The measurements of thermal neutron flux were made by means of electrical spark counting technique of etched nuclear tracks on polycarbonate foils of 18μm thickness. By this method, spark counts of etch-pits below 400/cm2 on the foil can be counted within about 10% relative standard deviation and the minimum detectable limit of thermal neutron fluence was about 105/cm2. The result of the thermal neutron distribution in a reactor (UTR-B) by electrical spark counting technique was in good agreement with that estimated by Au activation method within 6% deviation. The leakage of thermal neutron from the reactor operated at 0.1 watts for 3 hours was also determined to be 1.2×106/cm2 (about 0.5mrad/3hr) by this method.
