Masters Theses

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

Laurence F. Miller

Committee Members

E. Marcia Katz, Lawrence W. Townsend

Abstract

Neutron dosimetry is especially difficult due to spectral effects, the unavoidable presence of gamma rays, and the fact that most detection systems respond to both gamma rays and neutrons. Thermoluminescent dosimetry is a convenient method with which to measure gamma exposures in a medical facility. However, in medical facilities, neutrons are also a component of the dose received by personnel and patients. Therefore, a study was undertaken to determine the neutron dose using thermoluminescent materials and a relatively new detector, the bubble detector. Three methods are presented for estimating the radiation dose to be expected at various locations within the treatment room maze, where neutrons are present. Three types of detectors were used to determine the neutron dose expected in the maze of a linear accelerator: Panasonic Li2B4O7 TLDs, Harshaw LiF TLDs, and Apfel Neutrometer™ bubble detectors. In all comparisons, the TLDs primarily responded in a similar manner. The bubble detectors' responses were not consistent with either of the TLDs used. The TLD dose responses were within a factor of two for each location within the maze. The TLDs were within a factor of three of the dose response predicted by an analytical method, while the bubble dosimeters' dose response varied significantly (a factor of 85 times) from the expected dose. The advantage of using the TLDs was that a dose estimate could be obtained at the outer end of the maze, where the Neutrometer™ did not respond. An analytical method was used to determine the neutron dose at the inner and outer end of the maze. This method included the use of the inverse square relationship to determine the dose at the inner end of the maze and using the tenth value distance (TVD) to determine the neutron dose at the outer end of the maze. Using the inverse square relationship and the TVD to estimate the dose at the inner and outer end of the maze, the LiF detector produced the best result for the inner end of the maze, while none of the detectors gave an adequate response for the outer end of the maze. The Li2B4O7 and the LiF thermoluminescent dosimeters and the Neutrometer™ dosimeter are acceptable devices for measuring neutron doses in medical accelerator facilities. These devices could be used in combination to utilize each detector's strong qualities. The TLDs could be used to determine the dose at the outer end of the maze, with the Neutrometer™ being used at the inner end of the maze. Regardless of the detector type chosen, maze design and shielding will influence how the detectors respond to neutrons and to gammas if the detector is sensitive to these rays.

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