**3. Absorption of photons by chemical elements**

The photoionisation cross sections with photon energy of low Z (oxygen 8) and high Z (Gold 79) are shown in **Figure 2**. From **Table 1** we can see that oxygen may be used to approximate tissue absorption. In the low-energy region around 100 keV, it is clear from **Figure 2** that the absorption (and thus photoelectron production) of gold is several orders of magnitude greater than tissue. **Table 2** gives the photoionisation cross sections for a section of elements of interest [8].

If the absorption of gamma ray photons by chemical elements varies so widely, with such an increased cross section for the higher Z elements, it seems clear that the incorporation of high-Z elements in living tissue would be essentially harmful. There is evidence from evolution to support this idea, and this will be discussed below. Apart from contamination issues due to anthropogenic sources and the question of medical procedures, the problem arises because of the continuous irradiation of living creatures by natural background radiation (NBR). The gamma


#### **Table 2.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

**E (keV) Oxygen (8) Water Muscle (striated)** 2.950E−4 2.515E−4 2.536E−4 4.992E−3 4.320E−3 4.356E−3 1.647E−2 1.431E−2 1.443E−2 3.325E−2 2.817E−2 2.841E−2 2.018E−1 1.766E−1 1.781E−1

 *for electrons of different energies in oxygen, water and muscle tissue [6].*

**178**

**Figure 2.**

**Table 1.**

*Continuous slowing down range r0 in g cm<sup>−</sup><sup>2</sup>*

*Gamma ray absorption cross sections for oxygen (Z = 8) and gold (Z = 79).*

*Photoionisation cross sections for a selection of elements of interest at different incident energies in the natural background low-energy region (barns) (Hartree-Fock approximation) [8].*

spectrum of NBR increases rapidly to lower energies, roughly as the −7/2 power of the energy. From **Table 2**, it is clear that the absorption of photon energy in the NBR region (50 keV) from iodine is about 3000 times that from oxygen or water/tissue. It has been suggested that this may explain the radiosensitivity of the thyroid gland [9]. It should be noted in passing that the absorption coefficients at the energies tabulated do not generally reflect the overall absorption differences between the low-Z and high-Z elements over the whole-energy spectrum because of discontinuities in the absorption by the d- and f-orbital electrons in the heavier elements like gold and uranium. These discontinuities for gold are clear in **Figure 2**. For gold, the enhancement factor relative to water at the four energies tabulated (10, 50, 100 and 150 keV) are 246, 2592, 19,500 and 24,545. Similar variations in enhanced photon cross section are apparent for uranium which has 45,000 times the photoelectron cross section at 150 keV than the oxygen in water.

It is clear from this approach that the determining absorption of living tissue is defined not by water but by the higher Z elements present. This is starkly true for iron and iodine which must form centres for photon absorption and photoelectron production. It may therefore be plausible to argue that this is why that the main cancers associated with external radiation exposures are leukaemia and thyroid cancer.
