AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
© 2013, Science Huβ, http://www.scihub.org/AJSIR
ISSN: 2153-649X, doi:10.5251/ajsir.2013.4.1.89.94
Mass attenuation coefficients of fabricated Rhizophora spp. particleboard
for the 15.77 – 25.27 keV range
Shakhreet B. Z.1, Bauk S.2 and Shukri A.3
1
School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia.
Department of Diagnostic Radiology, Faculty of Applied Medical Sciences, KAU, Jeddah,
KSA, Tel: +966582958803, E-mail: [email protected]
2
Physical Sciences Program, School of Distance Education, Universiti Sains Malaysia,
11800 Penang, Malaysia.
3
School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia.
1
ABSTRACT
The Rhizophora spp. wood what is hard and heavy is considered strong in strength and is
suitable for structural purposes. Generally, Rhizophora spp. was used in radiation dosimetry
research and it is being used in this project too. There are some disadvantages in using the raw
(natural) material of Rhizophora spp. in radiation dosimetry research and there is a need to
modify this natural wood to be easily used as a phantom in radiation dosimetry. The mass
attenuation coefficients (/) of fabricated Rhizophora spp. particleboard were determined for
photons in the energy range of 15.77–25.27 keV. This was carried out by studying the attenuation
of X-ray fluorescent photons from zirconium, molybdenum, palladium, silver, indium and tin
targets. The results were compared with theoretical values for average breast tissues in youngage, middle-age and old-age groups calculated using photon cross section database (XCOM),
the well-known code for calculating attenuation coefficients and interaction cross-sections. The
measured mass attenuation coefficients were found to be very close to the calculated XCOM
values in breasts of old-age group.
Keywords: XCOM, Rhizophora spp., Attenuation Coefficients
INTRODUCTION
Natural wood is a very variable material both
between and within species, and not just in
appearance but, more importantly, in density,
strength, and durability. Although the strength
properties of wood composites are generally lower
than natural lumber they are more consistent. This
means that they can support loads with smaller
safety margins, which in effect reduces the apparent
difference in strength between natural wood and
composites.
Other benefits of wood composites come from the
fact that their properties can be engineered.
Rhizophora spp. lumber is limited to a large extent by
size, width in particular. It is difficult to obtain
Rhizophora spp. wood wider than 28 cm. Wood
composites can be made to have special properties
such low thermal conductivity, fire resistance, or have
their surfaces improved for decorative purposes.
In order to manufacture a board of adequate strength
the particle must be compressed to at least 5%
above their natural density. In practice, the raw
material is usually compressed to nearer 50% of its
natural density; so if a raw material of about 400
3
kg/m is used then the finished board will have a
3
density of approximately 600 kg/m . This degree of
compression is needed to achieve good chip to chip
contact.
The followings are however some disadvantages of
using the raw material of Rhizophora spp. in radiation
dosimetry studies especially with regard phantom
design:
-
-
-
The trunk diameter of the majority of trees is
limited where the maximum diameter which
is available is around 28 cm.
It is a hardwood and heavy to handle, so it is
difficult to cut and shape it.
With long periods of time, chunks are
observed to crack through the drying
process.
The slabs tend to be curved or arched with
time
which
causes
unsymmetrical
deformations problems in staking of slabs.
Am. J. Sci. Ind. Res., 2013, 4(1): 89-94
To account for the above disadvantages of raw
(natural) Rhizophora spp., the wood needs to be
fabricated for following benefits:
-
Their properties can be engineered with
unlimited size.
-
Easy to cut and shape it after fabrication.
-
No deformation over long periods of time.
-
Homogeneous slabs can be obtained.
-
Easy to modify by adding some materials or
adhesives.
The way a board is pressed will determine its density
profile, which is the change in density from one
surface to the other. Density of boards can
significantly affect its strength properties and
therefore its end use.
When a board is first placed into a press, its surfaces
begin to warm. A board with high density surface will
have a higher bending strength than a board which
does not. In addition, the high density makes the
surfaces hardwearing and less prone to absorb
paints and adhesives.
Studies have shown that this variation in density
profile can be reduced by closing the press very
slowly, for example; seven or more minutes (Moslemi
1974). Such long closing times inevitably lead to
procure and loss of production. The alternative
method would be to close the press extremely
quickly, say within a few seconds, so that the
surfaces do not have sufficient time to heat up and
plasticize, but this would require very large and
powerful hydraulic pumps. This relatively new
technique of steam injection pressing (Geimer 1982)
significantly reduces density variation and, at the
same time, greatly reduces total press time. The
density variation is reduced by the fact that the board
is more uniformly heated through out during
compression.
For economic production of particleboards the
adhesive must cure in the press very quickly but it
must also have a potlife of something in excess of 20
– 30 minutes so that the adhesive does not cure
before entering the press. Longer potlives are
desired, and achieved by many, to allow for line
stoppages.
The most commonly used particleboard adhesive is
phenol formaldehyde (PF). Phenol formaldehyde
(PF) resins are much more weather resistant and do
not have a formaldehyde problem. However, in using
it some additional production problems are
encountered higher temperatures and longer time
periods are required to cure PF. Not only does this
reduce productivity but it can also lead to significant
penetration of the adhesive into the wood chip; if the
glue has been absorbed by the particles then it is not
available to bond them together and poor board
strength results.
MATERIALS AND METHODS
A total of three trees with average diameters of 23 cm
from the 50 trees chunk samples were randomly
chosen. Three discs per tree were taken from the
middle part of those trees for density determination
(Shakhreet B. Z. et al. 2009). Logs were then sawn
3
into planks of about 20  20  2 cm before being cut
to small pieces. The small pieces of about 3 cm long
were then passed into a ring flaker to obtain flakes
(chips) of thickness about 0.02 cm and random width
(Figure 1d). The total amount of all flakes produced is
about 7 kg. It was kept under the same
environmental conditions for about 7 days (1 week) to
stabilize its moisture content.
Prior to pressing, a board should have the same
temperature throughout and have a uniform density.
The moisture content of the board should also be
consistent throughout. In addition, there will not be
any vapor pressure or internal stresses present in the
board before pressing. Once pressing starts,
however, all of these factors will begin to change at
various rates in different parts of the board.
Consequently, the geographical position of a wood
particle will determine the conditions it is subjected to
which in turn will give rise to the board possessing
inconsistent physical properties.
The flakes produced were then oven dried for 24
hours that was found to give 10.22 % moisture
content (mc). The moisture content of the flakes
produced was also determined using a moisture
reader (Figure 1e) and then compared with the oven
dried. The average moisture content using a moisture
reader of the Rhizophora spp. flakes was about 10.15
%.
Heat transfer in the board is by radiation, conduction
and convection (water vapor movement). The latter
transfer mechanism is predominant, particularly in the
early stages of pressing (Bolton et al. 1989). The
main reasons for this are that wood is a good
insulator and vapor flow will occur readily as soon as
a vapor pressure gradient is created (by surface
heating).
Particleboards were fabricated at four density levels
90
Am. J. Sci. Ind. Res., 2013, 4(1): 89-94
3
0.65, 0.75, 0.85, and 1.0 g/cm according to standard
laboratory
procedures.
The
resin
phenol
formaldehyde was used at different resin treatment
levels of 10, 13, 16 and 20% based on the oven dried
weight of wood particles.
Stainless steel plate
Stainless steel
deckle
Stainless steel mold
Rhizophora spp.
flakes
(a)
(b)
(c)
(d)
Moisture
reader
(e)
Balance
Flakes into deckle over
the plate
(f)
(g)
Particleboards produced with different
resin treatment level
The mat
(Resinated flakes)
(h)
Particleboards after cutting to small
pieces
(i)
(j)
Oven
(k)
Pressing machine
(l)
Fig. 1 The materials and apparatus used in fabricating Rhizophora spp. particleboard.
91
Am. J. Sci. Ind. Res., 2013, 4(1): 89-94
X-ray source
Collimators 4 mm dia.

Wood Sample
Target
Detector
Fig 2. Schematic diagrams showing the arrangement of the apparatus for the experiment.
pieces with a size of 5.0  5.0 cm with different
thicknesses (Figure 1j) as obtained from the last
section. Then all the pieces were irradiated with XRF
photon energies to check whether the big board has
a uniformity of density and physical properties. The
average values of the mass attenuation coefficient of
the Rhizophora spp. particleboard was measured and
determined.
2
Using an airless spray gun, the resin was sprayed on
weighted quantities of dried Rhizophora spp. wood
particles in a container. The resinated flakes were
then spread uniformly on square shape stainless
steel plate covered by a transparency paper (Figure
1a) and bounded with a square stainless steel frame
and deckle (Figure 1b). The frames used were
designed to have the same dimensions of the square
2
shape (21.1 × 21.1 cm ) and were only varied in
thicknesses. After completing the spreading of the
flakes, another stainless steel plate was used to
cover the top of the frame to keep the resinated
flakes between the two plates bounded with the
frame (Figure 1h). The mat was cold-pressed at 8
2
kg/cm pressure for about 3 minutes before hoto
pressing at 180 C. The mat was pressed to the
required thicknesses for 7 minutes.
The XRF set up used are shown in Figure 2. Four
millimeter diameter pinhole collimators were
employed. Power to the X-ray tube is maintained via
a generator (Philips PW 1830) which also provided a
selection of applied voltage in the range 10 – 60 kV,
tube current in the range 10 – 35 mA and
programmable and manual timing. Applied voltage
and tube current used in this experiment were
maintained at 55 kVp and 25 mA respectively.
Several metal targets were irradiated at a grazing
o
angle of 75 for a fixed period of 300 seconds. The
metal targets used were zirconium, molybdenum,
palladium, silver, indium and tin producing XRF at
15.77, 17.48, 21.18, 22.16, 24.21 and 25.27 keV
respectively (Shakhreet B. Z. et al. 2009). The
spectra were collected and analyzed using the Jandal
Peak Fit, Version 4, software package. Room
temperature was maintained at a constant value of
o
17 C throughout the experiment.
The pressure of the pressing machine (Figure 1l) was
decreased slowly to avoid any unexpected change or
curvature in the board. The mat (Figure 1h) was
removed from the hot-pressing after the pressure
reached zero. The boards were then conditioned in
an environment of (70  5) % relative humidity (RH)
o
and 28  2 C before being cut into test specimens
and left to be cool down and then the Rhizophora
spp. particleboard removed from the frame and
trimmed. The process and materials used in the
particleboard fabrication are shown in Figure 1.
The results obtained were compared with the mass
attenuation coefficient for young-age, middle-age and
The fabricated particleboards were cut into small
92
Am. J. Sci. Ind. Res., 2013, 4(1): 89-94
old-age breasts (designated as Breast 1, Breast 2
and Breast 3 respectively) calculated using XCOM
(Berger and Hubbell, 1987) and (Shakhreet B. Z. et
al. 2009) based on the elemental composition
suggested by Constantinou (1982).
different densities for each thickness with differences
in resin treatment level for each density.
The mass attenuation coefficients of Rhizophora spp.
at different densities are as shown in Figures 3, 4, 5
and 6. The mass attenuation coefficients of the
particleboard of Rhizophora spp. are closest to the
XCOM calculated mass attenuation coefficient of
Breast 3 for all densities. The best correlation of them
3
is the particleboard with a density of 1.00 g/cm .
RESULTS AND DISCUSSION
The particleboards were successfully fabricated
within the targeted densities and resin treatment
levels using the phenol formaldehyde adhesive.
The density of the particleboard was varied to have
1.60
Mass attenuation coefficients of RhizophoraBreast
spp. from
the
counts
under
the
K
x-ray
fluorescent
peaks
for

1
(XCOM)
1.40
3
particleboards with a density of 0.75 g/cm Breast
1.60
1.00
(cm
/g)
(cm
2
/g)
Mass Attenuation Coefficient (cm2/g)
1.20
Mass Attenuation Coefficient (cm2/g)
2
Breas
t 1 (XCOM)
Breas
t 2 (XCOM)
Breas
t 3 (XCOM)
1.40
0.80
0.60
0.40
0.20
2 (XCOM)
Breast
3 (XCOM)
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.00
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0.0
10.0
20.0
Energy (keV)
30.0
40.0
50.0
60.0
70.0
Energy (keV)
Fig 5 Mass attenuation coefficients of Rhizophora spp.
from the counts under the K x-ray fluorescent peaks
3
for particleboards with a density of 0.85 g/cm .
Fig 3. Mass attenuation coefficients of Rhizophora spp.
from the counts under the K x-ray fluorescent peaks
3
for particleboards with a density of 0.65 g/cm .
1.60
1.00
0.80
(c
m2/
g)
1.20
Breas
t 1 (XCOM)
Breas
t 2 (XCOM)
Breas
t 3 (XCOM)
1.40
Mass Attenuation Coefficient (cm2/g)
(c
m2/
g)
Mass Attenuation Coefficient (cm2/g)
1.60
Breast
1 (XCOM)
Breast
2 (XCOM)
Breast
3 (XCOM)
1.40
0.60
0.40
0.20
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.00
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0.0
Energy (keV)
10.0
20.0
30.0
40.0
50.0
60.0
70.0
Energy (keV)
Fig. 4. Mass attenuation coefficients of Rhizophora
spp. from the counts under the K x-ray fluorescent
3
peaks for particleboards with a density of 0.75 g/cm
Fig. 6 Mass attenuation coefficients of Rhizophora spp.
from the counts under the K x-ray fluorescent peaks
3
for particleboards with a density of 1.00 g/cm
93
Am. J. Sci. Ind. Res., 2013, 4(1): 89-94
REFERENCES
CONCLUSION
Berger M. J. and Hubbell J. H., (1987). XCOM: Photon
cross-sections on a personal computer. U.S.
Department of Commerce, 1-10.
The measured mass attenuation coefficients of
particleboard Rhizophora spp. wood in the 15.77 to
25.27 keV photon peak range was found to be very
close to the calculated XCOM values for old-age
breast (Breast 3).
Bolton A. J., Humphry P.E. and Kavvouras P. K., (1989).
The hot pressing of dry-formed wood-based
composites. Part III. Predicted Vapor pressure and
temperature variation with time, compared with
experimental
data
for
laboratory
boards.
Holzforschung Journal 43, 265-274.
These results indicated that the Rhizophora spp.
particleboard using phenol formaldehyde can be
suitable to be used as a mammographic phantom
material in radiation dosimetry.
Constantinou C., (1982). Phantom materials for radiation
dosimetry. I. Liquids and gels. British Journal of
Radiology 55, 217-224.
ACKNOWLEDGEMENT
Geimer R. L., (1982). Steam injection pressing.
th
Proceedings of the 16 international particleboard
symposium, W.S.U.
The authors would like to acknowledge support from
IRPA Research Grant no. 09-02-05-2135EAO04
awarded by the Ministry of Science, Technology and
Innovation, Malaysia. The authors would like to thank
Mr. Azmi Omar from the Biophysics laboratory,
School of Physics and Miss Siti Hazneza Abdul
Hamid from the School of Industrial Technology,
Universiti Sains Malaysia, Penang for their technical
assistance and help.
Moslemi A. A., (1974). Particleboard 1: Materials.
Particleboards 2: Technology. Southern Illinois
University Press. 244 -245.
Shakhreet B. Z., Bauk S., Tajuddin A. A. and Shukri A.,
(2009). Mass attenuation coefficients of natural
Rhizophora spp. wood for x-rays in the 15.77–25.27
keV range. Journal of Radiation Protection Dosimetry,
135 (1), 47-53.
94