Positron Annihilation Lifetime Spectrometer

• 详细介绍
• 详细参数

• 详细介绍

  Positron Annihilation Lifetime Spectrometer
  Introduction
  Positron lifetime spectroscopy is widely applied in the field of materials science for detecting defects in semiconductor materials. This is an integrated measurement and power supply system designed specifically for positron lifetime testing.
  Technical Features
  In lifetime testing, a 3 GSPS sampling rate is used to measure the positron lifetime, based on fast pulse signals generated by two BaF₂ scintillators. In Doppler Broadening Spectroscopy (CDB), a two-dimensional histogram is calculated from the pulse height distribution values of two high-purity germanium (HPGe) detectors.
  Additionally, the system can operate in AMOC mode to measure both positron lifetime and electron momentum density distribution in relevant materials.
  ✓ Lifetime Spectrum
  ✓ Doppler Broadening Spectrum (CDB)
  ✓ Age–Momentum Correlation (AMOC)
  Schematic of the Annihilation Process
  
  Positron Motion in Material
  After high-energy positrons are injected into a solid material, they lose energy by colliding with atoms and ionizing/exciting electrons. Within approximately 10⁻¹³ seconds, they thermalize to energies below 100 eV.
  Low-energy positrons continue to lose energy by exciting electron-hole pairs, eventually dropping below 1 eV.
  Positrons with energy below 1 eV lose additional energy via phonon scattering.
  Eventually, positrons reach thermal equilibrium with the material and maintain this state through the emission and absorption of phonons. The thermalized positron energy is approximately 0.0025 eV.
  Lifetime Calculation
  The positron source ²²Na emits a 1.27 MeV gamma photon simultaneously with a positron. The time difference between this gamma ray and the 511 keV gamma photon emitted during positron–electron annihilation is used to calculate the lifetime.
  
  
  CDBS Principle
  Doppler Broadening:
  If the positron–electron pair is at rest during annihilation, two 511 keV gamma photons are emitted in opposite directions (180° apart). However, if the electron in the material has a certain momentum, the energy of each annihilation photon deviates from 511 keV.
  DBS is used to record the energy deviation of each photon from 511 keV.
  According to the Doppler broadening principle, the Doppler energy shift of the annihilation photon is related to the longitudinal component Pₗ of the total momentum of the positron–electron pair before annihilation:
  
  The Doppler energy shift is proportional to the longitudinal momentum of the annihilated electron. By measuring the Doppler-broadened energy spectrum, the electron momentum density distribution can be obtained.
  Five Measurement Modes
  Lifetime Spectrum Mode
  Waveform Mode
  Energy Mode
  CDB (Coincidence Doppler Broadening Spectrum) Mode
  AMOC (Age–Momentum Correlation Spectrum) Mode
  Lifetime Spectrum Mode
  In Lifetime mode, waveforms from two BaF₂ scintillation detectors are acquired simultaneously. The time difference between the rising edges of the two waveforms is used to generate the positron lifetime spectrum.
  
  
  Waveform Mode
  In Waveform mode, the system collects waveforms from two BaF₂ scintillation detectors.
  
  
  Energy Mode
  In Energy mode, the system measures energy spectra from two HPGe semiconductor detectors.
  
  
  CDB Mode (Coincidence Doppler Broadening Spectrum)
  In CDB mode, the system counts coincidences from two HPGe detectors. Peak values from each detector are used to calculate the Doppler broadening of positron annihilation coincidence counts.
  
  
  AMOC Mode (Age–Momentum Correlation Spectrum)
  AMOC mode involves coincidence counting among two BaF₂ scintillation detectors and one HPGe detector to obtain the time-resolved distribution of positron momentum.
  
  
  

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