Beta Radiation

Beta particles are moderately charged, light and fast moving (electrons or positrons). Though the energy released during a specific radioactive decay process that results in beta emission is discrete (one energy), the betas actually emitted have a wide spectrum of energies (are polyenergetic), since each beta emission is accompanied by the emission of an anti-neutrino/neutrino (not of concern here, since their mass is very small and since they do not result in a significant risk) that shares the total energy released. Unless otherwise stated, the beta energy given in reference literature is the maximum or end-point energy. The average beta energy is about one-third of the indicated maximum beta energy.

Some isotopes are pure beta emitters, which means they emit only betas. Some examples of pure beta emitters are ³²P, ³H, ¹⁴C, ³⁵S, and ⁴⁵Ca. Table 1 gives some of the physical properties of these five commonly used pure beta emitters. Other beta emitters may release beta radiation accompanied by other types of radiation (see Appendix I).

Some beta emitters require special considerations relative to handling and shielding that are more involved than the straightforward methods used when shielding only gamma emitters.

Beta radiation will penetrate varying thicknesses of matter depending on the material and the energy of the beta radiation. The higher the energy of the beta particle, the greater the penetration through air or other materials. Generally, beta emitters whose energies are less than 200 keV, such as ³H (tritium), ³⁵S, and ¹⁴C, have limited ranges in tissue and air, so are not considered to result in an external radiation hazard. An exception is the case of skin contamination (examples of typical exposures are given on the bottom line of Table 1) where it can be seen that the dose rate to the basal cells of the skin ranges from 1.4-9.2 rad/hr, for skin contamination with 1 Ci/cm² for isotopes other than tritium.

Higher energy betas can result in an external radiation hazard since they can travel long distances in air and penetrate considerable distances in tissue. A beta particle will travel approximately four meters per MeV in air. Therefore, high-energy beta particles, such as those from ³²P and ⁹⁰Sr, can travel a long distance in air.

All beta emitters can cause internal exposure, so precautions must be taken to prevent the internal uptake of radioisotopes (orally, via air or wounds, or by transport directly through the skin).

In addition to producing exposure directly by ionization, all charged particles, including beta particles, lose energy in an absorbing material by excitation and radiation. Radiative energy losses of charged particles are called bremsstrahlung, which in German means "braking radiation". This process occurs when a charged particle slows down as it passes through an absorber and in the process creates an x-ray (or bremsstrahlung radiation). This radiation is more penetrating than the original beta radiation since it is a photon (x-ray). The fraction of the total beta energy that contributes to the production of bremsstrahlung radiation is proportional to both the atomic number of the absorber and the energy of the beta or other charged particle. To reduce the production of bremsstrahlung radiation, high-energy particle emitters (e.g., betas) must be shielded with a material that has a low-atomic number, (e.g., lucite or plastic), about l cm thick for ³²P. If excessive bremsstrahlung radiation still passes through the low atomic-number shield, lead must be added to attenuate this radiation. The lead must be placed on the side of the plastic shield that is away from the source, so as to reduce the bremsstrahlung radiation hazard due to the lead. The amount of lead usually required to attenuate the bremsstrahlung resulting from 32P is about 0.3 mm. Bremsstrahlung radiation produced as a result of 32P betas is characterized in Table 2.

Note 1: From Healy, 1971. The dose is from beta particles emitted in all directions equally from contamination on the surface of the skin. Basal cells are considered to be 0.007 cm below the surface.

Since beta emission can be either electrons (e-) or positrons (e+), we must consider the special radiation safety problems associated with positron emitters. After a positron enters an absorber, it slows down like a regular electron, but eventually interacts with an orbital electron in the absorbing material. When this interaction occurs, the orbital electron and positron annihilate one another to produce two gamma rays, each with an energy of .511 MeV. This annihilation radiation is more penetrating than the original positron radiation but can be attenuated by using lead shielding in the same way as for the bremsstrahlung radiation discussed above.

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