Master of Science / Department of Mechanical and Nuclear Engineering / J. Kenneth Shultis / Perforated neutron detectors suffer the unfortunate effect that their efficiency is a strong function of the direction of neutron incidence. It is found, by Monte Carlo simulation of many perforation shapes, that sinusoidal-type perforations greatly reduce the variation of detector efficiency. Detectors with rod-type perforations are modeled using a hybrid transport
method linking the MCNP transport code and a specialized ion-transport code to
calculate the probability that a neutron is detected. Channel, chevron, and sinusoidal perforations
are modeled using other customized transport codes. Detector efficiency calculations
are performed for neutrons incident at various polar and azimuthal angles. It is discovered
that the efficiency losses of the detectors result from the decreasing solid angle subtended
by the detector from the source and streaming through the detector at specific azimuthal
angles. Detectors achieving an efficiency in excess of 10% and having a relatively flat ± 1%
angular dependence in all azimuthal angles and polar angles between 0 and 60 degrees are predicted. Efficiencies up to 25% are achievable at the loss of directional independence.
In addition to minimizing the directional dependence of the perforated detectors, the
feasibility of developing a neutron detector for deployment in cargo containers to locate
nuclear weapon pits is investigated using the MCNP transport code. The detector considered
is a 7-mm diameter, 6LiF, rod-perforated detector surrounded in a cylinder of polyethylene.
The optimum thicknesses of surrounding polyethylene, to maximize the response of the
detector, is determined to be 10 cm of radial, 5 cm of front, and 5 cm of back polyethylene
for end-on neutron incidence. Such a detector is predicted to produce a count rate between 12
and 15 cpm from a nuclear-weapon pit composed of 90% 239Pu and 10% 240Pu at a distance
of 3 m. Side incidence is also considered, and the optimum moderator dimensions are 8 cm
of radial, 10 cm of front, and 10 cm of back polyethylene that produce approximately the
same count rate.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/328 |
| Date | January 1900 |
| Creators | Solomon, Clell J. Jr. |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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