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Title: Neutron dosimetry with a water calorimeter
Author: Galloway, Gordon
ISNI:       0000 0001 3487 285X
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1986
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In certain situations neutron radiotherapy has potential advantages over conventional X-ray therapy. It has yet to be conclusively proven that neutron therapy is more efficacious than X-ray therapy, and in many institutions worldwide neutron therapy is currently being evaluated. It is important that the dosimetry of neutron beams is consistent so that clinical results from different centres can be compared. Due to the relatively large uncertainties in the measurement of absorbed dose in neutron beams this consistency is difficult to achieve. This work describes the application of a recently developed dosimetry method, namely water calorimetry, to the field of neutron dosimetry. The concept of absorbed dose and its measurement are discussed. The theory behind radiation calorimetry in general, and specifically water calorimetry, is outlined followed by a review of water calorimetry and calorimeters for neutron dosimetry. The calorimeter, which consists simply of a thermally insulated tank of water, and its construction are described. The temperature rise in the water was detected by a thermistor which was incorporated in a Wheatstone bridge circuit. The merits of different configurations of bridge circuit are discussed and the literature on the subject is reviewed. The circuit employed was an AC bridge having asymmetric, inductive ratio arms. The output of the bridge was detected by a lock-in amplifier and the signal was fed directly to a microcomputer which stored and analysed the data. Absorbed dose measurements with the calorimeter were made in 4 and 9 MV photon beams, a 10 MeV electron beam and a d(15) + Be neutron beam. Calorimetric determinations of absorbed dose were compared, under identical conditions, with ionisation chamber dosimetry. In the 4 MV 10 and 9 MV photon beams calorimetric measurements of absorbed dose to water were respectively 4.0 and 4.5% higher than ionisation chamber measurements. In the 10 MeV electron beam calorimetric measurements were 5.5% higher. These differences are due to the heat defect in water which is discussed in detail. In the neutron beam absorbed dose measured with the calorimeter was compared with that measured using an Exradin tissue equivalent ionisation chamber. Doses measured with the ionisation chamber were calculated according to the European protocol for neutron dosimetry. Absorbed dose to tissue measured with the calorimeter was 4.4% lower than that measured with the ionisation chamber. Relative to ionisation chamber dosimetry, dose measurements with the calorimeter were approximately 9% lower in the neutron beam than in the photon beams. The implications of this result, and those from other water calorimeters, are fully discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Radiotherapy with neutrons