Research Reactor FRM Research Articles

Pencil beam kernel-based dose calculations on CT data for a mixed neutron-gamma fission field applying tissue correction factors.

Objective.For fast neutron therapy with mixed neutron and gamma radiation at the fission neutron therapy facility MEDAPP at the research reactor FRM II in Garching, no clinical dose calculation software was available in the past. Here, we present a customized solution for research purposes to overcome this lack of three-dimensional dose calculation.Approach.The applied dose calculation method is based on two sets of decomposed pencil beam kernels for neutron and gamma radiation. The decomposition was performed using measured output factors and simulated depth dose curves and beam profiles in water as reference medium. While measurements were performed by applying the two-chamber dosimetry method, simulated data was generated using the Monte Carlo code MCNP. For the calculation of neutron dose deposition on CT data, tissue-specific correction factors were generated for soft tissue, bone, and lung tissue for the MEDAPP neutron spectrum. The pencil beam calculations were evaluated with reference to Monte Carlo calculations regarding accuracy and time efficiency.Main results.In water, dose distributions calculated using the pencil beam approach reproduced the input from Monte Carlo simulations. For heterogeneous media, an assessment of the tissue-specific correction factors with reference to Monte Carlo simulations for different tissue configurations showed promising results. Especially for scenarios where no lung tissue is present, the dose calculation could be highly improved by the applied correction method.Significance.With the presented approach, time-efficient dose calculations on CT data and treatment plan evaluations for research purposes are now available for MEDAPP.

A new Multi Leaf Collimator for the fission neutron therapy facility at FRM II

As a part of the general upgrade program of the fission neutron therapy facility MEDAPP at the research reactor FRM II, a newly designed, electronically operated Multi Leaf Collimator (MLC) was introduced. The design of the new collimator was optimized in order to increase dose output and to allow patient positioning closer to the collimator. The new collimator allows divergent and convergent beam shaping in order to maximize the effective area of the fission source used for irradiation. In comparison to the old MLC, the radiation dose for the staff is reduced by the motorized operation of the new MLC. The advanced beam shaping abilities of the new collimator are investigated by a comparison with depth dose curves and beam profiles of the old MLC. For the separation of neutron and gamma dose deposition in water, the two chamber dosimetry method recommended for mixed field dosimetry was applied. A re-calibration of the set of ionization chambers used for dosimetry in mixed neutron-gamma radiation fields at FRM II was performed. The calibration was done in a reference neutron field from 3H(p,n)3He-reactions in a low scattering environment. An overall increased dose output as intended by the design of the new MLC was shown. Also, improved dosimetric characteristics of the treatment field due to the optimized beam shaping abilities of the new MLC were demonstrated. In the calibration measurements of the magnesium ionization chamber used in the two chamber method, an unexpected increase of the neutron sensitivity by a factor of 8 was measured. The new MLC will enable more conformal dose delivery to the target volume in patients and therefore help to reduce healthy tissue damage by the application of neutron radiation.

POWTEX visits POWGEN.

The high-intensity time-of-flight (TOF) neutron diffractometer POWTEX for powder and texture analysis is currently being built prior to operation in the eastern guide hall of the research reactor FRM II at Garching close to Munich, Germany. Because of the world-wide 3He crisis in 2009, the authors promptly initiated the development of 3He-free detector alternatives that are tailor-made for the requirements of large-area diffractometers. Herein is reported the 2017 enterprise to operate one mounting unit of the final POWTEX detector on the neutron powder diffractometer POWGEN at the Spallation Neutron Source located at Oak Ridge National Laboratory, USA. As a result, presented here are the first angular- and wavelength-dependent data from the POWTEX detector, unfortunately damaged by a 50g shock but still operating, as well as the efforts made both to characterize the transport damage and to successfully recalibrate the voxel positions in order to yield nonetheless reliable measurements. Also described is the current data reduction process using the PowderReduceP2D algorithm implemented in Mantid [Arnold et al. (2014). Nucl. Instrum. Methods Phys. Res. A, 764, 156-166]. The final part of the data treatment chain, namely a novel multi-dimensional refinement using a modified version of the GSAS-II software suite [Toby & Von Dreele (2013). J. Appl. Cryst.46, 544-549], is compared with a standard data treatment of the same event data conventionally reduced as TOF diffraction patterns and refined with the unmodified version of GSAS-II. This involves both determining the instrumental resolution parameters using POWGEN's powdered diamond standard sample and the refinement of a friendly-user sample, BaZn(NCN)2. Although each structural parameter on its own looks similar upon comparing the conventional (1D) and multi-dimensional (2D) treatments, also in terms of precision, a closer view shows small but possibly significant differences. For example, the somewhat suspicious proximity of the a and b lattice parameters of BaZn(NCN)2 crystallizing in Pbca as resulting from the 1D refinement (0.008 Å) is five times less pronounced in the 2D refinement (0.038 Å). Similar features are found when comparing bond lengths and bond angles, e.g. the two N-C-N units are less differently bent in the 1D results (173 and 175°) than in the 2D results (167 and 173°). The results are of importance not only for POWTEX but also for other neutron TOF diffractometers with large-area detectors, like POWGEN at the SNS or the future DREAM beamline at the European Spallation Source.

Measurement of k0 values for europium, lutetium and iridium at FRM\xa0II with a very well thermalized neutron spectrum

The k0 values of 6 non-1/v nuclides (152Eu, 152mEu, 154Eu, 177Lu, 192Ir and 194Ir) were determined using the extended Høgdahl formalism at the research reactor FRM II with very high f values. Standards were irradiated in 4 channels at different local temperatures between 40 °C and 55 °C measured using temperature sensitive irreversible labels. A good agreement with the recommended values was found for 152Eu, 154Eu and 177Lu using the original g(Tn) factors by Gryntakis, however, the k0 values for 152mEu in this work were 7% higher. New k0 values were also determined using the g(Tn) factors by Van Sluijs. Differences up to 6% were found for Eu isotopes compared with the recommended values. The recommended k0 values for 192Ir and 194Ir could be confirmed using g = 1. The theoretical k0 values for 177Lu were calculated using new nuclear data. They are up to 6% less than the recommended values. The present k0 values determined in this work showed a similar trend. The influence of different g(Tn) factors on the determination of the k0 values was investigated.

In situ measurement of neutron temperature for the extended k0 NAA at FRM II

The neutron temperature at different irradiation positions in the research reactor FRM II was measured using thermometer labels together with lutetium standards. This was the first time to measure the local temperature at the irradiation positions with this combination. The simplified Westcott formalism was used to calculate the neutron temperature using Lu and g(Tn) factors from different data sources. The results showed agreement between neutron temperature according to the Lu method and the temperature showed on the labels. They can be used as a reliable alternative to determine the neutron temperature instead of using Lu standards in the extended k0 NAA.

Design of the magnet system of the neutron decay facility PERC

The PERC (Proton and Electron Radiation Channel) facility is currently under construction at the research reactor FRM II, Garching. It will serve as an intense and clean source of electrons and protons from neutron beta decay for precision studies. It aims to contribute to the determination of the Cabibbo-Kobayashi-Maskawa quark-mixing element Vud from neutron decay data and to search for new physics via new effective couplings. PERC's central component is a 12 m long superconducting magnet system. It hosts an 8 m long decay region in a uniform field. An additional high-field region selects the phase space of electrons and protons which can reach the detectors and largely improves systematic uncertainties. We discuss the design of the magnet system and the resulting properties of the magnetic field.

Local Residual Stress Depth Distribution in the Inner Gearing of a Case Hardened Sliding Collar

With respect to residual stress analysis the inner gearing of an automotive sliding collar is a component with a challenging, complex geometry. The accessibility of the tooth root does not exist for conventional measuring approaches. However, the process steps like e.g. broaching and case hardening induce characteristic residual stress distributions, which must be known for the valuation of the mechanical integrity of the parts. For lab X-ray stress analysis approaches according to the sin2ψ-method [1] the ring like structure must be sectioned, which affects the process induced residual stress state. The tooth root is rather small, which further leads to shadowing effects during tilting of the sample. Standard mechanical approaches like incremental hole drilling can be excluded due to the narrow tooth root. Local neutron diffraction residual stress analysis in the tooth root by means of neutron through surface strain scanning at the STRESS-SPEC instrument at the research reactor FRM II, Garching (Germany) was successfully carried out for the inner gearing. A measuring and evaluation strategy is proposed, where special attention is paid to the compensation of the surface effect due to the incomplete immersion of the nominal gauge volume during through surface scanning and to the local variation of the D0-value as a consequence of the case hardening process.

Radio frequency elevator for a pulsed positron beam

An elevator increases the potential energy of a particle beam with respect to ground potential without any alteration of kinetic energy and other beam parameters. This elevator is necessary for the implementation of the Munich Scanning Positron Microscope (SPM) at the intense positron source NEPOMUC at the research reactor FRM II in Munich. The principles of the rf elevator for pure electrostatically guided positrons are described. Measurements of beam quality behind the elevator are reported, which confirm that after the implementation of elevator and SPM at NEPOMUC the SPM can be operated at a considerably improved resolution (~0.3μm) and event rate (~3.7kHz) compared to the laboratory based β+-source.

Paving the Road for Modern Particle Therapy - What Can We Learn from the Experience Gained with Fast Neutron Therapy in Munich?

While neutron therapy was a highly topical subject in the 70s and 80s, today there are only a few remaining facilities offering fast neutron therapy (FNT). Nevertheless, up to today more than 30,000 patients were treated with neutron therapy. For some indications like salivary gland tumors and malignant melanoma, there is clinical evidence that the addition of FNT leads to superior local control compared to photon treatment alone. FNT was available in Munich from 1985 until 2000 at the Reactor Neutron Therapy (RENT) facility. Patient treatment continued at the new research reactor FRM II in 2007 under improved treatment conditions, and today it can still be offered to selected patients as an individual treatment option. As there is a growing interest in high-linear energy transfer (LET) therapy with new hadron therapy centers emerging around the globe, the clinical data generated by neutron therapy might help to develop biologically driven treatment planning algorithms. Also FNT might experience its resurgence as a combinational partner of modern immunotherapies.

FaNGaS: A new instrument for [formula omitted] reaction measurements at FRM II

The FaNGaS instrument was set up at the research reactor FRM II, Garching. It performs measurements of (n,n′γ) reactions based on the γ-ray spectroscopy technique within the fission neutrons energy region. The optimization studies of the experimental setup were carried out using MCNP simulations. A full characterization of the detector system and the neutron beam is presented. The determination of the neutron energy spectrum was done using the foil activation method. An average neutron energy of about 2MeV and a neutron flux of 108cm−2·s−1 were measured.

KWS-1 high-resolution small-angle neutron scattering instrument at JCNS: current state

The KWS-1 small-angle neutron scattering (SANS) instrument operated by the Jülich Centre for Neutron Science (JCNS) at the research reactor FRM II of the Heinz Maier-Leibnitz Zentrum in Garching near Munich has been recently upgraded. The KWS-1 instrument was updated, from its active collimation apertures to the detector cabling. Most of the parts of the instrument were installed for the first time, including a broadband polarizer, a large-cross-section radio-frequency spin flipper, a chopper and neutron lenses. A custom-designed hexapod in the sample position allows heavy loads and precise sample positioning in the beam for conventional SANS experiments as well as for grazing-incidence SANS under applied magnetic field. With the foreseenin situpolarization analysis the main scientific topic of the instrument tends towards magnetism. The performance of the polarizer and flipper was checked with a polarized3He cell at the sample position. The results of these checks and a comparison of test measurements on a ferrofluid in a magnetic field with polarized and nonpolarized neutrons are presented.

High-flux PGAA for milligram-weight samples

With the high-intensity cold neutron flux available at the Prompt Gamma Activation Analysis (PGAA) instrument of the research reactor FRM II at the Heinz Maier-Leibnitz Zentrum (MLZ), samples with a weight of 1 mg or even less can be investigated for their elemental compositions using the (n,γ) capture reaction. In such cases, the typical sample packing material for PGAA experiments made of 25 μm thick PTFE foil (ca. 80 mg) can be orders of magnitude more massive than the sample weight itself. Proper choice of the packing material and measuring conditions are then of the highest importance [1].

Determination of trace concentrations of transmuted stable nuclides in TMD detectors using PGAA

The capability of prompt gamma-ray activation analysis (PGAA) for neutron fluence dosimetry by means of transmutation detectors is reported. The metallic foils of Ni, Au, Cu and Nb and the small piece of Ge crystal, which were irradiated at the LVR-15 reactor at Řež for several days, were selected for analysis by PGAA technique. Concentrations of transmuted stable nuclides in these foils were measured using the PGAA facility installed at the research reactor FRM II in Garching.

Applications of k 0 NAA at FRM II with high f values

The research reactor FRM II offers different irradiation facilities with highly thermalized neutron flux. 3 facilities for the k0 neutron activation analysis (k0 NAA) will be introduced shortly. The influence of flux parameter α on the concentration calculation of samples irradiated in a neutron field with very high ratio of thermal to epi-thermal neutron flux f > 1,000 are here investigated. Even for the most k0 isotopes with big Q0 values, the uncertainty of a concentration calculation without α correction is <3 %, when the f value larger than 3,000. The uncertainty is about 5 % for the isotope 96Zr in this case. The k0 library of the computer program MULTINAA is updated. A standard reference material IAEA/soil-7 was analyzed to verify the k0 NAA at FRM II.

Corrections on energy spectrum and scatterings for fast neutron radiography at NECTAR facility

Distortions caused by the neutron spectrum and scattered neutrons are major problems in fast neutron radiography and should be considered for improving the image quality. This paper puts emphasis on the removal of these image distortions and deviations for fast neutron radiography performed at the NECTAR facility of the research reactor FRM- II in Technische Universität München (TUM), Germany. The NECTAR energy spectrum is analyzed and established to modify the influence caused by the neutron spectrum, and the Point Scattered Function (PScF) simulated by the Monte-Carlo program MCNPX is used to evaluate scattering effects from the object and improve image quality. Good analysis results prove the sound effects of the above two corrections.

Neutron Residual Strain Surface Scans - Experimental Results and Monte Carlo Simulations

Precise determination of diffraction peak positions is of particular importance for the evaluation of residual strains. Neutrons are commonly used to probe residual strains from material volumes in depths of several millimetres under the sample surface. However, neutron strain analyses are critical for the near surface region. When scanning close to a sample surface, aberration peak shifts arise, which can be of the same order as the peak shifts related to residual strains [1]. Series of Monte Carlo (M.C.) simulations using the software package RESTRAX/SIMRES [2] were carried out to simulate the peak shift as a function of gauge volume depth, monochromator curvature and other instrumental parameters, which can be used to quickly optimise the experimental setup for direct measuring residual strains near the sample surface at an arbitrary surface orientation. The M.C. simulations were compared and agree very well with the experimental data, not only for a stress free steel sample but as well for a deep rolled steel sample, measured at the STRESS-SPEC diffractometer at the research reactor FRM II, Garching (Germany).

The Upgrade of the Neutron Induced Positron Source NEPOMUC

In summer 2012, the new NEutron induced POsitron Source MUniCh (NEPOMUC) was installed and put into operation at the research reactor FRM II. At NEPOMUC upgrade 80% 113Cd enriched Cd is used as neutron-gamma converter in order to ensure an operation time of 25 years. A structure of Pt foils inside the beam tube generates positrons by pair production. Moderated positrons leaving the Pt front foil are electrically extracted and magnetically guided to the outside of the reactor pool. The whole design, including Pt-foils, the electric lenses and the magnetic fields, has been improved in order to enhance both the intensity and the brightness of the positron beam. After adjusting the potentials and the magnetic guide and compensation fields an intensity of about 3·109 moderated positrons per second is expected. During the first start-up, the measured temperatures of about 90°C ensure a reliable operation of the positron source. Within this contribution the features and the status of NEPOMUC upgrade are elucidated. In addition, an overview of recent positron beam experiments and current developments at the spectrometers is given.

The ANCIENT CHARM project at FRM II: three-dimensional elemental mapping by prompt gamma activation imaging and neutron tomography

The aim of the EU FP6-funded ANCIENT CHARM project has been the development of new, and the integration of existing element-sensitive imaging methods utilizing neutrons. We present here the methodology and the first implementation of 3D elemental mapping at the Prompt Gamma Activation Analysis instrument of the research reactor FRM II, in order to determine spatially resolved elemental abundances in samples. After the design, optimization, and characterization of the new setup, measurements were successfully completed on archaeological objects of historical significance from the collection of the Hungarian National Museum.

RBE of thermal neutrons for induction of chromosome aberrations in human lymphocytes

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with slow neutrons was examined to assess the maximum low-dose RBE (RBE(M)) relative to (60)Co γ-rays. For the blood irradiations, cold neutron beam available at the prompt gamma activation analysis facility at the Munich research reactor FRM II was used. The given flux of cold neutrons can be converted into a thermally equivalent one. Since blood was taken from the same donor whose blood had been used for previous irradiation experiments using widely varying neutron energies, the greatest possible accuracy was available for such an estimation of the RBE(M) avoiding the inter-individual variations or differences in methodology usually associated with inter-laboratory comparisons. The magnitude of the coefficient α of the linear dose-response relationship (α=0.400±0.018Gy(-1)) and the derived RBE(M) of 36.4±13.3 obtained for the production of dicentrics by thermal neutrons confirm our earlier observations of a strong decrease in α and RBE(M) with decreasing neutron energy lower than 0.385MeV (RBE(M)=94.4±38.9). The magnitude of the presently estimated RBE(M) of thermal neutrons is-with some restrictions-not significantly different to previously reported RBE(M) values of two laboratories.

Quality assessment of neutron delivery system for small-angle neutron scattering diffractometers of the Jülich Centre for Neutron Science at the FRM II

Following the shutdown of FRJ-2 research reactor in Jülich, the pinhole small-angle neutron diffractometers KWS-1 and KWS-2 have been moved to the research reactor FRM II in Garching. The installation of these 40m long instruments required the design and setup of new neutron guides with geometrical and optical features imposed by the instruments' positioning in the neutron guide hall, such as, the predetermined length and beam height as well as the foreseen improvement of the instrument performance. We report here about the quality assessment of the newly constructed neutron guides with respect to the optical, geometrical and alignment characteristics and the positioning of the velocity selector integrated in the neutron guide system by comparing the features of the measured neutron beams (in terms of neutron flux, intensity distribution and beam profile) with the results of the simulations of optimal neutron guide systems.