2.1 Sample preparation

After the oil had been extracted from olives, the rest of the unused samples were packed in an open area, usually near a water pond, and exposed for approximately 1 month. The samples were collected from different areas in Jordan, namely, Mafraq (sample M), Kerak (sample K), Ajloun (sample A), and Irbid (sample I) and left for 30 days in the air to reduce their water content. Fig.1 shows the different locations from which the samples were obtained.

Fig. 1Full-screen View

The sampling sites of olive peat in Jordan.

Samples were dried to about 6% moisture and then sieved (size: 140 µm) to remove large particles. They were then compressed into pellets using a manual hydraulic press machine for 45 s at 31 MPa. No other reagent was added to the material to fabricate the binderless pellets. Four pellets of different thickness (i.e. 0.5, 1.0, 1.5, or 2.0 cm) and an area of 1.41 cm² were obtained. Table 1 lists the olive peat samples according to the area from where they were obtained and the densities measured after pelletisation. The densities of the pellets were calculated by dividing their mass (g) by their volume (cm³).

Energy dispersive X-ray analysis (EDXA) (FEI NovaSEM 450) was used to determine the elemental composition of the samples. The pellets were scanned at magnifications of 500× and 1000× at an acceleration voltage of 10 kV (Fig. 2). Details of the elemental composition of the olive peat samples and the results of chemical analysis of concrete are shown in Table 2.

Fig. 2Full-screen View

EDXA spectrum and elemental composition of olive peat made from sample A.

2.2 Mass attenuation coefficient measurements

The linear mass coefficient (µ) and μ/ρ are determined by measuring the transmission photon beam passing through samples of known thickness. The experimental setup in this work is shown in Fig.3. Point sources (10 mCi) of ²¹⁴Am (0.059 MeV), ¹³³Ba (0.356 MeV), ¹³⁷Cs (0.662 MeV), and ⁶⁰Co (1.17 and 1.332 MeV) were used to irradiate the samples. Each sample was irradiated thrice, and the average value was taken to calculate µ. Energy intensities were measured using a 2" × 2" NaI(Tl) scintillation detector (ORTEC Inc.) with an energy resolution of 7.6% at 662 keV. Signals were collected into a spectroscopic amplifier and multichannel pulse height analyser, and samples were irradiated for 3600 s to ensure good statistics. The gamma spectra were analysed using a Maestro-ORTEC instrument.

Fig. 3Full-screen View

Experimental setup

The detector shield was surrounded by a 5 cm-thick layer of Pb to reduce background and scattered radiation. The diameters of the collimator facing the detector and another collimator in front of the point source were 0.3 and 0.5 cm, respectively, and the distances between the point source and sample and between the sample and detector were 8 and 7 cm, respectively. The experimental setup is displayed in Fig. 3.

The µ (cm^-1) and µ/ρ (cm²/g) of the olive peat samples were calculated according to Beer–Lambert’s law using Eqs. (1) and (2), respectively.

where I_o denotes the photon intensity without sample; I is the photon intensity after sample and ρx is the mass thickness.

The error in the mass attenuation coefficient ($\mu /\rho$) was given by:

where ${\text{σ}}_{\text{μ}}=\pm \left({\text{μ}}_{\text{max}}-{\text{μ}}_{\text{min}}\right)/2$, $\rho$ is the density of the sample.

The attenuation experiment was performed by measuring the μ/ρ of Al with a thickness of 0.5 cm (Fig. 4). No significant difference between the theoretical and experimental μ/ρ of Al at different gamma ray energies was observed, and a high correlation of R² = 0.9981 was obtained. The theoretical μ/ρ of Al was calculated using XCOM software.

Fig. 4Full-screen View

The comparison of experimental and theoretical values of mass attenuation coefficients of Al.

3.1 Mass attenuation coefficient measurements

The µ/ρ of the olive peat samples obtained at photon energies of 0.059, 0.356, 0.662, 1.17, and 1.332 MeV are shown in Table 3. The intensities of the incident and transmitted gamma rays are determined from the net area under the k_α peak of the different sources. The errors in µ/ρ of the olive peat samples are between 0.002% and 0.020%. Table 3 reveals that μ/ρ decreases with increasing photon energy, as also shown in Fig. 5. The difference in measured μ/ρ of the olive peat samples can be attributed to differences in the elemental compositions of the samples. However, no difference in terms of basic elements, such as oxygen and carbon, which have similar atomic numbers, is found. No significant differences between the μ/ρ of the olive peat samples are observed in the range of 0.356–1.332 MeV, likely due to the similarity of the basic structure of the samples. Clear variations among the samples occur at 0.059 MeV.

Fig. 5Full-screen View

Mass attenuation coefficient at photon energy (0.059 – 1.133) MeV for olive peat (M, K, A, and I) samples compared with calculated XCOM for concrete and lead.

The calculated and measured μ/ρ of the olive peat samples are shown in Table 4. Good agreement between the experimental and theoretically calculated results is observed in the range of 0.356–1.332 MeV, but the former tend to be slightly lower than the latter. This difference can be attributed to differences in the elemental compositions of the samples.

Fig. 5 compares the experimental µ/ρ of the olive peat samples with the theoretical µ/ρ of concrete and Pb as calculated using XCOM software [22]. The µ/ρ of the olive peat samples is in close agreement with the calculated µ/ρ of concrete. Interestingly, the experimental μ/ρ of the olive peat samples and calculated µ/ρ of concrete and Pb decrease irregularly with increasing photon energy. Photoelectric and Compton effects appear to dominate at energy levels less than 0.356 MeV (Fig. 5). In addition, the different µ/ρ of the four studied samples, concrete, and Pb could be clearly observed in the low-energy region. This result can be attributed to the fact that photoelectric effects are different from varying material combinations of olive peat and concrete materials when compared with Pb material. The olive peat samples contain essential elements, namely, O and C, as well as slight differences in some heavy-metal elements; these differences can affect the µ/ρ of the samples at low photon energies. In addition, the olive peat material has the ability to capture these elements and therefore future studies can be carried out on this material. Fig.6 shows the experimental µ/ρ of the samples at 0.356–1.332 MeV in comparison with the XCOM-calculated µ/ρ of concrete and Pb. The µ/ρ of the olive peat samples is closer to the calculated µ/ρ of concrete than to the calculated µ/ρ of Pb at 0.0356 MeV. The level of agreement between the attenuation measurements for olive peat and concrete at the studied photon energy range has not been measured in previous work. These results indicate that olive peat can potentially be used as radiation shield or as a phantom material.

Fig. 6Full-screen View

Mass attenuation coefficient at photon energy (0.356 – 1.133) MeV for olive peat (M, K, A, and I) samples compared with the calculated XCOM for concrete and lead.

The percentage difference between the experimental μ/ρ of olive peat samples and the calculated µ/ρ of concrete at photon beam energies of 0.059, 0.356, 0.662, 1.17, and 1.332 MeV are shown in Table 5. No significant difference was found among the olive peat samples and the calculated concrete, where the percentage difference of µ/ρ values for olive peat samples from concrete are within 0.7% at photon energies of 0.356–1.332 MeV. However, the radiation behaviours of the same samples were consistent with previous results [23-24] at a low photon energy of 0.059 MeV. In addition, the percentage difference in the µ/ρ of sample K from the calculated µ/ρ of concrete was generally lower than those of other samples at all energies tested.

A comparison of the μ/ρ of olive peat samples with those of some polymeric materials by [23-25] is presented in Table 6. The μ/ρ of the olive peat samples at 0.059 MeV is compatible with those of Perspex [23] and polycarbonate [24]. These findings indicate that olive peat has potential use in radiation applications in the field of medical physics.

3.2 Half Value Layer (HVL) measurements

The measured X_1/2 of the olive peat samples and calculated X_1/2 of concrete and Pb are given in Table 7. The results indicate that X_1/2 depends on the photon energy. Fig. 7 compares the experimental X_1/2 of the samples with the calculated X_1/2 of concrete and Pb. The X_1/2 values of the samples increase with increasing photon energy. According to their X_1/2, the olive peat samples, concrete, and Pb exhibit the same behaviour. X_1/2 is an important quantitative factor of the penetration potential of specific radiation through specific materials [26]. Materials with values lower than HVL are better radiation shields in terms of thickness requirements [27]. Fig. 7 shows that the X_1/2 of Pb and concrete are minimal due to their high density. By comparison, the X_1/2 of the olive peat samples indicates lower shielding effectiveness than that of concrete. Therefore, to achieve the required radiation protection, a thicker layer of olive peat should be applied.

Fig. 7Full-screen View

The HVL results of olive peat (M, K, A, and I) samples and calculated concrete and lead.