The Secondary Photoelectron effect

The Secondary Photoelectron effect

Releases of Uranium giving rise to its incorporation in body tissue appear to be genotoxic despite Uranium's low radioactivity. For example, a wide-ranging review of the teratogenicity of parental prenatal exposure to DU aerosols has concluded that "the evidence, albeit imperfect, indicates a high probability of substantial risk". (1) This represents an extreme anomaly between actual risks and those expected on the basis of ICRP recommendations. It appears improbable that the reported effects depend on the intrinsic radioactivity of Uranium. The hazard is more likely to be mediated by a mechanism known as the Secondary Photoelectron effect (SPE) in combination with the affinity between atomic Uranium and the DNA molecule. Particulates are also likely agents of harm, with implications for the deployment of weapons containing Uranium. In principle, the Secondary Photoelectron effect may provide a mechanism to explain the observed toxicity of heavy metals.
Quantifying the discrepancy between ICRP and a new model that takes account of the Secondary Photoelectron effect
The absorption of gamma rays by any element is proportional to at least the fourth power of the element's atomic number Z. ICRP, in considering gamma ray absorption, models the human body as water, H2O. It has been proposed
(2) that the baseline of absorption in uncontaminated tissue should be established using Oxygen - the most massive of the atoms in the water molecules in the ICRP phantom. The atomic number of Oxygen is 8. 84 = 4096. The atomic number of Uranium is 92. 924 = 71639296. 71639296/4096 = 17490. This is the enhanced ability of an atom of Uranium to absorb incident gamma or X-rays, relative to an atom of oxygen. Energy absorbed in this way is re-emitted in the form of photoelectrons indistinguishable from beta radiation, potentially causing tissue damage.

The enhancement of external radiation by high atomic number materials was described as early as 1947 when Spiers calculated the enhancement of X-rays in bones, showing a ten-fold increase in radiation damage at the edge of bones due to photoelectrons induced in Calcium (Z=20). Others had tried to use Iodine (Z=53) to enhance X-ray therapy for brain tumours. Experiments in the USA on the photoelectron enhancement of X-rays by gold nanoparticles (Z=79) have been shown to cure breast cancer in mice. (3)
Uranium binds strongly to DNA. This is well known and has been described in the peer review literature since 1962.
(4) The affinity constant for UO2++ and DNA is 1010 This means that at very low concentrations of Uranium, the DNA is fairly well saturated with it. The reason for the affinity is that the ion UO2++, the uranyl ion, follows Calcium in its chemical properties in the body. Calcium is the element which stabilises the DNA through neutralising the negative charges on the phosphate backbone.

The quantity of DNA in a cell is about 7 picograms. The cell has a mass of 270 picograms, assuming an 8 micron diameter cell. So the DNA represents roughly 1/40th by mass on the basis of these BEIRV figures. (5) It is thus shown that at quite modest levels of Uranium in tissue, it is the Uranium that is the predominant absorbing material for natural background gamma radiation, and that the absorbed energy is converted into photoelectrons which attack the DNA - the principal target for radiation effects - both directly and indirectly though ionization of water. This argument is simple and immediate. The base line is that Uranium health effects are not mainly due to its intrinsic radioactivity, but to its high atomic number. Counter-intuitively, it is low energy incident radiation and the smallest particles that represent the greatest divergence from expectations based on LNT. (6) The photoelectron idea was presented by Busby at the CERRIE international workshop at St Catherine's College in 2003. (7) The UK Committee on Radioactive Waste Management commissioned work on the relevance of SPE on public exposure to Uranium. (8) , (9) and the argument outlined above was formally presented to the MoD Depleted Uranium Oversight Board in 2004. (10) Papers have been published. (11) , (12) , (13).

From 2008 the UK Health Protection Agency has engaged with LLRC in a limited dialogue on SPE. HPA has used inappropriate methods and has obstructed the dialogue. (14) A paper by Pattison et al. (15) has been criticised for inappropriate criteria on particulate Uranium, for inappropriate methodology, and for failing to address those aspects of the SPE hypothesis which involve atomic Uranium. (16)
The scientific issues remain unresolved.

1 Hindin R, Brugge D, Panikkar B. "Teratogenicity of depleted uranium aerosols: a review from an epidemiological perspective." Environmental Health: A Global Access Science Source 2005 Aug 26;4:17 doi:10.1186/1476-069X-4-17 (link)

2 pers. comm. the late Dr. Philip Day, 12th January 2009.

3 Hainfeld, J.F., Slatkin, D.N. & Smilowitz, H.M. 2004. The use of gold nanoparticles to enhance radiotherapy in mice. Phys. Med. Biol. 49: N309-N315

4 Nielsen, P.E, Hiort, C., Soennischsen, S.O., Buchardt, O., Dahl, O. & Norden, B. 1992. DNA binding and photocleavage by Uranyl VI salts. J. Am. Chem. Soc. 114: 4967-4975.
also Huxley, H.E. & Zubay, G. 1961. Preferential staining of nucleic acid containing structures for electron microscopy. Biophys. Biochem. Cytol. 11: 273.
also Constantinescu, D.G. 1974. Metachromasia through uranyl ions: a procedure for identifying the nucleic acids and nucleotides. Anal. Biochem. 62: 584-587.

5 1990 recommendations of the US National Academy of Sciences BEIRV committee.

6 C. V. Howard, A. Elsaesser & C. Busby (2009) The biological implications of radiation induced photoelectron production, as a function of particle size and composition. International Conference; Royal Society for Chemistry NanoParticles 2009 (link)

7 CERRIE MINORITY REPORT; Minority Report of the UK Department of Health / Department of Environment (DEFRA) Committee Examining Radiation Risks of Internal Emitters (CERRIE); Sosiumi Press Aberystwyth. ISBN 0-9543081-1-5

8 CoRWM Document 1633 link

9 CoRWM Document 2019 revised Jan 2007 link

10 Final Report of the Depleted Uranium Oversight Board submitted to the Under-Secretary of State for Defence - February 2007.

11 Busby C (2005) "Depleted Uranium weapons, Metal Particles, and Radiation Dose"; European Journal of Biology and Bioelectromagnetics Vol 1 No 1 p 82-93

12 Busby C (2005a) "Does Uranium Contamination amplify natural background radiation dose to DNA? European Journal of Biology and Bioelectromagnetics Vol 1 No 2 p

13 Busby CC and Schnug E (2007) ĎAdvanced Biochemical and Biophysical Aspects of Uranium Contaminationí in Loads and fate of Fertiliser Derived Uranium ed. LJ de Kok and E. Schnug, Backhuys Publishers, Leiden

14 "Health Protection Agency and the Secondary Photoelectron effect: Bad faith and bad science."

15 Enhancement of natural background gamma-radiation dose around uranium micro-particles in the human body John E. Pattison, Richard P. Hugtenburg and Stuart Green September 23, 2009, doi: 10.1098/rsif.2009.0300.

16 "Uranium and the Secondary Electron effect: appraisal of a paper by Pattison and others"

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