Volume 3 Number 1, January-June 2015
INVESTIGATION OF LINEAR ATTENUATION
COEFFICIENT OF MERCURY SHIELD BY USING
ACRYLIC CONTAINER FOR LOW-LEVEL GAMMA RAY
SPECTROMETERS
May Saung Chan Myae
Associate Professor Nay Win Oo
Department of Physics, Yadanabon University,
Mandalay, Republic of the Union of Myanmar.
ABSTRACT
INTRODUCTION
The aim of this research work
is to investigate the detector shielding
choosing the mercury to provide a
degree of isolation in laboratories
where other radiation sources may be
used or moved about during the
course of a measurement. First, a
good container for mercury shielding
is chosen by using gamma
attenuation technique. NaI (Tl)
scintillation detector and Maestro 32
software are used to determine the
attenuation of containers and
mercury. Acrylic container (0.076
cm-1 at 661.62 keV) is more suitable
than the glass container (0.090cm-1
at 661.62keV) for mercury shield.
Mercury is good gamma rays
absorber
comparable
to
lead,
therefore it is suitable shielding
material for measuring the low
activity radiation, including the
background radiation
Because of the cosmic
radiation that continuously bombards
the earth’s atmosphere and existence
of natural radioactivity in the
environment, all radiation detectors
record some background signal. The
nature of this background varies
greatly with the size and type of
detector and with the extent of
shielding that may be placed around
it. The background counting rate can
be as high as many thousands of
counts per second for large-volume
scintillators, to less than a count per
minute
in
some
specialized
applications. Because the magnitude
of the
background
ultimately
determines the minimum detectable
radiation level, it is most significant
in those applications involving
radiation sources of low activity.
However, background is often
important enough in routine usage so
that majority of radiation detectors
are provided with some degree of
external shielding to effect a
reduction in the measured level. A
109
Volume 3 Number 1, January-June 2015
second purpose of detector shielding
is to provide a degree of isolation in
laboratories where other radiation
sources may be used or moved about
during the course of a measurement.
Because both the atomic number and
density of steel are considerably
lower than those of lead, thicknesses
of several tens of centimeters are
normally required for very low
background applications .
Acrylic is a clear high grade
plastic
made
primarily
from
petroleum. It is a form of plastic, but
just as there are different grades of
metals or woods, there are different
grades of plastic. The misconceptions
about acrylic, because it is a form of
plastic, are that it either yellows or
scratches easily. Cheap plastic will
yellow in the sun, but high quality
cell cast acrylic will not, and unlike
glass or lacquers, scratches in acrylic
can be buffed out. Other names, such
as Lucite (Perspex)or Plexiglass
(Acrylic)have all been used when
referring to high grade Acrylic.
Acrylic is similar to glass, but acrylic
has characteristics that make it
superior to glass in many ways.
Mercury is a chemical element with
the symbol Hg and atomic number
80. The density of the mercury is
13.6 gcm–3,with a freezing point of
−38.83 °C and boiling point of
356.73 °C.
Although it is relatively
expensive it is a very effective
shielding material in low background
counting situations. It can be purified
to a high degree through distillation
and thus has an inherently low level
110
of residual radioactivity. It is often
used as the innermost component of
large gamma-ray shields. Because
mercury is a liquid at room
temperature, it must be held within a
suitable container, often constructed
of Lucite, a material of inherently
low background radioactivity which
can be configured to match of the
detector. Because it’s density is even
greater than that of lead and its
atomics
number
also
greater,
thickness of a few centimeters of
mercury will be relatively effective as
a gamma-ray shield.
The
behavior
of
the
background in a number of largevolume Ge (Li) detectors has been
studied. Using shielding of the type
shown in Figure 1 a typical detector
background is made up of a 30 %
contribution from cosmic radiation
60% from the radioactive contamination of shielding materials, and
10% from radioactivity within the
detector and unidentified sources.
Figure 1. A low-background shield
configuration for a germanium
detector.
Volume 3 Number 1, January-June 2015
ATTENUATION OF GAMMA
RAYS
If a parallel beam of monoenergetic ɤ -rays with intensity I (0)
strikes a target of thickness 'x' , the
number of photons, I (x), emerging
without having interaction in the
target is given by x I(x) I(0)e−μx
The probability that a photon
will traverse thickness 'x' without an
interaction is
number of transmitted I(0)ex

 ex
number of incident
I(0)
The total probability for
interaction μ called the total linear
attenuation coefficient is equal to the
sum of the three probabilities:
μ (cm) −1 = τ+σ+ κ
Physically, μ is the probability of
interaction per unit distance. where,
τ= photo electric effect coefficient , σ
=Compton scattering coefficient,
κ=pair production coefficient
EXPERIMENTAL SETUP
Gamma ray Spectroscopy is
measurement method that determines
the energy and count rate of gamma
rays emitted by radioactive substances. Gamma Spectroscopy is
extremely important measurement. A
detailed analysis of gamma energy
spectrum is used to determine the
identity and quantity of gamma
emitters present in a material.
SAMPLE PREPARATION
Firstly, The acrylic slab will
be made that is useful for many
things, especially for acrylic partial
denture. To make acrylic, it is needed
to make a mould for that. Photograph
of applied acrylic polymers powder
and monomer solution is shown in
Figure. 2. Then, 5.3 g (a cup) of acrylic powder and 2 cc solution have to
be mixed and pour the acrylic
solution into the mould. The reason
for use 5 cm of acrylic slabs and glass
slabs is that the surface area of
detector which is 2"×2". The surface
area of the lead shield of the defector
is also 1.5"×1.5".It will take for 15
minutes. Then, get the acrylic slab
from the mould. Finally, the rough
acrylic slab is made to smooth. It
would make different thickness of
four acrylic slabs.
The measurements of all the
slabs are described. The thickness of
the first slab is 2.07 mm, the second
one is 4mm, the third one is 4.85 mm
and the last is 6.69 mm. The length
and the width of all the slab is 5 cm
each. In addition, the density of the
first slab is 1.195 g/cm3, the second
one is 1.08 g/cm3, the third and the
last one are 1.076 g/cm3 and 1.028
g/cm3 each. Secondly, to compare
attenuation coefficient of acrylic
slabs, glass slabs will be used which
are 5 cm square of 5 slabs. The
densities of the glass slabs are 2.65
g/cm3, 2.63 g/cm3, 2.7 g/cm3, 2.65
g/cm3 and 2.7 g/cm3 each. Photograph of applied acrylic slabs and
glass slabs are shown in Figure 2.
111
Volume 3 Number 1, January-June 2015
Figure 2. (a) Acrylic powder and Solution. (b) Comparison of samples glass
and acrylic slab.
Next, the acrylic cup is made
for mercury container. To make
acrylic cup, a mould is needed. The
glass slab for mould is shown in
Figure 3. Then, the mixtures of
acrylic powder and monomers
solution is put into the mould and
pour the acrylic solution into the
glass mould. Glass slab inject into the
mould to get the acrylic cup. It takes
for 15 minutes. Then, get the acrylic
cup from the mould. Finally, the
rough acrylic cup is made to smooth.
The different thickness of three cups
would be made. Acrylic Cup is also
shown in Figure 3. The measurement
of all the cups is serially done. The
average inner thickness of first cup is
0.428 cm, the second is 0.558 cm and
the last one is 0.675 cm.
Figure 3. (a) To make acrylic cup used the glass mould, (b) acrylic cup.
EXPERIMENTAL PROCEDURE
In the present work, monoenergetic gamma ray source 137Cs is
used. It has photo peak (661.62 keV).
Its strength is 5μCiwhich available at
the Experiment Nuclear Laboratory
of the Department of Physics,
Yadanabon University. The manufacture date is 2012, June. The half
112
life of standard source is 30.07 yrs.
60
Co have two photo peaks (first peak
1173 keV and second peak 1332
keV). Its strength is 1μCi which is
available at the Experiment Nuclear
Laboratory of the Department of
Physics, Yadanabon University. The
manufacture date is 2012, June. The
half life of standard source is 5.27
yrs.
Volume 3 Number 1, January-June 2015
In gamma ray spectroscopy
system, sodium crystal mounted a
photomultiplier tube, preamplifier,
amplifier, a pulse sorter (MCA),
high-voltage power supply and data
readout devices are included. In this
experiment, NaI(Tl) 2 inches x 2
inches scintillation detector is used to
detect the gamma radiation after
passing through the absorbing
material and then passed information
(electron pulses) are amplified by the
preamplifier and the fast spectroscopy amplifier and collected by
using MCA based on personal
computer. The operating voltage used
in scintillation detector is 1000V dc.
This value is fixed for all
measurements and measuring time is
300 seconds each step. The collected
data and spectrum for each
measurement is stored and analyzed
by using Maestro 32 software.
The thickness of acrylic and
glass are measured by using slide
clipper. The average thickness of one
slab is measured ten times for
different position. The average
thickness and thickness error is
calculated.
The gamma spectrum of 137Cs
standard gamma source (5μ Ci)
without absorber is measured by the
experimental. And then gamma ray
spectra of various thickness of acrylic
absorber are measured. Also gamma
ray spectra of various thickness of
glass are also measured. Finally, the
linear attenuation coefficient of
acrylic, and glass were determined.
Next, for measuring mercury
attenuation by using mercury (80Hg)
250 mg and NaI(Tl) sodium Iodide
Scintillation detector are shown in
Figure 4a and Figure 4b. The
operating voltage used in scintillation
detector is 1000V dc. This value is
fixed for all measurements and
measuring time is 600 seconds each
step.
Figure 4. (a) Mercury (80Hg) 250 mg , (b) For measuring mercury
attenuation by NaI(Tl) Detector.
113
Volume 3 Number 1, January-June 2015
RESULTS AND DISCUSSION
The counts obtained from the spectrum by using MCA software were
analyzed.
Figure 5. Spectrum of 137 Cs without Absorber.
The spectra of acrylic sample and background were compared. To get
the region of interest (ROI), Gaussian shape (bell shape) is needed to take
from left to right channel. And also the ROI of the gamma peak from the
sample spectrum was marked and checked with library file and get peak
information. From this gross area and net area of full energy peak were
obtained. The ROI report includes energy range of the peak, gross area under
the peak, centroid of the peak and the corresponding energy level. Net area of
the photo peak is the lowest when the thickness of acrylic and glass are the
highest.
Table 1. The measurement results of acrylic with various thickness.
Thickness (cm)
The Ratio of Net Area (I/I0)
0.20745 ± 0.005
0.509 ± 0.41
0.40045 ± 0.020
0.502 ± 0.40
0.502 ± 0.40
0.490 ± 0.40
0.669 ± 0.031
0.475 ± 0. 41
When the ln (I/I0) is plotted
against the absorber thickness, the
slope of the straight line gives the
linear attenuation coefficient of
acrylic and glass graph are shown in
Figure 6. The linear attenuation
114
coefficients for acrylic and glass are
0.076 cm-1 and 0.09 cm-1 at gamma
energy 662 keV, thus, the value of
linear attenuation coefficient (μ) of
acrylic is smaller than the glass.
Volume 3 Number 1, January-June 2015
Next, the value of different
thicknesses of mercury were used for
662 keV, 1173 keV and 1332 keV
gamma energy are shown in table (2)
. The spectra of 137Cs and 60Co
without absorber were measured.
Similarity, the slopes of the straight
lines give which the attenuation
coefficient of mercury with 137Cs and
60
Co were determined. Then, the
linear attenuation coefficient of
mercury for 662 keV, 1173 keV and
1332 keV are 1.658,1.221 and 0.957
cm-1 were obtained.
Figure 6. Determination of Linear Attenuation coefficient of acrylic.
Table 2. The measurement results of Mercury (80Hg) with various thickness
for 137Cs and 60Co.
Thickness (cm)
ratio of net area
ratio of net area
ratio of net area
(I/I0)
(I/I0)
(I/I0)
(662 keV)
(1173 keV)
(1332 keV)
0.428 ± 0.291
0.669 ± 1.27
0.569 ± 1.03
0.345 ± 0.80
0.558 ± 0.043
0.589 ± 0.57
0.458 ± 1.18
0.293 ± 1.32
0.675 ± 0.065
0.442 ± 1.55
0.421 ± 2.17
0.272 ± 0.75
CONCLUSION
The
value
of
linear
attenuation coefficient of acrylic is
smaller than that of glass. Therefore
acrylic container (0.076 cm-1 at
661.62 keV) is more suitable than the
glass container (0.09 cm-1 at 661.62
keV) for mercury shield. According
to the results of Figure 7. mercury is
good
gamma
rays
absorber
comparable to lead. Therefore
mercury is suitable shielding material
for measuring the low activity
radiation, including the background
radiation.
115
Volume 3 Number 1, January-June 2015
Figure 7. Linear Attenuation coefficient of Mercury (80Hg) for 662 keV, 1173
keV and 1332 keV Gamma Energy.
BIBLIOGRAPHY
William R. Leo "Techniques for nuclear and particle physics experiments"
(Germany: Springer – Velar) (1994)"About Lucite and Mercury"
(Wikipedia).
116