Fabrication and characterisation of phantom material made of Tannin-added Rhizophora spp. particleboards for photon and electron beams

Particleboards made of Rhizophora spp. with addition of tannin adhesive were fabricated at target density of 1.0 g/cm3. The physical and mechanical properties of the particleboards including internal bond strength (IB) and modulus of rupture (MOR) were measured based on Japanese Industrial Standards (JIS A-5908). The characterisation of the particleboards including the effective atomic number, CT number and relative electron density were determined and compared to water. The mass attenuation coefficient of the particleboards were measured and compared to the calculated value of water using photon cross-section database (XCOM). The results showed that the physical and mechanical properties of the particleboards complied with Type 13 and 18 of JIS A-5908. The values of effective atomic number, CT number and relative electron density were also close to the value of water. The value of mass attenuation coefficients of the particleboards showed good agreement with water (XCOM) at low and high energy photon indicated by the χ2 values.


Introduction
Phantom material is defined as materials that simulated the absorption and scattering properties of human tissues towards ionising radiation (Khan, 2010). Phantom materials are used mainly in quality assurance and dosimetry works involving ionising radiation. Water is commonly used as phantom materials it has mas density near to human soft tissues at 1.0 g/cm 3 . The use of water however is not always convenience due to its liquidity. Several solid-state phantom materials are introduced and made water-equivalent to replace water such as acrylic and perspex. These water equivalent materials however do not always give attenuation properties similar to water as they still differs to water in term of effective atomic number and electron density. These phantom materials also may develop electron contaminations that could alter the dose measurement.
Earlier studies have suggested that Rhizophora spp. as potential phantom material as it has mass density and attenuation coefficient close to water (Bradley at al., 1991;Tajuddin et al., 1996). However the use of Rhizophora spp. solid woods as phantom materials has several limitations including tendency to crack over period of times. The size of Rhizophora spp. trunk is also limited to construct a full size phantom besides its density inhomogeneity across the trunk. particleboards made of Rhizophora spp. was introduced with advantages includes better density uniformity and able to be fabricated at various size and shapes without compromising its attenuation properties (Marashdeh et al., 2011). However, the strength of the binderless particleboards made of Rhizophora spp. becoming a concern due to rigidity and heavy workload as phantom materials. To date, studies had been carried out to improve the strength of the particleboards made of Rhizophora spp. Addition of adhesives or binder becoming major option to increase the strength of the particleboards with at the same time retaining its water-equivalent properties. The synthetic-based adhesives such as urea-formaldehyde (UF) commonly used in industrial particleboards manufacturing failed to retain the attenuation properties of Rhizophora spp. particleboards in comparison to water (Surani, 2008, Ngu et al., 2015. Tannin has been used as alternative binder for particleboards and plywood to replace commonly used formaldehyde-based adhesives (Pizzi and Merlin, (1981); Pizzi and Scharfetter, (1989); Oo, et al, (2008). Tannin is a biological-based adhesive that can be extracted mainly from barks including Rhizophora spp. (Mohd Yusoff et al., 1988). Previous study had suggested the suitability of tannin adhesive for Rhizophora spp. particleboards as phantom at low energy photons (Safian, 2012). However, the tannin-based particleboards with low treatment level of tannin (5%) still failed to satisfy the industrial standards for physical and mechanical properties. Previous studies had suggested that the suitable percentage of biological-based adhesives for Rhizophora spp. is within 10% without changing their attenuation properties (Ghasemi, 2012; Tousi et al., 2014;Abu Arra et al., 2014). This study focused on the fabrication and characterisations of Rhizophora spp particleboards with 10% percentage of tannin adhesive as phantom materials in application of photon and electron beams.

Preparation of Sample
The Rhizophora spp. trunk was obtained from a charcoal factory in Kuala Sepetang, Perak, Malaysia. Only trunk from the middle part of the tree was chosen according to the study by Shakhreet et al., (2013). The trunk was cleaned and dried before planed in order to reduce it into smaller wood chips. The wood chips were then ground into smaller wood particle and screened using horizontal screen machine to obtained only wood particle with particle size of 104 μm according to the study by Marashdeh et al., (2011). Particleboards were fabricated using hot press machine with target density of 1.0 g/cm 3 . The target density of the particleboards was determined based on the calculation of the mass wood particle required to fabricate the particleboards at density of 1.0 g/cm 3 . And amount of 10% tannin is added into the wood particle based on the dried mass of wood particle. The actual density of the fabricated particleboards were measured based on the external dimension and mass of the particleboards given by the equation of = (1) With m is the measured mass of the particleboards and V is the volume of the particleboards measured based on its external dimension given by the equation of = ℎ × ℎ × ℎ (2) The particleboards were cut into smaller piece sample with size 5.0 cm x 5.0 cm as shown in Figure  1(a) for mechanical and characterisation tests. A replication of particleboard was sawn circularly as shown in Figure 1 (4) where ai and zi are electron fraction and atomic number of i th element in the sample respectively. m is the experimental coefficient for biological materials and water with value of 3.4. The electron fraction of the i th element can be calculated by the equation where wi and Ai are fractional weight and atomic mass of the i th element respectively.

Measurement of CT Number and Relative Electron Density
A study by Saito, (2012) had suggested that the electron density of a medium is very much related to it CT number. Thus the electron density of a medium can be derived directly from it CT number. A density plug phantom was constructed from the particleboard made compatible with computed tomography (CT) electron density phantom model CIRS 062M. The particleboard was sawn circularly and stacked together using wood adhesive to obtain a cylindrical-shaped phantom with approximate diameter of 3.0 cm. The density plug phantoms were scanned using the CT scanner at 120 kVp CT energy along with the water, liver muscle, and fat equivalent density plug phantoms. The average CT numbers of samples were measured and the relative electron density curves were plotted based on the recommendation by the manufacturer as shown in Figure 2. The relative electron density of tanninadded Rhizophora spp. particleboards was determined from the electron density curve.   with I˳ is initial intensity of photons, and µ is linear attenuation coefficient of sample in. The value of linear attenuation coefficient is calculated using the equation µ = 1 ln (°) The value of mass attenuation coefficient is obtained by dividing the value of linear attenuation coefficient with the gravimetric calculation of sample density. The measured mass attenuation cofficient is compared to the calculated value of water using photon cross-section database (XCOM) according to the study by Berger and Hubbell, (1987).

Results
The average mass density of tannin-added Rhizophora spp. particleboards measured using gravimetric method is presented in Table 1. The results showed that the particleboards were able to be fabricated at target density of 1.0 g/cm 3 . The particleboards were also showed good density uniformity shown by the standard deviation (SD) of measured density.

CT Number and Relative Electron Density
The average CT number and relative electron density of Rhizophora spp. particleboards in comparison to various tissue equivalent density plug phantoms is presented in Table 3. The results showed that the average CT number of tannin-added Rhizophora spp. particleboards was close to the value of water equivalent density plug phantoms. The calculation of relative electron density also showed close value of tannin-added Rhizophora spp. to water. This had indicated good agreement of attenuation coefficient of tannin-added Rhizophora spp. to water as the CT number is very much related to the attenuation coefficient of medium to the x-ray. A study by Kurudirek, (2014) suggested that the electron density of tissue is very much related to the attenuation coefficient of the tissue. Therefore, the results also indicated the similarity of attenuation coefficient of tannin-added Rhizophora spp. particleboards to water in based on their close value of relative electron density.

Mass Attenuation Coefficient
The mass attenuation coefficient of tannin-added Rhizophora spp. particleboards in comparison to water (XCOM) and previous studies at low energy photon measured using XRF set up is illustrated in Figure 5. The detailed value of linear and mass attenuation coefficient of the samples is presented in Table 4. The results showed that the linear and mass attenuation coefficient of tannin-added Rhizophora spp. particleboards was in good agreement to the value of water (XCOM). This had indicated the water equivalent property and similar attenuation properties of tannin-added Rhizophora spp. particleboards. The results also showed that the mass attenuation coefficient of tannin-added Rhizophora spp. particleboards was in good agreement with the binderless Rhizophora spp.   The mass attenuation coefficient at high energy photon measured using Ladlum set up is presented in Table 5. The results showed that the mass attenuation coefficient of tannin-added Rhizophora spp. particleboards were close to the value of water (XCOM). The mass attenuation coefficient at 137 Cs gamma energy was slightly lower to the value of water with percentage of discrepancies of 17% to that in water. The mass attenuation coefficient at 60 Co gamma energy however was in good agreement to water with lower percentage of discrepancies of 11%. The results had indicated that tannin-added Rhizophora spp. having attenuation coefficient close to to the value of water at both low and high energy photons.