NEUTRON RESPONSE FUNCTION FOR A DETECTOR WITH 3He COUNTERS
FOR THE 0.39 – 1.54 MeV NEUTRON ENERGY RANGE
J. Černý1, Z. Dlouhý2, Z. Doležal1, M.P. Ivanov, P. Kubik1, E.A. Kuznetsova,Yu.E.
Penionzkevich,
E.A. Sokol, J. Švejda1 and I. Wilhelm1
1
-Institute of Particle and Nuclear Physics, Faculty of Mathematics and Physics, Charles University,
2180 00 Prague 8, Czech Republic;
2
-Nuclear Physics Institute, Academy of Sciences of the Czech Republic,
250 68 Rež near Prague, Czech Republic
In the present work1), we have
measured the response functions of two
neutron detector arrays, with 39 3He
proportional
counters
each,
for
monoenergetic neutrons within the 0.39 –
1.54 MeV neutron energy range. The main
goals of the measurements were to obtain
experimental data on the sensitivity of
neutron counting to varying neutron
energies and the influence of the thickness
of the polyethylene moderator. The
experimental data will be used for
constructing optimal neutron detector
arrays with modules for studies of the
properties of the delayed neutrons from
fission fragments within the framework of
the Dubna Radioactive Ion Beams
(DRIBs) and Accélérateur Linéaire auprés
du Tandem d’Orsay (ALTO) projects.
Each neutron detector array
consisted of 39 modules arranged in six
layers. In the first assembly, all the
modules were identical and each of them
consisted of a hexagonal polyethylene unit
(distance between parallel planes – 50
mm) with a cylindrical cavity (diameter 32
mm) for a neutron counter. The length of
the counter and the polyethylene
moderator was 50 cm. The counters were
filled with 3He up to a pressure of 7 atm.
Every neutron counter was supplied with a
pre-amplifier, which was mounted rigidly
to the counter and had a cylindrical case;
the cases of the pre-amplifiers were used
for fixing the neutron counters in
assembling. In the second assembly, the
polyethylene moderator was removed
from the modules of the 2nd and 4th rows.
The detection assemblies were surrounded
by a shield of boron loaded polyethylene
with a window in the front side; this
window was covered with 1 mm cadmium
foil. The experiments described here were
performed with the Van de Graaf
electrostatic accelerator HV2500AN at
Charles University, Prague, Czech
Republic. Monoenergetic neutrons were
produced as secondary particles from a
3
H(p,n)3He reaction on a Ti-T target (0.2
mg/cm2) with a molybdenum backing. The
target was mounted at an angle of 45o to
the incident proton beam, so the real
thickness of the target was about 0.28
mg/cm2. The counting rates of the separate
groups of neutron counters arranged in the
central part of each layer of the detection
assemblies and the monitor detectors were
measured at different energies of incident
neutrons. The good sensitivity of both
arrays to a change in neutron energy is
clearly seen, so the valid correlation of the
count rate ratios for different rows of
neutron counters with neutron energy was
obtained. If the detection sensitivity versus
neutron energy curve for one row of
counters is quite different from that for
another row of counters, the ratio of
counts for the two rows will be a function
of neutron energy.
It is seen from a comparison of the two
response functions that removing part of the
moderator leads to the displacement of the
maximum of the rate to the deeper layers of
the counter and the total detector sensitivity
drops with the decreasing of the volume of
the moderator. For the detection assembly
with a larger amount of moderator, the
registration sensitivity is systematically
higher for all neutron energies.
As neutron energy increases, the
neutron registration sensitivity increases.
This can be explained by the fact that at low
energies, a considerable fraction of neutrons
are reflected by the front layers of the
detectors and do not penetrate into the
assembly; neutrons of higher energies
penetrate deeper into the assembly and their
losses due to reflection are smaller.
4detectors will register low-energy
neutrons more effectively, because they do
not abandon detectors due to reflections in
the other internal layers, but high-energy
neutrons will leave the assembly.
The experimental sensitivity curves
were compared with calculated response
functions generated using the MCNP
code2) version B. During calculations good
agreement with experimental information
was achieved. Calculations with a larger
amount of moderator in detectors were
conducted, the distance between the
parallel planes of the polyethylene
hexahedrons being changed from 5 cm to
6, 7 and 8 cm. The calculations showed
that the distance between the parallel
planes of the polyethylene hexahedrons
about 6.5 cm is optimal. Taking into
account that in experiments with 4
detectors, neutrons get into detectors
isotropically, the effective thickness of
moderator in our modules is near to
optimal. It is also necessary to notice that
an increase in the thickness of the
moderator results in an increase in the
neutron life time in the detectors, which, in
experiments in studying delayed neutrons
in coincidences with β-particles or γ-rays
or in multiple neutron measurements,
results in an increase in the background
from random coincidences.
Consequently,
since
neutron
registration sensitivity depends weakly on
the energy of incident neutrons up to an
energy of 1.5 MeV, a neutron detector must
consist of no less than 4-5 layers of modules
comprising neutron detectors (7 atm 3Не, 32
mm in diameter) and moderator surrounding
the detectors by a layer approximately 1 cm
thick.
REFERENCES
1) Černý J.et al., Nucl. Instr. & Meth.,
A540 (2005) 430.
2) Briesmeister J.F. (Ed.), Program MCNP,
Version 4A, Los Alamos, Report LA12625-M, 1993.