Marco Silari

Secondary neutron and photon dose in proton therapy

BACKGROUND AND PURPOSE The dose due to secondary neutrons and photons in proton therapy was estimated with Monte Carlo simulations. Three existing facilities treating eye and deep-seated tumours were taken into account. The results of the calculations related to eye proton therapy were verified with measurements. MATERIALS AND METHODS The simulations were performed with the FLUKA code. Neutron fluence was measured inside an Alderson phantom (type ART) with activation techniques. RESULTS The maximum dose due to secondaries produced in a passive beam delivery system was estimated to be of the order of 10(-4) and 10(-2) Gy per therapy Gy for eye and deep tumour treatments, respectively. In the case of irradiations of deep-seated tumours carried out with an active system, the dose was of the order of 10(-3) Gy per therapy Gy. CONCLUSIONS The dose due to secondaries depends on the geometry of the beam delivery system and on the energy of the primary beam and is lower in the healthy tissues distant from the target volume.

A facility for the test of large-area muon chambers at high rates

Abstract Operation of large-area muon detectors at the future Large Hadron Collider (LHC) will be characterized by large sustained hit rates over the whole area, reaching the range of kHz cm −2 . We describe a dedicated test zone built at CERN to test the performance and the aging of the muon chambers currently under development. A radioactive source delivers photons causing the sustained rate of random hits, while a narrow beam of high-energy muons is used to directly calibrate the detector performance. A system of remotely controlled lead filters serves to vary the rate of photons over four orders of magnitude, to allow the study of performance as a function of rate.

Neutron measurements around medical electron accelerators by active and passive detection techniques

Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and 3He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x-ray dose of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.

Double differential distributions and attenuation in concrete for neutrons produced by 100–400 MeV protons on iron and tissue targets

Abstract Double differential distributions of neutrons produced by 100, 150, 200, 250 and 400 MeV protons stopped in thick iron and soft tissue targets were calculated by two Monte Carlo codes, FLUKA and LCS, from 0 (forward shielding) to 180°. The results are compared with calculated and experimental data available in the literature. The attenuation in ordinary concrete of the dose equivalent due to neutrons and other particles was calculated. The contribution due to photons and protons was found to range from a few percent up to 50% of the total dose equivalent. Source terms and attenuation lengths are given as a function of energy and emission angle.

Development of an Extended Range Bonner Sphere Spectrometer

To improve the response to high-energy neutrons of a conventional Bonner Sphere Spectrometer, the response functions of several detector configurations of different sizes and materials were calculated with the Monte Carlo programme FLUKA. The two most promising configurations were selected, built and afterwards exposed to neutrons of an Am-Be source and to a broad high-energy neutron spectrum at CERN. The comparison between the measured and calculated detector responses of the new spheres in these radiation fields confirms their simulated response functions and justifies their implementation into the conventional Bonner Sphere Spectrometer.

Maze design of a gantry room for proton therapy

Abstract Monte Carlo simulations with the LCS code were performed for the design of the access maze of an isocentric gantry room for proton therapy. The double distributions of secondary neutrons, produced by 250 MeV protons in the gantry structural materials and in the patient, were determined at the treatment room access for different irradiation directions. Neutron transport was simulated in a three-leg maze of various lengths. The secondary photons produced by neutrons in the maze walls were taken into account. The simulation results were fitted by analytical expressions based on the solution of the Hubbell integral for a rectangular radiation source. Some simulations were also performed with the FLUKA code for comparison.

Neutron production from 158 GeV/c per nucleon lead ions on thin copper and lead targets in the angular range 30-135°

Abstract The neutron emission from 5, 10 and 20 mm thick lead and 10 and 20 mm thick copper targets bombarded by a lead ion beam with momentum of 158 GeV/c per nucleon were measured at the CERN Super Proton Synchrotron. The neutron yield and spectral fluence per incident ion on target were measured with an extended range Bonner sphere spectrometer in the angular range 30–135° with respect to beam direction. Monte Carlo simulations with the FLUKA code were performed to establish a guess spectrum for the unfolding of the experimental data. The results have shown that, lacking Monte Carlo radiation transport codes dealing with ions with masses larger than 1 amu, a reasonable prediction can be carried out by scaling the result of a Monte Carlo calculation for protons by the projectile mass number to the power of 0.85–0.95 for a lead target and 0.88–1.03 for a copper target.

Predicting Induced Radioactivity at High-Energy Electron Accelerators

Radioactive nuclides are produced at high-energy electron accelerators by different kinds of particle interactions with accelerator components and shielding structures. Radioactivity can also be induced in air, cooling fluids, soil and groundwater. The physical reactions involved include spallations due to the hadronic component of electromagnetic showers, photonuclear reactions by intermediate energy photons and low-energy neutron capture. Although the amount of induced radioactivity is less important than that of proton accelerators by about two orders of magnitude, reliable methods to predict induced radioactivity distributions are essential in order to assess the environmental impact of a facility and to plan its decommissioning. Conventional techniques used so far are reviewed, and a new integrated approach is presented, based on an extension of methods used at proton accelerators and on the unique capability of the FLUKA Monte Carlo code to handle the whole joint electromagnetic and hadronic cascade, scoring residual nuclei produced by all relevant particles. The radiation aspects related to the operation of superconducting RF cavities are also addressed.

Neutron measurements in the stray field produced by 158 GeV c(-1) per nucleon lead ion beams.

This paper discusses measurements carried out at CERN in the stray radiation field produced by 158 GeV c(-1) per nucleon 208Pb82+ ions. The purpose was to test and intercompare the response of several detectors, mainly neutron measuring devices, and to determine the neutron spectral fluence as well as the microdosimetric (absorbed dose and dose equivalent) distributions in different locations around the shielding. Both active instruments and passive dosimeters were employed, including different types of Andersson-Braun rem counters, a tissue equivalent proportional counter, a set of superheated drop detectors, a Bonner sphere system, and different types of ion chambers. Activation measurements with 12C plastic scintillators and with 32S pellets were also performed to assess the neutron yield of high energy lead ions interacting with a thin gold target. The results are compared with previous measurements and with measurements made during proton runs.

Shielding calculations for a 250 MeV hospital-based proton accelerator

Abstract The accelerator shields (250 MeV protons, 400 MeV/u 16 O 8+ ions) and treatment rooms of the Hadrontherapy Centre, a hospital-based facility under design in Italy, were determined by means of Monte Carlo calculations. The LCS and FLUKA codes were employed, together with analytical estimates carried out by making use of empirical formulas from the literature, and the results compared. In the case of 250 MeV protons a 250 cm thick concrete wall ensures and annual dose equivalent lower than 2 mSv in the environments adjacent to the accelerator room. The best ceiling thickness was found to be 200 cm for a unitary occupancy factor. The photon dose equivalent beyond the concrete shield was also estimated using the LCS code. In the case of ions the shield thickness was calculated using empirical formulas from the literature; the concrete thicknesses calculated for protons should ensure the required dose equivalent when some local shields are added. Monte Carlo calculations of the treatment room shielding were also carried out using the FLUKA code.