Photoneutron intensity variation with field size around radiotherapy linear accelerator 18-MeV X-ray beam

H. Al-Ghamdi, Fazal-ur-Rehman, M. I. Al-Jarallah, N. Maalej

Research output: Contribution to journalArticlepeer-review

34 Scopus citations


In X-ray radiotherapy accelerators, neutrons are produced mainly by (γ, n) reaction when high energy X-rays interact with high Z materials of the linear accelerator head. These materials include the lead (Pb) used as shielding in the collimator, tungsten (W) target used for the production of X-rays and iron (Fe) in the accelerator head. These unwanted neutrons contaminate the therapeutic beam and contribute to the patient dose during the treatment of a cancer patient. Knowing the neutron distribution around the radiotherapy accelerator is therefore desired. CR-39 nuclear track detectors (NTDs) were used to study the variation of fast and thermal neutron relative intensities around an 18 MeV linear accelerator X-ray beam with the field sizes of 0, 10 × 10, 20 × 20, 30 × 30 and 40 × 40 cm2. For fast neutron detection, bare NTDs were used. For thermal neutron detection, NTDs were covered with lithium tetra borate (Li2 B4 O7) converters. The NTDs were placed at different locations in the direction perpendicular to the treatment couch (transversal) and in the direction parallel to the treatment couch (longitudinal) with respect to the isocenter of the accelerator. The fast neutron relative intensity is symmetrical about the beam axis and exhibits an exponential-like drop with distance from the isocenter of the accelerator for all the field sizes. At the primary beam (isocenter), the relative fast neutron intensity is highest for 40 × 40 cm2 field size and decreases linearly with the decrease in the field size. However, fast neutron intensities do not change significantly with beam size for the measurements outside the primary beam. The fast neutron intensity in the longitudinal direction outside the primary beam decreases linearly with the field size. The thermal neutron intensity, at any location, was found to be almost independent of the field size.

Original languageBritish English
Pages (from-to)S495-S499
JournalRadiation Measurements
Issue numberSUPPL.1
StatePublished - Aug 2008


  • Fast and thermal neutron
  • High energy X-ray
  • Neutron dosimetry
  • NTD
  • Nuclear track detectors
  • Photoneutrons
  • Radiotherapy accelerator


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