Ionization effects produced in diamonds subjected to mono-energetic β -ray bombardment

Abstract

The ionization produced within diamond specimens by the passage of high-energy β -rays has been investigated using the methods of conduction pulse counting, and a new technique has been developed to enable pulse-height spectra to be taken, rapidly and in quick succession, under widely different experimental conditions. It is shown that only a small proportion of the incident β -rays may dissipate their total energy within the specimen and that pulse-height spectra can be interpreted successfully only when a conduction pulse is related to the energy lost by the electron producing it. Under conditions of saturation field strength and low crystal polarization, the conduction pulse magnitude is proportional to the energy dissipated. The mean value of energy per ion pair is thus independent of the primary electron energy and the experimental value of 20 eV is shown to be remarkably consistent between diamond specimens. This value differs from the previously accepted value of 10 eV and, if used in preference to 10 eV, removes many apparent anomalies from previously published work. A theory is outlined in which it is proposed that the degradation of the primary electron energy takes place principally by interaction with the valence electrons of the crystal. The mean energy per ion pair depends, therefore, not only on the width of the forbidden gap in the solid, as previously suggested, but also on the width of the valence band and particularly on the position of the maximum in the density of states curve within the valence band. The available data for diamond suggest a value of approximately 18 eV for the mean ionization energy. This value is consistent with the experimental value from the maximum pulse height under saturation conditions. The finite breadth of the pulse spectra, however, can be explained only by some charge-reducing process occurring after the total dissipation of the incident energy. The process is tentatively linked with the scintillation response of the diamond crystal. Finally, criteria are suggested by which the conduction pulse response of various solids may be predicted.