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.
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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).
http://www.llrc.org/du/duframes.htm (click on the link at the top of the list "Secondary Photoelectron effect)