**4. The gamma energy spectrum of natural background radiation**

Since secondary photoelectrons will be generated from all exposures to gamma radiation and since the local ionisation density near the absorbing atom, particle or metal prosthesis is the quantity of interest, it is clear that the energy spectrum of gamma NBR is an important component of any assessment. External gamma radiation degrades in energy as it passes through tissue as a result of the various processes which occur. Energy is lost by Compton scattering resulting in the production of a Compton photon of lower energy than the initial energy. Electrons generated by the photoelectric effect lose energy through collisions and the generation of Bremsstrahlung photons of low energy and so forth. Thus, the further the initial photon travels in tissue, the greater the number of low-energy photons there are in the medium. The natural background radiation spectrum in Burnham-on-Sea, Somerset, UK, is reproduced in **Figure 3**.

Note the sharp increase in the number of photons at low energy: the cut-off is a result of absorption by the shielding of the thallium-doped sodium iodide scintillation detector. The degradation of photon energy inside the human body can be examined by placing an insulated scintillation detector inside a water-filled container and comparing the spectrum with that obtained in air. The spectrum obtained in this way, which compares well with that employed by Pattison et al. (who attempted to model the photoelectron effects in uranium particles [10]), is shown in **Figure 4**. The cut-off at low energy 15 cm inside the water jacket is due to the absorption of the low-energy short-range photons. By subtraction it is possible to show that the number of photons of low energy increases inside the water sphere of 30 cm diameter (used to approximate the body). Thus, the dispersion curve shifts to lower energy. The enhancement of photon numbers by energy is shown in **Figure 5**.

What is clear from these results is that NBR delivers mainly low-energy photons. It turns out that 60% of in-air NBR photons have energy below 150 keV and the peak in photon numbers increases to low energy below 50 keV. Photoelectrons of this energy have a mean CSDA range (**Table 1**) which is comparable with the dimensions of a single cell or cell nucleus. Therefore, a high-Z atom in a cell will be continuously amplifying NBR in proportion to the photoionisation cross section

**181**

**Figure 4.**

**Figure 3.**

*rollover at about 60 keV.*

*The Secondary Photoelectron Effect: Gamma Ray Ionisation Enhancement in Tissues from High…*

*Gamma ray spectrum obtained on beach at Burnham-on-Sea using a 2-in. NaI (Tl) Scionix detector. Note* 

shown in **Table 2** and delivering enhanced ionisation to that cell or cell nucleus relative to that calculated using the concept of absorbed dose which is based on the assumption that the absorber is effectively water (i.e. oxygen). Further, the biological effectiveness of NBR, its damage to tissues, will be defined by the highest Z atoms in the tissue. This will also be true for other exposures, for X-rays, medical examinations and exposures to anthropogenic sources, indeed the entire range of exposures which are regulated by the law on the basis of the current risk models.

*the energy dispersion of photons inside the body is very uncertain.*

*Energy dispersion in the low-energy region 0–500 keV of the natural background gamma photons at 15 cm depth inside a human body. Based on Pattison et al., Figure 3 and unpublished work using a gamma probe packed with bags of water. Shielding effects on the primary in-air dispersion below 100 keV are uncertain, and* 

*DOI: http://dx.doi.org/10.5772/intechopen.86779*

*The Secondary Photoelectron Effect: Gamma Ray Ionisation Enhancement in Tissues from High… DOI: http://dx.doi.org/10.5772/intechopen.86779*

**Figure 3.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

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.

**4. The gamma energy spectrum of natural background radiation**

Somerset, UK, is reproduced in **Figure 3**.

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.

Since secondary photoelectrons will be generated from all exposures to gamma radiation and since the local ionisation density near the absorbing atom, particle or metal prosthesis is the quantity of interest, it is clear that the energy spectrum of gamma NBR is an important component of any assessment. External gamma radiation degrades in energy as it passes through tissue as a result of the various processes which occur. Energy is lost by Compton scattering resulting in the production of a Compton photon of lower energy than the initial energy. Electrons generated by the photoelectric effect lose energy through collisions and the generation of Bremsstrahlung photons of low energy and so forth. Thus, the further the initial photon travels in tissue, the greater the number of low-energy photons there are in the medium. The natural background radiation spectrum in Burnham-on-Sea,

Note the sharp increase in the number of photons at low energy: the cut-off is a result of absorption by the shielding of the thallium-doped sodium iodide scintillation detector. The degradation of photon energy inside the human body can be examined by placing an insulated scintillation detector inside a water-filled container and comparing the spectrum with that obtained in air. The spectrum obtained in this way, which compares well with that employed by Pattison et al. (who attempted to model the photoelectron effects in uranium particles [10]), is shown in **Figure 4**. The cut-off at low energy 15 cm inside the water jacket is due to the absorption of the low-energy short-range photons. By subtraction it is possible to show that the number of photons of low energy increases inside the water sphere of 30 cm diameter (used to approximate the body). Thus, the dispersion curve shifts to lower energy. The enhancement of photon numbers by energy is

What is clear from these results is that NBR delivers mainly low-energy photons. It turns out that 60% of in-air NBR photons have energy below 150 keV and the peak in photon numbers increases to low energy below 50 keV. Photoelectrons of this energy have a mean CSDA range (**Table 1**) which is comparable with the dimensions of a single cell or cell nucleus. Therefore, a high-Z atom in a cell will be continuously amplifying NBR in proportion to the photoionisation cross section

**180**

shown in **Figure 5**.

*Gamma ray spectrum obtained on beach at Burnham-on-Sea using a 2-in. NaI (Tl) Scionix detector. Note rollover at about 60 keV.*

**Figure 4.**

*Energy dispersion in the low-energy region 0–500 keV of the natural background gamma photons at 15 cm depth inside a human body. Based on Pattison et al., Figure 3 and unpublished work using a gamma probe packed with bags of water. Shielding effects on the primary in-air dispersion below 100 keV are uncertain, and the energy dispersion of photons inside the body is very uncertain.*

shown in **Table 2** and delivering enhanced ionisation to that cell or cell nucleus relative to that calculated using the concept of absorbed dose which is based on the assumption that the absorber is effectively water (i.e. oxygen). Further, the biological effectiveness of NBR, its damage to tissues, will be defined by the highest Z atoms in the tissue. This will also be true for other exposures, for X-rays, medical examinations and exposures to anthropogenic sources, indeed the entire range of exposures which are regulated by the law on the basis of the current risk models.

#### **Figure 5.**

*Enhancement of photon energy at different energies on passage through 15 cm water. Internal photon fluence divided by external photon fluence. Unpublished measurements.*

It will be the location in the body of a high-Z atom or particle relative to the target DNA which will be the determinator of biological risk. This is a phantom radioactivity: the atom is radioactive by virtue of its high atomic number and its amplification of NBR gamma radiation through photoelectron emission.
