GB2090054A - Radiation detector - Google Patents

Abstract

A thermistor 1 with a positive temperature coefficient (PTC) of resistance acts as a radiation detector and is provided with means for maintaining its temperature within a specified range. Its enclosure 15 may include a second PTC thermistor which acts as a self regulating heater to maintain the detector element within said range. The figure shows two such thermistors 11, 12, one of which has an aperture 13 to allow the input radiation to reach the sensing thermistor 1, which is located between 11 and 12. <IMAGE>

Description

SPECIFICATION Radiation detector This invention relates to a radiation detector comprising an element of a thermistor ceramic material in an enclosure, the enclosure having an opening which is located such that radiation from an external source may fall on the element and raise the temperature thereof, the element being provided with electrodes for enabling the electrical resistance of said element to be measured as an indication of the intensity of said radiation.

Radiation detectors for detecting microwave or infrared radiation with a wide spectral bandwidth and which use negative temperature coefficient (NTC) of resistance thermistor materials for the element have been available for some time. The sensitivity of these detectors is partly dependent on the thermistor temperature coefficient (fez) of the material, where a is the percentage change of resistance of the material per degree Centigrade temperature change. For a NTC thermistor material, a can have a maximum value of about 5%/at. The fact that this is a relatively low number means that NTCbased radiation detectors, in general, are not highly sensitive to the effect of the radiation incident on the detector element.In addition, unless special means are taken to stabilise an NTC-based radiation detector against changes in the ambient temperature, these changes can have a detrimental effect on the accuracy of the detector output measurement.

It is an object of the present invention to provide a radiation detector which has an increased sensitivity to radiation incident on the detector element and in which the effect of ambient temperature changes on the element is reduced.

According to the invention, there is provided a radiation detector of the type defined in the opening paragraph hereof, characterised in that the element material has a positive temperature coefficient of resistance and its temperature/resistance curve has a maximum slope portion which is substantially linear over a given temperature range, the slope of which portion is such that the resistance changes by at least 8% for each degree Centrigrade change in temperature and in that the detector further includes means for maintaining the element at a temperature within said range.

The manufacture of bodies of PTC thermistor materials is disclosed in Patent Specification No.

1 1 71 ,756. The resistance-temperature characteristic curve of the PTC material shows a steep slope over a particular temperature range where the value of a can be very high. At temperatures up to 1 300C, cr-values greater than 75OC can be obtained. A radiation detector constructed to make use of this effect can thus be extremely sensitive.

The provision of the temperature control means ensures that the element operates on a part of the resistance-temperature characteristic curve of the material where the temperature coefficient will be high so that an optimum sensitivity for the device may be obtained. The temperature control means redices the effect of ambient temperature changes. The resulting increases in stability of the detector allow a greater accuracy in the detector output measurement.

Preferably, the value of a is at least 75%/ C.

In one embodiment, the detector is characterised in that the thermistor element is provided in the form of a flake, a flake being defined as a plate whose thickness is small in comparison with its length and breadth. This provision permits the detector to have a rapid response time since the quantity of radiation required to raise the temperature of the flake can be very small.

In another embodiment, the detector is characterised in that the temperature control means comprises a body of a second thermistor material having a positive temperature coefficient of resistance, which acts as a self-regulating heater when connected to a voltage supply, the temperature/resistance characteristic curve of the second thermistor material being selected to maintain the temperature of said element within said temperature range at a given voltage supply, the said body also forming at least part of the enclosure. The second thermistor material body comprising the temperature control means may be of the same PTC material as that used for the element, or the PTC materials may be of different kinds.With the temperature control means constructed in this way, the detector can be made in a compact form and at the same time the construction process can be effected in a very simple manner.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 is a graph showing the resistancetemperature characteristic of one PTC ceramic element material, Figure 2 is a plan view of a flake of the element material located in a mounting ring, Figure 3 is a side view partly in cross-section of the mounting ring supported on a base, and Figure 4 is a partial view similar to Figure 3 showing heater bodies carried an the mounting ring and an outer envelope located on the base.

The graph shown in Figure 1 gives the resistance R (ohms) on the vertical axis and temperature T (degrees Centigrade) on the horizontal axis for a flake of a particular PTC thermistor ceramic element material. The slope of the curve gives the thermistor temperature coefficient a. The vertical dotted line on the graph marks the centre of a portion of the curve where the slope remains substantially constant over a given temperature range. In this embodiment, the position of the dotted line corresponds to a temperature of 1 22.50C. For the particular ceramic material used, the temperature coefficient has a maximum slope (a:=75%/ C) within a temperature range of 122.5 + 2.50C.

This temperature range therefore is that over which a radiation detector should work to give a maximum sensitivity of response to the incident radiation.

One construction of a radiation detector according to the invention employs an element flake which is cut from a cylindrical pellet measuring 10 millimetres diameter and millimetres thickness of PTC ceramic material.

Bodies of suitable ceramic material are from Mullard Ltd. as Type No.2322 661 91003.

In the bulk form, the ceramic material showed a thermistor temperature coefficient a of 75%/0C.

From experience, it appears that even higher a- values can be achieved with extremely thin slices (flakes) of the thermister material. The reason for this is not known, but it may be due to a spread of the characteristics ot the material which can occur at intergrain boundries coupled with the fact that, with a thin slice, the number of intergrain boundaries likely to affect this value have been reduced very considerably. From a large number of prepared slices, a selection can be made to find slices having the highest a!-values.

For the present embodiment, flakes having the dimensions one millimetre square and 15 Hm thick were prepared and a selection was made to find those with the highest a!-values such as 100% or higher. The chosen flake 1 as shown in Figure 2 is provided with nickel electrodes and mounted on a membrane 2 of about 1 micron thickness which is carried on a membrane support ring 3 made of glass. The membrane 2 provides a support for the flake which support has a relatively low thermal conductance in order to minimise heat loss from the flake via the support.

The membrane 2 was prepared from a polyimide plastics film sufficiently large in area to make several detectors according to a method similar to that described in U.K. Patent Specification 1,508,299. The preparation took place initially on a circular glass microscope slide measuring 40 millimetres in diameter and 0.1 millimetre thick. Three volumes of PYRE-M.L.

(Trade Mark, Du Pont Co.) wire enamel type No.

R.C. 5044 were diluted with one volume of N methyl-2-pyrrolidone; a small quantity of this solution was placed on the cleaned glass slide, which was then spum at 4000 r.p.m. for two minutes, initially at ambient temperature and then under a lamp to dry the resulting film. The film was cured by baking at a temperature of 4009C for about an hour in oxygen-free nitrogen; the cured film had a thickness of about 0.85 micron. A copper washer was adhesively secured to the free surface of the film with an epoxy resin adhesive.

When the adhesive had set, the glass slide was carefully dissolved away with hydrofluoric acid, leaving the film held tautly on the washer.

A number of glass rings were then secured to the plastics film with adhesive. The rings had an outer diameter of 3.5 millimetres and an inner diameter of 3 millimetres. The adhesive used was again a solution of PYRE-M.L., one volume of wire enamel in this case being diluted with three volumes of solvent. The adhesive was lightly cured at about 1 000C for a quarter of an hour; the rings were then cut free from the large film around their outer peripheries, and the adhesive is finally fully cured at 4000C for about an hour in oxygen-free nitrogen, leaving each ring with a film held tautly thereon.

A glass membrane support ring 3 carrying the thin membrane 2 is then prepared for electrical connection of the flake thereto. The membrane 2 is provided with two evaporated gold electrode areas 4 and these became electrically connected by the flake in the mounting operation.

The electrode areas 4 are next secured to connection wires 5 which are joined to connection strips 6 carried on a mounting ring 7. The membrane support ring 3 is thus suspended on the connection wires 5 within the mounting ring 7.

As shown in Figure 3, the mounting ring 7 is next connected to a four-pin base 8 and further connection wires 9 are used to attach the connection strips 6 to respective pins 10 (only one of which is shown) of the base. The further connection wires 9 are copper strips 250 microns thick and one millimetre wide and two of these were sufficient to provide the sole mounting means for supporting the mounting ring 7 on the base 8.

It would be possible at this point, to place the assembly of Figure 3 in a component oven having an opening to admit the radiation to be detected, temperature control means and a power supply to provide a fully effective radiation detector.

However, the invention also provides a device construction having an integral heater and temperature control means. The construction of a detector with integral heater will now be described.

The construction of heater bodies for the detector begins with the preparation of a second PTC thermistor material which in this embodiment is chosen to have slightly different characteristics to those required for the flake 1 of the device. For the heater material, the thermistor temperature coefficient (a) was chosen to be about 1 0 /O/ C.

The thermistor material is usually a ceramic composition based particularly on barium titanate and by the addition of strontium to the composition the properties may be varied.

The selected composition of barium carbonate, strontium and titanium oxides and other materials depending on the required electrical characteristics are milled, mixed and pressed into a suitable form. After drying, the resulting bodies are sintered at a very high temperature, for example 140000, to form the required ceramic.

Alternatively, bodies of the already manufactured ceramic are available from Mullard Ltd. The ceramic composition used for the embodiment about to be described is identified as Type No. 2322 66491086.

In one practical embodiment, the bodies of ceramic material intended to form heater bodies for the detectors are machined to the shape of discs having a diameter of 12 millimetres and a thickness of 0.5 millimetres. To provide heater electrodes on the ceramic discs, an electroless nickel deposit is laid down on the two major surfaces of each disc. The deposits are subsequently made into electrical contacts with low resistance and non-rectifying properties by baking the discs at 4000C in a nitrogen atmosphere. An electrically conductive silver epoxy resin cement was also used to provide contact pads on the outer edges of each disc for external connection wires. In order eventually to provide an opening for the passage of incident radiation, some of the heater bodies are formed with a central hole.

Figure 4 is a view similar to that of Figure 3 showing the mounting ring 7 attached to the base 8. For the sake of clarity in the drawing, the membrane support ring and its connection wires have been omitted from this view. To the left hand side of the mounting ring 7, as viewed in the Figure, a heater body 11 is attached and to the right hand side of the ring 7 a heater body 12 having a central hole 1 3 is attached. Contact pads on the heater bodies are joined by connection wires 14 to pins 10 of the base 8. After testing the assembly, the unit is sealed in a vacuum within an outer envelope 1 5 having a window 1 6. The window 1 6 is made of a plate of germanium which is transparent to radiation of the wavelength which this embodiment of the detector is intended to measure.The outer surface of the window 1 6 was bloomed to reduce losses by reflection in the transmission of radiation therethrough.

The dimensions of the detector in the outer enveloper 1 5 and excluding the protruding pins 10 of the base 8, were: height 1 9 millimetres, width 1 8 millimetres and depth 7.5 millimetres.

In operation of the radiation detector, when a voltage from a power supply unit is applied across the heater body electrodes this causes selfheating of the heater body material. By use of the positive temperature coefficient ceramic material for the heater bodies, the heater material has a self-regulating behaviour which causes a rapid increase in the electrical resistance of the heater material if the temperature of this material should rise above some predetermined temperature. With the construction described, the heater bodies reach the required stabilisation temperature of 122.50C when a potential of 3.90 volts is applied across the heater electrodes. This voltage results in a mean power consumption of 760 mW thorugh the heater bodies when the room temperature is 250C.The power supply unit for energising the heater bodies is thus arranged to be stabilised to have an output voltage of 3.90 volts +0.02 volts.

If, with the element temperature stabilised at 1 22.50C the ambient temperature should rise, the thermistor material forming the heater bodies will tend to become heated above 122.50C and thus the electrical resistance of this material will increase. As already explained, the heater body thermistor material has a positive temperature coefficient a giving a 10% increase in the resistance of the material for a 1 OC temperature rise. Since the voltage across the heater bodies is fixed at 2.25 volts, an increase in the electrical resistance of the heater bodies will cause a reduction in the current taken and thus the thermal energy dissipated by the heater bodies will decrease such that the element temperature is maintained at 122.500.

Similarly a reduction of the ambient temperature will cause a reduction in the electrical resistance of the heater bodies and a greater current will flow causing an increase in the quantitiy of thermal energy dissipated by the heater bodies.

Changes of ambient temperature thus cause a corresponding change in the resistance of the thermistor material so that the heater body output will automatically be adjusted in order to keep the temperature of the bodies, the space between them and the flake 1, stable at the required 122.500.

In this way, with the heater bodies 1 2 maintaining the temperature of the flake 1 at 122.50C, the temperature that will give a maximum sensitivity of response to radiation incident on the flake, the detector may be set up with the window 1 6 facing the radiation source to be measured. The resulting radiation, falling on the flake will cause an increase in its temperature and thus a substantial increase in the electrical resistance across the flake. This electrical resistance change may be measured with a convention resistance bridge circuit to give an indication of the intensity of the incident radiation.

The detector was found in practice to have a performance comparable with that of a pyroelectric radiation detector but there was, of course, no need to provide a shutter to interrupt a steady flow of the incident radiation. The device of the invention thus has the advantage of providing a continuous measurement of the intensity of the incident radiation.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, instead of using a window in the outer envelope of germanium, a silicon window might be used. A germanium window is transparent to radiation having wavelengths between 1.8 and 23.0 microns these limits being considered to define a range outside which the transmittance of the window is reduced to 10% of the original transmittance. A silicon window is not very different, being transparent to radiation between 1.2 and 15.0 microns in wavelength.

Claims (9)

1. A radiation detector comprising an element of a thermistor ceramic material in an enclosure, the enclosure having an opening which is located such that radiation from an external source may fall on the element and raise the temperature thereof, the element being provided with electrodes for enabling the electrical resistance of said element to be measured as an indication of the intensity of said radiation, characterised in that the element material has a positive temperature coefficient of resistance and its temperature/resistance curve has a maximum slope portion which is substantially linear over a given temperature range, the slope of which portion is such that the resistance changes by at least 8% for each degree Centigrade change in temperature, and in that the detector further includes means for maintaining the element at a temperature within said range.

2. A detector as claimed in Claim 1, characterised in that the said slope is at least 75%/ C.

3. A detector as claimed in Claim 1 or 2, characterised in that the element has the form of a flake of said material.

4. A detector as claimed in any one of Claims 1 to 3, characterised in that the element is supported on a membrane having a relatively low thermal conductance.

5. A detector as claimed in any one of Claims 1 to 4, characterised in that said temperature control means comprises at least one body of a second thermistor material having a positive temperature coefficient of resistance which acts as a self-regulating heater when connected to a voltage supply, the temperature/resistance characteristic curve of the second thermistor material being selected to maintain the temperature of said element within said temperature range at a given voltage supply, the said body also forming at least part of the enclosure.

6. A detector as claimed in Claim 5, characterised in that the said temperature control means comprises two said bodies each forming a respective confronting wall of the enclosure.

7. A detector as claimed in any one of Claims 1 to 6, characterised in that it includes an outer envelope having a window which is transparent to the radiation intended to be measured and which is aligned with said opening.

8. A detector as claimed in Claim 7, characterised in that said window comprises a plate of germanium or silicon.

9. A radiation detector substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawing.