Pulse-Height Measurements of Recoils fromB10(n, α)Li7

Abstract

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.

High-resolution, vacuum-ultraviolet absorption spectrum of boron trifluoride

“…Boron-10 trifluouride, 10 BF 3 , was first used in a neutron detector by Amaldi et al, 1,2 and for many years, until the availability of 3 He, it was commonly used in gaseous proportional counters to detect thermal neutrons. The advantage of 10 BF 3 as a neutron detection medium is based on its relatively large thermal neutron capture cross-section, σ th = 3840 b (3.84 × 10 −21 cm 2 ), for the reactions: 10 B + n → 7 Li + α + 2.792 MeV,…”

“…3,4 It is followed by prompt gamma emission according to Eq. (3) In traditional BF 3 proportional counters, the energetic products of the reactions in Eqs. ( 1) and (2) precipitate a cascade of gas ionization in applied electric fields of approximately 100 kV/m.…”

“…The charged particles are then collected as a current pulse. In contrast, we aim to build a detector similar to that described in Hughes et al, 5 in which 3 He was mixed with various noble gases and irradiated with cold neutrons to produce the reaction:…”

“…(2) describes the major reaction branch with a branching ratio of 94%. 3,4 It is followed by prompt gamma emission according to Eq. (3) In traditional BF 3 proportional counters, the energetic products of the reactions in Eqs.…”