- Email: [email protected]
Optical digitization of wire spark chambers
NUCLEAR INSTRUMENTS
AND METHODS 81 ( I 9 7 0 )
310-316;
©
NORTH-HOLLAND
PUBLISHING
CO.
OPTICAL D I G I T I Z A T I O N OF WIRE SPARK CHAMBERS F. K R I E N E N
CERN, Geneva, Switzerland Received 29 January 1970 A new solution has been found for the operation of digitized wire spark chambers inside a magnetic field. The wire chambers have crossed wire planes and each wire is connected up to a secondary spark unit, which is located well outside the magnetic field volume. The spark unit is a two-dimensional array of closely packed spark points. A spark is produced on those points which
correspond to the position of the primary spark initiated by the charged particle trajectory, i.e. one sees two projected sparks for each primary spark. A television camera looks at this optical image. The electrical output of the camera may be processed to obtain the particle coordinates in a digitized form.
1. Introduction The automatization of spark chamber tracks by means of wire spark chambers has been most successful. The development started some eight years ago with the memory core read-out system1). Since then quite a variety of wire chambers have been presented amongst which we mention the sophisticated and as it were more ancient proportional wire chamber2). This chamber has the advantage of being able to work in a magnetic field and of having a short sensitive time. There is little doubt that wire chambers will remain the appropriate tools in many physics experiments. The trend is to propose experiments in which many large spark chambers are operating in a magnetic field3'4). However, some existing systems are not suitable for any one of the following reasons: (a) they are expensive, (b) they do not work in a magnetic field and (c) they have a large dead volume. We present here an economical solution in which a television camera is the recording medium. The proposed system differs from a previous proposition 5) in that the camera does not look at the primary sparks initiated by the particle trajectory, but looks at spark projections thereof. This comes about by making the usual wire chambers with crossed wire planes and connecting up the wires with printed
circuitry to a spark unit where secondary sparks are produced. It is attractive to propose television equipment because the instrumentation for high-energy physics may profit from the vast and continuing development which occurs in the commercial sector. In particular, the newer camera tubes 6'7) offer low lag, low noise, reasonable lifetime and resolution and high sensitivity. Of course a television system will have inherent problems of which we mention the drift, the linearity and the alignment. In the present proposition we believe to have found a means of overcoming this difficulty. An outstanding feature of a camera tube is the high density of information which can be stored on its photosensitive target. As a result, large spark chamber batteries may be looked at by few cameras. We give first a description of a prototype spark chamber built in this laboratory. In section 3 the principle of operation is discussed, in section 4 the spark projection chamber and in section 5 the electronic read-out. Finally in section 6 we present some more details and observations. 2. Discussion of the model In fig. 1 is shown the spark chamber and in fig. 2 its schematic representation, where for the sake of
Fig. 1. Model spark chamber with 128 wires.
310
OPTICAL
DIGITIZATION
OF W I R E
SPARK
this connedion is printed /I on the mylar
i
311
CHAMBERS
H.V.+ 1000PG:n .,ooK, R-~' ,r
r-:---e~ ................. 7?. . . . . - e : ~ . . . . i~28oP ........................................................ :." ;.;""
1
Fig. 2. Schematic representation o f model s p a r k c h a m b e r with 128 wires.
simplicity the crossed wire planes are drawn in parallel. The wire pitch is l mm and the gap is 8 mm. The useful area is 127 x 127 mm. The wires are brought out with the same pitch of 1 mm, but in the spark projection chamber where the secondary sparks are produced the wire pitch is 2 ram. The printing is done on mylar of 0.25 mm thickness. The spark chamber in this model is also printed on mylar but will be generally made with real wires as this method yields less scattering mass to the ionizing particles. On the back of the printed mylar are two capacitive coupling electrodes Ca and Cb. In this model they are made of metallized scotch tape, but presumably in large production schemes doublesided printed mylar would be used. C, is the coupling capacitor with the high-voltage pulser and is situated close to the spark chamber. Cb is a coupling capacitor located at the free end of the printed mylar. On the printed mylar the two outer conductors are reserved for interconnecting the electrodes C, and Cb. The projected sparks are drawn between the wire tip in question and the adjacent coupling electrode. In the present model the adjacent coupling electrode is a dummy electrode Cb, because we have only one printed strip per high voltage or earth side of the spark chamber. The length of these sparks is about 1.7 mm. The printed wires are galvanically connected by coating with a narrow strip of colloidal graphite. High ohmic
section
~.///////'///////f/~
0
resistors are connected up to prevent accumulation of free charges. The high-voltage pulser consists of a capacitor Cc charged to the required level and discharged in the load through a thyratron or a triggered spark gap. The leads to the pulser should be short and should start from the comer of the spark chamber where the high-voltage electrode C, and the ground electrode C~' cross over. A common clearing field supply is provided for; the clearing voltage across the spark chamber Cd is twice the clearing voltage across Cb. A helium neon gas mixture plus some organic vapour is used in the spark chamber, but in the projection chambers the gas is either pure helium or pure argon. 3. Principle of operation For the chamber to work well, the electric field in the gap should rise fast, say within l0 to 20 nsec. The load presented to the discharging capacitance Cc consists in the first instance of three capacitances in series: Ca, Cd and C~', to which is shunted a resistive load of R~ + R 2 + R ' ~. In addition, shunted to Cd, are two bundles of printed wires leading to the spark projection chambers. The bundle on the high-voltage side and on the grounded side form together an equivalent open line, typically 2 m long and having an average wave impedance which is fairly high, say
.../-C~ spacer
printed mylar
1
/copper50. V
spark
fcamne
direction of line scan
_
Fig. 3. Configuration of spark projection chamber with mesh points of 2 x 2 mm.
312
F. KRIENEN
Fig. 4a. Test battery of four wire spark chambers of 512 wires vertical and 256 wires horizontal. The wire pitch is 1 ram, and the chambers are 136 mm spaced apart. The spark-gap pulser is near the top right-hand corner of the spark chambers. The charging capacitor is 1000 pF per chamber. The spark projection chambers are at the very right of the photograph and seen from the side. 200 f~ or more. In o t h e r w o r d s the l o a d o f the open line is usually small when c o m p a r e d to the s p a r k c h a m b e r load. Cb in parallel with Ca via the p r i n t e d c o n d u c t o r s c o n t r i b u t e s little to the fast rise o f the electric field in the s p a r k c h a m b e r . T h e p a r a m e t e r s are by no means critical, nevertheless one w o u l d choose Ca ~ Cd so t h a t m o s t o f the voltage a p p e a r s across the c h a m b e r a n d n o t enough across Cb to p r o d u c e s p u r i o u s sparking, a n d Cc/Cd = 2 to 4, as usual. F u r t h e r m o r e , one w o u l d choose RI Ca = R2 Ca to minimize equalizing currents in the resistive strip across the p r i n t e d wires. A s s o o n as a s p a r k is f o r m e d in the c h a m b e r , the voltage between the c o r r e s p o n d i n g wires d r o p s to a low value. Hence on b o t h sides o f the c h a m b e r there will be a p o t e n t i a l difference between the wire in question a n d the s u r r o u n d i n g wires. Thus an electrom a g n e t i c wave will p r o p a g a t e t o w a r d s the open end o f the p r i n t e d wires. T h e i m p e d a n c e o f this localized system is e s t i m a t e d to be o f the o r d e r o f 50 YL A t the o p e n end a s p a r k will develop if the distance between the wire in question a n d the electrode Cb facing it, is small e n o u g h so t h a t the gap is overvolted. The resistive coating s h o u l d be a p p l i e d so t h a t the resistance between a d j a c e n t wires is large when c o m p a r e d to the wave i m p e d a n c e o f one wire against the s u r r o u n d i n g
Fig. 4b. Frontal view of the spark projection chambers, showing the vertical projection (left) and the horizontal projection (right), side by side, of a straight track of a charged particle traversing the spark chambers.
OPTICAL DIGITIZATION OF WIRE SPARK CHAMBERS wires, thus of the order of 500 ~q. The energy needed to sustain the spark is initially coming from the electric field of several adjacent wires, partly by direct coupling and partly via the coupling capacitance Ca. In this respect the system should have a good multiple spark efficiency. When the secondary sparks have formed, more wires participate in sustaining the three sparks. Presumably the coupling capacitance Cb has a stabilizing effect on spurious sparking and may be adjusted to an optimum value. Finally the resistive chain Rl Rz R'~ may be adjusted to obtain the proper pulse length.
4. Spark projection chamber The aspect of the spark projection chamber is a regular pattern of spark points. The direction of scanning is parallel to the printed mylar strips and the pitch in that direction is 2 mm. The trace width of the printing is 0.5 ram. in the model are 128 spark points on a line. The pitch in the frame scan direction is also 2 ram. With an aspect ratio of 4:3, there are 96 rows of spark points in a full sized television screen. Thus in this configuration 128 × 96 = 12 288 spark chamber wires may be accommodated, see fig. 3. The wear on the spark point, i.e. the copper trace of 35/~m on the mylar, is small. A test showed about 0.1 mm per 106 sparks on the same trace if the trace was connected
313
as cathode, and even less at opposite polarity. Large spark chambers with more wires per frame than go on one scan line will be subdivided and occupy successive rows on the spark projection chamber. Fig. 4 shows four wire spark chambers of 512 wires vertical and 256wires horizontal, thus in total 3072 wires connected up to the spark projection chamber. The vertical wires occupy 16 spark rows and the horizontal wires 8. This model is made to study the performance of a commercial television chain of studio quality. Fig. 5 shows on the left a photograph taken directly from the spark projection chamber, and on the right a photograph of the same event taken from the monitor. The event is a straight track and good correspondence of both images may be deduced. The spark projection chamber is supposed to be just outside a magnet, and the television camera looking at it would be even further away. Hence the residual magnetic shielding of the camera tube would be relatively simple. The object space is obviously reduced to a fiat surface, thus there are no focal depth problems. We emphasize also that no mirrors are required in this electro-optical system.
5. Discussion on electronic read-out The length of the secondary sparks (1.7 ram) is somewhat less than the distance between the spark
Fig. 5. Comparison between a direct photograph of the spark projection chambers (left) and a photograph of the television monitor (right) for the s a m e event. The 25 cm illuminated rulers have half cm divisions. Sparks on individual wires may be seen.
314
F. K R 1 E N E N
rows (2 mm). In our model with about 600 scan lines in a frame and 96 spark rows, there are about six scan lines per row and about five scan lines overlapping a spark. Only one of these, centred on the spark, will be used for electronic read-out, simply by gating out the video signal before processing. Thus only one out of six of the scan lines are occupied in a camera, hence a total of six cameras would fit in a read-out system, utilizing optimum real time. The final capacity of this model would therefore be 6 x 12 288 = 73 778 wires. We speak o f " true optical digitization" if the number of spark points on the screen is limited so that the distance between two adjacent points and the length of the spark are larger than the absolute accuracy of the television system. The parameters involved in this context are the non-linearity and the drift. Conservatively, one may assume that the parameters of the chosen model conform to this criterion. We would even think of putting 256 spark points in a row and 144 rows to fill out a television frame. Thus the capacity per television camera is 36 864 wires, using only one out of four scan lines. Hence a total of four cameras would fit in a read-out system bringing the final capacity to 147 456 wires. This exercise would require more refinements in the camera electronics, but what we emphasize here is the non-utilization of fiducials to reconstruct an event. We obtain thus an appreciable economy in computer time. Of course there will be fiducials on the screen for setting up and for internal feedback in the television electronics. I f we increase still more the number of spark points in a screen, we leave the domain of true optical digitization and deal with an optical analogue-to-digital conversion system. In this respect the limiting parameter will be the resolution of the television system. This automatically involves a liberal use of fiducials to correct for errors. The number of wires per screen becomes of course very large, but that may conflict with the construction of the screen. In any case, the load on the computer will increase and except for the feature of bringing an optical image out of a magnetic field volume, the complications are the same as if the
8 lef[oo 0 . 5 ~
O-ring 44 40 shore in " ' V / / / A Groove2.5x4
;
6
printed c mylar 0.25 V M3 !._..-----~_ 2kg i Fig. 6. C l a m p construction.
spark chambers were directly looked at by a television cameraS). This mode of operation might still find an application in cases where one wants to look at wide gap chambers or at chambers of unusual geometry9). 6. Some observations The conductors between spark chamber and spark projection chamber (figs. 1 and 4) show right-angled bends. Also their developed length varies by as much as 64 cm. The total length is more than the workshop could produce in one piece; hence we connected up two pieces by means of a clamp, see fig. 6. The copper tracings are gold-plated. The capacitance of one clamp against 132 conductors was measured to be less than 100 pF. The fanning out of the conductors at the spark projection plate from 1 m m to 2 mm is by no means essential; in the models, we tried to compromise between dimensional tolerances. However, the fanning out is one more example in which a change of format may be achieved. Another, even more important, format change is the display of a spark chamber wire plane on several scan lines. Finally the third format change is the reduction of the distance between spark chambers, fig. 4, which amounts to 136 m m in real space, to 4 m m for the horizontal wires and to 8 mm for the vertical wires. The projected sparks are not right on the tip of the trace but j u m p erratically on the side edge. This correlates with the observation that the etching on the side is more jagged than on the tip. Presumably in a large spark chamber project one would make the connection between spark chamber and spark projection chamber with commercially available flat cable. In the frequency range of consideration the loss on those cables is of the order of 1 db per metre, hence considerable distances could be tolerated. The flexible cable would allow us to mount the spark projection chamber outside the magnet and well above the gap so as to provide easy access to the equipment in the magnet gap or to provide adequate space for detection equipment outside the gap. The cable would be plugged in on the spark chamber frame which accommodates the printing of the coupling capacitance 6", and the resistive coating. Space requirements on the spark chamber are thus little and stand out favourably against the requirements of other systems using digital read-out. The spark projection chamber of the model, fig. 4, is assembled in two units: one for each projection. In principle, one unit for both projections could be considered. Also, we envisage some development in conjunction with commercially available flat
315
O P T I C A L D I G I T I Z A T I O N OF W I R E S P A R K CHAMBERS
cable. For instance, the coupling electrode Cb could perhaps be a simple wire plane crossing or opposing the partly denuded tips of the flat cables and making an inexpensive and easily exchangeable unit. Logically one ought also to study a "core memory projection unit" based on the same principle as outlined in section 3. Our prediction is that the conditions for flipping cores with the right amount of current are more difficult to achieve. This is in contrast to the production of projected sparks which is insensitive to ringing effects. In addition, we believe that a large system capacity is more economical to obtain via a television chain. Another interesting alternative would be to use magnetic tape as the recording medium1°). Again we propose the same principle of transporting the particle coordinates and to concentrate the information at some convenient distance from the magnet in a small surface. In this case, however, we do not provide light but localized magnetic fields, which would leave an image on the tape strobing the surface. The voltage pulse, as measured on the chamber, has the characteristic shape produced by a capacitor which discharges across another capacitor shunted by a resistor. On this general shape some ringing may be noticed. The general shape does not change whether there are particle tracks or not, only the ringing increases somewhat. Adjacent sparks on the spark projection chamber show up whenever the primary track branches out over adjacent spark chamber wires. As to the traditional question concerning the spark efficiency, we could answer that the efficiency is traditionally 100 per cent. The capacitive coupling C,, C,' between the pulser and the spark chamber proper has the additional advantage of accommodating a clearing field supply not requiring high power coupling capacitors. The various threshold voltages and their relation to the optimum working voltage are of considerable interest. In table 1 a typical case is presented measured on both models, in which the chambers are filled with a mixture of 70% neon and 30% helium, to which is added 13 torr of 2-propanol. In the spark projection chambers pure argon is used. Argon or helium are best for high threshold and high luminance in the spark projection chambers. The particles are from a 9°Sr source. If we define the optimum working voltage as the level at which practically 100%o efficiency is reached, then this level should be little higher than the threshold for producing projected sparks, because characteristic for an overvolted spark gap is the rapid decrease of
TABLE 1 Threshold level (kV on power supply).
Spurious on high-voltage side Spurious on grounded side Spurious in spark chamber Particle on high-voltage side Particle on grounded side Particle in spark chamber Optimum working voltage
Model fig. 1
Model fig. 4
12 > 13 > 13 8.5 8 7.5 8.5
> 9.5 9.5 > 9.5 6.0 6.5 5.5 7.0
spark formation time with overvoltage. The treshold for visible sparks in the spark chamber depends on the conditions of observation. However, they will never be bright, because of the limited amount of energy available, unless projected sparks are produced. The delay between primary sparks and projected sparks should favour high multiple spark efficiency, because spark robbing in conventional chambers is due to small differences in spark formation time, so that a spark channel is formed along the first well-developed streamer preventing the other streamers from maturing into spark channels. In the present chamber however the actual dissipation of field energy starts with the breakdown of the projected spark leaving ample time for all streamers to develop. The commercial television camera used with these models is equipped with a plumbicon tube. The video pulse, as measured on the output of the amplifier housed in the camera head, revealed the following details. Saturation occurs at a lens opening of f:4. The half width is 200 nsec. The pulse height during the first field scan is 400 mV, during the second field 60 mV, during the third field 10 mV, and for the following fields not measurable. The scanning in commercial television sets is 2:1 interlaced, so that the extrapolation to the possible event taking rate is quite difficult. However, the absence of any measurable noise gives some confidence that this rate will be at least 25 Hz. The rate could be possibly higher, eventually by sacrificing some system capacity, if the tube lag does not deteriorate with the ageing of the tube. It is probable that the chamber can be made to work in the trackfollowing mode because the rise-time of the electric field in the chamber is short and the voltage level or the pulse length is not critical. Track following is visually observed with the 9°Sr source. The advantage is obvious: one obtains enhanced accuracy at inclined tracks.
316
F. K R I E N E N
Re[erences 1) F. Krienen, Nucl. Instr. and Meth. 16 (1962) 262. z) G. Charpak et al., Nucl. Instr. and Meth. 62 (1968) 262. 3) A. Michelini et al., The Omega project, proposal for a large magnet and spark chamber system, CERN NP Internal Report 68-11 (1968). 4) L. Resegotti, Progress report on split field magnet, CERN Internal Report ISR-MAG 69-58 (1969). 5) H. Gelernter, Nuovo Cimento 92 (1961) 632. 6) T. M. Buck et al., Bell Syst. Tech. J. (USA) 47 (1968) 1827.
7) Plumbicon camera tubes and their applications, Mullard Tech. Commun. (G. B.) 10, no. 98 (1969) 244. s) N . H . Lipman et al., Paper presented at Intern. Seminar Filmless spark and streamer chambers (Dubna, April 15-18, 1969). 9) F. Krienen, Instrumentation for a large magnet to be used in the CERN Intersecting Storage Rings, Paper presented at Intern. Seminar Filmless spark and streamer chambers (Dubna, April 15-18, 1969). 10) M. Neumann and H. Sherrard, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 295.
AND METHODS 81 ( I 9 7 0 )
310-316;
©
NORTH-HOLLAND
PUBLISHING
CO.
OPTICAL D I G I T I Z A T I O N OF WIRE SPARK CHAMBERS F. K R I E N E N
CERN, Geneva, Switzerland Received 29 January 1970 A new solution has been found for the operation of digitized wire spark chambers inside a magnetic field. The wire chambers have crossed wire planes and each wire is connected up to a secondary spark unit, which is located well outside the magnetic field volume. The spark unit is a two-dimensional array of closely packed spark points. A spark is produced on those points which
correspond to the position of the primary spark initiated by the charged particle trajectory, i.e. one sees two projected sparks for each primary spark. A television camera looks at this optical image. The electrical output of the camera may be processed to obtain the particle coordinates in a digitized form.
1. Introduction The automatization of spark chamber tracks by means of wire spark chambers has been most successful. The development started some eight years ago with the memory core read-out system1). Since then quite a variety of wire chambers have been presented amongst which we mention the sophisticated and as it were more ancient proportional wire chamber2). This chamber has the advantage of being able to work in a magnetic field and of having a short sensitive time. There is little doubt that wire chambers will remain the appropriate tools in many physics experiments. The trend is to propose experiments in which many large spark chambers are operating in a magnetic field3'4). However, some existing systems are not suitable for any one of the following reasons: (a) they are expensive, (b) they do not work in a magnetic field and (c) they have a large dead volume. We present here an economical solution in which a television camera is the recording medium. The proposed system differs from a previous proposition 5) in that the camera does not look at the primary sparks initiated by the particle trajectory, but looks at spark projections thereof. This comes about by making the usual wire chambers with crossed wire planes and connecting up the wires with printed
circuitry to a spark unit where secondary sparks are produced. It is attractive to propose television equipment because the instrumentation for high-energy physics may profit from the vast and continuing development which occurs in the commercial sector. In particular, the newer camera tubes 6'7) offer low lag, low noise, reasonable lifetime and resolution and high sensitivity. Of course a television system will have inherent problems of which we mention the drift, the linearity and the alignment. In the present proposition we believe to have found a means of overcoming this difficulty. An outstanding feature of a camera tube is the high density of information which can be stored on its photosensitive target. As a result, large spark chamber batteries may be looked at by few cameras. We give first a description of a prototype spark chamber built in this laboratory. In section 3 the principle of operation is discussed, in section 4 the spark projection chamber and in section 5 the electronic read-out. Finally in section 6 we present some more details and observations. 2. Discussion of the model In fig. 1 is shown the spark chamber and in fig. 2 its schematic representation, where for the sake of
Fig. 1. Model spark chamber with 128 wires.
310
OPTICAL
DIGITIZATION
OF W I R E
SPARK
this connedion is printed /I on the mylar
i
311
CHAMBERS
H.V.+ 1000PG:n .,ooK, R-~' ,r
r-:---e~ ................. 7?. . . . . - e : ~ . . . . i~28oP ........................................................ :." ;.;""
1
Fig. 2. Schematic representation o f model s p a r k c h a m b e r with 128 wires.
simplicity the crossed wire planes are drawn in parallel. The wire pitch is l mm and the gap is 8 mm. The useful area is 127 x 127 mm. The wires are brought out with the same pitch of 1 mm, but in the spark projection chamber where the secondary sparks are produced the wire pitch is 2 ram. The printing is done on mylar of 0.25 mm thickness. The spark chamber in this model is also printed on mylar but will be generally made with real wires as this method yields less scattering mass to the ionizing particles. On the back of the printed mylar are two capacitive coupling electrodes Ca and Cb. In this model they are made of metallized scotch tape, but presumably in large production schemes doublesided printed mylar would be used. C, is the coupling capacitor with the high-voltage pulser and is situated close to the spark chamber. Cb is a coupling capacitor located at the free end of the printed mylar. On the printed mylar the two outer conductors are reserved for interconnecting the electrodes C, and Cb. The projected sparks are drawn between the wire tip in question and the adjacent coupling electrode. In the present model the adjacent coupling electrode is a dummy electrode Cb, because we have only one printed strip per high voltage or earth side of the spark chamber. The length of these sparks is about 1.7 mm. The printed wires are galvanically connected by coating with a narrow strip of colloidal graphite. High ohmic
section
~.///////'///////f/~
0
resistors are connected up to prevent accumulation of free charges. The high-voltage pulser consists of a capacitor Cc charged to the required level and discharged in the load through a thyratron or a triggered spark gap. The leads to the pulser should be short and should start from the comer of the spark chamber where the high-voltage electrode C, and the ground electrode C~' cross over. A common clearing field supply is provided for; the clearing voltage across the spark chamber Cd is twice the clearing voltage across Cb. A helium neon gas mixture plus some organic vapour is used in the spark chamber, but in the projection chambers the gas is either pure helium or pure argon. 3. Principle of operation For the chamber to work well, the electric field in the gap should rise fast, say within l0 to 20 nsec. The load presented to the discharging capacitance Cc consists in the first instance of three capacitances in series: Ca, Cd and C~', to which is shunted a resistive load of R~ + R 2 + R ' ~. In addition, shunted to Cd, are two bundles of printed wires leading to the spark projection chambers. The bundle on the high-voltage side and on the grounded side form together an equivalent open line, typically 2 m long and having an average wave impedance which is fairly high, say
.../-C~ spacer
printed mylar
1
/copper50. V
spark
fcamne
direction of line scan
_
Fig. 3. Configuration of spark projection chamber with mesh points of 2 x 2 mm.
312
F. KRIENEN
Fig. 4a. Test battery of four wire spark chambers of 512 wires vertical and 256 wires horizontal. The wire pitch is 1 ram, and the chambers are 136 mm spaced apart. The spark-gap pulser is near the top right-hand corner of the spark chambers. The charging capacitor is 1000 pF per chamber. The spark projection chambers are at the very right of the photograph and seen from the side. 200 f~ or more. In o t h e r w o r d s the l o a d o f the open line is usually small when c o m p a r e d to the s p a r k c h a m b e r load. Cb in parallel with Ca via the p r i n t e d c o n d u c t o r s c o n t r i b u t e s little to the fast rise o f the electric field in the s p a r k c h a m b e r . T h e p a r a m e t e r s are by no means critical, nevertheless one w o u l d choose Ca ~ Cd so t h a t m o s t o f the voltage a p p e a r s across the c h a m b e r a n d n o t enough across Cb to p r o d u c e s p u r i o u s sparking, a n d Cc/Cd = 2 to 4, as usual. F u r t h e r m o r e , one w o u l d choose RI Ca = R2 Ca to minimize equalizing currents in the resistive strip across the p r i n t e d wires. A s s o o n as a s p a r k is f o r m e d in the c h a m b e r , the voltage between the c o r r e s p o n d i n g wires d r o p s to a low value. Hence on b o t h sides o f the c h a m b e r there will be a p o t e n t i a l difference between the wire in question a n d the s u r r o u n d i n g wires. Thus an electrom a g n e t i c wave will p r o p a g a t e t o w a r d s the open end o f the p r i n t e d wires. T h e i m p e d a n c e o f this localized system is e s t i m a t e d to be o f the o r d e r o f 50 YL A t the o p e n end a s p a r k will develop if the distance between the wire in question a n d the electrode Cb facing it, is small e n o u g h so t h a t the gap is overvolted. The resistive coating s h o u l d be a p p l i e d so t h a t the resistance between a d j a c e n t wires is large when c o m p a r e d to the wave i m p e d a n c e o f one wire against the s u r r o u n d i n g
Fig. 4b. Frontal view of the spark projection chambers, showing the vertical projection (left) and the horizontal projection (right), side by side, of a straight track of a charged particle traversing the spark chambers.
OPTICAL DIGITIZATION OF WIRE SPARK CHAMBERS wires, thus of the order of 500 ~q. The energy needed to sustain the spark is initially coming from the electric field of several adjacent wires, partly by direct coupling and partly via the coupling capacitance Ca. In this respect the system should have a good multiple spark efficiency. When the secondary sparks have formed, more wires participate in sustaining the three sparks. Presumably the coupling capacitance Cb has a stabilizing effect on spurious sparking and may be adjusted to an optimum value. Finally the resistive chain Rl Rz R'~ may be adjusted to obtain the proper pulse length.
4. Spark projection chamber The aspect of the spark projection chamber is a regular pattern of spark points. The direction of scanning is parallel to the printed mylar strips and the pitch in that direction is 2 mm. The trace width of the printing is 0.5 ram. in the model are 128 spark points on a line. The pitch in the frame scan direction is also 2 ram. With an aspect ratio of 4:3, there are 96 rows of spark points in a full sized television screen. Thus in this configuration 128 × 96 = 12 288 spark chamber wires may be accommodated, see fig. 3. The wear on the spark point, i.e. the copper trace of 35/~m on the mylar, is small. A test showed about 0.1 mm per 106 sparks on the same trace if the trace was connected
313
as cathode, and even less at opposite polarity. Large spark chambers with more wires per frame than go on one scan line will be subdivided and occupy successive rows on the spark projection chamber. Fig. 4 shows four wire spark chambers of 512 wires vertical and 256wires horizontal, thus in total 3072 wires connected up to the spark projection chamber. The vertical wires occupy 16 spark rows and the horizontal wires 8. This model is made to study the performance of a commercial television chain of studio quality. Fig. 5 shows on the left a photograph taken directly from the spark projection chamber, and on the right a photograph of the same event taken from the monitor. The event is a straight track and good correspondence of both images may be deduced. The spark projection chamber is supposed to be just outside a magnet, and the television camera looking at it would be even further away. Hence the residual magnetic shielding of the camera tube would be relatively simple. The object space is obviously reduced to a fiat surface, thus there are no focal depth problems. We emphasize also that no mirrors are required in this electro-optical system.
5. Discussion on electronic read-out The length of the secondary sparks (1.7 ram) is somewhat less than the distance between the spark
Fig. 5. Comparison between a direct photograph of the spark projection chambers (left) and a photograph of the television monitor (right) for the s a m e event. The 25 cm illuminated rulers have half cm divisions. Sparks on individual wires may be seen.
314
F. K R 1 E N E N
rows (2 mm). In our model with about 600 scan lines in a frame and 96 spark rows, there are about six scan lines per row and about five scan lines overlapping a spark. Only one of these, centred on the spark, will be used for electronic read-out, simply by gating out the video signal before processing. Thus only one out of six of the scan lines are occupied in a camera, hence a total of six cameras would fit in a read-out system, utilizing optimum real time. The final capacity of this model would therefore be 6 x 12 288 = 73 778 wires. We speak o f " true optical digitization" if the number of spark points on the screen is limited so that the distance between two adjacent points and the length of the spark are larger than the absolute accuracy of the television system. The parameters involved in this context are the non-linearity and the drift. Conservatively, one may assume that the parameters of the chosen model conform to this criterion. We would even think of putting 256 spark points in a row and 144 rows to fill out a television frame. Thus the capacity per television camera is 36 864 wires, using only one out of four scan lines. Hence a total of four cameras would fit in a read-out system bringing the final capacity to 147 456 wires. This exercise would require more refinements in the camera electronics, but what we emphasize here is the non-utilization of fiducials to reconstruct an event. We obtain thus an appreciable economy in computer time. Of course there will be fiducials on the screen for setting up and for internal feedback in the television electronics. I f we increase still more the number of spark points in a screen, we leave the domain of true optical digitization and deal with an optical analogue-to-digital conversion system. In this respect the limiting parameter will be the resolution of the television system. This automatically involves a liberal use of fiducials to correct for errors. The number of wires per screen becomes of course very large, but that may conflict with the construction of the screen. In any case, the load on the computer will increase and except for the feature of bringing an optical image out of a magnetic field volume, the complications are the same as if the
8 lef[oo 0 . 5 ~
O-ring 44 40 shore in " ' V / / / A Groove2.5x4
;
6
printed c mylar 0.25 V M3 !._..-----~_ 2kg i Fig. 6. C l a m p construction.
spark chambers were directly looked at by a television cameraS). This mode of operation might still find an application in cases where one wants to look at wide gap chambers or at chambers of unusual geometry9). 6. Some observations The conductors between spark chamber and spark projection chamber (figs. 1 and 4) show right-angled bends. Also their developed length varies by as much as 64 cm. The total length is more than the workshop could produce in one piece; hence we connected up two pieces by means of a clamp, see fig. 6. The copper tracings are gold-plated. The capacitance of one clamp against 132 conductors was measured to be less than 100 pF. The fanning out of the conductors at the spark projection plate from 1 m m to 2 mm is by no means essential; in the models, we tried to compromise between dimensional tolerances. However, the fanning out is one more example in which a change of format may be achieved. Another, even more important, format change is the display of a spark chamber wire plane on several scan lines. Finally the third format change is the reduction of the distance between spark chambers, fig. 4, which amounts to 136 m m in real space, to 4 m m for the horizontal wires and to 8 mm for the vertical wires. The projected sparks are not right on the tip of the trace but j u m p erratically on the side edge. This correlates with the observation that the etching on the side is more jagged than on the tip. Presumably in a large spark chamber project one would make the connection between spark chamber and spark projection chamber with commercially available flat cable. In the frequency range of consideration the loss on those cables is of the order of 1 db per metre, hence considerable distances could be tolerated. The flexible cable would allow us to mount the spark projection chamber outside the magnet and well above the gap so as to provide easy access to the equipment in the magnet gap or to provide adequate space for detection equipment outside the gap. The cable would be plugged in on the spark chamber frame which accommodates the printing of the coupling capacitance 6", and the resistive coating. Space requirements on the spark chamber are thus little and stand out favourably against the requirements of other systems using digital read-out. The spark projection chamber of the model, fig. 4, is assembled in two units: one for each projection. In principle, one unit for both projections could be considered. Also, we envisage some development in conjunction with commercially available flat
315
O P T I C A L D I G I T I Z A T I O N OF W I R E S P A R K CHAMBERS
cable. For instance, the coupling electrode Cb could perhaps be a simple wire plane crossing or opposing the partly denuded tips of the flat cables and making an inexpensive and easily exchangeable unit. Logically one ought also to study a "core memory projection unit" based on the same principle as outlined in section 3. Our prediction is that the conditions for flipping cores with the right amount of current are more difficult to achieve. This is in contrast to the production of projected sparks which is insensitive to ringing effects. In addition, we believe that a large system capacity is more economical to obtain via a television chain. Another interesting alternative would be to use magnetic tape as the recording medium1°). Again we propose the same principle of transporting the particle coordinates and to concentrate the information at some convenient distance from the magnet in a small surface. In this case, however, we do not provide light but localized magnetic fields, which would leave an image on the tape strobing the surface. The voltage pulse, as measured on the chamber, has the characteristic shape produced by a capacitor which discharges across another capacitor shunted by a resistor. On this general shape some ringing may be noticed. The general shape does not change whether there are particle tracks or not, only the ringing increases somewhat. Adjacent sparks on the spark projection chamber show up whenever the primary track branches out over adjacent spark chamber wires. As to the traditional question concerning the spark efficiency, we could answer that the efficiency is traditionally 100 per cent. The capacitive coupling C,, C,' between the pulser and the spark chamber proper has the additional advantage of accommodating a clearing field supply not requiring high power coupling capacitors. The various threshold voltages and their relation to the optimum working voltage are of considerable interest. In table 1 a typical case is presented measured on both models, in which the chambers are filled with a mixture of 70% neon and 30% helium, to which is added 13 torr of 2-propanol. In the spark projection chambers pure argon is used. Argon or helium are best for high threshold and high luminance in the spark projection chambers. The particles are from a 9°Sr source. If we define the optimum working voltage as the level at which practically 100%o efficiency is reached, then this level should be little higher than the threshold for producing projected sparks, because characteristic for an overvolted spark gap is the rapid decrease of
TABLE 1 Threshold level (kV on power supply).
Spurious on high-voltage side Spurious on grounded side Spurious in spark chamber Particle on high-voltage side Particle on grounded side Particle in spark chamber Optimum working voltage
Model fig. 1
Model fig. 4
12 > 13 > 13 8.5 8 7.5 8.5
> 9.5 9.5 > 9.5 6.0 6.5 5.5 7.0
spark formation time with overvoltage. The treshold for visible sparks in the spark chamber depends on the conditions of observation. However, they will never be bright, because of the limited amount of energy available, unless projected sparks are produced. The delay between primary sparks and projected sparks should favour high multiple spark efficiency, because spark robbing in conventional chambers is due to small differences in spark formation time, so that a spark channel is formed along the first well-developed streamer preventing the other streamers from maturing into spark channels. In the present chamber however the actual dissipation of field energy starts with the breakdown of the projected spark leaving ample time for all streamers to develop. The commercial television camera used with these models is equipped with a plumbicon tube. The video pulse, as measured on the output of the amplifier housed in the camera head, revealed the following details. Saturation occurs at a lens opening of f:4. The half width is 200 nsec. The pulse height during the first field scan is 400 mV, during the second field 60 mV, during the third field 10 mV, and for the following fields not measurable. The scanning in commercial television sets is 2:1 interlaced, so that the extrapolation to the possible event taking rate is quite difficult. However, the absence of any measurable noise gives some confidence that this rate will be at least 25 Hz. The rate could be possibly higher, eventually by sacrificing some system capacity, if the tube lag does not deteriorate with the ageing of the tube. It is probable that the chamber can be made to work in the trackfollowing mode because the rise-time of the electric field in the chamber is short and the voltage level or the pulse length is not critical. Track following is visually observed with the 9°Sr source. The advantage is obvious: one obtains enhanced accuracy at inclined tracks.
316
F. K R I E N E N
Re[erences 1) F. Krienen, Nucl. Instr. and Meth. 16 (1962) 262. z) G. Charpak et al., Nucl. Instr. and Meth. 62 (1968) 262. 3) A. Michelini et al., The Omega project, proposal for a large magnet and spark chamber system, CERN NP Internal Report 68-11 (1968). 4) L. Resegotti, Progress report on split field magnet, CERN Internal Report ISR-MAG 69-58 (1969). 5) H. Gelernter, Nuovo Cimento 92 (1961) 632. 6) T. M. Buck et al., Bell Syst. Tech. J. (USA) 47 (1968) 1827.
7) Plumbicon camera tubes and their applications, Mullard Tech. Commun. (G. B.) 10, no. 98 (1969) 244. s) N . H . Lipman et al., Paper presented at Intern. Seminar Filmless spark and streamer chambers (Dubna, April 15-18, 1969). 9) F. Krienen, Instrumentation for a large magnet to be used in the CERN Intersecting Storage Rings, Paper presented at Intern. Seminar Filmless spark and streamer chambers (Dubna, April 15-18, 1969). 10) M. Neumann and H. Sherrard, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 295.