Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Patent
1994-06-14
1996-05-14
Fields, Carolyn E.
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
250374, 2503851, G01T 1205
Patent
active
055170307
DESCRIPTION:
BRIEF SUMMARY
SCOPE OF THE INVENTION
The present invention concerns detectors for ionizing radiation, of the gas proportional scintillation counter type for X-rays, gamma rays, electrons, protons, alpha particles as well as other charged particles like deuterons and mesons, or other nuclear radiation like neutrons. With the principles described in the present invention it is possible to build gas proportional scintillation counters with a high energy resolution, even if the radiation windows and/or the detection volumes have dimensions analogous to those of the secondary scintillation sensor, generally a photomultiplier.
PRIOR ART
Radiation detectors of the gas proportional scintillation counter type have been described in the scientific literature since 1967 (C.A.N.Conde and A.J.P.L.Policarpo, "A gas proportional scintillation counter", Nucl. Instrum. Meth. 53 (1967) 7) being the most commonly used type the one that uses two parallel grids (C.A.N. Conde, M.C.M. Santos, M. Fatima A. Ferreira and Celia A. Sousa, "An argon gas proportional scintillation counter with uniform electric field", IEEE Trans. Nucl. Sci. NS-22 (1) (1975) 104). The working principle has been described in detail in the scientific literature (C.A.N.Conde, "The gas proportional scintillation counter: evaluation of performances with interest for analysis by ion beam techniques", Nucl. Instrum. Meth. 149 (1978)685) and is briefly described in the following.
When ionizing radiation is absorbed in a very pure noble gas (typically xenon at atmospheric pressure) it produces a primary electron cloud with a number of electrons which is proportional, within a good approximation, to the energy of the absorbed radiation (F. P. Santos, T.H.V.T. Dias, A. D. Stauffer and C.A.N. Conde, "Variation of energy linearity and w value in gaseous xenon radiation detectors for X-rays in the 0.1 to 25 keV energy range: a Monte Carlo simulation study", Nucl. Instrum. Meth. A307 (1991)347). These electrons are subject to a weak reduced electric field (smaller than 1V/cm/Torr) produced by the application of a voltage (typically 500V) between a plane first grid and the detector window; then the primary electrons drift towards this grid. The region between the detector and the first grid is usually called "drift region". Next to the first grid and parallel to it there is a plane second grid, about 1 cm away. An electric high voltage (typically 5000 Volts) is applied to the second grid which produces a strong electric field with an intensity below or near the threshold for ionization (i.e. 5 V/cm/Torr). Once the primary electrons reach the region between the two grids (the so-called "scintillation region") they excite the noble gas atoms, which in the de-excitation process lead to the production of vacuum ultraviolet light, the so-called "secondary scintillation". One single electron may excite, successively, hundreds of noble gas atoms. Therefore the intensity of the secondary scintillation will be quite large and proportional to the number of primary electrons, i.e. proportional to the energy dissipated by the ionizing radiation in the gas.
The secondary scintillation is detected by a vacuum ultraviolet sensor (generally a photomultiplier) which produces a pulse with an amplitude proportional to the number of primary electrons. Thus, with a multichannel analyzer it is possible to obtain energy spectra, much in the same way as with a standard NaI(Tl) scintillation counter, but with much better energy resolution, at least for X-rays.
The energy resolution of gas proportional scintillation counter is limited by the Fano factor (T.H.V.T.Dias, F. P. Santos, A. D. Stauffer and C.A.N. Conde, "The Fano factor in gaseous xenon: A Monte Carlo calculation for X-rays in the 0.1 to 25 keV energy range", Nucl. Instrum. Meth. A307 (1991) 341) but depends also, amongst other factors on the grid parallelism (C.A.N. Conde, L. F. Requicha Ferreira and M. Fatima A. Ferreira, "The secondary scintillation output of xenon in a uniform field gas proportional scintillation counter", IEEE Trans. Nucl
REFERENCES:
patent: 4286158 (1981-08-01), Charpak et al.
patent: 4376892 (1983-03-01), Charpak et al.
patent: 4429228 (1984-01-01), Anderson
W. H.-M. KU, et al., "Properties of An Imaginag Gas Scintillation Proportional Counter", Nuc. Inst & Met., 196 (1982) 63-67.
Marques Ferreira dos Santos Joaquim
Nabais Conde Carlos A.
Sena Sao Miguel Benito Antonio C.
Fields Carolyn E.
Glick Edward J.
Ostrow Seth H.
Rabin Frederick H.
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