Low-source component positron annihilation lifetime spectrometer for micron-sized film measurement

Abstract

The invention discloses a low-source component positron annihilation lifetime spectrometer for micron-sized film measurement, and relates to the field of positron annihilation lifetime. In the spectrometer, a positron annihilation lifetime measuring device is used for detecting gamma photons generated by decay of a radioactive source as initial detector signals; detecting gamma photons generated by annihilation of positron in the film sample as termination detector signals; the anticoincidence device is used for detecting anticoincidence signals generated when positron does not annihilate in the film sample; the positron annihilation life measuring device performs time coincidence judgment on a starting detector signal and a terminating detector signal of a case that positron annihilation occurs in a film sample according to the anti-coincidence signal, calculates a time difference between the starting detector signal and the terminating detector signal meeting a time condition, and counts the time difference. And drawing a positron annihilation lifetime spectrum. According to the invention, the source components in the positron annihilation lifetime spectrum are greatly reduced, and the measurement of the film sample by using a simple device is realized.

Description

Technical Field

[0001] The invention belongs to the field of positron annihilation lifetime, and in particular relates to a low-source component positron annihilation lifetime spectrometer for measuring micron-level thin films. Background Art

[0002] After entering the sample, the positrons are rapidly slowed down until their kinetic energy reaches room temperature, and annihilate with the electrons in the sample. The gamma photons produced by annihilation can reflect information such as the electron density inside the material. Positrons are easily captured by defects inside the material, so they are currently the most sensitive material defect probe. The annihilation lifetime of the positron can reflect the electron concentration at the annihilation position, and further obtain the corresponding structural information. As a non-destructive and sensitive method for characterizing material defects, positron annihilation lifetime spectroscopy has unique advantages in the field of materials science.

[0003] In conventional positron annihilation lifetime measurements, the most commonly used 22 Na was used as the positron source. 22 When Na decays with a positron, it releases a gamma photon with an energy of about 1.275 MeV, which can be considered the starting signal of positron annihilation. When a positron annihilates an electron, it will be transformed into a pair of annihilation photons with an energy of 0.511 MeV, which is considered the end signal of positron annihilation. The positron annihilation lifetime can be obtained by measuring the time difference between the end signal and the start signal, and the positron annihilation lifetime spectrum can be obtained by statistically analyzing the positron annihilation lifetime.

[0004] The positron source is generally wrapped with a polyimide film or a metal film. The radiation source and the sample form a sandwich structure, and the radiation source is located between the two samples to be tested. Conventional positron annihilation lifetime spectrometers cannot distinguish whether the positrons are annihilated in the sample or in the polyimide film that wraps the positron source. Therefore, the obtained positron annihilation lifetime spectrum contains the positron annihilation lifetime components of the sample and the polyimide film, among which the positron annihilation lifetime component of the polyimide film is called the source component. When the sample has a positron annihilation lifetime that is similar to the positron annihilation lifetime of the source component, it is difficult to accurately analyze the positron annihilation lifetime of the sample.

[0005] At the same time, for thin film samples, some positrons will pass through the thin film samples without being annihilated in them, and will be annihilated at an uncertain position outside the thin film samples. Conventional positron annihilation lifetime spectrometers are also unable to distinguish whether positrons are annihilated in thin film samples. For thin film samples, the proportion of positrons annihilated in thin film samples is small, so the obtained positron annihilation lifetime spectrum is difficult to accurately interpret to obtain relevant positron annihilation lifetime information of thin film samples. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a low source component positron annihilation lifetime spectrometer for micron-level thin film measurement, which uses anti-coincidence technology to screen events and removes events in which positrons pass through thin film samples without annihilation, thereby achieving accurate measurement of the positron annihilation lifetime spectrum of thin film samples.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A low-source component positron annihilation lifetime spectrometer for measuring micron-level thin films, the spectrometer comprising a positron annihilation lifetime measurement device and an anti-coincidence device, wherein:

[0009] The positron annihilation lifetime measuring device is used to detect gamma photons generated by the decay of the radioactive source as a starting detector signal, and to detect gamma photons generated by the annihilation of positrons in the thin film sample as a termination detector signal;

[0010] The anti-coincidence device is used to detect the anti-coincidence signal generated by the positrons not being annihilated in the thin film sample;

[0011] The positron annihilation lifetime measurement device performs temporal coincidence judgment on the start detector signal and the end detector signal of the event in which the positron annihilates in the thin film sample according to the anti-coincidence signal, calculates the time difference between the start detector signal and the end detector signal that meet the time condition, counts the time difference, and plots the positron annihilation lifetime spectrum.

[0012] Furthermore, the positron annihilation lifetime measurement device includes a start detector, a stop detector, an anti-coincidence unit, a coincidence unit, and a data analysis module, wherein:

[0013] The initial detector is used to detect gamma photons generated by the decay of the radioactive source as initial detector signals;

[0014] The termination detector is used to detect gamma photons generated by positron annihilation as termination detector signals;

[0015] The anti-coincidence unit is used to classify the events into events in which positrons are annihilated in the thin film sample and events in which positrons are not annihilated in the thin film sample according to the amplitude of the anti-coincidence signal collected by the anti-coincidence device;

[0016] The coincidence unit is used to perform temporal coincidence determination on the start detector signal and the end detector signal of the event of positron annihilation in the thin film sample, and calculate the time difference between the start detector signal and the end detector signal that meet the time condition;

[0017] The data analysis module is used to count the time difference between the start detector signal and the end detector signal, and draw the positron annihilation lifetime spectrum.

[0018] Furthermore, the anti-coincidence device includes a radiation source, a first plastic scintillator and a second plastic scintillator of the same structure, a third photoelectric detection device, and a data acquisition module, wherein:

[0019] The first plastic scintillator is used to store the radiation source and convert the energy deposited by the positrons therein into fluorescent photons;

[0020] The second plastic scintillator is used for receiving positrons passing through the thin film sample and converting the energy deposited therein by the positrons into fluorescent photons;

[0021] The third photoelectric detection device is used to convert the received fluorescent photons generated in the first or second plastic scintillator into an electrical pulse signal, that is, an anti-coincidence signal;

[0022] The data acquisition module is used to collect the start detector signal, the end detector signal, and the anti-coincidence signal and send them to the anti-coincidence unit for processing.

[0023] Furthermore, a hole is dug in the middle of the first plastic scintillator for storing a radiation source.

[0024] Furthermore, the third photoelectric detection device, the first plastic scintillator, the thin film sample, and the second plastic scintillator are stacked in sequence from bottom to top to form an anti-coincidence detector.

[0025] Furthermore, the peripheries of the first plastic scintillator and the second plastic scintillator are wrapped with reflective material.

[0026] Further, the starting detector includes a coupled first scintillation crystal and a first photodetection device, and the terminating detector includes a coupled second scintillation crystal and a second photodetection device.

[0027] Furthermore, the first scintillation crystal and the second scintillation crystal are cylindrical barium fluoride crystals, lanthanum bromide or yttrium lutetium silicate; the first photoelectric detection device, the second photoelectric detection device and the third photoelectric detection device are photomultiplier tubes or silicon photomultiplier tubes.

[0028] Furthermore, during measurement, the starting detector and the ending detector are placed at right angles.

[0029] Furthermore, the data acquisition module is used to collect the starting detector signal, the ending detector signal, and the anti-coincidence signal and send them to the anti-coincidence unit for processing, including screening the anti-coincidence signal according to the amplitude of the anti-coincidence signal, and retaining the cases where the anti-coincidence signal amplitude is less than a threshold. The anti-coincidence signal amplitude less than the threshold indicates that the positron is directly annihilated in the thin film sample.

[0030] The beneficial effects of the present invention are:

[0031] Anti-coincidence technology is used to screen cases, which greatly reduces the source components in the positron annihilation lifetime spectrum of thin film samples, thereby reducing the impact of the source components on the analysis of thin film sample results, ensuring that the obtained positron annihilation lifetime spectrum corresponds to the information of the measured thin film sample, and realizing accurate measurement of thin film samples using simple equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic structural diagram of a low-source-component positron annihilation lifetime spectrometer for micron-level thin film measurement according to the present invention;

[0033] Figure 2 This is a schematic diagram of the data processing process of a low source component positron annihilation lifetime spectrometer for micron-level thin film measurement according to the present invention;

[0034] Figure 3 It is a schematic diagram of the structure of the anti-conformity device;

[0035] Figure 4 This is a schematic diagram of anti-coincidence signal;

[0036] Figure 5 This is a diagram showing the measurement results of the positron annihilation lifetime spectrum using the device of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] The present invention provides a low-source component positron annihilation lifetime spectrometer for micron-level thin film measurement, which uses anti-coincidence technology to screen events, retains events in which positrons are annihilated in thin film samples, reduces source components in the positron annihilation lifetime spectrum, reduces the influence of source components on the analysis of thin film sample results, and accurately measures the positron annihilation lifetime spectrum of the thin film sample.

[0039] like Figure 1 As shown, the spectrometer includes a positron annihilation lifetime measurement device and an anti-coincidence device, wherein the positron annihilation lifetime measurement device includes a hardware part and a software part, the hardware part includes a starting detector, a terminating detector, and a data acquisition module, wherein the starting detector includes a cylindrical first scintillation crystal with a diameter of 20-50 mm and a thickness of 10-30 mm and a first photoelectric detection device, and the terminating detector includes a cylindrical second scintillation crystal with a diameter of 20-50 mm and a thickness of 10-30 mm and a second photoelectric detection device; the data acquisition module is used to collect signals from each detector; as shown Figure 2 As shown, the software part includes an anti-conformity unit, a conformity unit, and a data analysis module, wherein the software part is mainly used for data processing, which is usually completed in a computer.

[0040] Start detector: used to detect 22 The 1.275 MeV gamma photons generated by the decay of the Na radioactive source serve as the initial detector signal.

[0041] Termination detector: used to detect 0.511 MeV gamma photons produced by positron annihilation as termination detector signal.

[0042] Anti-coincidence unit: used to classify the events into events where positrons are annihilated in the thin film sample and events where positrons are not annihilated in the thin film sample according to the amplitude of the anti-coincidence signal collected by the anti-coincidence device.

[0043] Coincidence unit: used to make a temporal coincidence judgment on the start detector signal and the end detector signal of the event of positron annihilation in the thin film sample, and calculate the time difference between the start detector signal and the end detector signal that meet the time condition.

[0044] Data analysis module: used to count the time difference between the start detector signal and the end detector signal, and draw the positron annihilation lifetime spectrum.

[0045] Anti-coincidence device such as Figure 3 As shown. Included 22 Na radiation source, a first plastic scintillator and a second plastic scintillator of the same structure in a cylindrical shape with a diameter of 20-50 mm and a thickness of 1-5 mm, a third photoelectric detection device, and a data acquisition module. The data acquisition module here can be the same as the data acquisition module of the positron annihilation lifetime measurement device, and the data acquisition module includes an oscilloscope.

[0046] The first plastic scintillator has a hole in the middle, the hole is cylindrical with a diameter of 5-10 mm and a thickness of 1 / 3 to 1 / 2 of the total thickness. 22 Na radiation source and converts the energy deposited by the positrons therein into fluorescent photons.

[0047] The second plastic scintillator is used to receive positrons that pass through the thin film sample and convert the energy deposited by the positrons into fluorescent photons. Such positrons are the main reason why it is difficult to accurately measure thin film samples. The second plastic scintillator can basically remove them.

[0048] The third photoelectric detection device, the first plastic scintillator, the film sample, and the second plastic scintillator are stacked in sequence from bottom to top to form an anti-coincidence detector. The peripheries of the first plastic scintillator and the second plastic scintillator are wrapped with reflective materials.

[0049] like Figure 4As shown, the third photodetector converts the received fluorescent photons generated in the first or second plastic scintillator into an electrical pulse signal, i.e., an anti-coincidence signal. When the positron is directly annihilated in the thin film sample, the generated gamma photon will quickly pass through the thin plastic scintillator, and the third photodetector will only output a very weak signal. When the positron passes through the first or second plastic scintillator, fluorescent photons will be generated in the first or second plastic scintillator, and after reflection, they will be received by the third photodetector, generating a stronger signal, and this event will be removed in subsequent processing.

[0050] Data acquisition module: performs high-speed digital sampling of detector pulses to digitize analog signals so that they can be subsequently digitally processed.

[0051] The first scintillation crystal and the second scintillation crystal of the present invention can be cylindrical barium fluoride crystals, lanthanum bromide or yttrium lutetium silicate and other scintillation crystals; the first photoelectric detection device, the second photoelectric detection device, and the third photoelectric detection device can be photomultiplier tubes or silicon photomultiplier tubes and other devices. The data acquisition module can also be a digital acquisition card, and the data collected by the digital acquisition card is processed by a computer terminal to obtain a correct positron annihilation lifetime spectrum.

[0052] In the positron annihilation lifetime experiment, the starting detector and the ending detector are placed at right angles, and the test film sample is placed between the first and second thin plastic scintillators. 22 The Na source is placed in the hole of the first plastic scintillator with holes at the bottom. Start the detector and measure 22 The 1.275 MeV gamma photons generated by the decay of the Na radioactive source terminate the detector and measure the 0.511 MeV gamma photons generated by the annihilation of the positron. The anti-coincidence detector measures the fluorescence photons generated when the positrons deposit energy in the first and second plastic scintillators. When the positrons are annihilated in the thin film sample, the signal amplitude output by the anti-coincidence detector is very small.

[0053] The pulse signals of the first, second, and third detection devices are simultaneously input to the data acquisition card through a coaxial cable. The data acquisition card collects data and transmits it to the computer for processing. The test data is screened by the amplitude of the anti-coincidence signal, and the events where the anti-coincidence signal amplitude is less than the threshold are retained. At the same time, the time is calculated by the timing module. The subsequent processing is the same as that of a conventional positron annihilation lifetime spectrometer. The data is input to the coincidence logic unit. When the time difference between the two signals is less than the given coincidence time window (100 ns) and is retained, it is a set of coincidence events. The time difference of the coincidence events is statistically processed to obtain the positron annihilation lifetime spectrum, such as Figure 5 shown.

[0054] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement, characterized in that: The spectrometer includes a positron annihilation lifetime measurement device and an anti-coincidence device, wherein: The positron annihilation lifetime measuring device is used to detect gamma photons generated by the decay of the radioactive source as a starting detector signal, and to detect gamma photons generated by the annihilation of positrons in the thin film sample as a termination detector signal; The anti-coincidence device is used to detect the anti-coincidence signal generated by the positrons not being annihilated in the thin film sample; The positron annihilation lifetime measurement device performs temporal coincidence judgment on the start detector signal and the end detector signal of the event in which the positron annihilates in the thin film sample according to the anti-coincidence signal, calculates the time difference between the start detector signal and the end detector signal that meet the time condition, counts the time difference, and plots the positron annihilation lifetime spectrum.

2. A low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 1, characterized in that: The positron annihilation lifetime measurement device comprises a start detector, a stop detector, an anti-coincidence unit, a coincidence unit, and a data analysis module, wherein: The initial detector is used to detect gamma photons generated by the decay of the radioactive source as initial detector signals; The termination detector is used to detect gamma photons generated by positron annihilation as termination detector signals; The anti-coincidence unit is used to classify the events into events in which positrons are annihilated in the thin film sample and events in which positrons are not annihilated in the thin film sample according to the amplitude of the anti-coincidence signal collected by the anti-coincidence device; The coincidence unit is used to perform temporal coincidence determination on the start detector signal and the end detector signal of the event of positron annihilation in the thin film sample, and calculate the time difference between the start detector signal and the end detector signal that meet the time condition; The data analysis module is used to count the time difference between the start detector signal and the end detector signal, and draw the positron annihilation lifetime spectrum.

3. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 2, characterized in that: The anti-coincidence device comprises a radiation source, a first plastic scintillator and a second plastic scintillator of the same structure, a third photoelectric detection device, and a data acquisition module, wherein: The first plastic scintillator is used to store the radiation source and convert the energy deposited by the positrons therein into fluorescent photons; The second plastic scintillator is used for receiving positrons passing through the thin film sample and converting the energy deposited therein by the positrons into fluorescent photons; The third photoelectric detection device is used to convert the received fluorescent photons generated in the first or second plastic scintillator into an electrical pulse signal, that is, an anti-coincidence signal; The data acquisition module is used to collect the start detector signal, the end detector signal, and the anti-coincidence signal and send them to the anti-coincidence unit for processing.

4. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 3, characterized in that: The first plastic scintillator has a hole in the middle for storing a radiation source.

5. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 3, characterized in that: The third photoelectric detection device, the first plastic scintillator, the thin film sample, and the second plastic scintillator are stacked in sequence from bottom to top to form an anti-coincidence detector.

6. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 3, characterized in that: The peripheries of the first plastic scintillator and the second plastic scintillator are wrapped by reflective material.

7. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 2, characterized in that: The starting detector includes a coupled first scintillation crystal and a first photodetection device, and the terminating detector includes a coupled second scintillation crystal and a second photodetection device.

8. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 7, characterized in that: The first scintillation crystal and the second scintillation crystal are cylindrical barium fluoride crystals, lanthanum bromide or yttrium lutetium silicate; the first photoelectric detection device, the second photoelectric detection device and the third photoelectric detection device are photomultiplier tubes or silicon photomultiplier tubes.

9. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 2, characterized in that: During measurement, the starting detector and the ending detector are placed at right angles.

10. The low source component positron annihilation lifetime spectrometer for micron-scale thin film measurement according to claim 2, characterized in that: The data acquisition module is used to collect the starting detector signal, the ending detector signal, and the anti-coincidence signal and send them to the anti-coincidence unit for processing, including screening the anti-coincidence signal according to the amplitude of the anti-coincidence signal, and retaining the cases where the anti-coincidence signal amplitude is less than a threshold. The anti-coincidence signal amplitude less than the threshold indicates that the positron is directly annihilated in the thin film sample.

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