*2.2. Fabrication of DSF/NaOH/IA-PAE/R. spp. Particleboard Phantoms*

At the start of the process, all sample formulations with particle size ≤ 74 μm were thoroughly mixed by hand as they were applied for 0.5 h to the DSF/NaOH/*R.* spp. mixture with different IA-PAE content. Thereafter, they were mixed evenly for another 10 min using a rotary mixer machine to ensure the uniformity of the samples. By using a mould of dimension (30 cm × 30 cm × 1.0 cm), the mixtures were subsequently cold-compressed for 10 min using a hydraulic press machine (0.49 MPa, 5 min, and 0 ◦C) fitted with stops to achieve a target density of 1.0 gcm−<sup>3</sup> at room temperature and relative humidity of 55%. The stacked mats were then constructed using a hydraulic hot press machine at 170 ◦C for 20 min with 20 MPa [6]. A total of 150 units of DSF/NaOH/IA-PAE/*R.* spp. particleboard phantoms were developed. Tables 1 and 2 have been tabulated to examine the physico-mechanical and dimensional stability properties (PMDSP) (MC—moisture content, SC—solid content, IB—internal bonding, MOR—modulus of rupture, MOE—modulus of elasticity, TS—thickness swelling, and WA—water absorption), elemental compositions, effective atomic numbers (*Zeff*), and electron densities (*Nel*) of the particleboards and standard phantom materials. As can be seen, Table 1 shows that sample A15 with 15 wt% IA-PAE concentrations provides the ascribable parameters and meets the minimum requirements of Type 8, Type 13, and Type 18, according to JIS A-5908 [26]. The *Zeff* and *Nel* of these phantoms, which were the exclusive parameters used to characterize various types of materials, were found to be comparable to those of water and other commercial phantom materials (Table 2).


**Table 1.** PMDSP of particleboard phantoms.

Remark: A0 = Uncured *R.* spp., A5 = DSF/NaOH/IA-PAE/*R.* spp., A10 = DSF/NaOH/IA-PAE/*R.* spp., and A15 = DSF/NaOH/IA-PAE/*R.* spp. indicate 0, 5, 10, and 15 wt% IA-PAE. Data are expressed as an average ± standard deviation (SD).



<sup>a</sup> Curent study, <sup>b</sup> Sahoo et al. [27], <sup>c</sup> Schoenfeld et al. [28].

Using the gravimetric technique (Equation (1)), the average particleboard densities (*ρ*) were assessed, and the propagation of uncertainty was deduced based on the external dimensions using Equation (2):

$$\rho = \frac{m}{l \times w \times h} \tag{1}$$

$$d\rho = \left(\frac{dm}{m} + \frac{dl}{l} + \frac{dw}{w} + \frac{dh}{h}\right)\rho\tag{2}$$

where, *m*, *l*, *w*, and *h* denote the respective particleboard mass, length, width, and thickness; *dm*, *dl*, *dw*, and *dh* are the uncertainties in *m*, *l*, *w*, and *h*, respectively.

The computed tomography (CT) image modality was achieved based on a previous technique detailed by Samson et al. [7]. The parameters of the various standard phantom materials, compared with DSF/NaOH/IA-PAE/*R.* spp., are listed in Table 3. The results of the average density show that the density of A15 is within the range found for water and other commercial phantom materials. According to the results, the mean HU values and ED of the A15 were near to those acceptable standard reference equivalent materials [7], while a significant variation is observed with Ao, A5, and A10, respectively. This might explain the fact that better attenuation abilities were observed when X-ray beams passed through the corresponding sample (A15). Therefore, the A15 sample formulation showed the potential to replicate human tissue because it has a comparable dynamic and is higher in terms of stability as a medical phantom.


**Table 3.** Parameters of various standard phantom materials compared with DSF/NaOH/IA-PAE/*R.* spp.

<sup>a</sup> Current study, <sup>b</sup> Sahoo et al. [27], <sup>c</sup> Schoenfeld et al. [28].

#### *2.3. Measurement of RAPs*

The attenuation properties were determined using a Ludlum lead equivalent setup, as depicted in Figure 1. 137Cs and 60Co sealed sources with effective gamma energies of 0.662 and 1.250 MeV were used to provide the incident photons. The sources and the Ludlum NaI(TI) detector, with diameters of 2.5 cm and 6.5 cm, were encapsulated in a lead container with collimation of diameter 0.5 cm and thickness of 2 cm to simulate the line source projection and avoid leakage. An aluminum (Al) plate of dimension 7 cm × 7 cm, with an approximate thickness of 0.1 cm, was used as an attenuator to produce the scattered photons. The optimum distance between the source compartment and the Al plate and between the Al plate and the detector compartment was 30 cm, whereas the distance between the phantom samples and the detector compartment was 6.2 cm. The transmitted photons from the source were collected and detected using the Ludlum scintillation detector connected to a single channel analyzer (SCA).

**Figure 1.** Ludlum setup: (**a**) schematic diagram and (**b**) actual experimental setup.

The linear attenuation coefficient (LAC) and mass attenuation coefficient (MAC) are the fundamental parameters to evaluate the dosimetric and radiation shielding performance of any composite material. These commonly used parameters provide some information on the possibility of photon interaction processes with matter per unit thickness. As a photon beam propagates through a homogeneous medium, the beam intensity at depth *t* is assigned as *It*, whereas the beam intensity at a reference point in the absorbing material (*t* = 0), is assigned as *Io*. This can be described by the familiar Beer-Lambert's law (Equation (3)) [6,7].

$$
\mu = \frac{1}{\chi} \ln \left( \frac{I\_o}{I\_t} \right) \tag{3}
$$

$$
\mu\_m = \frac{\mu}{\rho} = \frac{1}{\rho t} \ln \left( \frac{I\_o}{I\_t} \right) = \frac{A}{M} \ln \left( \frac{1}{T} \right) \tag{4}
$$

where *<sup>μ</sup>* (cm−1) denotes the LAC, *<sup>ρ</sup>* (g·cm−3) is the density, *<sup>x</sup>* (cm) and *<sup>t</sup>* (g·cm−2) are the physical thickness and mass thickness (mass per unit area), *T* is the transmittance, *M* (g) is the mass of the sample material, *A* is the cross-sectional area (cm2), and *μ<sup>m</sup>* (cm2g−1) indicates the total MAC. The total *μ<sup>m</sup>* values were calculated on the basis of the mixture rule by using the weight fraction (*ωi*) for each element *i* of the particleboard materials, as expressed in Equations (5) and (6):

$$\mu\_m = \left(\frac{\mu}{\rho}\right)\_{\text{DSF}-based\ particle} = \omega\_1 \left(\frac{\mu}{\rho}\right)\_1 + \omega\_2 \left(\frac{\mu}{\rho}\right)\_2 + \dots = \sum\_{i=1}^{N} \omega\_i \left(\frac{\mu}{\rho}\right)\_i \tag{5}$$

$$
\omega\_{\bar{i}} = \frac{n\_{\bar{i}} A\_{\bar{i}}}{\sum\_{\bar{i}} n\_{\bar{i}} A\_{\bar{i}}} = \frac{\tilde{\rho}\_{\bar{i}}}{\rho} \tag{6}
$$

where *ni* denote the number of atoms of the *i* th individual element, *Ai* is the atomic weight, and *<sup>ρ</sup><sup>i</sup>* is the actual mass density. The related cumulative discrepancies in the experimental MAC were obtained by using the propagation of error relationship from ambiguities in *Io*, *It*, *x* and areal density (*ρ*) [7]:

$$
\Delta \left( \frac{\mu}{\rho} \right) = \frac{1}{\rho} \sqrt{\left[ \left( \frac{\Delta I\_l}{I\_l} \right)^2 + \left( \frac{\Delta I\_o}{I\_o} \right)^2 + \left( \ln \frac{\Delta I\_o}{I\_l} \right)^2 + \left( \frac{\Delta \chi}{\chi} \right)^2 \right]} \tag{7}
$$

where Δ*It*, Δ*Io*, and Δ*x* are the errors in the intensities *It*, *Io*, and thickness *x* of the sample material, respectively. Paired *t*-test using SPSS (V22.0) was used to calculate any variations in *μ<sup>m</sup>* values, as compared with the value of water ascertained via the photon cross-section database (XCOM) [29]. The half-value layer (HVL–*X*1/2) is used to assess how far X-ray penetrates the particleboard samples, which were used to verify the performance of the patient's radiation exposure. It can be defined, as given in Equation (8), whereas Equation (9) is the relationship between the mean free path (MFP − *λ*) and *X*1/2 [7,8].

$$\text{HVL}\_{\text{\textdegree X}\_{1}/2} = \frac{0.693}{\mu\_{m} \times \rho} \tag{8}$$

$$\text{MFP}\_{\prime} \, \lambda = \frac{X\_{1/2}}{0.693} \,\tag{9}$$

#### *2.4. Dosimetric Evaluation of DSF/NaOH/IA-PAE/R. spp. Particleboard Phantoms*

Samples with up to 15 wt% IA-PAE addition were selected because of their optimum characteristics, and a total of 34 units of DSF/NaOH/IA-PAE/*R.* spp. particleboard phantoms of sizes 30 cm × 30 cm × 1.0 cm and 30 cm × 30 cm × 0.5 cm, simulating the dimensions of widely used solid water phantom slabs (CIRS Inc., Norfolk, VA, USA), were fabricated. Additionally, two of these slabs were designed with slots to accommodate the cylindrical IC. The Farmer-type IC was used due to its unique features, such as high precision, stability, dose rate independence, excellent linearity, little to no fading, and equivalency to soft tissue nature. All experimental measurements with both photon and electron beams were carried out on the medical Elekta Synergy PRIMUS LINAC at the Department of Oncological and Radiological Science, Advanced Medical and Dental Institute, Universiti Sains Malaysia (USM).
