MEASUREMENTS OF AIRBORNE RADIONUCLIDES 199 that occur in the atmosphere. Knowing the precise relative concentrations of a group of radionuclides is of particular value since this knowledge allows the relative deposition and circulation rates of each radionuclide to be followed. The accurate measurements of a large group of the airborne radionuclides in a sample require extremely Sensitive and selective analytical techniques that are often prohibitive both in cost and time. Measurements of the physical and chemical states of airborne radionuclides require fractionation into the desired species and add to the required analytical sensitivity. Studies of the type mentioned and others dealing with the characteristics and behavior of airborne radionuclides have become practical through our development of a highly sensitive and selective multidimensional gamma-ray spectrometer.! This instrument uses the cascade gamma-ray decay characteristics of each radionuclide for its identification and measurement and employs anticoincidence techniques for background and Compton suppression. The instrument permits an air-filter sample to be measured directly for Be, 7*Na, “Mn, °Co, >7r— Nb, By, Ru, @sb, cs, 8%cs, and “4Ce. Measurements of these 12 radionuclides after physical- and chemical-fractionation procedures are also possible. The use of these advance techniques plus conventional means have been applied in tropospheric- and stratospheric-distribution studies and in physical- and chemicalcharacteristics determinations. MULTIDIMENSIONAL SPECTROMETER The multidimensional spectrometer is described in detail elsewhere, ! but its basic operational characteristics and counting efficiency are covered here. The spectrometry system consists of a Nuclear Data 4096 channel analyzer used in a 64- by 64-channel grouping arrangement in conjunction with two anticoincidence shielded 4~in,-thick 6-in.-diameter NalI(Tl) detectors (see Fig. 1). One of these detectors has a 4-in. light pipe of pure sodium iodide which serves to shield the detector from gamma radiation originating in its phototube. The anticoincidence annulus is a 11'/, in.-diameter 12-in.-long Nal(T1) crystal with a 6'/,-in.-diameter center hole to accommodate the principal detectors. The analyzer is equipped with an Optikon photographic printer that permits a readout of the entire memory in about 4 min. A Tally Register Corporation paper-tape unit can be used as a readout for computer calculations. The sample to be measured is sandwiched between the two 6-in.-diameter detectors. The signals from the two detectors are fed to the two separate analog-to-digital converters of the analyzer for energy and coincidence analysis and from thereto the memory storage unit. When a single photon interacts with one of the