Front-end electronics for high rate neutron counters: its performance and radiation hardness improvement

Radiation-resistant circuits focused on the radiation hardness process have been developed for many years to protect them from radiation damage. In general, radiation hardened transistors are very expensive and sometimes not easy to acquire. In this study, we designed front-end electronics for high rate neutron counters. An aim of our study was to improve their radiation hardness by using commercial fast bipolar transistors not specifically processed for radiation resistance. This circuit includes a preamplifier, a comparator, and a monostable multivibrator. The preamplifier circuit is a kind of current-sensitive configuration. The TTL pulse width of the multivibrator output is 40 ns. This short TTL pulse width makes it possible to count high rate neutrons of over 5 MHz. For radiation hardness improvement experiments, the transistors were categorized into two groups: general purpose and high speed transistors. After a 100 Mrad irradiation from a Co-60 gamma-ray source, the reduction of the current gain of the general purpose transistors (KN3904 and KN3906) was over 80% and for the fast transistors (KTC9018 and KTC2347) the current gain reduction was within 68%. The signal to noise ratio of the preamplifier output voltage was reduced by 66% for the general purpose transistor (KN3904) and within 36% for the high speed ones.