The World of Nuclear Science

Measuring Radiation

The strength of a radiation source can be defined as the number of disintegrations per time (usually per second) - known as the source activity. (One disintegration is one decay event.) Often the curie unit (Ci) is used, and one curie is defined as 3.70 × 10¹⁰ disintegrations per second, and arises from the source activity of 1 gram of radium.

The common SI unit however is the becquerel (Bq), which is defined as 1 disintegration per second. Both units are named after famous scientists that played important parts in nuclear science.

Another unit of radioactivity measurement is the absorbed does. One unit of this type is the roentgen (R). One roentgen is now defined as the amount of X-ray or gamma radiation that deposits 0.878 × 10^-2J of energy per kilogram of air. However, the more common unit is the rad. One rad is the amount of radiation that deposits energy at the rate of 1.00 × 10^-2J per kilogram in an absorbing material. The SI unit is the gray (Gy), and one gray is equal to 100 rads.

Radiation measurement is particularly common in medicine. The quality factor of any type of radiation is the number of rads of X-ray or gamma radiation that produces the same amount of biological damage as 1 rad of that type of radiation. (Obviously different types of radiation have different biological effects.)

Effective dose is the dose in rads multiplied by the quality factor. Effective dose is measured by the SI unit sievert (Sv).

The first apparatus used to measure radiation was an ionisation chamber. The principle of these chambers are used today in smoke detectors.

Ionisation chambers consist of a vessel (often tube shaped) filled with a gas (sometimes just air), which is situated between two electrodes. When radiation penetrates through the gas, the gas molecules become ionised and flow to conduct an electric current between the two electrodes. This current is measured with electronic circuits to give a measure of the radiation intensity. The more radiation, the more ionised gas - conducting a larger current.

Some ionising chambers are specialised so as to detect individual ionising particles (produced from radioactive decay). One of the most commonly used instruments that perform this task is known as the Geiger-Muller counter (often shortened to Geiger counter). Invented by the German physicist Hans Geiger in the 1920s, the counter counts individual decay events by counting the pulses of current that occur when a ray of radiation passes through the tube. (One ray is the result of one decay.) The counter has no moving parts, and as a result is used often by geologists in the field because of its reasonably robust nature. Good quality counters can easily withstand large falls. When the Geiger counter is connected to special electronic equipment, a digital count of the radiation decay events that have taken place can be kept.

Another type of radiation detector is a scintillation counter. A scintillator (sometimes called a phosphor) is often a solid substance although it may also be liquid or gas. The atoms in a scintillator are excited by incoming particles from radiation (eg a helium nucleus from alpha radiation or an electron from beta radiation), and when they drop back down to their ground energy states they release light. Sodium iodide crystals are one example of a scintillator. Some special plastics also have scintillation properties.

The scintillator is placed inside a photomultiplier tube, and this tube is sealed from light. Inside the tube, photoelectric surfaces in combination with electrodes of varying voltages convert the released light from the scintillator into electric pulses, which can then be counted in a similar way to how the pulses from a Geiger counter are counted. Scintillation counters are often better than other methods at detecting gamma rays.

Radiation can also be seen instead of being just counted by a range of techniques collectively known as photographic emulsion.

A cloud chamber contains a gas cooled down to an extremely low temperature, below its boiling point. These gas molecules condense on charged particles. Because radiation can ionise air, these ionised air molecules act as surfaces (condensation nuclei) on which the gas molecules condense. These droplets then are seen as "clouds", that enable us to see the path of the radiation. One of the first types of cloud chamber was the Wilson cloud chamber. It is a very simple (and easy to make) device but not often used in scientific experiments today.

Other techniques include bubble chambers and wire chambers.

Dosimetry is the technique of measuring the amount of radiation that can cause a given amount of biological damage. It is common practice in medicine.

A radiation badge is a simple device that contains a sheet of film, with different regions of this film having different sensitivities to light. (When radiation hits a photosensitive film, the film becomes darkened. Less sensitive film requires more radiation to expose it.) The badge film can be developed to see which sensitivity of the film has not been exposed - giving an indication of the maximum strength that that badge has been exposed to. Workers who work in radiation environments wear these badges and have them developed at regular intervals to monitor their exposure to radiation.

There are many other newer and more sophisticated techniques of monitoring radiation exposure, although the badge one of the the simplest and cheapest ways.

The higher up you live, the more radiation you are exposed to every day. This is because cosmic rays from the sun are more common higher in the atmosphere. By the time they reach the Earth's ground, some of them will have been absorbed - explaining the lower level of radiation closer to the ground. Additionally, the intensity of cosmic radiation also obeys the inverse square law. The ground of the Earth is also further away from the sun than higher up in the atmosphere. However, note that living in a canyon below sea level does not necessarily mean you are exposed to less radiation. In fact, you may be exposed to more because of the canyon's walls containing minerals that may be naturally radioactive.

At sea-level, the average radiation level is approximately 0.03 microsieverts per hour. As the altitude increases, the radiation exposure increases exponentially. Mexico City, 2240 m above sea-level, is exposed to about 0.09 microsieverts per hour; La Paz (in Boliva, South America) - the highest city in the world - has radiation of about 0.23 microsieverts per hour.
Click here for a map of where La Paz is located. (Map courtesy of Lonely Planet.)

Radon is a radioactive gas formed from the decay of metals like uranium and thorium. It is in the air that we breathe, but only in very small proportions - in trace amounts of less than 0.01%. Nevertheless, prolonged and concentrated exposure to radon gas is believed to cause lung cancer.

Frequent international aircraft travellers will be exposed to more radiation because they will spend more of their time up in the atmosphere, closer to the cosmic rays.

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