HPGe detector shielding optimization with MCNPX for the MEDINA PGNAA cell
- CEA, DEN, Cadarache, Nuclear Measurement Laboratory, F-13108 Saint-Paul-lez-Durance (France)
- FZJ, Institute for Energy and Climate Research - Nuclear Waste Management and Reactor Safety, Wilhelm-Johnen-Strasse, D-52428 Juelich (Germany)
- Laboratoire de Physique Subatomique et de Cosmologie, Universite Grenoble Alpes, CNRS/IN2P3, Grenoble (France)
Radioactive waste repositories must guarantee the non-toxicity of the waste in the long term, not only regarding radioactivity but also regarding other environmental contamination such as toxic chemicals. Analytical methods already exist for chemical characterization (ICP-MS, ICP-AES...) but they are based on test sampling. A possible alternative, for waste packages with an appropriate gamma radiation level, is to use Prompt Gamma Neutron Activation Analysis (PGNAA), a non-destructive measurement technique sensitive to several toxic chemicals. In view of the characterization of radioactive wastes in Germany and France, collaboration between the CEA Cadarache (France) and the Forschungszentrum Juelich (Germany) was initiated a few years ago. FZJ holds a PGNAA graphite cell called MEDINA (Multi Element Detection based on Instrumental Neutron Activation), allowing the characterization of 225 L drums. Fast neutrons are emitted from a D-T pulsed 14 MeV neutron generator and thermalized in graphite to induced radiative captures in the waste materials. Prompt capture gamma rays are detected using a 104% relative efficiency n-type HPGe. However, HPGe crystal is sensitive to fast neutron damage and to thermal neutron activation. A thermal neutron shield made of lithium fluorine and lithium carbonate is already used around the detector. In order to further decrease the current of fast and thermal neutrons coming into the crystal without penalizing MEDINA sensitivity (by decreasing the thermal neutron flux and neutron die away time of the cell, the gamma detection efficiency, or increasing the gamma background), some configurations based on easy-to-implement modifications of MEDINA have been simulated with MCNPX with a model of the cell already validated by experiments. Results show that fast and thermal neutron incoming current in the HPGe could easily be reduced by about a factor of 2 by additional quantities of graphite and by replacing lithium carbonate by lithium fluorine with a higher {sup 6}Li concentration. In addition, these modifications slightly increase the thermal neutron flux in the cell without deteriorating the neutron die away time, and reduce the gamma background about a factor of 2 during the neutron pulse but 5 times less after it. More important changes have also been tested, such as the addition of polyethylene and lead between the neutron generator and the HPGe detector, which is more effective regarding neutron shielding but decreases the neutron die away time, partly compensated by a larger initial thermal neutron flux. Concerning gamma background, hydrogen capture gamma ray (2.23 MeV) is increased due to the presence of polyethylene but lead around the HPGe decreases the total gamma background. In conclusion, simple modifications are possible to improve detector shielding and life time before thermal annealing of the crystal, without reducing MEDINA cell performances. Some of these modifications will be tested in the coming months. (authors)
- Research Organization:
- Institute of Electrical and Electronics Engineers - IEEE, 3 Park Avenue, 17th Floor, New York, N.Y. 10016-5997 (United States)
- OSTI ID:
- 22531468
- Report Number(s):
- ANIMMA-2015-IO-87; TRN: US16V0401102409
- Resource Relation:
- Conference: ANIMMA 2015: 4. International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications, Lisboa (Portugal), 20-24 Apr 2015; Other Information: Country of input: France; 3 Refs.
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
AUGER ELECTRON SPECTROSCOPY
CEA CADARACHE
CRYSTALS
EFFICIENCY
FAST NEUTRONS
FEDERAL REPUBLIC OF GERMANY
FLUORINE
GAMMA DETECTION
GRAPHITE
HIGH-PURITY GE DETECTORS
ICP MASS SPECTROSCOPY
LITHIUM 6
LITHIUM CARBONATES
MEV RANGE
NEUTRON ACTIVATION ANALYSIS
NEUTRON GENERATORS
POLYETHYLENES
RADIOACTIVE WASTES
RADIOACTIVITY
THERMAL NEUTRONS