Measurements of the decay constant require good (short) time resolution at early times (t small, I large) when the changes in gamma-exposure rate are most rapid. At later times (t large, I small), the rate of change of the gamma-exposure rate of the gamma radiation is much smaller, and the instrument system need not have such good time resolution. The instrument for the measurement of residual-garama radiation was designed to cover a range from 1 r/hr with a time resolution of 5 minutes, to 10‘ r/hr with a time resolution of 0.05 minutes. The basic circuit is shown .n Figure 2.1, where + —- 22% o oe I ° o x + WV > 'sensing|_ T~pELEMENY ELEMENT! ¢ | +}——---—~ onee a enneee ae Figure 2.1 Schematic diagram showing the basic circuit for the Conrad and Gustave detectors. The Conrad detector used an unsaturated ion chamber as the sensing element, whereas the Gustave detector used a scintillation detector. the sensing element is an unsaturated ion chamber. The ion chamber was designed to have a current output proportional to the square root of the gamma~exposure rate. The overall detector response is given by: f= kr¥? (2.2) Where: f = the output frequency r= the gamma-exposure rate in r/hr k = a parameter chosen to meet specific design objectives In laboratory calibrations on a 250-kv X-ray beam, these detectors showed a preci- 20