GB2198878A - Radiation detector - Google Patents

Abstract

A radiation detector, particularly a pyroelectric radiation detector, in which the radiation receiving surface of an element 11 of radiation sensitive material is provided with a radiation absorbing electrode 17. The radiation absorbing electrode 17 has cut away portions coincident with grooves in the subjacent radiation sensitive material. The grooves have radiation absorbing side walls 27 of depth within an order of magnitude of the wavelength of a radiation to be detected and the groove width is similarly dimensioned. <IMAGE>

Description

RADIATION DETECTOR This invention relates to radiation detectors, particularly thermal radiation detectors such as pyroelectric detectors. The invention is equally applicable to other thermal detectors such as those of the dielectric bolometer type.

Figure 1 of the accompanying drawing diagrammatically illustrates a known pyroelectric detector. The detector comprises an element 10 of a pyroelectric material. The element 10 is metallised on opposed faces 12,14 to provide electrodes to which wires 16,18 may be attached. The electrode on the face 12 may be treated or specially constructed so as to act as an absorber of radiation to be detected. The element 10 may be part -' a sa of similar material but of larger area. The sla3 may comprise a plurality of the elements 10 arranged as an array, the elements 10 being separated from one another by unmetallised regions or by partial or complete grooves or cuts.

The collecting area of each of such elements is equivalent to that of its absorbent metallised face 12.

Upon the absorption of radiation by the face 12 of an element, and a consequent increase in the temperature of the subjacent pyroelectric material, a voltage signal is obtained from the electrodes which voltage is dependent upon the ratio of the collecting area (area 12) to the capacitance of the element 10 (between the metallised faces 12,14).

Noise generated is approximately proportional to (capacitance)~ 2 . The signal-to-noise ratio is then proportional to (collecting area)/(capacitance) 2 .

A known improvement of the above-described detector is shown in Figure 2 of the accompanying drawing. The element lOa has "extended area". The metallised front (absorbent) surface 12a, is larger than that of the underlying pyroelectric material, and the other face 14a.

The face 12a is arranged to transfer energy, by conduction, absorbed over the whole of its area, to the subjacent pyroelectric material.

The collecting area of the element shown in Figure 2 is the same as that shown in Figure 1 but tne area of the subjacent pyroelectric material and of the opposite metallised face is less so that the capacitance is decreased and the ratio (collecting area)/(capacitance) is increased. An improvement of factor (old capacitance/new capacitance)1/2 is obtained.

Although the element of Figure 2 offers improvements over thct of Figure 1 insofar as electrical output is concerned, it is difficult to fabricate and hence relatively costly.

It is an object of the present invention, to provide an improved radiation detector wherein the electrical signal obtainable therefrom is enhanced.

According to the present invention, there is provided a radiation detector comprising an element of a material responsive to received radiation to provide a change in an electrical property thereof, the element having a radiation receiving first surface and a second surface, respective first and second electrode overlying and substantially coextensive with the first and second surfaces, parts of the first electrode being cut away so as to have a reduced area relative to that of the first s=ce, edges cf the first electrode coir.idins with absorption surfaces, perpendicular to the first surface, in the material and of depth within an order of magnitude of the wavelength df a radiation, to which the material is responsive, to be detected.

Preferably, the or at least one of the cut away portions in the first electrode has two edges spaced apart a distance within an order of magnitude of the wavelength of a radiation to be detected. The spaced apart edges may be substantially parallel.

The spaced apart edges may be longitudinal edges of an aperture or a respective aperture constituting a cut away portion of the first electrode.

The first electrode may have a plurality of parallel slots, each of width within an order of magnitude of the wavelength of a radiation to be detected, the slots constituting the cut away portions, and wherein the element is grooved or apertured coincident with the slots, to provide the absorption surfaces of depth within an order of magnitude of the wavelength of a radiation to be detected.

The element may be formed of a modified lead zirconate and the radiation to be detected may be infra-red radiation.

Tne surface of the apertured first electrode may be constructed to act as an absorber of to radiation to be detected. The thickness of the element may be witl a order of magnitude of the wavelength of a radiation to Dc detected.

This invention will be described further, by way of example, with reference to Figure 3 of the accompanying drawing which is a diagrammatic representation of a preferred radiation detector according to the present invention.

The radiation detector shown in Figure 3 is constructed for the detection of infra-red radiation.

It comprises an element 11 of a material which provides a change in an electrical property thereof upon a change occurring in its temperature. A suitable material is a modified lead zirconate such as the material described in our copending UK Patent Application No. 8506805. The element 11 is in the form of a rectangular plate having two opposed major surfaces 13 and 15. The first surface 13 is arranged to receive energy from radiation incident thereon, or more particularly, from radiation incident upon the surface of a first electrode 17 formed by metallisation of the surface 13. The first electrode 17 has its radiation receiving surface treated so as to be absorbent of incident radiation such as by blackening in a well known manner. The second surface 15 is also metallised to provide a second electrode 19 thereon.

wires 21 and 23 serve tO connect the electrodes 17 and 19 to ancillary electrical apparatus (not shown).

Portions of the first electrode 17 are cut away.

This may be effected, for example, by ion beam milling.

Additionally, the subjacent element 11 is grooved preferably in the same ion beam milling operation.

The edges 25 of the cut away portions of the first electrode 17 are thereby made coincident with absorption surfaces 27 perpendicular to the first surface 13, within the thickness of the element 11. The depth of the grooves, and hence of the absorption surfaces 27, are arranged to be within an order of magnitude of the wavelength of a radiation to be detected. For example, a depth of 20 micrometres is within an order of magnitude of the wavelength of substantially the whole of the spectrum of infra-red wavelengths. Similarly, the slots made in the first electrode by the ion beam milling operation, and the subjacent grooves are made so as to have a width within an order of magnitude of the wavelength of a radiation to be detected. A width of 20 micrometres may be chosen if infra-red radiation is to be detected. The absorption of incident radiation by the absorption surfaces 27 is believed to be due to diffraction of the incident radiation at the edges of the cut away portions of the first electrode 17. The incident radiation transfers its enemy by absorption on the resonent portion of the blackened first electrode 17 (and thence by conduction to the subjacent material of the element 11) and by absorption by the surfaces 27 and the grooves in the material. Although the material of the element 11 is normally less absorbent of incident radiation than the blackened surface of the first electrode 17, by providing te grooves and the absorption surfaces 27,an increased absorption is obtained.

Further, and as stated above, the effective collecting area and hence the consequent change in the electrical property of the material, for a given amount of incident radiation, is maintained whilst the capacitance, provided by the first and second electrodes 17 and 19, is reduced due to the reduced area of the first electrode 17.

An improved collecting area to capacitance ratio is obtained and an improved signal to noise ratio is also obtained.

If the area of the first electrode 17 was to be halved by the ion beam milling operation, a resultant improvement of 22 is theoretically possible. In practice, a number of secondary effects reduce the magnitude of this theoretical improvement but a 20% improvement in response is readily obtainable.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto. For example, the detector may be one of a plurality of similar detectors forming a linear or a two dimensional array of detectors. If grooves of appropriate dimensions separate adjacent elements, then the walls 29 of such grooves provide further absorption surfaces for the elements of the array.

The invention has been described with particular reference to pyroelectric detectors arranged to detect radiation in the infra-red. Other radiation detectors for the same or different parts of the radiation spectrum may be formed in a similar manner to utilise the present invention.

Ion beam milling has been described as a preferred manner of forming the cut away portions of the first electrode and the grooves, but other methods, such as etching, could readily be employed.

Claims (9)

CLAIMS:

1. A radiation detector comprising an element of a material- responsive to received radiation to provide a change in an electrical property thereof, the element having a radiation receiving first surface and a second surface, respective first and second electrodes overlying and substantially coextensive with the first and second surfaces, parts of the first electrode being cut away so as to have a reduced area relative to that of the first surface, edges of the first electrode coinciding with absorption surfaces, perpendicular to the first surface, in the material and of depth within an order of magnitude of the wavelength of a radiation, to which the material is responsive, to be detected.

2. A detector as claimed in Claim 1, wherein the or at least one of the cut away portions in the first electrode has two edges spaced apart a distance within an order of magnitude of the wavelength of a radiation to be detected.

3. A detector as claimed in Claim 2, wherein the spaced apart edges are substantially parallel.

4. A detector as claimed in Claim 2 or Claim 3, wherein the spaced apart edges are longitudinal edges of an aperture or a respective aperture constituting a cut away portion of the first electrode.

5. A detector as claimed in any one of Claims 1 to 4, wherein the first electrode has a plurality of parallel slots, each of width within an order of magnitude of the wavelength of a radiation to be detected, the slots constituting the cut away portions, and wherein the element is grooved or apertured coincident with the slots, to provide the absorption surfaces of depth within an order of magnitude of the wavelength of a radiation to be detected.

6. A detector as claimed in any preceding claim, wherein the element is formed of a modified lead zirconate an the radiation to be detected is infra-red radiation.

7. A detector as claimed in any preceding claim, wherein the surface of the apertured first electrode is constructed to act as an absorber of the radiation to be detected.

8. A detector as claimed in any preceding claim, wherein the thickness of the element is within an order of magnitude of the wavelength of a radiation to be detected.

9. A radiation detector, substantially as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying drawing.