millimeter twist drill gives a hole of just the right size. For our Varian A60 probe (Varian V6031 modified according to V6057 using a ruggedized dewar insert) the weight required to prevent lifting is 27 grams, and the actual weight of the cylinder is 30 grams. For short sample tubes, such as the sealed standard reference samples (Varian 943346), the cylinder shown in Figure 1b is preferred. A weighting cylinder made out of antimonial lead was not
satisfactory. An electrical current is induced in the metal cylinder as it spins in the magnetic field. The extra work required for this induction slows down the turbine. More serious are the side bands which result in the spectrum, perhaps because the induced circulating current affects the homogeneity of the magnetic field. RECEIVED for review April 26, 1968. Accepted May 21, 1968.
Modified Scintillation Counting Technique for Determination of Low Level Krypton-85 N. I. Sax, J. D. Denny, and R. R. Reeves RadiologicalSciences Laboratory, Dicision of Laboratories andResearch, New York State Department of Health, Albany, N . Y. 12201 A VARIETY OF TECHNIQUES are available for determining 85Kr in air samples (1-4). The most successful of these use vacuumcryogenic techniques for concentrating Kr to bring the specific activity of the sample with respect to 85Kr up to a point where reasonably sized samples of air are sufficient to supply, under normal condilions, enough 85Krto count, at a high confidence level in a time interval such as 100 minutes. Because planetary concentration of Kr in dry air is accepted as 1.14 ppm, a maximum of 1.14 ml of Kr can be recovered from 1 cubic meter of air sample. However, methods of counting the sample vary widely in efficiency. Proportional counting has many drawbacks as do methods involving ionization-chamber and vibrating-reed measurements. Krypton has been counted satisfactorily when dissolved in organic liquid scintillator media via liquid scintillation spectrometry ( I ) . In this paper, a modified scintillation counting technique has been developed which is sensitive, convenient, and economical for routine atmospheric 85Kr measurements. It uses inexpensive, plastic-scintillator shavings contained within a gas-tight vial which can be filled with Kr-containing gas and counted efficiently by scintillation spectrometry. EXPERIMENTAL Apparatus and Reagents. Gas-sample vials were made from 12-mm borosilicate glass tubing approximately 40 mm long, sealed on both ends with inserted neoprene septums. The vials, which contained 20-40 mesh scintillating plastic shavings (an inexpensive by-product of Pilot Chemicals, Inc., Watertown, Mass.), were evacuated through a hypodermic needle and then filled to a known pressure (-600 mm Hg) with the sample. The void volume of each vial was predetermined so that the volume of the Kr sample to be counted is readily calculated. Pure krypton, reagent grade as supplied by Matheson Company, was used for each background counting and dilution of standard samples without further purification. Procedure. The counting efficiency was determined by adding a known amount of 8jKr to a sample vial. For this purpose krypton of known activity of 85Kr was obtained
(1) M. L. Curtis, S. L. Ness, and L. I. Betz, ANAL.CHEM.,38, 636 (1966). (2) D. L. Horrocks and M. H. Studier, ibid., 36, 2077 (1964). (3) M. B. Reynolds, Nucleonics, 13, 54 (1955). (4) K. F. Wylie, G . R. Hager, and R. W. Ostinson, Mound Research Laboratory Progress Report MLM-1247, March 1965.
(NBS Standard No. 204-10/9/62 = 60.8 X lo6 dps/grammole) and diluted with the reagent grade pure krypton. The volume of the standard was measured and transferred to a one-liter reservoir to which a known amount of the reagent grade krypton was added. Measured volumes of this secondary standard were then added to the vials containing scintillator and the counting rate was compared with the known disintegration rate. The counting efficiency was found to be 94.4% with an average deviation of less than 0.7%. These were measured using several different ampoules of standard. Because little variation was found in counting efficiency, an average value of 94 % was generally used. RESULTS AND DISCUSSION
The gas-phase-solid scintillator spectrum observed by means of a liquid scintillation spectrometer displayed the shape characteristic for a beta emitter. Little variation was noted with changes in mesh size of scintillator. The average counting efficiency of 94% was found to be constant over various filling pressures. The fully prepared sample is stable as evidenced by repeated recounts at 400- and 500-minute intervals remaining within statistical limits (Table I). Some diffusion of Kr occurs into but this appears not to affect the the scintillator plastic (3, counting rate. When large amounts of 85Kr were charged into the vessel for calibration purposes, the vials could not be quickly evacuated and reused for low-level 85Krmeasurements. Over 99% of the Kr is pumped away almost immediately and low-level samples can be run relatively rapidly in consecutive order in the same vial if desired. In order to apply this technique to atmospheric surveillance, it was necessary to achieve a counting sensitivity capable of resolving background levels of 85Krin the atmosphere which on a planetary basis has reached about 10 pCi/m3 of air. The concentration of Kr in air is about 1.14 ppm. Krypton-85 content of pure, concentrated Kr is about 9 pCi/ml (STP) of Kr. This procedure allows precise determinations to be made on
