The copper filling of the MWCNTs is confirmed via a combination of various modalities, where EDS and XPS data showed the existence of copper within samples, and the TEM images provided proof of the filling of the MWCNTs with copper.
Following the confirmation of the copper filling of the carbon nanotubes, the effect of
these Cu-filled MWCNTs on enhancing the radiation dose in a water cube is studied, in megavolt x-ray beams where the dominant type of radiation interaction in that energy range (between 2.5 and 10 MeV) is Compton scattering, an interaction between the incident photon and a free electron11. This type of interaction is independent of the atomic number “Z” of the absorbing material, as opposed to the photoelectric effect, which has a mass-dependent cross-section but is a less likely interaction in the studied energy range. Figures 6 and 7 show the dose increases when the MWCNTs are filled with copper, which has a higher atomic number than carbon and water equivalent, suggests that the photoelectric interaction is responsible for this dose enhancement. Both photoelectric effect and Compton scattering decrease with increasing photon energy, the decrease in the dose enhancement shown in Fig. 6 is consistent with both types of interactions.
Furthermore, the dose increase just below the water cube was measured by placing a radiochromic film between the cube and the solid water slab (Figs. 7 and 8). We see a dose increase with values that are different than the ion chamber measurements. When unfilled MWCNTs are added to water, there’s no photoelectric effect introduced, given that the density of the MWCNTs is less than the effective density of water, therefore the main increase would be due to Compton scattering. This increase is equal at two different distances from the water cube, at 0 cm and at 1 cm away. When the pure MWCNTs are replaced with copper filled ones, for the same photon energy (6 MV in this case), we notice a large increase in the proximal measurements (the film that is placed just under the cube) and a small increase in the distal point of measurement ( the ion chamber, 1 cm below the water cube). This suggests that at the vicinity of the interaction, photoelectrons due to the high Z of the copper contribute most to the dose increase wit this effect decreasing with distance. Further measurements will be done to support this hypothesis.
The dose increase when pure carbon nanotubes were added to water (Figs. 7 and 8) can be explained by the probability of Compton scattering. Compton scattering probability is a function of the classical radius of the electron12, as in the Klein-Nishina formula such that:
Compton cross section per unit solid angle (dσ/dΩ) α r2
with “r” being the electron’s classical radius, which is inversely proportional to the rest mass of the electron. The effective mass of the electron in carbon nanotubes was studied by Roy and Maksym13 in units of the electron’s rest mass, a function of the radius of the MWCNT. This leads to an expected increase in the probability of Compton scattering.
Adding copper filled carbon nanotubes to water would increase the measured dose, due to higher probability of photoelectric effect from the low energy photons in the spectrum. As we increase the energy, this probability decreases and so does the probability of Compton effect, therefore the increase in the measured dose (due to dose enhancement), decreases with increasing the energy of the beam. In the studied material, pair production is negligible. Also, secondary electrons in the beam react with nanoparticles in a different way than they would in bulk material (Krasheninnikov and Nordlund 2004), owing to the large surface area of the nanotubes the increased probability of interaction.
In addition, CNTs would potentially have a different effect, compared to other nanoparticles (e.g. Au-NPs) due to their unique honeycomb like shape, allowing electrons to escape with no self absorbance16.
Cu-MWCNTs concentration in the water cube is 5 × 10− 2 g/cc, this is much lower than the recommended concentration of nanoparticles to achieve dose coverage at the cellular level14.
However, these recommendations were given to nanoparticles of sizes ranging between 2 nm and 100 nm and carbon nanotubes have diameters in this range but lengths in µm range. The metal particles are also distributed along the axis of the nanotubes. This could alter the recommended concentrations; further studies will be conducted to find the optimum number.