Self-Powered Neutron Detectors (SPND) are widely used in reactors to monitor neutron flux. While they have several advantages such as small size, and relatively simple electronics required in conjunction with those usage, they have some intrinsic problems of the low level of output current, a slow response time, and the rapid change of sensitivity which makes it difficult to use for a long term. Monte Carlo simulation was used to calculate the escape probability as a function of the birth position of emitted beta particle for geometry of rhodium-based SPNDs. A simple numerical method calculated the initial generation rate of beta particles and the change of generation rate due to rhodium burn-up. Using the simulation result, the burn-up profile of rhodium number density and the neutron sensitivity were calculated as a function of burn-up time in reactors. In addition, for improvement of some properties of rhodium-based SPNDs which are currently used, a new material and modified geometry were proposed in this work. Ag/sup 109/ was chosen as a replacing material for rhodium. Also, this work compared the initial sensitivity of a solid type with its of a tube type.