What is the dosimeter, radiometer?
The dosimeter is a device for doze measurement or ionizing radiation dose rate measurement received by the device (and by who uses it) for some time interval, for example, for the period of finding in some territory or for a labor shift. Measurement of the above-described quantities is called dosimetry.
Sometimes "dosimeter" not absolutely precisely is named radiometer – is the device for measurement of radionuclide activity in source or in sample (in liquid volume, gas, aerosol, on the polluted surfaces) or ionizing radiant flux density for control on radio-activity of suspicious goods and estimation of radiating conditions in the given place at the moment. Measurement of the above-described sizes is called radiometry. Roentgenmeter – is a variety of radiometer for gamma rays rate measurement.
Household equipment, as a rule, is combined, has both operating mode with changeover "dosimeter"-"radiometer", light and (or) audio signaling and display for measurements reading. The weight of household equipment is from 400 up to several tens of grams, the size allows putting them in a pocket. Some modern models can be put on hand like watches. Time of continuous operation from one battery is from several days up to several months.
Measurement range of household radiometers, as a rule, from 10 microroentgen per hour up to 9.999 milliroentgen per hour (0.1-99.99 microsievert per hour), measurement accuracy is up to ±30%.
The detector (sensitive element of dosimeter or radiometer, used for transformation of the phenomena caused by ionizing radiation in electric or other signal, easily accessible to measurement) can be the ionization chamber, Geiger counter, scintillator, semiconductor diode etc.
Ionization chamber is gas-filled sensor intended for measurement of level of ionizing radiation.
Measurement of radiation level is provided by measurement of ionization gas level in operating volume of the chamber, which is between two electrodes. The potential difference is created between electrodes. There is ionic current, which can be measured under availability of ions in gas between electrodes. The current under other conditions being equal is proportional to speed of ions origin and, accordingly, to dose rate.
Proportional counters and Geiger-Muller counters in the wide sense are also referred to ionization chambers. The phenomenon of so-called gas amplification due to secondary ionization - in a strong electric field electrons, originated during transit of ionizing particle, are dispersed up to energy, sufficient to ionize molecules of gas in turn is used in these devices. In the narrow sense ionization chamber is gas-filled ionization detector operating outside of gas amplification mode.
Gas, by which the ionization chamber is filled, usually is inert gas (or its mix) with adding of easily ionized compounds (usually hydrocarbon, for example methane or acetylene). Opened ionization chambers (for example, ionization detectors of smoke) are filled by air.
Ionization chambers are current (integrating) and pulsed. In the latter case on the anode of chamber quickly moving electrons are collected (during the order 1 microsecond) whereas slowly drifting heavy positive ions have not time to reach the cathode for this time. It allows registering separate impulses from each particle. The third electrode - a grid located near to the anode and shielding it from positive ions is introduced into such chambers.
Geiger counter, Geiger-Muller counter is a gas-discharge device for calculation of number of the ionizing particles got in it. Represents gas-filled capacitor, which is breaching under transit of the ionizing particle through volume of gas. The additional electronic scheme provides the counter with a feed (as a rule, not less than 300V), provides, if necessary, discharge quenching and calculates discharge through the counter.
Geiger counters are divided on non-self-quenched and self-quenched (not demanding to the external scheme of discharge termination). Sensitivity of the counter is defined by gas structure, its volume and material (and thickness) of it wall.
are usually applied in household dosimeters and radiometers of the USSR’s
and Russia manufacture:
Wide application of Geiger-Muller counter is down to high sensitivity, opportunity to register a different sort of radiation, comparative simplicity and cheapness of installation. The counter was invented in 1908 by Geiger and improved by Muller.
Cylindrical Geiger-Muller counter consists of metal tube or glass tube metallized from within and the thin metal fiber tightened on the axis of cylinder. The fiber serves as the anode, a tube – as the cathode. The tube is filled with the rarefied gas, in most cases use noble gases argon and neon. Between cathode and anode the pressure of about 1500 V is created.
Operation of the counter is based on collision ionization. Gamma-quantum which are let out by radioactive isotope, getting on the counter’s walls, beat out from it electrons. Electrons, moving in gas and colliding with atoms of gas, beat out electrons from atoms and create positive ions and free electrons. The electric field between cathode and anode accelerates electrons up to energies under which collision ionization begins. There is avalanche of ions, and the current through the counter sharply increases. Thus on resistance R the impulse of pressure which moves in the registering device is formed. The avalanche charge needs to be extinguished in order that the counter could register the following particle, which got in it. It occurs automatically. There is considerable voltage drop during the moment of current impulse occurrence on resistance R, therefore a pressure between the anode and cathode sharply decreases and so that the discharge stops, and the counter is again ready for operation.
The important characteristic of the counter is its efficiency. Not all the gamma-ray photon, got on the counter, will give secondary electrons and will be registered, as interaction act of gamma rays with substance are rather rare, and the part of secondary electrons is absorbed in walls of device, without reaching gas volume.
of the counter depends on thickness of walls of counter, its material
and energy of gamma - radiation. The greatest efficiency has counters
walls of which are made of material with greater nuclear number Z as under
this formation of secondary electrons increases. Besides, walls of the
counter should be thick enough. Thickness of wall of the counter gets
out of condition of its equality to length of free run of secondary electrons
in the material of wall. Under big thickness of a wall secondary electrons
will not pass in operating volume of the counter and impulse current occurrence
will not happen. As gamma - radiation poorly interacts with substance
usually efficiency of the gamma - counters is also small and makes only
1-2%. Other disadvantage of Geiger-Muller counter is that it does not
enable to identify particles and to define its energy. These disadvantages
don’t have scintillation counters
Scintillators – are the substances which have ability to radiate light at absorption of ionizing radiation (gamma-ray quantum, electrons, alpha particles and etc.). As a rule, radiated quantity of photons for the given type of radiation is approximated to proportionally absorbed energy that allows receiving power radiation spectra. Scintillation detectors of nuclear radiations analysis – are the basic application of scintillators. The light, radiated under scintillation in the scintillation detector, is collected on the photo receiver (as a rule, it is the photocathode of the photoelectronic multiplier - FEM, are much less often used photo diodes and other photoreceivers), is transformed to the impulse of current, is intensified and is inscribed by this or that registering system.