Angular dependence of dose sensitivity of surface diodes.

Commercially available surface diodes are found to have as great as +/-12% change in sensitivity with the angle of incidence of radiation. This work is a study of the cause of angular dependence in diode sensitivity and how it can be decreased. A number of different surface diodes were used in these measurements: A commercially available diode and four prototype diodes. A number of the diodes were constructed with the silicon chip, the die, mounted on a circuit board that had a plane of copper on its back side. These diodes had angular dependence of sensitivity as great as +/- 10%. It was hypothesized that the copper plane on the circuit board was the cause of the anisotropy in sensitivity of the diodes. To test this hypothesis, diodes with a new design [Patent No. 61/035,257 (pending)], without a copper back plane, were fabricated and characterized in this work. These diodes were found to have the following characteristics: A dependence on incident angle of radiation of +/- 3.6%; after 10 kGy of pre-irradiation, a 1.6% change in sensitivity for a 260-fold change in dose per pulse; an areal density of 0.08 g/cm2.