Observation of the 3He(n,tp) reaction by detection of far-ultraviolet radiation

Abstract

We have detected Lyman alpha radiation, 121.6 nm light produced from the n = 2 to n = 1 transition in atomic hydrogen, as a product of the 3He(n, tp) nuclear reaction occurring in a cell of 3He gas. The predominant source of this radiation appears to be decay of the 2p state of tritium produced by charge transfer and excitation collisions with the background 3He gas. Under the experimental conditions reported here we find yields of tens of Lyman alpha photons for every neutron reaction. These results suggest a method of cold neutron detection that is complementary to existing technologies that use proportional counters. In particular, this approach may provide single neutron sensitivity with wide dynamic range capability, and a class of neutron detectors that are compact and operate at relatively low voltages.

Far-ultraviolet signatures of the H3e(n,tp) reaction in noble gas mixtures

“…We found that, at a 3 He pressure of 100 kPa, tens of FUV photons are produced for every reacted neutron. This number is already encouraging as concerns the prospects for an absolute neutron detector based on FUV emissions vs. the electrical discharges that drive proportional counters, as discussed previously [11]. When mixtures of Ar, Kr or Xe are added to the 3 He cell, we found much stronger FUV signals than were observed in He alone, in some cases by factors exceeding 1000.…”

“…Such high conversion efficiencies have been encountered previously in electric discharges [3][4][5][6] , irradiation of noble gases by beams of electrons [7-9] and proton [10] bombardment of noble gases. Here they are reported for the first time in the context of a nuclear reaction and put on a quantitative basis needed to evaluate their applicability in a new FUV-based neutron detector.Following previous work [11], we started with a gas cell containing pure 3 He and 3 He/ 4 He mixtures, exposed to neutrons with a deBroglie wavelength of 0.384 ± 0.004 nm, produced at the NIST Center for Neutron Research. At this wavelength, the cross section for the reaction 3 He(n,tp) reaction is (1.135 ± 0.012) x 10 -20 cm 2 .…”

“…This package is discussed below, as are the procedures for absolute measurements of the neutron flux and modeling of the optical detection system. The combination of measurement and modeling enables us to determine the number of FUV photons produced per neutron reaction.In the work with pure 3 He and 3 He/ 4 He mixtures the source of the optical signal was determined to be Lyman α radiation from the 2p states of the 1 H and 3 H formed as neutralization and excitation products of the 3 He(n,tp) process [11,12]. We found that, at a 3 He pressure of 100 kPa, tens of FUV photons are produced for every reacted neutron.…”

“…Following previous work [11], we started with a gas cell containing pure 3 He and 3 He/ 4 He mixtures, exposed to neutrons with a deBroglie wavelength of 0.384 ± 0.004 nm, produced at the NIST Center for Neutron Research. At this wavelength, the cross section for the reaction 3 He(n,tp) reaction is (1.135 ± 0.012) x 10 -20 cm 2 .…”

“…In the work with pure 3 He and 3 He/ 4 He mixtures the source of the optical signal was determined to be Lyman α radiation from the 2p states of the 1 H and 3 H formed as neutralization and excitation products of the 3 He(n,tp) process [11,12]. We found that, at a 3 He pressure of 100 kPa, tens of FUV photons are produced for every reacted neutron.…”