Photoneutron Flux Measurement via Neutron Activation Analysis in a Radiotherapy Bunker with an 18 MV Linear Accelerator

Abstract

Abstract. In cancer treatment, high energy X-rays are used which are produced by linear accelerators (LINACs). If the energy of these beams is over 8 MeV, photonuclear reactions occur between the bremsstrahlung photons and the metallic parts of the LINAC. As a result of these interactions, neutrons are also produced as secondary radiation products (Ȗ,n) which are called photoneutrons. The study aims to map the photoneutron flux distribution within the LINAC bunker via neutron activation analysis (NAA) using indium-cadmium foils. Irradiations made at different gantry angles (0°, 90°, 180° and 270°) with a total of 91 positions in the Philips SLI-25 linear accelerator treatment room and location-based distribution of thermal neutron flux was obtained. Gamma spectrum analysis was carried out with high purity germanium (HPGe) detector. Results of the analysis showed that the maximum neutron flux in the room occurred at just above of the LINAC head (1.2x10 5 neutrons/cm 2 .s) which is compatible with an americium-beryllium (Am-Be) neutron source. There was a 90% decrease of flux at the walls and at the start of the maze with respect to the maximum neutron flux. And, just in front of the LINAC door, inside the room, neutron flux was measured less than 1% of the maximum.

Assessment of radio-activation using spectroscopy in medical linear accelerators

“…The resulting influence can be confirmed in the measurement results. Many of the nuclides identified in linac (4) were similar to those in (5), but the cps level was 1/10 times lower at the peak of 60 Co. In the case of linac (5), with the highest 15 MV usage rate, the most radionuclides were detected among the nine of equipment.…”

“…Many of the nuclides identified in linac (4) were similar to those in (5), but the cps level was 1/10 times lower at the peak of 60 Co. In the case of linac (5), with the highest 15 MV usage rate, the most radionuclides were detected among the nine of equipment. The case of linac (6) using only 6 MV, it can be judged that it rarely activated and that the maximum cps was 410, indicating a shallow level of radioactivity and the lowest number of detected nuclides.…”

“…In addition, for equipment with high utilization rates of 15 and 10 MV, we consider that the measured dose rate will PLOS ONE be higher. Based on the dose rate for the linac equipment using 15 MV energy at 20% to 50% of linac (5) in the results section, the dose rate after treatment was observed to be as high as 2.773 uSv/h. For linac (9), even after approximately 5 h, it was as high as 0.532 uSv/h.…”

“…The emitted neutrons are recaptured via a reaction that emits gamma rays. The corresponding linac equipment becomes radioactive as these processes are repeated for 10-15 years [3][4][5].…”

Measurements of Neutron Activation and Dose Rate Induced by High-Energy Medical Linear Accelerator

“…In the case of cancer treatments using a photon beam with an energy ≥8 MV, radiation occurs because of a chain reaction of the following two reactions [3,4]. With continuous irradiation, neutrons are generated through a photonuclear reaction (γ, n) inside the LINAC's head, and these then interact with the components inside the LINAC head, generates gamma rays [5][6][7].…”