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The well-known interaction of low-energy neutrons and 3He produces a proton and a triton with 764 keV of kinetic energy shared between the two particles. For many years, this reaction has been used in neutron detectors by sensing electric discharges initiated by the energetic particles. The Lyman-Alpha Neutron Detector (LAND) uses the characteristic hydrogen emission at 121.6 nm (10.2 eV) from the proton and triton as the detection signal. Recently, the addition of noble gas buffer gases has been shown to increase the far ultraviolet signal by a factor of up to 1000 or more, probably due to the formation and subsequent radiative dissociation of rare-gas excimers.
The 3He(n,tp) process, in which a low-energy neutron reacts with a helium nucleus to produce a proton and a triton with excess energy of 764 keV, is one of the best-characterized neutron reactions. It is the trigger mechanism of the 3He proportional counter, which is presently one of the most widely-used neutron detectors. The Lyman-α Neutron Detector (LAND) is an alternative neutron detector which uses this same trigger reaction to initiate far-ultraviolet optical emissions within a gas cell, rather than electrical discharges. In a gas cell containing mixtures of 3He and 4He, which does not significantly interact with the neutrons, we found that, at atmospheric pressure, tens of far-ultraviolet photons were produced per reacted neutron. If the 4He is replaced with a noble gas, for example Ar, Kr or Xe, we find far-ultraviolet emissions that are significantly stronger than those found in all-helium mixtures, in some cases by factors exceeding 1000. Spectrally-resolved measurements using filter detector packages suggest that this radiation is predominantly due to X2* excited dimer (excimer) emissions.
A LAND instrument will require much less 3He than current proportional counters. Since 3He has become extremely expensive and difficult to obtain, reducing the amount of 3He required to effectively detect low-energy neutrons could result in a larger supply of less expensive neutron detectors to be used for national security - for example port-of-entry screening for clandestine nuclear material and the detection and monitoring of nuclear programs for non-proliferation purposes - oil-well logging, neutron diffraction and scattering experiments, and other applications.
R&D 100 Award
DoC Bronze Medal
Lyman alpha radiation detector, shown here in prototype, could dramatically improve performance in key applications.
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