This is the basis for the controversy and concerns on the subject of nuclear power generation, waste disposal and nuclear weapons. The particles and photons that result from nuclear decay carry most of the energy released from the original unstable nucleus. The value of this energy is expressed in electron Volts, or eV. The energy of beta and alpha rays is invested in the particles' speed. A typical beta particle from Cesium-137 has an energy of about 500,000 eV, and a speed that approaches that of light. Beta energies can cover a wide range, and many radioisotopes are known to emit betas at energies in excess of 10 million eV. The penetration range of typical beta particles is only a few millimeters in human skin.
Alpha particles have even shorter penetration ranges than beta particles.
Typical alpha energies are on the order of 5 million eV, with ranges so short that they are extremely difficult to measure. Alphas are stopped by a thin sheet of paper, and in air only travel a few inches at most before coming to a stop. Therefore, alpha particles cannot be detected without being in close contact with the source, and even then only the alphas coming from the surface of the source can be detected. Alphas generated within the source are absorbed before reaching the surface. Due to short range, alpha particles are not a serious health hazard unless they are emitted from within the body when their high energy, in close contact with sensitive living tissue, is an extreme hazard.