Quartz fiber dosimeter
Encyclopedia
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
received by the device. It is usually clipped to a person's clothing and worn to measure one's actual exposure to radiation. As with other types of personal radiation dosimeter
Dosimeter
Dosimeters measure an individual's or an object'sexposure to something in the environment — particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime...
, it is worn by workers who are occupationally exposed to radiation, so their employers can keep a record of their exposure, to verify that it is below legally prescribed limits. It works by measuring the decrease in electrostatic charge on a metal conductor in an ionization chamber
Ionization chamber
The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
, due to ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
of the air in the chamber by radiation. It was invented in 1937 by Charles Lauritsen.
The dosimeter must be periodically recharged. It is usually read before it is charged, and the dose is logged, to chart one's exposure. In many organizations, possession of the recharger is limited to health physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
s to prevent falsification of exposures. Magnifying lenses (a low-power microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
) and an illumination lens allow one to directly read the dose at any time by aiming the illumination lens at a light source and looking into the device.
The device is mainly sensitive to gamma
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
and x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, but it also detects beta radiation above 1 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
. Neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
sensitive versions have been made.
Quartz fiber dosimeters are made in different ranges. Peace-time occupational exposure ranges usually measure up to 500 mrem (5 mSv), which exceeds the normal US yearly dose of 360 mrem (3.6 mSv). War-time fallout meters measure up to 500 rem (5 Sv), roughly the lethal dose.
The quartz fiber device is an older dosimeter design. It suffers from these disadvantages:
- Low accuracy: Because of the analog mechanical design, accuracy is around 15%, less than other dosimeters.
- Reading errors: Since it can only be read manually it is prone to human reading errors.
- Small dynamic range: The range of the device is limited by the charge on the electrode. Once the charge is gone the device stops recording exposure. So unexpected large radiation doses can quickly saturate devices designed to monitor the more usual low level exposures.
- Susceptibility to moisture: the charging terminalTerminal (electronics)A terminal is the point at which a conductor from an electrical component, device or network comes to an end and provides a point of connection to external circuits. A terminal may simply be the end of a wire or it may be fitted with a connector or fastener...
is sealed with a cap, but if this should be removed during use dampness can accelerate the leakage of the charge, imitating the effect of radiation exposure. Deliberate falsification is possible, by breathing onto the exposed terminal.
How it works
The quartz fiber dosimeter is a rugged form of a device called a Lauritsen electroscopeElectroscope
An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert...
. It consists of a sealed air-filled cylinder called an ionization chamber
Ionization chamber
The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
. Inside it is a metal electrode strip that is attached to a terminal on the end of the pen for recharging. The other end of the electrode has a delicate gold-plated quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
fiber attached to it, which at rest lies parallel to the electrode. The ends of the chamber are transparent and the microscope is focused on the fiber.
During recharging, the charger applies a high DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltage, usually around 150-200 volts, to the electrode, charging it with electrostatic charge. The quartz fiber, having the same charge, is repelled by the surface of the electrode due to the coulomb force and bends away from the electrode. After charging, the charge remains on the electrode because it is insulated.
When a particle of ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
passes through the chamber, it collides with molecules of air, knocking electrons off them and creating positively and negatively charged atoms (ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s) in the air. The ions of opposite charge are attracted to the electrode and neutralize some of the charge on it. The reduced charge on the electrode reduces the force on the fiber, causing it to move back toward the electrode. The position of the fiber can be read through the microscope. Behind the fiber is a scale graduated in units of radiation, with the zero point at the position of the fiber when it is fully charged.
Since each radiation particle allows a certain amount of charge to leak off the electrode, the position of the fiber at any time represents the cumulative radiation that has passed through the chamber since the last recharge. Recharging restores the charge that was lost and returns the fiber to its original deflected position.
The charger is a small box, usually powered by a battery. It contains an electronic circuit that steps the battery voltage up to the high voltage needed for charging. The box has a fixture that requires one to press the end of the dosimeter on the charging electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
. Some chargers include a light to illuminate the measurement electrode, so that measurement, logging and recharging can occur with one routine motion.
Units with larger ranges are made by adding a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
attached between the electrode and the case. The capacitor stores a larger amount of charge on the device for a given voltage on the electrode. Since each radiation particle allows a fixed amount of charge to escape, a larger number of radiation particles is required to move the fiber a given amount.
Pocket ionization chamber
A version of the above dosimeter without the self-reading capabilities, called a pocket ionization chamber or just pocket chamber, was widely used in World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
and postwar government and military projects, particularly the Manhattan project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
. This consisted of a simple ionization chamber with an electrode running down the center, but no electroscope for reading. Instead the exposure was read by plugging the device into a separate precision electrometer
Electrometer
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices...
/charger, which measured the decline in charge on the electrode and displayed it on a meter, before recharging the electrode. These had the advantage that they were simpler, more rugged, and cheaper than the electrometer type, but the disadvantage (considered desirable in some military applications) that the exposure couldn't be read by the wearer without the electrometer/charger. They are no longer used.
Rate meter
A similar device, used with the same charger, is a rate meter. This is an inexpensive method for civil defenseCivil defense
Civil defense, civil defence or civil protection is an effort to protect the citizens of a state from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery...
persons to measure radiation rates. One measures the rate of change of the rate meter for a timed exposure after charging the rate meter. Usually one measures heavy fallout of a thirty-second period, and light fallout
Fallout
Fallout or nuclear fallout is the residual radiation hazard from a nuclear explosion.Fallout may also refer to:*Fallout , a 1997 post-apocalyptic computer role-playing game released by Interplay Entertainment...
over a ten-minute period. The rate meter has two internal scales that read the radiation flux directly in rems for each period.