Multiplication in the Fission Fragment Pulse Height Response of Silicon Surface Barriers

A number of investigators have observed multiplication phenomenon in the response of silicon surface barriers to fission fragments. In many cases, this multiplication is anomalous in the sense that it occurs at electric field intensities well below that required for avalanche multiplication. One group has suggested that this phenomenon is related to a "threshold depth". 1 However, other results from investigations of the response of surface barriers to fission fragments2 and heavy ions3 for a wide range of silicon resistivities and bias voltages have exhibited no systematic evidence to support the concept of a "threshold depth". In the work reported here, we demonstrate that multiplications at anomalously low field strengths can be significantly reduced by surface treatment after fabrication and can be reduced or eliminated by the proper surface treatment and control of the thickness of the front electrode. A possible multiplication mechanism based on the concept of tunneling injection through the oxide layer, which was first suggested by Gibson and Miller4, is discussed in terms of these experimental results. Fabrication techniques which minimize the tendency to multiply, together with other detailed design criteria for optimizing the performance of fission fragment and heavy ion spectrometers, are also presented.