Dosimetric Instruments

Dosimetric Instruments

 

dosimeters, devices designed to measure doses of ionizing radiation (or quantities related to doses).

Dosimetric instruments can be used to measure doses of a single type of radiation (gamma dosimeters, neutron dosimeters, and so forth) or doses of mixed radiation. Dosimetric instruments used in measuring exposure doses of X-radiation and gamma radiation are usually graduated in roentgens (R) and called roentgenometers. Instruments that measure an equivalent dose that describes the degree of radiation danger are sometimes graduated in rems; they are often known as rem meters. Radiometers measure the activity or concentration of radioactive substances.

In a typical dosimetric instrument a detector absorbs the radiation energy, leading to radiation effects; the magnitude of these effects is measured by a meter. The detector is the signal pickup. The readings of the dosimetric instrument register in an output device (for example, needles, recorders, electromechanical counters, or audio or visual indicators).

A distinction is made, according to method of operation, between stationary, movable (only when turned off), and portable dosimetric instruments. Dosimetric instruments that measure the radiation doses received by individuals in a radiation zone are called personal dosimeters.

According to the nature of the detector used, dosimetric instruments are grouped as ionization, scintillation, luminescence, semiconductor, and photodosimeters.

In ionization chambers the composition of the gas and of the wall materials are selected such that there is identical energy absorption (per unit mass) under identical irradiation conditions in the chamber and in the biological tissue. In exposure dosimeters the chambers are filled with air. An example of an ionization dosimeter is the MRM-2 microroentgenometer, which is equipped with a spherical ionization chamber and provides for a range of measurements from 0.01 to 30 microroentgens (µR) per sec for irradiations with a photon energy from 25 kiloelectron volts (keV) to 3 mega-electron (MeV) volts. The reading is shown by a needle.

The SD-l-M instrument is used for warning in the event that the specified dosage of gamma radiation is exceeded. The detector is a Geiger-Müller counter in a cylindrical casing. The instrument is equipped with audible and visible warning signals, which activate when the set dose rate is exceeded. The trigger threshold is regulated within limits of 2 to 10 milliroentgens (mR) per sec. An external alarm can be set up as much as 250 m away from the sensor; it shuts off automatically when the radiation level declines below the trigger threshold.

The SU-1 instrument is designed for automatic monitoring of the level of contamination by alpha- and beta-active particles on body surfaces and clothing. It has several gas-discharge counters, arranged such that they register radiation over the entire surface of the body. On a special illuminated indicator board depicting a human silhouette, signal lights go on to indicate where the tolerable contamination rates have been exceeded.

The personal DK-0.2 dosimeters, in the form of cylinders the size of an ordinary pencil, are designed to be carried in the pocket. The cylinder contains a miniature ionization chamber and a single-fiber electrometer. The degree of deflection of the fiber and the reading of the dose are noted visually with an optical device that has a scale graduated in milliroentgens (mR). The ionization chamber plays the role of a condenser which discharges as a result of air ionization (between the electrodes) under the effect of ionizing radiation. The degree of discharge is recorded by the deflection of the fiber, at the same time determining the radiation dose (the dosimeter is first charged by a special charging unit).

In scintillation dosimetric instruments, flashes of light arising in the scintillator under the effect of radiation are converted by a photoelectric multiplier into electrical signals, which are then recorded by a meter.

Luminescent dosimetric instruments make use of the fact that luminophors are able to accumulate absorbed radiation energy and release it later as luminiscence after the additional excitation provided by heating or irradiating the luminophor. The intensity of the flash of luminescence, measured by a special device, is proportional to the radiation dose. Depending on the mechanism of luminescence and the method of additional excitation, these dosimetric instruments are called thermoluminescent or radiophotoluminescent dosimeters. A particular feature of luminescent dosimeters is the ability to store information concerning the dose. At the necessary moment the information can be obtained by additional excitation.

A further development in the luminescent dosimeter is a type of dosimetric instrument based upon thermoexoelectron emission. In heating certain luminophors, which have first been irradiated with ionizing radiation, electrons (exoelectrons) are emitted from their surface. The number of these exoelectrons is proportional to the radiation dose in the luminophor. The exoelectrons have very low energies (to 10 eV) and are difficult to record. In an experimental variation for such a dosimeter the luminophor is placed inside a gas discharge counter, making it possible to record the exoelectrons. In some personal thermoluminescent dosimeters (produced by a Belgian firm) the luminophor is soldered into a glass tube along with a heating coil, the electrodes of which lead outside. The tube is placed in a metal or plastic case with a pocket catch. To measure a dose the tube, with its electrodes, is inserted into a measuring device in which the luminophor is heated by passing an electric current through the heating coil, and the intensity of thermoluminescent light is measured. The entire measuring procedure takes several minutes. The dosimeter is again ready for use after warming up sufficiently.

Also among the instruments that accumulate information on the radiation dose are dosimeters in which a special type of photosensitive film serves as the detector. The optical density of the darkening (after chemical treatment) is the measure of the radiation dose.

REFERENCE

Ivanov, V. I. Kurs dosimetrii, 2nd ed. Moscow, 1970.

V. I. IVANOV

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