Attenuation coefficient for Agarwood and shade tree sections at 59.6keV and 356keV

Gamma ray measurements were carried out on wood sections of agarwood and shade trees to obtain its mass attenuation coefficients. Diameter of the blocks of agarwood (Aquilaria Malaccensis) is 17.0cm and shade tree (Khaya Senegalensis) sections are 24.0cm (branch) and 37.2cm (trunk). The radiation transmission measurement setup used a 0.5” x 0.5” thallium-activated sodium iodide detector, scaler ratemeter Ludlum model 2200 and collimated radioactive sources 3.5GBq Am-241 and 0.05GBq Ba-133 for the attenuation measurements. The dry density of agarwood trunk was 0.40±0.01 gcm−3. For the shade tree, the dry densities were 0.52±0.02 gcm−3 (branch) and 0.49±0.01 gcm−3 (trunk). At 59.6keV, the mass attenuation coefficient for the agarwood at diameter is 0.114±0.007 cm2g−1, and for shade tree (branch) is 0.136±0.004 cm2g−1 and 0.120±0.004 cm2g−1 shade tree (trunk). At 356keV, the mass attenuation coefficient for the agarwood at diameter is 0.081±0.003 cm2g−1, and for shade tree (branch) is 0.086±0.004 cm2g−1 and 0.068±0.028 cm2g−1 shade tree (trunk).


Introduction
Previous studies reported various ways to determine wood attenuation coefficient and density using water immersion/density metric method, photon and x-ray measurements, ultrasonic goniometry as well as MCNP calculations [1][2][3][4][5][6][7][8][9][10].Density and strength of wood indicate the quality of wood [11].It has been established that these efforts provide a way to identify particular types of wood, including Rhizophora spp, Agarwood, soft and hard wood, Pinus Radiata and Tectona Grandis through direct measurements or indirectly [1][2][3][4][5][6]9].The mass attenuation coefficient is a measure of the probability of the interaction that occurs between incident photons and the matter of the unit mass per unit area.The factors that affect attenuation are related to the incident photon beam and the properties of the material through which the radiation traverses.These factors include the incident beam energy, the thickness, atomic number and the density of the material.The density of wood, exclusive of water, varies greatly both within and between species.In this present study, we describe the measurements of mass attenuation coefficients for local shade tree and agarwood tree sections both in dry and wet conditions.Dry densities are recorded as 0.40±0.01,0.49±0.01 and 0.52±0.02gcm -3 for agarwood, shade tree(branch) and shade tree(trunk) respectively.Both Am-241 and Ba-133 are suitable gamma ray radioactive sources due to their excellent penetrability and sensitivity through low-density materials.This study provides the density data to use in gamma densitometry whereby it may be used to reasonably estimate locations of gaharu resin within a tree.Classification of agarwood (gaharu) by resin content has been described by MA Nor Azah et.al [12].Agarwood is valued for its distinctive fragrance and highly valuable fragrant heartwood [12][13].Due to its pharmalogical properties, the species of Aquilaria spp also provides health applications from the use of its leaves, stem, bark and wood.This includes having antioxidant, anti-bacterial and antimicrobial effects [14].

Material and methods
Woods sections were first measured and weighed to obtain wood density for both shade tree and agarwood samples.This was done for both dry sample and wet sample.The shade tree sample came from a species of Khaya Senegalensis, a common shade tree planted for landscaping in Malaysia [15].Measurements for this wood was performed within six months from the day the tree was cut down, however the agarwood sample is at least 10 years old.The shade tree density is approximately 20-30% higher than agarwood density for this case.Table 1 shows the physical properties of the wood samples.For sample A, the wood was intact with no obvious signs of rotting or damage to the wood.Data was first collected in this intact condition.Measurements were taken for two (2) conditions; dry and wet.Wet condition is achieved by fully immersing the wood in plain water for 24 hours.Next, we drilled holes into the wood to create voids within the section.The holes were 2mm, 5mm and 10mm in diameter.Measurements were repeated in the same experimental setup to detect small voids within the wood.The detection of the smallest voids indicates the sensitivity of the gamma ray transmission technique in detecting density differences within the solid wood.This step is useful in simulating detection of smallest resin formation within an Agarwood tree.
For sample B, the Agarwood was acquired from an Agarwood plantation.The resin has been taken out for processing leaving a hole within the wood section.This sample has been kept for more than 10 years in room temperature storage unit and is dry.Measurements were taken for two (2) conditions; dry and wet.Wet condition is achieved by fully immersing the wood in plain water for 24 hours.
For sample C, the wood has some rotting in the middle area.This section was cut from the bottom section of the tree trunk.Masking tape was used to keep the internals in its original position and for marking positions.Measurements were taken for two (2) conditions; dry and wet.Wet condition is achieved by fully immersing the wood in plain water for 24 hours.
Background count measurements were taken before and after experiments.These readings were taken to monitor experimental conditions (electronics and environment) were maintained throughout the experiment.Counting time on the scaler/ratemeter was set at 6 seconds for all data collection.The dial setting (DS)/high voltage (HV) setting is adjusted accordingly for each gamma source and for each cable-scaler/ratemeter setup combination.The sodium iodide detector was placed in a lead holder that has a 5mm dia.collimator.The radioactive source was also placed in a lead holder with a 15mm x 5mm collimator.Collimation prevents unnecessary exposure of sample and equipment that are outside the area of interest.The source-detector were aligned and directly facing each other so that the counting system measures the decrease in intensity along a linear path.Measurements were taken at every 1cm interval so as to obtain a series of intensities across the wood section.Three readings were taken for each point (linear path) and corrected for background.
Two radioisotopes were used in this work; Am-241 (59.5409 keV) and Ba-133 (356keV).We studied the gamma ray attenuation of wood using different radioisotope energies and activities along its cross section.The detection sensitivity for voids (holes) within the wood section is examined.Each sample was exposed to both Am-241 and Ba-133 separately and attenuation data were collected and analysed.The averaged background and air counts are listed in Table 2.The mass attenuation coefficient, µm is given by the division of the linear attenuation coefficient, µ and the density, ρ.
The counts represent intensity of transmitted beam, I and the mass attenuation coefficient,μ m in equation (1) was calculated and tabulated.These measurements were taken for solid wood at thickness range A: 3.6-24.0cm,B: 4.2-16.6cm,C:.4.1-35.8cm.Mass attenuation coefficient, µm is calculated and tabulated.Special attention is given to mass attenuation coefficient values at diameter to compare material discrimination ability between Ba-133 (higher energy) and Am-241 (lower energy).Figure 1 shows the experimental setup for the measurements.

Results and discussion
The count measurement results are presented in figures 2-5 below.Figures 2 and 3 show the results of density measurement on sample A using Ba-133 and Am-241 respectively.Each figure gives the experimental results for measurement conditions that is dry without void, dry with void and wet with void.The baseline data is taken as the result obtained for dry without void; wood section is intact in original shape.
Sample B (17.0 cm dia.) is taken from a section of an Aquilaria tree.This sample is dry, not freshly cut from the tree and has been kept in the store for many years.The gaharu resin has been taken out and left a void.Its measurement results for both Ba-133 and Am-241 is shown in figure 4.There are two sample conditions, dry and wet.For both Ba-133 and Am-241, both conditions displayed similar trendline respectively; the dry sample allowed relatively more gamma rays through wood compared to the wet sample.In the results for Ba-133, the wet and dry counts trendline were close to each other.The count difference between the thickest (position 7, 16.6 cm) and thinnest (position 1, 4.2 cm) through wood thickness is less obvious, i.e. dry 1678 counts, wet 2190 counts (∆~512 counts).For Am-241, both wet and dry count trendlines are similar, but are further apart from each other.The wet sample having more attenuation due to the water absorbed in the wood after 24 hours soaking.The count difference between the thickest (position 7) and thinnest (position 1) wood thickness is more obvious, i.e. dry 18618 counts, wet 17885 counts (∆733 counts).From the trendlines, Am-241 is the better choice in differentiating thickness, both in wet and dry conditions.In figure 4 at positions 8, 9, 10, 11 and 12 a void space is present.Void path lengths are 1.6cm, 2.9cm, 3.1cm, 2.5cm, 1.0cm respectively within the middle-area of the wood section.This void was created when the gaharu resin was taken out.We can see a slight uptrend of counts in this segment.The combined thickness (path length) of wood and void at positions 9 and 10 is 17.0cm, however their void sizes are 2.9cm and 3.1cm respectively.We see that this measurement is less sensitive to voids when it is dry compared to wet condition, especially when using Ba-133.
Figures 5 and 6 are results from density measurements of sample C (37.2 cm highest thickness) using Ba-133 and Am-241 respectively.Sample C has some rotting at the middle-segment of the wood, the void size is highest at positions 20 and 21 (void size 10.5cm).We see that in wet condition, both Ba-133 and Am-241 display mostly flat trendline compared to dry condition, and that gamma rays is further attenuated by the moisture within the wood.The presence of void at locations 15 through 28 with void sizes ranging between 2.0cm and 10.5cm is more obvious in the Am-241 results in figures 5 and 6, where we can see an upward drift at these locations.In Table 3, the attenuation coefficients for both wet and dry conditions is tabulated.This result supports the assertion that water content does provide additional attenuation, therefore suggest that if the measurements were done on-site at the plantation the moisture level would affect the data.

Conclusion
Mass attenuation coefficient is a parameter that describes how much attenuation the incident ray undergoes by passing through a material.This parameter lends to quantitative analysis of a material.In density/thickness measurement and computed tomography, this coefficient is an important parameter for differentiating material composition and physical properties.This paper gives the estimated gamma ray attenuation coefficient for Khaya Senegalensis, a common shade tree planted for landscaping in Malaysia and Agarwood tree.Two radioisotopes, Am-241 and Ba-133 were utilised and results were compared to determine their suitability in wood section examination.It is found that for wood sections with densities between 0.40±0.01g/cm 3 and 0.70±0.02g/cm 3 , Am-241 is more sensitive towards distinguishing thicknesses and densities compared to Ba-133.The results from the experiments can be a guideline for development of a gamma densitometer.Wet (Am-241)

Figure 1 :
Figure 1: Source-detector arrangement for measurement of the wood sections.Taking background and air count measurement (left).Taking sample count measurement (right)

[ 1 ]
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Table 1 .
Physical properties of wood samples.

Table 2 .
Averaged counts for samples A, B and C in their respective setups.