Would you like to join our teams for an internship or a post-doc?

You will find hereafter our current proposals.

• Post-doctoral position
  Study of X-ray emitting radionuclides for dosimetry in reactors
  

  Topic
  Engineering sciences, metrology

  Short description
  The LNHB (Laboratoire National Henri Becquerel) is the French metrology laboratory in the field of ionizing radiation. One of the tasks of the laboratory is the measurement of radionuclides activity (in becquerels) and the determination of radionuclides decay data. The measurement of the activity of X-ray emitting radionuclides, in the energy range below 100 keV, presents several difficulties that limit the accuracy of the result. These include the calibration of the detector efficiency and, in general, the large uncertainties associated with X-ray emission intensities. In addition, the effects of self-attenuation of X-rays in standard sources or samples lead to important corrections that must be controlled.
  Among the important applications of the measurement of X-ray emitters, the dosimetry in reactor, which allows to determine the neutron fluence received during an irradiation and to characterize the spectrum, is based on the analysis of the activity of irradiated dosimeters. These dosimeters are made of pure metals or alloys of perfectly known composition, some of which are subject to activation or fission reactions under the effect of neutrons. For example, the reactions ⁹³Nb(n,n’)^93mNb and ¹⁰³Rh(n,n’)^103mRh are of primary importance for reactor dosimetry and are particularly interesting for characterizing neutron fluxes around 1 MeV.
  
  The task of the postdoc will consist in:
  

  • Exploiting the results of the efficiency calibration of a high purity germanium (HPGe) detector using a cryogenic electrical substitution radiometer and transfer photodiodes. This will be used to measure the absolute X-ray emission intensities of radionuclides used as standard for the calibration of detectors (¹³³Ba, ¹⁵²Eu, etc.);
  • Improving the knowledge of photon emission intensities following the decay of ^93mNb, ¹⁰³Pd and ^103mRh;
  • Critically analyzing published data on these radionuclides and contributing to the evaluation and publication of their decay schemes.

  Contract duration
  1 year (possible renewal 1 time)

  Desired starting date
  1/4/2023

  Location
  CEA Saclay

  Candidat profile
  The candidate must have a PhD in physics with knowledge in nuclear instrumentation and/or atomic physics. He/she must have a solid knowledge of radiation-matter interactions and an interest in laboratory experiments. He/she should demonstrate good critical thinking skills and be able to analyze the results in details.

  Application: Please send a curriculum vitae and a motivation letter to:
  Marie-Christine LÉPY
  DRT/LIST/DIN/SIMRI/LNHB-MA
  Bât. 602
  CEA-Saclay
  91191 Gif-sur-Yvette FRANCE
  Tél. : +33 1 69 08 24 48
  Fax. : +33 1 69 08 26 19
  E-mail : marie-christine.lepy@cea.fr

  Detailed description of the subject
  The measurement of the activity of X-ray emitting radionuclides in the energy range below 100 keV encounters several difficulties that limit the accuracy of the results. These include the difficulty in calibrating the efficiency of detectors and, in general, in the large uncertainties associated with X-ray emission intensities. In addition, the effects of self-attenuation of X-rays in standard sources or samples lead to important corrections that must be controlled.
  Among the important applications of the measurement of X-ray emitters, the dosimetry in reactor, which allows to determine the neutron fluence received during an irradiation and to characterize the spectrum, is based on the analysis of the activity of irradiated dosimeters. These dosimeters are made of pure metals or alloys of perfectly known composition, some of which are subject to activation or fission reactions under the effect of neutrons. For example, the reactions ⁹³Nb(n,n’)^93mNb and ¹⁰³Rh(n,n’)^103mRh are of primary importance for reactor dosimetry and are particularly interesting for characterizing neutron fluxes around 1 MeV.
  The data used for the experimental calibration of X-ray spectrometers in the energy range below 100 keV correspond to the emission intensities of the XKα and XKβ lines of radionuclides used as standards. These data are fully correlated, as they are calculated from the same basis for the fluorescence yields, and it appeared that they are partially inconsistent, including for the Kα/Kβ ratios of the same radionuclide. A detailed analysis of the atomic parameters used to calculate the emission intensities and the evaluation of new values, based on a critical analysis of the most recent publications, is necessary.
  For the calibration of X-ray spectrometers, an innovative approach consists in being free from radioactive standards by using monochromatic photon beam whose energy and intensity are well characterized. This is being achieved with an electrically substituted cryogenic radiometer that has allowed the “absolute” calibration of photodiodes, which in turn are used as a transfer detector to calibrate a spectrometer based on a high-purity germanium detector (HPGe). With an efficiency calibration obtained independently from the tabulated X-ray emissions, it will be possible to measure the X-ray emission intensities in a completely independent way and thus to provide valuable information for all analyses relying on spectrometry in the energy range below 40 keV. This will in particular be applied to the measurement of X-rays emitted in the decay of europium-152, which has two branches, towards samarium-152 and gadolinium-152, thus emitting two series of X-ray lines. It appears that the X-ray emission intensities of the two branches are inconsistent. New measurements are thus necessary to remove this ambiguity. The measurements consist in using radioactive sources of known activity and to measure the photon emission intensities by X-ray spectrometry. Relative uncertainties of the order of 1% are expected, which requires a strict metrological approach.
  For the measurement of the activity of dosimeters used in the reactor, it is also necessary to improve knowledge of the intensities of low-energy X-ray and gamma emissions from niobium-93m and rhodium-103m. The measurement of the second radionuclide is delicate, because its radioactive period is less than 1 hour. A specific approach is thus necessary, using palladium-103 which decays to the metastable level of rhodium-103m: the two decay schemes of ¹⁰³Pd and ^103mRh are thus directly linked. Moreover, there is an inconsistency between the intensity of the gamma emission at 40 keV and the K X-ray emissions around 20 keV, common to these two radionuclides (the X emission corresponding to the internal conversion of the transition at 40 keV for ^103mRh). A new evaluation of these two decay schemes will be based on the analysis of recent experimental data (including those obtained in this study) and on the use of codes for the calculation of internal conversion coefficients. This work will lead to “recommended values” and will allow updating the “Table of radionuclides” distributed by the Laboratoire National Henri Becquerel (http://www.lnhb.fr/nuclear-data/nuclear-data-table/).
  
  Several of these studies may lead to publication
  

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• Post-doctoral position
  Development of a modular multi-detector experimental setup for the measurement of atomic and nuclear parameters
  

  Field
  Physics, instrumentation

  Description of the proposal
  The PLATINUM project (PLATeforme d’Instrumentation NUmérique Modulable – Modular Numerical Instrumentation Setup) is aiming to develop a modular setup to test new instrumentation based on, at least, two detectors working in coincidence.
  The underlying idea behind the project is to detect simultaneously the interaction of particles in two independent detectors, while measuring physical information such as the type of particle and its energy (particle spectroscopy). This detection principle is used in absolute intensity measurements or with active systems (in order to improve detection limit by performing continuous background suppression). Besides, such experimental setup is also capable to provide parameters needed to reconstruct the decay scheme of nuclei such as: the internal coefficients, the fluorescence yields or the angular correlations between photons produced during a gamma cascade process.
  The Laboratoire National Henri Becquerel (LNHB) researchers are worldly known for their expertise in the completion of the nuclear scheme of various nuclides. Theses nuclear schemes are built from the evaluation of the existing literature, which is sometimes inconsistent or incomplete. This is particularly true in case of very weak gamma ray transitions or states with high internal conversion yields (for example, recent studies have revealed that ¹⁰³Pa, ¹²⁹I and ¹⁴⁷Nd exhibit such kind of inconsistency). It is therefore critical for LNHB to master coincidence detection methods using the latest developments in numerical acquisition (and time stamping) to access complementary information improving the knowledge on decay scheme.
  
  The work proposed to the candidate is to develop and optimize the acquisition part of the coincidence setup using a fast numerical module with time stamp capability before performing off-line analyses. The development will be divided in three steps
  
  

  1. Selection and optimization of the numerical acquisition
     Various commercial modules are available in the laboratory and will be compared for the coincidence experimental setup. Several tests with different particle energies and counting rates will be necessary in order to optimize acquisition parameters such as the decay constants and time coincidence gates.
  2. Instrumental validation
     The performances of the acquisition will validated after integrating the selected modules in a two-gamma spectrometer coincidence setup. The setup will be developed to perform autonomous rotation from one detector to another to measure angular correlation of gamma rays emitted by the decay of a radioactive source (⁶⁰Co and ²²Na). These tests will be performed to assert the metrological stability of the detection system and to evaluate the experimental uncertainties associated.
  3. Coincidence correction application
     The developed experimental setup will be used to study radionuclides with more complex decay schemes (such as ¹³³Ba or ¹⁵²Eu). The effects on the coincidence corrections in gamma spectrometry will be quantified (in various geometry) using an experimental approach coupled with Monte Carlo simulation.

  Contract
  1 year

  Expected employment start date
  In 2021

  Location
  CEA Saclay

  Host laboratory
  The LNHB, located in CEA Paris-Saclay, is a laboratory from CEA (Commissariat à l’Énergie Atomique et aux Énergies Alternatives – French Alternative Energies and Atomic Energy Commission) responsible for the French ionizing radiation metrology. It is one of the national metrology institutes federated by the Laboratoire National de métrologie et d’Essais (LNE) since 2005. The LNHB is composed of around 50 permanent members among which about 25 are part of the LMA (Laboratoire de Métrologie de l’Activité – Activity Metrology Laboratory). LMA is in charge of primary metrology for the measurement of activity and the transfer of references to accredited calibration laboratories and users in the fields of application such as: nuclear medicine, nuclear industry, environmental monitoring.

  Contact
  Benoît SABOT or Christophe BOBIN
  DRT/LIST/DM2I/LNHB/LMA
  B. 602
  CEA-Saclay
  91191 Gif-sur-Yvette FRANCE
  Tél. : +33 1 69 08 46 52 or +33 1 69 08 29 64
  Fax. : +33 1 69 08 26 19
  E-mail : benoit.sabot@cea.fr or christophe.bobin@cea.fr
  

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