Abstract-AREVA Mines and the Nuclear Measurement Laboratory of CEA Cadarache are collaborating to improve the sensitivity and precision of uranium concentration evaluation by means of gamma measurements. This paper reports gamma-ray spectra, recorded with a high-purity coaxial germanium detector, on standard cement blocks with increasing uranium content, and the corresponding MCNP simulations. The detailed MCNP model of the detector and experimental setup has been validated by calculation vs. experiment comparisons. An optimization of the detector MCNP model is presented in this paper, as well as a comparison of different nuclear data libraries to explain missing or exceeding peaks in the simulation. Energy shifts observed between the fluorescence X-rays produced by MCNP and atomic data are also investigated. The qualified numerical model will be used in further studies to develop new gamma spectroscopy approaches aiming at reducing acquisition times, especially for ore samples with low uranium content.
Fission chambers are devices widely used in the zero power facilities of the CEA within the scope of the various experimental programs conducted in the Cadarache center. Those detectors gave commonly good quality results; however, improvements could still be done to enhance them. It concerns in particular the behaviour of the chamber itself as well as the modelling of the downstream electronics. Within this framework a PhD work has been launched by the SPEx/LPE in order to improve that knowledge and to reduce the uncertainties linked to the measurements performed. Two major issues are to be addressed first, the calibration process of the fission chambers and the dead time phenomenon.
Investigations of spectrum adjustment by unfolding techniques for different locations of critical and driven subscritical MUSE4 configurations are presented. The current analysis is based on the recent accomplishment of specific foil activation measurements and on the use of the unfolding code STAY'SL in conjunction with IRDF-90 and IRDF-2002 dosimetry libraries. Specific developments have been conducted to consider the uncertainties associated with the determination of the experimental absolute reaction rates and the initial calculated spectrum in the most realistic manner. In this context, the iterative adjustment process for the neutron spectrum is established on the basis of variance/covariance matrices defined for the three types of input parameters, i.e., the experimental reaction rates, the initial spectrum, and the dosimetry nuclear data files. The results highlight the spectral variations between different locations of the MUSE4 cores, in particular the characteristic behavior of a subcritical configuration driven by an external T(d,n)He4 (14 MeV neutrons) source.
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