The MADERE Radio-activity Measurement Platform: Developments for a Better Addressing to the Experimental Needs.

摘要

The main goal of the Reactor Dosimetry is to provide information (reaction rates, fluence, fluence rate...) for the interpretation of experiments irradiated in critical mock-up, test reactors or power nuclear reactors. Various techniques are used, including analysis of irradiated activation or fission dosimeters whose radioactivity is measured afterwards. The MADERE platform (Measurement Applied to DosimEtry for REactors) is a CEA facility which is dedicated to the activation dosimeters manufacturing and their activity measurement after irradiation in a nuclear reactor. The laboratory is accredited by the French Accreditation Committee for specific activity measurements of solid samples using gamma and X-rays spectrometry. The choice of dosimeters takes into account limitations coming from the characteristics of the measurement devices. To meet experimenter's new demands, the MADERE platform set out to improve its offer by lowering the energy of measured radiations down to 10 keV, and the activity level down to the tenth of Becquerel (Bq). Doing so, the range of usable dosimeters and by the way, the energy range of the neutron spectrum is expanded. Dosimeter, wires or foils, few millimeters large, are manufactured using ultra-pure material (Gold, Iron, Nickel, ...). Some of them are encapsulated in quartz containers for integration into experimental devices. In order to determine the activity of irradiated metal solid samples, the MADERE platform makes use of several measurement equipments: 1. Three Gamma spectrometry devices equipped with semiconductor diode detectors HPGe set to work on an energy field ranging from 50 keV to 2 MeV. The range of activity covered by these devices spreads from a few tenths of Bq to a few tens of MBq. The ability to measure samples with activities of the order of one tenth of Bq may be used to implement a nondestructive determination of the isotopic composition of low-active objects. 2. One X-ray spectrometry device equipped with a LEGE (Low Energy GErmanium) GeHP semiconductor detector. The settings of this equipment have been made to work in an energy range from 10 keV to 300 keV. This range allows covering almost all the X energy domain and also easily measuring low energy gamma emitters. For a long time, this device has been exclusively used for relative activity measurements of irradiated niobium using a ~(93)Nb~m reference sample (17 keV Xk emitter). This direct comparison measurement method allows getting free from the determination of different measurement parameters, since they are considered proportional between the reference and the measured samples, under the hypothesis that geometry, activity level and measurement conditions are identical. A need has emerged for measuring the ~(103)(Rh~m) activity. It would have required the use of a reference ~(103)(Rh~m) source which is in fact excluded due to the short half-life (56 minutes) of this radionuclide. It was therefore decided to establish the total absorption yield curve in order to perform absolute activity measurement. 3. One precise micro-balance allowing the measurement of sample mass with an accuracy of few micrograms. 4. One fissile measurement device equipped with 4 Nal detectors associated to an automated samples holder. The quality of gamma spectrometry measurements mainly depends on the knowledge of the detection efficiency yield associated to the Source-Detector system geometry. Thus, the MADERE platform uses twenty different calibrated measurement geometries. In order to optimize the time spent on the experimental determination Of each associated detection yield, a modeling work is underway, using the ETNA software, developed by the Henri Becquerel French National Primary Laboratory, to derived transfer efficiency and correction coincidence parameters. The precise determination of the sample activity also requires the use of correction factors related to the effects of self absorption, sum peak and geometry. Thus, the MADE