Aaron J. Schmidt

Investigation of aerogel, saturated foam, and foil for thermal neutron detection

A recent shortage in ³He for neutron detection has caused an increase in research efforts for a viable replacement. Three materials, borosilicate aerogel, ⁶LiF saturated foam, and natural lithium foil, have been investigated as neutron detection materials. The materials of focus are designed in such a way that allow both reaction products from the ⁶Li(n,α)³H and ¹⁰B(n,α)⁷Li interactions to escape the absorber and enter a detector volume simultaneously. This attribute dramatically increases the neutron detection efficiency of the detector. The neutron response pulse-height spectra of borosilicate aerogel, 4.5 percent saturated ⁶LiF foam, and 10 layers of 30, 50, and 75 µm thick Li foil are presented. The theoretical neutron detection efficiencies of saturated foam and Li foils of different thicknesses are presented and compared to experimentally obtained values.

Nuclear reactor pulse calibration using a CdZnTe electro-optic radiation detector

A CdZnTe electro-optic radiation detector was used to calibrate nuclear reactor pulses. The standard configuration of the Pockels cell has collimated light passing through an optically transparent CdZnTe crystal located between crossed polarizers. The transmitted light was focused onto an IR sensitive photodiode. Calibrations of reactor pulses were performed using the CdZnTe Pockels cell by measuring the change in the photodiode current, repeated 10 times for each set of reactor pulses, set between 1.00 and 2.50 dollars in 0.50 increments of reactivity.

A simulation of a layered 6 Li foil multi-wire proportional counter

The shortage of 3He gas for neutron detection has led to alternative technologies for large-area high-efficiency neutron detectors. Recently, MCNP6-beta has been released and has the ability to track the energy of low mass particles, which now allows neutron detector pulse-height spectra to be simulated. A Monte-Carlo code generated in MatLab was used to validate the MCNP6-beta neutron response pulse-height spectra of a 6Li foil multi-wire proportional counter. After validation, MCNP6 was used to optimize HDPE thicknesses in front and back of the detector against a 252Cf neutron source. Additionally, the pulse-height spectra was obtained using MCNP6 for detector pressures of 1.0, 1.5, 2.0, and 2.8 atm. Lastly, the angular response was investigated and remained relatively constant over angles ranging from 0 to 45 degrees from normal incidence.

A layered 6 Li foil multi-wire proportional counter

The shortage of 3He gas for neutron detection has led to alternative technologies for large-area high-efficiency neutron detectors. Large area sheets of 6Li foils covering more than 625 cm2 were positioned 1.6 cm apart in a multi-wire proportional counter each with an anode wire bank positioned on each side of the absorber sheet. The intrinsic neutron detection efficiency was measured at P-10 pressures of 1.1,1.5,2.0, and 2.8 atm, using 252Cf as the neutron source, and was calculated to be approximately 18% at all pressures. Additionally, the intrinsic thermal neutron detection efficiency was measured to be greater than 50% for a detector with four 6Li foil layers. The gamma-ray sensitivity, measured with the same gas pressures, was determined with a 137CS source positioned at a distance to yield an average field strength of 60 mRlhr. The relatively high gamma-ray exposure rate had minimal impact on the neutron measurements, as evidenced from the pulse-height spectra.