An alpha-particle energy spectrometer based on the BBC micro

Abstract

Semiconductor nuclear radiation detectors are widely employed as the transducers in systems used to measure nuclear radiation energies. The majority of such detectors make use of some form of reversebiased p-n junction. The depletion region formed around the junction defines the sensitive volume of the detector. Incident ionising nuclear radiation releases electron-hole pairs in the depletion region, the number of electron-hole pairs being proportional to the energy the radiation deposits in the detector. These charges are then swept by the applied electric field to the boundaries of the depletion region. which effectively act as the collecting electrodes. The resulting charge on these electrodes produces an output signal (voltage pulse) whose amplitude is proportional to the number of electron-hole pairs created and hence to the energy deposited in the detector by the incident radiation.Thus a measurement of the pulse amplitudes leads, with suitable calibration, to the energies of the incident radiation. The amplitudes of the transducer output signals are typically of the order of 0.1-10mV, the pulses having risetimes of tens of nanoseconds.In order to be able to use an analogue-to-digital converter ( A D C ) to convert these pulse amplitudes to digital signals it is first necessary to amplify them using a low noise amplifier and then to 'hold' the peak amplitude whilst conversion takes place. Once in a digital format the signal can be processed by a