“…Geiger counters operating at low gas density, electron multiplier tubes, crystal counters, and scintillation counters generally have efficiencies below unity. Gamma-ray counter efficiencies {17, 19,35,37,102,103,116,126,127) are low and depend on the energy and the material of construction. Brass-wall counters of ordinary size have efficiencies of roughly 0.01 times the 7-energy in million electron volts in the range 0.3 to 3 m.e.v.…”