64 SAMPLING AND RESEARCH PROGRAM WORLDWIDE EFFECTS OF ATOMIC WEAPONS meters, and J ts the integral in Eq. (1). (For an anisotropic source the ' 80 integral J is deleted:) The acttvity from the combined Sr’ and Y® in a sample of thickness T is given by __ Ts, 0.693T 0.693T Aux =Aaloges]! exp(—2O°7) 410277) ages|' ee (- oe) +1(PT) / @) If the yttrium were separated from this same sample with a small amount of carrier, tts activity would be 65 Ay (separated ¥) 60 = > °o * 2 Ag, y(Sr+Y) 40 3 ira ‘s 20 Ay == AT. Figure 6 shows a plot of these two functions using 14 mg/cm? and 134 mg/cm’ as the half-thicknesses for Sr’ and Y°’, respectively. The intersection of the two curves occurs at 90 mg/cm’. Thus, with a given sample of material to be analyzed and a particular counting arrangement, one could decide whether to simply isolate strontium and count Sr”’ and Y“° together or to separate and count the Y° alone. If the sample thickness would be much greater than 38 mg/cm’ when both were together, then it would be advantageous to separate the yttrium, For very thin samples of, say 20 mg/cm? or less, strontium and yttrium should be counted together. In the intermediate range of 20 to 55 mg/cm’ tt would make little difference as far as counting rate were concerned and the choice would be a matter of convenience. The principal task in measurement of extremely small amounts of radio-. activity remaining after one has supplied a sufficiently sensitive counter is the reduction of the extraneous background rate due to cosmic radiation TacOncTVeexist in all laboratories in such amounts as to give very appreciable countrates. The latter component of the background is best removed by selecting solid material that is free of uranium and thorium and of their disinte- gration products such as radon. Consideration of this problem, together 0 0 20 40 Thickness 60 (mg/cm?) 80 100 Fig. 6—Relationship between the activity and sample thickness for Sr°° and Y** in equilibrium and for separated Y*° with the problem of cost, has led to the belief that stee! should be an excellent material for a low-level counter shield, The method used to eliminate the cosmic-ray component of the background consists in surrounding the counter and sample with a complete layer of Geiger counters that are in tangential contact. With these “anti- coincidence counters in place the background is reduced to a very small value. It can be reduced further by incorporating a device developed by Kulp™ who introduced a 1.5-in.-thick shield of distilled mercury between the anti-coincidence shielding counters and the central measuring counter. FlRT ER Se two counts per minute. The apparatus as described involves no extraordinarily complicated parts. In particular, since the electronics is very simple, few difficulties develop during operation. Occasionally oue of the vacuum tubes in the