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A cylindrical wire chamber
N U C L E A R I N S T R U M E N T S A N D M E T H O D S 57
(I967) 346-350; © N O R T H - H O L L A N D P U B L I S H I N G
CO.
A CYLINDRICAL WIRE CHAMBER D. MILLER and W. H O F F M A N
Haverford College, Haverford, Pennsylvania, U.S.A. Received 14 August 1967 A wire spark chamber has been fabricated in a geometry natural for scattering and production processes and used to study protonproton scattering. The construction of the chamber was possible as soon as new fabrication principles supplemented the magneto-
strictive readout. Also discussed are a simple digitizer requiring only one scaler and a strategy for recognizing straight tracks. Particular care is taken to expose the few assumptions necessary to achieve a logically transparent system.
1. Introduction To date, wire spark chambers 1) and sonic spark chambers 2) have attracted experimenters who found them less expensive in time than optical spark chambers 3) and less expensive in research funds than scintillation counter hodoscopes4). Either type of attraction is soundly based. Actually, the comparison with optical spark chambers, which led to such an early misnomer as "digital discharge plane"S), seems to have obscured some of the promise of wire spark chambers. It is just as easy to bend a wire as to bend a plastic scintillator; to bend light requires lenses. So, without the mechanical impedance of a lens, wire spark chambers may be matched to the natural geometry of the physical problem. This natural geometry may or may not be Cartesian. Together with a uniform field electromagnet, wire spark chambers complete the ubiquitous spectrometer6). Here the geometry of the uniform magnetic field makes Cartesian geometry the natural choice for chambers behind the magnet. Chambers between the target and the magnet contribute to angular measurement as well as to momentum measurement. Consequently, the geometry natural to the physics is no longer so clear. Indeed, the solid angle limitation imposed by the magnet aperture represents a real handicap. Such considerations cause the designer of a system of wire chambers to look at homogeneous detectors such as bubble chambers 7) and wide gap spark chambers 8) for lessons. These optical detectors feature magnetic analysis with much larger detection solid angles and less inherent danger of systematic instrumental bias. During a study of 160 MeV proton-proton scattering with high angular resolution 9) (completed in 1964), a cylindrical wire chamber was used. Its evolution continues today. Wires in the chamber are circles coaxial with the incident beam. For a point target, all points on each circular wire have the same value for the
polar angle in a spherical polar coordinate system. For a long target, several coaxial cylinders of different diameter permit a direct measurement of the scattering angle (fig. 1). By bending the wires in the chamber, azimuthal angular information has been suppressed in the detector with microsecond resolution. Our experience has been that, for azimuthal knowledge, spatial precision is less useful than the contribution of temporal precision in the logical requirement for triggering the wire chambers. The fabrication of the cylindrical wire chamber is described in section 2, including the contribution of the magnetostrictive readout 1°) to this development. Section 3 describes our technique for converting delay lineinformation into spark locations. Section 4 includes a discussion of our recent experience with track recognition routines ~1). 2. Fabrication of the cylindrical wire chamber If each electrode is only as massive as necessary for
346
"
I
SCALE I0:1 TARGET • 2
R
L
Fig. 1. Cylindrical spark chamber for p-p scattering.
A CYLINDRICAL
its electrical function, a detector remains as flexible as possible. Experience with homogeneous optical detectors ~2) has shown how important it is to be able to locate range material or photon converters without affecting the basic operation of the detector. Yet, to maintain uniform spacing between two thin, flexible electrodes (each of which forms part of a large cylindrical surface) is a formidable mechanical problem. Early schemes involved suspending electrodes into the sensitive volume from stronger members outside the volume. They were uniformly unsuccessful, for observable mechanical imperfections have observable electrical consequences. Reproducible large chambers of uniform sensitivity were achieved only when uniform spacing was insured by inserting a positive spacer in the gas between electrodes. For this spacer we selected a fabrication with a very large ratio of strength to weight. Mylar honeycomb, 0.952 cm thick, was purchased 1~) in panel sizes. Cells of 1.905 cm dia. and 0.0076 cm wall thickness have the attractive feature that they will conform to the large diameter cylindrical shape and still preserve the 0.952 cm thickness. Such a positive spacer reasserts the 0.952 em spacing every 1.905 cm without the need for significant mass and solves the mechanical problem. This honeycomb spacer does not disturb the uniform sensitivity of the chamber. To prove this, we fabricated a chamber such that the cell structure was aligned with the structure of the wire electrode. When the chamber was detecting cosmic muons, the computer display of the spatial distribution showed no evidence of the cell structure. That there was no evidence of the cell structure for even a tilted chamber is certainly traceable to our use of a grounded wire anode and a plane pulsed cathode. The sensitivity of the chamber is confined close to the anode region~4). The Mylar honeycomb has such a high ratio of rigidity to mass that it has earned another structural role in our fabrication process. Each inexpensive disposable form is a laminated paper cylinder with a 0.5 cm wall thickness. These forms are laminated and cured on steel mandrels used to make generator bushings and are available 15) in many diameters. On this rigid form, a very light rigid cylinder is easily constructed. A thin Mylar foil is wrapped around, glued to itself, and coated with a very thin film of epoxyl6). Then a Mylar honeycomb panel is cut to size, wrapped around the cylinder, and attached to itself with transparent tape. Finally, a 0.0025 cm aluminum cathode is coated with a very thin film of epoxy, wrapped around the outside, and glued to itself. This forms a rigid cylinder which has little mass, and
WIRE
CHAMBER
347
can be slipped off the paper form when complete. N o w the sensitive volume of the chamber can be fabricated outside this rigid cylinder. Without coating the aluminum cathode with epoxy, a 0.952 cm thick Mylar honeycomb panel is cut to size, wrapped around the cathode, and attached to itself with transparent tape. Then the wire anode is formed by wrapping woven wire cloth 17) around the honeycomb. Wires are 0.0127 cm dia. copper, 20 lines per cm. Cylinder elements are 0.0114 cm dia. acrylic fiber, 16 lines per cm. The end wires are used as fiducial markings. The sensitive volume is completed with a 0.0025 cm thick cylinder of Kapton film. There need be no thick frames, for all the foils are wrapped, rather than stretched. The electrode spacing is assured by the honeycomb. Only the inner cylinder is a structural member. The advent of the magnetostrictive readout ~°) made it attractive to use this natural cylindrical geometry. For the first time, an electrical readout mechanism presented so little mechanical interference compared to cores 1) that it could literally be inserted within the volume of the detector and extract the spatial information. The magnetostrictive wire is positioned outside the K a p t o n film as follows. A long barrier 1.0 cm wide and 0.025 cm high is made with two thicknesses of double coated, pressure sensitive tape. 0.5 cm away, another identical barrier is laid parallel to it. These two barriers form the sides of a tunnel whose top is formed by a piece of 0.0076 cm Mylar. When placed inside the tunnel, the magnetostrictive delay line [-a 0.0127 cm vanadium permendur wire~8)] is kept near the anode wires yet free to support elastic waves. 2.54 cm of transparent tape forms a simple damping pad. Disturbances are attenuated by less than 10% per meter. Above the top of the tunnel, an aluminum foil is added to support image currents and thus increase the output powerS°). Note how simple this readout can be.
3. Digitizing spark locations Our fiducials are observed by passing simulated spark current from the pulser through the two extreme wires of a gap. The distance from the near fiducial wire to the pick-up coil, which is accurately reproduced from gap to gap, is long enough to permit spark chamber noise to subside before the near fiducial pulse reaches the pick-up coil. Under computer control, a different gap passes simulated spark current through its fiducial wires each event. Our discrimination system is straightforward. Fiducial and spark pulses from the pick-up coil are differentiated and preamplified before entering the
348
D. M I L L E R
A N D W. H O F F M A N
50 ohm transmission line. The discriminator terminates the line and yields a standard output pulse when the input pulse crosses zero if the input pulse has also met an earlier amplitude requirement. This unit was fashioned from a difference amplifier to accommodate polarity reversals traceable to the effect of external magnetic fields longitudinal to the delay line. To design a digitizer for converting the occurrence of each standard pulse into a digital number requires considerable judgment. Should one choose the logical techniques of nuclear instrumentation 6) or those of digital instrumentation? The experience of nuclear instrumentation would argue for assigning one scaler to each spark pulse which may occur, and using a scaler scanner to transfer a fixed sequence of scaler readings along a data bus to the computer after the delay line is cleared. The experience of digital scanning would argue for scalers whose contents are transferred onto the data buss whenever a discriminator output occurs. Our choice has been the latter. The digitizer control performs the following functions. When the logical requirement for applying high voltage to the spark chambers is met, the digitizer control disables the nanosecond logic to inhibit subsequent sparks during the digitization, and informs our computer that an event is about to be transferred, so the stored program can accomplish any necessary bookkeeping. After a delay (with clamping) to permit the spark chamber noise to subside, the digitizer control resets all its bistable devices to the zero state and starts the 10 Mc crystal clock. The digitizer includes a master scaler, a gap encoder, and a buffer. The 10 Mc master scaler which counts the crystal clock pulses has 10 binary stages. Its carry propagation delay was decreased by designing the six lower frequency stages with dc carry (for simultaneous transitions) and accepting only the carry propagation delay for the six least significant bits. The result is a carry propagation delay of 60 nsec, and 40 nsec each cycle during which the scaler can be examined without complicating transitions. Whenever a discriminator output occurs, the gap encoder expresses the gap from which it came as a binary number. The gap encoder makes the assumption, excellent for spark pulses, that only one gap will produce a spark pulse during any one of the 4096 periods of the clock. This is the assumption which noticeably simplifies the system. Each discriminator output also is synchronized with the clock, so that the contents of the master scaler can be strobed into the buffer while the scaler can be examined free of transitions. At the same time, the contents of the gap
encoder are strobed into the buffer. No data collection problem arises because a single scaler is used. So each 18 bit word entering the buffer contains 12 less significant bits which give the spark position across the gap and 6 more significant bits which give the gap number. Call this word an uncalibrated spark location. The buffer consists of four 18 bit 10 Mc shift registers, arranged for j a m transfer. The first register receives words from the master scaler and gap encoder. The last register sends words to the input multiplexer of the computer memory. This arrangement permits the buffer, whenever an uncalibrated spark location reaches the last shift register, to defer the central processor for one computer cycle (1/~sec) and transfer a spark location directly into the computer memory. Each of the four shift registers in the buffer has a status bit, so that, whenever any shift register is empty and the next lower shift register is full, the uncalibrated spark location will be jammed up one step in the buffer and the status bits reset appropriately. Consequently, the computer memory can remove a word from the buffer every one microsecond and the lower registers of the buffer are always as empty as possible. Four stages accommodate the stochastic problem with ease. Since the elastic disturbance propagates on the magnetostrictive delay line at 5 km/sec, our clock period corresponds to 0.5 ram. This is comparable with the wire spacing and adequate to extract the available precision6). The entire digitizer has also been used with our electrostrictive readout developed for use in high magnetic fields.
4. Track recognition The first step in the analysis is to apply the fiducial data to calculate distance ratios and thus to convert uncalibrated spark locations into calibrated spark locations. The arrangement for our proton-proton scattering in fig. 1 shows that a cylindrical coordinate system is natural. Each calibrated spark location contains the position of the spark along the axis in units of 1/4096 of the sensitive length as its 12 least significant bits and also contains the radial position of the spark in units of the gap spacing. Each gap spacing is expressed in units of 1/4096 of the sensitive length. Such a fixed number system is very attractive, both for diagnostic display and for following tracks in machine language. Since the calibrated spark locations are listed in memory in order of increasing size, the basic loop of the track recognition routine simply processes this list. Our logical requirement for the application of high voltage to the chambers was a coincidence between
A CYLINDRICAL
counters 1, 2, R and L as shown in fig. 1. Since all experiments will have a similar logical requirement, the track recognition routine itself is reentrant, useful for other problems such as near a magnetic spectrometer, and therefore of general interest. The class of track recognition problems which have a scintillation counter which enters the trigger requirement either upstream or downstream of a field-free chamber is very large. For example, a charged particle passes through the chamber, or a charged particle enters the chamber and expends its range within the chamber, or a charged particle leaves the chamber because a neutral particle was converted within the chamber. One criterion for a good track recognition program is flexibility. If a gap is inoperative or absent, only one constant (one intergap spacing) should have to be changed. The more difficult criterion to achieve is that the track recognition program be logically transparent. This lucidity is what makes it possible to analyze trouble during the experimental period. Experience has proven that the program must be free from issues secondary to the recognition strategy and the track criterion. In every case, confusing secondary issues have been traced to using a list processing technique when it complicates rather than simplifies. The basic tools of our track recognition program are a spark counter, which counts the number of gaps in which a spark has been found within the predicted range, and a miss counter, which counts the number of successive gaps in which a spark has not been found within the predicted range. Our flexible criterion for a track is that the spark counter reads higher than an assigned number before the miss counter exceeds one. The track recognition strategy is based firmly on the fixed point number system, with the highest numbers at the end of the chamber nearest that scintillation counter which triggered the chamber. Originally, the last spark in the event and the first spark in the event are used to predict a road through the chamber which is straight and connects them. In each gap, the region cut by the road is calculated f r o m the road's endpoints and the chamber gap spacings. Then our routine processes the list of calibrated spark locations to determine whether or not each spark is on the road in the intervening gaps. Whenever the spark counter is indexed, the address of that spark is stored temporarily in a list of road members. Whenever the track criterion has been met, the participating road members are stored in the list of output data and the participating road members are erased both from the list of road members and from the
WIRE CHAMBER
349
list of calibrated spark locations. This latter erasure simplifies the recognition strategy. It is justifiable, because it is extremely unlikely that a spark would participate in more than one track. Then the routine uses the last spark in the event and the next spark after the preceding first spark to define a new road. This process continues until each spark location in the last two gaps has terminated a series of roads originating on all other spark locations for which the track criterion could possibly be met. Note that this strategy automatically avoids the logical problems of lateral exit. The characteristic advantage of this recognition strategy is that the predicted road is narrow and the routine is very unlikely to find extra sparks on the road within one gap. This permits the routine to assume that only one spark per gap will be found on the road. This important assumption logically decouples the miss criterion from the predictive stage of the routine and thus reduces the number of working lists to one, a list of road members. The overall consequence of this assumption is a logically tranparent track recognition program. The display of output data for the proton-proton scattering experiment was a plot of the number of events versus the center-of-mass scattering angle. A very useful feature of this general display routine is the capability of changing bin size for the abscissa or magnifying the ordinate from the computer console. As the counting period unfolds, these console features permit intelligent judgments about the counting interval and the best bin size for printout. 5. Conclusions
Our experience throughout the development of this instrument has been that the experimenter has rarely prepared himself to cope with instrumental problems in portions of the instrument developed by others. This problem can be alleviated by making justifiable simplifying assumptions which clarify the entire system logically. Examples of this are the natural geometry, the single scaler, and the erasure of accepted sparks from the processed list. Moreover, the experimenter has rarely prepared himself to cope with the interpretive problems which arise when so much analyzed information is available during the experimental period. That is a problem to which we will have to grow up. After all, herein lies the fun of studying physics experimentally. We should spend more time faced with this problem. Others made contributions to an earlier version of the detector. In particular, P. deBruyne worked with the chamber and digitizer and C. Freed worked with the track recognition program.
350
D. MILLER AND W. HOFFMAN
References 1) F. Krienen, Nucl. Instr. and Meth. 16 (1962) 262. 2) B. C. Maglic, Nucl. Instr. and Meth. 20 (1963) 165. 8) S. Fukui and S. Miyamoto, Nuovo Cimento 11 (1959) 113. 4) K. J. Foley, S. J. Lindenbaum, W. A. Love, S. Ozaki, J. J. Russell and L. C. L. Yuan, Nucl. Instr. and Meth. 30 (1964) 45. 5) G. B. Collins, Proc. C E R N Conf. on Filmless Spark Chamber Techniques and Associated Computer Use, C E R N 64-30 (June, 1964). 0) V. Perez-Mendez, T. J. Devlin, J. Solomon and T. F. Droege, Nucl. Instr. and Meth. 46 (1967) 197. 7) D. A. Glaser; Phys. Rev. 91 (1953) 762. 8) A. I. Alikhanian, T. L. Asiatiani, E. M. Matevosian and R. O. Sharkhatunian, Phys. Letters 4 (1963) 295. 9) C. Freed, P. deBruyne and D. G. Miller, Bull. Am. Phys. Soc. Series II, 10, no. 1 (1965) 79.
10) V. Perez Mendez and J. M. Pfab, Nucl. Instr. and Meth. 33 (1965) 141. 11) D. Miller and C. Freed, Proc. CERN Conf. on Filmless Spark Chamber Techniques and Associated Computer Use, CERN 64-30 (June, 1964). 12) R. W. Williams, Can. J. Phys. 37 (1959) 1085. la) Hexcel Products, Inc., Berkeley, Calif., U.S.A. 14) D. Miller and P. de Bruyne, Proc. CERN. on Filmless Spark Chamber Techniques and Associated Computer Use, CERN 64-30 (June, 1964). 15) General Electric, Bushing Products Division, Pittsfield, Mass. U.S.A. 16) Allaco Products, Braintree, Mass. U.S.A. 17) Science Accessories Corp., Sourthport, Conn., U.S.A. 18) Telcon Metals, Ltd., Crawley, Sussex, U.K.
(I967) 346-350; © N O R T H - H O L L A N D P U B L I S H I N G
CO.
A CYLINDRICAL WIRE CHAMBER D. MILLER and W. H O F F M A N
Haverford College, Haverford, Pennsylvania, U.S.A. Received 14 August 1967 A wire spark chamber has been fabricated in a geometry natural for scattering and production processes and used to study protonproton scattering. The construction of the chamber was possible as soon as new fabrication principles supplemented the magneto-
strictive readout. Also discussed are a simple digitizer requiring only one scaler and a strategy for recognizing straight tracks. Particular care is taken to expose the few assumptions necessary to achieve a logically transparent system.
1. Introduction To date, wire spark chambers 1) and sonic spark chambers 2) have attracted experimenters who found them less expensive in time than optical spark chambers 3) and less expensive in research funds than scintillation counter hodoscopes4). Either type of attraction is soundly based. Actually, the comparison with optical spark chambers, which led to such an early misnomer as "digital discharge plane"S), seems to have obscured some of the promise of wire spark chambers. It is just as easy to bend a wire as to bend a plastic scintillator; to bend light requires lenses. So, without the mechanical impedance of a lens, wire spark chambers may be matched to the natural geometry of the physical problem. This natural geometry may or may not be Cartesian. Together with a uniform field electromagnet, wire spark chambers complete the ubiquitous spectrometer6). Here the geometry of the uniform magnetic field makes Cartesian geometry the natural choice for chambers behind the magnet. Chambers between the target and the magnet contribute to angular measurement as well as to momentum measurement. Consequently, the geometry natural to the physics is no longer so clear. Indeed, the solid angle limitation imposed by the magnet aperture represents a real handicap. Such considerations cause the designer of a system of wire chambers to look at homogeneous detectors such as bubble chambers 7) and wide gap spark chambers 8) for lessons. These optical detectors feature magnetic analysis with much larger detection solid angles and less inherent danger of systematic instrumental bias. During a study of 160 MeV proton-proton scattering with high angular resolution 9) (completed in 1964), a cylindrical wire chamber was used. Its evolution continues today. Wires in the chamber are circles coaxial with the incident beam. For a point target, all points on each circular wire have the same value for the
polar angle in a spherical polar coordinate system. For a long target, several coaxial cylinders of different diameter permit a direct measurement of the scattering angle (fig. 1). By bending the wires in the chamber, azimuthal angular information has been suppressed in the detector with microsecond resolution. Our experience has been that, for azimuthal knowledge, spatial precision is less useful than the contribution of temporal precision in the logical requirement for triggering the wire chambers. The fabrication of the cylindrical wire chamber is described in section 2, including the contribution of the magnetostrictive readout 1°) to this development. Section 3 describes our technique for converting delay lineinformation into spark locations. Section 4 includes a discussion of our recent experience with track recognition routines ~1). 2. Fabrication of the cylindrical wire chamber If each electrode is only as massive as necessary for
346
"
I
SCALE I0:1 TARGET • 2
R
L
Fig. 1. Cylindrical spark chamber for p-p scattering.
A CYLINDRICAL
its electrical function, a detector remains as flexible as possible. Experience with homogeneous optical detectors ~2) has shown how important it is to be able to locate range material or photon converters without affecting the basic operation of the detector. Yet, to maintain uniform spacing between two thin, flexible electrodes (each of which forms part of a large cylindrical surface) is a formidable mechanical problem. Early schemes involved suspending electrodes into the sensitive volume from stronger members outside the volume. They were uniformly unsuccessful, for observable mechanical imperfections have observable electrical consequences. Reproducible large chambers of uniform sensitivity were achieved only when uniform spacing was insured by inserting a positive spacer in the gas between electrodes. For this spacer we selected a fabrication with a very large ratio of strength to weight. Mylar honeycomb, 0.952 cm thick, was purchased 1~) in panel sizes. Cells of 1.905 cm dia. and 0.0076 cm wall thickness have the attractive feature that they will conform to the large diameter cylindrical shape and still preserve the 0.952 cm thickness. Such a positive spacer reasserts the 0.952 em spacing every 1.905 cm without the need for significant mass and solves the mechanical problem. This honeycomb spacer does not disturb the uniform sensitivity of the chamber. To prove this, we fabricated a chamber such that the cell structure was aligned with the structure of the wire electrode. When the chamber was detecting cosmic muons, the computer display of the spatial distribution showed no evidence of the cell structure. That there was no evidence of the cell structure for even a tilted chamber is certainly traceable to our use of a grounded wire anode and a plane pulsed cathode. The sensitivity of the chamber is confined close to the anode region~4). The Mylar honeycomb has such a high ratio of rigidity to mass that it has earned another structural role in our fabrication process. Each inexpensive disposable form is a laminated paper cylinder with a 0.5 cm wall thickness. These forms are laminated and cured on steel mandrels used to make generator bushings and are available 15) in many diameters. On this rigid form, a very light rigid cylinder is easily constructed. A thin Mylar foil is wrapped around, glued to itself, and coated with a very thin film of epoxyl6). Then a Mylar honeycomb panel is cut to size, wrapped around the cylinder, and attached to itself with transparent tape. Finally, a 0.0025 cm aluminum cathode is coated with a very thin film of epoxy, wrapped around the outside, and glued to itself. This forms a rigid cylinder which has little mass, and
WIRE
CHAMBER
347
can be slipped off the paper form when complete. N o w the sensitive volume of the chamber can be fabricated outside this rigid cylinder. Without coating the aluminum cathode with epoxy, a 0.952 cm thick Mylar honeycomb panel is cut to size, wrapped around the cathode, and attached to itself with transparent tape. Then the wire anode is formed by wrapping woven wire cloth 17) around the honeycomb. Wires are 0.0127 cm dia. copper, 20 lines per cm. Cylinder elements are 0.0114 cm dia. acrylic fiber, 16 lines per cm. The end wires are used as fiducial markings. The sensitive volume is completed with a 0.0025 cm thick cylinder of Kapton film. There need be no thick frames, for all the foils are wrapped, rather than stretched. The electrode spacing is assured by the honeycomb. Only the inner cylinder is a structural member. The advent of the magnetostrictive readout ~°) made it attractive to use this natural cylindrical geometry. For the first time, an electrical readout mechanism presented so little mechanical interference compared to cores 1) that it could literally be inserted within the volume of the detector and extract the spatial information. The magnetostrictive wire is positioned outside the K a p t o n film as follows. A long barrier 1.0 cm wide and 0.025 cm high is made with two thicknesses of double coated, pressure sensitive tape. 0.5 cm away, another identical barrier is laid parallel to it. These two barriers form the sides of a tunnel whose top is formed by a piece of 0.0076 cm Mylar. When placed inside the tunnel, the magnetostrictive delay line [-a 0.0127 cm vanadium permendur wire~8)] is kept near the anode wires yet free to support elastic waves. 2.54 cm of transparent tape forms a simple damping pad. Disturbances are attenuated by less than 10% per meter. Above the top of the tunnel, an aluminum foil is added to support image currents and thus increase the output powerS°). Note how simple this readout can be.
3. Digitizing spark locations Our fiducials are observed by passing simulated spark current from the pulser through the two extreme wires of a gap. The distance from the near fiducial wire to the pick-up coil, which is accurately reproduced from gap to gap, is long enough to permit spark chamber noise to subside before the near fiducial pulse reaches the pick-up coil. Under computer control, a different gap passes simulated spark current through its fiducial wires each event. Our discrimination system is straightforward. Fiducial and spark pulses from the pick-up coil are differentiated and preamplified before entering the
348
D. M I L L E R
A N D W. H O F F M A N
50 ohm transmission line. The discriminator terminates the line and yields a standard output pulse when the input pulse crosses zero if the input pulse has also met an earlier amplitude requirement. This unit was fashioned from a difference amplifier to accommodate polarity reversals traceable to the effect of external magnetic fields longitudinal to the delay line. To design a digitizer for converting the occurrence of each standard pulse into a digital number requires considerable judgment. Should one choose the logical techniques of nuclear instrumentation 6) or those of digital instrumentation? The experience of nuclear instrumentation would argue for assigning one scaler to each spark pulse which may occur, and using a scaler scanner to transfer a fixed sequence of scaler readings along a data bus to the computer after the delay line is cleared. The experience of digital scanning would argue for scalers whose contents are transferred onto the data buss whenever a discriminator output occurs. Our choice has been the latter. The digitizer control performs the following functions. When the logical requirement for applying high voltage to the spark chambers is met, the digitizer control disables the nanosecond logic to inhibit subsequent sparks during the digitization, and informs our computer that an event is about to be transferred, so the stored program can accomplish any necessary bookkeeping. After a delay (with clamping) to permit the spark chamber noise to subside, the digitizer control resets all its bistable devices to the zero state and starts the 10 Mc crystal clock. The digitizer includes a master scaler, a gap encoder, and a buffer. The 10 Mc master scaler which counts the crystal clock pulses has 10 binary stages. Its carry propagation delay was decreased by designing the six lower frequency stages with dc carry (for simultaneous transitions) and accepting only the carry propagation delay for the six least significant bits. The result is a carry propagation delay of 60 nsec, and 40 nsec each cycle during which the scaler can be examined without complicating transitions. Whenever a discriminator output occurs, the gap encoder expresses the gap from which it came as a binary number. The gap encoder makes the assumption, excellent for spark pulses, that only one gap will produce a spark pulse during any one of the 4096 periods of the clock. This is the assumption which noticeably simplifies the system. Each discriminator output also is synchronized with the clock, so that the contents of the master scaler can be strobed into the buffer while the scaler can be examined free of transitions. At the same time, the contents of the gap
encoder are strobed into the buffer. No data collection problem arises because a single scaler is used. So each 18 bit word entering the buffer contains 12 less significant bits which give the spark position across the gap and 6 more significant bits which give the gap number. Call this word an uncalibrated spark location. The buffer consists of four 18 bit 10 Mc shift registers, arranged for j a m transfer. The first register receives words from the master scaler and gap encoder. The last register sends words to the input multiplexer of the computer memory. This arrangement permits the buffer, whenever an uncalibrated spark location reaches the last shift register, to defer the central processor for one computer cycle (1/~sec) and transfer a spark location directly into the computer memory. Each of the four shift registers in the buffer has a status bit, so that, whenever any shift register is empty and the next lower shift register is full, the uncalibrated spark location will be jammed up one step in the buffer and the status bits reset appropriately. Consequently, the computer memory can remove a word from the buffer every one microsecond and the lower registers of the buffer are always as empty as possible. Four stages accommodate the stochastic problem with ease. Since the elastic disturbance propagates on the magnetostrictive delay line at 5 km/sec, our clock period corresponds to 0.5 ram. This is comparable with the wire spacing and adequate to extract the available precision6). The entire digitizer has also been used with our electrostrictive readout developed for use in high magnetic fields.
4. Track recognition The first step in the analysis is to apply the fiducial data to calculate distance ratios and thus to convert uncalibrated spark locations into calibrated spark locations. The arrangement for our proton-proton scattering in fig. 1 shows that a cylindrical coordinate system is natural. Each calibrated spark location contains the position of the spark along the axis in units of 1/4096 of the sensitive length as its 12 least significant bits and also contains the radial position of the spark in units of the gap spacing. Each gap spacing is expressed in units of 1/4096 of the sensitive length. Such a fixed number system is very attractive, both for diagnostic display and for following tracks in machine language. Since the calibrated spark locations are listed in memory in order of increasing size, the basic loop of the track recognition routine simply processes this list. Our logical requirement for the application of high voltage to the chambers was a coincidence between
A CYLINDRICAL
counters 1, 2, R and L as shown in fig. 1. Since all experiments will have a similar logical requirement, the track recognition routine itself is reentrant, useful for other problems such as near a magnetic spectrometer, and therefore of general interest. The class of track recognition problems which have a scintillation counter which enters the trigger requirement either upstream or downstream of a field-free chamber is very large. For example, a charged particle passes through the chamber, or a charged particle enters the chamber and expends its range within the chamber, or a charged particle leaves the chamber because a neutral particle was converted within the chamber. One criterion for a good track recognition program is flexibility. If a gap is inoperative or absent, only one constant (one intergap spacing) should have to be changed. The more difficult criterion to achieve is that the track recognition program be logically transparent. This lucidity is what makes it possible to analyze trouble during the experimental period. Experience has proven that the program must be free from issues secondary to the recognition strategy and the track criterion. In every case, confusing secondary issues have been traced to using a list processing technique when it complicates rather than simplifies. The basic tools of our track recognition program are a spark counter, which counts the number of gaps in which a spark has been found within the predicted range, and a miss counter, which counts the number of successive gaps in which a spark has not been found within the predicted range. Our flexible criterion for a track is that the spark counter reads higher than an assigned number before the miss counter exceeds one. The track recognition strategy is based firmly on the fixed point number system, with the highest numbers at the end of the chamber nearest that scintillation counter which triggered the chamber. Originally, the last spark in the event and the first spark in the event are used to predict a road through the chamber which is straight and connects them. In each gap, the region cut by the road is calculated f r o m the road's endpoints and the chamber gap spacings. Then our routine processes the list of calibrated spark locations to determine whether or not each spark is on the road in the intervening gaps. Whenever the spark counter is indexed, the address of that spark is stored temporarily in a list of road members. Whenever the track criterion has been met, the participating road members are stored in the list of output data and the participating road members are erased both from the list of road members and from the
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list of calibrated spark locations. This latter erasure simplifies the recognition strategy. It is justifiable, because it is extremely unlikely that a spark would participate in more than one track. Then the routine uses the last spark in the event and the next spark after the preceding first spark to define a new road. This process continues until each spark location in the last two gaps has terminated a series of roads originating on all other spark locations for which the track criterion could possibly be met. Note that this strategy automatically avoids the logical problems of lateral exit. The characteristic advantage of this recognition strategy is that the predicted road is narrow and the routine is very unlikely to find extra sparks on the road within one gap. This permits the routine to assume that only one spark per gap will be found on the road. This important assumption logically decouples the miss criterion from the predictive stage of the routine and thus reduces the number of working lists to one, a list of road members. The overall consequence of this assumption is a logically tranparent track recognition program. The display of output data for the proton-proton scattering experiment was a plot of the number of events versus the center-of-mass scattering angle. A very useful feature of this general display routine is the capability of changing bin size for the abscissa or magnifying the ordinate from the computer console. As the counting period unfolds, these console features permit intelligent judgments about the counting interval and the best bin size for printout. 5. Conclusions
Our experience throughout the development of this instrument has been that the experimenter has rarely prepared himself to cope with instrumental problems in portions of the instrument developed by others. This problem can be alleviated by making justifiable simplifying assumptions which clarify the entire system logically. Examples of this are the natural geometry, the single scaler, and the erasure of accepted sparks from the processed list. Moreover, the experimenter has rarely prepared himself to cope with the interpretive problems which arise when so much analyzed information is available during the experimental period. That is a problem to which we will have to grow up. After all, herein lies the fun of studying physics experimentally. We should spend more time faced with this problem. Others made contributions to an earlier version of the detector. In particular, P. deBruyne worked with the chamber and digitizer and C. Freed worked with the track recognition program.
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