An electron multiplier has been developed which has increased the sensitivity of an existing mass spectrometer toy a factor of 10^5. This increased sensitivity has permitted determination of fission yields of substances that have undergone little fission. Two such samples, for which the rare gas isotopes have been analysed, are Bohemian pitchblende and the products resulting from the neutron fission of Th232, The Bohemian ore exhibited more neutron fission than any uranium deposit previously analysed. An explanation of this effect may be obtained by considera­tion of the geological and chemical nature of the deposit. The neutron fission of thorium, which shows fine structure in both the xenon and krypton regions, is similar to the patterns of u235 U235 and pu239, / Thesis / Master of Science (MS)