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.

Previous work showed that the 3 He(n,tp) reaction in a cell of 3 He at atmospheric pressure generated tens of far-ultraviolet photons per reacted neutron. Here we report amplification of that signal by factors of 1000 and more when noble gases are added to the cell. Calibrated filter-detector measurements show that this large signal is due to noble-gas excimer emissions, and that the nuclear reaction energy is converted to far-ultraviolet radiation with efficiencies of up to 30%. The results have been placed on an absolute scale through calibrations at the NIST SURF III synchrotron. They suggest possibilities for highefficiency neutron detectors as an alternative to existing proportional counters.The 3 He(n,tp) process, in which a neutron reacts with a helion to produce a proton and a triton with excess energy of 764 keV, is one of the best-characterized neutron reactions [1]. This reaction is the trigger mechanism for the 3 He proportional counter, which is presently one of the most widely-used neutron detectors [2]. Here we discuss using the same trigger reaction to initiate far-ultraviolet (FUV) optical emissions, rather than electrical discharges. We have found cases in which about one-third of the nuclear reaction energy is converted to FUV emissions by noble-gas excimer molecules. 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 . We measured FUV emissions in the cell with a filter-detector package that was calibrated at the NIST SURF III Synchrotron Ultraviolet Radiation Facility. 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. 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...

In the course of investigations of thermal neutron detection based on mixtures of 10 BF 3 with other gases, knowledge was required of the photoabsorption cross sections of 10 BF 3 for wavelengths between 135 and 205 nm. Large discrepancies in the values reported in existing literature led to the absolute measurements reported in this communication. The measurements were made at the SURF III synchrotron radiation facility at the National Institute of Standards and Technology. The measured absorption cross sections vary from 10 −20 cm 2 at 135 nm to less than 10 −21 cm 2 in the region from 165 to 205 nm. Three previously unreported absorption features with resolvable structure were found in the regions 135 to 145 nm, 150 to 165 nm and 190 to 205 nm. Quantum mechanical calculations, using the TD-B3LYP/aug-cc-pVDZ variant of time-dependent density functional theory implemented in Gaussian 09, suggest that the observed absorption features arise from symmetry-changing adiabatic transitions.

First principles resonance widths for Li near an Al(001) surface: Predictions of scattered ion neutralization probabilities

The observation of Lyman alpha (Lα) radiation produced by the end products of the 3He (n,tp) reaction has suggested the possibility of a new method of cold thermal neutron detection. In order for this goal to be achieved, a basic understanding of how the Lα radiation is formed and how it may be detected, is needed. The model study described here is an attempt to provide this basic understanding and to provide quantitative results that can be used in designing future experiments.