Indian Journal of Pure & Applied Physics
Vol. 48, December 2010, pp. 860-868
Study of shielding behaviour of some multilayer shields containing PB and BX
M S Hossain*1, S M A Islam2, M A Quasem3 & M A Zaman4
1
2,3,4
Physics Department, National University, Gazipur, Bangladesh
Physics Department, Jahangirnagar University, Savar, Dhaka, Bangladesh
1
*E-mails: [email protected]; [email protected]; [email protected]; [email protected]
Received 22 June 2009; revised 3 June 2010; accepted 27 July 2010
Neutron shielding properties of poly boron (PB), borax mixed ilmenite-magnetite concrete (BX) and the multilayers
PB+BX and their reversed combinations have been studied using a 252Cf point source whose emission characteristics are
similar to that of inside a reactor core. For the detection of neutron, a BF3 long counter detector has been used. The shielding
characteristics such as transmission factors, removal cross-sections and fluence spectra have been measured. MCNP code
has been adopted for the calculations. The multilayered shields are found to show better performance than the single layered
ones, thus supporting applicability of it for practical purpose.
Keywords: Neutron shielding, poly boron (PB), Fluence spectra, Borax mixed ilmenite-magnetite concrete (BX),
Transmission factors
1 Introduction
Shielding of neutrons and gamma rays is an
important aspect in any nuclear establishment. Boron
(B) having very high neutron absorption cross-section
plays an important role in this aspect. It is well known
that neutrons lose their energy by elastic scattering
with hydrogenous materials. Neutron shielding
effectiveness of hydrogenous materials is expected to
be improved by adding boron. Karni and Greenspan1
suggested that neutron shielding ability of
polyethylene can be improved by adding a good
shielding material such as boron. Addition of boron
would reduce the production of energetic secondary
gamma photon.
The scattering cross-section of hydrogen is
inversely proportional to neutron energy. For effective
slowing down of very high energy neutrons,
hydrogenous materials are mixed with moderate and /
or high mass number elements such as lead (Pb). It
reduces the energies of fast neutrons through inelastic
collisions. Hence for designing shielding for fast
neutrons, the hydrogen content is maximised in the
shielding materials by using hydrogenous materials.
Lead is toxic and its use has now become restricted.
Concrete, on the other hand, is a mixed material
susceptible to tailoring by adding/changing the
ingredients and matrix of composition. A wide variety
of concrete compositions are accessible and hence
concretes have now become more popular. A number
of heavy concretes (HC) had been tested and found to
serve the purpose better. Efforts are continued to
improve the system.
Suitability of multilayered shields for the mixed
gamma and neutron fields has been discussed
elsewhere2. In this paper the performance of PB and
BX multi-layer shielding arrangement has been
described. Researchers3-5 have found that multi-layers
are more effective. In the present study experimental
investigation has been conducted for the neutrons
emitted from the fission source 252Cf. MCNP
calculations are also made for the same shielding
arrangements for 14 MeV neutrons. Removal crosssections are calculated to observe their dependence on
the thickness of shields. Fluence values were also
determined for the fission and 14 MeV neutrons at the
some selected shield thicknesses.
Total neutron fluence as a function of shield
thickness has been measured. Using the data, thus
obtained, the effective transmission factors and the
removal cross-sections are calculated from the
following formula for instantaneous removal cross
section, [∑r (t)]:
∑r (t) = − [1/F(t)] × [dF(t)/dt] cm−1
…(1)
where F(t) = I(t)/I(0) is the transmission factor, I(0)
and I(t) are the integrated counts in the absence and
presence of the shield of thickness t, respectively. I(0)
and I(t) are the intensity values for the absence and
presence of the shield of thickness t, respectively. For
HOSSAIN et al.: SHIELDING BEHAVIOUR OF SOME MULTILAYER SHIELDS CONTAINING PB AND BX
861
the present purpose the quantities can be considered
as integrated counts in the absence and presence of
the shield of the mentioned thickness, respectively.
2 Materials and Methods
2.1 Specification of the shielding materials
Polyboron (PB), a locally developed shield, was
made with the following elemental densities of its
components:
H : 0.1202 g . cm−3
O : 0.2212 g . cm−3
C : 0.5814 g . cm−3
B : 0.0475 g . cm−3
The shield was found to have good mechanical
strength as well as neutron shielding properties6,7.
Borax
(BX)
is
a
sodium
pyroborate
(Na2B4O7.10H2O) and Ilmenite magnetite mixed
heavy concrete named as BOXIM. The BX shield is a
heavy concrete. It has been made of cement, sand and
stone chips mixed together in the ratio 1:2:3 by
weight. The sand used for this purpose was made of
ordinary sand, ilmenite, magnetite and borax in the
ratio 6:9:9:1 by weight. For the multilayer settings
slabs PB and BX were placed alternately.
Measurements were taken after placing every slab.
2.2 Experimental arrangement
The experiment was done at the Institute of Nuclear
Science and Technology (INST), Atomic Energy
Research Establishment (AERE), Savar, Dhaka. The
experimental arrangement consisting of a radiation
source 252Cf (point source of strength 3.7 × 104 Bq),
the shielding arrangement and the detector is shown
in Fig. 1. The source was chosen as its neutron and
gamma ray spectra are similar to that of 235U used in a
nuclear reactor. The source was placed at a distance
of 110 cm away from the detector. The shielding
materials investigated were poly-boron (PB), Borax
(BX), and the multilayers PB+BX and their reverse
arrangement BX+PB. These materials in the shape of
slabs of lateral dimensions 20 cm × 10 cm or 26.5 cm
× 13 cm were placed between the source and the
detector (Fig. 1). First, a single slab was placed very
close to the source and subsequently one by one, more
and more slabs were added taking care that the gap
between successive slabs remains as minimum as
possible. A broad beam geometry was adopted in the
experiment. In the case of multilayer shield, slabs of
different components were alternately placed. The
Fig. 1 —Block diagram of the experimental set-up
Table 1 — Important characteristics of the shielding materials
used in this study
Shielding
materials
Physical
density
(g/cm3)
Compressive
strength
(kg/cm2)
Slab
dimension
(cm3)
PB
BX
0.971
2.42
Unknown
278.92
20 × 10 × 4
26.5 × 13 × 7.5
relevant information about the slabs is given in
Table 1.
For each setting neutron fluence was measured
initially for 200 s without placing any shielding slab
between the source and the detector. Then the first
slab closest to the source was placed and the fluence
was measured. The subsequent measurements were
made for increased thickness of the shield by
increasing the number of slabs one by one at a time.
The maximum shielding thickness studied was
100 cm. The shielding materials PB and BX have
been fabricated in the present work.
A Hanson-Mckibben type long counter8 was used
for neutron detection. It was constructed incorporating
the modifications suggested by other researchers9.
The counter was found to be almost independent of
energy over a long range of energy that made it most
suitable for monitoring neutrons.
3 MCNP Code
The experimental results were compared with the
predictions of the general purpose MCNP code
MCNP4A. The MCNP code was developed on the
basis of Monte Carlo method. An arbitrary threedimensional configuration of materials in geometric
cells bounded by first- and second-degree surfaces
and fourth-degree elliptical core is used by the code.
Exactly three-dimensional simulation of the
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INDIAN J PURE & APPL PHYS, VOL 48, DECEMBER 2010
experimental set-up of the shield with plain surfaces
normal to X, Y, Z directions was performed.
To generate a continuous probability density
function for the point isotropic 252Cf fission source, a
built- in analytic function of MCNP4A was utilized.
Point detectors with the axial symmetry were
considered to calculate the highest fluence at the point
of interest. The detector was considered to lie just
outside the maximum shielding surface which is, in
the present case, about 100 cm away from the source
position. Enough particles were considered to keep
the relative error below 0.04. It may be mentioned
here that this number actually depends on the
thickness of the shield. The required particle number
that should be given in the input for the error to be
under 0.04 ranges from 0.2 to 100 millions. With the
increasing of shield thickness the particle number in
the input must also be increased. The code uses
continuous-energy, nuclear and atomic data libraries,
the primary sources of which are the Evaluated
Nuclear Data File10 (ENDF) system, the Evaluated
Nuclear Data Library11 (ENDL), the Activation
Library12 (ACTL) compilations from Livermore, and
evaluations from the Applied Nuclear Science
Group13-15 (ANS Group) at Los Alamos. The
evaluated data are processed into a format appropriate
for MCNP with the help of codes16,17 such as NJOY.
Nuclear data tables exist for different types of neutron
interactions from which appropriate ones may be
selected through unique identifiers for each table,
called ZAIDs.
4 Results and Discussion
4.1 Transmission of fission neutrons
The attenuating characteristics of the neutrons
while
passing
through
different
shielding
arrangements are shown in Figs 2 and 3. At first
Fig. 2 — Variation of fission neutron transmission factors (Experimental and MCNP) with thicknesses of the shields PB, BX and PB+BX
Fig. 3 — Variation of fission neutron transmission factors (Experimental and MCNP) with thicknesses of the shields PB+BX and BX+PB
HOSSAIN et al.: SHIELDING BEHAVIOUR OF SOME MULTILAYER SHIELDS CONTAINING PB AND BX
fission neutrons were investigated. These neutrons
were mostly of energy greater than cadmium cut off
value. This was observed previously18 while the effect
of placing a 0.7 mm thick cadmium sheet in front of
the detector was studied for the same source used in
the work. In all the multilayer cases, the attenuation
curves are found to lack in smoothness. This happens
because
of
the
inhomogeneous
shielding
arrangements of the multilayers.
Multilayer slabs were placed one after another; as a
result, when a particular slab is placed, its attenuation
influences the multilayer curve. On changing the slab
type the attenuation also is suddenly changed. This is
how the curves produced lost smoothness.
Figure 2 shows the neutron attenuation
performance of the shields PB, BX and their
multilayer PB+BX. It lies in between the components
being closer to the PB shield. The reverted
arrangement e.g., BX+PB is less effective (Fig. 3)
than the PB+BX set-up. The HVLs of the shields PB,
BX, PB+BX and BX+PB obtained by experiment are,
respectively 6.5, 11.4, 7.55, and 8.5 cm.
Corresponding TVLs are, respectively 15.0, 34.5,
19.9 and 20.2 cm. As of the previous cases the
variation patterns observed are all well supported by
MCNP calculation.
4.2 Removal cross-section of fission neutrons
4.2.1 PB shield
The experimental values of fission neutron removal
cross section, ∑r for PB at 5 cm shield thickness is
0.097 cm−1 which rises up to 0.196 cm−1 at 90 cm
863
thickness (Fig. 4). These values have been calculated
using Eq. (1). The thickness dependence is a smooth
function and there is a sharp increase up to 15 cm and
then the increment rate decreases and remains almost
constant.
4.2.2 BX shield
The ∑r for BX at 5 cm is 0.056 cm−1 which rises up
to 0.078 cm−1 for fission neutrons, the maximum
variation being 0.003 cm−1. Agreement between
calculated and experimental results are good which
can be observed from the existing maximum variation
value of 4%. It is found that the removal cross-section
values are very high for PB followed by that of BX at
all the penetration distances. This is expected and can
be explained in terms of the hydrogen content of the
shielding materials.
4.2.3 PB, BX, PB+BX and BX+PB shields
The set-ups PB+BX and BX+PB are similar in
behaviour with 14 MeV neutrons for multilayer
shields. A slight variation can be observed (Figs 4 and
5) due to the variation of shielding effectiveness
between PB and BX. PB shows better shielding
capability for 14 MeV neutrons than BX. As of the
previous cases the shielding arrangements are less
effective for shielding 14 MeV neutrons than the
fission neutrons.
Figure 5 shows the variation of fission neutron
removal cross-sections for the PB+BX multilayer set.
The removal cross sections increase smoothly with
increase of thickness. The curve possesses a minimum
of 0.098 cm−1 (PB+BX, Experimental) at 5 cm which
Fig. 4 — Variation of fission neutron removal cross-section (Experimental and MCNP) with thicknesses
of the shields PB, BX and PB+BX
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INDIAN J PURE & APPL PHYS, VOL 48, DECEMBER 2010
Fig. 5 — Variation of fission neutron removal cross-section (Experimental and MCNP) with thicknesses of the shields PB+BX as
compared to BX+PB
Fig. 6 — Fluence spectra for fission neutrons at 5, 10, 25 and 50 cm thicknesses for the shield PB
rises to 0.131 cm−1 (PB+BX, Experimental) at
100 cm. This aspect observed from the experimental
data is also evident when one observes the MCNP
results for removal cross-section. The feature is
obvious for the reversed set BX+PB and is supported
also by the MCNP results (Fig. 5). The maximum
difference between these two data sets is 2.53%
(PB+BX), 2.26% (BX+PB) at 100 cm shield
thickness.
For the multilayer, e. g., PB+BX although the
removal cross-section values are less (Figs 4 and 5)
than that of PB but much higher than BX single
layers. This observation is also similar to the one
found during the discussion of the transmission
factors for the shielding layers. The multilayers of all
type do have similarity in behaviour to PB, and not so
much to other component shield. As we determine the
characteristics of the shields for neutrons, hence, as
usual, PB will have a greater effect than the other
heavier shield.
4.3 Neutron fluence spectra for the single layer shields
4.3.1 Neutron fluence spectra for PB
Fission neutron fluence for PB at 5, 10, 25 and
50 cm thicknesses of the shield has been plotted in
Fig. 6. Neutrons are found to exist in 2 different
groups of energy range, that is, 0 to 2.5 MeV and 2.5
to 10 MeV corresponding peaks being observed at
1.35 MeV and 4.97 MeV. The figure shows that the
second lot of neutrons have a long tail towards its
HOSSAIN et al.: SHIELDING BEHAVIOUR OF SOME MULTILAYER SHIELDS CONTAINING PB AND BX
865
higher energy side meaning that on the higher energy
side energy loss is not a sharp process, rather it is
done at a slow rate. The ratio of these energy neutrons
for 5, 10 and 25 cm are 1.09, 0.90 and 0.58,
respectively. From the figure it is observed that with
the increase of shield thickness the neutron fluence
spectrum decreases. At 25 cm thickness the fluence is
very low and at 50 cm thickness all the features
whatever it had almost disappear. It is also observed
that the neutron fluence of different energies are very
small (maximum at 5 cm thickness of the order of
10−6). A remarkable feature can be noticed – at energy
2.34 MeV there is a minimum fluence irrespective of
shield thicknesses which probably has happened due
to a resonance total cross section (5b) peak of 12C at
this energy.
BX. The common features for all other shields are
also present here. The peak to peak ratio for both the
shields are same. Maximum number of neutrons
possess energy 1.3 MeV at shield thickness 5 and
10 cm. The maximum number of neutrons
corresponding to 4.5 MeV, again are available at both
the thicknesses. Before 50 cm almost all the neutrons
are absorbed by the shield. The small proportion of
the neutrons that could be observed at 50 cm
thickness possess energy in between 1 and 2 MeV for
the shield BX. Again the shield possesses two minor
groups each on the lower energy side of the first
group – the feature being prominent in thicknesses 5,
10 and 25 cm. The number of neutrons found for the
different shield thicknesses are found to decrease with
thickness.
4.3.2 Neutron fluence spectra for BX
4.3.3 Neutron fluence spectra for PB+BX and BX+PB shields
Figure 7 shows the fluence spectra of fission
neutrons for various shield thicknesses and gives the
calculated spectra at various thicknesses of the shield
The energy spectra for the multilayers PB+BX and
BX+PB show (Figs 8 and 9) that on interchange of
the component layers to form the multilayer PB+BX
Fig. 7 — Fluence spectra for fission neutrons at 5, 10, 25 and 50 cm thicknesses for the shield BX
Fig. 8 — Fluence spectra for fission neutrons at 5, 10, 25 and 50 cm thicknesses of the shields PB+BX
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INDIAN J PURE & APPL PHYS, VOL 48, DECEMBER 2010
Fig. 9 — Fluence spectra for fission neutrons at 5, 10, 25 and 50 cm thicknesses of the shields BX+PB
Fig. 10 — Fluence spectra for 14 MeV neutrons at 5, 10 and 100 cm thicknesses PB+ BX
the peak values of the second major group becomes
less strong meaning that the number of neutrons
available are less now. For the first major group the
situation is rather different; the peak value now
increases on reversing the component shields. There
is also a shifting of the neutrons towards the higher
energy range at 25 cm thickness depicting that at this
thickness more number of comparatively higher
energy neutrons exist.
4.3.4 PB, BX, PB+BX and BX+PB shields
In the set-ups PB+BX and BX+PB multilayer
shields neutron fluence spectra were studied. The
curves shown in the figures for multilayers are
jumbled up showing their shielding performance. A
slight variation can be observed (Figs 8 and 9) due to
the variation of shielding effectiveness of BX. PB
shows better shielding capability for 14 MeV neutrons
than BX2. As of the previous cases the shielding
arrangements are less effective for shielding 14 MeV
neutrons than the fission neutrons as shown in
Figs 10 and 11.
4.4 Comparison of the multilayer shields
It can be observed from Figs 8 and 9 that of all the
PB multilayers, the multilayer PB+BX shield is found
to be the most effective neutron shield. Some
description here as regards the shielding ability, the
setup PB+BX is then followed by BX+PB
multilayers2. It may also be mentioned that the
shielding capability of the PB multilayers is found to
have the same serial as that of their uncommon single
layer components, that is, BX.
HOSSAIN et al.: SHIELDING BEHAVIOUR OF SOME MULTILAYER SHIELDS CONTAINING PB AND BX
867
Fig. 11 — Fluence spectra for 14 MeV neutrons at 5, 10 and 100 cm thicknesses BX+PB
Table 2 — The 5, 10, 25 and 50% transmission thickness values (experimental and MCNP calculations) of all the shielding arrangements
for fission neutrons
Shielding
arrangement
PB
BX
PB+BX
BX+PB
5% transmission, (cm)
MCNP
Expt.
18.8
42.0
23.5
25.8
19.3
44.0
24.0
26.6
10% transmission, (cm)
MCNP
Expt.
14.6
32.8
19.0
19.6
It appears from experimental results (Fig. 9) that at
15 cm thickness the multilayers PB+BX transmit only
15.1% whereas their reverted set-ups BX+PB transmit
21.3% of the neutrons. Thus the relative performances
of the shields are not altered even though the shield
components are interchanged (Fig. 8). This
observation is supported also by the MCNP
calculations in Table 2 (Figs 8 and 9). At 20 cm
thickness the results quickly change. Now the
corresponding experimental transmission is 9.78% for
the multilayers PB+BX. The multilayers are thus
found to be influenced by the presence of PB to a
large extent.
5 Conclusions
In the present study single layer shields PB and BX
and their combinations PB+BX and BX+PB have
been studied. Transmission factors, removal crosssections and fluence spectra for neutron and 14 MeV
neutrons shielding have been investigated. MCNP
calculations have been done to compare with the
experimental measurements.
15.0
34.5
19.9
20.2
25% transmission, (cm)
MCNP
Expt.
9.3
20.1
12.5
14.0
9.9
21.5
13.0
14.8
50% transmission, (cm)
MCNP
Expt.
6.0
10.0
6.7
8.0
6.5
11.4
7.6
8.5
The present study reveals that for fission neutron
shielding, the two single layers studied show a clear
pattern of their comparative behaviour of shielding
ability up to around 100 cm shield thickness. Of the
two single layer shields, PB for 14 MeV energy
neutrons show maxium variation in the transmission
factors and removal cross-section in the range of 5 cm
to about 40 cm of shield thickness; removal cross
section shows a tendency to rise within these limits.
However, BX removal cross-sections show a
tendency to decrease up to around 10 cm and
therefore it shows a tendency to rise.
It appears from the present study that neutrons
having energy around 2.34 MeV are rarely
transmitted through the shields studied. These
neutrons, to a large extent, are removed in the
shielding process. On the other hand, two distinct
major groups of neutrons, e.g., 0.82 to 2.3 MeV are
found inadequate in numbers in the single layer
shields studied. In addition to these two major groups,
two other minor groups between 0 to 0.36 MeV and
0.36 to 0.82 MeV energy were observed in the shields
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INDIAN J PURE & APPL PHYS, VOL 48, DECEMBER 2010
(Figs 2 and 3). Maximum number of neutrons
detected are found to have energies around 1.5 and 5
MeV. From the fluence (Figs 6 - 9) investigations of
14 MeV neutrons (Figs 10 and 11), peaks are found at
4.966, 7.408 and 12.214 MeV and the absorption
troughs are found at 0.821, 2.365, 6.065 and 8.607
MeV in these shields. It is also observed that the
behaviour of the transmitted neutron fluence through
the PB-multilayers do not change appreciably on
reversing the components of the multilayer
arrangements.
Boron plays an important role in the production of
shielding materials. It is a good thermal neutron
absorber. Moreover, nuclei, of this material decrease
the production of buildup (secondary) gamma rays.
Thus its presence influences the shielding
characteristics to a considerable extent. For these
reasons shielding materials have been fabricated using
boron.
On the basis of the above discussions it may be
concluded that multilayer shields can be made by
adding two single layers alternately, one of which
may be a light/hydrogenous suitable for shielding
neutrons effectively. Multilayers formed by placing a
hydrogenous material (PB) first are found to attenuate
neutrons more effectively than the reverse set-ups.
Characteristics of PB-multilayers for fission
neutrons are found to be dependent on their
uncommon component shields. On reversing the
components the shielding characteristics change, but
the relative characteristics of the uncommon
components do not. Among the single layers studied
PB is found to be the best fission neutron shield.
Among the multilayers thus formed PB+BC is found
to be the best shielding arrangement for fission
neutron shielding when small thickness is concerned
(up to 50% cut off). This observation is well
supported by experiments as well as MCNP
calculations (Table 2).
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