Introduction. Controversy over the health effects of the depleted uranium munitions used during the first Persian Gulf war in 1991 and subsequent wars.
The Enhancement of Natural Background Radiation Dose around Uranium Micro-particles Swansea University
University Hospital Birmingham
John E Pattison1, Richard P Hugtenburg2, and Stuart Green3,4
Birmingham University
1 School of EIE – Applied Physics, University of South Australia, Adelaide, Australia. 2 School of Medicine, University of Swansea, Swansea, UK. 3 Department of Medical Physics, University Hospital Birmingham NHS Trust, UK. 4 School of Physics & Astronomy, University of Birmingham, Birmingham, UK.
Introduction • Controversy over the health effects of the depleted uranium munitions used during the first Persian Gulf war in 1991 and subsequent wars. • The combustion products of depleted uranium have been suggested as one cause, amongst others, for the “Gulf War Syndrome”. • Many reviews held to determine the health effects of depleted uranium, including the UK Royal Society, the European Commission, the IAEA and WHO, which concluded that it does not present as serious a radiological hazard as some have asserted. The chemical toxicity of uranium was acknowledged. • Attention then focussed on the dose enhancement that uranium particles in the body would produce upon exposure to naturally occurring background gamma radiation. • The main proponent of this view is Dr Chris Busby, a former member of the UK Ministry of Defence Depleted Uranium Oversight Committee (2001-2006)(Busby 2005). • He estimated, based on rough calculations, that the enhancement would be a factor of 500 to 1000, and that this enhanced dose of the background gamma radiation would contribute a significant radiation dose in addition to the dose received from the radioactivity of the depleted uranium. • The aim of this project was to obtain more accurate estimates of the dose enhancement in tissue immediately surrounding micron-sized particles of uranium upon exposure to naturally occurring background gamma radiation using the Monte Carlo code EGSnrc. • EGSnrc is an extended and improved version of EGS4.
Corner
Convex surface
Figure 4 Typical Animal Cell (~10 µm diameter)
Figure 5 Dose Scoring Regions around the Uranium Particles, 1 µm and 5 µm thicknesses.
shoulders
x
x xx
• All of the results are shown in Table 1, and the highest values are shown in Figure 10.
Table 1
10 cm
mid-height
• The above calculations were then repeated with the uranium replaced by tissue.
Results
7 cm
60cm
• The radiation dose was calculated using EGSnrc for each of the three (four) regions (Figure 5) around each of the three uranium particles (Figure 3), for particles at each of the three positions (x in Figure 6) within the body.
• The enhancement factors were then calculated for each situation: Dose Enhancement Factor = Dose with Uranium/Dose without Uranium.
Particle axis
Isotropic Natural Background Gamma Radiation
hips
16 cm
Body axis
Figure 6 Body (Torso) Model Homogenous cylindrical torso, made from ICRU 4-element tissue. Compare this body model with the one used by Pattison et al (2001, 2004).
Counts/channels/1000s)
1000 K-40 1460 keV
100
Th-232 2600 keV
10
500
1000 1500 2000 Photon Energy (keV)
2500
3000
Figure 7 Measured Natural Background Gamma Pulse Height Spectra
Natural background G-rays Uranium particle
10000
e's
Region of enhanced local dose
Natural background G-rays
Figure 2 Photoelectric effect Water Tissue Bone Iodine Gadolinium
Z=7.5 Z=12 Z=53 Z=64
• The average measured natural background gamma spectrum, shown as a black curve in Figure 7, was corrected for: • detection efficiency, and • effective cross-sectional area, of the NaI detector, and then unfolded to give the spectrum incident on the outside of the detector. • The corrected spectrum, shown as the black curve in Figure 8, was then transported through the body using EGSnrc to obtain the photon fluence spectra at three points within the body. • The electron fluence spectra, at the same points within the body, are shown in Figure 9. Photon Fluence Rate (/cm2/channels/1000s)
At low photon energies, g-rays are absorbed in high atomic number materials by the Photoelectric Effect; where the absorption is proportional to fourth power of atomic number˙
Platinum Z=78 Gold Z=79 Uranium Z=92
Incident Isotropic NBG Gamma Radiation Fluence Rate Spectrum and Photon Fluence Rate Spectra Tissue Body
2 µm 2 µm
4 µm
10 µm
4 µm
Small solid particle (~3.0 µBq)
Large solid particle (~370 µBq)
Large particle with cavity (~340 µBq)
Figure 3 Model Particles Studied Homogeneous cylindrical particles, made from pure uranium for worst case.
Electron fluence Rate (/cm2/channels/1000s)
10 µm
Convex Surface
Corner Surface
1.6 1.2
1.5 1.1
1.2 1.0
2.2 1.4
2.0 1.2
1.4 1.1
2.6 1.9
2.5 1.7
1.7 1.3
4.1 2.7
3.8 2.3
2.3 1.6
Small Solid Particle (2μm) Just inside body 1μm layer 5μm layer On body axis. 1μm layer 5μm layer
Large Solid Particle (10μm) Just inside body 1μm layer 5μm layer On body axis 1μm layer 5μm layer
Large Particle (10μm) with 4μm Cavity Outside Particle Just inside body 1μm layer 2.8 2.5 5μm layer 2.0 1.7 On body axis 1μm layer 4.4 3.9 5μm layer 2.8 2.3 Inside Cavity Just inside body 1μm layer 9.9 9.8 Whole cavity 10.1 On body axis 1μm layer 16.5 16.4 16.7 Whole cavity
1000
100
10
1 0
500
1000 1500 2000 Photon Energy (keV)
2500
3000
•
0.01 0.001
•
1000 1500 2000 Photon Energy (keV)
Figure 9 Internal Electron Spectra
3.8
4.1
2.3
4.4
3.9
17
17.2
For worst case scenario for micron-sized Uranium particles: • The enhancement factors are considerably less than 500 – 1000. • Enhancement factors are largest (and relatively uniform) within cavities, with values between 10 and 17 depending upon position of particle within the body. • The enhancement factors are largest deep inside the body compared with near the body surface. • The enhancement factors increase with the size of the particle from 2 to 10 µm. • For a given particle size and position, the enhancement factors outside of the particle decrease going from flat to convex to (double convex) corner.
•
500
2.3
9.8
Figure 10 Largest Enhancement Factors, 1 µm scoring regions around Uranium particles on body axis
• Just inside body Halfway inside body On axis inside body
0.1
0.0001 0
2.2
2.3 1.6
References
Electron Fluence Rate Spectra in Tissue Body Exposed to Isotropic NBG Gamma Radiation
1
1.4 2.0
1.7 1.3
Conclusions
Incident photon spectrum Just inside body Halfway inside body On axis inside body
Figure 8 Internal Photon Spectra
Materials and Method
Flat Surface
75mm x 75mm cylindrical NAL(TI), minimum shielding, 256 (12 keV) channels, 1000s count time
10000
1 0
• Not just Uranium Oxide, but may also contains other materials • Oxidation may not be complete • Typical sizes range 1 -10 µm
10 cm 3 cm
Figure 1 Shapes and Sizes of Uranium Oxide Particles • SEM micrographs from Cho et al (2002) & Milodowski (2001)
Flat surface
Nature of Uranium • Naturally occurring uranium, 99.3% U-238 (4,500 Myrs), 0.7% U235 (710 Myrs), 0.006% U-234 (0.25 Myrs) Weakly radioactive • Depleted uranium, 99.8% U-238 (4,500 Myrs), 0.2% U-235 (710 Myrs), 0.001% U-234 (0.25 Myrs) Its activity is only ~ 60% that of natural uranium • Atomic number, Z = 92 • Density, r ≈ 19,000 kg/m3 • Chemically toxic as a heavy metal • Pyrophoric; in fine powder form spontaneously combusts producing various oxides, but mainly U3O8
Dose and Enhancement Calculations
2500
3000
•
Busby, C. 2005 Depleted uranium weapons, metal particles, and radiation dose. Euro. J. Biol. Bioelectromag., 1, 82-93. Cho, W. D., Han, M-H., Bronson, M. C. & Zundelevich, Y. 2002 Processing of uranium oxide powders in a fluidized-bed reactor. I. Experimental. J. Nucl. Mat., 305, 106-111. Milodowski, A. E. 2001 A trial investigation by scanning electron microscopy of uranium particulate in building debris associated with a depleted uranium munitions strike site during the Kosovo conflict. Commercial Report CR/01/145. Keyworth: British Geological Survey. Pattison, J. E., Hugtenburg, R. P., Charles, M. W. & Beddoe, A. H. 2001 Experimental simulation of A-bomb gamma-ray spectra for radiobiology studies. Radiat. Prot. Dosim., 95, 125-136. Pattison, J. E., Paine, L. C., Hugtenburg, R. P., Charles, M. W. & Beddoe, A. H. 2004 Experimental simulation of A-bomb gamma-radiation: Revisited. Radiat. Prot. Dosim., 109, 175-180.
