Cerenkov Counting of 40K in KC1 Using a Liquid Scintillation An Undergraduate Experiment B. P. Pullen Southeastern Louisiana University, Hammond, LA 70402
Nuclear counting techniques have usually been excluded from most undergraduate laboratory courses for a number of reasons. A Radioactive Material License is required for use of radioactive material. Radiation exposure records are usuallv reauired for each individual. which is a laree in " exoense itself. Records of radiation contamination, surveys, radiation levels. and inventories are iust a few of the other safety requirements which must he met in the license. andl ling radioactive chemicals usuallv involves soecial clothinp, .safety precautions, and radiation safety monitoring equipment. Consideriue the fadors oresented above, an experiment for determining the counting efficiency of naturaily occurring radioactive 4oK in potassium chloride ("dietary salt suhstitute") using Cerenkov counting with a Beckman LS lOOC liquid scintillation spectrometer has been developed. Since O ' K has a very long half-life, the same salt can he used indefinitely. For example, a 0.4226-g sample of KC1 contains ' K producing 411 transformations per 3.93 x 1017atoms of O minute. Although Cerenkov counters have been used extensively in various fields of high-energy physics for many years, the use of the Cerenkov light in aqueous solutions containing energetic @-emittersas a means of measuring radioactivity has achieved importance onlv in recent years.l Cerenkov ra&ation is due to the movement of charged particles in a medium with a velocity greater than that of light in the medium. In water the minimum electron energy for Cerenkov effect is 0.260 MeV. The number of photons, dN. nroduced in the snectral ranee between XI and A7 alone the'path dl by the ~ e i e n k o veffect is given2 b;
transparent medium. By integrating between the initial and cut-off energy, the total number, N,, of photons radiatedcan he obtained
" .
B-%,
Ross3 developed a numerical integration procedure for N. usine both Simoson and traoezoidd methods. S&ce ~ e r e n d o vradiation is a shock wave, it contains components of all frequencies with energy flux larger toward the violet region, and in water the radiation exhibits a bluish1 white aooearance to the eve. The efficiency of detection is a fuuctiohbf the beta particle energy, sampie volume, nature of the dispersive medium and the presence of wavelength shifter. Haberer4 has shown that a linear relationship exists between the sample count rate and volume in polyethylene vials. In glass vials the linear dependence is only approximate, and the presence of 40K in glass increases the background. Experimental Procedure
Small samples of from 0.1 g to 1 g of potassium chloride were weighed intu polyethylene counting vial3 to which 10 mi of water were added. These w d e s then counted wine the Beckman liauid scintillation counter. ~ifieienciesof countine from 34%to 55%&re ohtaincd. As an example of t h effirrency ~ calrulation, a 0.1226 g sample of potassium chluride contarns (0.4226 gl (39.1171.56) (1.18 X lW'140) (6.02 X 10"l = 3.93 X 1O1~atomdof"K~hrre 1.18 X I W 4 is the fractional abundance of @K.Using the equation ~~~~~~~~
~
~
~
~~
~~
~
~~
where m is the fine structure constant and is equal to e2/hc = 11137,B is the velocity of the particle relative to the velocity of light in vacuum, and n is the refractive index of the
'
Peng. C. T.; Horrocks. D. L.: Alpen. E. L. "Liquid Scintillation Counting. Recent Applications and Development"; Academic: New York. 1980: p 357. Bebher, E. H. Proc. Roy. SOC.(London) 1953, A216 90. ROSS. H. H. Anal. Chem. 1969,41, 1260. Haberer, K. Atomwirf. 1965, 10. 36.
-
where A is the decay constant (0.1046 X 10-I4 min-I), one obtains a value of 411 disintegrationsper minute for the 0.4226 g sample. 89% of the transformations in 40Kare p8.The net counts per minute obtained were 201. This gives a value of 100 X 2011411 (0.89) = 55% for the Cerenkov counting efficiency(% of disintegrations)using the liquid scintillation counter. From the efficiency,the 90Kin dietary salt substitute may be determined. Acknowledgment The author wishes t o thank the following students for their assistance: Dominic Milioto, Jaime Adams, Auroras Gondolfo, Lyle Lamhert, Marie Moniotte, Lauren Pugh, and Craig Showers.
Volume 63 Number 11 November 1966
971
