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.

METU-Defocusing Beam Line (METU-DBL) project aims to perform Single Event Effect (SEE) tests for space, nuclear and other applications. Turkish Atomic Energy Authority (TAEA) has a cyclotron which can accelerate protons up to 30 MeV kinetic energy at the Proton Accelerator Facility (PAF) mainly for radioisotope production and for research and development (R&D) purposes. In the facility, the stable proton beam current is variable between 0.1 µA to 1.2 mA and the beam size is nearly 1 cm x 1 cm. METU-DBL pre-test setup, which has been installed in the R&D room, enlarges the beam size with two quadrupole magnets and it reduces the proton flux with a collimator. The pretest setup beam size is about 10 cm x 10 cm and the beam flux is 10 8 p/cm 2 /s. The first tests of electronic cards, detectors and also commercial and experimental solar cells have been performed using this setup. Also, the final configuration of METU-DBL is now under construction to provide a beam according to ESA ESCC No. 25100 standard. MCNP Monte Carlo codes were used for the calculations of secondary particles (neutrons, gammas) and residuals.

Abstract:In order to simulate radiation transport, various algorithms, codes, and programs have been developed. In this study Monte Carlo N-particle code is used to simulate a medical electron linear accelerator gantry for research purposes.Detailed geometry of the LINAC head and water phantom are modeled and simulated for calculations. Analyses are made for filtered and flattening filter-free (FFF) systems. Percent depth dose and dose profile measurements are calculated with Monte Carlo simulations and compared with experimental and theoretical values for quality assurance of the model. Flux, dose, and spectrum analyses are performed for filtered and FFF systems separately. In this study, it was aimed to run the linear accelerator in a computer environment for different purposes, and this aim was achieved.

Purpose. Nearly all Cobalt-60 teletherapy machines were removed around the world during the last two decades. The remaining ones are being used for experimental purposes. However, the rooms of these teletherapy machines are valuable because of lack of space in radiotherapy clinics. In order to place a new technology treatment machine in one of these rooms, one should re-shield the room since it was designed only for 1.25 MeV gamma beams on average. Mostly, the vendor of the new machine constructs the new shielding of the room using their experience. However, every radiotherapy room has different surrounding work areas and it would be wise to shield the room considering these special conditions. Also, the shield design goal of the clinic may be much lower than the International Atomic Energy Agency (IAEA) or the local association accepts. The study shows re-shielding of a Cobalt-60 room, specific to the clinic, using Monte Carlo simulations.Materials & Methods. First, a 6 MV Tomotherapy machine, then a 10 MV conventional linear accelerator (LINAC) was placed inside the Cobalt-60 teletherapy room. The photon flux outside the room was simulated using Monte Carlo N-Particle (MCNP6.1) code before and after re-shielding.For the Tomotherapy simulation, flux distributions around the machine were obtained from the vendor and implemented as the source of the model. The LINAC model was more generic with the 10 MeV electron source, the tungsten target, first and secondary collimators. The aim of the model was to obtain the maximum (40x40 cm 2 ) open field at the isocenter. Two different simulations were carried out for gantry angles 90 o and 270 o . The LINAC was placed in the room such that the primary walls were A' (Gantry 270 o ) and C' (Gantry 90 o ) (figure 1).The second part of the study was to model the re-shielding of the room for Tomotherapy and for the conventional LINAC, separately. The aim was to investigate the recommended shielding by the vendors. Left side of the room was adjacent to a LINAC room with 2 meters thick concrete wall (figure 1). No shielding was necessary for that wall. Behind wall A-A' there was an outdoors forbidden area; behind wall B-B' was the contouring room for the doctors; and the control room was behind wall C-C' (figure 1). After some modifications, the final shielding was designed.Results. The photon flux distributions outside the room before and after the re-shielding were compared. The reshielding of Tomotherapy reduced the flux down to 1.89 % on average with respect to pre-shielding (table 1). For the conventional LINAC case; after re-shielding, the photon flux in the control room -which corresponds to gantry 90°-decreased down to 0.57% with respect to pre-shielding (table 2). The photon flux behind wall A' -which corresponds to gantry 270°-decreased down to 2.46%. Everybody was all safe behind wall B' even before re-shielding.