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The hybrid chamber: A proportional chamber with gated spark readout
NUCLEAR
INSTRUMENTS
AND
METHODS
IOl
(I972)
4OI-404;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E H Y B R I D CHAMBER: A P R O P O R T I O N A L C H A M B E R W I T H GATED SPARK R E A D O U T * J. F I S C H E R a n d S. S H I B A T A
Brookhaven National Laboratory, Upton, N.Y. 11973, U.S.A. Received 24 J a n u a r y 1972 T h e hybrid c h a m b e r c o m b i n e s the fast resolving time ( ' ~ 75 ns) o f the multiwire p r o p o r t i o n a l c h a m b e r with the event selectivity, spatial resolution ( ~ ' 0 . 3 m m ) , a n d " i n e x p e n s i v e " r e a d o u t o f the spark c h a m b e r . Some characteristics are described.
The efficient utilization of high flux, external particle beams in experiments at the AGS and other high energy particle accelerators makes stringent requirements on the time resolution of the particle locating devices. The new device, the '° hybrid chamber", improves this time resolution and maximum event recording rates by about a factor of ten over present spark chamber practice. It combines the advantages of the short time resolution of the multiwire proportional chamber 1) with the event-selecting ability and the inexpensive readout of the pulsed spark chamber as shown in table 1. TABLE 1 C o m p a r i s o n o f characteristics for two conventional c h a m b e r s a n d the hybrid chamber.
Resolving time in experiments Recovery time for recorded events Spatial resolution
Spark chamber
Hybrid chamber
Proportional chamber
400-800 ns
~ 75 ns
30-100 ns
" 1 ms ~ 0.3 m m
' ~ 80/~s ~ 0.3 m m
~< 1/is 1 mm
The hybrid chamber, described here and earlier at several conferences 2-5) and seminars, consists of a proportional wire chamber gap (without amplifiers), which is read out, on selected events, by a narrow gap pulsed spark chamber. The two gaps are separated by an electron drift space which provides a variable delay time for synchronization with associated event selecting counter logic of experiments. * This w o r k was p e r f o r m e d u n d e r the auspices o f the U.S. A t o m i c Energy C o m m i s s i o n .
401
The arrangement is shown in fig. 1. It is similar to the "Transfer Chamber ''2) presented by the authors in 1968. Successive electrodes are connected to successively higher positive dc potentials. However, the last gap is operated in the pulsed spark chamber mode, a variation mentioned earlier2'3). The hv pulse applied to spark this gap serves as a time-gate for reading out the track positions of a selected event. A small test chamber was developed and operated by the authors in December 1968 and several others up to 15x 15 cm early in 19693'4). The operation of the chamber is as follows. The electrons from any particle track in the proportional gap, which is the only gap sensitive to tracks, drift to the high field region at the anode wires and form avalanches which spread around the wire into the field of the second gap. A large number of the electrons of these avalanches drift through the delay gap. The drift time depends on the gap width, gas, and field strength. It is adjusted to compensate for the delays in the associated counters and logic circuits, which recognize a particular type of nuclear interaction in experiments. The electron cloud then drifts into the last narrow gap and normally vanishes on the last electrode. If, however, the event was selected by the counter logic, then a high voltage pulse of short duration is applied to the last gap
D.C. (--) PROPORTIONAL MODE GAP •
,,:~ •
•
•
•
,,,$'k° '~
•
•
•
•
•
•C-)
VARIABLE DRIFT-DELAY GAP ::: \ • (q,,) PULSED SPARK GAP : : : : : i ~\" : : : : "{+)end J-L FOR X,Y READOUT ~ H V PULSE PARTICLE TRACK Fig. 1. Hybrid c h a m b e r diagram. T h e avalanche f o r m e d on the second electrode drifts into the last gap where a gated high voltage pulse f o r m s a s p a r k for readout.
402
J. F I S C H E R
A N D S. S H I B A T A
while the electron cloud drifts in it. A spark is formed which can be read out (X and Y) by conventional methods, e.g. magnetostriction, memory cores, or nonmagnetic means. If desired, a third coordinate direction readout can be obtained from an additional spark gap adjacent to the last one. The high voltage pulse amplitude and duration must be controlled, so that sparks form only from electron clouds, but not from the few electrons of particle tracks. The position readouts, even for inclined tracks, always refer to the track position in the proportional plane only, since the avalanches drift normal to the electrodes. The time resolution of the hybrid chamber is mainly determined by the drift time of the track electrons in the proportional gap. Except for electron diffusion effects, it is independent of the delay in the associated event-recognition system of the high energy experiment. Normal spark chambers store tracks during this delay time, but the transfer or hybrid chamber preserves the time sequence of events by continuously drifting the track information out of the sensitive region and because the delay gap and read out gap are not sensitive to tracks. The recovery time after a spark is shorter than in standard spark chambers because of the much narrower gap, which permits rapid clearing of ions left by a spark. The spatial resolution is better than in standard proportional chambers because the anode wires in the proportional gap of the hybrid chamber can be closer together as less gasmultiplication is required. The hybrid chamber also reads out the track position along the direction of the proportional gap anode wires where no significant quantization is present.
The hybrid chamber's characteristics and some effects of parameters will be discussed next. The performance curves of figs. 2 and 3 refer to a chamber with electrodes of ~ 50/~m diam. stainless steel wires spaced '~ 0.6 mm, with gaps " 6.6, "-~6.4, and "-~ 1.6 mm. The dimensions are not critical if the best time resolution is not required. The proportional wires can be of smaller diameter (e.g. 20 pm) or more widely spaced, e.g. 1 mm, for improved amplification with little loss of spatial or time resolution. One of the faster electron drift gases permitting close wire spacing and low operating voltage is spark chamber neon ( N e + 1 0 % He) with 7 0 t o r r methyl alcohol (spectral grade). This vapor forms only little deposits in the spark gap. The electric fields for this gas mixture are indicated in figs. 2 and 3. Less vapor pressure or other quench gases also work. A little argon helps the efficiency but mixtures rich in argon require higher fields, and may not permit close wire spacing, nor very narrow spark gaps. The maximum efficiency was about 97%. With a larger proportional gap, e.g. ~ 10 mm, the efficiency can be about 100% but the resolving time increases almost proportionately with the proportional gap width. The short resolving time of the hybrid chamber is apparent from the curves in fig. 2. The sparking efficiency versus delay between passage of a particle and the high voltage pulse is plotted for various delay-gap voltages. The resolving time is about 75 ns (fwhm). A slight broadening to 85 ns at longer delays may be due to diffusion in the delay gap. Similar resolving times were obtained with tests of two particles occuring at various delays from each other. The delay range can be increased by using a wider delay gap, up to 25 mm was tried, because a high field
)
,
90
P R O P A M P L G A P G.6rnm ' - ..... D E L A Y G ; A P ,~,,4-mm PULSED SPARK G A P : - - - : - - " - ' l I,~mrn
80
?.~--- 70 ~
f
-,,~
. . . . . . . , ~ - ~ ) - H V-7_ +E¢
. v , - HV2 = :~oo,Z.
- 85
H V 2 = $ 0 0 - IOCO V. EC = -t- 2 0 0 ~ D C ELECTRODE WIRES~I6/cm,~50P .mD~A.~.~. GA~: Ne+ IO%He + 70 l"orr METHVL ALCOHOL
~l 4-0
30
(,3
~-~-~,,
T "rR,GG='R~r, "BY" ~ A~.i.°KsVo:,~s E PAR'r IC LE ~£LECTINql " COUI~ITERS
')-
W ~Jso
• •
IO i
,~k
,
,
,
Oleo ZOO Z50 3o0 ~5D 400 4do 50o 550 TOTAL DELAy
(nsec)
Fig. 2. Resolving time of the hybrid chamber. Curves of sparking efficiencyvs delay time of hv pulse for various delay gap fields.
THE HYBRID
403
CHAMBER
HYBRID CHAMBER GAS: Ne +03 He+7Otorr CH30H PROPORTIONAL GAP:6.6mm; DELAY GAP: 6.4rnm; PULSED SPARK GAP: 1.6mrn H.V. PULSE: + 2 KV; 50nsec. ELECTRODE WIRES: SSO.05mrn DIA,16 WIRES PER crn.
I00 -
\
-
~ 60
|
rr
//
a.
PROPORTIONAL GAP
\
\~
~
(--)
(-1 (o) (÷)
D.C. BIAS: + 200V. "L I ~ ~ PULSED BIAS(tp) J / /. /
. ;;÷
_z 40
_~ 3BOO.V.
DELAY GAP ~ 700V. PULSED SPARK GAPZ ~ - ~ - ~ - -~.~
/
IstSPARK
o/
H.V. TESTPULSE
td
.[
hi
4/~ sec.
20
o
0
~.~,,~ I
I i I I ~x::==~l 20 40 60 DELAY OF SECOND H.V. PULSE AFTER FIRST SPARK
t p = t d - IO/u,sec. I 80
.t d (p.sec)
Fig. 3. Recovery time o f 1.6 m m s p a r k gap. Curves o f retiring probability vs time after a read o u t s p a r k ; at 200 V dc with additional pulsed clearing fields.
strength reduces diffusion effects on the time resolution as shown in table 2. TABLE 2
Relative resolving times in ns for two drift gap widths at various delays. Drift gap width
370
500
600
Delay time (ns)
6.6 m m 13 m m
90 80
110 90
130 100
Resolving time (fwhm) (ns)
It was observed in the ranges of interest that for given high field strengths at various drift gaps, the resolving time increases by about 5 ns per 100 ns of additional delay. An increase in the spark gap from 1.6 m m to 3 . 2 m m increases the resolving time only about 10-15%. However, a wider spark gap can disturb the discrimination between avalanches and other track electrons. The short recovery time after a read-out spark on selected events is presented in fig. 3. The curves show
the retiring probability at various times after a read out spark when tested with a second high voltage pulse for the 1.6 m m gap. The recovery time is only ~_ 80/ts and even less with a pulsed clearing field. Larger gaps, e.g. 3.2 mm, have also been tried. They increase the recovery time which is usually not important. The spatial resolution, as indicated by preliminary tests, is about the same as in regular wire spark chambers or about 0.3 m m for normal incident tracks and about 0.4 m m for tracks at 30 degrees to the normal. A small systematic shift in the apparent track position in the direction of the track projection on the proportional gap anode plane can be observed. A correction can be applied. Encouraged by the success with the small hybrid chambers described here, a larger chamber ~ 70 cm has been built by Garelick and Moromisato6). The preparation of several medium size hybrid chambers by a group from Karlsruhe for an experiment at C E R N has also been reported recently7). The hybrid chambers can also be operated in other modes. Early proportional chamber monitor signals are obtainable from the cathode and the first anode. The anode wires can be combined in groups and connected
404
J. FISCHER AND S. SHIBATA
to one or several amplifiers, if desired, for rough kinematic analysis and decision making; e.g. to inhibit or select spark triggers or for labelling. Self-triggering of the hybrid chamber can also be initiated from this signal, which permits the chamber to operate even with particles stopping in the first gap. This mode is useful for low energy particles and for X-ray and neutron applications. In another simplified, but not necessarily gated mode, even the last gap can be operated at a fixed dc potential only (without any pulsing). Depending on the selected field strength and wire geometry, the avalanches can be made to develop into sparks at low rates or the avalanches can just be multiplied in the semi-proportional mode for easier readout at high rates.
References 1) G. Charpak et al., Nucl. Instr. and Meth. 62 (1968) 262. z) j. Fischer and S. Shibata, Proc. Intern. Syrup. Nuclear electronics, Versailles, Sept. 1968, vol. 3 (Documentation Franfalse, Paris, 1969) p. 2-1 ; also BNL 12804. 8) Brookhaven National Laboratory Annual Report for July 68 to June 69, BNL 50169 or TID 4500 (July 1969) p. 69. 4) J. Fischer and S. Shibata, Brookhaven National Laboratory High Energy Discussion Group Report BNL 14766 (13 Dec. 1969) unpublished. 5) J. Fischer and S. Shibata, Intern. Conf. Instrumentation for high energy physics (Dubna, Sept. 1970); also BNL 14899.
6) D. Garelick and J. Moromisato, Northeastern Univ., NUB 7)
2004 Rev. (1970) unpublished; and submitted to D ubna Conf., see ref. 5; also Letter in this issue. CERN Courier 11, no. 8 (1971) 231.
INSTRUMENTS
AND
METHODS
IOl
(I972)
4OI-404;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E H Y B R I D CHAMBER: A P R O P O R T I O N A L C H A M B E R W I T H GATED SPARK R E A D O U T * J. F I S C H E R a n d S. S H I B A T A
Brookhaven National Laboratory, Upton, N.Y. 11973, U.S.A. Received 24 J a n u a r y 1972 T h e hybrid c h a m b e r c o m b i n e s the fast resolving time ( ' ~ 75 ns) o f the multiwire p r o p o r t i o n a l c h a m b e r with the event selectivity, spatial resolution ( ~ ' 0 . 3 m m ) , a n d " i n e x p e n s i v e " r e a d o u t o f the spark c h a m b e r . Some characteristics are described.
The efficient utilization of high flux, external particle beams in experiments at the AGS and other high energy particle accelerators makes stringent requirements on the time resolution of the particle locating devices. The new device, the '° hybrid chamber", improves this time resolution and maximum event recording rates by about a factor of ten over present spark chamber practice. It combines the advantages of the short time resolution of the multiwire proportional chamber 1) with the event-selecting ability and the inexpensive readout of the pulsed spark chamber as shown in table 1. TABLE 1 C o m p a r i s o n o f characteristics for two conventional c h a m b e r s a n d the hybrid chamber.
Resolving time in experiments Recovery time for recorded events Spatial resolution
Spark chamber
Hybrid chamber
Proportional chamber
400-800 ns
~ 75 ns
30-100 ns
" 1 ms ~ 0.3 m m
' ~ 80/~s ~ 0.3 m m
~< 1/is 1 mm
The hybrid chamber, described here and earlier at several conferences 2-5) and seminars, consists of a proportional wire chamber gap (without amplifiers), which is read out, on selected events, by a narrow gap pulsed spark chamber. The two gaps are separated by an electron drift space which provides a variable delay time for synchronization with associated event selecting counter logic of experiments. * This w o r k was p e r f o r m e d u n d e r the auspices o f the U.S. A t o m i c Energy C o m m i s s i o n .
401
The arrangement is shown in fig. 1. It is similar to the "Transfer Chamber ''2) presented by the authors in 1968. Successive electrodes are connected to successively higher positive dc potentials. However, the last gap is operated in the pulsed spark chamber mode, a variation mentioned earlier2'3). The hv pulse applied to spark this gap serves as a time-gate for reading out the track positions of a selected event. A small test chamber was developed and operated by the authors in December 1968 and several others up to 15x 15 cm early in 19693'4). The operation of the chamber is as follows. The electrons from any particle track in the proportional gap, which is the only gap sensitive to tracks, drift to the high field region at the anode wires and form avalanches which spread around the wire into the field of the second gap. A large number of the electrons of these avalanches drift through the delay gap. The drift time depends on the gap width, gas, and field strength. It is adjusted to compensate for the delays in the associated counters and logic circuits, which recognize a particular type of nuclear interaction in experiments. The electron cloud then drifts into the last narrow gap and normally vanishes on the last electrode. If, however, the event was selected by the counter logic, then a high voltage pulse of short duration is applied to the last gap
D.C. (--) PROPORTIONAL MODE GAP •
,,:~ •
•
•
•
,,,$'k° '~
•
•
•
•
•
•C-)
VARIABLE DRIFT-DELAY GAP ::: \ • (q,,) PULSED SPARK GAP : : : : : i ~\" : : : : "{+)end J-L FOR X,Y READOUT ~ H V PULSE PARTICLE TRACK Fig. 1. Hybrid c h a m b e r diagram. T h e avalanche f o r m e d on the second electrode drifts into the last gap where a gated high voltage pulse f o r m s a s p a r k for readout.
402
J. F I S C H E R
A N D S. S H I B A T A
while the electron cloud drifts in it. A spark is formed which can be read out (X and Y) by conventional methods, e.g. magnetostriction, memory cores, or nonmagnetic means. If desired, a third coordinate direction readout can be obtained from an additional spark gap adjacent to the last one. The high voltage pulse amplitude and duration must be controlled, so that sparks form only from electron clouds, but not from the few electrons of particle tracks. The position readouts, even for inclined tracks, always refer to the track position in the proportional plane only, since the avalanches drift normal to the electrodes. The time resolution of the hybrid chamber is mainly determined by the drift time of the track electrons in the proportional gap. Except for electron diffusion effects, it is independent of the delay in the associated event-recognition system of the high energy experiment. Normal spark chambers store tracks during this delay time, but the transfer or hybrid chamber preserves the time sequence of events by continuously drifting the track information out of the sensitive region and because the delay gap and read out gap are not sensitive to tracks. The recovery time after a spark is shorter than in standard spark chambers because of the much narrower gap, which permits rapid clearing of ions left by a spark. The spatial resolution is better than in standard proportional chambers because the anode wires in the proportional gap of the hybrid chamber can be closer together as less gasmultiplication is required. The hybrid chamber also reads out the track position along the direction of the proportional gap anode wires where no significant quantization is present.
The hybrid chamber's characteristics and some effects of parameters will be discussed next. The performance curves of figs. 2 and 3 refer to a chamber with electrodes of ~ 50/~m diam. stainless steel wires spaced '~ 0.6 mm, with gaps " 6.6, "-~6.4, and "-~ 1.6 mm. The dimensions are not critical if the best time resolution is not required. The proportional wires can be of smaller diameter (e.g. 20 pm) or more widely spaced, e.g. 1 mm, for improved amplification with little loss of spatial or time resolution. One of the faster electron drift gases permitting close wire spacing and low operating voltage is spark chamber neon ( N e + 1 0 % He) with 7 0 t o r r methyl alcohol (spectral grade). This vapor forms only little deposits in the spark gap. The electric fields for this gas mixture are indicated in figs. 2 and 3. Less vapor pressure or other quench gases also work. A little argon helps the efficiency but mixtures rich in argon require higher fields, and may not permit close wire spacing, nor very narrow spark gaps. The maximum efficiency was about 97%. With a larger proportional gap, e.g. ~ 10 mm, the efficiency can be about 100% but the resolving time increases almost proportionately with the proportional gap width. The short resolving time of the hybrid chamber is apparent from the curves in fig. 2. The sparking efficiency versus delay between passage of a particle and the high voltage pulse is plotted for various delay-gap voltages. The resolving time is about 75 ns (fwhm). A slight broadening to 85 ns at longer delays may be due to diffusion in the delay gap. Similar resolving times were obtained with tests of two particles occuring at various delays from each other. The delay range can be increased by using a wider delay gap, up to 25 mm was tried, because a high field
)
,
90
P R O P A M P L G A P G.6rnm ' - ..... D E L A Y G ; A P ,~,,4-mm PULSED SPARK G A P : - - - : - - " - ' l I,~mrn
80
?.~--- 70 ~
f
-,,~
. . . . . . . , ~ - ~ ) - H V-7_ +E¢
. v , - HV2 = :~oo,Z.
- 85
H V 2 = $ 0 0 - IOCO V. EC = -t- 2 0 0 ~ D C ELECTRODE WIRES~I6/cm,~50P .mD~A.~.~. GA~: Ne+ IO%He + 70 l"orr METHVL ALCOHOL
~l 4-0
30
(,3
~-~-~,,
T "rR,GG='R~r, "BY" ~ A~.i.°KsVo:,~s E PAR'r IC LE ~£LECTINql " COUI~ITERS
')-
W ~Jso
• •
IO i
,~k
,
,
,
Oleo ZOO Z50 3o0 ~5D 400 4do 50o 550 TOTAL DELAy
(nsec)
Fig. 2. Resolving time of the hybrid chamber. Curves of sparking efficiencyvs delay time of hv pulse for various delay gap fields.
THE HYBRID
403
CHAMBER
HYBRID CHAMBER GAS: Ne +03 He+7Otorr CH30H PROPORTIONAL GAP:6.6mm; DELAY GAP: 6.4rnm; PULSED SPARK GAP: 1.6mrn H.V. PULSE: + 2 KV; 50nsec. ELECTRODE WIRES: SSO.05mrn DIA,16 WIRES PER crn.
I00 -
\
-
~ 60
|
rr
//
a.
PROPORTIONAL GAP
\
\~
~
(--)
(-1 (o) (÷)
D.C. BIAS: + 200V. "L I ~ ~ PULSED BIAS(tp) J / /. /
. ;;÷
_z 40
_~ 3BOO.V.
DELAY GAP ~ 700V. PULSED SPARK GAPZ ~ - ~ - ~ - -~.~
/
IstSPARK
o/
H.V. TESTPULSE
td
.[
hi
4/~ sec.
20
o
0
~.~,,~ I
I i I I ~x::==~l 20 40 60 DELAY OF SECOND H.V. PULSE AFTER FIRST SPARK
t p = t d - IO/u,sec. I 80
.t d (p.sec)
Fig. 3. Recovery time o f 1.6 m m s p a r k gap. Curves o f retiring probability vs time after a read o u t s p a r k ; at 200 V dc with additional pulsed clearing fields.
strength reduces diffusion effects on the time resolution as shown in table 2. TABLE 2
Relative resolving times in ns for two drift gap widths at various delays. Drift gap width
370
500
600
Delay time (ns)
6.6 m m 13 m m
90 80
110 90
130 100
Resolving time (fwhm) (ns)
It was observed in the ranges of interest that for given high field strengths at various drift gaps, the resolving time increases by about 5 ns per 100 ns of additional delay. An increase in the spark gap from 1.6 m m to 3 . 2 m m increases the resolving time only about 10-15%. However, a wider spark gap can disturb the discrimination between avalanches and other track electrons. The short recovery time after a read-out spark on selected events is presented in fig. 3. The curves show
the retiring probability at various times after a read out spark when tested with a second high voltage pulse for the 1.6 m m gap. The recovery time is only ~_ 80/ts and even less with a pulsed clearing field. Larger gaps, e.g. 3.2 mm, have also been tried. They increase the recovery time which is usually not important. The spatial resolution, as indicated by preliminary tests, is about the same as in regular wire spark chambers or about 0.3 m m for normal incident tracks and about 0.4 m m for tracks at 30 degrees to the normal. A small systematic shift in the apparent track position in the direction of the track projection on the proportional gap anode plane can be observed. A correction can be applied. Encouraged by the success with the small hybrid chambers described here, a larger chamber ~ 70 cm has been built by Garelick and Moromisato6). The preparation of several medium size hybrid chambers by a group from Karlsruhe for an experiment at C E R N has also been reported recently7). The hybrid chambers can also be operated in other modes. Early proportional chamber monitor signals are obtainable from the cathode and the first anode. The anode wires can be combined in groups and connected
404
J. FISCHER AND S. SHIBATA
to one or several amplifiers, if desired, for rough kinematic analysis and decision making; e.g. to inhibit or select spark triggers or for labelling. Self-triggering of the hybrid chamber can also be initiated from this signal, which permits the chamber to operate even with particles stopping in the first gap. This mode is useful for low energy particles and for X-ray and neutron applications. In another simplified, but not necessarily gated mode, even the last gap can be operated at a fixed dc potential only (without any pulsing). Depending on the selected field strength and wire geometry, the avalanches can be made to develop into sparks at low rates or the avalanches can just be multiplied in the semi-proportional mode for easier readout at high rates.
References 1) G. Charpak et al., Nucl. Instr. and Meth. 62 (1968) 262. z) j. Fischer and S. Shibata, Proc. Intern. Syrup. Nuclear electronics, Versailles, Sept. 1968, vol. 3 (Documentation Franfalse, Paris, 1969) p. 2-1 ; also BNL 12804. 8) Brookhaven National Laboratory Annual Report for July 68 to June 69, BNL 50169 or TID 4500 (July 1969) p. 69. 4) J. Fischer and S. Shibata, Brookhaven National Laboratory High Energy Discussion Group Report BNL 14766 (13 Dec. 1969) unpublished. 5) J. Fischer and S. Shibata, Intern. Conf. Instrumentation for high energy physics (Dubna, Sept. 1970); also BNL 14899.
6) D. Garelick and J. Moromisato, Northeastern Univ., NUB 7)
2004 Rev. (1970) unpublished; and submitted to D ubna Conf., see ref. 5; also Letter in this issue. CERN Courier 11, no. 8 (1971) 231.