Thermal and epithermal neutrons in the vicinity of the Primus Siemens biomedical accelerator

Abstract

In this paper, the thermal and epithermal neutron fluence distributions in the vicinity of the Primus Siemens accelerator are presented. The measurements were carried out by the use of the neutron activation method for 15 MV X-rays and electron beams of 18 MeV and 21 MeV. From the radiation safety point of view for the hospital personnel, it is important to know the thermal and epithermal neutron fluence distribution in the vicinity of the accelerator because the neutrons interacting with atoms of a medium by various processes induce the activity of objects (accelerator, other apparatus etc.) and walls in the treatment room. The thermal and epithermal neutron capture, particularly, in high atomic number materials of the accelerator head can be a significant source of gamma radiation and it has to be taken into account for estimation of the work safety of the personnel. Values of the neutron fluence were normalized to the maximum photon (or electron) dose Dmax,γ (e) measured at the central axis of therapeutic X-ray (or electron) beam in a water phantom. The thermal neutron fluences measured during the 15 MV X-ray emission varied between 1.1 × 105 n · cm−2· Gy−1 and 4.4 × 105 n · cm−2· Gy−1 whereas the epithermal neutron fluences ranged from 0.2 × 105 n · cm−2· Gy−1 to 1.8 × 105 n · cm−2· Gy−1. In the case of electron beams, the neutron fluence measurements were performed only at the isocentre. The obtained thermal and epithermal neutron fluences were 1.2 × 104 n · cm−2· Gy−1 and 0.6 × 104 n · cm−2· Gy−1, respectively, for the 18 MeV electrons. In the the case of the 21 MeV electron beams the thermal neutron fluence was 2.0 × 104 n · cm−2· Gy−1 whereas the epithermal neutron fluence was 0.8 × 104 n · cm−2· Gy−1.