An analysis of the causes of the pulse height defect and its mass dependence for heavy-ion silicon detectors

Abstract

The mass dependence of the pulse height defect observed with heavy-ion radiation detectors is, like the defect itself, a useful parameter in comparisons of observed detector responses with theoretical predictions. Such a survey is made in which are assessed the contributions of non-ionizing nuclear collision losses and charge carrier recombination losses to the defect and its mass dependence as observed with fission fragments and similar ions incident on silicon surface barrier detectors. In particular recombination effects are evaluated by relating the charge losses, through expressions for the recombination lifetime of the carriers in the plasma formed in the wake of each fragment and the lifetime of the plasma itself, to previously established experimental data for heavy-ion stopping powers in solids. The correlation between hot carrier effects in the decay of the plasma and the relatively small observed field dependences of the defect and its mass variation is emphasized. Agreement can be obtained between the predictions and the rather wide range of experimental results which has been observed if in the recombination theory the silicon minority carrier lifetime is regarded as a variable parameter for which an upper limit can be determined from the resistivity of the detector material. For a lifetime of 10 −4 s, which can be expected in good detector material of resistivity a few ohm-m, a value typical in heavy-ion detectors, the predictions for the total defect and its mass dependence for fission fragments are about 10 MeV and 0.1 MeV amu −1, respectively.