The E LiBANS project: Thermal and epithermal neutron sources based on a medical Linac.

The E_LIBANS project (INFN) aims at producing neutron facilities for interdisciplinary irradiation purposes among which pre-clinical research for BNCT. After the successful setting-up of the thermal neutron source based on a medical LINAC, a similar apparatus for epithermal neutrons has been developed. Both structures are based on an Elekta 18 MV coupled with a photoconverter-moderator system which deploys the (γ,n) reaction to convert the X-rays into neutrons. This communication describes the two neutron sources and the results obtained in their characterization.

INTENSE THERMAL NEUTRON FIELDS FROM A MEDICAL-TYPE LINAC: THE E_LIBANS PROJECT.

The e_LiBANS project aims at producing intense thermal neutron fields for diverse interdisciplinary irradiation purposes. It makes use of a reconditioned medical electron LINAC, recently installed at the Physics Department and INFN in Torino, coupled to a dedicated photo-converter, developed within this collaboration, that uses (γ,n) reaction within high Z targets. Produced neutrons are then moderated to thermal energies and concentrated in an irradiation volume. To measure and to characterize in real time the intense field inside the cavity new thermal neutron detectors were designed with high radiation resistance, low noise and very high neutron-to-photon discrimination capability. This article offers an overview of the e_LiBANS project and describes the results of the benchmark experiment.

DEVELOPING RADIATION RESISTANT THERMAL NEUTRON DETECTORS FOR THE E_LIBANS PROJECT: PRELIMINARY RESULTS.

Radiation-resistant, gamma-insensitive, active thermal neutron detectors were developed to monitor the thermal neutron cavity of the E_LIBANS project. Silicon and silicon carbide semiconductors, plus vented air ion chambers, were chosen for this purpose. This communication describes the performance of these detectors, owing on the results of dedicated measurement campaigns.

Design and simulation of an optimized e-linac based neutron source for BNCT research.

The paper is focused on the study of a novel photo-neutron source for BNCT preclinical research based on medical electron Linacs. Previous studies by the authors already demonstrated the possibility to obtain a mixed thermal and epithermal neutron flux of the order of 10(7) cm(-2) s(-1). This paper investigates possible Linac's modifications and a new photo-converter design to rise the neutron flux above 5 10(7) cm(-2) s(-1), also reducing the gamma contamination.

Comparison of different MC techniques to evaluate BNCT dose profiles in phantom exposed tovarious neutron fields.

The absorbed dose in BNCT (boron neutron capture therapy) consists of several radiation components with different physical properties and biological effectiveness. In order to assess the clinical efficacy of the beams, determining the dose profiles in tissues, Monte Carlo (MC) simulations are used. This paper presents a comparison between dose profiles calculated in different phantoms using two techniques: MC radiation transport code, MCNP-4C2 and BNCT MC treatment planning program, SERA (simulation environment for radiotherapy application). In this study MCNP is used as a reference tool. A preliminary test of SERA is performed using six monodirectional and monoenergetic beams directed onto a simple water phantom. In order to deeply investigate the effect of the different cross-section libraries and of the dose calculation methodology, monoenergetic and monodirectional beams directed toward a standard Snyder phantom are simulated. Neutron attenuation curves and dose profiles are calculated with both codes and the results are compared.

Test of a bubble passive spectrometer for neutron dosimetry.

A passive system for neutron spectrometry has been tested in view of neutron dose evaluation in mixed radiation fields. This system, based on bubble detectors (Bubble Technology Industries, Ontario, Canada), is suitable to evaluate the neutron energy distribution in the range 10 keV-20 MeV even in the presence of intense gamma radiation, as required in various fields: medical x-ray accelerators, nuclear reactors, cosmic ray exposures on commercial high-altitude flights and space missions. A new unfolding code BUNTO has been especially developed for this application. In the present work, the results of two experimental tests are summarized. In the first one, the device has been exposed to a standard AmBe neutron source (Joint Research Centre, Ispra, Varese, Italy). In the second one, measurements have been carried out at the MAX-Lab photonuclear facility in Sweden, with a bremsstrahlung photon beam impinging on thick targets of different materials and generating a giant dipole resonance neutron spectrum. Simulations of the experimental apparatus have been performed with MCNP4B (AmBe source) and with MCNP4B-GN (MAX-Lab). Results of the comparison between experimental and calculated spectra are shown and discussed. A good agreement between measurements and simulation data is obtained in both the experiments.

Neutron spectra in a tissue equivalent phantom during photon radiotherapy treatment by LINACS.

Bremsstrahlung photon beams produced by LINAC accelerators are currently the most used radiotherapy method for tumour treatments. When the photon energy exceeds the (gamma,n) reaction threshold energy, the patient receives an undesired dose due to photoneutron production both in the accelerator head and in the human body. In this paper, a method is presented for the assessment of the photoneutron spectra produced by Giant Dipole Resonance (GDR) during cancer radiotherapy with energetic photon beams. Experimental and numerical results have been obtained for the facility at Onkologik Klinik, Lund (Sweden), which is based on an ELEKTA 18 MV LINAC. Neutron spectra are evaluated both at the patient plane and inside an anthropomorphic phantom.

Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems.

Bremsstrahlung photon beams produced by linac accelerators are currently the most commonly used method of radiotherapy for tumour treatments. When the photon energy exceeds 10 MeV the patient receives an undesired dose due to photoneutron production in the accelerator head. In the last few decades, new sophisticated techniques such as multileaf collimators have been used for a better definition of the target volume. In this case it is crucial to evaluate the photoneutron dose produced after giant dipole resonance (GDR) excitation of the high Z materials (mainly tungsten and lead) constituting the collimator leaves in view of the optimization of the radiotherapy treatment. A Monte Carlo approach has been used to calculate the photoneutron dose arising from the GDR reaction during radiotherapy with energetic photon beams. The simulation has been performed using the code MCNP4B-GN which is based on MCNP4B, but includes a new routine GAMMAN to model photoneutron production. Results for the facility at IRCC (Istituto per la Ricerca e la Cura del Cancro) Candiolo (Turin), which is based on 18 MV x-rays from a Varian Clinac 2300 C/D, are presented for a variety of different collimator configurations.