by a scintillator and a photomultiplier tube Into, the described fast coincidence time spectrometer contains a fast coincidence circuit, a constant-ratio timing discriminator with an energy window of the start track, a constant-ratio timing discriminator and a time-amplitude converter with an energy window of the stop track, and is characterized in that : The scintillator adopts lanthanum chloride scintillator. 2. The positron annihilation lifetime spectrometer according to claim 1, characterized in that: the energy window of the start track is an energy window in the energy range of 1.2MeV-1.30MeV; the energy window of the stop track is in the energy range Energy window at 0.49MeV-0.53MeV.

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