Quartz Fiber Radiation Dosimeter
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A quartz fiber dosimeter, sometimes called a self indicating pocket dosimeter (SIPD) or self reading pocket dosimeter (SRPD) or quartz fibre electrometer (QFE), is a type of radiation dosimeter, a pen-like device that measures the cumulative dose of
ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
received by the device, usually over one work period. It is clipped to a person's clothing, normally a breast pocket for whole body exposure, to measure the user's exposure to radiation. It is now being superseded by more modern dosimeter types such as the
electronic personal dosimeter The electronic personal dosimeter (EPD) is a modern electronic dosimeter for estimating uptake of ionising radiation equivalent dose, dose of the individual wearing it for radiation protection purposes. The electronic personal dosimeter has the a ...
(EPD).


Use

As with other types of personal radiation
dosimeter A radiation dosimeter is a device that measures the equivalent dose, dose uptake of external ionizing radiation. It is worn by the person being monitored when used as a personal dosimeter, and is a record of the radiation dose received. Modern el ...
, 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 The ionization chamber is the simplest type of gaseous ionisation detector, and is widely used for the detection and measurement of many types of ionizing radiation, including X-rays, gamma rays, alpha particles and beta particles. Conventionall ...
, due to
ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...
of the air in the chamber by radiation. It was invented in 1937 by Charles Lauritsen. The dosimeter must be periodically recharged to restore it to a zero dose reading after being exposed to radiation. It is normally read immediately after use, and the dose is logged to record the user's exposure. In some organizations, possession of the recharging device is limited to
health physicist Health physics, also referred to as the science of radiation protection, is the profession devoted to protecting people and their environment from potential radiation hazards, while making it possible to enjoy the beneficial uses of radiation. H ...
s to ensure accurate recording of exposures. It contains a low-power
microscope A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...
and an illumination lens which allow direct reading of exposure at any time by aiming the illumination lens at a light source and looking into the microscope. The device is mainly sensitive to
gamma Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...
and
x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
s, but it also detects
beta Beta (, ; uppercase , lowercase , or cursive ; or ) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Ancient Greek, beta represented the voiced bilabial plosive . In Modern Greek, it represe ...
radiation above 1 
MeV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...
.
Neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
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 Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...
: The range of the device is limited by the charge on the electrode. Once the charge is gone the device stops recording exposure. This means unexpected large radiation doses can quickly saturate devices designed to monitor the more usual low level exposures. Susceptibility to moisture is dealt with by separating the charging pin from the ion chamber by a small gap. The device is pushed firmly onto the charger, closing the gap and allowing the dosimeter to be reset. Releasing the dosimeter disconnects the charger pin from the ion chamber but does induce a small change in the zero which is relatively unpredictable.


Theory of operation

The quartz fiber dosimeter is a rugged form of a device called a Lauritsen
electroscope The electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects this by the movement of a test charge due to the Coulomb's law, Coulomb electrostatic force on it. The amount of charge on ...
. It consists of a sealed air-filled cylinder called an
ionization chamber The ionization chamber is the simplest type of gaseous ionisation detector, and is widely used for the detection and measurement of many types of ionizing radiation, including X-rays, gamma rays, alpha particles and beta particles. Conventionall ...
. 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 fiber Quartz fiber is a fiber created from high-purity quartz crystals. It is made by first softening quartz rods (in an oxyhydrogen flame) and then creating filaments from the rods. Since the creation of high-purity quartz crystals is an energy int ...
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 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 Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the ''electrostatic ...
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 (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
passes through the chamber, it collides with molecules of air, knocking electrons off them and creating positively and negatively charged atoms (
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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 An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or Conductive trace, traces through which electric current can flow. It is a t ...
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 An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
. 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 In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
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 II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
and postwar government and military projects, particularly the
Manhattan project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
. 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 An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern ...
/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 had 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 defense Civil defense or civil protection is an effort to protect the citizens of a state (generally non-combatants) from human-made and natural disasters. It uses the principles of emergency management: Risk management, prevention, mitigation, prepara ...
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 Nuclear fallout is residual radioactive material that is created by the reactions producing a nuclear explosion. It is initially present in the radioactive cloud created by the explosion, and "falls out" of the cloud as it is moved by the ...
over a ten-minute period. The rate meter has two internal scales that read the radiation flux directly in rems for each period.


See also

*
Geiger counter A Geiger counter (, ; also known as a Geiger–Müller counter or G-M counter) is an electronic instrument for detecting and measuring ionizing radiation with the use of a Geiger–Müller tube. It is widely used in applications such as radiat ...
*
Scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...


References

{{reflist


External links


A photographic list of radiation dosimeters
Dosimeters Ionising radiation detectors