L’IPHC | Thèses et stages Master » Les Doctorants de l’IPHC / The PhD Students of the IPHC » Pages pro » Anciens doctorants » VANSTALLE Marie
Dernière mise à jour mardi 19 juillet 2011, par
Institut Pluridisciplinaire Hubert Curien - IPHC
Département de Recherche Subatomique - DRS
Centre National de la Recherche Scientifique - CNRS
UMR 7178 CNRS/Université De Strasbourg - UDS
23 rue du loess, 67037 Strasbourg cedex 2, France
Contact : Marie VANSTALLE
Titre de la thèse : Dosimétrie électronique et métrologie neutrons par capteurs à pixels actifs de dernière génération
Période : 2008-2011
Electronic (or operational) dosimetry of workers exposed to neutrons is subject to new and stringent european policy. About 60000 workers are affected in Europe, mainly those in nuclear power plants and in medical physics.
The RaMsEs group is currently developing a new real-time dosemeter based on CMOS technology, referred as DOSIPIX-N. This chip was initially developed for tracking purpose in high energy physics. To be used as an operational neutron dosemeter, the sensor has to be γ-transparent, to be able to detect neutrons on a wide energy range with high detection efficiency and also to have a constant response as a function of distance and angle. Successful tests have already shown the flat response of this kind of sensor as a function of the distance in a previous PhD.
The response of the device, made of the CMOS sensor MIMOSA-5 and a converter in front of it, has been compared with Monte Carlo simulations carried out with MCNPX and GEANT4. These codes have been beforehand validated to check they can be used properly for our application. The choice of the converter material depends on neutron energy : we use polyethylene for fast neutron detection (the sensor will detect recoil protons produced by elastic scattering of neutron on 1H) and 10B for thermal neutron detection (inelastic reaction 10B(n,α) 7Li will produce detectable α-particle and 7Li). Experiments to characterize the sensor have been performed at IPHC and at IRSN/LMDN (Cadarache).
An important part of this work was to demonstrate the γ-transparency of the device, as neutron environments usually consist of mixed γ/n fields. The results of the sensor irradiation to photon sources (241Am, 60Co) and mixed field (241AmBe source which emits MeV-neutrons and γ-rays of 4.438 MeV) show the γ-transparency of the sensor by applying an appropriate threshold on the deposited energy (around 100 keV). The associated detection efficiency is satisfactory with a value of 10-3, in good agreement with MCNPX and GEANT4. Other features of the device have been tested with the same source, like the angular response. This response can be flat if the space between the converter and the sensor is reduced.
The last part of this work deals with the detection of thermal neutrons (eV-neutrons). Assays have been done in Cadarache (IRSN) with a 252Cf source moderated with heavy water (with and without cadmium shell). Results asserted a high detection efficiency (up to 6.10-3 for a pure 10B converter, 2.10-3 for natural boron) in good agreement with GEANT4.
All these measurements allow us to conclude that the CMOS technology is suitable for neutron dosimetry. The final system will be a miniaturized mono-pixel chip, with low noise and low power consumption. Tests with a prototype of this sensor, the ALPHARAD-2, are already in progress as part of another PhD.
Keywords : CMOS Active Pixel Sensor, neutron, dosimetry, γ-transparency, simulation, MCNPX, GEANT4.