source was Calibrated in the field over the exposure-rate region used with a set of Victoreen r-meters calibrated at National Bureau of Standards (NBS) in March 1956. 2.2.6 High-Range Initial-GammaStation Calibration. There were no sources available for direct gamma-radiation calibration up to the maximum ranges of the initialgammainstruments. Because of this lack, scintillation detectors were used, thereby enabling calibration with a light source. In practice, the instruments were directly calibrated by the use of the 200-curie Co™ source in the field and a Van de Graaff generator in the laboratory to the limit of the available radiation rates. The calibration was then extended to the maximum range through the use of a light calibration, which was normalized to the radiation calibration. The light calibrator consisted of a light source filtered to provide a beam having approximately the same spectral quality as the light output of the scintillator, and a series of neutral-density filters that varied the light output in known discrete steps. Errors due to the direct response of the circuit elements to gamma radiation were introduced into the calibration; hawever, these errors were shownto be small in the ranges where the light and radiation calibrations overlapped. There were no reasons why the relative error should have increased beyond the range of dual calibration. 2.3 READOUT ERROR AND ACCURACY OF THE GUSTAVE AND CONRAD SYSTEMS In general, the output of the Gustave and Conrad detectors may be given as: r= kt" Where: . (2.4) r = gamma exposure rate t = tirne between output pulses n, k = design parameters If the error in reading time between pulses (i.e. time base) is At, then: r+ Ar =k(t + Aty? Ar = K[¢e + AtyA - n | Ar — (t + At? - «n r a (2.5) At For + << 1,.this formula reduces to the definition of differentials. r Where: t (2.6) Ar . . —— = the relative error in gamma-exposure rate due to errors in the time measurement TO the relative time-measurement error 25