Helmholtz-Zentrum Dresden-Rossendorf Research Articles

First test of the prompt gamma ray timing method with heterogeneous targets at a clinical proton therapy facility

Ion beam therapy promises enhanced tumour coverage compared to conventional radiotherapy, but particle range uncertainties significantly blunt the achievable precision. Experimental tools for range verification in real-time are not yet available in clinical routine. The prompt gamma ray timing method has been recently proposed as an alternative to collimated imaging systems. The detection times of prompt gamma rays encode essential information about the depth-dose profile thanks to the measurable transit time of ions through matter. In a collaboration between OncoRay, Helmholtz-Zentrum Dresden-Rossendorf and IBA, the first test at a clinical proton accelerator (Westdeutsches Protonentherapiezentrum Essen, Germany) with several detectors and phantoms is performed. The robustness of the method against background and stability of the beam bunch time profile is explored, and the bunch time spread is characterized for different proton energies. For a beam spot with a hundred million protons and a single detector, range differences of 5 mm in defined heterogeneous targets are identified by numerical comparison of the spectrum shape. For higher statistics, range shifts down to 2 mm are detectable. A proton bunch monitor, higher detector throughput and quantitative range retrieval are the upcoming steps towards a clinically applicable prototype. In conclusion, the experimental results highlight the prospects of this straightforward verification method at a clinical pencil beam and settle this novel approach as a promising alternative in the field of in vivo dosimetry.

Response of multi-strip multi-gap resistive plate chamber

A prototype of Multi-strip Multi-gap Resistive Plate chamber (MMRPC) with active area 40 cm × 20 cm has been developed at SINP, Kolkata. Detailed response of the developed detector was studied with the pulsed electron beam from ELBE at Helmholtz-Zentrum Dresden-Rossendorf. In this report the response of SINP developed MMRPC with different controlling parameters is described in details. The obtained time resolution (σt) of the detector after slew correction was 91.5 ± 3 ps. Position resolution measured along (σx) and across (σy) the strip was 2.8±0.6 cm and 0.58 cm, respectively. The measured absolute efficiency of the detector for minimum ionizing particle like electron was 95.8±1.3 %. Better timing resolution of the detector can be achieved by restricting the events to a single strip. The response of the detector was mainly in avalanche mode but a few percentage of streamer mode response was also observed. A comparison of the response of these two modes with trigger rate was studied.

Scatter analysis and correction for ultrafast X-ray tomography.

Ultrafast X-ray computed tomography (CT) is an imaging technique with high potential for the investigation of the hydrodynamics in multiphase flows. For correct determination of the phase distribution of such flows, a high accuracy of the reconstructed image data is essential. In X-ray CT, radiation scatter may cause disturbing artefacts. As the scattering is not considered in standard reconstruction algorithms, additional methods are necessary to correct the detector readings or to prevent the detection of scattered photons. In this paper, we present an analysis of the scattering background for the ultrafast X-ray CT imaging system ROFEX at the Helmholtz-Zentrum Dresden-Rossendorf and propose a correction technique based on collimation and deterministic simulation of first-order scattering.

Drag and turbulence modelling for free surface flows within the two-fluid Euler–Euler framework

Two-phase flows are regularly involved in the heat and mass transfer in industrial processes. To ensure the safety and efficiency of such processes, an accurate prediction of the flow field and phase distribution by means of Computational Fluid Dynamics (CFD) is required. Nowadays, Direct Numerical Simulations (DNS) of large-scale two-phase flow problems are not feasible due to the computational costs involved. Therefore, the Euler–Euler framework is often employed for large-scale simulations, which involves macro-scale modelling of turbulence, mass and momentum transfer. The research activities at Helmholtz–Zentrum Dresden–Rossendorf (HZDR) focus on general closure models for multiphase flows that are closer to physics and include less empiricism. As part of this effort, an Algebraic Interfacial Area Density model (AIAD) is developed for the morphology detection in the two-fluid Euler–Euler approach. Drag models for free surface flows are often based on experimental correlations, their applicability thus being limited to certain flow regimes. In this paper, a modified free-surface drag model based on local shear stress is investigated that avoids this limitation. For this purpose, the algebraic morphology detection mechanism of the AIAD model is revised. In DNS of free surface flow a dampening of the gas side turbulent fluctuations in the near surface region was found by previous investigators. This effect has also been accounted for in Euler–Euler simulations by means of dampening functions. In this work, the significance of turbulence dampening, in case of free surface flows, is examined quantitatively for the k–ω turbulence model. Model validation is performed with the commercial CFD code ANSYS CFX by means of experimental data of countercurrent, supercritical stratified air–water flow. The revised morphology detection mechanism is seen as an improvement with respect to the detection of sharp interfaces. Satisfactory quantitative agreement is achieved for the modified free surface drag model based on experimental pressure difference, liquid levels and interfacial shear stress. Furthermore, it is demonstrated that turbulence dampening has to be accounted for in the k–ω model to qualitatively reproduce the mean flow and turbulence quantities from the experiment. More CFD grade experimental data is required for further model validation.

Evaluation of the cone-shaped pickup performance for low charge sub-10 fs arrival-time measurements at free electron laser facilities

An evaluation of the cone-shaped pickup performance as a part of the high bandwidth bunch arrival-time monitors (BAMs) for a low charge sub-10 fs arrival-time measurements is presented. Three sets of pickups are installed at the free electron laser FLASH at Deutsches Elektronen-Synchrotron, the quasi-cw SRF accelerator ELBE at the Helmholtz-Zentrum Dresden-Rossendorf and the SwissFEL injector test facility at Paul Scherrer Institute. Measurements and simulations are in good agreement and the pickups fulfill the design specifications. Utilizing the high bandwidth BAM with the cone-shaped pickups, an improvement of the signal slope by a factor of 10 is demonstrated at ELBE compared to the BAM with a low bandwidth.

Numerical simulations for the DRESDYN precession dynamo

The next generation dynamo experiment currently under development at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) will consist of a precessing cylindrical container filled with liquid sodium. We perform numerical simulations of kinematic dynamo action applying a velocity field that is obtained from hydrodynamic models of a precession driven flow. So far, the resulting magnetic field growth-rates remain below the dynamo threshold for magnetic Reynolds numbers up to Rm=2000.

Towards a precession driven dynamo experiment

The most ambitious project within the DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is the set-up of a precession-driven dynamo experiment. After discussing the scientific background and some results of water pre-experiments and numerical predictions, we focus on the numerous structural and design problems of the machine. We also outline the progress of the building's construction, and the status of some other experiments that are planned in the framework of DRESDYN.

S-Factor measurement of the12C(p,γ)13N reaction in inverse kinematics

Hydrogen rich solid targets have been developed and produced to investigate the 12 C(p, γ ) 13 N reaction in inverse kinematics. The SRIM simulation software has been used to determine the parameters for ion implantation in various materials. Nuclear Resonant Reacton Analysis (NRRA) with the resonant reaction 15 N(p, αγ ) 12 C has been carried out to measure the hydrogen content of the produced targets. Measurements of the produced targets at the energy range from E cm = 577 keV down to E cm = 191 keV, were performed at the 3-MV Tandetron of Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

Fast neutron measurements at the nELBE time-of-flight facility

The compact neutron-time-of-flight facility nELBE at the superconducting electron accelerator ELBE of Helmholtz-Zentrum Dresden-Rossendorf has been rebuilt. A new enlarged experimental hall with a flight path of up to 10 m is available for neutron time-of-flight experiments in the fast energy range from about 50 keV to 10 MeV. nELBE is intended to deliver nuclear data of fast neutron nuclear interactions e.g. for the transmutation of nuclear waste and improvement of neutron physical simulations of innovative nuclear systems. The experimental programme consists of transmission measurements of neutron total cross sections, elastic and inelastic scattering cross section measurements, and neutron induced fission cross sections. The inelastic scattering to the first few excited states in 56 Fe was investigated by measuring the gamma production cross section with an HPGe detector. The neutron induced fission of 242 Pu was studied using fast ionisation chambers with large homogeneous actinide deposits.

Static and transient pin-by-pin simulations of a full PWR core with the extended coupled code system DYNSUB

The evolutionary multi-physics tool developed at the Karlsruhe Institute of Technology is the homogeneous pin-by-pin reactor simulator DYNSUB, an internal coupling of the 3D neutron kinetics code DYN3D developed by Helmholtz Zentrum Dresden Rossendorf and the in-house sub-channel code SUBCHANFLOW. The ultimate goal of the on-going efforts concerning DYNSUB is to provide a cost-effective improved description of light water reactor core behavior with pin-by-pin resolution for both static and transient safety relevant scenarios. A cost-effective computer code is defined to be executable on commodity computing clusters which users/customers commonly have access to. Efforts undertaken to improve DYNSUB’s numerical performance and parallelize the code system are presented in this work. Moreover, the coupled code system has been extended in terms of fuel pin level homogenization corrections and flexible mapping schemes. After optimization and extension DYNSUB is successfully applied to study the OECD/NEA and U.S. NRC PWR MOX/UO2 core transient benchmark with both fuel assembly/channel and pin level/sub-channel model resolution. Even though further improvements in terms of numerical performance and accuracy of physical models are required, the applicability of DYNSUB pin-by-pin simulations for light water reactor safety analysis is proven in principle in this work.

Precision measurement of timing RPC gas mixtures with laser-beam induced electrons

The main goals of a new test facility at Helmholtz-Zentrum Dresden-Rossendorf are precision measurements of the electron drift velocity and the Townsend coefficient of gases at atmospheric pressure in the strongest ever used homogenous electrical fields and the search for new RPC gas mixtures to substitute the climate harmful Freon. Picosecond UV laser pulses were focused into a sub-millimeter gas gap to initialize a defined tiny charge. These gaps are formed by electrodes of low-resistive ceramics or high-resistive float glass. The charge multiplication occurs in a strong homogeneous electric field of up to 100 kV/cm. Electron-ion pairs were generated in a cylindrical micro-volume by multi-photon ionization. The laser-pulse repetition rate ranges from 1 Hz to a few kHz. The RPC time resolution has been measured for different gases. First results of the Townsend coefficient at 100 kV/cm show a strong disagreement between the present measurement and Magboltz simulations for the typical timing RPC gas mixture C2F4H2/SF6/i-C4H10, while the measured electron drift velocities are in a good agreement with the model predictions.

Oxygen depth profiling with subnanometre depth resolution

A High-depth Resolution Elastic Recoil Detection (HR-ERD) set-up using a magnetic spectrometer has been taken into operation at the Helmholtz-Zentrum Dresden-Rossendorf for the first time. This instrument allows the investigation of light elements in ultra-thin layers and their interfaces with a depth resolution of less than 1nm near the surface. As the depth resolution is highly influenced by the experimental measurement parameters, sophisticated optimisation procedures have been implemented. Effects of surface roughness and sample damage caused by high fluences need to be quantified for each kind of material. Also corrections are essential for non-equilibrium charge state distributions that exist very close to the surface. Using the example of a high-k multilayer SiO2/Si3N4Ox/SiO2/Si it is demonstrated that oxygen in ultra-thin films of a few nanometres thickness can be investigated by HR-ERD.

Effects of Shape and Size on Countercurrent Flow Limitation in Flow Channels Simulating a PWR Hot Leg

A numerical study is presented to examine the effects on countercurrent flow limitation (CCFL) of the shape and size of hot leg models with a rectangular cross section. The CCFL was described in terms of Wallis parameters using the channel height H as the characteristic length. Numerical simulations, using the computational fluid dynamics software code FLUENT 6.3.26, were done for the air-water CCFL experiments carried out previously at Helmholtz-Zentrum Dresden-Rossendorf in a 1/3-scale hot leg model with a rectangular channel (H×W = 0.25×0.05 m2), and the results were compared with the air-water CCFL data obtained at Kobe University in a 1/5-scale hot leg model with rectangular cross section (H×W = 0.15×0.01 m2) and the results of simulations. It was found that both the height-to-width ratio and the size of the cross section affected the CCFL characteristics in the Wallis diagram. Comparison of CCFL characteristics in rectangular channels with those in circular channels showed that the hydraulic diameter Dh was a major cross-section geometry term influencing the CCFL characteristics. CCFL constants of the Wallis correlation were ~0.61 on average for the range 0.05 m ≤ Dh ≤ 0.75 m but became small for Dh ≤ 0.0254 m, and these tendencies were well reproduced by the numerical simulations.

Observation of Anisotropic Exchange in a Spin Ladder by ESR

E. ƒiomara,∗, M. Ozerov, K.W. Kramer, Ch. Ruegg, S.A. Zvyagin Institute of Physics, Faculty of Sciences, P.J. ’afarik University, Park Angelinum 9, Ko2ice, Slovakia Dresden High Magnetic Field Laboratory (HLD), Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland Laboratory for Neutron Scattering, Paul Scherrer Institut, Villigen PSI, Switzerland

Experimental study of a simplified 3 × 3 rod bundle using DPTV

Computational fluid dynamics (CFD) has evolved over the last years as an important tool for the simulation and prediction of rod bundles in plant components under various scenarios, contributing to the continuing improvement of performance and safety of nuclear power plants. However, these tools must be validated and verified in order to assure, with enough confidence, the reliability and quality of CFD predictions. The present work focuses in the basic study and a benchmark case of rod bundles using a top-bench experimental set up. The experimental set-up provides complete optical access to the test section by using a matching refractive index between the liquid flow, rods and flow envelope. Dynamic particle tracking velocimetry (DPTV) was used to measure the time resolved velocity fields in the rod bundle. Experimental turbulence parameters and main flow characteristics are presented in this work. This study seeks to provide a basic yet enough comprehensive benchmark case for CFD under single-phase conditions. The geometry associated with the bundle problem has been simplified in order to provide high quality data (high temporal and spatial resolution) in order to directly compare with CFD results. The measurement uncertainties in these experiments are discussed and evaluated yielding an uncertainty of 11% in the axial and lateral velocity components of the measured velocity vectors. This work is a collaborative effort between Texas A&M University and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) Institute in Germany.

Test of Compton camera components for prompt gamma imaging at the ELBE bremsstrahlung beam

In the context of ion beam therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a Compton camera setup is being developed consisting of two scatter planes: two CdZnTe (CZT) cross strip detectors, and an absorber consisting of one Lu2SiO5 (LSO) block detector. The data acquisition is based on VME electronics and handled by software developed on the ROOT framework.The setup has been tested at the linear electron accelerator ELBE at HZDR, which is used in this experiment to produce bunched bremsstrahlung photons with up to 12.5 MeV energy and a repetition rate of 13 MHz. Their spectrum has similarities with the shape expected from prompt gamma rays in the clinical environment, and the flux is also bunched with the accelerator frequency.The charge sharing effect of the CZT detector is studied qualitatively for different energy ranges. The LSO detector pixel discrimination resolution is analyzed and it shows a trend to improve for high energy depositions.The time correlation between the pulsed prompt photons and the measured detector signals, to be used for background suppression, exhibits a time resolution of 3 ns FWHM for the CZT detector and of 2 ns for the LSO detector. A time walk correction and pixel-wise calibration is applied for the LSO detector, whose resolution improves up to 630 ps. In conclusion, the detector setup is suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the quantitative comparison with simulations and the test of imaging algorithms. Experiments at proton accelerators have also been performed and are currently under analysis.

Target Characterization of Large Area Minor Actinide Layers and Fission Chamber Simulations for Fast Neutron-induced Fission Cross Section Experiments at nELBE

At the Center for High-Power Radiation Sources at Helmholtz-Zentrum Dresden-Rossendorf fast neutron-induced fission cross section experiments on 235U and 242Pu will be investigated by a parallel plate fission ionization chamber. An optimization of chamber parameters was performed using extensive Geant4 simulations with GEF code generated fission observable inputs. Pile-up effects due to the high α activity of the plutonium targets have been considered in a realistic geometry. Beyond that, a setup for the determination of the areal density and homogeneity of targets will be presented with a focus on current simulation work.

Fast-neutron Induced Reactions at the nELBE Time-of-flight Facility

The compact neutron-time-of-flight facility nELBE at the superconducting electron accelerator ELBE of Helmholtz-Zentrum Dresden-Rossendorf is being rebuilt and extended with a low-background experimental hall. The neutron radiator consists of a liquid lead circuit without additional neutron moderators. The useful neutron spectrum extends from some tens of keV to about 10 MeV. nELBE is intended to deliver cross section data of fast-neutron nuclear interactions e.g. for the transmutation of nuclear waste and improvement of neutron physical simulations of innovative nuclear systems. Before the extension of the facility, the photon production cross section of 56Fe was measured with an HPGe detector and the inelastic neutron scattering cross section to the first few excited states in 56Fe was determined. The neutron total cross sections of Au and Ta were determined in the energy from 200 keV to 7 MeV in a transmission experiment.

Tomographic Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy serves as a perfect tool for studies of open-volume defects in solid materials such as vacancies, vacancy agglomerates, and dislocations. Moreover, structures in porous media can be investigated ranging from 0.3 nm to 30 nm employing the variation of the Positronium lifetime with the pore size. While lifetime measurements close to the material's surface can be performed at positron-beam installations bulk materials, fluids, bio-materials or composite structures cannot or only destructively accessed by positron beams. Targeting those problems, a new method of non-destructive positron annihilation lifetime spectroscopy has been developed which features even a 3-dimensional tomographic reconstruction of the spatial lifetime distribution. A beam of intense bremsstrahlung is provided by the superconducting electron linear accelerator ELBE (Electron Linear Accelerator with high Brilliance and low Emittance) at Helmholtz-Zentrum Dresden-Rossendorf. Since the generation of bremsstrahlung and the transport to the sample preserves the sharp timing of the electron beam, positrons generated inside the entire sample volume by pair production feature a sharp start time stamp for lifetime studies. In addition to the existing technique of in-situ production of positrons inside large (cm3) bulk samples using high-energy photons up to 16 MeV from bremsstrahlung production, granular position-sensitive photon detectors have been employed. The detector system will be described and results for experiments using samples with increasing complexity will be presented. The Lu2SiO5:Ce scintillation crystals allow resolving the total energy to 5.1 % (root-mean-square, RMS) and the annihilation lifetime to 225 ps (RMS). 3-dimensional annihilation lifetime maps have been created in an offline-analysis employing well-known techniques from PET.

3 × 3 rod bundle investigations, CFD single-phase numerical simulations

The work here presented has been performed in the framework of a research project aimed to investigate two-phase (boiling) flows in pressurized water reactors (PWR). CFD investigations of a rod bundle have been conducted while a new experimental facility (ROFEX) was constructed in Helmholtz Zentrum Dresden-Rossendorf (HZDR) for the generation of quality validation data. The apparatus consists of a 3×3 rod bundle inside a Plexiglas vertical pipe. The results summarized in this paper are considered as a pre-investigation, being the final goal to be able to predict accurately boiling water flows under high pressure around rods. For this purpose, three steps were defined: analysis of single-phase flows in such geometry, analysis of the multiphase flow when using a refrigerant as a working fluid and, finally, the analysis of a multiphase flow using water. The single-phase approach allows gaining experience regarding the turbulence behaviour of the flow, while the multiphase investigation of the refrigerant simplifies the experimental conditions since it is possible to get boiling situations at lower pressure level. At the moment of writing this paper, the authors were focused on the first step (single-phase flows at low pressure), since this not only made possible to better understand the turbulence in that geometry, but it also resulted in valuable feedback to the experimentalists on improving the construction of the facility. In parallel, HZDR researchers have been developing a new tomography measurement technique to measure gas content in multiphase flows.