Enhancement of natural background gamma-radiation dose around uranium microparticles in the human body

Abstract

Ongoing controversy surrounds the adverse health effects of the use of depleted uranium (DU) munitions. The biological effects of gamma-radiation arise from the direct or indirect interaction between secondary electrons and the DNA of living cells. The probability of the absorption of X-rays and gamma-rays with energies below about 200 keV by particles of high atomic number is proportional to the third to fourth power of the atomic number. In such a case, the more heavily ionizing low-energy recoil electrons are preferentially produced; these cause dose enhancement in the immediate vicinity of the particles. It has been claimed that upon exposure to naturally occurring background gamma-radiation, particles of DU in the human body would produce dose enhancement by a factor of 500-1000, thereby contributing a significant radiation dose in addition to the dose received from the inherent radioactivity of the DU. In this study, we used the Monte Carlo code EGSnrc to accurately estimate the likely maximum dose enhancement arising from the presence of micrometre-sized uranium particles in the body. We found that although the dose enhancement is significant, of the order of 1-10, it is considerably smaller than that suggested previously.

Figure 1.

Photon interaction cross sections for uranium as a function of photon energy. Note that the total attenuation below a photon energy of about 200 keV is due nearly entirely to the photoelectric effect. As the photon energy increases above 200 keV, Compton scattering increases until it is the dominant interaction mechanism above about 700 keV. Solid line, PE absorption; dashed line, Compton scatter; dash-dotted line, pair production; dotted line, total attenuation. Adapted from Berger & Hubbell (1987).

Figure 2.

Measured pulse-height spectra using 76 mm × 76 mm cylindrical Na(Tl) crystal with minimum shielding, 256 (12 keV) channels, and 1000 s counting time for five different locations in North America and Europe shown as faint dashed curves. The weighted-average curve is shown as a bold continuous curve.

Figure 3.

Incident isotropic free-in-air gamma-radiation fluence-rate spectrum, and internal photon fluence-rate spectra. Solid line, incident outside body; long-dashed line, just inside body; small-dashed line, half-way inside body; dashed-dotted line, on axis inside body.

Figure 4.

Body (torso) model. The positions on the midplane where the enhancement factors are calculated are shown by ‘X’.

Figure 5.

Particle models. (a) Small solid particle. (b) Large particle with closed cavity. (c) Large particle with open cavity. These particles are made of uranium and are surrounded by, and the cavities filled with, ICRU four-element tissue. Particle (c) has the same outer dimensions as particle (b). The closed cavity in particle (b) is centrally located within that particle. The open cavity in particle (c) opens at the bottom of the particle shown.

Figure 6.

Radial dose-rate distribution across the midplane and half-way planes of the model body due to natural background gamma-radiation. Solid line, midplane; dashed line, average of two half-way planes.

Figure 7.

Internal electron fluence-rate spectra. Long-dashed line, just inside body; small-dashed line, half-way inside body; dashed-dotted line, on axis inside body.

Figure 8.

Values of the estimated enhancement factors around uranium particles for the 5 µm dose-scoring region. (a) Particles just below the body surface. (b) Particles mid-way between the body surface and the body axis. (c) Particles on the body axis. The 1 µm size particle is shown on the left, the 10 µm size particle with closed cavity is shown in the centre and the 10 µm size particle with open cavity is shown on the right. The smaller of the two values shown in the open cavity is for the entrance to the open cavity.