Devices and methods used to measure external exposure to ionizing radiation can be grouped into four categories: dosimeters, beta and gamma radiation detectors, alpha radiation detectors, and neutron detection methods. Though less straightforward, there are also methods of detecting internal exposure to radiation. We will also discuss measurement of radionuclides in air, water, vegetation, and soil.

Dosimeters

Dosimeters are devices that monitor an individual's external radiation dose. The two most commonly used dosimeters are thermoluminescent dosimeters (TLDs) and film badges. Both devices measure the dose accumulated over a given period of time. For example, film badges might be worn for a month. When they are collected and analyzed, the total exposure for that month can be determined.

One widely used type of thermoluminescent dosimeter uses a crystal of lithium fluoride. When radiation is absorbed by the lithium fluoride, it raises electrons in the crystal to higher energy levels. Some of these electrons are trapped by impurities in the crystal where they remain in their excited states until the crystals are heated. When the crystal is heated, the electrons are released from these trapping sites and give off light. The light emitted can be measured and is proportional to the amount of radiation to which the TLD crystal, and presumably the individual, were exposed. Once the crystal is heated to a sufficiently high temperature, all the trapped electrons are released, and the dosimeter may be reused. Some TLDs are sensitive enough to measure a dose of beta or gamma radiation of a few tens of microrads. Some TLDs can also detect neutrons.

Film badges are used to monitor personal exposure to beta and gamma radiation. To assess various radiations simultaneously, a strip of film is covered with absorbers. By varying the type and thickness of the absorbers, one can determine skin dose, dose to the lens of the eye, and whole-body dose. Some film badges have a small window shielded by a sheet of mylar which can detect beta radiation, and one or more sections shielded by metal foils for detecting gamma radiation. The radiation exposure of the film is determined by the degree of darkening of the film after the film is developed. Film badges look like badges and can be clipped onto a pocket or a belt.

While film badges and TLDs measure a workers dose over an extended period of time, pocket dosimeters measure a worker's radiation dose each day. Rather than waiting weeks, pocket dosimeters can detect whether a worker might have received a dangerous dose during a given workshift. In principle, one should wear a film badge or TLD and a pocket dosimeter at the same time. Pocket dosimeters can measure gamma radiation with energies up to two MeV. They are basically devices that can store an electric charge. They consist of an exterior wall, which is essentially a plastic tube coated with a conducting material, and an interior central wire which is insulated from the outer wall. An additional device, called the charger-reader, is used to place a positive charge on the central wire. When exposed to radiation, some of this positive charge is neutralized by ions created by the radiation. The dosimeters are read directly or by being placed in the charger-reader to determine the actual radiation dose received. Pocket dosimeters look like pens, and are clipped onto a shirt pocket.

Film badge (Source: ICN Biomedicals, Inc.)		Pocket dosimeters (Source: Biodex Medical Systems)

Beta and Gamma Radiation Detectors

Radiation detectors are devices used to detect beta and gamma radiation in air. They differ from dosimeters in that they can measure radiation directly, in real time. Most radiation detectors detect the interaction of radiation with gas molecules. As radiation slows down in a gas, it ionizes gas atoms by ejecting electrons from them and leaving behind positive ions. In a Geiger-Müller detector, or Geiger counter as it is more commonly known, the result of this ionization produces a constant output electrical pulse, regardless of the amount of energy deposited in the detector or the nature of the ionizing radiation. On the other hand, the output of scintillation counters and gas flow proportional counters is proportional to the amount of energy deposited in the detector.

A Geiger counter can count beta particles and gamma rays. If equipped with a window thin enough (as in, for example, a "pancake" detector), a Geiger counter can also detect alpha particles. The entire instrument is actually made up of two components: a Geiger-Müller tube (the detector in which the ionizations are produced) and an electronic amplifier (which activates a device that counts the ionizations). The Geiger-Müller (GM) tube consists of a cylindrical chamber with a metal wire stretched along its center which is insulated from the outer wall. The tube contains an inert gas such as helium or neon. The positive lead of the high voltage supply is connected to the central wire; the negative lead is connected to the outer shell of the tube.

The process of measuring a source of radiation begins by holding the Geiger counter near the source. An incident beta particle or gamma ray will then ionize atoms of the gas. The resulting electrons are strongly attracted to the positive wire. In the path of the electrons are other gas molecules which will also be ionized. These new electrons will produce further ionizations, resulting is a cascade of ionizations. One initial ionization results in billions of ionizations which are collected on the central positive wire. An electronic amplifier is then used to activate a counting device.

Beta particles that reach the gas in the detector and cause ionizations will register a count. Many gamma rays, however, will pass through the entire gas without any interaction, and thus will not be recorded (unless thicker absorbers are used to capture the high-energy gamma rays). While a GM tube is more efficient at detecting beta particles than gamma rays, the tube must be designed so that the window is thin enough to allow the beta particles to penetrate. Its output signal cannot be used to provide information on the type of incident particle that produces the count. To distinguish between beta particles or gamma rays, absorbers can be used. For instance, a thin absorber between the radiation source and the GM tube will stop all the beta particles, allowing the gamma rays to enter the detector. The counting rate with and without the absorber can be used to distinguish between beta particles and gamma rays.

Whereas Geiger counters count the ionizations resulting from the incident radiation's interaction with gas atoms, scintillation counters are sensitive to the energy of the incident radiation itself. A scintillation counter is made with a material which glows (a scintillator) when it is struck by radiation, and a light amplifier. When a beta particle slows down in a scintillator, a fraction of the energy it imparts to the atoms in the scintillator is converted to light. When gamma rays pass through the scintillator, they produce electrons which in turn behave just like beta particles and convert some of their energy into light.

Scintillators come in all shapes and sizes. Some are plastic, and some are dense sodium iodide crystals. Large, dense scintillators are necessary to detect gamma rays since energetic gamma rays can pass through moderate thicknesses of ordinary matter (human tissue, concrete walls, water, etc.) with little interaction. The amount of light produced in the scintillator can be measured with a light amplifier, called a photomultiplier. The amount of each pulse of light represents a measure of the energy deposited in the scintillator. The ability to measure this energy means that radiation from various sources can be identified and at the same time one can evaluate the magnitude of the source. The other devices discussed above do not enable us to discern the amount of energy of the photons (that is, the type of gamma rays). Determining the type of gamma rays allow us to infer the type of radionuclide that emitted them.

Geiger counters can be made as small hand held instruments. They are easy to use as portable radiation monitors. Scintillation counters are generally large laboratory instruments.

Alpha Radiation Detectors

Detecting alpha particles is technically more difficult than detecting beta particles and gamma rays. Like beta and gamma radiation, alpha particles can produce ionizations, but they are not as penetrating.

In principle, alpha particles could be detected with an ordinary GM tube. GM counters equipped with a detector made with a very thin mylar window (e.g. pancake GM probe) can be used to detect alpha as well as gamma and beta radiation. However, alpha particles are best measured by what are called gas flow proportional counters.

In some proportional counters, the radioactive source that is to be measured, or the sample, is placed directly inside the detector. In these "windowless" tubes, the sample is in direct contact with the counter gas (the gas in the detector). As in Geiger counters, the signal produced by a proportional counter results from the electric charge, which in turn is produced by the ionization of the gas by the incident radiation. The gas in the detector is usually 90% argon and 10% methane and flows through the chamber at atmospheric pressure.

Hand-held instruments that measure alpha, beta and gamma radiation combined (in terms of the amount of ionization they produce) with readings in counts per minute or milliroentgens per hour are commercially available (see photo of portable survey meter, page 12). Alpha counters are used in, for example, places where workers are handling plutonium (an alpha-emitter).

Survey meter Used with scintillation probe (not shown) or pancake GM probe (right) (Source: Biodex Medical Systems)		Pancake Geiger-Müller probe Also called a "frisker" (Source: Biodex Medical Systems)

Portable survey meter
Measures alpha, beta and gamma radiation
(Source: Biodex Medical Systems)

Neutron Detection

Gamma rays are classified as ionizing radiation. They are electromagnetic rays (just like light), and have no charge associated with them. They remove electrons from neutral atoms, leaving behind a positive ion. Alpha and beta particles are ions; in other words, they carry a net charge. Alpha particles have a net +2 charge and beta particles have a single negative or positive charge. Various schemes, discussed above, have been employed to turn ions, or the products of ionizations, into measurable events (counts).

Neutrons, on the other hand, are neutral particles. They carry no electric charge and cause no direct ionization. Neutrons can be detected indirectly through the charged particles they produce in a nuclear reaction or by gamma rays produced by indirect ionization. For instance, a typical capture reaction would involve a neutron being captured by the isotope boron-10. This would initiate a nuclear reaction that would produce a characteristic gamma ray which could be detected by one of the gamma radiation detection methods described above. However, the gamma detector must be able to discriminate between the gamma rays produced in the nuclear reaction and the gamma rays arising from other sources.

Doing this is not so easy, but if one has a way of detecting the energy associated with the radiation (see scintillation counters, above), then one can be on the look out for energies of certain ranges that might be associated with the source of radiation (for example, gamma rays that are known to be decay products). It is possible to build electronic circuits that discriminate the characteristic gamma rays from all others.

Environmental Measurements

Radionuclides can be measured in air, water, vegetation, and soil using the instruments described above in conjunction with air monitoring stations, water sampling with lab analysis, soil sampling, and other equipment and methods. To detect the amount of radiation in the workspace air, a certain quantity of the air would be drawn through a paper filter which would then be measured with one of the detectors described above.

Radioactivity in liquids is measured with a liquid scintillation counter. If the liquid is water, this is a fairly routine procedure. For example, facilities using radioactive materials have to measure the radioactivity in liquid waste to determine if it is below the standards set for disposal as waste water. Determining the level of radioactivity in other liquids, particularly unknown liquids, is more difficult.

Measuring the concentration of a gamma-emitting radionuclide in soil can be done in the field with a simple hand-held Geiger counter. However, to detect specific alpha- or beta-emitting radionuclides, a soil sample would be analyzed with a scintillation or gas flow proportional counter, usually in a laboratory.

Many laboratories that perform radionuclide measurements submit themselves to testing protocols. The Department of Energy's Environmental Measurements Laboratory (EML) evaluates participating laboratories through its Quality Assessment Program. This program compares the analytical performance of participating laboratories. The EML publishes their evaluations roughly twice a year and makes them available on its Web site, http://www.eml.doe.gov/qap/.

Internal Dose¹

External monitoring devices, such as TLDs, can measure how much external radiation a worker has been exposed to, but not the radiation dose due to radionuclides taken into the body through inhalation, ingestion, or other means. It is generally much harder to estimate doses from substances inside the body. The size of an internal dose will depend on the chemical form of the material, its pathways and distribution in the body, and the rate of its elimination from the body (called biological half-life), among other factors. Since metabolic factors vary considerably from one person to the next, the internal dose that any individual gets from a particular radionuclide may be considerably different from the dose calculated using its average biological half-life.

Internal doses can be monitored in various ways. One common way is to measure radionuclide concentrations in urine. If one knows the rates of excretion corresponding to various body burdens, then it is possible to calculate these body burdens and thereby infer the radiation dose.

Another method is to measure the gamma radiation being emitted by the radionuclide inside the body. Since a portion of gamma radiation penetrates the body, a fraction of the gamma rays emitted by radionuclides inside the body escape outside it. This is measured by putting the worker or part of his or her body into a "counter," which is a chamber that measures gamma radiation. Thus, we have "whole body counters," "lung counters," and so on. Care must be taken to exclude or adjust for other sources of environmental radioactivity in the measurement of internal body burdens, notably radon and its decay products.

Internal doses to workers can also be assessed indirectly by measuring the concentrations of radionuclides in the air in the workplace. In areas where exposure is more likely, workers can wear portable air monitoring devices to measure concentrations of radionuclides in the "breathing zone" that is, in the air very close to their faces. Internal worker doses can be estimated if breathing rates, efficiencies of protective devices worn by workers (if any), and other factors are known.

It is essential that radiation monitoring be carried out accurately and in sufficient detail. For instance, film badges and TLDs must be stored properly when not in use, so that they are not contaminated between worker exposure times. Also, workers at risk of internal exposures must be monitored frequently enough to accurately determine internal body burdens of radionuclides.

In the nuclear weapons industry, worker dosimetry and exposure records are seriously deficient. In 1994, the Department of Energy admitted that its records for worker exposure to external radiation are incomplete, unreliable, and misleading, and that this was partly due to poor calibration of measuring devices, issuance of multiple badges, and poor placement of dosimeters.² More recently, a study that evaluated the performance of approximately 1,000 personal monitoring devices in Europe found that 25% of the external doses recorded by the beta and neutron dose monitors were significant underestimates.³

Glossary of Radiation-Related Terms
Measuring Radiation: Units and Measurements

³ J.M. Bordy, et al. "Performance Test of Dosimetric Services in the EU Member States and Switzerland for the Routine Assessment of Individual Doses (Photon, Beta and Neutron)," Radiation Protection Dosimetry 89(1­2), pp 107­154 (2000), as reported in New Scientist, 26 August 2000.