Institutionen för fysik och astronomi

Licentiatseminarium 29/5


Kaj Jansson presenterar sin licentiatsavhandling om två experiment som ska förbättra standarden för två olika neutrontvärsnitt.

Seminariet hålls i sal 4101, och opponenten är Karin Schönning från avdelningen för kärnfysik.

Avhandlingens titel: Improving the Neutron Cross-section Standards 238U(n,f) and 6Li(n,a).

Abstract: 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.

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