How Does Pb Work as Radiation Shielding?

Shields against ionizing radiation reduce the radiation intensity and serve to protect people (see radiation protection), other living beings, objects, or components against radiation damage and reduce the background during radiation measurements.

Shields against ionizing radiation reduce the radiation intensity and serve to protect people (see radiation protection), other living beings, objects, or components against radiation damage and reduce the background during radiation measurements.

Charged Particles

Charged particles, for example, alpha or beta radiation, each have a specific range R in the matter, which depends on the type of particle, particle energy, and material. A shield that is thicker than R completely suppresses the incident radiation. The range in the air from atmospheric pressure is z. B. for electrons with the energy of 10 MeV about 3 m, for alpha radiation of the same energy only 10 cm.

The specific range is explained by the fact that the particles give their energy to the absorber in many small steps-by exciting outer shell electrons. To a lesser extent, it knocks out internal electrons from the atom (see also linear energy transfer, LET).

Elastic scattering processes on the electron shell and the Coulom scattering on atomic nuclei contribute somewhat to absorption by lengthening the path of the charged particles in the absorber.

For non-relativistic particles of mass m and charge z, the linear energy transfer D line is inversely proportional to the energy E. It depends on the density ρ and the ratio of the atomic number to the mass number Z/A of the material:

• D inel/ρ ~ (Z/A) * z 2 * m/E

The inverse dependence on the (continuously decreasing along the way) energy is reflected in the Bragg curve. Your abrupt drop (Bragg peak) defines the range R. For alpha particles with emission energy E 0 in air. It corresponds to Geiger’s empirical non-relativistic range law:

• R= 3.1 * E 0 (3/2) (E 0 in MeV, R in mm)

At high energies and large absorber, atomic numbers also provide a significant contribution to energy dissipation D:

• D radiation/ρ ~ (z * e/m) 2 * E * Z* (Z/A)

For electrons, this range starts at around 5 MeV, for protons only in the TeV range is:

• D radiation/D inel ~ E * Z/800 (for electrons, Ein MeV)

X-rays and Gamma Rays

High-energy electromagnetic radiation such as X-rays or gamma radiation is attenuated exponentially in the matter. It, therefore, has no specific range, but its reduction can be described by a half-value thickness that depends on the quantum energy and material. This is z. B. for 2 MeV quanta in the lead at 1.3 cm.

The attenuation described by the mass attenuation coefficient is essentially based on three different interaction processes, the relative contributions of which depend on the quantum energy and the atomic number (atomic number) Z and mass number A of the shielding material. Two other mechanisms, nuclear photo effect, and normal elastic scattering are generally negligible for the shielding effect.

The single most crucial process is

• at energies up to approx. 1 MeV the photoelectric effect; Attenuation ~ Z 4/A
• at energies between approx. 1 MeV and 5 MeV the Compton scattering; Attenuation ~ Z/A
• pair formation at energies above approx. 5 MeV; Attenuation ~ Z 2/A

At low and high energies, heavy elements, i.e., elements with a high atomic number, absorb particularly well. This is why most gamma and X-ray shields are made of lead. Glass with a high lead shielding content is used for viewing windows. In the middle energy range, where Compton scattering is significant, the properties of the shielding materials differ only slightly.

Low-neutron light nuclides are somewhat more effective with the same mass occupancy but are not very common in practice because of the greater layer thickness required.

When the radiation is weakened, electrons are released as secondary particles, and, at very high energies, protons and neutrons are released due to the nuclear photo effect. In turn, they may be new sources of gamma radiation and tertiary particles. To avoid spallation reactions with heavy elements, shields against high-energy radiation consist of a combination of heavy and light materials.