The absol ute emissio n rate of ne ut rons fr om t he National Bureau of Standa rds rad iumber yllium p hotoneutron standard source has been determined from t he volume in teg ra l of t he neu tr on absorp tion rate in wat er . Thin ind ium a nd manganese fo ils were employcd to m easure the t hermal-ne utron d ensity as a fun ction of distance fro m t he source. The foil activities were converted to neu tron d ensities by cali bration of the foils in a reference t hermalne u tr on fl ux p reviously calibrated with boron films. A m otor dri ven m ec ha nical integra t or , which m oves foils radi ally from t h e so urce a t a rate that takes into acco unt the incr ease in area wi th d istance from the so urce and t he decay of t he fo ils, provided over 80 percc nt of t he volume integ ral. A value of 1. 265 X 106 ne utrons per second was d etermincd for t he emission rate of t hc phot oneutron stand ard, wit h a n estimated standard error of 3 percent.
Thin films of boron deposited over a small area by vacuum evaporation have been exposed to the National Bureau of Standards thermal-neutron flux in pulse ionization chambers. Pulse ionization measurements in the past have yielded He 4 -to-Li 7 * pulse ratios considerably higher than the energy ratio of 1.753 from momentum conservation. Under suitable operating conditions we have obtained ratios as low as 1.778. Columnar recombination appears to cause most of this variation. The probability of the reaction B 10 (w,a)Li 7 going to the ground state was determined to be 6.52±0.05 percent.
A moderating geometry providing a constant thermal neutron flux from two radiumberyllium (a,n ) sources has been constructed. The flux at the center of the geometry has been calibrated by absolute alpha counting from boron films exposed in pulse ionization chambers. Th e boron capture cross section has b een obtained by comparison of the NBS boron with the Argonne "s tandard" boron , and a value of nv = 3918 ( ± 2 percent) n / cm 2 sec has been d etermined for t he flux .
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