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Many people might fear radiation, but there is no reason to. Everybody experiences small doses. Even exposures during nuclear accidents and clean-up are relatively low. While ionizing radiation has been linked to causing cancer many other substances, some naturally occurring, are more dangerous. Let’s look at some basics.

Ionizing radiation

Radiation is all around us. All light is radiation. When people talk about radiation protection, they really mean “ionizing radiation” protection. Ionizing radiation is any radiation that has high enough energy to ionize (eg. eject an electron from) an atom. Ionizing radiation can include high energy UV rays and X-rays, but in nuclear engineering there are two main categories of radiation.

  • Charged particles
    • Alpha – a helium-4 nucleus stripped of electrons (positively charged)
    • Beta – a high energy electron (or positron) usually emitted from a nuclear reaction
  • Neutral particles
    • Gamma rays (and x-rays) – high energy photons usually emitted from a nuclear reaction
    • Neutron – a free neutron usually emitted from a nuclear reaction

Charged particles interact all atoms, so they do not penetrate very far into solid material (eg. skin). The main risk to humans is only if the charged particles are taken into the body (eg. ingested or inhaled).

Neutral particles, however, can travel much further in even solid material. When they do interact, charged particles are created which then can cause damage internally.

Biological effects

The unit of absorbed dose is the rad. It is defined as 0.01 joules absorbed per kilogram of the absorbing material. It depends on the energy deposited, but also the material. Dose rate is also important; it is measured in rads per unit time.

Ionizing radiation interacts on different length scales, certain radioactive atoms can accumulate in specific organs, and different organs respond differently to radiation. This means that the biological effect of a dose, or dose equivalent (measured in rem), can be as much as 100x the dose (measured in rad) though usually just 1 to 3 times higher. The goal is to estimate the effect of radiation on humans, so the dose equivalent allows different interactions in different areas of the body to be represented as one number. That said, dose equivalent rate (rem per time) is also important in estimating detrimental effects.

Background radiation

We all experience some exposure to background ionizing radiation. Cosmic rays rain down from the skies and the ground emits radiation from uranium and radon. This varies based on where one lives. We even ingest some naturally occurring radioisotopes like carbon-14 and potassium-40.

In addition to natural background radiation, we also experience some man-made background radiation. These include sources like certain medical treatments and nuclear weapons testing.

Here are some data for the average American in 2009.

  • natural background: 310 millirem
  • medical: 300 millirem (this is the average; most is received by those with health issures)
  • nuclear weapons testing: 0.5 millirem
  • total: 624 millirem

It should be pretty clear that natural background and medical treatments are the main sources to which Americans are exposed. Even in the spent fuel pool of a nuclear power plant, radiation doses can be surprisingly safe.

credit: what-if.xkcd.com/29/

credit: what-if.xkcd.com/29/