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Thermal neutron capture cross sections of 403 ± 8 b. for Pu238, 18.6 ± 0.8 b. for Pu242, and 73.6 ± 1.8 b. for Am243 have been measured by the activation method assuming 36.4 b. for the thermal neutron cross section of Co59. Samples of plutonium (90% Pu242 – 10% Pu238) were irradiated for 1 to 5 months in the NRX reactor. The growth of Pu239, Am243 (the daughter of the 5 hr. Pu243), and Cm244 (the daughter of the 25 min. Am244) was determined by mass analysis and by alpha spectrum analysis and total alpha counting. Resonance capture integrals (from 0.5 ev. to ∞) of 3260 ± 280 b. for Pu238, 1275 ± 30 b. for Pu242, and 2290 ± 50 b. for Am243 were found using cobalt and U238 as standards. Values of 48.6 b. and 282 b. were taken as the resonance integrals of Co59 and U238 respectively.The fission cross section of Pu242 was zero within experimental error.

Using the 4 r o -y coincidence method the self-absorption of 47rB counting sources prepdred by a number of source techniques has been n~easured for the negatron emitters Csl34 (86-kev branch only), Nb", MgZ0', CoGO, ScdG, RbsG, A L I~~~, Nazi, and the positron branch of CUG'. The self-absorption varied from a few tenths of one per cent to about ten per cent for sources with a mean supcrfcial density of 2 ~g / c r n~ for most of the n~~clides st~~cliecl. Subsidiary experiments were carried out to indicate the variation of self-absorption with source superficial density.'The comparative simplicity of l a p counting has lecl to its general use for absolute disintegration rate measurements. I t is \\?ell known that the chief corrections are those required for absorption in the source and mount. The latter has been extel~sively discussed (1, 2, 3) and is small for all but the softest beta emitters if thin source ~llounts :Ire used. Self-absorption cannot be so readily determined, and this is the principal uncertainty in -lap counting. I-Ialvkings ct al. (-1) progressively diluted a stock solution, and observed the activity (expressed in terms of the original solution) as a functioli of the dilution factor. .9t sufficiently high clilutions a constant activity was observecl, ~vhich was taken to indicate that self-absorption had been eliminated. 'This procedure has been criticized (3) on the grouncls that diluting a stock solution may not decrease the size of the crystals forming a11 active cleposit but merely increase the distance between them. Pate and Yaffe (5) have clescribed a distillation technique for preparing unifori~~ly thin sources ancl have used this ~uethod to set up self-absorption curves for several isotopes (6). Seliger and Sch\vebel (7) have estimated the self-absorptiol~ of a rlumber of isotopes from electron ~nicroscope lueasurements of the crystal sizes. The lap-y coincidence methocl has been used by Gunnink ct al. (8) to study self-absorptio~l in thin sources prepared by conventio~~al techniques. Assumi~~g that the observed inefficiencies are entirely due to self-absorption they have obtained a curve of per cent loss in 4a counting against maximum beta particle energy. These authors are careful to poi11t out that these losses are "best" averages, and that the value for any one source may deviate considerably from the n~ean.In this laboratory a lap-y coincide~~ce counting system consisting of a l a p proportional counter sancl\viched between two 3 in. X 3 in. sodium iodide crystals (9) has been used to ii~vestigate the relative merits of various source preparation techniques. The results of six such techniques appliecl to sevel-a1 isotopes of widely differing maxiinurn beta particle energy are reported here. 13XPERI MENTALIn their simplest fornz the ecluations relating the observed counting rates in a coinciclence determination to the absolute disintegration rate, No, are:'.\Ianuscr~pt rccecvcd AIarclz 11, 1!15!1.

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