= ee ee tl, Lanett me Nace Sey + nll nent NinaaEDM Sn matt Chapter 2 PROCEDURE 2.1 INSTRUNENTAT ICN A lew-sensitivity systea was usod for moesuring hightinitial- and high-residual-gasca radiation rates. For maasuring lowr residual~gama rediation ratos from fallout, th inatruientation waa of high seiusitivity. (See Appendix for circuit theory of tha instrumntation.) 2.1.1 Low-Sensitivity System. The detecting element was a stilbene crystal mounted in a graphite block for clactron equilibrium. This crys- tal asserbly was installed inside a hlast-resistant housing at the top of alight pipe. The crystal cutput passed through the light pipe and was cetacted by a 1F21 phcotonultiplier tube. This tube was used in a 100 percent feed~beck circuit, which held the anode current nearly constant (regardless of the incident light) by reducing the photomultipller-tube dynode voltage. The gain of the tube was approximately a direct function of the antilog of the dynode voltage. In this mamner, a useful dynamic range of about 10 was realized. Tw different recording systems wero ved to take edvantage of the wide dynante rangs of tha instrecant. Insitial-ge-ma rates from 10° r/br to 10° x/hr wore rocosvied by a high-spsed recorder, which was capable of rosolving to 0.01 c2cond with a recording tina to 20 minutes. This recordur, wonufcotured by tha Sanborn Cocnany, Cavoridge, Massachusetts, uced heat-sensitiva paper and @ hot-wire stylus. It was found to be will suited to recording under the adverra conditieng encountorsd during nae clear testa, since it vas virtunlly free of the troubles that may arise with ink and photoprephic recorders. Initiol-pacna rates fron 0.20 r/hr to lof r/ar with low resolution over a long-tins period were recorded by a Bristol recording millivoltmeter. This recorder used stoked paper, hed a minning tine of about 24 hours, and a tine resoluticn that was proportional to tho running tire (1 rinute at tha start acd ntinutes at the end of 24 hours). A double station, consictins of two detectora, one with a lou- cid ong with a hipghtresolution recording sysiom, covered tha rarge from 0.2 rfhr to 10? r/fhr and reescded unsful data from 0,01 cecond to 24 hours. 2.).2 HieheS nalsivity S.sten, The detecting olecent was en anthras scene eryst\l countedinaEreposeablock for electron ecnilitrsum end ate teched to tha wiadow of a 5219 photemultiplior tube. This tubs, dike the U2l mentioned in Section 2.1.1, wa3 ured dn a l0O-percent fecicback elrevit, and the ancda current was kept virtually constant. Its gain was approximately a direct function of the antilog of the dynoda voltage. 4 SECRET ee a meee tee ee me ee Sem merece car emcee ee me ae oe