Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Patent
1987-10-15
1989-10-24
Howell, Janice A.
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
25037001, 25037007, G01T 102, H01L 3100
Patent
active
048764540
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a radiation detector which is particularly though not exclusively intended for use in dosemeters, for example small, active personal dosemeters for the same general purposes as Geiger-Muller tubes.
The object of the present invention is to provide novel, and at least in some respects, improved detection of radiation.
SUMMARY OF THE INVENTION
The present invention is based on the use, in a radiation meter comprising a detector for generating pulses in response to incident radiation and means for indicating the rate of pulses which exceed a threshold, of a detector comprising at least one PIN photodiode structure disposed to operate in a self biasing mode.
According to a further aspect of the invention, a detector for generating pulses in response to incident radiation comprises two PIN photodiodes each disposed to operate in a self biasing mode, one of the diodes having an effective sensitive area substantially larger than that of the other and being provided with means for attenuating radiation incident thereon. Preferably the diodes are selected such that the variation of output with incident radiation is compensated by the output from the other. There may be provided means for reducing the amplitude of pulses produced by the larger diode; this means may comprise a capacitor in series with that diode.
The aforementioned means for attenuating may comprise a metallic screen, preferably in the form of a brass cap or shield.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates schematically a radiation meter incorporating a detector according to the invention; and
FIG. 2 illustrates one possible physical configuration of the components in the meter according to FIG. 1.
DETAILED DESCRIPTION
FIG. 1 illustrates by way of example a circuit comprising a detector, constituted in this embodiment by two silicon PIN photodiodes 1 and 3, together with other circuit elements forming a complete meter. In this particular embodiment of the invention the other elements comprise a capacitor 4 in series with the diode 1, a preamplifier 5 which is arranged to receive the combined output of the diodes 1 and 3, an amplifier 6, a threshold detector 7, and a counter and timer 8, the elements 5 to 8 constituting, in this embodiment, a means for indicating the rate of pulses which exceed a threshold. The counter may be coupled to a processing circuit 9, which in known manner may produce denoting the dose rate and the total dose and provide an alarm signal if either the dose rate or the dose exceeds a threshold. These signals may control respective sections of a display device 10 (such as a liquid crystal display).
Considering first the diode 1, which is preferably a silicon photodiode having an intrinsic layer between appropriately doped P and N regions, it will be understood that photon radiation incident on the diode can generate energetic electrons from photoelectric and Compton interactions with the silicon crystal lattice. Such electrons then lose energy by interaction with the crystal lattice and can generate electron-hole pairs within the intrinsic layer. The electrons thus generated are separated by the inherent field within the device to form a charge pulse which can be amplified by the preamplifier 5 and the amplifier 6. In this embodiment of the invention, if the charge pulse is in excess of said threshold as determined by the threshold detector 7, the pulse may be counted.
Although a device employing only a single PIN diode in a self-biassing mode has some utility, it exhibits a response which is substantially non-uniform with variation in radiation energy and it is preferably to provide compensation by means of at least one further diode in order to obtain a combined output which is substantially uniform with variation of radiation energy. For this purpose the diode 1, which may be a BPX61 PIN diode, is provided with means for attenuating incident radiation, preferably constituted by a brass cap, typically of the order of 2.5 millimeters thick. The c
REFERENCES:
patent: 4110621 (1978-08-01), Horn
Article: "Silicon diode dosimetry", Dixon et al., Int. J. Applied Radiation and Isotopes, vol. 33, 1982 (Oxford, GB), pp. 1171-1176.
Article: "A silicon-diode pocket radiation chirper", Nowotny, Health Physics, vol. 44, Feb. 1983, pp. 158-160.
Hannaher Constantine
Howell Janice A.
National Radiological Protection Board
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