Modeling the response of a diamond detector in the zero power reactor CROCUS

The present manuscript documents the development of a practical calculation scheme to model the response of a diamond detector in the mixed (neutron and gamma) radiation field of the CROCUS experimental reactor at EPFL. The model is shown to perform reasonably well in terms of energy spectra shapes for the limited amounts of irradiations considered; the relative magnitude of the neutron and gamma contributions to the detector output is, however, not properly captured. The gamma contribution is underestimated by a factor of four if the neutron contribution is correctly captured. A sensitivity study to three parameters (the azimuthal position of the detector, the magnitude of the neutron and gamma sources, and the threshold voltage used in the detector) is also carried out to analyze the effect of modeling assumptions and experimental choices on the results. Then, a detailed analysis of the computational results is carried out to provide quantitative information about the response of a diamond detector in a mixed radiation field. It is shown that the low-energy interactions are mainly produced by gamma interactions; that the high energy interactions are generated by proton recoil interactions; and that 60% of the recorded neutron interactions are produced by thermal neutron captures in Li-6, while the rest come from scattering interactions with fast neutrons. Finally, it is demonstrated that considering some of the effects of the electronics on the pulses (RC time constant) is required to reproduce the trends observed in the experimental results, especially the increased detection rate within the diamond crystal.

Testing of a sCVD diamond detection system in the CROCUS reactor

Pavel Frajtag, Mathieu Hursin, Vincent Pierre Lamirand, Andreas Pautz

The paper describes the testing of the NEUTON detection system into CROCUS, the zero-power reactor of the École Polytechnique Fédérale de Lausanne (EPFL). NEUTON is composed of a 4mm×4 mm sCVD diamond detector with a 6Li converter and the associated acquisition electronics. It is developed by CIVIDEC Instrumentation GmbH. The use of a diamond detector with converter in the mixed radiation field of a nuclear reactor is challenging because these detectors are sensitive to gamma-rays, fast neutrons and thermal neutrons through conversion in 6Li . In NEUTON, the rejection of gamma-rays is achieved in real time, via the analysis of the signal pulse shape from the detector. To do so, a few signal characteristics (amplitude, area and FWHM) are recorded in the integrated Field Programmable Gate Arrays (FPGA) of the system. This treatment does not induce any dead time. Measurements in CROCUS demonstrated for the first time the capability of a system like NEUTON to detect and separate fast neutrons, thermal neutrons, and gamma-rays. The system response was shown to be linear with respect to the reactor power (up to 35W) and its thermal sensitivity was found to be (3.5±0.2)×10−5 cps/nv.

2018

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

EPFL2015

Design and Simulation of Gamma Spectrometry Experiments in the CROCUS Reactor

Mathieu Hursin, Vincent Pierre Lamirand, Oskari Ville Pakari, Andreas Pautz, Fanny Vitullo

Gamma rays in nuclear reactors, arising either from fission or decay processes, significantly contribute to the heating and dose of the reactor components. Zero power research reactors offer the possibility to measure gamma rays in a purely neutronic environment, allowing for validation experiments of computed spectra, dose estimates, reactor noise and prompt to delayed gamma ratios. This data then contributes to models, code validation and photo atomic nuclear data evaluation. In order to contribute to aforementioned experimental data, gamma detection capabilities are being added to the CROCUS reactor facility. The CROCUS reactor is a two-zone, uranium-fueled light water moderated facility operated by the Laboratory for Reactor Physics and Systems Behaviour (LRS) at the Swiss Federal Institute of Technology Lausanne (EPFL). With a maximum power of 100 W, it is a zero power reactor used for teaching and research, most recently for intrinsic and induced neutron noise studies. For future gamma detection applications in the CROCUS reactor, an array of four detectors - two large 5 '' x10 '' Bismuth Germanate (BGO) and two smaller Cerium Bromide (CeBr3) scintillators - was acquired. The BGO detectors are to be arbitrarily positioned in the core reflector and out of the vessel for measurements at arbitrary distances. The CeBr3 detectors on the other hand are small enough to be set in the guide tubes of the control rods for in-core measurements. We present a study of the neutron and gamma flux in the core and reflector using the MCNP 6.2 and Serpent 2 Monte Carlo codes for coupled neutron and photon transport criticality calculations. More specifically, we investigate and compare predicted spectra as well as reactivity worth of different envisioned experimental setups. We further predict pulse height spectra as well as doses to the crystals with and without cadmium shielding to estimate allowable reactor powers with respect to detector radiation hardness. The results serve as basis for calibration and aid in the design and regulatory approval of the experiments.