Abstract: A compact circular magnetic induction accelerator (betatron) for use as a borehole gamma ray source includes a field magnet and generally circular pole pieces composed of a class of ferrite having the general formula M.sup.2+ Fe.sub.2.sup.3+ O.sub.4, where M represents two or more divalent metal ions from the group consisting of Mn, Zn and Ni. The core magnet is in the form of two symmetrical closed loops, with one leg of each loop passing axially through the circular pole pieces. The field coil and the core coil may be arranged in series or in parallel, and switching circuits are provided for effecting electron beam capture and ejection. In an illustrative borehole application, the betatron is used as a gamma ray source in a bulk density logging tool.