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Improving the Neutron Cross-section Standards 238U(n,f) and 6Li(n,a): Measurements and Simulations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Improving or extending the range of cross-section standards, which in general are believed to be well known, require good control of the experimental conditions and the uncertainties involved. Two experiments that aspire to improve two different neutron cross-section standards, 238U(n,f) and 6Li(n,a, are presented in this thesis. Both standards have previously been extensively measured, but outside certain energy ranges discrepancies exist. In this thesis, a future precision measurement of the 238U(n,f) standard, relative to the elastic neutron scattering on hydrogen, is analysed through simulations. The status of the currently ongoing measurement of the second standard, 6Li(n,a), is also reported and some preliminary results are presented.Measurements of the uranium standard, with a total uncertainty better than 2%, are planned at the upcoming NFS facility. The new experimental situation requires our existing setup to be upgraded with new detectors. The simulation study has supplied limits on the target and detector designs but also provided estimates of the uncertainties that show the feasibility of a precision measurement. The design of the whole setup and the development of new detectors are guided by the simulation study presented in this thesis. When the upgrade is complete, the setup will consist of two parallel plate avalanche counters (PPACs), in addition to the eight detector telescopes already present in the existing setup.The 6Li(n,a) measurement is ongoing at the GELINA facility at IRMM in Geel, Belgium. A twin Frisch-grid ionisation chamber is employed measuring both 6Li(n,a) and 235U(n,f) in separate compartments. Although a problematic background was found, the preliminary cross section in the resonance region around 240 keV reproduces evaluated neutron library data fairly well. A recent move of the setup to a position closer to the neutron production shows promising improvements in the background situation.

Place, publisher, year, edition, pages
Polacksbackens repro , 2015.
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-255235OAI: oai:DiVA.org:uu-255235DiVA, id: diva2:821395
Presentation
2015-05-29, 11:58 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilAvailable from: 2015-06-29 Created: 2015-06-15 Last updated: 2015-06-29Bibliographically approved
List of papers
1. Designing an upgrade of the Medley setup for light-ion production and fission cross-section measurements
Open this publication in new window or tab >>Designing an upgrade of the Medley setup for light-ion production and fission cross-section measurements
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2015 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 794, p. 141-150Article in journal (Refereed) Published
Abstract [en]

Abstract Measurements of neutron-induced fission cross-sections and light-ion production are planned in the energy range 1-40 MeV at the upcoming Neutrons For Science (NFS) facility. In order to prepare our detector setup for the neutron beam with continuous energy spectrum, a simulation software was written using the Geant4 toolkit for both measurement situations. The neutron energy range around 20 MeV is troublesome when it comes to the cross-sections used by Geant4 since data-driven cross-sections are only available below 20 MeV but not above, where they are based on semi-empirical models. Several customisations were made to the standard classes in Geant4 in order to produce consistent results over the whole simulated energy range. Expected uncertainties are reported for both types of measurements. The simulations have shown that a simultaneous precision measurement of the three standard cross-sections H(n,n), 235U(n,f) and 238U(n,f) relative to each other is feasible using a triple layered target. As high resolution timing detectors for fission fragments we plan to use Parallel Plate Avalanche Counters (PPACs). The simulation results have put some restrictions on the design of these detectors as well as on the target design. This study suggests a fissile target no thicker than 2 µm (1.7 mg/cm2) and a PPAC foil thickness preferably less than 1 µm . We also comment on the usability of Geant4 for simulation studies of neutron reactions in this energy range.

Keywords
Neutron-induced fission, Light-ion production, Geant4, Standard cross-section, PPAC
National Category
Accelerator Physics and Instrumentation
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-255210 (URN)10.1016/j.nima.2015.05.001 (DOI)000356356100019 ()
Funder
Swedish Research Council
Available from: 2015-06-15 Created: 2015-06-15 Last updated: 2017-12-04Bibliographically approved
2. Measuring Light-ion Production and Fission Cross Sections Normalised to H(n,p) Scattering at the Upcoming NFS Facility
Open this publication in new window or tab >>Measuring Light-ion Production and Fission Cross Sections Normalised to H(n,p) Scattering at the Upcoming NFS Facility
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2014 (English)In: Nuclear Data Sheets, ISSN 0090-3752, E-ISSN 1095-9904, Vol. 119, p. 395-397Article in journal (Refereed) Published
Abstract [en]

The Medley detector setup is planned to be moved to and used at the new neutron facility NFS where measurements of light-ion production and fission cross-sections are planned at 1-40 MeV. Medley has eight detector telescopes providing Delta E-Delta E-E data, each consisting of two silicon detectors and a CsI(Tl) detector at the back. The telescope setup can be rotated and arranged to cover any angle. Medley has previously been used in many measurements at The Svedberg Laboratory (TSL) in Uppsala mainly with a quasi-mono-energetic neutron beam at 96 and 175 MeV. To be able to do measurements at NFS, which will have a white neutron beam, Medley needs to detect the reaction products with a high timing resolution providing the ToF of the primary neutron. In this paper we discuss the design of the Medley upgrade along with simulations of the setup. We explore the use of Parallel Plate Avalanche Counters (PPACs) which work very well for detecting fission fragments but require more consideration for detecting deeply penetrating particles.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-246695 (URN)10.1016/j.nds.2014.08.110 (DOI)000347706200226 ()
Available from: 2015-03-10 Created: 2015-03-09 Last updated: 2017-12-04

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