43: JOURNAL OF THE A0.a.c. (Vol. 42, No. 2, 1959) changer and print-out which recorded the time necessary to accumulate 600 counts from each sample. The Geiger tube sensing element was a Tracerlab TGC 14 carbon-counter with essentially 100% response to any beta particles entering, but of low response to gamma photons. This tube was provided with a thin, aluminized mylar window (09 mg/cm’) and the internal atmosphere was maintained with a mixture of 99% helium and 1% isobutane (Geiger gas), flowing at the rate of one bubble per second. Because of the small sensitive volume and additional shielding within the lead 31%. In expressing all readings on ash samples in terms of the potassium standard, counts per pig, the background response of the tube aver- aged 11 counts per minute (c/m). All ash samples were measured in 1” stainless steel planchettes at fixed geometry. One hundred mg of ash was used for each determination and special care was taken in adjusting the thickness and surface uniformity of the sample. One standard and two empty (background) planchettes spaced 180° apart were carried with each revolution of the 25 position sample changer. Depending on the amount of radioactivity present, the time for one revolution of the turntable varied from 12 to 24 hours. 3. Standard—-The reference standard was potassium chloride. According to Nier (1) potassium contains 0.011% of K*, the naturally occurring radioactive isotope, and so far as can be ascertained, this species is uniformly distributed in nature. Suttle and Libby (2) have determined the absolute numbers of beta and gamma emissions to be, respectively, 29.6 dis- integrations and 2.96 disintegrations per gram of the metal. Because of the low counting efficiency of the Geiger tube for gamma photons, this component was ignored, and the standard was applied as a pure beta standard. One hundred milligrams of dried and finely powdered reagent grade KCI was used as the working standard. This amount, deposited in a 1” stainless steel planchette, reproduced the geometry of all ash samples very closely. The mass absorption error of 100 mg of salt was of the order of 5%. The density of most ash samples closely approached that of KCl; hence, mass absorption error of the ash was assumed to be of the same order. For ash weights greater or less than 100 mg, corresponding weights of KCi standards were applied. As derived from 29.6 disintegrations/second/g of potassium, 100 mg of KCl produces 93.13 disintegrations/min. A number of determinations of 100 mg samples of KCl has established that the Geiger tube “sees,” on the average, 289 counts/min. This is an overall efficiency of min. (¢/m) have been converted to disintegra- tions per min. (d/m) by multiplying c/m by an average factor of 3.22. Potassium Analyses All ash samples were analyzed for potassium (flame photometer) with an accuracy of +5%. Milligrams potassium per gram of original sample were converted to disintegrations per minute per gram by multiplying by 1.776. This is the factor derived from the data published by Suttle and Libby (2). Net Radioactivity This was derived by subtracting from the total radioactivity the contribution due to the presence of potassium. On the average the net value approached zero except in those cases where other radioactive substances or fission products were present. Wherever the term “total beta radioactivity” has been used sub- sequently, a net or potassium corrected value is meant. Errors 1, Mass absorption—From experiments with a variety of ash samples wherein radioactivity as read was plotted against varying weights of sample, it was determined that mass absorption at 100 mg was of the order of 5%. However, since a 100 mg KCI standard also exhibited mass absorption of the same order, these errors were considered to cancel out. Hence with strictly fixed geometries, no corrections for mass absorption were applied. 2, Counting—Theoretically, the probable counting error for 600 counts, regardless of the time necessary to accumulate this number, is =~ 3.2%. However, an error of this low order obtains only when the ratio of total count to background is 10 to 1 or better. Such a favorable situation rarely occurred because most of the samples were low in radioactive content. Generally the ratios varied from 1:1 to 4:1. Under these conditions the probable error was much greater, and at times as great as 20%. 8. Net radioactivity-——Errors in measuring net radioactivity are not directly determinable, but it is clear that they will be significantly influenced by the errors of the analvtical operations, viz., (@) flame photometric determina- tion of potassium, and (6) total radioactivity. In extreme instances the errors may be additive and a range of error of = 25% is possible.