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A long scintillation counter for precise measurement of time of flight and impact position
N U C L E A R I N S T R U M E N T S AND METHODS
I2 5
(i975) 357-363; © N O R T H - H O L L A N D P U B L I S H I N G CO.
A L O N G S C I N T I L L A T I O N COUNTER FOR PRECISE M E A S U R E M E N T OF T I M E O F F L I G H T AND I M P A C T P O S I T I O N B. GHIDINI
lstituto di Fisica dell' Universitd, Bari, Italy and lstituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy K. MI~LLER
Physikalisches Institut der Universitiit, Bonn, Germany J. EADES, B. FRENCH, L. MANDELLI, F. NAVACH* and V. PICCIARELLI
CERN, European Organization for Nuclear Research, Geneva, Switzerland J. BAILEY, M. EDWARDS and I. L. SMITH
Science Research Council, Daresbury Laboratory, Warrington, England D. N. EDWARDS and J. FRY
University of Liverpool, England R. CAR, P. D'ANGELO, S. GRABAR and G. MINELLI
Istituto di Fisica dell" Universitd di Milano, Italy and Istituto Nazionale di Fisica Nueleare, Sezione di Milano, Italy Received 14 November 1974 Plastic scintillator counters made of long slabs (200 x 20 cm 2) of different thickness have been tested with 120 cm long plexiglas light guides coupled to 58 AVP photomultipliers. The accuracies achieved in measuring the time of flight of 600 MeV/c protons, which hit the counter at different distances from the photo-
multiplier, range from 350 to 450 ps fwhh. The impact position of the particles is determined along the counter by the time difference, as measured from the two opposite ends, with an accuracy ranging from 4,5 to 5.5 cm fwhh.
1. Introduction
defined range of missing masses to the proton. A complete description is given elsewherea). In order to reduce problems caused by double hits and to achieve more uniform response, the large sensitive surface (200 x 100 c m 2) w a s divided into five slabs of reasonable size (200 × 20 cm2). Pulse-height and time resolutions, achieved with three such slabs of lowattenuation scintillator (NE 110), are given hereafter for three different thicknesses (1, 3 and 5 cm). Results obtained with a prototype of the counter to be used in the experiment are reported, with emphasis on its performance relative to the determination of the impact position.
The results reported in this paper have been obtained during a series of measurements performed at C E R N in proton, pion, and electron beams. The aim of these tests was to design a plastic scintillator counter with a large acceptance and good time resolution, to perform an experiment 1) at the 12-spectrometer2). The experiment, set up to study the production of non-strange mesons in 7r-p interactions, also required that the counter worked in a strong magnetic field and determined the impact position of the proton along the scintillator. Therefore, a special shape was necessary for the long light guides such that the two photomultipliers (PM) could be placed in a region where the strength of the magnetic field fell to an acceptable value. The essential feature of the triggering system was the ability to identify recoiling protons from the other particles (essentially pions) and to select a well* Present address: Istituto di Fisica dell'UniversitY., Bari, Italy.
357
2. Time and pulse-height resolution These measurements were performed at the synchrocyclotron, in a proton beam of variable momentum ranging from 370 MeV/c to 510 MeV/c, according to the experimental requirements at 12 1,3). A 160 MeV/c electron beam was used to study the performance of the counter for minimum-ionizing particles.
358
B. GHIDINI et al. - Bz r/~/~/~//~/~/~/~/~//~/~///~/////~///~///////////~////////////~/////~/~77/7/~///////~///77/~/////////3
I x=Ocm
I
\
T
T counter ( NellO scintillator ) -B 1 l
300cm
/
x =I 200cm
¢
I~::
PM(58 AVP)
Plexiglas light guide
Beam
Fig: 1. Setup used at the synchro-cyclotron for time of flight and pulse-height resolution studies. 2.1. SETUP AND ELECTRONICS The beam was defined by two narrow (5 x 15 cm 2) counters B1 and B E (fig. l) and by a small (1 x I cm 2) S T A R T counter S, which was placed near to the counter T under test. The time resolution was studied by measuring the time of flight of beam particles between S and T. The electronics logic is shown in fig. 2. The overlap between the shaped signals S and T is proportional to the S - T time difference and it is encoded through a chain of three standard C E R N - N I M circuits. The measured electronical jitter of ~ 80 ps is included in all numbers
T
S
B1
B2
DY
AN
AN
D¥
E
given in this paper. Both S and T shaped signals are obtained by the well-known " h i g h - l o w " discrimination technique. The PM anode signal goes through a lowthreshold ( ~ 5 0 mV) pulse shaper to be gated in an A N D by the inverted signal from the last dynode of the PM, which has been shaped in a discriminator with the threshold as high as required to reject unwanted signals (PM noise as well as minimum-ionizing particles). Some typical tests of the "high-low" technique are illustrated in figs. 3a and 3b. Fig. 3a shows that both peak position and time resolution do not depend on the " h i g h " threshold, as long as this is not much larger than the average pulse height of the dynode ( ~ 400 mV). In fact there is an appreciable change of 50 ps in fwhh and 140 ps in peak position for a threshold setting of 800 mV. Fig. 3b shows that the time walk observed by fixing the " h i g h " threshold and varying the " l o w " one is linear and that the time resolution remains good until the threshold is about half the average pulse height of the anode ( ~ 1.2 V). Due to the small range of pulse heights expected in the experiment, the " h i g h - l o w " technique was perfectly adequate, and use of a more refined zero-crossing circuit* did not improve the resolution. 2.2. RESULTS Fig. 4a shows the time resolution for the 1 cm thick scintillator with three differently ionizing particles, namely: protons of 510 MeV/c and 370 MeV/c, and electrons t. As expected, the time resolution depends on the impact position along the counter and on the ionization. The fact that it improves considerably both with decreasing distance from the PM and with increasing ionization confirms the dependence of resolutions on photon statistics. The same sort of improvement is found by increasing the thickness of the scintillators, as is borne out by the
Fig. 2. Electronics logic for time-of-flight and pulse-height measurements. All units are CERN-NIM standard circuits. DISC: disciminator; SH: pulse shaper; LG: linear gate; SA: shaping amplifier; DB: delay box; PHA: pulse-height analyser.
* EGG TI40/N zero-crossing discriminator. t For this particular set of measurements the resolution of the START counter was not very good (~400 ps).
PRECISE MEASUREMENT
OF TIME OF F L I G H T
o) "Low"threshold fixed at 125 mV ~;.I;br2tsi::/C h
l b)"High"threshold fixed Qt 125 mV
_ / 300
~
t/k\
359
AND IMPACT POSITION
"--..j
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-100 I
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20
40
60
80
100
120
0
I
1
200
Channels
I
I
400
I
I
I
600
800
Low threshold (mV)
Fig. 3. (a) Variation of time-of-flight spectra for fixed "low" threshold and different "high" thresholds. (b)Time of flight and resolution versus "low" threshold for fixed "high" threshold. a) Time resolution
1500
~o e160 MeVlc l c m i x p510 .... /e p370 . . . .
150(
b) Time resolution ~x lcrn p 510 MeVIc/• 3c rn tn 5cm
I
i
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¢) Pulse height (O • 160 MeVlc 1cm I I p370 . . . . )I 5c Ix p510 . . . . P~4
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0.5 250[
250 I
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50
I
100 x (Cm)
I
150
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2OO
0
I
50
I
100 150 200 x(cm) Position along the counter
0.2
I
1
I
50
100
150
200
x(cml
Fig. 4. (a) Time-of-flight resolution versus position along the 1 cm thick counter for various beam momenta. (b) Time-of-flight resolution versus position along counters o f different thickness. (c) Pulse height versus position along the 1 cm thick counter for various beam momenta. The bars indicate the fwhh resolution obtained with pulse-area analysis. The values obtained with a 5 cm thick slab are also reported.
data for the 3 and 5 cm slabs which gave a uniform resolution along the counter (fig. 4b). The pulse-height variation along the counter is shown in fig. 4c. F o r a given thickness the apparent attenuation length depends on ionization due to the P M saturating. The pulse-height resolution improves
when the beam hits the scintillator closer t o the P M and when the thickness is increased from 1 cm to 5 cm. For the 5 cm slab the variation o f resolution along the counter is negligible, likewise for the time measurement (fig. 4b), proving that the best one expected from the p h o t o n statistics has been achieved.
360
B. GHIDINI et al. L= 200 cm
i Aluminized
Plexiglas light guide
y /" ," J ~.~//~//////////////////f////////////////f/~//////~/////////////j//////////////f//~ x=0cm
'
120cm
"1
/
2'
Ne 110 scintillator
OF
~
I~
_ ,
PM 2(58 AvP)
I i ~ PM 1 (se AVP)
x
Start
I
$ilcoloid cpticol joints Prototype of the counter
Fig. 5. The 200 × 20 x 5 cma scintillation counter seen by two 58 AVP PMs. The symbols help to define the electronical quantities TF1, TFz, TF, P O S (see fig. 6).
3. The prototype with two photomultipliers As mentioned before, the counter had to be placed deep inside the magnetic field as close as possible to the Bt
E
!s B2
START AN
DY
PM2 AN
PM1
D¥
AN
D¥
spark chambers in order to have the best acceptance for the wanted recoiling protonsa). Even so, it was required to determine the impact position by the time-difference method [CDF technique4)], which needs two PMs each coupled to one edge of the scintillator. 3.1. DESIGN AND ELECTRONICS
7
)
I 1 ,1
IsAI
Fig. 6. Electronicslogicused for time-of-flight(TOF) and position (x) measurements using the prototype seen by two PMs: TF1 = T O F + ( L - - x ) / v l , TFg = T O F + x/v2, TF = ½(TF1 + TF~), P O S = ½(TF2-- TF1).
DISC: discriminator; SH: pulse shaper; LG: linear gate; SA: shaping amplifier; DB: delay box; PHA: pulse-height analyser.
The above-mentioned requirements were met with the light guides shown in fig. 5. A good transmission of light at the far edge of the counter is obtained by gluing a half cylinder ( ~ = 12 cm) of aluminized plexiglas to the 5 cm thick slab of scintillator and to the 7 cm thick slab of plexiglas. It was shown by Monte Carlo calculations that, out of several geometrical configurations tried, this one had a good efficiency for collecting photons around a corner 5). The light guide to the PM was segmented in order not to freeze the position of the counter in ~2 at the time the design was made. Liquid silcoloid-201 was poured into a gap of ~0.5 cm, ensuring a good optical contact when solidified. The mechanical support of the whole counter was designed to allow the light guide to be lengthened as required by the final setup in ~. The standard C E R N bases for PMs were implemented with four potentiometers which allowed the gain to be easily and continuously changed from linear to logarithmic3). The electronics used to define the time of flight T F , the impact position P O S and the times of flight T F l and T F 2 , as measured from sides 1 and 2, respectively (fig. 6), is similar to the one described in section 2.1. A smaller START counter (0.2 × l c m 2) w a s used for these measurements. An extra circuit (CERN - Mean Timer) has been introduced to define a signal having the average time of the two PM signals, which then provides a measurement of the time-of-flight. Furthermore, the overlap of the two signals from the PMs coupled to the opposite ends of the scintillator, determines the impact position4).
PRECISE
MEASUREMENT
u
OF T I M E OF F L I G H T
POS colibtotion 0.392 c m / C h
AND IMPACT
361
POSITION
(o)
20(~ x =lOcm
x = 50cm
x= 100 cm
~
O
c
r
n
150 .20 o
100
~o
50
o
0 0
100
200
300
/,00
500
Channels
iI=63cmns1b f 50
0
I 100
I 150
I 200
~- Vz = 1/,.3 cm/ns
-e
(c)
I 50
I 100
I 150
I 200
Position along the counter (cm)
Fig. 7. (a) Position calibration a n d position resolution along the counter. (b) a n d (c) TFI a n d TF2 variation versus position along the counter, yielding the a p p a r e n t velocities o f light travelling in the scintillator.
Impoct
5 o -r "i-
position
resolution
e Ref. 6
•
& Ref. 7 + Ref. 8
•
• This looper
-I-
•
4
Ll. 3-
2-
1-
J
i 5
10
50
100
I ,,,,I 5OO
_ IOO0
Distance between photomultiplie~ (era) Fig. 8. Position resolution versus distance between P M s as f r o m table 1 c o m p a r e d with o t h e r results. T h e resolution is a factor l0 worse w h e n the distance increases by a factor 100.
3.2. RESULTS
Table 1 gives a summary of typical resolutions obtained using a 600 MeV/c proton beam at the proton synchrotron. No observable deterioration was found of both the resolution and the efficiency provided
the particles hit the counter within 0.2 cm of its edge. Fig. 7a shows a typical calibration for POS, obtained by moving the counter across the beam. Figs. 7b and 7c show the corresponding variations in TF1 and T F 2 . The slopes of 16.3 cm/ns and 14.3 cm/ns are
362
B. GHIDINI et al.
Be(am momentum
p = 600 MeV/c
Phl 1.5 -
a.
Ph2
~
,
,
~
~
0.5
0
50
100
150
200
Position cttong the counter(cm) Fig. 9. Pulse height for 600 MeV/c pions a n d protons. T h e bars c o r r e s p o n d to the f w h m resolutions.
TABLE 1
Typical resolutions (fwhh) (200 × 20 × 5 cm 3 counter). T h e contribution o f ~ 1 2 0 ps due to the S T A R T is n o t subtracted. x (cm)
TF1 (ps)
TF2 (ps)
TF (ps)
POS (cm)
10 100 190
450 400 350
350 500 650
400 450 500
4.5 5.0 5.5
the apparent velocities of light travelling from the impact position to PM~ and PM2, respectively. The difference is due to the different geometry of the two light guides and makes T F slightly dependent on the impact position ( < 1 ns from one edge to the other). Another consequence of the sharp bend of the light guide is that the resolution observed for T F 2 is worse than for T F 1 (see table 1). The PM on side 2 is, however, essential to determine P O S and TF. It is interesting to note that the accuracy on P O S is strongly dependent on the total distance between the PMs (fig. 8). The results obtained with the present configuration fit well with those obtained by other groups 6' S). It would appear that it is difficult to improve the accuracy beyond the limits imposed by the distance between PMs. The performances of the prototype, as far as pulse height is concerned, are shown in fig. 9. A good discrimination of 600 MeV/c protons against minimum-
ionizing particles is possible both from side 1 and side 2. A more detailed discussion about the use of pulse heights in the trigger is given in ref. 3. 4. Conclusions The results reported in this paper show that, using a long plastic scintillation counter, one can achieve time accuracies as good as +200 ps. This compares well with the best results obtained so far with counters of dimensions one order of magnitude smaller6). Furthermore, the impact position, even in the difficult conditions dictated by the use in the Qspectrometer, can be determined to ___2.5 cm. In order to achieve this accuracy with the standard technique of hodoscope counters, one needs 40 elements to cover such a large area. Therefore, in such cases, reliability, simplicity, and cost considerations strongly favour a system based on the C D F technique against hodoscope counters to provide impact position of particles, both for triggering and off-line analysis. We are indebted to M. P. Manfredi for his helpful suggestions in the early stage of this work. We would like to thank the C E R N plexiglas workshop and in particular Messrs. L. Thornhill and A. Malavallon whose skillful contributions have been appreciated during the design and construction of the counters. We also thank the C E R N - T C Electronics laboratory staff for their continuous assistance throughout the settingup and the running time.
P R E C I S E M E A S U R E M E N T OF TIME OF F L I G H T AND I M P A C T P O S I T I O N
Note added in proof A r e c e n t p a p e r b y K h r e n o v et al. [ N u c l . I n s t r . a n d M e t h . 123 (1975) 471] r e p o r t s o t h e r r e s u l t s f o r t h e spatial resolution of large scintillation counters. None o f t h e s e d a t a falls b e l o w t h e p o i n t s s h o w n in fig. 8.
References 1) N. Armenise, V. Picciarelli, A. Romano, A. Silvestri, V. Idschock, B. Nellen, K. Miiller, H. W. Atherton, J. Eades, B. French, B. Ghidini, A. Grant, L. Mandelli, J. Moebes, F. Navach, G. Bellini, A. Cantore, M. dl Corato, M. P. Manfredi and G. Vegni, Proposal for a systematic study of the strangeness zero charged boson spectrum, using the Omega and a proton time-of-flight trigger, PH 1/COM-70/63 (CERN, December 1970). e) The Omega project, NP Internal Report 68-11, CERN (May 1968); O. Gildemeister, The CERN Omega spectrometer, Intern. Conf. on: Instrumentation for high-energy physics (Frascati, May 1973) p. 669.
363
z) B. Ghidini, A. Palano, K. Miiller, B. Nellen, J. Eades, B. French, K. Knudson, L. Mandelli, F. Navach, V. Picciarelli, A. Werbrouck, M. Edwards, I. L. Smith, D. N. Edwards, J. Fry, A. Cantore, R. Car, P. D'Angelo and M. di Corato, The slow proton trigger for the CERN .Q-spectrometer; to be submitted to Nucl. Instr. and Meth. 4) G. Charpak, L. Dick and L. Feuvrais, Nucl. Instr. and Meth. 15 (1962) 325. 5) G. Minelli, Misure di tempo di volo per uno spettrometro di massa mancante al protone, Tesi (Milano 1971) unpublished. 6) D. Bollini, P. Dalpiaz, P. L. Frabetti, T. Massam, F. Navach, F. L. Navarria, M. A. Schneegans and A. Zichichi, Nucl. Instr. and Meth. 81 (1970) 50. 7) D. Bollini, A. Buhler-Broglin, P. Dalpiaz, T. Massam, F. Navach, F. L. Navarria, M. A. Schneegans, F. Zetti and A. Zichichi, Nuovo Cimento 61A (1969) 125. s) U. Stier, Ein Szintillationsz~hler zum Ortsnachweis von Neutronen, lnstitut ffir Experimentelle Kernphysik, Universitat Karlsruhe (Februar 1970) private communication.
I2 5
(i975) 357-363; © N O R T H - H O L L A N D P U B L I S H I N G CO.
A L O N G S C I N T I L L A T I O N COUNTER FOR PRECISE M E A S U R E M E N T OF T I M E O F F L I G H T AND I M P A C T P O S I T I O N B. GHIDINI
lstituto di Fisica dell' Universitd, Bari, Italy and lstituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy K. MI~LLER
Physikalisches Institut der Universitiit, Bonn, Germany J. EADES, B. FRENCH, L. MANDELLI, F. NAVACH* and V. PICCIARELLI
CERN, European Organization for Nuclear Research, Geneva, Switzerland J. BAILEY, M. EDWARDS and I. L. SMITH
Science Research Council, Daresbury Laboratory, Warrington, England D. N. EDWARDS and J. FRY
University of Liverpool, England R. CAR, P. D'ANGELO, S. GRABAR and G. MINELLI
Istituto di Fisica dell" Universitd di Milano, Italy and Istituto Nazionale di Fisica Nueleare, Sezione di Milano, Italy Received 14 November 1974 Plastic scintillator counters made of long slabs (200 x 20 cm 2) of different thickness have been tested with 120 cm long plexiglas light guides coupled to 58 AVP photomultipliers. The accuracies achieved in measuring the time of flight of 600 MeV/c protons, which hit the counter at different distances from the photo-
multiplier, range from 350 to 450 ps fwhh. The impact position of the particles is determined along the counter by the time difference, as measured from the two opposite ends, with an accuracy ranging from 4,5 to 5.5 cm fwhh.
1. Introduction
defined range of missing masses to the proton. A complete description is given elsewherea). In order to reduce problems caused by double hits and to achieve more uniform response, the large sensitive surface (200 x 100 c m 2) w a s divided into five slabs of reasonable size (200 × 20 cm2). Pulse-height and time resolutions, achieved with three such slabs of lowattenuation scintillator (NE 110), are given hereafter for three different thicknesses (1, 3 and 5 cm). Results obtained with a prototype of the counter to be used in the experiment are reported, with emphasis on its performance relative to the determination of the impact position.
The results reported in this paper have been obtained during a series of measurements performed at C E R N in proton, pion, and electron beams. The aim of these tests was to design a plastic scintillator counter with a large acceptance and good time resolution, to perform an experiment 1) at the 12-spectrometer2). The experiment, set up to study the production of non-strange mesons in 7r-p interactions, also required that the counter worked in a strong magnetic field and determined the impact position of the proton along the scintillator. Therefore, a special shape was necessary for the long light guides such that the two photomultipliers (PM) could be placed in a region where the strength of the magnetic field fell to an acceptable value. The essential feature of the triggering system was the ability to identify recoiling protons from the other particles (essentially pions) and to select a well* Present address: Istituto di Fisica dell'UniversitY., Bari, Italy.
357
2. Time and pulse-height resolution These measurements were performed at the synchrocyclotron, in a proton beam of variable momentum ranging from 370 MeV/c to 510 MeV/c, according to the experimental requirements at 12 1,3). A 160 MeV/c electron beam was used to study the performance of the counter for minimum-ionizing particles.
358
B. GHIDINI et al. - Bz r/~/~/~//~/~/~/~/~//~/~///~/////~///~///////////~////////////~/////~/~77/7/~///////~///77/~/////////3
I x=Ocm
I
\
T
T counter ( NellO scintillator ) -B 1 l
300cm
/
x =I 200cm
¢
I~::
PM(58 AVP)
Plexiglas light guide
Beam
Fig: 1. Setup used at the synchro-cyclotron for time of flight and pulse-height resolution studies. 2.1. SETUP AND ELECTRONICS The beam was defined by two narrow (5 x 15 cm 2) counters B1 and B E (fig. l) and by a small (1 x I cm 2) S T A R T counter S, which was placed near to the counter T under test. The time resolution was studied by measuring the time of flight of beam particles between S and T. The electronics logic is shown in fig. 2. The overlap between the shaped signals S and T is proportional to the S - T time difference and it is encoded through a chain of three standard C E R N - N I M circuits. The measured electronical jitter of ~ 80 ps is included in all numbers
T
S
B1
B2
DY
AN
AN
D¥
E
given in this paper. Both S and T shaped signals are obtained by the well-known " h i g h - l o w " discrimination technique. The PM anode signal goes through a lowthreshold ( ~ 5 0 mV) pulse shaper to be gated in an A N D by the inverted signal from the last dynode of the PM, which has been shaped in a discriminator with the threshold as high as required to reject unwanted signals (PM noise as well as minimum-ionizing particles). Some typical tests of the "high-low" technique are illustrated in figs. 3a and 3b. Fig. 3a shows that both peak position and time resolution do not depend on the " h i g h " threshold, as long as this is not much larger than the average pulse height of the dynode ( ~ 400 mV). In fact there is an appreciable change of 50 ps in fwhh and 140 ps in peak position for a threshold setting of 800 mV. Fig. 3b shows that the time walk observed by fixing the " h i g h " threshold and varying the " l o w " one is linear and that the time resolution remains good until the threshold is about half the average pulse height of the anode ( ~ 1.2 V). Due to the small range of pulse heights expected in the experiment, the " h i g h - l o w " technique was perfectly adequate, and use of a more refined zero-crossing circuit* did not improve the resolution. 2.2. RESULTS Fig. 4a shows the time resolution for the 1 cm thick scintillator with three differently ionizing particles, namely: protons of 510 MeV/c and 370 MeV/c, and electrons t. As expected, the time resolution depends on the impact position along the counter and on the ionization. The fact that it improves considerably both with decreasing distance from the PM and with increasing ionization confirms the dependence of resolutions on photon statistics. The same sort of improvement is found by increasing the thickness of the scintillators, as is borne out by the
Fig. 2. Electronics logic for time-of-flight and pulse-height measurements. All units are CERN-NIM standard circuits. DISC: disciminator; SH: pulse shaper; LG: linear gate; SA: shaping amplifier; DB: delay box; PHA: pulse-height analyser.
* EGG TI40/N zero-crossing discriminator. t For this particular set of measurements the resolution of the START counter was not very good (~400 ps).
PRECISE MEASUREMENT
OF TIME OF F L I G H T
o) "Low"threshold fixed at 125 mV ~;.I;br2tsi::/C h
l b)"High"threshold fixed Qt 125 mV
_ / 300
~
t/k\
359
AND IMPACT POSITION
"--..j
V / /
/
~,,o /
,,o, ,~o
,
,w~.
_,oi _o~ -_,o
/
o~
j
500 i
-100 I
0
20
40
60
80
100
120
0
I
1
200
Channels
I
I
400
I
I
I
600
800
Low threshold (mV)
Fig. 3. (a) Variation of time-of-flight spectra for fixed "low" threshold and different "high" thresholds. (b)Time of flight and resolution versus "low" threshold for fixed "high" threshold. a) Time resolution
1500
~o e160 MeVlc l c m i x p510 .... /e p370 . . . .
150(
b) Time resolution ~x lcrn p 510 MeVIc/• 3c rn tn 5cm
I
i
I
¢) Pulse height (O • 160 MeVlc 1cm I I p370 . . . . )I 5c Ix p510 . . . . P~4
'~ 1250I
7,~r
\
0.5 250[
250 I
0
50
I
100 x (Cm)
I
150
01
2OO
0
I
50
I
100 150 200 x(cm) Position along the counter
0.2
I
1
I
50
100
150
200
x(cml
Fig. 4. (a) Time-of-flight resolution versus position along the 1 cm thick counter for various beam momenta. (b) Time-of-flight resolution versus position along counters o f different thickness. (c) Pulse height versus position along the 1 cm thick counter for various beam momenta. The bars indicate the fwhh resolution obtained with pulse-area analysis. The values obtained with a 5 cm thick slab are also reported.
data for the 3 and 5 cm slabs which gave a uniform resolution along the counter (fig. 4b). The pulse-height variation along the counter is shown in fig. 4c. F o r a given thickness the apparent attenuation length depends on ionization due to the P M saturating. The pulse-height resolution improves
when the beam hits the scintillator closer t o the P M and when the thickness is increased from 1 cm to 5 cm. For the 5 cm slab the variation o f resolution along the counter is negligible, likewise for the time measurement (fig. 4b), proving that the best one expected from the p h o t o n statistics has been achieved.
360
B. GHIDINI et al. L= 200 cm
i Aluminized
Plexiglas light guide
y /" ," J ~.~//~//////////////////f////////////////f/~//////~/////////////j//////////////f//~ x=0cm
'
120cm
"1
/
2'
Ne 110 scintillator
OF
~
I~
_ ,
PM 2(58 AvP)
I i ~ PM 1 (se AVP)
x
Start
I
$ilcoloid cpticol joints Prototype of the counter
Fig. 5. The 200 × 20 x 5 cma scintillation counter seen by two 58 AVP PMs. The symbols help to define the electronical quantities TF1, TFz, TF, P O S (see fig. 6).
3. The prototype with two photomultipliers As mentioned before, the counter had to be placed deep inside the magnetic field as close as possible to the Bt
E
!s B2
START AN
DY
PM2 AN
PM1
D¥
AN
D¥
spark chambers in order to have the best acceptance for the wanted recoiling protonsa). Even so, it was required to determine the impact position by the time-difference method [CDF technique4)], which needs two PMs each coupled to one edge of the scintillator. 3.1. DESIGN AND ELECTRONICS
7
)
I 1 ,1
IsAI
Fig. 6. Electronicslogicused for time-of-flight(TOF) and position (x) measurements using the prototype seen by two PMs: TF1 = T O F + ( L - - x ) / v l , TFg = T O F + x/v2, TF = ½(TF1 + TF~), P O S = ½(TF2-- TF1).
DISC: discriminator; SH: pulse shaper; LG: linear gate; SA: shaping amplifier; DB: delay box; PHA: pulse-height analyser.
The above-mentioned requirements were met with the light guides shown in fig. 5. A good transmission of light at the far edge of the counter is obtained by gluing a half cylinder ( ~ = 12 cm) of aluminized plexiglas to the 5 cm thick slab of scintillator and to the 7 cm thick slab of plexiglas. It was shown by Monte Carlo calculations that, out of several geometrical configurations tried, this one had a good efficiency for collecting photons around a corner 5). The light guide to the PM was segmented in order not to freeze the position of the counter in ~2 at the time the design was made. Liquid silcoloid-201 was poured into a gap of ~0.5 cm, ensuring a good optical contact when solidified. The mechanical support of the whole counter was designed to allow the light guide to be lengthened as required by the final setup in ~. The standard C E R N bases for PMs were implemented with four potentiometers which allowed the gain to be easily and continuously changed from linear to logarithmic3). The electronics used to define the time of flight T F , the impact position P O S and the times of flight T F l and T F 2 , as measured from sides 1 and 2, respectively (fig. 6), is similar to the one described in section 2.1. A smaller START counter (0.2 × l c m 2) w a s used for these measurements. An extra circuit (CERN - Mean Timer) has been introduced to define a signal having the average time of the two PM signals, which then provides a measurement of the time-of-flight. Furthermore, the overlap of the two signals from the PMs coupled to the opposite ends of the scintillator, determines the impact position4).
PRECISE
MEASUREMENT
u
OF T I M E OF F L I G H T
POS colibtotion 0.392 c m / C h
AND IMPACT
361
POSITION
(o)
20(~ x =lOcm
x = 50cm
x= 100 cm
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n
150 .20 o
100
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100
200
300
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500
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0
I 100
I 150
I 200
~- Vz = 1/,.3 cm/ns
-e
(c)
I 50
I 100
I 150
I 200
Position along the counter (cm)
Fig. 7. (a) Position calibration a n d position resolution along the counter. (b) a n d (c) TFI a n d TF2 variation versus position along the counter, yielding the a p p a r e n t velocities o f light travelling in the scintillator.
Impoct
5 o -r "i-
position
resolution
e Ref. 6
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& Ref. 7 + Ref. 8
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•
4
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10
50
100
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Distance between photomultiplie~ (era) Fig. 8. Position resolution versus distance between P M s as f r o m table 1 c o m p a r e d with o t h e r results. T h e resolution is a factor l0 worse w h e n the distance increases by a factor 100.
3.2. RESULTS
Table 1 gives a summary of typical resolutions obtained using a 600 MeV/c proton beam at the proton synchrotron. No observable deterioration was found of both the resolution and the efficiency provided
the particles hit the counter within 0.2 cm of its edge. Fig. 7a shows a typical calibration for POS, obtained by moving the counter across the beam. Figs. 7b and 7c show the corresponding variations in TF1 and T F 2 . The slopes of 16.3 cm/ns and 14.3 cm/ns are
362
B. GHIDINI et al.
Be(am momentum
p = 600 MeV/c
Phl 1.5 -
a.
Ph2
~
,
,
~
~
0.5
0
50
100
150
200
Position cttong the counter(cm) Fig. 9. Pulse height for 600 MeV/c pions a n d protons. T h e bars c o r r e s p o n d to the f w h m resolutions.
TABLE 1
Typical resolutions (fwhh) (200 × 20 × 5 cm 3 counter). T h e contribution o f ~ 1 2 0 ps due to the S T A R T is n o t subtracted. x (cm)
TF1 (ps)
TF2 (ps)
TF (ps)
POS (cm)
10 100 190
450 400 350
350 500 650
400 450 500
4.5 5.0 5.5
the apparent velocities of light travelling from the impact position to PM~ and PM2, respectively. The difference is due to the different geometry of the two light guides and makes T F slightly dependent on the impact position ( < 1 ns from one edge to the other). Another consequence of the sharp bend of the light guide is that the resolution observed for T F 2 is worse than for T F 1 (see table 1). The PM on side 2 is, however, essential to determine P O S and TF. It is interesting to note that the accuracy on P O S is strongly dependent on the total distance between the PMs (fig. 8). The results obtained with the present configuration fit well with those obtained by other groups 6' S). It would appear that it is difficult to improve the accuracy beyond the limits imposed by the distance between PMs. The performances of the prototype, as far as pulse height is concerned, are shown in fig. 9. A good discrimination of 600 MeV/c protons against minimum-
ionizing particles is possible both from side 1 and side 2. A more detailed discussion about the use of pulse heights in the trigger is given in ref. 3. 4. Conclusions The results reported in this paper show that, using a long plastic scintillation counter, one can achieve time accuracies as good as +200 ps. This compares well with the best results obtained so far with counters of dimensions one order of magnitude smaller6). Furthermore, the impact position, even in the difficult conditions dictated by the use in the Qspectrometer, can be determined to ___2.5 cm. In order to achieve this accuracy with the standard technique of hodoscope counters, one needs 40 elements to cover such a large area. Therefore, in such cases, reliability, simplicity, and cost considerations strongly favour a system based on the C D F technique against hodoscope counters to provide impact position of particles, both for triggering and off-line analysis. We are indebted to M. P. Manfredi for his helpful suggestions in the early stage of this work. We would like to thank the C E R N plexiglas workshop and in particular Messrs. L. Thornhill and A. Malavallon whose skillful contributions have been appreciated during the design and construction of the counters. We also thank the C E R N - T C Electronics laboratory staff for their continuous assistance throughout the settingup and the running time.
P R E C I S E M E A S U R E M E N T OF TIME OF F L I G H T AND I M P A C T P O S I T I O N
Note added in proof A r e c e n t p a p e r b y K h r e n o v et al. [ N u c l . I n s t r . a n d M e t h . 123 (1975) 471] r e p o r t s o t h e r r e s u l t s f o r t h e spatial resolution of large scintillation counters. None o f t h e s e d a t a falls b e l o w t h e p o i n t s s h o w n in fig. 8.
References 1) N. Armenise, V. Picciarelli, A. Romano, A. Silvestri, V. Idschock, B. Nellen, K. Miiller, H. W. Atherton, J. Eades, B. French, B. Ghidini, A. Grant, L. Mandelli, J. Moebes, F. Navach, G. Bellini, A. Cantore, M. dl Corato, M. P. Manfredi and G. Vegni, Proposal for a systematic study of the strangeness zero charged boson spectrum, using the Omega and a proton time-of-flight trigger, PH 1/COM-70/63 (CERN, December 1970). e) The Omega project, NP Internal Report 68-11, CERN (May 1968); O. Gildemeister, The CERN Omega spectrometer, Intern. Conf. on: Instrumentation for high-energy physics (Frascati, May 1973) p. 669.
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z) B. Ghidini, A. Palano, K. Miiller, B. Nellen, J. Eades, B. French, K. Knudson, L. Mandelli, F. Navach, V. Picciarelli, A. Werbrouck, M. Edwards, I. L. Smith, D. N. Edwards, J. Fry, A. Cantore, R. Car, P. D'Angelo and M. di Corato, The slow proton trigger for the CERN .Q-spectrometer; to be submitted to Nucl. Instr. and Meth. 4) G. Charpak, L. Dick and L. Feuvrais, Nucl. Instr. and Meth. 15 (1962) 325. 5) G. Minelli, Misure di tempo di volo per uno spettrometro di massa mancante al protone, Tesi (Milano 1971) unpublished. 6) D. Bollini, P. Dalpiaz, P. L. Frabetti, T. Massam, F. Navach, F. L. Navarria, M. A. Schneegans and A. Zichichi, Nucl. Instr. and Meth. 81 (1970) 50. 7) D. Bollini, A. Buhler-Broglin, P. Dalpiaz, T. Massam, F. Navach, F. L. Navarria, M. A. Schneegans, F. Zetti and A. Zichichi, Nuovo Cimento 61A (1969) 125. s) U. Stier, Ein Szintillationsz~hler zum Ortsnachweis von Neutronen, lnstitut ffir Experimentelle Kernphysik, Universitat Karlsruhe (Februar 1970) private communication.