Quartz fiber dosimeter

Last updated December 24, 2023

Quartz fiber radiation dosimeter, showing clip for securing it to clothing; normally a breast pocket.

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 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.

Use

As with other types of personal radiation dosimeter, 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, due to ionization of the air in the chamber by radiation. It was invented in 1937 by Charles Lauritsen.^[2]

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 physicists to ensure accurate recording of exposures. It contains a low-power microscope 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 and x-rays, but it also detects beta radiation above 1 MeV. Neutron sensitive versions have been made.^[1]

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:^[3]

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. 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.^[3]^[4] It consists of a sealed air-filled cylinder called an ionization chamber. 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 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,^[1] 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 passes through the chamber, it collides with molecules of air, knocking electrons off them and creating positively and negatively charged atoms (ions) 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. 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 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 and postwar government and military projects, particularly the Manhattan project.^[1] 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/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 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 over a ten-minute period. The rate meter has two internal scales that read the radiation flux directly in rems for each period.

Related Research Articles

A radiation dosimeter is a device that measures 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 electronic personal dosimeters can give a continuous readout of cumulative dose and current dose rate, and can warn the wearer with an audible alarm when a specified dose rate or a cumulative dose is exceeded. Other dosimeters, such as thermoluminescent or film types, require processing after use to reveal the cumulative dose received, and cannot give a current indication of dose while being worn.

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 electrometers based on vacuum tube or solid-state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges.

The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a practical tube that could detect a number of different radiation types.

Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.

Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposure can be from a source of radiation external to the human body or due to internal irradiation caused by the ingestion of radioactive contamination.

In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation. It is used in at least two applications:

<span class="mw-page-title-main">Health physics</span>

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. Health physicists normally require a four-year bachelor’s degree and qualifying experience that demonstrates a professional knowledge of the theory and application of radiation protection principles and closely related sciences. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation are used or produced; these include research, industry, education, medical facilities, nuclear power, military, environmental protection, enforcement of government regulations, and decontamination and decommissioning—the combination of education and experience for health physicists depends on the specific field in which the health physicist is engaged.

The electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it. The amount of charge on an object is proportional to its voltage. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. An electroscope can only give a rough indication of the quantity of charge; an instrument that measures electric charge quantitatively is called an electrometer.

The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is proportional to the radiation energy absorbed by the detector due to an ionizing event; hence the detector's name. It is widely used where energy levels of incident radiation must be known, such as in the discrimination between alpha and beta particles, or accurate measurement of X-ray radiation dose.

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. Conventionally, the term "ionization chamber" refers exclusively to those detectors which collect all the charges created by direct ionization within the gas through the application of an electric field. It uses the discrete charges created by each interaction between the incident radiation and the gas to produce an output in the form of a small direct current. This means individual ionising events cannot be measured, so the energy of different types of radiation cannot be differentiated, but it gives a very good measurement of overall ionising effect.

<span class="mw-page-title-main">Film badge dosimeter</span>

A film badge dosimeter or film badge is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.

Geiger counter is a colloquial name for any hand-held radiation measuring device in civil defense, but most civil defense devices were ion-chamber radiological survey meters capable of measuring only high levels of radiation that would be present after a major nuclear event.

Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing particles, and in radiation protection applications to measure ionizing radiation.

The hot-filament ionization gauge, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure (vacuum) measuring device for the region from 10⁻³ to 10⁻¹⁰ Torr. It is a triode, with the filament being the cathode.

An electrostatic fieldmeter, also called a static meter is a tool used in the static control industry. It is used for non-contact measurement of electrostatic charge on an object. It measures the force between the induced charges in a sensor and the charge present on the surface of an object. This force is converted to volts, measuring both the initial peak voltage and the rate at which it falls away.

<span class="mw-page-title-main">Kearny fallout meter</span> Design of radiation meter

The Kearny fallout meter, or KFM, is an expedient radiation meter. It is designed such that someone with a normal mechanical ability would be able to construct it before or during a nuclear attack, using common household items.

<span class="mw-page-title-main">Electronic personal dosimeter</span>

The electronic personal dosimeter (EPD) is a modern electronic dosimeter for estimating uptake of ionising radiation dose of the individual wearing it for radiation protection purposes. The electronic personal dosimeter has the advantages over older types that it has a number of sophisticated functions, such as continuous monitoring which allows alarm warnings at preset levels and live readout of dose accumulated. It can be reset to zero after use, and most models allow near field electronic communications for automatic reading and resetting.

Francis Rudolph Shonka was a physicist and inventor. Shonka was known for his pioneering work with ionizing radiation measurement devices and equipment. This equipment bears his name today as the: Shonka ionization chamber, the Shonka electrometer, and Shonka plastics.

References

1 2 3 4 Frame, Paul (October 11, 2021). "Pocket Chambers and Pocket Dosimeters". ORAU Museum of Radiation and Radioactivity. Oak Ridge Associated Universities. Retrieved October 11, 2021.
↑ Frame, Paul (October 11, 2021). "Robley Evan's Lauritzen Electroscope". ORAU Museum of Radiation and Radioactivity. Oak Ridge Associated Universities. Retrieved October 11, 2021.
1 2 Ahmed, Syed Naeem (2007). Physics and Engineering of Radiation Detection. USA: Academic Press. pp. 647–648. ISBN 978-0-12-045581-2.
↑ Raj, Baldev; Venkataramen B. (2004). Practical Radiography. UK: Alpha Science Int'l. pp. 162–163. ISBN 1-84265-188-9.

External links

A photographic list of radiation dosimeters

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[Frame-1] 1 2 3 4 Frame, Paul (October 11, 2021). "Pocket Chambers and Pocket Dosimeters". ORAU Museum of Radiation and Radioactivity. Oak Ridge Associated Universities. Retrieved October 11, 2021.

[2] Frame, Paul (October 11, 2021). "Robley Evan's Lauritzen Electroscope". ORAU Museum of Radiation and Radioactivity. Oak Ridge Associated Universities. Retrieved October 11, 2021.

[Ahmed-3] 1 2 Ahmed, Syed Naeem (2007). Physics and Engineering of Radiation Detection. USA: Academic Press. pp. 647–648. ISBN 978-0-12-045581-2.

[4] Raj, Baldev; Venkataramen B. (2004). Practical Radiography. UK: Alpha Science Int'l. pp. 162–163. ISBN 1-84265-188-9.

[2]

[1]

[3]

[4]