Reply #142: #5. Technetium is a pure beta emitter with no decay chain.

[NNadir]

Not all radioactivity is created equal. Some radioactivity, alpha particles - which are helium nuclei traveling at relativistic speeds - can be stopped by a sheet of paper. Some very energetic gamma rays on the other hand, can pass through thick layers of concrete. Beta particles, the third general class of radiations available to neutron rich nuclei are somewhat intermediate. In general, though, they can be stopped rather easily, by light shielding or by a few centimeters of air. The shielding actually required in most cases depends on the characteristic energy of the particle or electromagnetic radiation (gamma, x-ray, or hard UV).

Not all radioactive nuclei decay with only one kind of emission. Many alpha emitters, for instance, also release considerable amounts of gamma radiation. For instance, Americium-241, the radionuclide contained in nearly every smoke detector on the planet, is an alpha emitter, but it actually works by ionizing smoke particles with its high energy gamma radiation that is released simultaneously with the alpha particle. In this case, the characteristic energy of decay is the sum of the alpha particles kinetic energy and the electromagnetic energy of the gamma rays emitted.

The same situation holds true for beta emitters. Some beta emitters, like Cesium-137 that I have discussed in posts above, emit powerful gamma rays and can be very dangerous without extensive shielding. However some nuclei, like strontium-90, its daughter yttrium-90 and technetium are pure beta emitters. None of the radiation is carried off in the form of electromagnetic radiation like gamma or x-rays. All of it is contained in the kinetic energy of the beta particle (an electron) that is emitted from the nucleus undergoing decay. This means that in general, the use of these materials requires very little shielding.

Basically this means that a person standing next to a few tons of technetium would face very little risk if he or she merely stood a meter away. In order for technetium to represent a real danger one needs to be in direct contact with it. In the case of technetium, even eating it would probably have little effect. This is because, as described above, technetium has very little tendency to remain in the human body. It is easily excreted, as many imaging experiments and imaging diagnostic tests have shown.

Many nuclei decay into other radioactive nuclei, creating a series of decays called a decay chain. The number of decays in the uranium decay chain is 14 for instance. Since uranium has long since the formation of the earth had sufficient time to reach radioactive equilibrium in its decay chain, the amount of activity attributed to uranium in its ores (as opposed to in isolated uranium) must actually be multiplied by 14. (This is one factor that accounts for the reason that it is actually possible to reduce the radioactivity of the earth via the use of nuclear power.) Technetium isotopes formed in nuclear reactors (almost 100% Tc-99) however do not have decay chains. Technetium-99 decays directly into stable non-radioactive ruthenium-99. If technetium-99 captures a neutron in a neutron flux, it becomes technetium-100 which decays with a 17 second half-life into stable ruthenium-100.