**5. Internal particles**

The radiobiological issue of photoelectron emission by internal high atomic number particles was raised in 2005 by Busby in connection with depleted uranium weapons which create respirable submicron particles on impact [11]. Research in Iraq, where DU weapons were deployed in 1991 and later in 2003, were shown to have caused high levels of congenital effects and cancer in a number of studies both of civilians in Iraq and of military veterans [12–14]. The concerns about the genotoxicity of DU particles led to research by a number of groups in the early 2000s. The laboratory researches demonstrated that both uranium and uranium particles were capable of causing measurable genetic effects, chromosome breakages and so forth [15–17]. In one study with mice, both embedded uranium and tungsten particles caused local cancer effects [18]. These findings have been reviewed in Busby [19, 20] and will not be rehearsed here. What will be presented here are some results from nanoparticle mathematical modelling studies carried out at the University of Ulster between 2009 and 2012 which looked at photoelectron production from water, gold and uranium spheres [21, 22].

#### **5.1 The University of Ulster studies**

Photoelectron emission from nanoparticles of water, gold and uranium was investigated by Elsaessar, Busby and Howard from 2009 to 2012. Preliminary results were presented at a conference [21], and the studies contributed to a PhD thesis [22]. The CERN FLUKA code was employed. The beam geometry is shown in **Figure 6**, and in **Figure 7** results are given for 10 nm particles of water, gold and uranium. Referring

**183**

**Figure 7.**

**Figure 6.**

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

*Beam and target geometry for FLUKA calculations. A photon beam of cross-sectional diameter equal to that of* 

*a particle of water, gold (Z = 79) and uranium (Z = 92) [21, 22].*

to the numbering in **Figure 7**, which is from the conference presentation [21], the top row of **Figure 2a–c** shows photoelectron tracks induced by an incident photon beam of 150 keV involving 1000 photons in the cases of 10 nm diameter gold and uranium particles, whilst for the water particle, the number of photons was 10,000. Thus, it is clear that the photoelectron tracks of various energies (lengths) induced

*Secondary escaping photoelectron production (seen in two dimensions following incident 100 keV photon beam into 10 nm particles of water (Z = 7.5) [Figure 2a and d], gold (Z = 79) [Figure 2b and e] and uranium (Z = 92) [Figure 2c and f]). Below: corresponding energy deposition. Monte Carlo calculation with 1000 incident photons for gold and uranium but 100,000 for water [21, 22]. Individual figure numbers are from [21].*

*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 6.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

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 amplifica-

*Enhancement of photon energy at different energies on passage through 15 cm water. Internal photon fluence* 

The radiobiological issue of photoelectron emission by internal high atomic number particles was raised in 2005 by Busby in connection with depleted uranium weapons which create respirable submicron particles on impact [11]. Research in Iraq, where DU weapons were deployed in 1991 and later in 2003, were shown to have caused high levels of congenital effects and cancer in a number of studies both of civilians in Iraq and of military veterans [12–14]. The concerns about the genotoxicity of DU particles led to research by a number of groups in the early 2000s. The laboratory researches demonstrated that both uranium and uranium particles were capable of causing measurable genetic effects, chromosome breakages and so forth [15–17]. In one study with mice, both embedded uranium and tungsten particles caused local cancer effects [18]. These findings have been reviewed in Busby [19, 20] and will not be rehearsed here. What will be presented here are some results from nanoparticle mathematical modelling studies carried out at the University of Ulster between 2009 and 2012 which looked at photoelectron production from

Photoelectron emission from nanoparticles of water, gold and uranium was investigated by Elsaessar, Busby and Howard from 2009 to 2012. Preliminary results were presented at a conference [21], and the studies contributed to a PhD thesis [22]. The CERN FLUKA code was employed. The beam geometry is shown in **Figure 6**, and in **Figure 7** results are given for 10 nm particles of water, gold and uranium. Referring

tion of NBR gamma radiation through photoelectron emission.

*divided by external photon fluence. Unpublished measurements.*

**5. Internal particles**

**Figure 5.**

water, gold and uranium spheres [21, 22].

**5.1 The University of Ulster studies**

**182**

*Beam and target geometry for FLUKA calculations. A photon beam of cross-sectional diameter equal to that of a particle of water, gold (Z = 79) and uranium (Z = 92) [21, 22].*

#### **Figure 7.**

*Secondary escaping photoelectron production (seen in two dimensions following incident 100 keV photon beam into 10 nm particles of water (Z = 7.5) [Figure 2a and d], gold (Z = 79) [Figure 2b and e] and uranium (Z = 92) [Figure 2c and f]). Below: corresponding energy deposition. Monte Carlo calculation with 1000 incident photons for gold and uranium but 100,000 for water [21, 22]. Individual figure numbers are from [21].*

to the numbering in **Figure 7**, which is from the conference presentation [21], the top row of **Figure 2a–c** shows photoelectron tracks induced by an incident photon beam of 150 keV involving 1000 photons in the cases of 10 nm diameter gold and uranium particles, whilst for the water particle, the number of photons was 10,000. Thus, it is clear that the photoelectron tracks of various energies (lengths) induced

#### *Use of Gamma Radiation Techniques in Peaceful Applications*

in the particles of the high atomic number elements gold and uranium are orders of magnitude greater than those in water. The emission of secondary photoelectron tracks from the three materials is roughly in agreement with a fourth or fifth power law. **Figure 2d–f** shows the energy deposition in the particles on a coloured scale given also in the picture. It is immediately clear from **Figure 7** how the internal particles of high-Z elements result in increased absorption of background radiation and its reemission by photoelectrons and associated enhanced biological damage relative to the absorption by tissue (water). Due to self-absorption of the induced photoelectrons, the danger exists mainly from smaller particles. Results for different sizes of particles of gold and three different photon energies are shown in **Figure 8**. This shows the variation secondary photoelectron production with photon energy (100 keV, 250 keV, 500 keV and 1 MeV) in a gold target. Photon penetration depth decreases as energy decreases, but the number of electrons escaping the target increases.

To examine the deposition of photoelectron energy into the tissue surrounding the particles examined in the analysis presented in **Figures 7** and **8**, particles were modelled surrounded by water spheres, and the deposition of energy into the

#### **Figure 8.**

*Upper: secondary electrons/primary photons in gold particles of different diameters and photon energies 2, 10 and 100 keV. Lower: electrons per target volume/photons per beam projection for gold particles of different diameters and photon energies of 2, 10 and 100 keV [21, 22].*

**185**

**Figure 9.**

*escaping the target increases [21].*

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

spheres was obtained. In **Figure 9**, results for different photon energies of 100, 250, 500 and 1000 keV are presented. As the photon energy was decreased, the penetration also decreased, as expected, but the photoelectron density in the local volume near the particle increased. This is not unexpected since the photoelectron range

*The variation of secondary photoelectron production with photon energy (100 keV, 250 keV, 500 keV and 1 MeV) in a gold target. Photon penetration depth decreases as energy decreases, but the number of electrons* 

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

would be shorter with the low-energy photoelectrons.

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

spheres was obtained. In **Figure 9**, results for different photon energies of 100, 250, 500 and 1000 keV are presented. As the photon energy was decreased, the penetration also decreased, as expected, but the photoelectron density in the local volume near the particle increased. This is not unexpected since the photoelectron range would be shorter with the low-energy photoelectrons.

#### **Figure 9.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

in the particles of the high atomic number elements gold and uranium are orders of magnitude greater than those in water. The emission of secondary photoelectron tracks from the three materials is roughly in agreement with a fourth or fifth power law. **Figure 2d–f** shows the energy deposition in the particles on a coloured scale given also in the picture. It is immediately clear from **Figure 7** how the internal particles of high-Z elements result in increased absorption of background radiation and its reemission by photoelectrons and associated enhanced biological damage relative to the absorption by tissue (water). Due to self-absorption of the induced photoelectrons, the danger exists mainly from smaller particles. Results for different sizes of particles of gold and three different photon energies are shown in **Figure 8**. This shows the variation secondary photoelectron production with photon energy (100 keV, 250 keV, 500 keV and 1 MeV) in a gold target. Photon penetration depth decreases as energy

To examine the deposition of photoelectron energy into the tissue surrounding the particles examined in the analysis presented in **Figures 7** and **8**, particles were modelled surrounded by water spheres, and the deposition of energy into the

decreases, but the number of electrons escaping the target increases.

**184**

**Figure 8.**

*Upper: secondary electrons/primary photons in gold particles of different diameters and photon energies 2, 10 and 100 keV. Lower: electrons per target volume/photons per beam projection for gold particles of different* 

*diameters and photon energies of 2, 10 and 100 keV [21, 22].*

*The variation of secondary photoelectron production with photon energy (100 keV, 250 keV, 500 keV and 1 MeV) in a gold target. Photon penetration depth decreases as energy decreases, but the number of electrons escaping the target increases [21].*

The Ulster results can be used to obtain enhancement factors for photoelectron production from 10 nm diameter gold and uranium particles relative to a water particle of the same size. This enhancement factor is compared with a fourth-power law comparison in **Table 3** [19].

The range of the photoelectrons increases as the photon energy increases, but the number of photons increases at low energy for natural background radiation as has been discussed above. The trade-off is shown in **Figure 10**. Dose enhancement (energy per unit mass) falls off rapidly with distance from the high-Z particle but is significant in the micron region. Results for a 400 nm uranium particle are given in **Figure 11** [19].

**Figure 11** shows enhancement of dose close to a 400 nm uranium particle embedded in tissue and exposed to natural background radiation. For the method of obtaining this, see [19].

#### **5.2 Other modelling studies of the secondary photoelectron effect**

Because of the use in the battlefield of uranium weapons and the fact that there are other sources of uranium particles (which will be discussed below), there is considerable financial and military investment in showing that these photoelectron effects are not biologically important. The author was a member of the UK Ministry of Defence Depleted Uranium Oversight Board [23] from 2001 to 2005 and also the UK Committee Examining Radiation Risks from Internal Emitters (CERRIE) [24]. He also gave evidence to the Royal Society Committee on Depleted Uranium in 2001. In 2009 a paper describing the secondary photoelectron effect entitled "Phantom Radioactivity of Uranium" was sent by him to the chair of the Royal Society Committee which had published reports on the issue in 2001 and 2002. These reports argued that DU could have no adverse health effects as the absorbed doses from the particles were too low [25]. At the suggestion of the chair, Brian Spratt, the photoelectron paper was submitted to the *Journal of the Royal Society Interface* and sent for peer review. The three reviewers all advised that the idea was important and should be published. Despite this, the editor of the journal, William Bonfield, rejected the paper because of "lack of space". Nevertheless, the idea was next presented in a German conference [9] and was covered by the New Scientist in an article in 2009 [26]. Shortly after this a Monte Carlo study appeared in the same journal that had refused to publish the original idea, the *Journal of the Royal Society Interface*, by Pattison et al. arguing that there was no enhancement of dose by uranium particles [10]. A year later, another Monte Carlo study was published by Eakins et al. of the UK National Radiological Protection Board [27]. Both studies were badly flawed for various reasons which will be briefly summarised.

Pattison et al. carried out Monte Carlo modelling using a different code to that employed by Elsaessar, EGSnrc [10]. They modelled two sizes of cylindrical particles and hollow cylindrical particles of 10 μ diameter and length. Using input photons of 200 keV, they concluded that the enhancement of dose was significant and of the order of one to tenfold. Apart from the fact that the particles they modelled were too large to represent the respirable DU particles found in Iraq, and the input photons too energetic, the key to dismissing their approach was their finding that the dose enhancement was largest for the larger particles, the opposite result to that obtained at Ulster. This was because their method was to fix the spherical volume into which the photoelectrons were emitted and calculate energy per unit mass in the annular water shell. Clearly as the particle diameter approached the water shell diameter, the dose would become infinite, showing that the method was nonsensical, and it is hard to see how the paper passed the reviewers.

**187**

**Figure 11.**

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

**Calculation Water Z = 7.5 Gold Z = 79 Uranium Z = 92** Elsaessar et al. [21] 1 12,900 29,200 Z4 1 12,300 22,600 *Ratio of gold and uranium photoelectron numbers to water photoelectron numbers. Also shown is the Z<sup>4</sup>*

*Number of photoelectrons emitted following exposure of a 10 nm particle of water, gold and uranium to* 

*Percentage of all photoelectrons with energies equal to natural background radiation photons (blue diamonds) and range in tissue in microns (red triangles) (from results presented in Figures 5 and 6). Thirty percent of all* 

*Dose enhancement (energy per unit mass of tissue) by distance in nm from a 400 nm uranium particle.*

 *predicted* 

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

*ratio [19].*

*100 keV photons.*

**Table 3.**

**Figure 10.**

*NBR photons have energy <60 keV.*

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


*Ratio of gold and uranium photoelectron numbers to water photoelectron numbers. Also shown is the Z<sup>4</sup> predicted ratio [19].*

**Table 3.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

law comparison in **Table 3** [19].

**Figure 11** [19].

of obtaining this, see [19].

will be briefly summarised.

The Ulster results can be used to obtain enhancement factors for photoelectron production from 10 nm diameter gold and uranium particles relative to a water particle of the same size. This enhancement factor is compared with a fourth-power

The range of the photoelectrons increases as the photon energy increases, but the number of photons increases at low energy for natural background radiation as has been discussed above. The trade-off is shown in **Figure 10**. Dose enhancement (energy per unit mass) falls off rapidly with distance from the high-Z particle but is significant in the micron region. Results for a 400 nm uranium particle are given in

**Figure 11** shows enhancement of dose close to a 400 nm uranium particle embedded in tissue and exposed to natural background radiation. For the method

Because of the use in the battlefield of uranium weapons and the fact that there are other sources of uranium particles (which will be discussed below), there is considerable financial and military investment in showing that these photoelectron effects are not biologically important. The author was a member of the UK Ministry of Defence Depleted Uranium Oversight Board [23] from 2001 to 2005 and also the UK Committee Examining Radiation Risks from Internal Emitters (CERRIE) [24]. He also gave evidence to the Royal Society Committee on Depleted Uranium in 2001. In 2009 a paper describing the secondary photoelectron effect entitled "Phantom Radioactivity of Uranium" was sent by him to the chair of the Royal Society Committee which had published reports on the issue in 2001 and 2002. These reports argued that DU could have no adverse health effects as the absorbed doses from the particles were too low [25]. At the suggestion of the chair, Brian Spratt, the photoelectron paper was submitted to the *Journal of the Royal Society Interface* and sent for peer review. The three reviewers all advised that the idea was important and should be published. Despite this, the editor of the journal, William Bonfield, rejected the paper because of "lack of space". Nevertheless, the idea was next presented in a German conference [9] and was covered by the New Scientist in an article in 2009 [26]. Shortly after this a Monte Carlo study appeared in the same journal that had refused to publish the original idea, the *Journal of the Royal Society Interface*, by Pattison et al. arguing that there was no enhancement of dose by uranium particles [10]. A year later, another Monte Carlo study was published by Eakins et al. of the UK National Radiological Protection Board [27]. Both studies were badly flawed for various reasons which

Pattison et al. carried out Monte Carlo modelling using a different code to that employed by Elsaessar, EGSnrc [10]. They modelled two sizes of cylindrical particles and hollow cylindrical particles of 10 μ diameter and length. Using input photons of 200 keV, they concluded that the enhancement of dose was significant and of the order of one to tenfold. Apart from the fact that the particles they modelled were too large to represent the respirable DU particles found in Iraq, and the input photons too energetic, the key to dismissing their approach was their finding that the dose enhancement was largest for the larger particles, the opposite result to that obtained at Ulster. This was because their method was to fix the spherical volume into which the photoelectrons were emitted and calculate energy per unit mass in the annular water shell. Clearly as the particle diameter approached the water shell diameter, the dose would become infinite, showing that the method was nonsensi-

cal, and it is hard to see how the paper passed the reviewers.

**5.2 Other modelling studies of the secondary photoelectron effect**

**186**

*Number of photoelectrons emitted following exposure of a 10 nm particle of water, gold and uranium to 100 keV photons.*

#### **Figure 10.**

*Percentage of all photoelectrons with energies equal to natural background radiation photons (blue diamonds) and range in tissue in microns (red triangles) (from results presented in Figures 5 and 6). Thirty percent of all NBR photons have energy <60 keV.*

**Figure 11.**

*Dose enhancement (energy per unit mass of tissue) by distance in nm from a 400 nm uranium particle.*

Eakins et al. study was carried out by employees of the UK National Radiological Protection Board (NRPB) [27]. They used the computer code MCNP5 to model an arrangement consisting of concentric spheres with the particle at the centre and tissue shells surrounding the particle as had the Ulster modelling. However, like Pattison et al., Eakins et al. fixed the volume into which the photoelectron energy was converted into absorbed dose. The authors did, however, model a range of uranium particles, obtaining enhancements of 3-fold at 100 nm diameter and 20-fold for the 2.5 nm diameter particles. Like the Pattison et al. study, this was an absurd analysis since having a fixed volume for the dose absorption but increasing the particle size, the enhancement factor eventually becomes infinite.
