Patent GB-1279400-A (GB)
Title: Improvements in or relating to nuclear explosion detection systems
Inventor: FOTHERGILL REGINALD ALLAN
IPC: G21J5/00
Filed: 1968-12-10
Abstract: 1279400 Nuclear explosion detection system UNITED KINGDOM ATOMIC ENERGY AUTHORITY 1 Dec 1969 [10 Dec 1968] 58686/68 Heading G1A [Also in Division G4] A nuclear explosion is detected by a system including means receiving the radiated transient optical signal and means determining whether it satisfies selected criteria. Analysis of the radiated transient radio frequency signal also takes place in the detection operation. Information as to the yield of the explosion, time of occurrence of alarms, for up to three explosions, is also provided. Transient signal detectors. The R.F. signal is detected by a rod aerial AEI and the optical signal by two arrays SOI, SO 2  of photocells similar to those described in Patent No. 1165331. R.F. processing channel. The immediate detector circuit is designed to deal with a positive or a negative R.F. pulse, either of which is possible. If the rate of rise of the pulse is above a certain level (5V/m/per sec), as determined by differentiator A 3  and level detectors D2, D3, a pulse is produced from NAND gate N1, the differentiated trailing edge of the detected pulse being removed in circuit CG1. If the pulse from N1 is less than 2À5Ás, as determined by I1-N2, a pulse delayed by monostable M3, is delivered to NAND gate N3. If the amplitude of the detected RF pulse uses above 3V/ metre, as detected by D1, monostable M1 also delivers a pulse to NAND gate N3. This results in a 200 Ás "RF criteria pulse" from M4. Optical processing channels. The optical signal has two peaks, T1 MAX and T2MAX, Fig.2 The fast-rising portion to T1MAX being processed by the 01 channel, which also provides an 01 interior pulse for the 02 channel, which is arranged to determine whether predetermined requirements for the slow-rising portion TMIN to T2 MAX are present in the signal from detector array SO2. 01 channel. The signal from SO1 is differentiated by transformer T1, and if the resultant pulse is above a predetermined level set by Schmitt trigger D4, it is tested for coincidence with the R.F. criteria pulse by NAND gate N4-or occurrence within 200 Ásec of the start of the R.F. pulse as provided by M4. The duration of a pulse from N4 is measured by counter C1 having associated NAND gates N5-N12 and inverters I2-I4. The pulse from N4 starts C1 counting up 10 Ásec clock pulses and continues until (a) the end of the 01 pulse or (b) a count representing a >6À3 Ásec duration has been attained. If the latter case, the circuit also determines through level detector D5, whether the 01 pulse represents a high level signal above a predetermined level. If so the mode of operation reverts to condition (a). If not, the counter is reset by F2 through NII and the circuit is returned to the "wait" condition. Condition (a) operation comprises the counter being reversed to count down 2 Á sec clock pulses, thereby producing an 01 criteria pulse representing [200 x (length of 01 pulse) + 20 Á sec]. The 20 Ásec is intro duced, by means of the set 10 facility in C1, to avoid loss of genuine alarms. 02 channel. The signal from detector array SO2 passes through similar transformer and level circuits as in the 01 channel and is tested for coincidence with the 01 criteria pulse by NAND gate N22, but only after a minimum delay of 1 Ásec after the start of the 01 criteria pulse (or end of the 01 pulse). This minimum delay is provided by flip-flop F3 which is set after the counter C2 has accumulated 5 200 Ásec clock pulses (it having been started by the commencement of the 01 criteria pulse). The counter C2 thus counts 200 Ásec clock pulses until the front edge of the 02 pulse occurs and is passed by NAND gate N22 to stop and reverse the counter, which proceeds to count down 600 Ásec clock pulses, to zero and thereupon produce an alarm pulse to set alarm flip flop F5. If however, the end of the 02 pulse occurs before the counter reaches zero, the circuit is reset to the wait condition and there is no alarm pulse. Operation of alarm F5 indicates that all criteria have been met and that the signals are from a nuclear explosion. Yield measurement. Yield in kilotons is calculated from W = 0À349 T<SP>2À25</SP> where T = time between start of 01 pulse to start of 02 hold pulse from F4, and the measurement is made by NAND gates N22, N23 and binary coded decimal counter C3. The counter C3 holds yield information a short time, after which it is transferred to memory ME1, under control of counter C4, decodes DE2, DE3, flip-flop F6 and gate N24 which receives each alarm pulse from F4. The memory ME1 may store up to three yield measurements, each alarm being indicated as is the number of alarms occurring (ND1), and the fact that stores are full (SF). AV1 gives an audible and visual alarm for each detection event by means of F1, C6 and I16. Readout of the stores is at ND2. Time of alarms may be recorded by equipment C6 - ND3. Modifications and alternative embodiments. Different parts of the optical signal may be compared from those parts used in the arrangement described in detail. For example, the interval TIMAX to TMIN may be checked in the 01 channel which is suitably modified. This check may be additional to those of the main embodiment or substitute the 02 check the channel for which is dispensed with. Pulse durations may be measured by charging and discharging capacitors instead of by the use of reversible counters.