Georg Fehrenbacher

Unfolding the response of a Ge detector used for in-situ gamma-ray spectrometry

Abstract In environmental radiation protection portable Ge detectors are used for in-situ gamma-ray spectrometry. In order to determine the complete photon fluence rate spectra including the continuum component due to photons scattered in the environment from measured pulse height distributions one needs to apply unfolding methods. A prerequisite of the unfolding is the knowledge of the response functions of the detector to monoenergetic photons. In this work it is investigated whether response functions for modern portable high purity Ge detectors can be calculated by Monte Carlo simulation of photon histories with sufficient accuracy for a reliable determination of the incident photon fluence spectrum. The codes MCNP and EGS4 were used for calculating detector response functions for photon energies ranging from 20 keV to 2.6 MeV. It is demonstrated that a detector response to photons from a 152 Eu source can be unfolded with good precision. A comparison of measured and calculated responses shows that for applications in in-situ spectrometry response functions calculated by Monte Carlo simulations are more appropriate for unfolding than those determined experimentally since there is no need to separate the contribution to the response of photons scattered in the laboratory. The dependence of the response functions on the angular distribution of the incident photons was investigated by calculations for various irradiation geometries. Systematic errors in the photon fluence spectra resulting from applying for the unfolding response functions which do not correspond to the real irradiation geometry are estimated.

Heavy ion radiotherapy during pregnancy

OBJECTIVE To provide a safe particle therapy treatment for a pregnant woman with skull-base cancer. DESIGN Case report. SETTING University clinic. PATIENT(S) A 27-year-old woman diagnosed for a skull-base chordoma and whose pregnancy was found during the course of radiotherapy with accelerated carbon ions. INTERVENTION(S) Therapy was continued as scheduled, and fetal dose produced by photons and neutrons was measured at each radiotherapy fraction using passive and active monitors. MAIN OUTCOME MEASURE(S) Radiation dose to the uterus. Health of the mother and the newborn. RESULT(S) Total dose to the uterus was <0.2 mSv. About 30% of this dose was caused by neutrons. Magnetic resonance imaging of the skull base showed no evidence of recurrent disease in the mother. The child was healthy with normal development. CONCLUSION(S) Heavy ion cancer therapy produces a very low dose in distal organs.

SHIELDING OF THE TARGET AREA OF THE FRAGMENT SEPARATOR SUPER-FRS

Abstract The Super-FRS is designed as a versatile partially superconducting fragment separator for the planned international Facility for Antiprotons and Ion Research. It will be able to separate all kinds of nuclear projectile fragments of primary heavy-ion beams including uranium with energies of up to 1.5 GeV/u and intensities of up to 1012 particles/s. The primary beam power of up to 50 kW has to be dumped in six shaped beam catchers in accordance with the ion optical setting of the separator in order not to enter the main separator, which will have accordingly weaker shielding. A key issue for such a high-power facility is the activation of several components and thus their access by maintenance personnel. Both the prompt and the residual dose due to activation are calculated by means of the Monte Carlo particle transport code FLUKA. The biological shielding in the target area will be realized by massive iron blocks (thickness [approximate] 2 m) around the beam tube and the magnets. This will be surrounded by up to 6 m of concrete in order to reduce the dose rates below the design value of 0.5 μSv/h, which is in agreement with the German radiation protection ordinance for public access. A dedicated maintenance channel is foreseen in which the residual dose rates are tolerable for short time access after a certain cooling time.

On the neutron radiation field and air activation around a medical electron linac.

In high-energy photon therapy, several radiation protection issues result from photonuclear reactions. In this study, the photoneutron radiation field around a Varian Clinac linear accelerator in 18 MV-X mode within two different radiotherapy bunkers was investigated by means of Monte Carlo simulations using the FLUKA code as well as ambient dose-equivalent measurements. Furthermore, the activation of the air inside the treatment room due to photonuclear reactions (13N and 15O) and the capture of photoneutrons moderated down within the bunker (41Ar) was studied by FLUKA simulations. From the simulation results, the annual effective dose to medical workers due to photoneutrons and activated air was estimated. The emission of radioactivity due to ventilation of the treatment room was found to be negligible.

DESIGN DEVELOPMENT OF A PASSIVE NEUTRON DOSEMETER FOR THE USE AT HIGH-ENERGY ACCELERATORS

For the radiation survey at intermediate and high-energy accelerators, there is a need for a neutron dosemeter which provides reliable readings of the neutron dose in a wide energy range for continuous and pulsed radiation. The objective of this development is to find a dosemeter that fulfils the necessary requirements and can be reliably used to prove that the radiation levels in areas around accelerators are in accordance with the limits of the respective radiation protection legislation. A simple layout with small dimensions and light weight as well as the usage of common materials to lower the production costs is to be achieved.

Bonner Sphere System with Active Detector for Measurements in Pulsed Neutron Fields

Abstract Recent developments in accelerator physics have led to new challenges for radiation protection dosimetry. As is well known, the ambient dose equivalent indicated by area monitors in high-energy neutron fields behind shielding is often unreliable. Therefore, it is desirable to measure the spectrum and to do “in-field” calibrations using the spectral information as reference. For this purpose, the PTB NEMUS (an extended-range Bonner sphere spectrometer) was modified with a new active thermal neutron detector based on silver activation capable of measuring in intense pulsed fields with high energies and fluence rates.

SU-F-T-656: Monte Carlo Study On Air Activation Around a Medical Electron Linac

PURPOSE In high energy photon therapy, several radiation protection issues result from photonuclear reactions. The activation of air - directly by photonuclear reactions as well as indirectly by capture of photoneutrons generated inside the linac head - is a major point of concern for the medical staff. The purpose of this study was to estimate the annual effective dose to medical workers due to activated air around a medical high energy electron linac by means of Monte Carlo simulations. METHODS The treatment head of a Varian Clinac in 18 MV-X mode as well as the surrounding concrete bunker were modeled and the radiation transport was simulated using the Monte Carlo code FLUKA, starting from the primary electron striking the bremsstrahlung target. The activation yields in air from photo-disintegration of O-16 and N-14 nuclei as well as from neutron capture on Ar-40 nuclei were obtained from the simulations. The activation build-up, radioactive decay and air ventilation were studied using a mathematical model. The annual effective dose to workers was estimated by using published isotope specific conversion factors. RESULTS The oxygen and nitrogen activation yields were in contrast to the argon activation yield found to be field size dependent. The impact of the treatment room ventilation on the different air activation products was investigated and quantified. An estimate with very conservative assumptions gave an annual effective dose to workers of < 1 mSv/a. CONCLUSION From the results of this study it can be concluded that the contribution of air activation to the radiation exposure to medical workers should be negligible in modern photon therapy, especially when it is compared to the dose due to prompt neutrons and the activation of heavy solid materials such as the jaws and the collimators inside the linac head.

Radiation protection update for the FAIR APPA building

In 2011 the FAIR [1] application for construction approval was submitted to the radiation protection (RP) authorities based on -at that timeactual construction plans. In May 2014 the 11 and final part of the application was approved. However, since 2011 these plans underwent a series of changes due to ascertainment of additional needs by e.g. the scientific users, fire protection authorities or just the increasing grade of detail in the evolving FAIR project. These changes have to be reported to the RP authorities as an update of the 2011 application, because they might affect -to some gradethe radiation protection layout. This update-process requires a close monitoring and counseling of the necessary planning steps by the resident radiation protection at GSI, which is mandated to supervise all FAIR RP concerns. In a first step, the needs of all involved parties are acquired by the RP coordination team involving the architectural layout, requirements of the scientific user and of course the existing RP layout. The new layout is then tested by Monte Carlo calculations deploying FLUKA [2,3] following the basic principle that the new layout has to perform equally or better concerning RP needs. This maxim is laid out to achieve a smooth approval procedure of the updated construction application. These updates range from minor adjustment of shielding walls or niches therein to an entire new wall layout of a whole building like the APPA building “G50”. The original layout of the APPA cave is based on a double concrete wall design, where in-between the walls soil layers were used to improve the shielding effect (see Fig.1).

SHIELDING CALCULATIONS FOR THE NEW EXPERIMENTAL STORAGE RING FOR THE FACILITY FOR ANTIPROTON AND ION RESEARCH

Abstract The new experimental storage ring (NESR) is one of the new facilities planned for the Facility for Antiproton and Ion Research (FAIR) project. It is conceived as a versatile storage ring used for experiments with stored ion beams (up to 740 MeV/u for uranium beams) and for the deceleration of antiprotons from the injection energy of 3 GeV, which are subsequently extracted and used for experiments elsewhere. The planning of the shielding requires particular accuracy because rooms adjacent to the NESR are desired to be accessible at all times. Extensive shielding calculations have been done using the Monte Carlo code FLUKA. Calculations were performed separately for the different operation modes of the storage ring, as well as for the different parts of the facility. Because of the large shielding thicknesses required (up to ~4 m), biasing techniques had to be employed. While the results of the calculations confirmed that the planned shielding is sufficient in most places, two areas have been identified where a reinforcement of the shielding is recommended.

MONTE CARLO SIMULATIONS FOR THE SHIELDING OF THE COLLECTOR RING AND THE RECYCLED EXPERIMENTAL STORAGE RING AT FAIR

Abstract In the next years the Gesellschaft fuer Schwerionenforschung and international partners will realize the new international accelerator Facility for Antiproton and Ion Research (FAIR) for research with heavy ions, radioactive ions, and antiprotons. Two important storage rings of FAIR are the collector ring (CR) and the recycled experimental storage ring (RESR), which are located together in the same building: The CR is optimized for fast cooling of heavy ions and antiprotons, while the RESR is mainly used for accumulation of antiprotons. The concrete shielding for the CR and RESR is presented on the basis of several Monte Carlo simulations for radiation transport with the latest version of the FLUKA code. Extensive shielding calculations had to be done because of diverse beam types including different locations of beam losses. The goal of the simulations is to reveal possible weak points in the shielding and to ensure a dose rate outside the storage rings and in the technical supplies’ room to a value of <0.5 μSv/h so that this area is accessible without any restrictions.