Gaseous Detector Laboratory

Thermal Neutron Detection with 3He

The Nuclear Reaction:

Thermal neutrons convert in 3He through this reaction:
3He + n --> 3H + p + 764 keV.

The reaction products, a 191 keV triton and a 573 keV proton, are emitted in opposite directions. The ionization centroid of the two particle tracks is displaced from the neutron interaction position because the proton is more heavily ionizing than the triton, and also has a larger range. This displaced centroid is measured by the position-sensitive detector.

Position Resolution Limit

When projected in one dimension, the loci of centroids from many events describe a rectangular distribution whose width is equal to the diameter of the sphere. To a good approximation, the FWHM position resolution is 80% of the proton range, which varies inversely with the gas density. To achieve resolution in the millimeter range, an additional gas is needed to provide stopping power for the proton and triton. Propane is chosen as the optimum additive for these proportional chambers because of its high stopping power and low sensitivity to gamma radiation.

Detection Efficiency

For a thermal neutron chamber with a gas depth of 1.5 cm, the detection efficiency over a range of operating gas pressures is shown to the right. Very high detection efficiencies can be achieved for cold neutrons, around 9, with just 1 to 2 atm. of 3He. As wavelength decreases, efficiency falls, but even at 1 an efficiency of about 50% can be achieved with 6 atm. of 3He. The detection efficiency of this class of detector is better than, or as good as, that of other detecting media.

Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST