F. Fasolo

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.

Photoneutron yields from tungsten in the energy range of the giant dipole resonance

Photoneutron production on the nuclei of high-Z components of medical accelerator heads can lead to a significant secondary dose during a course of bremsstrahlung radiotherapy. However, a quantitative evaluation of secondary neutron dose requires improved data on the photoreaction yields. These have been measured as a function of photon energy, neutron energy and neutron angle for natW, using tagged photons at the MAX-Lab photonuclear facility in Sweden. This work presents neutron yields for natW(gamma, n) and compares these with the predictions of the Monte Carlo code MCNP-GN, developed specifically to simulate photoneutron production at medical accelerators.

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.

Biodevices for Space Research

This review focuses on the realisation of optical sensors able to monitor the effect of complex space radiation on biological components, based on the biosensor concept. A biosensor is a device that can reveal a biochemical variable using a biological component interfaced with a transducer. It issues an electric signal which is easy to process, depending on the analysed variable. Biosensors are useful to study the effect of stress conditions on living organisms. One of the goals of this research was to develop two types of biosensors able to monitor directly the response of oxygenic photosynthetic organisms to radiation present in space in view of their importance for future space colonization. In ground experiments and in balloon stratosphere flights, the photosynthetic process has been analysed at the level of photosystem II (PSII), the supramolecular pigment-protein complex in the chloroplast which catalyses the light-induced transfer of electrons from water to plastoquinone; PSII splits water into molecular oxygen, protons and electrons, thereby sustaining an aerobic atmosphere on Earth and providing the reducing equivalents necessary to fix carbon dioxide to organic molecules, creating biomass, food and fuel. The results indicated that presence of space radiation in the dark has a synergistic effect on photosystem II activity, suggesting that PSII D1 protein turnover may be involved in resistance to space stress. The resistance of the tested microorganisms to space stress seems to be related to their position on the evolutive scale of photosynthesis. The present studies allow to establish a regular and reliable correlation between measured physical characteristics of space radiation and biological radiation effect.