Ionising radiation detectors

Detectors. Two general types of detectors are used in x-ray medical imaging scintillation and gas ionisation. Scintillation detectors are used for both conventional projection and computerized tomographic imaging. Ionization detectors have been used only in CT appHcations. All detectors used in detection of x-ray radiation must be linear and have a maximum efficiency at the wavelength of the x-ray photon to be detected. 

There are three types of smoke detectors utilising ionising radiations, light scatter and obscuration. 

The principal workplaces which could have ionising radiation present are the nuclear industry, medical centres (hospitals and research centres) and educational centres. Radioactive processes are used for the treatment of cancers and radioactive isotopes are used for many different types of scientific research. X-rays are used extensively in hospitals, but they are also used in industry for non-destructive testing (e.g. crack detection in welds). Smoke detectors, used in most workplaces, also use ionising radiations. 

Ar. Ns This detector employs a source of P radiation (usually 6JNi-cf. helium ionisation detector). The 1000-10.000 velocities of the high energy electrons are reduced to thermal velocities by coOisioo with atoms of inert gas with which the detector chamber is purged. When a sample of a gas with a greater election affinity than the inert gas is introduced into the ccC some of the electrons are captured and form negative ioas. 

This uses ultraviolet radiation to produce ionised species which can be collected and detected as a current. Lamps of various energies (8.3, 9.5, 10.2, 11.7 eV) can be used to change the selectivity of tiie detector. Sensitivity is of the same order as the flame ionisation detector for amenable compounds, although it can be as low as 2 pg for benzene, with a dynamic range of 7 orders of magnitude. 

The standard sources have been designed in order to allow the calibration of all the classical detectors of a, p, e, y, n, X radiation (ionisation chambers, Geiger-Miiller or proportional counters, scintillation or solid-state counters, etc.). They are classified as alpha sources, electron sources, beta sources, gamma sources, neutron sources. X-ray sources, heat flux sources, and sources for radiation protection dose meters. 

An ionisation detector works on the principle that ions are absorbed by smoke particles, thus when smoke enters a chamber which contains an ionised gas some of the ions are absorbed by the smoke particles and the current flow in the chamber is reduced. In practice a radiation source is used to maintain a pre-determined level of ionisation in both a sealed and open chamber. Both chambers will therefore pass the same current. When smoke enters the open chamber the ionisation is reduced and less current flows in... 

Certain interactions with matter of the radiation accompanying the decay of unstable nuclides (a- and /9-particles, y rays) are the basis for the detection and measurement of radioactivity These include photochemical processes, by which a radioactive sample placed in close contact with photographic emulsion causes blackening of the latter upon development (autoradiography) gas ionisation and the deriving production of current pulses that can be analysed and measured by suitable devices excitation of orbital electrons of special molecules, either in a crystalline form or in solution, with subsequent emission of light pulses to be converted into electric current by a photoelectric detector (scintillation)... 

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