HILE some of the hitherto published results 1-3 on the capture/beta-branching ratio of K 40 indicate a value of the order of 2, others, deduced from determinations of the argon content in old potassium minerals, seem to yield a value 4-6 smaller than 0.10.In view of the geophysical significance of this branching ratio, 7 -8 it seemed to be of interest to obtain further evidence on its magnitude. The aim of the present letter is to report the preliminary result of experiments started here to that purpose about a year ago.The intensity of the x-rays accompanying electron capture was measured by using a method of selective detection. An internalsource G-M counter was filled alternately with two gases, one possessing low, the other high efficiency for the K a x-rays of argon (4.18A). If the counter response to particles and to gammarays be assumed to be the same for the two fillings, the difference of the counting rates obtained with them should be proportional to the intensity of the x-rays emitted by the source. The proportionality factor could be calculated fairly accurately by numerical integration over the entire counting volume. As filling gases argon and krypton were used at a total pressure of 20 cm of Hg each, together with ethyl alcohol at 1.5 cm. The absorption coefficient of krypton for x-rays of 4.18A, computed with the help of the tables of Jonsson, 9 turned out to be even higher than that of xenon. Repeated measurements made with the internal source covered with a 2.3-mg/cm 2 Al foil (to absorb the x-rays), and with external pure beta-sources, showed that for the same overvoltage the counting rates were identical within less than 0.2 percent. The computed mean values of the x-ray efficiency were 8 and 69 percent, respectively (counter diameter 1.34 cm, wire length 7.4 cm).A layer of KF of about optimum thickness (3.44 mg/cm 2 ) was obtained by evaporation in vacuum. 10 KF being hygroscopic precautions had to be taken to keep the layer dry. The active surface was of 5.92X0.49 cm 2 on each of the six Al holders which, placed side by side, formed the greatest part of the counter wall. This inside source was covered with a graphite layer of 0.5 mg/cm 2 , which absorbed about eight percent of the x-rays, but appeared at the same time to improve greatly the stability of the counter operation.Computation yielded a difference of 0.87 count/min., when assuming a branching ratio equal to one. This represents about two percent of the counting rate with argon filling (47 counts/ min.). Actually, in three long runs, differences of 0.66±0.16, 0.73=b0.13, and 0.43db0.11 count/min. were obtained, with the indicated standard deviations. Their mean value corresponds to a branching ratio of 0.67±0.20, which figure also accounts for electron capture from the Li shell. 10 The limits given include no systematic errors. Owing to the higher counter voltage with krypton, small spurious effects, if present, would tend to increase the observed rate difference. Therefore, the value of 0.67 should rather be considered as an uppe...
