In present work, the ۹۰Sr radioisotope with half-life of ۲۸.۸ years and activity of ۱۶.۳۲۸ mCi was considered as a source. The purpose of this paper is to compare a direct charge nuclear battery (DCNB) with a betavoltaic nuclear battery in constant volume. In this comparison the volume of the batteries, the dimensions of the source and the activity of the source are constant. First, the beta spectrum of ۹۰Sr + ۹۰Y was calculated. Then, the type of collector (materials), the distance between the source and the collector, and collector thickness were investigated and optimized for the case of a DCNB by Monte Carlo N–Particle (MCNPX) code. It was found that the best parameters for this DCNB are the use of the Be as a collector, the distance between the source and the collector ۲۵ μm and collector thickness ۱۴۰۰ μm. Under these parameters, the short circuit current and open circuit voltage are ۰.۰۸۹۹۷ nA and ۵۴۹ kV, respectively. If there is a semiconductor material instead of a vacuum in the DCNB, it will be converted into a betavoltaic nuclear battery. Therefore, the semiconductor material (Si) was considered in the distance between the source and the collector instead of the vacuum. Then the best thickness for silicon was determined.

By discussing the effect of doping concentration and minority carrier diffusion length on the optimization of betavoltaic battery utilizing a۹۰Sr radioisotope source and Si as the converter material, it was found that junction depth of ۰.۹ μm, semiconductor thickness ۷۲۸ μm, Na (doping concentration in P-type semiconductor) = ۱.۵ × ۱۰۱۵ cm−۳ and Nd (doping concentration in N-type semiconductor) = ۳.۰ × ۱۰۱۵ cm−۳. Therefore, the short circuit current and open circuit voltage are ۳۹۲۰ nA and ۰.۳۰۹ V, respectively. In other words, at constant volume, constant source dimensions and constant activity, the use of the semiconductor instead of the vacuum increases the short circuit current of the nuclear battery by about ۴۴,۰۰۰ times.