Neutron dosimetry in high energy X-ray beams of medical accelerators

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Abstract
High energy X-ray beams from medical accelerators are used in [human] cancer therapy. These beams produce neutrons due to photon-neutron interactions in the target material, collimator and beam flattening filter. The feasibility of employing fast-neutron-induced recoil particle tracks in polycarbonate foils developed by the electrochemical etching (ECE) method for the dosimetry of these neutrons was investigated. The experiments were carried out in, and out of, the beam of a Varian Clinac-18, an Allis-Chalmers 25 MeV betatron and a Brown Boveri 45 MeV betatron. Neutron dose equivalents were found as a function of the photon dose and of the distance away from the center of the beam. The neutron dose equivalent was 0.6 and 1.3% of the X-ray dose at the center position of the beam for a TSD [target to skin distance] = 100 cm and a 10 .times. 10 cm2 field for the 25 and 45 MeV betatrons, respectively. Although the dosimeter showed adequate sensitivity to neutrons in the beam of the betatrons, it showed insensitivity to neutrons in the beam of the Clinac-18 which has a softer spectrum than the neutron energy threshold of the dosimeter. This dosimetry method provides a new approach with a number of advantages compared to some other existing techniques for such measurements above the energy threshold of the dosimeter.

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