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Ionizing radiation, which can form ions when it strikes a molecule, can cause biological effects that are frequently harmful to organisms. Instruments to detect ionizing radiation include Geiger counters, scintillation counters, and dosimeters.

Radioisotopes, isotopes that can undergo radioactive decay at relatively fast rates, are found throughout the natural environment. Organisms are exposed to different kinds of radiation throughout their lives.

Matter can block ionizing radiation, and different types of ionizing radiation can move different distances in matter before they are blocked. Alpha particles have the least penetrating power—they can be blocked by a sheet of paper. This is because $\rm\alpha$ particles have very large mass; they are helium nuclei. Alpha particles are mostly blocked by skin but are still dangerous if ingested or inhaled.

Beta particles have slightly more penetrating power. They have much less mass than alpha particles, making it easier for them to pass between molecules, such as those in a sheet of paper. About 5 mm of acrylic plastic or a thin sheet of aluminum, however, can stop a $\rm\beta$ particle.

Gamma rays have the least mass—none at all. This makes it possible for them to pass through materials that can stop alpha and beta particles. About 5 cm of lead can stop gamma rays. Note that these figures represent the amount required to stop a single particle or photon. In practice, the amount of material required to block radiation depends largely on the quantity of radioisotope present or on the amount of radiation emitted.

#### Penetrating Power of Different Types of Radiation

The biological effects of ionizing radiation can be broken down into categories in different ways. One division is whether the effects are deterministic or stochastic. Deterministic effects depend on the amount of exposure, or dose, and do not occur below a particular threshold. Stochastic effects do not depend on the dose, and no threshold exists. Instead, the chance of the effect happening depends on the dose. For example, cataracts of the eye are a deterministic effect. Below 5.0 sieverts (Sv) of exposure, cataracts will not form. Radiation-induced cancer is an example of a stochastic effect. The likelihood of developing cancer increases with increased dose.

Another division between types of effects of radiation is whether the effects are acute or latent. Acute effects happen immediately and go away over time if exposure stops. Latent effects are defined as those that appear six months or more following exposure. Acute effects are called radiation sickness and include symptoms such as nausea and vomiting and reduced red blood cells. The severity of acute effects depends on the dose. Latent effects are further divided into somatic effects and genetic effects. Somatic effects occur in the individual who received the exposure, while genetic effects occur in the descendants of that individual.

### Biological Effects of Ionizing Radiation

Characteristics of Effects Timing of Effects Location of Effects Examples of Effects
Deterministic effects Acute effects Somatic effects Skin damage
Infertility
Damage to blood-forming organs
Damage to digestive system
Damage to central nervous system
Latent effects Cataracts
Damage to immune system
Stochastic effects Cancer
Genetic effects Hereditary effects

Effects of ionizing radiation on humans can be deterministic (occurring only once an exposure threshold has been exceeded) or stochastic (occurring without a threshold). The timing can be acute (occurring soon after exposure) or latent (delayed; possibly occurring years later). Genetic effects are passed on to offspring, but somatic effects occur only in an individual.

It is impossible to completely protect against radiation exposure. However, limiting exposure to levels below which no harmful deterministic effects occur greatly reduces an individual's risk of developing any type of effect. In order to measure how much radiation a person (or other object) is exposed to, various instruments and devices have been invented.

The best-known instrument used to measure radiation is a Geiger counter, a device that measures ionizing radiation using a tube filled with inert gas and metal electrodes. This tube is called a Geiger-Müller tube and gives the counter its name. When ionizing radiation hits a Geiger-Müller tube, some of the gas in the tube is ionized. Ions have an electrical charge and move toward the electrodes of the tube. This motion changes the electric field inside the tube, which can be measured. Geiger counters are portable, inexpensive, and durable, making them quite popular.

A scintillation counter is an instrument that uses the photoelectric effect, the emission of electrons from a surface when light shines on it, to measure ionizing radiation. Liquid scintillation counters, in which beta particle emitters are immersed in a liquid before being placed in the counter, can precisely measure the quantities of radioisotope present in a sample. They can also be connected to storage or output devices, such as computers and printers, making them ideal for academic and industrial uses.

A dosimeter is a device that uses a metal chip or crystal to absorb radiation and then indicates the total absorbed amount later when subjected to light or heat respectively. Dosimeters measure exposure over time and are used to track exposure of people or objects with high exposure risk, such as X-ray technicians and nuclear power plant workers. Dosimeters are formed as a small badge that can be carried in a pocket, affixed to a lapel, or worn as a ring on a finger.

Because the reasons for measuring radioactivity vary, so too do the units. Radiation detectors often describe amounts of radiation in terms of counts per minute (cpm). The amount of radiation emitted by radioactive material, or the amount released into the environment by an accidental release of radiation, is often described using the unit curie (Ci) or the more modern SI unit becquerel (Bq). The amount of radiation absorbed by a person exposed to radiation is described by the unit radiation absorbed dose (rad) or the SI unit gray (Gy), where $1\;\rm{Gy} = 100\;\rm{rad}$. The biological risk of exposure from radiation is described by the unit roentgen equivalent man (rem) or the SI unit sievert (Sv).