Program - Brookhaven National Laboratory

We have recently re-measured the neutron capture cross section of the stable 25 Mg isotope at the neutron
time-of-flight facility n TOF at CERN, using C6D6 liquid scintillators. This experiment aims at the
improvement of existing data of interest for nuclear astrophysics and is part of an ongoing study for a
comprehensive discussion of the s-process abundances in red giant stars. The measurement is carried
out under similar conditions as for the Mg-experiment that was completed at n TOF during 2003, with
some important improvements in the experimental setup. In particular i) a metallic 25 Mg-enriched sample
is used instead of a MgO powder sample used in the previous measurement, which prevented us from
minimizing the uncertainty of the measured cross section; ii) the use of such a self-sustaining sample does
not require an aluminum canning, which introduced an important background contribution in the past;
iii) borated water is used as neutron-moderator, a new feature of the n TOF facility that strongly reduces
the background due to gamma rays traveling in the beam. In the present contribution we show the impact
of these improvements on the quality of the measured cross section. The preliminary results and the
astrophysical implications will be presented as well.
[1] C. Massimi, P. Koehler et al. (The n TOF collaboration), Phys. Rev. C 85, 044615 (2012)
PA 8 5:45 PM
Measurement of the 237 Np(n,f) Cross Section with the MicroMegas Detector
M. Diakaki, M. Kokkoris, A. Kyrtsos, N. G. Nicolis, E. Skordis, R. Vlastou
Department of Physics, National Technical University of Athens, 157 80 Athens, Greece
S. Andriamonje, E. Berthoumieux
Commissariat a l’Energie Atomique (CEA), Saclay, France
A. Lagoyannis
Institute of Nuclear Physics, N.C.S.R. ”Demokritos”, Athens, Greece
In the present work, the measurement of the 237 Np(n,f) cross section with reference to the standard
238 U(n,f) reaction was attempted with a new MicroMegas detector [1], based on the innovative Microbulk
technology, especially developed at CERN [2] for these measurements. The incident monoenergetic neutron
beams with energies in the range 4-6 MeV were produced via the 2 H(d,n) reaction at the neutron beam
facility of the Institute of Nuclear Physics at the NCSR “Demokritos”. Four actinide targets ( 237 Np,
two 238 U and 235 U) and the corresponding MicroMegas detectors were assembled in an especially made
chamber full of Ar/CO2 at a proportion 80/20 and around atmospheric pressure. The actinide targets used
are in the form of very thin disks of actinide oxides deposited on a 100 µm Al backing. The mass of the
isotope of interest and impurities was quantitatively determined via alpha spectroscopy. Furthermore, their
thickness and homogeneity have been examined via the RBS (Rutherford Backscattering Spectrometry)
technique. Monte-Carlo simulations were performed with the code MCNP5 [3] implementing the neutron
beam setup and the MicroMegas assembly in order to determine the neutron flux at the position of each
target. Additional simulations with FLUKA [4] were performed, studying the energy deposition of the
fission fragments in the active area of the detector, in order to accurately estimate the efficiency. Good
discrimination of the heavy and light mass peaks of the fission fragments was achieved. The cross section
results are in agreement with previous data and will be presented and discussed. This research has been
co-financed by the European Union (European Social Fund - ESF) and Greek national funds through the
Operational Program ”Education and Lifelong Learning” of the National Strategic Reference Framework
(NSRF) - Research Funding Program: Heracleitus II. Investing in knowledge society through the European
Social Fund.
[1] Giomataris, Ph. Rebourgeard, J. P. Robert and G. Charpak, Nucl. Instrum. Methods Phys. Res. Sect.
A 376, 29, (1996). [2] S. Andriamonje et al. (The n TOF Collaboration), Journal of the Korean Physical
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