Beta-Ray Spectrum ofK40

Abstract

I N a previous communication 1 the results of a spectrometer measurement on K 40 beta-rays were presented. The source was KC1, 18.5 rag/cm 2 in thickness, enriched to 1.2 percent K 40 , giving a maximum yield less than background. The end point occurred at 1.40±0.03 Mev and the apparent shape was that of an allowed type spectrum above 500 kev. Such a shape is in contradiction to the theory of forbidden beta-decay and the predictions of Marshak 2 for K 40 . Since the known spin change is 4, Marshak showed that with parity change the transition should be third-forbidden and, for either the axial vector or tensor interactions permitted in this case, the factor c = q 6 -{-7q 4 p 2 -\-7q 2 p 4 -\-p Q [where q -Wo-W, p~(W 2 -1) J , W is the energy in units of mc 2 , and Wo is the end-point energy] multiplies the usual Fermi function. The actual spectrum should therefore be distinctly different from the allowed form.During publication of this earlier result on K 40 the first of the newly discovered series of first-forbidden type beta-ray emitters, namely Y 91 , was reported by Langer and Price. 3 Making use of Y 91 mixed in various amounts of KG, Feldman and Wu 4 showed that thicknesses of 20 mg/cm 2 distorted the Y 91 spectrum into the allowed shape in the upper two-thirds range of energies. They concluded from this that the K 40 beta-ray spectrum probably did not have the allowed shape. The first clear evidence was given recently by Bell, Weaver, and Cassidy. 5 Using a scintillation spectrometer and 2.5-mg/cm 2 thick sources of KC1 enriched to 1.3 percent K 40 , they found a forbidden type spectrum agreeing with the factor, c, from 700 kev to the end point at 1.36±0.05 Mev.Improved techniques of electromagnetic separation have recently enabled C. P. Keim and co-workers at Oak Ridge to increase substantially the K 40 enrichment factor. The result is that a small quantity of potassium (in KC1) containing 7.13 percent K 40 has become available. 6 This has made possible conventional spectrometer measurements without excessive distortion due to source thickness. In the present work a source 4 cm 2 in area was deposited on 0.5-mg/cm 2 Nylon by precipitating a saturated drop of KC1 solution with acetone and rapidly evaporating the remaining liquid by hot air. Examination showed that small crystals were formed although a certain amount of clustering of crystals could not be prevented resulting in noticeable nonuniformities. The average thickness was 2.4 mg/cm 2 .To study the effects of this type and thickness of deposit, comparison runs were made on P 32 which is known to have the allowed shape. 7 In the lower curve of Fig. 1 is shown the Kurie plot for a thin source (^0.3 mg/cm 2 deposited on 0.5-mg/cm 2 Nylon) FIG. 1. Kurie plots of P 32 beta-ray spectrum showing effects of 17 percent resolution and 2.5-mg/cm 2 source thickness. THE EDITOR 629 25 j 1 20 r \ \ \ 10 r ^"v \ I I I I I I I ^^ I I O Z 4 6 .8 10 12 14 1.6 ENERGY (Mev) FIG. 2. Kurie plot of K 40 beta-ray spectrum with and without the theoretical correction factor....

Measurement of the shape-factor functions of the long-lived radionuclides 87Rb, 40K and 10Be

“…[38][39][40][41], the shapefactor function CðW Þ ¼ q 6 þ 7ðp 4 q 2 þ p 2 q 4 Þ þ p 6 estimated theoretically for the third forbidden unique transitions agrees well with the data obtained by experiment. However, a careful study of the Kurie plot linearizations reported in these references reveals a considerable deviation between theory and experiment below 500 keV.…”

“…The observed tendency is in agreement with the Kurie plot extrapolations reported in Refs. [38,39,41]. When comparing the curves with each other, the yield curve computed for E 0 ¼ 1360 keV agrees better with the values obtained by experiment when E4800 keV, whereas the curve 40 K, (c) 10 Be with the values obtained by experiment.…”

“…The shifting effect of the coefficient p 6 may be a good explanation for the energy gap detected in the Kurie plot extrapolations of the Refs. [38][39][40][41]. However, according to 1.8q 6 +7(q 4 p 2 +q 2 p 4 )+1.23p 6 1.4q 6 +7(q 4 p 2 +q 2 p 4 )+1.23p 6 q 6 +7(q 4 p 2 +q 2 p 4 )+p 6 0 Fig.…”