Développement des détecteurs de neutron

par Nesrine Dinar

Projet de thèse en Physique des accélérateurs

Sous la direction de Patrick Puzo.

Thèses en préparation à Paris Saclay , dans le cadre de Particules, Hadrons, Énergie et Noyau : Instrumentation, Imagerie, Cosmos et Simulation , en partenariat avec LAL - Laboratoire de l'Accélérateur Linéaire (laboratoire) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 01-10-2015 .


  • Résumé

    Development of Neutron Detectors Description Over the past years, several activities related to neutron measurements and instrumentation have been pursued in the Radiation Protection (RP) Group at CERN. This thesis aims at further developing neutron detectors and investigating new techniques for neutron detection and spectrometry. The PhD work will focus on three principal research lines: 1/ CONVENTIONAL NEUTRON DETECTORS The first step is a further understanding of existing neutron detectors, in particular the extended-range CERN Bonner Sphere Spectrometer (BSS) [1]. The CERN BSS and other state-of-the-art neutron detectors, such as the LUPIN rem counter specifically conceived for applications in pulsed neutron fields, recently developed in a collaboration with the participation of CERN and now available in a commercial version, will be tested at the CERN-EU high-energy Reference Facility (CERF) facility [2]. A wide community of users coming to CERF every year to test and calibrate passive dosimeters and active instrumentation lead CERN RP group to consider to legally certify CERF as workplace calibration for the next years. In this context, the intercomparison of detector performance will permit to validate the latest FLUKA simulations of the CERF field and to participate in the CERF accreditation. In order to improve the CERN BSS response and increase the number of applications in which it could be efficiently employed, two complementary spheres will be designed: one sphere for enhancing the sensitivity above 20 MeV and one for increased the response of the BSS in the thermal energy region. First, FLUKA simulations will be performed in order to select the appropriate material and dimension of these two additional spheres and to determine their response function. If sufficiently promising, the two BS will be fabricated and tested at CERF and in the new RP CALibration LABoratory (Cal Lab) built at CERN in 2014 to compare their influence on the final unfolded spectra. Finally a very important point concerning BSS is the unfolding code related to their use. Currently at CERN, two codes are used, MAXimum Entropy Deconvolution (MAXED) [3] and GRAVEL, available in the PTB U.M.G. package [4], which require an a priori estimation of the neutron spectrum, called guess spectrum, typically derived via Monte Carlo simulations. The idea is to develop an unfolding code in which we can have a control of all parameters and a deep understanding of how it works without using a guess spectra. An intercomparison of the new unfolding code with MAXED and GRAVEL will be done with various data obtained in different neutron field exposures to check the reliability of the code. The final aim is that it can be used routinely within the RP group at CERN. 2/ MPGD NEUTRON SPECTROMETER A prototype of a MPGD (Micro-pattern gaseous detector) for neutron spectrometry was designed, constructed and tested within the ARDENT project. It is composed by a conversion board divided in different regions, each one dedicated to a specific neutron energy range. The charged particles produced from neutron interactions are read-out by a triple GEM detector [5], while the neutron spectrum is acquired by unfolding the data from different regions, resembling the BSS but with planar geometry. The main advantages of the new device are wide energy range response from thermal to 100 MeV, reasonable counting efficiency, short irradiation time and low weight. Since the device is still at the prototype stage, it needs to be fully characterized through a campaign of measurements and data analysis, in order to investigate its capability to reconstruct a variety of different neutron spectra. The prototype is planned to be tested in different kinds of neutron fields. One of these is the isotropic field produced by a neutron emitting radioactive source, such as 241AmBe at the CERN Cal Lab. Of great interest is the neutron field at CERF, where reference positions are defined and characterized by FLUKA simulations. At these positions, the spectra measured with the new device can be compared with those acquired by BSS in the past and its overall response can be evaluated. The results of these measurements will be used for improvements in the design and response matrix of the spectrometer with FLUKA, with the aim to obtain more accurate and thus more reliable spectral measurements. 3/ B-RAD The idea of developing an innovative radiation survey meter capable of measuring residual dose rates in the presence of an intense magnetic field came from the requirements of the LHC experiments, which need to perform measurements of the residual radioactivity in the experimental halls with the magnetic field still on. Since no commercially available radiation survey meter could work in an intense magnetic field, this led to a collaboration between CERN and Politecnico of Milano for the development of a portable radiation survey meter (called B-RAD) for use in intense magnetic fields. To date, one prototype and five units, available for routine use within the CERN RP group, have been built. B-RAD has been patented in 2014 and the version equipped with a dose rate probe is near to commercialisation phase. Within this PhD work, a further development will be pursued for the foreseen extension of the device with a portable probe for neutrons, based on recently developed scintillating crystals (CLYC). This scintillating material should be able to measure γ‐rays, thermal neutrons and fast neutrons, allowing a good n/γ discrimination. The subject of this part of the PhD thesis will consist in studying newly developed neutron detecting materials, like CLYC or diamond , that have the potential to work as neutron detectors for portable probes. Since these are relatively new materials, their properties will need to be characterized for neutron detection. In addition to designing a probe for neutron dose rate/count rate, its application in the field of neutron spectrometry will be investigated. Skills Physics – Neutron detection – Radiation Protection – Training Value Readout electronics, detector development and calibration References [1] C. Birattari, E. Dimovasili, A. Mitaroff and M. Silari, A Bonner sphere spectrometer with extended response matrix, Nucl. Instrum. Meth. A 620 (2010) 260-269. [2] A. Mitaroff and M. Silari, The CERN-EU high-energy Reference Field (CERF) facility for dosimetry at commercial flight altitudes and in space, RPD 102, 7-22, 2002 . [3] M. Reginatto and P.Goldhagen, MAXED a computer code for maximum entropy deconvolution of multisphere neutron spectrometer data, 1999. [4] M. Reginatto, The “few-channel” unfolding program in the UMG Package: MXD_FC33, GRV_FC33 and IQU_FC33 (UMG Package, Version 3.3.), 2004. [5] GEM: A new concept for electron amplification in gas detectors, NIM A 386, 1997.

  • Titre traduit

    Development of neutron detectors


  • Résumé

    Development of Neutron Detectors Description Over the past years, several activities related to neutron measurements and instrumentation have been pursued in the Radiation Protection (RP) Group at CERN. This thesis aims at further developing neutron detectors and investigating new techniques for neutron detection and spectrometry. The PhD work will focus on three principal research lines: 1/ CONVENTIONAL NEUTRON DETECTORS The first step is a further understanding of existing neutron detectors, in particular the extended-range CERN Bonner Sphere Spectrometer (BSS) [1]. The CERN BSS and other state-of-the-art neutron detectors, such as the LUPIN rem counter specifically conceived for applications in pulsed neutron fields, recently developed in a collaboration with the participation of CERN and now available in a commercial version, will be tested at the CERN-EU high-energy Reference Facility (CERF) facility [2]. A wide community of users coming to CERF every year to test and calibrate passive dosimeters and active instrumentation lead CERN RP group to consider to legally certify CERF as workplace calibration for the next years. In this context, the intercomparison of detector performance will permit to validate the latest FLUKA simulations of the CERF field and to participate in the CERF accreditation. In order to improve the CERN BSS response and increase the number of applications in which it could be efficiently employed, two complementary spheres will be designed: one sphere for enhancing the sensitivity above 20 MeV and one for increased the response of the BSS in the thermal energy region. First, FLUKA simulations will be performed in order to select the appropriate material and dimension of these two additional spheres and to determine their response function. If sufficiently promising, the two BS will be fabricated and tested at CERF and in the new RP CALibration LABoratory (Cal Lab) built at CERN in 2014 to compare their influence on the final unfolded spectra. Finally a very important point concerning BSS is the unfolding code related to their use. Currently at CERN, two codes are used, MAXimum Entropy Deconvolution (MAXED) [3] and GRAVEL, available in the PTB U.M.G. package [4], which require an a priori estimation of the neutron spectrum, called guess spectrum, typically derived via Monte Carlo simulations. The idea is to develop an unfolding code in which we can have a control of all parameters and a deep understanding of how it works without using a guess spectra. An intercomparison of the new unfolding code with MAXED and GRAVEL will be done with various data obtained in different neutron field exposures to check the reliability of the code. The final aim is that it can be used routinely within the RP group at CERN. 2/ MPGD NEUTRON SPECTROMETER A prototype of a MPGD (Micro-pattern gaseous detector) for neutron spectrometry was designed, constructed and tested within the ARDENT project. It is composed by a conversion board divided in different regions, each one dedicated to a specific neutron energy range. The charged particles produced from neutron interactions are read-out by a triple GEM detector [5], while the neutron spectrum is acquired by unfolding the data from different regions, resembling the BSS but with planar geometry. The main advantages of the new device are wide energy range response from thermal to 100 MeV, reasonable counting efficiency, short irradiation time and low weight. Since the device is still at the prototype stage, it needs to be fully characterized through a campaign of measurements and data analysis, in order to investigate its capability to reconstruct a variety of different neutron spectra. The prototype is planned to be tested in different kinds of neutron fields. One of these is the isotropic field produced by a neutron emitting radioactive source, such as 241AmBe at the CERN Cal Lab. Of great interest is the neutron field at CERF, where reference positions are defined and characterized by FLUKA simulations. At these positions, the spectra measured with the new device can be compared with those acquired by BSS in the past and its overall response can be evaluated. The results of these measurements will be used for improvements in the design and response matrix of the spectrometer with FLUKA, with the aim to obtain more accurate and thus more reliable spectral measurements. 3/ B-RAD The idea of developing an innovative radiation survey meter capable of measuring residual dose rates in the presence of an intense magnetic field came from the requirements of the LHC experiments, which need to perform measurements of the residual radioactivity in the experimental halls with the magnetic field still on. Since no commercially available radiation survey meter could work in an intense magnetic field, this led to a collaboration between CERN and Politecnico of Milano for the development of a portable radiation survey meter (called B-RAD) for use in intense magnetic fields. To date, one prototype and five units, available for routine use within the CERN RP group, have been built. B-RAD has been patented in 2014 and the version equipped with a dose rate probe is near to commercialisation phase. Within this PhD work, a further development will be pursued for the foreseen extension of the device with a portable probe for neutrons, based on recently developed scintillating crystals (CLYC). This scintillating material should be able to measure γ‐rays, thermal neutrons and fast neutrons, allowing a good n/γ discrimination. The subject of this part of the PhD thesis will consist in studying newly developed neutron detecting materials, like CLYC or diamond , that have the potential to work as neutron detectors for portable probes. Since these are relatively new materials, their properties will need to be characterized for neutron detection. In addition to designing a probe for neutron dose rate/count rate, its application in the field of neutron spectrometry will be investigated. Skills Physics – Neutron detection – Radiation Protection – Training Value Readout electronics, detector development and calibration References [1] C. Birattari, E. Dimovasili, A. Mitaroff and M. Silari, A Bonner sphere spectrometer with extended response matrix, Nucl. Instrum. Meth. A 620 (2010) 260-269. [2] A. Mitaroff and M. Silari, The CERN-EU high-energy Reference Field (CERF) facility for dosimetry at commercial flight altitudes and in space, RPD 102, 7-22, 2002 . [3] M. Reginatto and P.Goldhagen, MAXED a computer code for maximum entropy deconvolution of multisphere neutron spectrometer data, 1999. [4] M. Reginatto, The “few-channel” unfolding program in the UMG Package: MXD_FC33, GRV_FC33 and IQU_FC33 (UMG Package, Version 3.3.), 2004. [5] GEM: A new concept for electron amplification in gas detectors, NIM A 386, 1997.