Browsing by Subject "ultralow temperature"

Positron Annihilation Spectroscopy is a powerful tool for defect characterisation, especially vacancies. Various defect properties can be studied, including defect behaviour at low and high temperatures. Despite the technique having its roots in the mid-20th century, there is little research on fundamental positron behaviour at ultralow temperatures. In this thesis, Positron Annihilation Lifetime Spectroscopy and Doppler Broadening Spectroscopy, two sub-methods of the spectroscopy technique, were used to measure positron trap-free Ge in the temperature range of 14 mK-300 K. Since a positron trap-free sample was used, the purpose was not to study defect processes. Instead, the aim of the thesis was to investigate whether any interesting positron processes could be seen at ultralow temperatures in the annihilation data. Previous research in Al has shown no change in either lifetime or Doppler broadening below 77 K. Measuring the positron lifetime in the sample located in a cryostat required designing a special detector setup, as the count rate was greatly reduced due to geometry. To tackle this, lifetime detectors consisting of BaF2 scintillators and quartz-windowed photomultiplier tubes were used. In addition, both analogue and digital signal processing techniques were tested for the lifetime setup, with the digital method proving to be preferable. Doppler Broadening was measured with a high-purity germanium detector connected to a digital gamma spectrometer. The results show a decrease in S-parameter and an increase in W-parameter with decresing temprature, with the rate of change being greatest at ultralow temperatures. This behaviour is concluded to be due to incomplete positron thermalization. The positron lifetime results are more difficult to interpret, as setup challenges resulted in results of questionable accuracy. Still, the trend suggests no change in lifetime over the whole temperature interval, which is in accordance with previous research.