GAMMA RADIATION FROM DEPOSITED FISSION PRODUCTS 235 SPECTROMETRIC TECHNIQUES Figure 1 shows a typical field measurement in progress. The site chosenis usually a large flat open space situated in or near a populated area, A 5- by 3-in. cylindrical NalI(T1) detector is placed 1 m above- z a Tee sy ae 3 AE 0mtitsal not a +g Boeae PEEsecs cancl gaia600 ON, | ne Fig. 1—-HASL pressurized ionization chamber and spectrometer set up for atypical measurement. Saratoga Springs, N. Y., 1963. ground facing downward through a hole in a small wooden platform. A 50-ft coaxial cable connects the detector to a multichannel pulse-height analyzer mounted in the HASL vehicle. Readout is by both typewriter and paper-tape punch. Power is supplied by a 300-watt 12-volt d-c—115volt a-c rotary inverter operating off the car battery. A 20-minlive time for each reading has been found to provide sufficiently detailed spectra. A high-pressure ionization chamber is placed nearby. This chamber contains 8 liters of argon at 40atm and is carefully calibrated against standard radium sources and known cosmic-ray fields.’ A battery-operated vibrating reed electrometer is mounted directly on the chamber head, thus making the instrument completely self-contained. Two typical gamma spectra are shown in Fig. 2. All field spectra obtained during 1962 and 1963 show the same four conspicuoustotal-