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Neutron spectral measurements in an intense photon field associated with a high‐energy x‐ray radiotherapy machine
Author(s) -
Holeman G. R.,
Price K. W.,
Friedman L. F.,
Nath Ravinder
Publication year - 1977
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.594339
Subject(s) - neutron , x ray , photon , physics , field (mathematics) , dosimetry , optics , medical physics , nuclear physics , materials science , nuclear medicine , medicine , mathematics , pure mathematics
High‐energy x‐ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x‐ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25‐MV x rays) located at Yale–New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2‐, 3‐, 5‐, 8‐, 10‐ and 12‐in.‐diameter polyethylene spheres with 6 Li and 7 Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium‐covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum. Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time‐of‐flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.