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Large planar drift chambers
NUCLEAR
INSTRUMENTS
AND
METHODS
14I 0977) 43-56;
© NORTH-HOLLAND
PUBLISHING
CO.
LARGE P L A N A R DRIFT C H A M B E R S G. M A R E L , P. B L O C H , S. B R E H I N , B. D E V A U X , A. M. D I A M A N T - B E R G E R , C. L E S C H E V I N , J. M A 1 L L A R D , Y. M A L B E Q U I , H. M A R T I N , A. P A T O U X , J. PELLE, J. P L A N C O U L A I N E , G. T A R T E and R. T U R L A Y
D(partement de Physique des Partict, les Eidmentaires, CEN Saclay, France Received 18 October 1976 We describe 14 m 2 hexagonal planar drift c h a m b e r s designed for the neutrino experiment o f the C E R N - D o r t m u n d - H e i d e l berg-Saclay Collaboration. Details on mechanical construction, electronic read-out, results on efficiency and accuracy are presented.
1. Introduction The principle and the development of the localisation of tracks of charged particles by measurement of drift time of primary ionization electrons have already been discussed by different authors1-3). The problems of electric field structure and gas mixture leading to the saturation and stability of drift velocity are now solved4). High accuracy fir < 100 #m) drift chambers are currently used in experiments with specific solution to solve the left-right ambiguity problem. In a drift chamber, the number of anode wires is smaller than for a multiwire proportional chamber, thus reducing the price of the associated electronics and simplifing the mechanical construction particularly if a not too high spatial resolution is demanded. These advantages are particularly suitable for experiments in which large detectors are necessary. This is the case, for example, in high energy neutrino interaction studies, and the Harvard University Group s) in the H P W F Collaboration has already adopted large planar drift chambers (4 m x 4 m). For a neutrino experiment with the SPS at C E R N , the C E R N - D o r t m u n d - H e i d e l b e r g - S a c l a y Collaboration 6) proposed the construction of twenty large drift chambers for the localisation of a muon track within a spatial resolution of -+ 1 ram. In the present paper we report on specific problems of the construction of 720 m z of usable detectors with such a constraint on the resolution. 2. Choice of parameters 2.1. GEOMETRY Each drift chamber used in the neutrino experiment is placed behind an iron core magnet of 3.8 m in diameter. The right-left ambiguity solution and correct association for multitrack events can be done with
three planes with different drift directions. For these reasons we decided to construct chambers of hexagonal shape (the distance between two opposite edges is 4 m) composed of three independent gaps. The wire directions chosen are plus and minus 60 ° relative to the horizontal one, the hexagonal shape beingwell adapted to these directions. The gap width, which has to be as thin as possible to keep the average density of the detector maximum along the trajectories is 3 cm. This value is sufficient to obtain electrostatic stability for the long wires with the electric field structure chosen.
2.2. ELECTRIC STRUCTURE OF THE CHAMBERS Most of the time these chambers will detect only one muon, but we retained the possibility to measure two tracks or more from the same event with a spatial separation larger than 1 cm. This could be achieved either by a sense wire spacing smaller than the minimum fixed spatial resolution between two tracks, or by a dead time after an avalanche on a wire corresponding to a distance smaller than the above resolution. The first solution requests a high number of sense wires and thus increases the price of the read-out electronics, whereas the second possibility requires special attention to the electric structure. We adopted this last solution with a distance between two sense wires of 6 cm. The simplest structure for a multiwire drift chamber is almost the same as for a normal multiwire proportional chamber; the anode wires (sense wires) are equidistant and parallel to the two equipotential cathode plates, the strength of the field between two sense wires being increased by a field wire (potential wire)3). Cheng et al. have adopted this structure for their large drift chamber (4 m x 4 m) with a large drift space of 5 cm (half distance between two sense wires) and a gap of +2.5 cm. it was very tempting to adopt
44
G. M A R E L et al.
30mm
.
.
.
.
.
.
J
~b
- 2 KV cathode
r
3.5 KV
Equipotential lines
Anode 50/~ m
~1200~ m
Fig. 1. Equipotential lines of the field for a drift chamber with cathode at constant potential. Distance between sense wires = 6 cm. Field wires at - 2 0 0 0 V separate the sense wires. MexJrnul11 Electric
the same conception for our chambers because of its simplicity but studies done with such a structure (drift space + 3 cm, gap -+ 1.5 cm) have shown some problems whenever large wire spacings are used.
field
500(
2.2. I. Pulse size; efficiency of the chamber For a non-electronegative gas mixture, the electrons of the primary ionization of a charged particle are drifting along the electric field lines with a drift velocity which depends on the field strength and the nature of the gas. For some mixtures like argon-isobutane, argon-ethylene or argon-propane, the velocity is an increasing function of the field and then becomes roughly a constant4), which depends on this gas mixture. For a proportional counter one measures the integrated signal on the anode wire which is proportional to the sum of all elementary charges produced in the primary ionization, however in a drift chamber one measures a current which varies with time. The peak value of this current depends on the convolution of the shape of the pulse for an elementary avalanche produced by one electron by the drift time dispersion of the initial electrons. This value is maximum for all primary
400~
300G
200C
100C
Cat lode wire
An ,de wire
110
20
310 rnm~"
Fig. 2. Electric field along the path (a) of fig. I.
LARGE
PLANAR
electrons reaching the anode wire at the same time (if no space charge effect occurs) and decreases with the drift time spread. With such a structure this spread is important for two reasons: first, the field lines are not parallel (fig. 1), then the difference between paths of electrons increases with the distance from the track to the sense wire. Secondly along all drift paths, the field strength does not reach the value corresponding to the drift velocity saturation (fig. 2). For these reasons the amplitude of the output pulse is a decreasing function of the track distance along the drift space. For a not too large drift space (less than 1 cm), this effect is weak, but it becomes important for a larger one. This is illustrated by fig. 3, where the curves (a) and (b) are the efficiencies as a function of the potential on the sense wire for a track passing near the sense wire (a) and a track near the field wire (b). The difference in potential between the two plateaus means that the decreasing peak current has to be compensated by increasing the
~Efficiency ~o
Threshold of the amplifier 3t/A G a s mixture Argon 7 0 ~ i s o b u t a n e 30,70
lOq (a)
5Q
(b.
it(c)
er-uler /,f~ o/ f "region GeigM
/l
i I 3.5
f
f
Sense wire HV , i [ :.~ 4 KV
Fig. 3. Efficiency curves on a c h a m b e r with cathode at c o n s t a n t potential. (a) Source on sense wire, (b) Source on potential wire. (c) Background.
DRIFT
CHAMBERS
45
multiplication on the anode outside the proportional region of amplification as can been seen from curve (c) of fig. 3. In these conditions the dark current increases and some large pulses appear ( > 1 mA), saturating the amplifiers. These large pulses are not strictly Geiger-Mfiller pulses, as we have shown v) but they are limited in pulse length, and this limitation is a function of the amount of quenching gas (isobutane in the present case). The useful width of the plateau may be improved by decreasing the amplifier threshold (I /~A threshold for instance) but then problems of noise and pick-up increase. 2.2.2. Pulse shape; multitrack separatiot, The electron distribution along the ionizing track follows a Poisson law. As we saw above with such a structure, the clusters of electrons reach the sense wire at different times, so the resulting anode pulse is a sum of successive avalanches during the total drift time (fig. 4). Then the pulse has many maxima and triggers the amplifier many times. For that reason it is difficult and even quite impossible to distinguish several tracks passing through the same drift space at the same time. This limits the performance of such a type of chamber to the detection of only one particle per each sense wire during the maximum drift time (600 ns for 3 cm). 2.2.3. Adopted structure The best electron collection and multitrack resolution are obtained with a uniform electric field, A cathode plane made of parallel equidistant wires at increasing potentials gives a good approximation of a constant gradient potential as has been tested by Charpak and Sauli for high accuracy drift chambers with a 2 m m wire spacing. For obvious reasons of cost we could not extend this solution to a chamber of 4 m × 4 m / " / "
//S
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oor,so,
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200)-~
%
c~ 5mm/5m
oI
5m
p.
5m
30ram
. 4
J Fig. 4. Shape o f pulses with cathode at c o n s t a n t potential, T h e o u t p u t resistor is 10 k,Q.
"-
GROUND
Fig. 5. A d o p t e d structure for the drift chambers.
46
G. M A R E L et al.
~_
.
.
.
.
.
.
30mm
_~
Honeycomb
panel
,
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+3.SKV
Anode Wire
Equipotential
Cathode
Lines
O2OO~m
£~40)LI m
Fig. 6. Equipotential lines o f the field with the structure presented in fig. 5.
and instead chose a compromise between this sophisticated solution and the cathode plane at uniform potential. An approximation of uniform field along the 3 cm of drift space can be obtained by only four wires (figs. 5 and 6). This structure gives a field strength sufficient to saturate the drift velocity (fig. 7) and to obtain a mean pulse size independent of the position of the ionizing track (fig. 8).
Maximum Electric Field
4000~
2.3. WIRE DIAMETER
2.3.1. Cathode wires W i t h a 5 m m cathode wire spacing, the field a r o u n d the wires is high a n d can induce a corona current. The
300
CHAMBER
WITH POTENTIAL CORRECTOR WIRES
30% Isobutana - 70% Argon
200C
,100%
,,~/'~--~x--
x On . . . . . .
ire
o -lcm 50 YgI~s__ 1OO0
-50%
& On potential wire Pot, wire HV =-2kV S. wire 40~Jm
'5 ;=
Cathode wire Anode wire
10
20
Fig. 7. Electric field for the structure o f fig. 5.
>'mm 30
HV (S.w.)
314
3.5
3:6
317
318
319
Fig. 8. Efficiency curves for the adopted solution drift chamber.
LARGE PLANAR
strength of this field can be reduced by using sufficiently thick wires. Thus we chose cathode wires of 200/~m diameter. 2.3.2. Sense wire As shown in fig. 9, the width of the efficiency plateau increases when the sense wire diameter decreases. For this reason, then wires of 20 pm diameter or less are usually used. For long wires, as in our case (4 m), the resistance of the wires is not negligible and introduces an attenuation of the pulse. In fig. 10 attenuation as a function of the wire length is plotted for wires of 20 pm and 40/~m diameter. In fig. [ l, we represent the amplification factor as a function of the high voltage for the 20 pm wires. One can see that the attenuation Gas mixture
707oIsobutane30~
Argon
Wire length I m Threshold of the amplifier 3 / t A o 20~/l m £1 Wire l + 3 0 / z m O
~Hits~sec/wire
i • 40~Jm~
103
for the 20/~m diameter wire has to be compensated by approximatively 200 V. This reduces the plateau which then becomes as short as the one for the 40 ~(m diameter wire. Thus, the use of 20 l~m wires has no advantages and we decided to use 40 #m wires which are mechanically more convenient. 2.4. GAS MIXTURE The gas mixture is composed of 70% argon, 30% isobutane. The choice of this mixture was made according to the work of Charpak et al. concerning drift velocity saturation4). The mixture is prepared online. The relative proportion of argon is obtained by passing each gas through a thermal ftowmeter. Each flowmeter provides an analog signal which is proportional to the mass flux and is independent of ambient temperature variations. The ratio of the two signals is c o m p a r e d against a reference level for the chosen proportion, and an electrovalve on the isobutane supply is actuated if the ratio deviates from the reference by more than 2%. The volume of each drift Amplificat on factor Arbitrary normalisation
/
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f
/
5.1O:
0.5_
I/ I
I
/
~
I
I
I
i
/
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Sense wire HV (kV) I I ! .3L
3.5
3.0
47
DRIFT CHAMBERS
4.
Fig. 9. Effect of the wire diameter on the length of the plateau. I 3.2
I 3.4
3.6
[ 3.8
I H V-(Kv) 4 anode wire
~Attenuation factor Anode of 20/Lm of diameter
o 2 0 / I m £1 • 40//~ m O
Isobutane 3 0 ~ Argon 70~o
1
}, 0 75. _
Fig. 11. Amplification factor vs high voltage on anode wi~e.
x Argoff.ff
05.
Thermal flowmeter ~.
-_
~,
,xt;r
ComparatorI53-/ :Chambers
O 2 .~
Nire length 0
I 1
[ 2
I 3
L 4
t~ 5
Isobu~ne ~
~
m
Fig. 10. Attenuation factor as a function of the wire's length and diameter.
Thermal flowmeter
~ Electrovalve
Fig. 12. Scheme for the gas system.
48
G. M A R E L et al.
TABLE 1 Specification o f wires.
(gf) Tension
T
3L/L (%)
~nn
Elongation
Resist.
140± l0 1000 :[: 80
0.28 0.26
110 31
(pra)
40 200
Elasticity limit
T
AL/L
~rljB
o',,,~ ul~
R (Q/m)
128 86
238 135
200 50
(%)
162 2700
0.33 0.7
~"
E
F
(p'm)
Young modulus
Linear weigth (g/m)
L = 2 ra
3m
4 ra
L = 2m
(gap 5 ram) 3m
4 m
39.7 ×103 12.27 × 103
2 3 . 4 × 1 0 -3 2.6 × 10- ~
60 48
40 32
29 24
4.5 7
4.3 6.8
4 6.5
40 200
Frequency n = I (Hz)
chamber gap is 500 1, yielding a total volume of 25 m 3 for the twenty chambers. The gas flow for each gap is adjusted and supplied independently. In order to compensate for small gas leaks, and minimize pollution of the gas by oxygen, the minimum flow is 7.5 1/h.gap. The gas system is shown schematically in fig. 12.
P
I
~
~ . p3FJ_ 1~-
hv vi bra t i ons (kV)
3. Chamber construction Each standard chamber consists of four sandwich panels (held by stainless steel pins) enclosing the three sense wire planes described above (one horizontal, the other two oriented +_60° with respect to the first, fig. 13a and b).
Gap1 ~ap,: - ~ --
f-- ~
li I J ~
Read out planes
~, Panels'~
Gap3
]
• Read out plane
(a) Fig. 13. Mechanical details of the chambers.
(b)
LARGE
PLANAR
a) Sandwich panels " N I D A " (fig. 14a) The sandwich panel is composed of two aluminium skins glued to the aluminium honeycomb. This aluminium honeycomb consists of 6 mm diameter cells, length (20_+0.2) mm and wails of thickness 38 40 Fm. This gives an average density of 48 da N/m 3 _+ 10%. The two aluminium skins are (1 _+0.06) m m thick. They have to be reinforced by inserts all around the hexagonal perimeter and along the aluminium plate joints to resists the forces due to the clamping and to avoid gas leaks. b) Frames and printed circuits As is shown in fig. 14b, the panels are separated by frames made of aluminium plate and "stesalite" fiber glass epoxy. The printed circuits are glued on the "stesalite" frames. No electronic circuits are mounted on these printed circuits, thus the mechanical construction is separated from the electronics system. c) Wire characteristics For a typical gap, the central plane has 62 goldplated tungsten sense wires of 40 pm diameter and 63 gold-plated Cu-Be field wires of 200/~m diameter. On each side of the central plane 372 field wires complete the gap. All the wires are gold-plated to avoid oxidation with time. The wires are stretched with a pneumatic wiring machine with a stress of 140 g for 40/xm diam-
DRIFT
49
CHAMBERS
eter and 1000 g for 200/~m diameter. They are soldered by hand on the printed circuits. All the characteristics of these wires are summarized in table 1. d) Deflections and maximum eJJects With one pin every 277 m m to clamp the four sandwich panels and the frames, excellent rigidity is achieved and no deformation outside the "tolerance", (-+2/10ram) can be measured. To maintain the reproducibility and the position of panels and frames, a drilling jig is used during the construction. The system is sealed by O-rings. The gas leaks are less than 0.7 1/h per chamber at a pressure of l mbar; at this pressure, the maximum deformation value of the panels is 0.2 m m (1.5 mm without central insert). 4. Electronics read-out
The design of the read-out system must take into account the chamber characteristics and experimental specifications. The output pulse on the chamber wire is low and we need an input threshold of 400-600 pV on 150 f2 on an ac coupled preamplifier. The beam spill in the neutrino experiment is short (26 ps) but the event rate is low. In the narrow band beam for instance we expect 2-3 events including background. They have to be recorded in 26 ps and
ALUMINIUM SKINS
/
'
/
Printed circuits
\\\
i
(a) Fig. 14. Frames and printed circuits,
(b)
50
G. MAREL et al.
this requires a buffering possibility and a fast rejection for bad events. The signals delivered by the drift chambers arrive in the counting room before a trigger decision can be made. Since it is not possible to introduce a delay line for each wire, we must start the time conversion with the drift signal and stop it by the retarded trigger signal (late time reference - LTR). We have stressed the need to record several tracks on the same wire which requires several scalers per wire. In order to avoid a large number of scalers, since we expect only one or two tracks on average, we provide sixteen sense wires with four scalers located in one time-to-digital converter (TDC) module. These wires are not adjacent in space; instead every fourth wire is connected to the same module. The resolution demanded (_+ 1 ram) fixes the frequency of the clock to 100 MHz. For easy connection with the other parts of the experiment, the system is interfaced following the C A M A C standards. All these points lead us to split the read-out system in two parts: -preamplifiers and concentrators located on the chamber, - time-to-digital converters in C A M A C crates located in the counting room. A test system has been added to check the read-out. a) Preamplifiers Preamplifiers are implemented on a card with an edge
connector directly plugged on the printed circuit board supporting the chamber's wires. This printed circuit board also brings dc power for the preamplifiers and an adjustable dc voltage line driving the amplifier's threshold. Each preamplifier board is designed to accept 4 sense wire channel signals in the basic diagram shown in fig. 15. First a 220 pF/6 kV capacitor isolates the amplifier input from sense wire hv, it is followed by a 150 (~ input resistor. This value was chosen to match the sense wire line impedance. Another 150 (2 resistor with a serial hv capacitor matches the other end of the line. It is difficult to correctly match this line principally due to the high resistivity of the sense wire. Two diodes protect the amplifier against sparks or hv leakage; and a high value ( 8 0 k ~ ) resistor is used to introduce a test signal in amplifier. Amplifiers are LD 604 (Lecroy hybrid circuits) with a minimum threshold of 400/~V, adjustable for the entire chamber up to 2 inV. Amplifier's outputs, in ECL differential mode, are carried out by flat cables to the "concentration" printed circuit board which groups wires in 16 packages. Signals go to the T D C inputs in the counting room, by a 16 twisted pair shielded cable, 40 m long. The flat cables also carry signals for amplitude and time tests as described below. A twisted pair line is matched at the T D C line receiver input by 100(2 resistor. The overall time
Threshold Adjustment
~
220pF
+ HV ~.
-.%';; _ :
]
}
50
S e n s e], Wir.
ff
6 IKIv
--'6
n
~
~ [ i-
i~
~
Diff ~
• [ I I
-- Output to'rOC
I ' Discri" -
'0oI11 i "i/////, "////////////;
~S
TEST CIRCUIT
Fig. 15. Diagram of the amplifier.
~, ~ ~.
Time Command Amplitude Command Address Command
LARGE PLANAR
DRIFT CHAMBERS
slewing measured from anaplifier input to T D C line receiver output was found to be 7 ns from 2 times to 20 times over threshold. According to the L D 604's specifications the output pulse duration depends o f the c h a m b e r ' s pulse amplitude and threshold setting. However with a typical chamber pulse one can distinguish two hits on the same wire separated by 100 ns at a threshold of 400 pV. b) Time-to-digital converter The relatively modest precision required led us to choose, in designing the T D C system, the direct counting m e t h o d with a m a x i m u m clock frequency of 100 MHz. This frequency makes it easy to build a counter in E C L 10.000 Technology. The small multiplicity o f neutrino events allowed us to group a
51
signifiant number of wires in the same encoder. So the T D C exhibit the following characteristics, in a single C A M A C bin: - 16 input channels with enable/disable c o m m a n d , - 4 wire number-time encoding possibilities per event, -12ns resolving time between two inputs to be encoded, - 80-100 M H z clock frequency, - fast reset for rejected events, - d c signal acts as " S t a r t " signal; trigger signal acts as common "Stop", - 40 events first in-first out buffer. The T D C scheme is shown in fig. 16. The leading edge of the chamber signal, after the line receiver, is differentiated in a 12 ns pulse and multiplexed on
Line
~_ Reieivers - -
E>
Wire #1
I I
an,,
Memory J
Lb.
I
::! 2xlO1851
L
I
F
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I
I t I From
I
Preamplifiers
I. . . . .
I
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Ill O
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t I
1
I
j
, L
~
:1 --I
I
Wire
I
A
D
Flip Flop
#16
I Io.r.ndo.Ir
li Enable 'Disable
Fig. 16. Block diagram of the time-to-digital converter.
Latetime Reference
IOOMHz Clock
Re ect
Load Buffer
X/
52
G. M A R E L et al.
4 channels of 16 input priority encoders and latches (10165). The OR output of the encoder is used to set a flip-flop (F). The " l " state of this flip-flop latches the wire number in the 10165 encoder. The output of the (F) flip-flop releases the reset condition on the next channel which is now ready to encode a new chamber signal, and so on. The output of the (F) flip-flop is also sent as data in a clocked type D master-slave flip-flop in order to derandomize the drift chamber signal. The output of this last flip-flop opens the gate to the 100 MHz scaler. After the maximum possible drift time a signal (late time reference) stops all the scalers. This LTR signal is also derandomized in the TDC and it disables the TDC inputs. Thus a dc level is available at an analog output indicating the number of coded hits. For a good event a " L o a d Buffer" signal pushes scaler's contents and wire number codes into the FIFO memories in 500 ns. For a bad event a "Reject" signal resets scalers and encoders. In both cases input gates are then enabled. If a channel is busy from a spurious signal at the end of the maximum counting time, no LTR signal occurs and the scaler resets itself. Thus the channel is free again to accept a new signal. The read-out of F I F O buffers is done by CAMAC. We use 7 bits for drift time, 4 bits for wire number and
~_-_-~/////~1,/I/ . . . . . . . . . . . . • l
.~
1 bit to indicate if the word is valid. We use one C A M A C crate for each chamber. In addition to the crate controller and 12TDCs, there are two fan-outs for clock and LTR signals, one module to check F I F O ' s signals and one other for testing all the drift chamber electronics. c) Permanent read-out test system In order to check the wires and the preamplifiers, each preamplifier card can be addressed by computer, via CAMAC, when misconnected or broken connections are detected a message is delivered. To check preamplifier, cables and TDC performance we can send a signal as input for any preamplifier. This signal can be selected, under computer control, from 400/~V to 2 m V and is used to check threshold stability. The delay of this signal can be varied with respect to a time reference by steps of 6 ns always under computer control. (Histograms of these checks are shown on a monitoring screen.) Signals for testing purposes are sent from the concentration board to the preamplifier by the same flat cable which transports preamplifier output signals. 5. Results We present the results we obtained with one of the completely equipped chambers. A cosmic ray test
Scintillator
PC
Drift
.!/
I: I. °i ......... ~ . / / / / ~ / .
Fig. 17. Set-up for the cosmic ray tests.
i
chamber"
J
V-
.,-.
....
......
tea'!
.....
oo,n,,,,a,or
-~ / / / / / " / / / / / / / / / -
_U_ ~
LARGE PLANAR DRIFT CHAMBERS (fig. 17) is set up at the end of the production chain in such a way that any mechanical mistake can be fixed rapidly. Two sets of scintillator counters with 10 cm of lead between them as shielding define a trigger for cosmic rays. In addition, with three proportional chambers we can define a straight track and thus determine the efficiency of the drift chamber and the drift velocity. These multiwire proportional chambers and the T D C are read on a computer CII 90.10 through a C A M A C system. These chambers define a trigger area of 3 0 × 3 0 cm 2 at the drift chamber which is placed on a movable table such that the whole area of the large chamber can be tested. a) Efficiency We show on fig. 18 the noise curves for non-gated output wire signals. This counting rate includes all the possible sources of noise: corona current, cross-talk and multitriggers of the amplifier for large pulses. The noise curve defines the end of the efficiency plateau at about 1.5 × 10 5 hits/gap/5 s. This plateau limit is lower than the Geiger-Miiller region and is chosen to avoid a large number of accidental counts, thus allowing an easy reconstruction of the event. We present in fig. 19 the efficiency curve for one gap for different thresholds
53
of the amplifier (the position of the area covered by the trigger is also shown). One can see that the chambers have a wide plateau (200 V) and that a factor two in amplification is obtained by increasing the high voltage by 100 V. The same efficiency curves are shown in fig. 20 for three gaps. We can see small differences at the beginning of the plateau which come from the fact that the trigger area is not at the same distance from the amplifiers in all gaps, however the same efficiency is achieved very rapidly showing that no strong effects come from the mechanical construction or gas inhomogeneity. The effect of the pulse attenuation along the wire is shown in fig. 21. A shift of 100 V corresponds to a longitudinal displacement of 3 m with respect to the read-out system. The plateau we obtain is large enough to allow for this effect. This efficiency is constant all along the drift length as shown in fig. 22. We must underline that these results are obtained with tracks practically normal to the drift chamber. The average angle to the normal plan is 7 ° . The average efficiency with such a trigger over the entire area of the chamber is 99.5% per gap. b) Linearity and precision For perpendicular tracks (0 < 5 °, 0 being the angle
!
GAP. N'- 2 CHAMBER. N!3
l OO~
/
/
AVI:RAGE" "'~ 100Hits~See/Wire
/
~,s.,o' ~o7oA,GON
/
I
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Thres'°ld /%'~90:#jv
k,o
//
0,o,o,
-
a 1.8reVolt ]
~o~
\ ol.2 mVolt
--i
\+9oo#~v
,"r"
/ i /!
; ....
~+-~1 3.4
a ~ ~
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f~,ilV
/
3oz.soB°....
O
....
I
3.5
3.6
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I
3.7
+
I,- 4~ HV 3.8Kv
Fig. 18. Noise counting rate for differentthresholds.
I
I
3600
3700
38OO Fig. 19. Efficiencycurves on a large drift chamberwith different amplifierthresholds. 3400
3500
54
G. MAREL I
et al.
! CHAMBER• 3
~
• GAP 1
Threshold 90O~tV
-{- GAP 2 GAP
HV 3.7 KV
A T
Efficiency
Adjacent sense wire
JJ 00 L
i / Distance I I to the
,+J~rk,~nse wire
4Amplifier Tr~tE,~r~olO 1 2 m \
-30
t
GAP~
GAP 2
GAP 1
--40
--20
Potential wire
--10
O
;
+lPO q-20
Sense wire
+30
+40
t
Potential wire
Fig. 22. Measured efficiency vs the impact of the particles in the drift cells.
Y 3500
3600
3700
HV Vo,'~
3800
Fig. 20. Efficiency curves for the three gaps of a drift chamber.
Efficiency
.loo% /
C - ~ . _ ~ = _ __ 3.-+ /.).~ "
f - - o -
oJ
o.~"
.+/ " ÷ .~
1OO VOLTa
/
/"
/ / /
/
d
_5o)~ 8m
13 8m I
I
between the projected track on the plane orthogonal to the wires and the normal to the chamber) the measured drift time is almost a linear function o f the track distance to the sense wire (fig. 23). For non-normal tracks the relation becomes non-linear and we have to make some corrections which increase with the angle and the mean distance along the drift space (fig. 24). The measured precision is 0.95 m m (fig. 25). This error is the sum o f the uncertainty in the track Iocalisation and the error on the drift time measurement. These two errors are o f the same order and we can conclude that the spatial resolution with these drift chambers is better than 0.7 mm. This precision is not affected by corrections for tracks at large angles. These tests are not presented with methylal in the gas. We k n o w n that methylal is useful for long runs with high intensities of incoming particles. As a test we have taken a run with 1.5% methylal added to the gas mixture, with the result that the efficiency curve is translated by 50 V. We have not seen other sizeable effects on either drift velocity or the high voltage efficiency curve. (We will use methylal for the running time although the flux is very small.) 6. C o n c l u s i o n
HV Volts 3500
3600
3700
380
0
Fig. 21. Efficiency curves for different distances from the read-out system.
We have achieved the construction o f very large dr!fl chambers, 12 m 3, within the frame o f the proposed track precision of +1 ram. The performance o f these chambers is quite reproducible for the first eighteen chambers already available out o f the twenty demanded by the experiment. This study and its realization owe a lot to G. Charpak, to w h o m we are very grateful for his knowl-
LARGE :
PLANAR
DRIFT
CHAMBERS
55
1 600
ts
6OC
s
t
401 GAP N~I
0 4 + 3 nSAm
I '
--30
--20
I
GAP N92
I :203+_ 3 nSAm
/ Distance I 10
--10
T 20
[ 30
Distance mm
m~ --30
--20
-10
(a)
10
O
20
30
(h) iDriff t i m e difference
ns - -
----T
\
r
V6ooIts--7-,
-- I ; +2C +11
/
i
Distance to the
30 sense wire
m~ -10 400 GAP. N93
/ __203__4.3 nSAm
-21 -3(
wt
/Track
e/
°
2O0
°
?.s
7°
¢
I
~
t
--30
--20
--10
0
I 10
t 20
(c) Fig. 23. Drift velocity in the three gaps.
Distance t mm, 30
0 0 (~) (~) (~
0°40< 5o~0 15~ ~ 0 25o~0 35°40
5o < is ~ <~ 25 ° <~ 35 o <45 °
Fig. 24. Difference in drift time between the m e a s u r e d time a n d the theoretical time vs the distance of the track to the sense wire, for different incident angles. T h e theoretical time is defined by fitting a straight line on the m e a s u r e d times for n o r m a l tracks (0 < 5°).
56
G. M A R E L et al. Events ~SO
G= Oo95mrn -~-
This standard deviation include the mwpc error
~0(
~'~'°~pc -- 0 - 7 m m
150
'00
edgeable and pleasant guidance and advice. We also profited from F. Sauli's great experience in this field. We thank the members of the CDHS Collaboration for their comments on the project, and in particular J. Steinberger who participated in the conception of the chambers' construction. We acknowledge the help of the engineering office and of the mechanical production group of the STIPE. We would like to recall more particularly the memory of J. Bernard, who started the project with us but died on 12th February 1975. The realization of this project was made possible only through the support of Prof. A. Berthelot, and J. F. Detoeuf and P. Prugne. We would like to thank them very sincerely.
50
References
-•. - 3 - 2 -1
0
1
2:3
Spatial resolution mm~
Fig. 25. Spatial resolution o f the chamber for normal tracks. Distance between the observed position on the chamber and the point given by the extrapolation o f the proportional chamber coordinates.
~) 2) 3) 4) 5) 6)
G. Charpak et al., Nucl. Instr. and Meth. 80 (1970) 13. R. Chaminade et al., Nucl. Instr. and Meth. 111 (1973) 77. A. H. Walenta et al., Nucl. Instr. and Meth. 92 (1971) 373. A. Breskin et al., Nucl. Instr. and Meth. 119 (1974) 9. D. C. Cheng et al., Nucl. Instr. and Meth. 117 (1974) 157. C E R N - D o r t m u n d - H e i d e l b e r g - S a c l a y Collaboration, C E R N Report S P S C - P . 7 3 . 1 - S P S C - P 731 ; A d d e n d u m - S P S C 74.6 PI and 75.33 P1.
INSTRUMENTS
AND
METHODS
14I 0977) 43-56;
© NORTH-HOLLAND
PUBLISHING
CO.
LARGE P L A N A R DRIFT C H A M B E R S G. M A R E L , P. B L O C H , S. B R E H I N , B. D E V A U X , A. M. D I A M A N T - B E R G E R , C. L E S C H E V I N , J. M A 1 L L A R D , Y. M A L B E Q U I , H. M A R T I N , A. P A T O U X , J. PELLE, J. P L A N C O U L A I N E , G. T A R T E and R. T U R L A Y
D(partement de Physique des Partict, les Eidmentaires, CEN Saclay, France Received 18 October 1976 We describe 14 m 2 hexagonal planar drift c h a m b e r s designed for the neutrino experiment o f the C E R N - D o r t m u n d - H e i d e l berg-Saclay Collaboration. Details on mechanical construction, electronic read-out, results on efficiency and accuracy are presented.
1. Introduction The principle and the development of the localisation of tracks of charged particles by measurement of drift time of primary ionization electrons have already been discussed by different authors1-3). The problems of electric field structure and gas mixture leading to the saturation and stability of drift velocity are now solved4). High accuracy fir < 100 #m) drift chambers are currently used in experiments with specific solution to solve the left-right ambiguity problem. In a drift chamber, the number of anode wires is smaller than for a multiwire proportional chamber, thus reducing the price of the associated electronics and simplifing the mechanical construction particularly if a not too high spatial resolution is demanded. These advantages are particularly suitable for experiments in which large detectors are necessary. This is the case, for example, in high energy neutrino interaction studies, and the Harvard University Group s) in the H P W F Collaboration has already adopted large planar drift chambers (4 m x 4 m). For a neutrino experiment with the SPS at C E R N , the C E R N - D o r t m u n d - H e i d e l b e r g - S a c l a y Collaboration 6) proposed the construction of twenty large drift chambers for the localisation of a muon track within a spatial resolution of -+ 1 ram. In the present paper we report on specific problems of the construction of 720 m z of usable detectors with such a constraint on the resolution. 2. Choice of parameters 2.1. GEOMETRY Each drift chamber used in the neutrino experiment is placed behind an iron core magnet of 3.8 m in diameter. The right-left ambiguity solution and correct association for multitrack events can be done with
three planes with different drift directions. For these reasons we decided to construct chambers of hexagonal shape (the distance between two opposite edges is 4 m) composed of three independent gaps. The wire directions chosen are plus and minus 60 ° relative to the horizontal one, the hexagonal shape beingwell adapted to these directions. The gap width, which has to be as thin as possible to keep the average density of the detector maximum along the trajectories is 3 cm. This value is sufficient to obtain electrostatic stability for the long wires with the electric field structure chosen.
2.2. ELECTRIC STRUCTURE OF THE CHAMBERS Most of the time these chambers will detect only one muon, but we retained the possibility to measure two tracks or more from the same event with a spatial separation larger than 1 cm. This could be achieved either by a sense wire spacing smaller than the minimum fixed spatial resolution between two tracks, or by a dead time after an avalanche on a wire corresponding to a distance smaller than the above resolution. The first solution requests a high number of sense wires and thus increases the price of the read-out electronics, whereas the second possibility requires special attention to the electric structure. We adopted this last solution with a distance between two sense wires of 6 cm. The simplest structure for a multiwire drift chamber is almost the same as for a normal multiwire proportional chamber; the anode wires (sense wires) are equidistant and parallel to the two equipotential cathode plates, the strength of the field between two sense wires being increased by a field wire (potential wire)3). Cheng et al. have adopted this structure for their large drift chamber (4 m x 4 m) with a large drift space of 5 cm (half distance between two sense wires) and a gap of +2.5 cm. it was very tempting to adopt
44
G. M A R E L et al.
30mm
.
.
.
.
.
.
J
~b
- 2 KV cathode
r
3.5 KV
Equipotential lines
Anode 50/~ m
~1200~ m
Fig. 1. Equipotential lines of the field for a drift chamber with cathode at constant potential. Distance between sense wires = 6 cm. Field wires at - 2 0 0 0 V separate the sense wires. MexJrnul11 Electric
the same conception for our chambers because of its simplicity but studies done with such a structure (drift space + 3 cm, gap -+ 1.5 cm) have shown some problems whenever large wire spacings are used.
field
500(
2.2. I. Pulse size; efficiency of the chamber For a non-electronegative gas mixture, the electrons of the primary ionization of a charged particle are drifting along the electric field lines with a drift velocity which depends on the field strength and the nature of the gas. For some mixtures like argon-isobutane, argon-ethylene or argon-propane, the velocity is an increasing function of the field and then becomes roughly a constant4), which depends on this gas mixture. For a proportional counter one measures the integrated signal on the anode wire which is proportional to the sum of all elementary charges produced in the primary ionization, however in a drift chamber one measures a current which varies with time. The peak value of this current depends on the convolution of the shape of the pulse for an elementary avalanche produced by one electron by the drift time dispersion of the initial electrons. This value is maximum for all primary
400~
300G
200C
100C
Cat lode wire
An ,de wire
110
20
310 rnm~"
Fig. 2. Electric field along the path (a) of fig. I.
LARGE
PLANAR
electrons reaching the anode wire at the same time (if no space charge effect occurs) and decreases with the drift time spread. With such a structure this spread is important for two reasons: first, the field lines are not parallel (fig. 1), then the difference between paths of electrons increases with the distance from the track to the sense wire. Secondly along all drift paths, the field strength does not reach the value corresponding to the drift velocity saturation (fig. 2). For these reasons the amplitude of the output pulse is a decreasing function of the track distance along the drift space. For a not too large drift space (less than 1 cm), this effect is weak, but it becomes important for a larger one. This is illustrated by fig. 3, where the curves (a) and (b) are the efficiencies as a function of the potential on the sense wire for a track passing near the sense wire (a) and a track near the field wire (b). The difference in potential between the two plateaus means that the decreasing peak current has to be compensated by increasing the
~Efficiency ~o
Threshold of the amplifier 3t/A G a s mixture Argon 7 0 ~ i s o b u t a n e 30,70
lOq (a)
5Q
(b.
it(c)
er-uler /,f~ o/ f "region GeigM
/l
i I 3.5
f
f
Sense wire HV , i [ :.~ 4 KV
Fig. 3. Efficiency curves on a c h a m b e r with cathode at c o n s t a n t potential. (a) Source on sense wire, (b) Source on potential wire. (c) Background.
DRIFT
CHAMBERS
45
multiplication on the anode outside the proportional region of amplification as can been seen from curve (c) of fig. 3. In these conditions the dark current increases and some large pulses appear ( > 1 mA), saturating the amplifiers. These large pulses are not strictly Geiger-Mfiller pulses, as we have shown v) but they are limited in pulse length, and this limitation is a function of the amount of quenching gas (isobutane in the present case). The useful width of the plateau may be improved by decreasing the amplifier threshold (I /~A threshold for instance) but then problems of noise and pick-up increase. 2.2.2. Pulse shape; multitrack separatiot, The electron distribution along the ionizing track follows a Poisson law. As we saw above with such a structure, the clusters of electrons reach the sense wire at different times, so the resulting anode pulse is a sum of successive avalanches during the total drift time (fig. 4). Then the pulse has many maxima and triggers the amplifier many times. For that reason it is difficult and even quite impossible to distinguish several tracks passing through the same drift space at the same time. This limits the performance of such a type of chamber to the detection of only one particle per each sense wire during the maximum drift time (600 ns for 3 cm). 2.2.3. Adopted structure The best electron collection and multitrack resolution are obtained with a uniform electric field, A cathode plane made of parallel equidistant wires at increasing potentials gives a good approximation of a constant gradient potential as has been tested by Charpak and Sauli for high accuracy drift chambers with a 2 m m wire spacing. For obvious reasons of cost we could not extend this solution to a chamber of 4 m × 4 m / " / "
//S
%//
/ / /~ G R O U N D
"
' "
'
oor,so,
+1 k v
"
"
/
/ "
,.r.
+ l , 5 k v 4 2 k v 4-1.5kv + l k v
E,.T
+3.Tkv
200)-~
%
c~ 5mm/5m
oI
5m
p.
5m
30ram
. 4
J Fig. 4. Shape o f pulses with cathode at c o n s t a n t potential, T h e o u t p u t resistor is 10 k,Q.
"-
GROUND
Fig. 5. A d o p t e d structure for the drift chambers.
46
G. M A R E L et al.
~_
.
.
.
.
.
.
30mm
_~
Honeycomb
panel
,
,
+3.SKV
Anode Wire
Equipotential
Cathode
Lines
O2OO~m
£~40)LI m
Fig. 6. Equipotential lines o f the field with the structure presented in fig. 5.
and instead chose a compromise between this sophisticated solution and the cathode plane at uniform potential. An approximation of uniform field along the 3 cm of drift space can be obtained by only four wires (figs. 5 and 6). This structure gives a field strength sufficient to saturate the drift velocity (fig. 7) and to obtain a mean pulse size independent of the position of the ionizing track (fig. 8).
Maximum Electric Field
4000~
2.3. WIRE DIAMETER
2.3.1. Cathode wires W i t h a 5 m m cathode wire spacing, the field a r o u n d the wires is high a n d can induce a corona current. The
300
CHAMBER
WITH POTENTIAL CORRECTOR WIRES
30% Isobutana - 70% Argon
200C
,100%
,,~/'~--~x--
x On . . . . . .
ire
o -lcm 50 YgI~s__ 1OO0
-50%
& On potential wire Pot, wire HV =-2kV S. wire 40~Jm
'5 ;=
Cathode wire Anode wire
10
20
Fig. 7. Electric field for the structure o f fig. 5.
>'mm 30
HV (S.w.)
314
3.5
3:6
317
318
319
Fig. 8. Efficiency curves for the adopted solution drift chamber.
LARGE PLANAR
strength of this field can be reduced by using sufficiently thick wires. Thus we chose cathode wires of 200/~m diameter. 2.3.2. Sense wire As shown in fig. 9, the width of the efficiency plateau increases when the sense wire diameter decreases. For this reason, then wires of 20 pm diameter or less are usually used. For long wires, as in our case (4 m), the resistance of the wires is not negligible and introduces an attenuation of the pulse. In fig. 10 attenuation as a function of the wire length is plotted for wires of 20 pm and 40/~m diameter. In fig. [ l, we represent the amplification factor as a function of the high voltage for the 20 pm wires. One can see that the attenuation Gas mixture
707oIsobutane30~
Argon
Wire length I m Threshold of the amplifier 3 / t A o 20~/l m £1 Wire l + 3 0 / z m O
~Hits~sec/wire
i • 40~Jm~
103
for the 20/~m diameter wire has to be compensated by approximatively 200 V. This reduces the plateau which then becomes as short as the one for the 40 ~(m diameter wire. Thus, the use of 20 l~m wires has no advantages and we decided to use 40 #m wires which are mechanically more convenient. 2.4. GAS MIXTURE The gas mixture is composed of 70% argon, 30% isobutane. The choice of this mixture was made according to the work of Charpak et al. concerning drift velocity saturation4). The mixture is prepared online. The relative proportion of argon is obtained by passing each gas through a thermal ftowmeter. Each flowmeter provides an analog signal which is proportional to the mass flux and is independent of ambient temperature variations. The ratio of the two signals is c o m p a r e d against a reference level for the chosen proportion, and an electrovalve on the isobutane supply is actuated if the ratio deviates from the reference by more than 2%. The volume of each drift Amplificat on factor Arbitrary normalisation
/
/
f
/
5.1O:
0.5_
I/ I
I
/
~
I
I
I
i
/
/
Sense wire HV (kV) I I ! .3L
3.5
3.0
47
DRIFT CHAMBERS
4.
Fig. 9. Effect of the wire diameter on the length of the plateau. I 3.2
I 3.4
3.6
[ 3.8
I H V-(Kv) 4 anode wire
~Attenuation factor Anode of 20/Lm of diameter
o 2 0 / I m £1 • 40//~ m O
Isobutane 3 0 ~ Argon 70~o
1
}, 0 75. _
Fig. 11. Amplification factor vs high voltage on anode wi~e.
x Argoff.ff
05.
Thermal flowmeter ~.
-_
~,
,xt;r
ComparatorI53-/ :Chambers
O 2 .~
Nire length 0
I 1
[ 2
I 3
L 4
t~ 5
Isobu~ne ~
~
m
Fig. 10. Attenuation factor as a function of the wire's length and diameter.
Thermal flowmeter
~ Electrovalve
Fig. 12. Scheme for the gas system.
48
G. M A R E L et al.
TABLE 1 Specification o f wires.
(gf) Tension
T
3L/L (%)
~nn
Elongation
Resist.
140± l0 1000 :[: 80
0.28 0.26
110 31
(pra)
40 200
Elasticity limit
T
AL/L
~rljB
o',,,~ ul~
R (Q/m)
128 86
238 135
200 50
(%)
162 2700
0.33 0.7
~"
E
F
(p'm)
Young modulus
Linear weigth (g/m)
L = 2 ra
3m
4 ra
L = 2m
(gap 5 ram) 3m
4 m
39.7 ×103 12.27 × 103
2 3 . 4 × 1 0 -3 2.6 × 10- ~
60 48
40 32
29 24
4.5 7
4.3 6.8
4 6.5
40 200
Frequency n = I (Hz)
chamber gap is 500 1, yielding a total volume of 25 m 3 for the twenty chambers. The gas flow for each gap is adjusted and supplied independently. In order to compensate for small gas leaks, and minimize pollution of the gas by oxygen, the minimum flow is 7.5 1/h.gap. The gas system is shown schematically in fig. 12.
P
I
~
~ . p3FJ_ 1~-
hv vi bra t i ons (kV)
3. Chamber construction Each standard chamber consists of four sandwich panels (held by stainless steel pins) enclosing the three sense wire planes described above (one horizontal, the other two oriented +_60° with respect to the first, fig. 13a and b).
Gap1 ~ap,: - ~ --
f-- ~
li I J ~
Read out planes
~, Panels'~
Gap3
]
• Read out plane
(a) Fig. 13. Mechanical details of the chambers.
(b)
LARGE
PLANAR
a) Sandwich panels " N I D A " (fig. 14a) The sandwich panel is composed of two aluminium skins glued to the aluminium honeycomb. This aluminium honeycomb consists of 6 mm diameter cells, length (20_+0.2) mm and wails of thickness 38 40 Fm. This gives an average density of 48 da N/m 3 _+ 10%. The two aluminium skins are (1 _+0.06) m m thick. They have to be reinforced by inserts all around the hexagonal perimeter and along the aluminium plate joints to resists the forces due to the clamping and to avoid gas leaks. b) Frames and printed circuits As is shown in fig. 14b, the panels are separated by frames made of aluminium plate and "stesalite" fiber glass epoxy. The printed circuits are glued on the "stesalite" frames. No electronic circuits are mounted on these printed circuits, thus the mechanical construction is separated from the electronics system. c) Wire characteristics For a typical gap, the central plane has 62 goldplated tungsten sense wires of 40 pm diameter and 63 gold-plated Cu-Be field wires of 200/~m diameter. On each side of the central plane 372 field wires complete the gap. All the wires are gold-plated to avoid oxidation with time. The wires are stretched with a pneumatic wiring machine with a stress of 140 g for 40/xm diam-
DRIFT
49
CHAMBERS
eter and 1000 g for 200/~m diameter. They are soldered by hand on the printed circuits. All the characteristics of these wires are summarized in table 1. d) Deflections and maximum eJJects With one pin every 277 m m to clamp the four sandwich panels and the frames, excellent rigidity is achieved and no deformation outside the "tolerance", (-+2/10ram) can be measured. To maintain the reproducibility and the position of panels and frames, a drilling jig is used during the construction. The system is sealed by O-rings. The gas leaks are less than 0.7 1/h per chamber at a pressure of l mbar; at this pressure, the maximum deformation value of the panels is 0.2 m m (1.5 mm without central insert). 4. Electronics read-out
The design of the read-out system must take into account the chamber characteristics and experimental specifications. The output pulse on the chamber wire is low and we need an input threshold of 400-600 pV on 150 f2 on an ac coupled preamplifier. The beam spill in the neutrino experiment is short (26 ps) but the event rate is low. In the narrow band beam for instance we expect 2-3 events including background. They have to be recorded in 26 ps and
ALUMINIUM SKINS
/
'
/
Printed circuits
\\\
i
(a) Fig. 14. Frames and printed circuits,
(b)
50
G. MAREL et al.
this requires a buffering possibility and a fast rejection for bad events. The signals delivered by the drift chambers arrive in the counting room before a trigger decision can be made. Since it is not possible to introduce a delay line for each wire, we must start the time conversion with the drift signal and stop it by the retarded trigger signal (late time reference - LTR). We have stressed the need to record several tracks on the same wire which requires several scalers per wire. In order to avoid a large number of scalers, since we expect only one or two tracks on average, we provide sixteen sense wires with four scalers located in one time-to-digital converter (TDC) module. These wires are not adjacent in space; instead every fourth wire is connected to the same module. The resolution demanded (_+ 1 ram) fixes the frequency of the clock to 100 MHz. For easy connection with the other parts of the experiment, the system is interfaced following the C A M A C standards. All these points lead us to split the read-out system in two parts: -preamplifiers and concentrators located on the chamber, - time-to-digital converters in C A M A C crates located in the counting room. A test system has been added to check the read-out. a) Preamplifiers Preamplifiers are implemented on a card with an edge
connector directly plugged on the printed circuit board supporting the chamber's wires. This printed circuit board also brings dc power for the preamplifiers and an adjustable dc voltage line driving the amplifier's threshold. Each preamplifier board is designed to accept 4 sense wire channel signals in the basic diagram shown in fig. 15. First a 220 pF/6 kV capacitor isolates the amplifier input from sense wire hv, it is followed by a 150 (~ input resistor. This value was chosen to match the sense wire line impedance. Another 150 (2 resistor with a serial hv capacitor matches the other end of the line. It is difficult to correctly match this line principally due to the high resistivity of the sense wire. Two diodes protect the amplifier against sparks or hv leakage; and a high value ( 8 0 k ~ ) resistor is used to introduce a test signal in amplifier. Amplifiers are LD 604 (Lecroy hybrid circuits) with a minimum threshold of 400/~V, adjustable for the entire chamber up to 2 inV. Amplifier's outputs, in ECL differential mode, are carried out by flat cables to the "concentration" printed circuit board which groups wires in 16 packages. Signals go to the T D C inputs in the counting room, by a 16 twisted pair shielded cable, 40 m long. The flat cables also carry signals for amplitude and time tests as described below. A twisted pair line is matched at the T D C line receiver input by 100(2 resistor. The overall time
Threshold Adjustment
~
220pF
+ HV ~.
-.%';; _ :
]
}
50
S e n s e], Wir.
ff
6 IKIv
--'6
n
~
~ [ i-
i~
~
Diff ~
• [ I I
-- Output to'rOC
I ' Discri" -
'0oI11 i "i/////, "////////////;
~S
TEST CIRCUIT
Fig. 15. Diagram of the amplifier.
~, ~ ~.
Time Command Amplitude Command Address Command
LARGE PLANAR
DRIFT CHAMBERS
slewing measured from anaplifier input to T D C line receiver output was found to be 7 ns from 2 times to 20 times over threshold. According to the L D 604's specifications the output pulse duration depends o f the c h a m b e r ' s pulse amplitude and threshold setting. However with a typical chamber pulse one can distinguish two hits on the same wire separated by 100 ns at a threshold of 400 pV. b) Time-to-digital converter The relatively modest precision required led us to choose, in designing the T D C system, the direct counting m e t h o d with a m a x i m u m clock frequency of 100 MHz. This frequency makes it easy to build a counter in E C L 10.000 Technology. The small multiplicity o f neutrino events allowed us to group a
51
signifiant number of wires in the same encoder. So the T D C exhibit the following characteristics, in a single C A M A C bin: - 16 input channels with enable/disable c o m m a n d , - 4 wire number-time encoding possibilities per event, -12ns resolving time between two inputs to be encoded, - 80-100 M H z clock frequency, - fast reset for rejected events, - d c signal acts as " S t a r t " signal; trigger signal acts as common "Stop", - 40 events first in-first out buffer. The T D C scheme is shown in fig. 16. The leading edge of the chamber signal, after the line receiver, is differentiated in a 12 ns pulse and multiplexed on
Line
~_ Reieivers - -
E>
Wire #1
I I
an,,
Memory J
Lb.
I
::! 2xlO1851
L
I
F
I;-I
I
I t I From
I
Preamplifiers
I. . . . .
I
.JI
Ill O
I I
I
t I
1
I
j
, L
~
:1 --I
I
Wire
I
A
D
Flip Flop
#16
I Io.r.ndo.Ir
li Enable 'Disable
Fig. 16. Block diagram of the time-to-digital converter.
Latetime Reference
IOOMHz Clock
Re ect
Load Buffer
X/
52
G. M A R E L et al.
4 channels of 16 input priority encoders and latches (10165). The OR output of the encoder is used to set a flip-flop (F). The " l " state of this flip-flop latches the wire number in the 10165 encoder. The output of the (F) flip-flop releases the reset condition on the next channel which is now ready to encode a new chamber signal, and so on. The output of the (F) flip-flop is also sent as data in a clocked type D master-slave flip-flop in order to derandomize the drift chamber signal. The output of this last flip-flop opens the gate to the 100 MHz scaler. After the maximum possible drift time a signal (late time reference) stops all the scalers. This LTR signal is also derandomized in the TDC and it disables the TDC inputs. Thus a dc level is available at an analog output indicating the number of coded hits. For a good event a " L o a d Buffer" signal pushes scaler's contents and wire number codes into the FIFO memories in 500 ns. For a bad event a "Reject" signal resets scalers and encoders. In both cases input gates are then enabled. If a channel is busy from a spurious signal at the end of the maximum counting time, no LTR signal occurs and the scaler resets itself. Thus the channel is free again to accept a new signal. The read-out of F I F O buffers is done by CAMAC. We use 7 bits for drift time, 4 bits for wire number and
~_-_-~/////~1,/I/ . . . . . . . . . . . . • l
.~
1 bit to indicate if the word is valid. We use one C A M A C crate for each chamber. In addition to the crate controller and 12TDCs, there are two fan-outs for clock and LTR signals, one module to check F I F O ' s signals and one other for testing all the drift chamber electronics. c) Permanent read-out test system In order to check the wires and the preamplifiers, each preamplifier card can be addressed by computer, via CAMAC, when misconnected or broken connections are detected a message is delivered. To check preamplifier, cables and TDC performance we can send a signal as input for any preamplifier. This signal can be selected, under computer control, from 400/~V to 2 m V and is used to check threshold stability. The delay of this signal can be varied with respect to a time reference by steps of 6 ns always under computer control. (Histograms of these checks are shown on a monitoring screen.) Signals for testing purposes are sent from the concentration board to the preamplifier by the same flat cable which transports preamplifier output signals. 5. Results We present the results we obtained with one of the completely equipped chambers. A cosmic ray test
Scintillator
PC
Drift
.!/
I: I. °i ......... ~ . / / / / ~ / .
Fig. 17. Set-up for the cosmic ray tests.
i
chamber"
J
V-
.,-.
....
......
tea'!
.....
oo,n,,,,a,or
-~ / / / / / " / / / / / / / / / -
_U_ ~
LARGE PLANAR DRIFT CHAMBERS (fig. 17) is set up at the end of the production chain in such a way that any mechanical mistake can be fixed rapidly. Two sets of scintillator counters with 10 cm of lead between them as shielding define a trigger for cosmic rays. In addition, with three proportional chambers we can define a straight track and thus determine the efficiency of the drift chamber and the drift velocity. These multiwire proportional chambers and the T D C are read on a computer CII 90.10 through a C A M A C system. These chambers define a trigger area of 3 0 × 3 0 cm 2 at the drift chamber which is placed on a movable table such that the whole area of the large chamber can be tested. a) Efficiency We show on fig. 18 the noise curves for non-gated output wire signals. This counting rate includes all the possible sources of noise: corona current, cross-talk and multitriggers of the amplifier for large pulses. The noise curve defines the end of the efficiency plateau at about 1.5 × 10 5 hits/gap/5 s. This plateau limit is lower than the Geiger-Miiller region and is chosen to avoid a large number of accidental counts, thus allowing an easy reconstruction of the event. We present in fig. 19 the efficiency curve for one gap for different thresholds
53
of the amplifier (the position of the area covered by the trigger is also shown). One can see that the chambers have a wide plateau (200 V) and that a factor two in amplification is obtained by increasing the high voltage by 100 V. The same efficiency curves are shown in fig. 20 for three gaps. We can see small differences at the beginning of the plateau which come from the fact that the trigger area is not at the same distance from the amplifiers in all gaps, however the same efficiency is achieved very rapidly showing that no strong effects come from the mechanical construction or gas inhomogeneity. The effect of the pulse attenuation along the wire is shown in fig. 21. A shift of 100 V corresponds to a longitudinal displacement of 3 m with respect to the read-out system. The plateau we obtain is large enough to allow for this effect. This efficiency is constant all along the drift length as shown in fig. 22. We must underline that these results are obtained with tracks practically normal to the drift chamber. The average angle to the normal plan is 7 ° . The average efficiency with such a trigger over the entire area of the chamber is 99.5% per gap. b) Linearity and precision For perpendicular tracks (0 < 5 °, 0 being the angle
!
GAP. N'- 2 CHAMBER. N!3
l OO~
/
/
AVI:RAGE" "'~ 100Hits~See/Wire
/
~,s.,o' ~o7oA,GON
/
I
/ :,/
Thres'°ld /%'~90:#jv
k,o
//
0,o,o,
-
a 1.8reVolt ]
~o~
\ ol.2 mVolt
--i
\+9oo#~v
,"r"
/ i /!
; ....
~+-~1 3.4
a ~ ~
o
/ 17-
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/
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f~,ilV
/
3oz.soB°....
O
....
I
3.5
3.6
..J l-
I
3.7
+
I,- 4~ HV 3.8Kv
Fig. 18. Noise counting rate for differentthresholds.
I
I
3600
3700
38OO Fig. 19. Efficiencycurves on a large drift chamberwith different amplifierthresholds. 3400
3500
54
G. MAREL I
et al.
! CHAMBER• 3
~
• GAP 1
Threshold 90O~tV
-{- GAP 2 GAP
HV 3.7 KV
A T
Efficiency
Adjacent sense wire
JJ 00 L
i / Distance I I to the
,+J~rk,~nse wire
4Amplifier Tr~tE,~r~olO 1 2 m \
-30
t
GAP~
GAP 2
GAP 1
--40
--20
Potential wire
--10
O
;
+lPO q-20
Sense wire
+30
+40
t
Potential wire
Fig. 22. Measured efficiency vs the impact of the particles in the drift cells.
Y 3500
3600
3700
HV Vo,'~
3800
Fig. 20. Efficiency curves for the three gaps of a drift chamber.
Efficiency
.loo% /
C - ~ . _ ~ = _ __ 3.-+ /.).~ "
f - - o -
oJ
o.~"
.+/ " ÷ .~
1OO VOLTa
/
/"
/ / /
/
d
_5o)~ 8m
13 8m I
I
between the projected track on the plane orthogonal to the wires and the normal to the chamber) the measured drift time is almost a linear function o f the track distance to the sense wire (fig. 23). For non-normal tracks the relation becomes non-linear and we have to make some corrections which increase with the angle and the mean distance along the drift space (fig. 24). The measured precision is 0.95 m m (fig. 25). This error is the sum o f the uncertainty in the track Iocalisation and the error on the drift time measurement. These two errors are o f the same order and we can conclude that the spatial resolution with these drift chambers is better than 0.7 mm. This precision is not affected by corrections for tracks at large angles. These tests are not presented with methylal in the gas. We k n o w n that methylal is useful for long runs with high intensities of incoming particles. As a test we have taken a run with 1.5% methylal added to the gas mixture, with the result that the efficiency curve is translated by 50 V. We have not seen other sizeable effects on either drift velocity or the high voltage efficiency curve. (We will use methylal for the running time although the flux is very small.) 6. C o n c l u s i o n
HV Volts 3500
3600
3700
380
0
Fig. 21. Efficiency curves for different distances from the read-out system.
We have achieved the construction o f very large dr!fl chambers, 12 m 3, within the frame o f the proposed track precision of +1 ram. The performance o f these chambers is quite reproducible for the first eighteen chambers already available out o f the twenty demanded by the experiment. This study and its realization owe a lot to G. Charpak, to w h o m we are very grateful for his knowl-
LARGE :
PLANAR
DRIFT
CHAMBERS
55
1 600
ts
6OC
s
t
401 GAP N~I
0 4 + 3 nSAm
I '
--30
--20
I
GAP N92
I :203+_ 3 nSAm
/ Distance I 10
--10
T 20
[ 30
Distance mm
m~ --30
--20
-10
(a)
10
O
20
30
(h) iDriff t i m e difference
ns - -
----T
\
r
V6ooIts--7-,
-- I ; +2C +11
/
i
Distance to the
30 sense wire
m~ -10 400 GAP. N93
/ __203__4.3 nSAm
-21 -3(
wt
/Track
e/
°
2O0
°
?.s
7°
¢
I
~
t
--30
--20
--10
0
I 10
t 20
(c) Fig. 23. Drift velocity in the three gaps.
Distance t mm, 30
0 0 (~) (~) (~
0°40< 5o~0 15~ ~ 0 25o~0 35°40
5o < is ~ <~ 25 ° <~ 35 o <45 °
Fig. 24. Difference in drift time between the m e a s u r e d time a n d the theoretical time vs the distance of the track to the sense wire, for different incident angles. T h e theoretical time is defined by fitting a straight line on the m e a s u r e d times for n o r m a l tracks (0 < 5°).
56
G. M A R E L et al. Events ~SO
G= Oo95mrn -~-
This standard deviation include the mwpc error
~0(
~'~'°~pc -- 0 - 7 m m
150
'00
edgeable and pleasant guidance and advice. We also profited from F. Sauli's great experience in this field. We thank the members of the CDHS Collaboration for their comments on the project, and in particular J. Steinberger who participated in the conception of the chambers' construction. We acknowledge the help of the engineering office and of the mechanical production group of the STIPE. We would like to recall more particularly the memory of J. Bernard, who started the project with us but died on 12th February 1975. The realization of this project was made possible only through the support of Prof. A. Berthelot, and J. F. Detoeuf and P. Prugne. We would like to thank them very sincerely.
50
References
-•. - 3 - 2 -1
0
1
2:3
Spatial resolution mm~
Fig. 25. Spatial resolution o f the chamber for normal tracks. Distance between the observed position on the chamber and the point given by the extrapolation o f the proportional chamber coordinates.
~) 2) 3) 4) 5) 6)
G. Charpak et al., Nucl. Instr. and Meth. 80 (1970) 13. R. Chaminade et al., Nucl. Instr. and Meth. 111 (1973) 77. A. H. Walenta et al., Nucl. Instr. and Meth. 92 (1971) 373. A. Breskin et al., Nucl. Instr. and Meth. 119 (1974) 9. D. C. Cheng et al., Nucl. Instr. and Meth. 117 (1974) 157. C E R N - D o r t m u n d - H e i d e l b e r g - S a c l a y Collaboration, C E R N Report S P S C - P . 7 3 . 1 - S P S C - P 731 ; A d d e n d u m - S P S C 74.6 PI and 75.33 P1.