same gamma-emitting isotopes. The conversion of total gamma counts to curies requires a knowledge of the energy spectra observed by the counter, an identification of the radionuclides present, and a knowledge of the nuclear leve] schemes of each isotope. In many cases, the rapid determination of radioisotopes in the field may be performed by measuring the energy of the gamma photons emitted from specimens. The gamma nls energy spectrum, combined with a simple chemical group separation or ion exchange aaimen ean = natal energy range may be used to compare the activity of similar samples containing the separation, often is sufficient to quantitatively identify a mixture of fission products. sea compared to the 150 cpm background experienced in a concrete building at La Jolla, California. , Gammaenergy spectrum studies were made with a Harshaw 1°/ inch by 2 inch Nal (T1) well-type scintillation crystal and Dumont 6292 photomultiplier tube, mounted. together with its cathode follower ina 24-inch thick lead shield and feeding into a commercial, automatic, single-channel, stepwise type of pulse height analyzer; and the information was read on a decade scaler, and a digital printer. The counters and associated electronic equipment operated on the ship’s 110 volt ac power supply through isolation transformers. No difficulty was encountered with variations of supply voltage. The instruments were mounted in an air-cooled counting room separated from the main laboratory. 1.5.3 Instrument Calibration. The beta counters were calibrated with a commercial UX, beta source. The calibration was also checked with known amounts of potassium carbonate in order to correct for self absorption and geometry variations among different samples of salts or dried tissues. Self absorption curves were run with aliquots of a standardized Sr™ solution added to simulated samples. tetahachambe amie ne de The calibration of the gross gammacounter involved considerable difficulties. Originally a Co™ calibrated commercial standard was used. The Co® source was sub- sequently discovered to be inerror. A pair of Co®? sources, from the same company, calibrated in the same units, were found to differ by a factor of 2. Secondary standards of Zn®, cs¥*, ce, Ru€, and Co® and K,CO, were prepared and calibrated by gamma energy spectrum analysis using a calibrated Ra”6 (+ decay daughters) standard. A subsequent check against a set of calibrated gamma sources showed an absolute standardization of less than 10 percent error. The calibration of the gross gamma count for energies between 0.075 and 1.5 Mev was valid only when measuring isotopes with energy and nuclear level scheme similar to the standard. Zn®§, Co®, and Cs¥" were used as standards when determining the 18 on ae 6 eretreeesettee. mele aur scaler. The gamma background count varied from 95 to 110 cpm aboard this ship at MR mo Gammaradiation was detected and counted with a Harshaw 114-inch diameter Nal (T1) scintillation crystal, shielded by lead and plastic, an RCA 5819 photomultiplier tube, and a preamplifier. Both beta and gamma counters were connected to a decade ot manele. mattis Reape Gelman Chet eteek both beta and gamma radiation. Differential gamma pulse height analyses were run on those samples that were sufficiently active to produce significant results. An end-window Geiger-Miiller tube, with a mica window of 1.4 mg/cm? thickness, was used to count the beta particles. The counting tube was shielded by 2 inches of lead to reduce the background due to cosmic and external radiation to about 25 cpm. The sample planchets were mounted in a plastic and aluminum shield to reduce the effect of scattering and bremsstrahlung. ch ee ot Niteiadectip Sat \ 1.5.2 Instrument Types. Radiation detection and assay equipment was selected for rapid estimates of radioactive substances on board ship. Gross counts were made for