Activation studies for a beta-beam Decay Ring (DR): residual dose rates during maintenance and airborne activity

In a future beta-beam facility radioactive ions (6He and 18Ne) are produced, accelerated and then stored in a large Decay Ring (DR), where they eventually produce anti-neutrino and neutrino beams through β± decay. CERN is one of the candidate sites for the beta-beam facility, as existing machines like the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) could be used for the acceleration. The DR is composed of two long straight sections and two arcs. The straight sections host the collimators and two bumps. The beam is injected into the ring at nearly 92 GeV per nucleon. Beam losses occur in different sections of the machine: relevant losses include collimation losses in the collimation section and the bumps, and decay losses in the magnets in the arcs. This work focuses on two radiation protection aspects related to the operation of the DR, namely the induced radioactivity and the air activation generated by collimation and decay losses. All the calculations are performed with the Monte Carlo transport code FLUKA and are based on a continuous three-month operation. The induced radioactivity in the machine components and the expected residual dose rates inside the tunnel during maintenance are calculated for three different waiting times. Airborne activity is evaluated through the convolution of predicted particle spectra in the tunnel with isotope-production cross-sections. Using activity-to-dose coefficients, previously calculated for the ISOLDE facility at CERN, and a laminar flow model for the air diffusion, the airborne activity is converted into effective dose to the reference population group. The results show that residual dose rates during maintenance decrease significantly in a week after the shutdown of the machine, reaching values that correspond, according to CERN area classification, to a limited stay area. The effective dose given to the reference population in one year of operation is below the reference value for CERN emissions into the environment

Thermal limits on MV x-ray production by bremsstrahlung targets in the context of novel linear accelerators.

PURPOSE: To study the impact of target geometrical and linac operational parameters, such as target material and thickness, electron beam size, repetition rate, and mean current on the ability of the radiotherapy treatment head to deliver high-dose-rate x-ray irradiation in the context of novel linear accelerators capable of higher repetition rates/duty cycle than conventional clinical linacs. METHODS: The depth dose in a water phantom without a flattening filter and heat deposition in an x-ray target by 10 MeV pulsed electron beams were calculated using the Monte-Carlo code MCNPX, and the transient temperature behavior of the target was simulated by ANSYS. Several parameters that affect both the dose distribution and temperature behavior were investigated. The target was tungsten with a thickness ranging from 0 to 3 mm and a copper heat remover layer. An electron beam with full width at half maximum (FWHM) between 0 and3 mm and mean current of 0.05-2 mA was used as the primary beam at repetition rates of 100, 200, 400, and 800 Hz. RESULTS: For a 10 MeV electron beam with FWHM of 1 mm, pulse length of 5 μs, by using a thin tungsten target with thickness of 0.2 mm instead of 1 mm, and by employing a high repetition rate of 800 Hz instead of 100 Hz, the maximum dose rate delivered can increase two times from 0.57 to 1.16 Gy/s. In this simple model, the limiting factor on dose rate is the copper heat removers softening temperature, which was considered to be 500°C in our study. CONCLUSIONS: A high dose rate can be obtained by employing thin targets together with high repetition rate electron beams enabled by novel linac designs, whereas the benefit of thin targets is marginal at conventional repetition rates. Next generation linacs used to increase dose rate need different target designs compared to conventional linacs

Modelling the radiation action for the estimation of biological effects in humans.

It is well known that ionizing radiation can induce biological effects at different levels, from DNA, chromosomes and cells up to tissues, organs and entire organisms. Theoretical models and Monte Carlo codes, especially those based on radiation track structure, can be of great help to elucidate the underlying mechanisms and to perform reliable predictions where data are lacking. In this work we will present and discuss a mechanistic ab initio model and a Monte Carlo code able to simulate the induction of chromosome aberrations (CAs) in human cells. This endpoint is particularly relevant, since some aberration types can lead to cell death, while others can lead to cell conversion to malignancy. The model is based on the hypothesis that only clustered lesions (CLs) of the DNA double-helix can evolve into aberrations. Simulated dose-response curves for CAs induced by different radiation types (including heavy ions) will be shown, together with applications to cancer risk estimation and biodosimetry. In this framework, we will also discuss examples of medical applications - including astronauts’ exposure to space radiation - obtained with the FLUKA code, also taking into account the role of nuclear interactions

Update on the status of the FLUKA Monte Carlo transport code

The FLUKA Monte Carlo transport code is a wellknown simulation tool in High Energy Physics. FLUKA is a dynamic tool in the sense that it is being continually updated and improved by the authors. We review the progress achieved since the last CHEP Conference on the physics models, some technical improvements to the code and some recent applications. From the point of view of the physics, improvements have been made with the extension of PEANUT to higher energies for p, n, pi, pbar/nbar and for nbars down to the lowest energies, the addition of the online capability to evolve radioactive products and get subsequent dose rates, upgrading of the treatment of EM interactions with the elimination of the need to separately prepare preprocessed files. A new coherent photon scattering model, an updated treatment of the photo-electric effect, an improved pair production model, new photon cross sections from the LLNL Cullen database have been implemented. In the field of nucleus-nucleus interactions the electromagnetic dissociation of heavy ions has been added along with the extension of the interaction models for some nuclide pairs to energies below 100 MeV/A using the BME approach, as well as the development of an improved QMD model for intermediate energies. Both DPMJET 2.53 and 3 remain available along with rQMD 2.4 for heavy ion interactions above 100 MeV/A. Technical improvements include the ability to use parentheses in setting up the combinatorial geometry, the introduction of pre-processor directives in the input stream. a new random number generator with full 64 bit randomness, new routines for mathematical special functions (adapted from SLATEC). Finally, work is progressing on the deployment of a user-friendly GUI input interface as well as a CAD-like geometry creation and visualization tool. On the application front, FLUKA has been used to extensively evaluate the potential space radiation effects on astronauts for future deep space missions, the activation dose for beam target areas, dose calculations for radiation therapy as well as being adapted for use in the simulation of events in the ALICE detector at the LHC

Physics to understand biology: Monte Carlo approaches to investigate space radiation doses and their effects on DNA and chromosomes

Exposure of biological structures to ionizing radiation can induce different damage types at various levels, from DNA and chromosomes up to cells, tissues, organs and entire organisms. Although these multi-step processes involve many orders of magnitude both in the space and in the time scale, the pattern of initial energy deposition in matter strongly influences the subsequent evolution of the process. Great help to elucidate the underlying mechanisms and to perform reliable predictions is provided by mechanistic models and Monte Carlo codes, which allow one to take into account the stochastic aspects characterizing energy deposition in matter. Concerning radiation damage at the level of tissues and organs, in this paper we will focus on organ doses to astronauts exposed to Galactic Cosmic Rays (GCR) in deep space, under different shielding conditions. The calculations were carried out by means of the FLUKA transport and interaction MC code, coupled with two anthropomorphic model phantoms inserted into an Al shielding box of variable thickness. Besides organ-averaged absorbed doses and dose equivalents we calculated “biological doses”, defined as yields of clustered DNA breaks (“Complex Lesions”) in a given organ. CL have been obtained by “event-by-event” radiation track-structure simulations at the nm level and they are integrated on-line into a purposely modified version of FLUKA, which adopts a “condensed-history” approach. To quantify the role of nuclear interactions, for each shield thickness the dose contributions from secondary hadrons (including ions) were calculated separately. Furthermore, the neutron contribution was separated from that of all other nuclear reaction products. Concerning damage at the molecular and cellular level, herein we will present and discuss examples of application of a Monte Carlo code developed at the University of Pavia, which can simulate chromosome aberration induction by different radiation types. The focus will be both on the role played by the particle track structure at the nm level and on the relationship between aberrations and relevant cellular endpoints such as cell death and cell conversion to malignancy. Main assumption of the model is the hypothesis that only clustered lesions (CLs) of the DNA double-helix can “evolve” and lead to aberrations. A combination of the two approaches (condensed-history and event-by-event) allowed estimation of yields of chromosome aberrations following exposure to GCR in deep space, which were found to be consistent with aberration yields observed in lymphocytes of astronauts involved in long-term missions onboard the Mir station and the International Space Station

GCR and SPE organ doses in deep space with different shielding: Monte Carlo simulations based on the FLUKA code coupled to anthropomorphic phantoms

Astronauts’ exposure to space radiation is of high concern for long-term missions, especially for those in deep space such as possible travels to Mars. In these cases shielding optimization is a crucial issue, and simulations based on radiation transport codes and anthropomorphic model phantoms can be of great help. In this work the FLUKA Monte Carlo code was coupled with two anthropomorphic phantoms (a mathematical model and a ‘‘voxel’’ model) to calculate organ-averaged dose, dose equivalent and ‘‘biological dose’’ in the various tissues and organs following exposure to the August 1972 Solar Particle Event and to Galactic Cosmic Rays under different shielding conditions. The ‘‘biological dose’’ was characterized by the average number of induced ‘‘Complex Lesions’’ (CLs) per cell in a given organ or tissue, where CLs are clustered DNA breaks which can play an important role in chromosome aberration induction. Separate calculation of the contributions from secondary hadrons – in particular neutrons – with respect to primary particles allowed us to quantify the role played by nuclear interactions occurring in the shield and in the human body. Specifically for GCR, the contributions from the different components of the incident primary spectra were calculated separately as well. As expected, the SPE doses showed a dramatic decrease with increasing Al shielding. Furthermore, for SPEs internal organs received much lower doses with respect to skin, and nuclear interactions were found to be of minor importance. A 10 g/cm2 Al storm shelter turned out to be sufficient to respect the NCRP limits for 30-days LEO missions in case of a SPE similar to the August 1972 event. In contrast with SPEs, GCR absorbed doses remained roughly constant with increasing Al shielding. The organ-averaged dose equivalent and biological dose showed a (slight) decrease starting from a shield thickness of 2 g/cm2, probably due the lower LET of projectile fragments