Current mode response of phototransistors to gamma radiation

• Current mode response of four commercial phototransistors to gamma radiation was investigated.
• Dose rate dependence of the current response followed a power law for dose rates from 0.65 Gy/h to 32.1 Gy/h.
• Significant current degradation was observed during continuous irradiation up to the dose of 20 Gy.
• Relation between absorbed dose and induced charge was fitted with a second order polynomial.
• Only one of the investigated phototransistors may be applicable as a gamma radiation dosimeter.

This paper investigates the current mode response of four commercial NPN phototransistors under gamma radiation exposure from Co-60 source, with the aim to evaluate their applicability for gamma radiation dosimetry. The radiation-induced collector current was measured while the phototransistors were successively biased with 10 V and 20 V between collector and emitter. The main aspects analyzed in the experiment were the stability of the induced current, dependence of the induced current with respect to dose rate, short-term repeatability of the induced current and relation between the induced charge and the absorbed dose. The current response of all samples was stable during the 2-min irradiation sessions at the dose rates from 0.65 Gy/h to 32.1 Gy/h, with the relative uncertainty less than 5% in all cases, and the variation of the mean effective current as a function of dose rate was well fitted with the standard power relation. During the three consecutive 2-min irradiation sessions at the dose rate of 32.1 Gy/h, the repeatability of the induced charge was very good for all samples with the relative uncertainty below 3%. However, for continuous irradiation up to the absorbed dose of 20 Gy (10-min irradiation at the dose rate of 60 Gy/h), all samples exhibited significant current fall which can be attributed to the radiation-induced current gain degradation. As a result of the current degradation, the relationship between the induced charge and the absorbed dose was non-linear and it was demonstrated that the second order polynomial can be used as a calibration equation for determination of absorbed dose. The obtained results have shown that only one of the examined phototransistors may be a suitable candidate for gamma radiation dosimetry employing Co-60 source, since it has exhibited the highest current sensitivity, best linearity of the induced current with respect to dose rate and smallest dose rate dependence of the charge sensitivity.