Book excerpt: The response to various neutron spectra of a 2-inch diameter BF3 proportional counter in a Halpern-type geometry has been studied as a function of moderator thickness. Neutron sources used were a calibrated RaDBe(a,n) source and a series of photoneutron sources using a number of bremsstrahlung spectra. The reactions used were: 16PO(gamma,n), 31P(gamma,n) and d(gamma,n). For these sources absolute neutron yields were determined either from the residual B+ activity or from the known cross section and the absolute bremsstrahlung intensity. Using these empirical data a final detector with 13 BF3 counters was constructed. Over the spectral range covered by the above sources the measured efficiency for counting neutrons of the final detector was 9.6 percent with an estimated uncertainty of plus or minus 0.5 percent, independent of spectrum. The detector was used to measure the neutron yields from Pb, Au, Ta, Ho, Ag, Cu, Co, Ca, P, Al, O and C for a series of bremsstrahlung energies between 12.0 and 29.0 MeV. These yields were compared with those calculated from previously published neutron yield cross sections. The data indicate that previously reported discrepancies in neutron yield data can probably be traced to the determination of neutron detector efficiencies. (Author).