Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

ACoM - 6
6 th Meeting of the Collaboration on Advanced Cold Moderators
Jülich, 11 – 13 September 2002
Experimental studies of neutronic characteristics of hydrogen moderators at para rich
concentrations and comparison with calculations
ABSTRACT
Y. Kiyanagi, H. Ogawa, M. Ooi and H. Iwasa
Graduate School of Eng., Hokkaido University
Kita-13, Nishi-8, Kita-ku, Sapporo, 060-8628, Japan
Hydrogen is the most important material as a cold neutron moderator for the future high
power spallation neutron sources. However, experimental data for the moderators of hydrogen
with high para concentration are very scares and not suitable for design of the future moderator
system. We made equipments to convert ortho to para-hydrogen and to measure the ortho/para
ratio. After then, we measured the neutronic performance of the hydrogen moderators
with 5 cm thickness and compared with simulation. The results indicated that overall
characteristics of the experimental result agreed with the calculated results but there exist
difference in the absolute values. Therefore, the simulation is useful for the basic design but
experiments are necessary to evaluate the precise neutronic performance.
1. INTRODUCTION
Hydrogen is used as cold neutron moderator for the Japan spallation source and also in
the spallation neutron source in the USA. The hydrogen consist of para and ortho-hydrogen
and the neutronic performance is different between them since the neutron cross section below
about 20 meV is very different. Neutron is transparent below about 10 meV for the parahydrogen
due to the very low cross section of para-hydrogen. Simulation calculation is very
important for the design of the cold moderator. The simulation results showed that the para
rich hydrogen moderator gave better neutronic characteristics than the normal hydrogen[1]. In
the decoupled moderator a para-rich moderator gives narrow pulse width higher pulse peak
intensity and in the coupled moderator the para-rich moderator gives higher integrated neutron
intensity by using the thicker moderator with keeping the pulse shape good. Such features
are very useful for the neutron scattering experiment.
However, the neutron cross section of the para-hydrogen is complicated since it has
coherent like nature of the scattering although hydrogen is essentially incoherent. Therefore,
synthesis of the cross section is not so easy. Furthermore, the validity check is not performed
sufficiently since the experimental cross section data and spectrum data are not enough and
have not given sufficient information.
Therefore, it is very important to check the validity of the simulation results by the
experiments. Here, we present experimental results of the para-rich hydrogen moderator as
well as equipments to convert ortho to para and to measure ortho/para ratio. Comparison with
simulation is also mentioned.
2. EXPERIMENTAL
At first we have to make an equipment to convert ortho to para since the conversion time is
too long, about 43,000 hours even at 20 K. We used catalyst of 30% of Al2O3 and 70% of
Cr2O3 at first, and later Fe(OH)3. The system we used is indicated in Fig. 1. Hydrogen gas is
169