second time constant and has a small standard air since the number of contributing gamma ray photons absorbed in the phosphor is lar ge. The plastic type phosphor was selected for the following reas Ons ¢ A. Its response to the gamma ray onergies, Figure 3, is practically equivalent to that of air. The response of a thallium activated sodium iodide phosphor which is widely used for scintillation cowmters is included for comparison, B, It is based on organic scintillatars which have short decay factors. Thus, there is little residual light in the phosphor after incident radiation has been removed. Therefore, it is feasible to survey low intensity areas immediately after leaving a high intensity field, C. The phosphor is strong and will not shatter from thermal or mechanical shock. With a given phosphor size the photomultiplier tube is limited to the range over which it will operate without saturation, There- fore, two phosphors and photomultiplier tubes were used to extend the total range, The low radiation level detector responds from 0,005 mr/hr to 100 mr/hr and the high radiation level detector responds from 10 mr/hr to 200 r/hro In a single flight line, the radiation intensity change may Span several decades, The D.C, amplifier was designed with a logarithmic response to present a large dynamic range on a single scale. The logarithmic response of this instrument is achieved by using the plate current-grid current characteristic of the D.C. amplifier tubes, 1, The two detectors have an overall range, IlaVine, H. D., “Logarithmic D.C. Ratemeters for Scintillation Counters”, Nucleonics, - Feb, 1954, pp 36=39 - 5 «=