Low energy α-p pulse-shape discrimination with silicon surface barrier detectors

Abstract

When an ionizing particle enters a semiconductor surface barrier counter, the pulse height that is produced is determined by the particle energy, while the range of the particle affects the shape of the pulse. The voltage pulse therefore contains information about the particle type. In this paper a low energy α-p identification system by the pulse-shape discrimination is described. The characteristics of the optimal detector for this purpose, as well as the electronic circuitry, are given. Particles which normally give a difference in rise time of 3 ns (in the range of 15-20 ns) can be discriminated with the artifice of bombarding the back side of the detector (hole collection). The lowest discrimination level attainable is 2.7 MeV. The energy to form an electron-hole pair in semiconductor nuclear particle detectors is independent on the nature of the impinging heavy particle. Hence, a simple pulse height analysis does not allow the distinction between different particles, with energies that are either the same or slightly different. A first method of pulse-shape discrimination has been proposed by Amsel,^l Funsten² and Scheer.³ It is primarily based on the difference between the collecting time of the charges created within the depletion layer on the one hand, and outside this region on the other hand. In the latter case, the charges created must diffuse toward the depletion layer, which leads to a slow component in the observed rise time of the pulse. This method requires the use of a detector which is suitable to the problem under a study and its application is limited to a rather small range of energies. Moreover, the pulse heights are no longer proportional to the energies of the impinging particles. A second method of pulse-shape discrimination, using a dE/dx. E telescope, gives good results for high energy particles, but is not so successful at low energies. This is mainly due to the fact that it is difficult to produce very thin detectors. Finally, a third method uses the particular pulse shapes resulting from the collected charges created in the depleted region of a detector. Such a pulse-shape discrimination has been described by Ammerlaan⁴ for particles of energies from 10 to 25 MeV. Using the principle of this last method, we have set up an experimental arrangement, which allows the discrimination between protons and alphas particles with energies greater than 3 MeV. The detector that was used, was a surface barrier Au-Si N counter. Under these conditions, the main difficulties are: firstly, the weak signal amplitudes involved (a few mV), secondly, the speed of the pulses (10-20 ns), and finally, above all the small difference in the rise time of pulses due to protons and alphas particles.