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The use of neon flash tubes at high repetition rates
NUCLEAR
INSTRUMENTS
AND METHODS
IO0 (1972) 4 2 9 - 4 3 2 ;
© NORTH-HOLLAND
PUBLISHING
CO.
T H E USE OF N E O N F L A S H TUBES AT H I G H R E P E T I T I O N RATES F. W. HOLROYD and J. M. BREARE
Department of Physics, University of Durham, England Received 22 December 197l When flash tubes are used at high repetition rates their efficiency at long time delays between the passage of the ionizing particle and the application of the high voltage pulse decreases faster than theoretical predictions. An explanation of the phenomena is
proposed in terms of clearing fields resulting from charges deposited on the inside glass surface by previous discharges. Methods of avoiding the build up of such clearing fields are discussed.
1. I n t r o d u c t i o n
support is given to this argument and some possible methods of removing the clearing fields are investigated. Two types of flash tubes have become c o m m o n in experimental arrangements: low pressure tubes filled at 600 torr containing 98% neon and 2% helium and high pressure tubes filled at 2.3 atm with the same gas mixture. The inside diameter of the low pressure tubes is 1.6 cm and the glass wall thickness 0.1 cm. The high pressure tubes are of 0.65 cm inside diameter with wall thickness 0.1 cm. The glass used is type S.95 soda glass. Here, only tests carried out on the low pressure tubes are described, but it is known that similar effects occur in the smaller, high pressure tubes7).
Neon flash tubes have been used extensively in cosmic ray experiments and their characteristics have been studied by several groups1-3). In most experiments where cosmic rays have been used, the rate at which an individual flash tube is operated is generally very low even when the event rate in the apparatus is large, since detectors usually contain many tubes. Under condition of low rate of flashing the characteristics of the tubes are reproducible. However, if the rate is increased it is observed 4'5) that the detection efficiency of the tubes varied with the rate of flashing. A similar dependence of efficiency on repetition rate has been found in sealed current limited glass spark chambers6). It has been suggested that in both cases, electric fields, caused by charges deposited on the walls of the detectors, are responsible for these effects. In this paper, further I00'
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Using cosmic rays as the ionizing particles, the efficiency of the tubes was measured at a very low repetiaoO°lo
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Fig. 2. Variation of internal efficiency with ht pulse length. Time delays of zero and 50/~sec. Peak ht pulse height = 4 kV/cm.
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r . W. H O L R O Y D AND J. M. BREARE
tion rate (each tube only flashing about once per I I I 30 min). The efficiency as a function of time delay between the passage of the particle and the application of the ht pulse to the tubes, is shown in fig. 1. The ht pulse had an amplitude of 2.8 kV/cm and a rise time of approximately 50 nsec. It was derived from a conventional switched RC decay circuit and the decay time could be varied. It is seen in fig. 1 that the efficiency of the tubes was independent of the decay time, insofar as values of 0.4/~sec and 26.8/~sec give the same efficiency curve at low repetition rate. Also, the measured values fit very well to the theoretical curve derived by Lloyd 8) using his parameters f l = 0.3 and Q = 33.6 ion pairs/cm atmospheric pressure. Similar efficiency curves were obtained at a higher rate of flashing (approximately 1 flash per minute per tube). These points are also given in fig. 1 and show I I I I I that for a short pulse (CR = 0.4/~sec) the efficiency still o I 2 a 4 (p. ~,~) follows the predicted curve. However, for a pulse of Fig. 4. Profiles of photomultiplier pulses of light output from 40/tsec decay time, the efficiency falls more rapidly. flash tubes. (a) Particle passing through front of the tube. (b) ParThe dependence of efficiency on repetition rate and ticle passing through rear of the tube. Ht pulse amplitude 4 kV/cm. pulse length is further illustrated by fig. 2 and fig. 3. In Decay time 4/~sec. fig. 2 the efficiency as a function of the ht pulse length is plotted for the case of a 50 psec pulse delay. It is seen repetition rate was approximately 1 flash per tube per that the efficiency decreases as the pulse length is minute. Fig. 3 on the other hand shows the variation increased, until pulses have a characteristic time of of efficiency with repetition rate for a fixed high voltage 60/~sec. For longer pulses the efficiency remains pulse, namely amplitude 2.8 kV/cm and decay time constant. Also shown are points for zero delay and 26.8 psec. It is observed that an approximate interval of several pulse lengths: these points, as expected, are of 12 min between flashes is required to obtain an effihigh efficiency. For these tests the ht pulse had an ciency consistent with Lloyd's theory. This result is amplitude of 4 kV/cm a rise time of 70 nsec, and the similar to that of Ashton et al. 4) where a time of approximately 20 min between flashes was required to give full efficiency- however, Ashton et al. were triggering their array on cosmic ray showers where the initial ' I ~ I ' I ionization on the tubes may have been somewhat different from that of single cosmic ray particles. In all .-------O O--80 cases the deviations from Lloyd's theoretical curve are thought to be due to clearing fields caused by the previous discharges in the tube. ~ 6o
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Fig. 3. Variation of internal efficiency with average interval between flashes. Peak ht pulse height = 2.8 kV/cm. Decay time 26.8 ,usec. Time delay 50/~sec.
The application of the ht pulse to the flash tube causes the electrons, produced by the primary ionizing particles to avalanche and produce locally a spark discharge. This persists unti! the charges, deposited on the glass walls at that point, produce a reverse field which counteracts the applied field sufficiently to extinguish the spark. Many of the excited states of neon and helium give rise to ultra-violet radiation and such radiation from the initial spark produces photoelectrons from the walls of the tube at points remote from the initial discharge. The discharge thus spreads
U S E OF N E O N F L A S H
T U B E S AT H I G H
down the tube. Fig. 4 shows oscilloscope traces of a photomultiplier output observing the visible light from a flash tube. In fig. 4a the particle passed through the flash tube close to the light output end, whereas in fig. 4b the initial discharge was approximately 1 m from the end. It can be deduced from these traces that the speed of propagation of the discharge along the tube is approximately 2.4 x 10~ cm/sec and that the total duration of the discharge is only 1 /~sec. It was also found that this discharge time was independent of the length of the applied ht pulse, provided that the latter was at least 1 /~sec long itself. This supports the argument that the discharge is self quenched by charges deposited on the inside wall of the flash tube. It had been observed previously 9) that the discharge in a flash tube appeared as a series of discs of light at random positions in the tube. This is now explained as due to the charges, deposited on the inside walls by the first discharge in that section, inhibiting the build up of further discharges close by. It is estimated from fig. 4 that the duration of the discharge at any point in the tube is approximately 150 nsec, which is in quite good agreement with a calculated time of 200 nsec for the discharge to cross the t u b e - t h i s value is obtained by considering the drift velocity measurements of Pack and Phelps 1°) for pure neon and assuming a uniform field of 4 kV/cm. In reality, space charge distortion of the field by the discharge itself will occur and the discharge can then be expected to progress more rapidly towards a streamer type discharge. Although in the time of approximately 150 nsec the electrons will be deposited on the wall of the tube, the positive ions remain almost stationary and can only be removed to the opposite wall if the applied high voltage pulse is sufficiently long. For a peak field of 4 kV/cm, and taking the drift velocity measurements referred to by von Engel~t), it can be shown that an ht pulse with a decay constant of 80/~sec is required to deposit all the ions onto the glass. This agrees well with fig. 2 which shows that the efficiency does not decrease any further for pulse decay times greater than approx. 60 l~sec - positive ions swept to the walls of the tube by the ht pulse contribute to the clearing field but those remaining in the tube after the ht pulse has finished will presumably drift to electrons on the walls of the tube and neutralize them, thus reducing the clearing field. All the positive ions have been swept to the walls of the tube by 60/tsec and the efficiency does not decrease any further for longer pulses. The clearing fields produced in this manner may persist for a long time if they can only be removed by electron ion recombination on the glass surface, since glass has a very high resistance.
REPETITION
RATES
431
Measurements of the resistance of glass show considerable variation with type of glass and with surface conditions13). The surface resistivity may be as low as 107CL but values as high a s 1016Q are known for some glasses in dry conditions. When flash tubes are manufactured they are evacuated for several hours and are often heated to remove water vapour. Also the tubes are heated, almost to annealing temperature in order to straighten them. All these processes are expected to increase the surface resistance of the interior wall of the final flash-tube. The highest measured surface resistivity for these tubes was 5 x 1013 f2 but this may be an underestimation since water vapour may not have been completely removed. However, using this value, together with a measured capacitance of 5 pF for the flash tube, a decay constant of the charges on the tubes of 3.8 sec is obtained. This leads to a time of approximately 40 sec for the field inside the tube to decrease from the value required to quench the discharge to 0.1 V/cm. This is in reasonable agreement with the rate of decrease of clearing field measured by Ashton +) at small times but it does not predict the slower decay rate which is still significant after several minutes. Bearing in mind the sensitivity of glass resistance to atmospheric conditions, it may be that the resistance inside the flash tube is greater than the measured value, and the fast decay at small times is supplemented by other processes such as recombination by the emission of charges from surface impurities into the gas, through which they drift to charges on the far wall.
4. Means of removing the clearing fields The clearing fields decay by conduction of charges over the glass surface in a time which may be several minutes. F r o m earlier work 2) it is known that the type of glass used had a considerable effect on the efficiency of flash tubes. It was found that pyrex glass, whose resistance is higher than soda glass, was generally unsatisfactory for use in flash tubes, because of a high spurious rate of flashing and a low efficiency even at small delays. Investigations have been made of increased surface conductivity as a means of reducing the effect of clearing fields. Since the resistance of glass is known to be markedly dependent on humidity, tubes have been manufactured containing water vapour. The efficiency of these tubes falls off more rapidly with increasing delay than ordinary tubes because water molecules have a high affinity for free e l e c t r o n s - the attachment coefficient at 20 °C is 4 x 10 -4 per collision~Z). It was found that some of these tubes, even when operated at the rate of 1 flash
432
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Fig. 5. Variation of efficiency with delay time. (a) Tubes at 100 °C. Pulse amplitude 4 kV/cm, decay time 40/tsec and repetition rate 1 flash per minute per tube. (b) Bi-polar ringing pulse (2 MHz). Amplitude 7.5 kV/cm, decay time ~5/zsec and repetition rate 1 flash per 10 sec per tube. (c) Tubes containing an addition of 0.04 torr water vapour. Pulse amplitude 7.5 kV/cm, decay time 40 #sec and repetition rate 1 flash per 10 sec per tube. per 10 sec p e r tube s h o w e d no change in efficiency, while o t h e r tubes actually s h o w e d an increase. This increase is p o s s i b l y due to the water v a p o u r being increasingly a b s o r b e d by the glass tube. A l t h o u g h the effect o f clearing fields does n o t seem to be evident, these tubes have the d i s a d v a n t a g e t h a t the light o u t p u t f r o m t h e m is greatly reduced. It was o b s e r v e d by means o f a p h o t o m u l t i p l i e r that the light o u t p u t pulse was also very s h o r t ( ~ 200 nsec) indicating that the water v a p o u r tends to inhibit the s p r e a d i n g o f the discharge, a n d it m a y be t h a t the restriction o f the discharge, r a t h e r than the increased surface c o n d u c t i o n , is the reason for the absence o f clearing field effects in these cases. A technique which reduces the clearing field effect w i t h o u t i m p a i r i n g the brightness o f the flash is the use o f a b i p o l a r ringing high voltage pulse, o b t a i n e d by e m p l o y i n g an inductive l o a d in place o f the resistor in the c o n v e n t i o n a l R C decay n e t w o r k . Since the a p p l i e d field swings positive a n d negative at a frequency o f a p p r o x i m a t e l y 2 M H z the ions will oscillate instead o f m o v i n g in one direction a n d hence the n u m b e r d e p o s i t e d on the flash tube wails will be less. In any case a p p r o x i m a t e l y the same n u m b e r will be d e p o s i t e d on b o t h sides o f the tube so that a n y effect due to t h e m will be cancelled out. The efficiency against delay for a repetition rate o f 1 flash per 10 sec per tube is shown in fig. 5, where it is seen that the p o i n t s agree well with
AND
J. M. B R E A R E
L l o y d s theoretical curve f o r f l Q = 10, showing that no clearing fields are present. A l t h o u g h this type o f pulse effectively reduces the clearing fields, it m a y present difficulties when a p p l i e d to very large flash tube arrays. Since the resistance o f glass is very m u c h d e p e n d e n t o n temperature13), the v o l u m e resistivity decreasing a p p r o x i m a t e l y by a factor 10 for every 25°C rise in t e m p e r a t u r e , studies o f the t e m p e r a t u r e sensitivity o f tubes have been made. I n p a r t i c u l a r m e a s u r e m e n t s o f flash tube efficiency were carried out at a t e m p e r a t u r e o f 100 °C. U s i n g a repetition rate o f a p p r o x i m a t e l y 1 flash p e r m i n u t e per tube a n d a pulse o f a m p l i t u d e 4 k V / c m a n d decay c o n s t a n t 40/tsec, tubes which h a d previously shown a m a r k e d clearing field at r o o m t e m p e r a t u r e showed no such effect when h e a t e d to 100°C. A l t h o u g h this test is a useful c o n f i r m a t i o n o f the n a t u r e o f the clearing fields it does n o t offer an attractive practical m e t h o d o f eliminating the effect. H o w e v e r it indicates that if a layer o f material o f sufficient c o n d u c t i v i t y could be p l a c e d on the inside o f the tubes, clearing fields s h o u l d be removed. The a u t h o r s wish to t h a n k Prof. A. W. W o l f e n d a l e for his e n c o u r a g e m e n t a n d helpful suggestions. Mr. R. J. Stubbs a n d Mr. J. A. Lightfoot are t h a n k e d for useful discussions. W e are indebted to Mr. A. R o b e r t shaw o f the I n t e r n a t i o n a l Research a n d D e v e l o p m e n t C o m p a n y , Newcastle u p o n Tyne, U . K . who p r o v i d e d the special tubes for these tests a n d to the Science Research Council for financial s u p p o r t for one o f us (F.W.H.).
References 1) M. Conversi, S. Focardi, C. Franzinetti, A. Gozzini and P. Murtas, Suppl. Nuovo Cimento 4 0955) 234. 2) M. Gardener, S. Kisdnasamy, E. Rossle and A. W. Wolfendale, Proc. Phys. Soc. 70 (1957) 687. 3) H. Coxell and A. W. Wolfendale, Proc. Phys. Soc. 75 0960) 378. 4) F. Ashton, J. M. Breare, F. W. Holroyd, K. Tsuji and A. W. Wolfendale, Nuovo Cimento Letters 2 (1971) 707. 5) F. W. Holroyd and J. M. Breare, Proc. Intern. Conf. Cosmic rays (Hobart, 1971). 6) R. J. Stubbs and J. M. Breare, Acta Phys. Acad. Sci. Hung. 29 (1970) 473. 7) C. A. Ayre, private communication. s) J. L. Lloyd, Proc. Phys. Soc. 35 (1960) 387. 9) H. Coxell, M. A. Meyer, P. S. Scull and A. W. Wolfendale, Suppl. Nuovo Cimento 21 (1961) 7. lO) J. L. Pack and A. V. Phelps, Phys. Rev. 121 (1961) 798. 11) A. von Engel, Ionized gases (Oxford University Press, Oxford, 1965). 12) S. C. Brown, Basic data in plasma physics (Wiley, New York, 1959). 13) P. M. Sutton, Prog. Dielectrics 2 (1960) 115.
INSTRUMENTS
AND METHODS
IO0 (1972) 4 2 9 - 4 3 2 ;
© NORTH-HOLLAND
PUBLISHING
CO.
T H E USE OF N E O N F L A S H TUBES AT H I G H R E P E T I T I O N RATES F. W. HOLROYD and J. M. BREARE
Department of Physics, University of Durham, England Received 22 December 197l When flash tubes are used at high repetition rates their efficiency at long time delays between the passage of the ionizing particle and the application of the high voltage pulse decreases faster than theoretical predictions. An explanation of the phenomena is
proposed in terms of clearing fields resulting from charges deposited on the inside glass surface by previous discharges. Methods of avoiding the build up of such clearing fields are discussed.
1. I n t r o d u c t i o n
support is given to this argument and some possible methods of removing the clearing fields are investigated. Two types of flash tubes have become c o m m o n in experimental arrangements: low pressure tubes filled at 600 torr containing 98% neon and 2% helium and high pressure tubes filled at 2.3 atm with the same gas mixture. The inside diameter of the low pressure tubes is 1.6 cm and the glass wall thickness 0.1 cm. The high pressure tubes are of 0.65 cm inside diameter with wall thickness 0.1 cm. The glass used is type S.95 soda glass. Here, only tests carried out on the low pressure tubes are described, but it is known that similar effects occur in the smaller, high pressure tubes7).
Neon flash tubes have been used extensively in cosmic ray experiments and their characteristics have been studied by several groups1-3). In most experiments where cosmic rays have been used, the rate at which an individual flash tube is operated is generally very low even when the event rate in the apparatus is large, since detectors usually contain many tubes. Under condition of low rate of flashing the characteristics of the tubes are reproducible. However, if the rate is increased it is observed 4'5) that the detection efficiency of the tubes varied with the rate of flashing. A similar dependence of efficiency on repetition rate has been found in sealed current limited glass spark chambers6). It has been suggested that in both cases, electric fields, caused by charges deposited on the walls of the detectors, are responsible for these effects. In this paper, further I00'
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2. M e a s u r e m e n t s
Using cosmic rays as the ionizing particles, the efficiency of the tubes was measured at a very low repetiaoO°lo
]~ I. flosh per ]rninute" "~, I. flash per) 30 rain.
o f efficiency
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Fig. 2. Variation of internal efficiency with ht pulse length. Time delays of zero and 50/~sec. Peak ht pulse height = 4 kV/cm.
430
r . W. H O L R O Y D AND J. M. BREARE
tion rate (each tube only flashing about once per I I I 30 min). The efficiency as a function of time delay between the passage of the particle and the application of the ht pulse to the tubes, is shown in fig. 1. The ht pulse had an amplitude of 2.8 kV/cm and a rise time of approximately 50 nsec. It was derived from a conventional switched RC decay circuit and the decay time could be varied. It is seen in fig. 1 that the efficiency of the tubes was independent of the decay time, insofar as values of 0.4/~sec and 26.8/~sec give the same efficiency curve at low repetition rate. Also, the measured values fit very well to the theoretical curve derived by Lloyd 8) using his parameters f l = 0.3 and Q = 33.6 ion pairs/cm atmospheric pressure. Similar efficiency curves were obtained at a higher rate of flashing (approximately 1 flash per minute per tube). These points are also given in fig. 1 and show I I I I I that for a short pulse (CR = 0.4/~sec) the efficiency still o I 2 a 4 (p. ~,~) follows the predicted curve. However, for a pulse of Fig. 4. Profiles of photomultiplier pulses of light output from 40/tsec decay time, the efficiency falls more rapidly. flash tubes. (a) Particle passing through front of the tube. (b) ParThe dependence of efficiency on repetition rate and ticle passing through rear of the tube. Ht pulse amplitude 4 kV/cm. pulse length is further illustrated by fig. 2 and fig. 3. In Decay time 4/~sec. fig. 2 the efficiency as a function of the ht pulse length is plotted for the case of a 50 psec pulse delay. It is seen repetition rate was approximately 1 flash per tube per that the efficiency decreases as the pulse length is minute. Fig. 3 on the other hand shows the variation increased, until pulses have a characteristic time of of efficiency with repetition rate for a fixed high voltage 60/~sec. For longer pulses the efficiency remains pulse, namely amplitude 2.8 kV/cm and decay time constant. Also shown are points for zero delay and 26.8 psec. It is observed that an approximate interval of several pulse lengths: these points, as expected, are of 12 min between flashes is required to obtain an effihigh efficiency. For these tests the ht pulse had an ciency consistent with Lloyd's theory. This result is amplitude of 4 kV/cm a rise time of 70 nsec, and the similar to that of Ashton et al. 4) where a time of approximately 20 min between flashes was required to give full efficiency- however, Ashton et al. were triggering their array on cosmic ray showers where the initial ' I ~ I ' I ionization on the tubes may have been somewhat different from that of single cosmic ray particles. In all .-------O O--80 cases the deviations from Lloyd's theoretical curve are thought to be due to clearing fields caused by the previous discharges in the tube. ~ 6o
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The application of the ht pulse to the flash tube causes the electrons, produced by the primary ionizing particles to avalanche and produce locally a spark discharge. This persists unti! the charges, deposited on the glass walls at that point, produce a reverse field which counteracts the applied field sufficiently to extinguish the spark. Many of the excited states of neon and helium give rise to ultra-violet radiation and such radiation from the initial spark produces photoelectrons from the walls of the tube at points remote from the initial discharge. The discharge thus spreads
U S E OF N E O N F L A S H
T U B E S AT H I G H
down the tube. Fig. 4 shows oscilloscope traces of a photomultiplier output observing the visible light from a flash tube. In fig. 4a the particle passed through the flash tube close to the light output end, whereas in fig. 4b the initial discharge was approximately 1 m from the end. It can be deduced from these traces that the speed of propagation of the discharge along the tube is approximately 2.4 x 10~ cm/sec and that the total duration of the discharge is only 1 /~sec. It was also found that this discharge time was independent of the length of the applied ht pulse, provided that the latter was at least 1 /~sec long itself. This supports the argument that the discharge is self quenched by charges deposited on the inside wall of the flash tube. It had been observed previously 9) that the discharge in a flash tube appeared as a series of discs of light at random positions in the tube. This is now explained as due to the charges, deposited on the inside walls by the first discharge in that section, inhibiting the build up of further discharges close by. It is estimated from fig. 4 that the duration of the discharge at any point in the tube is approximately 150 nsec, which is in quite good agreement with a calculated time of 200 nsec for the discharge to cross the t u b e - t h i s value is obtained by considering the drift velocity measurements of Pack and Phelps 1°) for pure neon and assuming a uniform field of 4 kV/cm. In reality, space charge distortion of the field by the discharge itself will occur and the discharge can then be expected to progress more rapidly towards a streamer type discharge. Although in the time of approximately 150 nsec the electrons will be deposited on the wall of the tube, the positive ions remain almost stationary and can only be removed to the opposite wall if the applied high voltage pulse is sufficiently long. For a peak field of 4 kV/cm, and taking the drift velocity measurements referred to by von Engel~t), it can be shown that an ht pulse with a decay constant of 80/~sec is required to deposit all the ions onto the glass. This agrees well with fig. 2 which shows that the efficiency does not decrease any further for pulse decay times greater than approx. 60 l~sec - positive ions swept to the walls of the tube by the ht pulse contribute to the clearing field but those remaining in the tube after the ht pulse has finished will presumably drift to electrons on the walls of the tube and neutralize them, thus reducing the clearing field. All the positive ions have been swept to the walls of the tube by 60/tsec and the efficiency does not decrease any further for longer pulses. The clearing fields produced in this manner may persist for a long time if they can only be removed by electron ion recombination on the glass surface, since glass has a very high resistance.
REPETITION
RATES
431
Measurements of the resistance of glass show considerable variation with type of glass and with surface conditions13). The surface resistivity may be as low as 107CL but values as high a s 1016Q are known for some glasses in dry conditions. When flash tubes are manufactured they are evacuated for several hours and are often heated to remove water vapour. Also the tubes are heated, almost to annealing temperature in order to straighten them. All these processes are expected to increase the surface resistance of the interior wall of the final flash-tube. The highest measured surface resistivity for these tubes was 5 x 1013 f2 but this may be an underestimation since water vapour may not have been completely removed. However, using this value, together with a measured capacitance of 5 pF for the flash tube, a decay constant of the charges on the tubes of 3.8 sec is obtained. This leads to a time of approximately 40 sec for the field inside the tube to decrease from the value required to quench the discharge to 0.1 V/cm. This is in reasonable agreement with the rate of decrease of clearing field measured by Ashton +) at small times but it does not predict the slower decay rate which is still significant after several minutes. Bearing in mind the sensitivity of glass resistance to atmospheric conditions, it may be that the resistance inside the flash tube is greater than the measured value, and the fast decay at small times is supplemented by other processes such as recombination by the emission of charges from surface impurities into the gas, through which they drift to charges on the far wall.
4. Means of removing the clearing fields The clearing fields decay by conduction of charges over the glass surface in a time which may be several minutes. F r o m earlier work 2) it is known that the type of glass used had a considerable effect on the efficiency of flash tubes. It was found that pyrex glass, whose resistance is higher than soda glass, was generally unsatisfactory for use in flash tubes, because of a high spurious rate of flashing and a low efficiency even at small delays. Investigations have been made of increased surface conductivity as a means of reducing the effect of clearing fields. Since the resistance of glass is known to be markedly dependent on humidity, tubes have been manufactured containing water vapour. The efficiency of these tubes falls off more rapidly with increasing delay than ordinary tubes because water molecules have a high affinity for free e l e c t r o n s - the attachment coefficient at 20 °C is 4 x 10 -4 per collision~Z). It was found that some of these tubes, even when operated at the rate of 1 flash
432
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(p. sec)
Fig. 5. Variation of efficiency with delay time. (a) Tubes at 100 °C. Pulse amplitude 4 kV/cm, decay time 40/tsec and repetition rate 1 flash per minute per tube. (b) Bi-polar ringing pulse (2 MHz). Amplitude 7.5 kV/cm, decay time ~5/zsec and repetition rate 1 flash per 10 sec per tube. (c) Tubes containing an addition of 0.04 torr water vapour. Pulse amplitude 7.5 kV/cm, decay time 40 #sec and repetition rate 1 flash per 10 sec per tube. per 10 sec p e r tube s h o w e d no change in efficiency, while o t h e r tubes actually s h o w e d an increase. This increase is p o s s i b l y due to the water v a p o u r being increasingly a b s o r b e d by the glass tube. A l t h o u g h the effect o f clearing fields does n o t seem to be evident, these tubes have the d i s a d v a n t a g e t h a t the light o u t p u t f r o m t h e m is greatly reduced. It was o b s e r v e d by means o f a p h o t o m u l t i p l i e r that the light o u t p u t pulse was also very s h o r t ( ~ 200 nsec) indicating that the water v a p o u r tends to inhibit the s p r e a d i n g o f the discharge, a n d it m a y be t h a t the restriction o f the discharge, r a t h e r than the increased surface c o n d u c t i o n , is the reason for the absence o f clearing field effects in these cases. A technique which reduces the clearing field effect w i t h o u t i m p a i r i n g the brightness o f the flash is the use o f a b i p o l a r ringing high voltage pulse, o b t a i n e d by e m p l o y i n g an inductive l o a d in place o f the resistor in the c o n v e n t i o n a l R C decay n e t w o r k . Since the a p p l i e d field swings positive a n d negative at a frequency o f a p p r o x i m a t e l y 2 M H z the ions will oscillate instead o f m o v i n g in one direction a n d hence the n u m b e r d e p o s i t e d on the flash tube wails will be less. In any case a p p r o x i m a t e l y the same n u m b e r will be d e p o s i t e d on b o t h sides o f the tube so that a n y effect due to t h e m will be cancelled out. The efficiency against delay for a repetition rate o f 1 flash per 10 sec per tube is shown in fig. 5, where it is seen that the p o i n t s agree well with
AND
J. M. B R E A R E
L l o y d s theoretical curve f o r f l Q = 10, showing that no clearing fields are present. A l t h o u g h this type o f pulse effectively reduces the clearing fields, it m a y present difficulties when a p p l i e d to very large flash tube arrays. Since the resistance o f glass is very m u c h d e p e n d e n t o n temperature13), the v o l u m e resistivity decreasing a p p r o x i m a t e l y by a factor 10 for every 25°C rise in t e m p e r a t u r e , studies o f the t e m p e r a t u r e sensitivity o f tubes have been made. I n p a r t i c u l a r m e a s u r e m e n t s o f flash tube efficiency were carried out at a t e m p e r a t u r e o f 100 °C. U s i n g a repetition rate o f a p p r o x i m a t e l y 1 flash p e r m i n u t e per tube a n d a pulse o f a m p l i t u d e 4 k V / c m a n d decay c o n s t a n t 40/tsec, tubes which h a d previously shown a m a r k e d clearing field at r o o m t e m p e r a t u r e showed no such effect when h e a t e d to 100°C. A l t h o u g h this test is a useful c o n f i r m a t i o n o f the n a t u r e o f the clearing fields it does n o t offer an attractive practical m e t h o d o f eliminating the effect. H o w e v e r it indicates that if a layer o f material o f sufficient c o n d u c t i v i t y could be p l a c e d on the inside o f the tubes, clearing fields s h o u l d be removed. The a u t h o r s wish to t h a n k Prof. A. W. W o l f e n d a l e for his e n c o u r a g e m e n t a n d helpful suggestions. Mr. R. J. Stubbs a n d Mr. J. A. Lightfoot are t h a n k e d for useful discussions. W e are indebted to Mr. A. R o b e r t shaw o f the I n t e r n a t i o n a l Research a n d D e v e l o p m e n t C o m p a n y , Newcastle u p o n Tyne, U . K . who p r o v i d e d the special tubes for these tests a n d to the Science Research Council for financial s u p p o r t for one o f us (F.W.H.).
References 1) M. Conversi, S. Focardi, C. Franzinetti, A. Gozzini and P. Murtas, Suppl. Nuovo Cimento 4 0955) 234. 2) M. Gardener, S. Kisdnasamy, E. Rossle and A. W. Wolfendale, Proc. Phys. Soc. 70 (1957) 687. 3) H. Coxell and A. W. Wolfendale, Proc. Phys. Soc. 75 0960) 378. 4) F. Ashton, J. M. Breare, F. W. Holroyd, K. Tsuji and A. W. Wolfendale, Nuovo Cimento Letters 2 (1971) 707. 5) F. W. Holroyd and J. M. Breare, Proc. Intern. Conf. Cosmic rays (Hobart, 1971). 6) R. J. Stubbs and J. M. Breare, Acta Phys. Acad. Sci. Hung. 29 (1970) 473. 7) C. A. Ayre, private communication. s) J. L. Lloyd, Proc. Phys. Soc. 35 (1960) 387. 9) H. Coxell, M. A. Meyer, P. S. Scull and A. W. Wolfendale, Suppl. Nuovo Cimento 21 (1961) 7. lO) J. L. Pack and A. V. Phelps, Phys. Rev. 121 (1961) 798. 11) A. von Engel, Ionized gases (Oxford University Press, Oxford, 1965). 12) S. C. Brown, Basic data in plasma physics (Wiley, New York, 1959). 13) P. M. Sutton, Prog. Dielectrics 2 (1960) 115.