DetectionandMeasurementofRadioactivity

Radioactive Decay

Isotopes of a given element have nuclei with the same number of protons but different numbers of neutrons. Some isotopes are stable, however radioisotopes are unstable and disintegrate, with the emission of three main types of radiation.

Alpha emitters release a particle composed of 2 neutrons and 2 protons. The atomic number is therefore reduced by 2, and the atomic mass by 4. Alpha particles are so heavy that even with low velocity their momentum is high. They don't travel far, but when they collide with other molecules they do a lot of damage, therefore alpha emitters are considered to be quite hazardous.

When a beta particle emitter decays, one of its extra neutrons is converted to a proton, increasing its atomic number by 1 without changing its atomic mass. The breakdown is accompanied by the emission of a negatively charged particle of low mass, called the beta particle, and an uncharged particle of low mass, called a neutrino. For example, hydrogen consists of just one proton and one electron. Deuterium (2-H), a component of "heavy water," consists of a proton, an electron, and one neutron, and is a stable isotope. Tritium (3-H) is an unstable isotope of hydrogen, consisting of a proton, an electron, and two neutrons. When an atom of tritium decays, one of the neutrons is converted to a proton, one beta particle and one neutrino are released, and a helium isotope (3-He) remains. Tritium is called a "soft" beta emitter, because its beta particles have relatively low velocities. A hard beta emitter such as 32-P (phosphorous) is more dangerous because its beta particles carry more kinetic energy (however it is easier to detect - read on).

Gamma rays consist of electromagnetic radiation resembling X-rays. An example of a gamma emitter is 131-I (iodine). Gamma radiation may accompany either alpha or beta particle emission.

A traditional unit of radioactivity is the Curie (Ci), which is defined as that quantity of any radioisotope undergoing 2.22 x 10^12 atomic disintegrations per minute (DPM). A milliCurie (mCi) of a radioisotope undergoes 2.22 x 109 DPM, and a microCurie produces 2.22 x 10^6 DPM. Since 1975, the becquerel (Bq) has replaced the Curie as the preferred international unit of radioactivity. One Bq is defined as one atomic disintegration per second, or 2.703 x 10^-11 dpm. Working amounts in a laboratory might be described in microCuries or milliCuries, kiloBecqerels or megaBecquerels.

When the product of an atomic disintegration is a stable isotope, atomic decay leaves less radioactive material behind. Therefore, as time passes, the amount of activity declines logarithmically. The half-life of a radioisotope is the time it takes for one-half of the unstable atoms to disintegrate. Each radioisotope has a characteristic rate of decay and pattern of radiation. For example, 14-C is a low energy beta emitter with a half life of 5500 years.

A radioisotope, or any compound that contains a radioisotope, is said to be radiolabeled and is called a radionuclide. The specific activity of a radionuclide is the amount of radioactivity, Bq or Ci, per mole of the compound. Clearly, as radioactive decay proceeds, the specific activity of any radionuclide declines.

Methods of detection

The method employed to detect radiation depends on the type of emitter and the intended purpose of detection. The most well known method of detecting radiation is with an ionization chamber. A high energy particle can dislodge electrons from the atoms it strikes, producing pairs of ions. Particles are allowed to pass between parallel plates, one with a positive charge and one with a negative charge. As ionization takes place the ions each move to the plate with the opposite charge, producing a current. The current is read on a meter. The Geiger-Mueller counter is based on the ionization detection principle.

Photographic film can be exposed by all types of radiation, and is used to monitor exposure of personnel working with high energy emitters. A visible track in a cloud or bubble chamber can pick up radioactivity, as can a calorimeter if the energy emitted is quite high.

A problem with many methods of detection is that the energy of the emitter must be high enough to travel some distance through air. The particles emitted by many radionuclides, especially 14-C and 3-H labeled compounds, do not travel a significant distance in air, but pose a danger if internalized because of their proximity to molecules such as DNA. For example, a beta particle emitted by tritium cannot penetrate a sheet of paper, yet tritium in the body fluids can pose a significant hazard. A liquid scintillation detector can pick up radiation from "soft" beta emitters as well as from other radioisotopes. An ionizing particle is allowed to pass through a crystal or liquid phosphor, which absorbs its energy and re-emits the energy as flashes of light. Usually the emitter must be dissolved in liquid containing the luminescent compound, so that the distance traveled by the particle is very short.