The Response of Stilbene to Internally Generated Protons

A thick radiator neutron spectrometer has been constructed in which protons recoiling forward in the stilbene escape from the crystal and are detected in a silicon surface barrier counter. The coincident pulses from the two counters are recorded using a two-parameter analyzer. The instrument has been used to investigate (1) the response of stilbene to protons originating within the scintillator and (2) the behavior of the pulse shape discrimination as a function of the energy deposited in the stilbene crystal. The response function, which is required for the analysis of the neutron spectra, is compared with one obtained for protons incident externally on the same crystal. Small differences between the two response functions are observed and analyzed. Concerning the behavior of the pulse shape discrimination between protons and electrons it is found that, for internally generated protons, the ability to separate protons from electrons worsens as the energy deposited in the scintillator decreases. This contrasts with the behavior for protons which stop in the scintillator. The results support the hypothesis that the fast component of the pulse decreases with increasing stopping power whereas the slow component depends primarily on the energy lost. The results are analyzed and may be interpreted with certain assumptions to give the ratio of the fast and slow scintillation components.