Detection of electromagnetic radiation

Abstract: A system for detecting UV radiation has a cold cathode discharge tube 10 which produces a pulse on a line 18 each time it avalanches. Logic 20 steers these pulses successively to separate timers 22,24,26,28,30,22,24,.. (and so on) in that order. In response to each pulse, each timer sends its own output line HIGH for a preset period, the same for all the timers. An AND gate 32 receives all the timer outputs and produces an alarm signal when all these are HIGH at the same time.

Description: The invention relates to a method of a method of detecting incident radiation using detecting means for that radiation whose response to it is at least in part random but with a predictable probability and which requires to be energised to produce such response and to be de-energised after each response before being re-energised for the next response, comprising the steps of detecting consecutive responses of the detecting means and defining a respective time period starting at the instant of each such response, de-energising the detecting means immediately after each response, re-energising the detecting means after de-energising it, and producing an output indication when a predetermined plurality of responses occur within a predetermined time duration. The invention also relates to a system for detecting radiation using detecting means for that radiation whose response to it is at least in part random but with a predictable probability, including energising means for energising the detecting means to enable it to produce such response, de-energising means for de-energising the detecting means after each said response to enable it to be re-energised by the energising means, and output means connected to react to each response of the detecting means and to define a respective time period starting at the instant of each such response, whereby to produce an output indication when a predetermined plurality of responses of the detecting means occur within a predetermined time duration. Such a method and such a system are shown in GB-PS-1 515 116. In the prior method and system, the detecting means is a cold cathode gas discharge tube to which is applied a predetermined rectangular-shaped voltage waveform defining consecutive energisation periods, each of predetermined fixed duration, during which the gas discharge tube is energised (so as to be capable of discharging in response to incident of radiation), these periods being separated by de-energisation periods of predetermined duration in which the voltage across the tube is reduced to a de-energising level. A monitoring circuit produces a warning output when the tube discharges during each of a predetermined plurality of consecutive energisation periods. The method and system are arranged such that the tube can only discharge once during each energisation period. Therefore, even if, for example, the tube discharges very near the beginning of a particular energisation period, it is impossible for the tube to discharge again until the whole of the remainder of that energisation period has elapsed and so has the immediately following de-energisation period - and this is so even if the level of incident radiation on the tube is such that the tube would discharge if suitably energised. Therefore, there is a minimum effective overall response time for the system, however great the level of the incident radiation. The prior method and system may therefore not produce a sufficiently rapid response to high levels of incident radiation. This is the problem which the invention aims to overcome. Accordingly, the prior method is characterised by the step of re-energising the detecting means immediately following and in response to each de-energising thereof, and in that the length of each said time period is the same as the said predetermined time duration. Furthermore, the prior system is characterised in that the energising means re-energises the detecting means immediately following and in response to each de-energising thereof, and in that the length of each said time period is the same as the said predetermined time duration. UV radiation detection arrangements embodying the invention, and methods of UV radiation detection according to the invention, will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block circuit diagram of one of the arrangements; and Figures 2 and 3 show waveforms for explaining the operation of the arrangement of Figure 1. The arrangements to be described are for use in detecting UV radiation and use a cold cathode gas-discharge tube as the radiation detector. Such a tube has an anode and a cathode across which a suitable potential difference is applied. Provided that this potential difference is sufficient (that is, provided that it is at least equal to the "striking voltage", V s ) then, in the presence of incident radiation of the correct wavelength and after a time lag, t s , the tube will "avalanche", that is, a gas discharge will occur. Such discharge occurs by virtue of the fact that electrons emitted from the cathode by the incident UV radiation trigger a pulse of current in the tube. The time lag t s , which may be several minutes, is referred to as the "statistical time lag" and depends (for the particular tube) on the intensity and wavelength bandwidth of the incident radiation. In response to a steady level of radiation, it can be shown that the time delay (the statistical time lag) between the application of the radiation and the occurrence of the current pulse conforms to Poisson statistics. The statistical nature of the process is due to the statistical fluctuations in the physical processes of emission and ionisation in the tubes. The striking voltage V s can be defined as that voltage at which the probability of the tube avalanching to produce a current greater than one microamp (due to the release of the electrons from the cathode and the subsequent ionisation of the gas under the field effect) goes from zero to a finite value. In other words, when the probability has such a finite value, it follows that if a voltage V is added to V s , where V is very small, then, after time t s a gas discharge will occur. As V is increased, the probability increases and consequently t s falls (for a given UV radiation stimulus). In normal operation, such a tube is operated by switching the applied potential difference off (or reducing it well below V b , the "burning voltage") immediately after each current pulse has been produced and detected, so as to allow the tube to restore itself to its initial conditions. This requires a finite recovery time. After this a voltage at lease as great as and preferably greater than the striking voltage V s is re-applied, so as to produce a fresh current pulse after the statistical time lag. V b , the burning voltage, is the voltage which is at least necessary across the tube to maintain it conducting once it has started to conduct. The arrangements to be described take account of the statistical nature of the response of the tube. As shown in Figure 1, the cold cathode gas discharge tube 10 is connected across a suitable HT power supply 11 via a resistor 12 and a resistor 14, and in parallel with a capacitor 16. Assuming that the tube 10 is not conducting, capacitor 16 is charged through resistor 12 to a voltage exceeding V s . If the tube 10 now discharges (that is, a current pulse occurs) due to UV radiation in the manner explained, the conduction of tube 10 discharges capacitor 16 through resistor 14 faster than it can be recharged by the supply 11 through resistor 12. This discharge of capacitor 16 causes the potential across the tube 10 to fall. When it falls below the burning voltage (V b ) of the tube 10, the tube ceases to conduct. The current pulse produced by conduction of the tube produces an output voltage pulse across resistor 14 which is fed on a line 18 to the remainder of the circuit, to be described below. When the tube 10 ceases to conduct, capacitor 16 starts to recharge with a time constant RC, where R is the resistance of resistor 12 and capacitance C is the capacitance of capacitance 16. The tube cannot re-conduct before the voltage across capacitor 16 has once more exceeded V s . The time taken for capacitor 16 to recharge from V b to V s determines the recovery time which the tube 10 is allowed. Line 18 is connected to the input of steering logic 20 having five outputs, "0", "1", "2", "3", and "4". The steering logic 20 responds to the pulses received on line 18 by outputting them on the five output lines in the sequential order "0","1","2","3","4","0","1","2"... and so on. The five outputs of the steering logic 20 are respectively connected to five timer units 22,24,26,28 and 30 each of which has a respective output line 22A,24A,26A,28A and 30A. When each timer receives a pulse from the respective output line of the steering logic 20, it sends its own output line HIGH for a preset period, T g (which is the same for all the timers). The output lines of all the timers are connected to an AND gate 32. When all the inputs to the AND gate 32 are HIGH at the same time, it produces an alarm output on a line 34. The operation of the arrangement of Figure 1 will now be described with reference to Figures 2 and 3. Figure 2 illustrates the operation of the arrangement in a situation in which a sudden and relatively intense UV radiation-producing event occurs, such as an explosion. It is assumed that this produces five output pulses on line 18, in rapid succession as shown in waveform 2A. Due to statistical fluctuations, these five pulses will not occur at exactly regular intervals. In the manner explained, the steering logic 20 will direct the incoming pulses to respective ones of its five output lines, in sequence, and the five timers will thus be sequentially activated. The outputs on the output lines 22A,24A,26A,28A and 30A are illustrated respectively in waveforms 2B,2C,2D,2E and 2F. It will be apparent that at a time instant T1 the AND gate 32 is receiving five HIGH inputs and it therefore produces an alarm on its output line 34. Figure 3 illustrates the operation of the circuit where the intensity of the radiation received by the tube is increasing. Thus, for example, this illustrates the situation in which the distance between a source of radiation and the discharge tube is decreasing as for example occurs when a rocket or missile is approaching the detection source. As shown in waveform 3A, therefore, the time separation between the successive pulses occuring on line 18 decreases. Seven such pulses are shown as occurring in waveform 3A, and the steering logic 20 directs these to the "0","1","2","3",4","0" and "1" output lines, respectively, of the steering logic 20. Therefore, as illustrated in waveforms 3B to 3H respectively, the outputs of the corresponding timers go HIGH. As is apparent, however, the five inputs of the AND gate 32 are not all at HIGH until time T2. This is because it is not until time T2 that the pulses on line 18 are occurring sufficiently close together for five of them all to occur within a time T a , where T a T g . The first two pulses occurring in waveform 3A are therefore discounted. From the foregoing, it will be apparent that the circuit arrangement described initially waits for incident UV radiation of sufficient intensity to trigger the tube. If such triggering does occur, the circuit arrangement effectively establishes a time period T g and then detects whether four further pulses occur within that time period. Only if four such further pulses do occur within that time period is an alarm produced. However, the circuit arrangement re-starts the time measuring and pulse detection process in response to each arriving pulse and therefore produces an alarm output as soon as five consecutive pulses have occurred within a time period T g measured from the first of those five. The arrangement described and illustrated with reference to Figures 1 to 3 is advantageous over prior arrangements in which a predetermined sequence of fixed time periods is initiated in response to the first pulse sensed and a warning output is produced only when a pulse is also detected in each other one of the time periods. The arrangement described and illustrated with reference to Figures 1 to 3 is able to respond much more rapidly, particularly to high intensity radiation, because the successive time periods which it employs can overlap in the manner described whereas this is not possible with the prior arrangement.

Claims: A method of detecting incident radiation using detecting means (10) for that radiation whose response to it is at least in part random but with a predictable probability and which requires to be energised to produce such response and to be de-energised after each response before being re-energised for the next response, comprising the steps of detecting consecutive responses of the detecting means and defining a respective time period starting at the instant of each such response, de-energising the detecting means immediately after each response, re-energising the detecting means after de-energising it, and producing an output indication when a predetermined plurality of responses occur within a predetermined time duration, characterised by the step of re-energising the detecting means immediately following and in response to each de-energising thereof, and in that the length of each said time period is the same as the said predetermined time duration. A method according to claim 1, characterised in that the radiation is UV radiation and the detecting means is a cold cathode gas discharge tube (10), and in that each said response is the production of a current pulse therein as a result of discharge of the tube (10). A system for detecting radiation using detecting means (10) for that radiation whose response to it is at least in part random but with a predictable probability, including energising means (11,16) for energising the detecting means (10) to enable it to produce such response, de-energising means (16) for de-energising the detecting means (10) after each said response to enable it to be re-energised by the energising means (11,16), and output means (20 - 32) connected to react to each response of the detecting means (10) and to define a respective time period starting at the instant of each such response, whereby to produce an output indication (34) when a predetermined plurality of responses of the detecting means (10) occur within a predetermined time duration, characterised in that the energising means (11,16) re-energises the detecting means (10) immediately following and in response to each de-energising thereof, and in that the length of each said time period is the same as the said predetermined time duration. A system according to claim 3, characterised in that the radiation is UV radiation and the detecting means is a cold cathode gas discharge tube (10), in that each said response is the production of a current pulse in the tube (10) as a result of discharge of the tube (10), and in that the output means includes timing means (22 - 30) operative to react to a response of the tube (10) to initiate the start of the said predetermined time duration, and in that the energising means (11,16) and the de-energising means (16) react to discharge of the tube (10) and operate independently of the timing means (20 - 30).

Inventors: BROWN ALFRED ROBERT, MACKRELL ANDREW DAVID, BROWN, ALFRED ROBERT, MACKRELL, ANDREW DAVID
Applicants: GRAVINER LTD KIDDE, KIDDE-GRAVINER LIMITED
Application year: 1987
Main classification: G08B17
IPC codes: G08B17, G01J1