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Design and performance of a straw tube drift chamber
Nuclear Instruments and Methods in Physics Research A303 (1991) 277-284 North-Holland
277
Design and performance of a straw tube drift chamber S.H. Oh, D.K. Wesson, J. Cooke, A.T. Goshaw, W.J. Robertson and W.D. Walker Department of Physics, Duke University, Durham, NC 27706, USA Received 19 November 1990
The design and performance of the straw drift chambers used in E735 is reported. The chambers are constructed from 2.5 cm radius aluminized mylar straw tubes with wall thickness less than 0.2 mm. Also, presented are the results of tests with 2 mm radius straw tubes. The small tube has a direct detector application at the Superconducting Super Collider.
1. Introduction
2. Design and construction of chambers
Drift c h a m b e r s are now used in almost every high energy physics experiment. There are m a n y different designs of drift c h a m b e r s a n d one of t h e m uses straw tubes as detector elements [1]. In this article, we report the design, construction, a n d p e r f o r m a n c e of large straw tube drift c h a m b e r s used in the spectrometer a r m of E735 at the F e r m i l a b collider. E735 is an experiment to study low transverse m o m e n t u m particles to search for a p h a s e transition of h a d r o n gas to q u a r k - g l u o n p l a s m a from the highest center of mass energy ~ p interactions [2]. We also present preliminary results of a study d o n e with 2 m m radius tubes which have a direct detector application at the Superconducting Super Collider (SSC). Using straw tubes has several advantages. First, since the cells are isolated from one another, cross talk is minimized. If a sense wire is broken, then the c h a n n e l can be easily removed without turning off all channels. Second, straw tubes can be pressurized for better resolution, a n d for mechanical rigidity. Third, the resolution of tracks is i n d e p e n d e n t of a particle's incident angle, so one does not have to incorporate angular correction factors when the drift distance is calculated from the drift time, as with typical drift chambers. This was the m a i n reason we chose straw drift c h a m b e r s since tracks in the spectrometer arm have large slopes. This article is organized as follows. In section 2, we discuss the design a n d construction of chambers, a n d in section 3, testing a n d p e r f o r m a n c e of the c h a m b e r s are discussed. In section 4 we describe a m e t h o d to measure the drift velocity a n d resolution. Section 5 includes the track reconstruction in the chambers, a n d section 6 contains a study d o n e with 2 m m radius tubes, which m a y have a detector application for the Superconducting Super Collider.
T h e basic ingredient of the c h a m b e r is a 2.5 cm radius tube m a d e of aluminized mylar. The wall of the tube consists of 0.018 m m of a l u m i n u m , 0.075 m m of p a p e r a n d 0.075 m m of mylar, a thickness of a b o u t 0.04% of a r a d i a t i o n length. The tubes were w o u n d by Stone Industrial [3] with length of a b o u t 110 cm. The tubes are strong e n o u g h to s u p p o r t a b o u t 10 kg of weight placed o n one end, a n d an internal pressure of at least 3 atm. The tubes are positioned between two a l u m i n u m plates as s h o w n in fig. 1. T w o a l u m i n u m plates on b o t h the top a n d the b o t t o m provide protection for the tube ends, feed-throughs a n d electronics. The size of the holes a n d the distance between holes were milled to be b e t t e r t h a n 0.1 mm. There are a b o u t 60 to 100 holes in each c h a m b e r . T h e holes in each c h a m b e r are in two staggered rows. After the holes were made, the center of each hole was m e a s u r e d with respect to a survey point to d e t e r m i n e the exact position of the sense wire. As shown in fig. 2, each tube is c o n n e c t e d to the plates b y m e a n s of a l u m i n u m end-pieces. A hose clamp is used to ensure the mechanical a n d electrical connection between the a l u m i n u m end-piece a n d the tube. Each a l u m i n u m piece is g r o u n d e d to the frame by a g r o u n d cable. A n end-piece m a d e of delrin a n d a feedt h r o u g h are inserted into the a l u m i n u m end-piece. 100 ~ m d i a m e t e r gold plated C u - B e wire is strung between feed-throughs u n d e r 250 g of tension. T h e elastic limit of the wire is a b o u t 1000 g. C u - B e wire is used because its t h e r m a l e x p a n s i o n coefficient is close to that of the a l u m i n u m frame. A larger d i a m e t e r wire is chosen to provide a higher electric field near the outer wall of the tube. T h e drift velocity saturates if the electric field is high enough. G a s leakage, especially a r o u n d the ends of the tubes,
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
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S.H, Oh et al. / A straw tube drift chamber
amplifier-discriminator through a 0.001 rtF capacitor. The amplifier-discriminators are mounted on the top part of the frames of the chambers near the ends of the tubes. The amplifier-discriminator is designed using MC10116 and its performance is quite comparable with LeCroy 2735DC whose minimum threshold is 2 ~tA. 130 ft of twisted pair ribbon cables connect the output of the amplifier-discriminator to LeCroy TDCs. We have used 4299B with 1 ns time resolution. There are seven chambers with two planes of tubes in each chamber. The size varies from 75 cm (height) × 220 cm (length) to 110 cm x 340 cm. Four out of seven chambers have tubes positioned vertically to measure the horizontal position of tracks. The strong component of the magnetic field is along the vertical direction. The tubes in the other three chambers are slanted by 4 ° for stereo reconstruction. When in place, chambers with slanted tubes alternate between chambers with vertical tubes. There are about 700 channels in the detector. Fig. 1. The aluminum frame structure of a straw drift tube chamber for E735. A few tubes between the two aluminum plates are also shown.
was a major problem during construction. The gas seal is accomplished by pouring low viscosity RTV-615 around the tubes. RTV-112 is used to provide the gas seal around the feed-throughs and around the gas tubing. Gas is provided by 0.6 cm diameter plastic tubing at the top of the aluminum end-piece. 16 tubes are connected in series and an independent gas line is provided for each group of 16. This is to minimize contamination to other tubes in a chamber in case a tube develops a leak. When data is being taken, each chamber receives gas at a rate of about 1 ft3/h. The output from the sense wire is connected to an
3. Testing and performance of chambers In order to find the operating voltage for the chambers, the singles rate as a function of voltage was studied. Fig. 3 shows a typical plateau taken with cosmic rays, and 137Cs source with Ar-ethane (50%-50% mixture). The operating voltage is selected at the end of the plateau curve as shown in the figure. After chambers were constructed, we measured the resolution by using cosmic rays. A chamber is sandwiched between two scintillators. A trigger is generated from the coincidence signal from the two scintillators and used to start the TDC. The measured time is converted to distance from the sense wire. For the 10000
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37'50
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(V)
Fig. 3. Two plateau curves obtained using cosmic rays and 137Cs. The gas used is Ar-ethane (50%-50% mixture).
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S.H. Oh et al. / A straw tube drift chamber
conversion, the drift time as a function of distance is measured (section 4) a n d plotted. This curve was fitted with a polynomial a n d used to convert the m e a s u r e d time to distance. Sometimes it was necessary to divide the data into two regions to get a good overall fit. After the conversion of time to distance, tracks were reconstructed (section 5) a n d the residual was calculated. W e have tried two different gas mixtures, A r - e t h a n e (50%-50%) a n d P10 ( A r - m e t h a n e , 90%-10%). The A r - e t h a n e mixture p r o d u c e d slightly better resolution t h a n A r - m e t h a n e mixture. Fig. 4 shows the residuals for the two gases, a n d the o is less t h a n 200 itm. The residual is calculated from tracks with more t h a n 6 hits. A n earlier study [4] showed that the radiation level from the accelerator at the c h a m b e r position was expected to be high. Therefore a radiation test was performed on a prototype consisting of a single tube. The tube was exposed to a n intense source (9°Sr) such that the sense wire drew a b o u t 1 ~tA/cm. This corresponds to a b o u t 0.09 C cm - l d -1. We used the A r - e t h a n e (50%-50%) mixture for the test because of the higher h y d r o c a r b o n content. H y d r o c a r b o n is a k n o w n aging agent. The exposure lasted for four m o n t h s with periodic checks m a d e of the current draw, single c o u n t a n d the shape of plateau. The results of the current draw a n d the single c o u n t as a function of integrated c h a r g e / c m on the sense wire is s h o w n in fig. 5. They remained c o n s t a n t during the period. After the test, the tube was o p e n e d a n d e x a m i n e d for radiation damage. N o signs of whisker growth or other deterioration were found. It was reported that some c h a m b e r s using the
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same gas b e c a m e inoperative after m u c h less dosage [5]. W e believe that this is due to the lower electric field at the c a t h o d e a n d a m u c h larger c a t h o d e area.
4. Drift velocity measurement
W e have used a simple m e t h o d to measure the relationship b e t w e e n the drift time a n d the distance from the sense wire. A 1 m C i 9°Sr source is placed in front of a slit followed by a c h a m b e r a n d a n o t h e r slit as s h o w n in fig. 6. The two slits (1 m m wide a n d 1 c m in height) d e t e r m i n e the b e a m position. A scintillator is placed b e h i n d the second slit a n d used to start a T D C a n d the signal from the tube is used to stop the TDC. Since the energy of electrons from the 9°Sr source is low, we did not use the coincidence of two scintillators. W e used one small scintillator large enough to cover the slit to reduce the r a n d o m b a c k g r o u n d start. This m e t h o d provides a fast way to measure the drift velocity and resolution of different gases u n d e r operating conditions as s h o w n below. T h e time d i s t r i b u t i o n for a given slit position is plotted a n d fitted with a G a u s s i a n to determine the m e a n time a n d o. T h e m e a n time a n d o are plotted as a function of the slit position. In fig. 7 the m e a n time is plotted as a function of distance for methane. The position of the sense wire is the intersection point between two curves which fit the data points in the left a n d right side of the sense wire. By differentiating the fitted curve, the drift velocity can be calculated as a
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S.H. Oh et al. / A straw tube drift chamber
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I
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Fig. 6. Schematic to measure drift velocity and dispersion using a 9°Sr source. The size of the slit is 1 mm (width) by 1 cm (height). See text for details.
function of the distance from the sense wire or the electric field since the electric field is easily calculable for the cylindrical geometry. In fig. 8, the measured drift velocity as a function of electric field is plotted for several gases. Fig. 8a shows the electron drift velocity for A r - e t h a n e (50%-50%) and P10 (90% Ar-10% methane). Fig. 8b shows the same for pure methane and methane with 5% ethyl-alcohol. This mixture is tried because mixing alcohols increases the lifetime of chambers. Mixing alcohol changes the characteristics of the drift velocity as a function of the electric field. In fig. 9, the drift velocity using mixtures of C F 4 and C H 4 is presented. The mixtures are tested because of a possible application to an SSC detector. For an SSC drift chamber, gas with fast electron drift velocity (larger than 100 ~ m / n s ) is desirable since the time between bunches is 16 ns (see section 4). As seen from the figure, pure CF 4 as well as the mixtures of C F 4 and C H 4 have
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drift velocities larger than 100 ~tm/ns. Although not plotted, some of our data points are compared with the existing data [6] and they are in fair agreement. The o of the time distribution is related to the resolution as a function of drift distance. We convert the o, measured in time to distance using the time to distance conversion plot, i.e., t _+ o,, in time to d + o d in distance. We point out that the od does not measure the absolute resolution directly because of the slit size and the multiple Coulomb scattering of electrons. However od provides a good way to compare the relative resolution as a function of the drift distance between different gases. In fig. 10a the o d (dispersion) is plotted as a function of drift distance. For P10 and A r - e t h a n e , the dispersion is fairly constant as a function of distance. In fig. 10b, the same is plotted for methane and methane with 5% ethanol. It is interesting that the dispersion rises steeply
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Fig. 8. Drift velocity as a function of electric field as measured using the setup in fig. 6 for P10, Ar-ethane (50%-50%), CH 4, and CH 4 + 5% ethanol.
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S.H. Oh et al. / A straw tube drift chamber
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at a c e r t a i n d i s t a n c e f r o m t h e s e n s e wire w h e n t h e a l c o h o l is m i x e d . T h i s m a y b e d u e to t h e e l e c t r o n a t t a c h m e n t to a l c o h o l m o l e c u l e s . W e h a v e tried several m i x t u r e s of d i f f e r e n t g a s e s w i t h d i f f e r e n t k i n d o f alc o h o l s , a n d we f o u n d t h a t m i x i n g a l c o h o l a l w a y s inc r e a s e s t h e d i s p e r s i o n at a l a r g e d i s t a n c e ( - 1 c m f r o m t h e s e n s e wire). In fig. 11, t h e d i s p e r s i o n is p l o t t e d for t h e m i x t u r e s o f C F 4 a n d C H 4 as a f u n c t i o n of d i s t a n c e . It w a s s h o w n earlier t h a t t h e s e m i x t u r e s p r o d u c e d f a s t e l e c t r o n drift velocity as r e q u i r e d for a n S S C d e t e c t o r a p p l i c a t i o n . Fig. 11 s h o w s t h a t C F 4 a l o n e is n o t a s u i t a b l e g a s for a d r i f t c h a m b e r d u e to a l a r g e d i s p e r s i o n . A s will b e s h o w n later, p u r e C F 4 n o t o n l y r e s u l t s in a large d i s p e r sion, b u t also p r o d u c e s a l o n g tail in t h e t i m e d i s t r i b u tion. T h e d i s p e r s i o n is r e d u c e d w h e n C H 4 is m i x e d . T h e r e d u c t i o n i n c r e a s e s as t h e f r a c t i o n of C H 4 i n c r e a s e s .
I 1.5
I 2.0
D (cm)
Fig. 11. Dispersion as a function of drift distance for different mixtures of C F 4 and C H 4.
M i x i n g o t h e r h y d r o c a r b o n gases, s u c h as e t h a n e p r o d u c e s t h e s a m e effect. It s e e m s to u s t h a t a m i x t u r e w i t h m o r e t h a n 50% o f C H 4 m a y be a c c e p t a b l e in t e r m s of dispersion. I n fig. 12, t h e c o n v e r t e d t i m e d i s t r i b u t i o n (to dist a n c e ) is p l o t t e d w h e n t h e slit is a b o u t 2 m m a w a y f r o m t h e s e n s e wire for d i f f e r e n t m i x t u r e s o f C F 4 a n d C H 4. A s p o i n t e d o u t earlier, p u r e C F 4 p r o d u c e d a l o n g tail.
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Fig. 12. Converted time (to distance) distribution when the slit is about 2 m m away from the sense wire for different mixtures of C F 4 and CH 4. The width of the slit is 1 mm.
S.H. Oh et al. / A strawtube drift chamber
282
The dispersion and the tail get smallei ~ as the C H 4 content increases.
5. Track reconstruction
Fig. 13 shows some tracks passing through the chambers. Since a drift time represents the distance from the sense wire, a hit is represented by a circle a r o u n d the sense wire. Tracks are reconstructed in the c h a m b e r s with vertical tubes a n d then the c h a m b e r s with slanted tubes are used for the stereo reconstruction. The first step to reconstruct a track is to calculate the four possible tangential lines between two circles as shown in fig. 14. The two circles are chosen in the furthest c h a m b e r from the interaction region to avoid any field from the analysis magnet. The magnetic field strength is a G a u s s i a n with m a x i m u m strength of 3 kG. The field strength drops to a b o u t 700 G at the first c h a m b e r (20 cm from the center) a n d to a b o u t 10 G at
radius r 1
~
~z
/
;
x
radius r 2
Fig. 14. Tracks which are tangential to two circles (hits). The number 1, 2, 3 and 4 and symbols correspond to the same in eq. (1)
the last c h a m b e r location (100 cm from the center of the magnet). The lines satisfy the following equations (see fig. 14 for the definition of variables):
s i n ( t ) ( x 2 - xl) - c o s ( t ) ( z 2 - z]) + (r2- rl) =O, (1) sin(t)(x 2-x~)
- c o s ( t ) ( z 2 - z,) - ( ~ - r,) = 0 ,
sin(t)(x2-xl) -cos(t)(z2-z])sin(t)(x2
--
1112 3 4
m
,5 6 7
JJ"~J
! \
O* °O
--1..._
Fig. 13. Tracks passing through the E735 spectrometer arm. E735 is one of the collider experiments at Fermilab. Hits in the straw chambers are represented by a circle with a radius corresponding the converted drift time to distance from the sense wire. The tubes in the chambers with even numbers are tilted by 4 o. The directions of protons and antiprotons are also shown.
-Xl)
- cos(,t)(z2 -Zl)
(r2+q) ~- (F 2 + r l )
(2)
=0,
(3)
:0.
(4)
After t is f o u n d from each equation, the slope ( t a n ( t ) ) a n d intercept are calculated for each line. Once the e q u a t i o n of a line is found, the line is projected to the next c h a m b e r (toward the interaction region) a n d the predicted position is c o m p a r e d with the measured position. If the difference is within a given limit, the hit is tagged to belonging to the line. A hit can be tagged by more t h a n one line. T h e search continues until there are five good points or the search reaches c h a m b e r s with appreciable magnetic field (first chamber). W h e n the search is over, those tracks with more than four points are fitted with a straight line. If only one track is reconstructed a n d its xZ/df is less than 2, then the track is accepted for further processing. If more than two tracks with x2/df less t h a n 2 resulted from the four tangential lines, the track with the most points is selected. If the n u m b e r of points is the same, then the line with best x2/df is chosen. Those hits belonging to good tracks are deleted from the data b a n k a n d not used further. This process is c o n t i n u e d until all hits are tried. Using the slope a n d intercept of the selected track, the a p p r o x i m a t e m o m e n t u m of the track is calculated using the interaction vertex a n d the integrated magnetic field. Using this m o m e n t u m , the track is swum through the magnetic field to the r e m a i n i n g c h a m b e r s a n d the nearest hits within a given limit are selected. After all hits belonging to a track are found, the track is finally fitted with a second order polynomial.
S.H. Oh et al. / A straw tube drift chamber
283
After the horizontal position of a track is found, the hits in the slanted tubes along the track are collected. F r o m the hits and k n o w n horizontal positions of the track, the c o r r e s p o n d i n g vertical position of each hit in the slanted tubes is calculated. A linear fit is p e r f o r m e d with the hits and the track with most hits (with good x 2 / d f ) is chosen. W e f o u n d this procedure reconstructs tracks correctly with better t h a n 95% efficiency.
6. Application for
the Superconducting
Super Collider
The concept of using straw tubes can be easily extended to a detector for the Superconducting Super Collider. As discussed earlier, there are several advantages of using tubes for a drift chamber. The chamber could be either a cylindrical central c h a m b e r or just a rectangular type tracking c h a m b e r [7]. The upper limit of the tube radius is set by the b u n c h spacing of the SSC. Since electrons p r o d u c e d from tracks of an interaction should have reached the sense wire before the next b u n c h arrives, the radius of the tubes should be a b o u t 1.5 m m if a fast gas with electron drift velocity 100 ~ t m / n s is used. As s h o w n previously, there are gases with drift velocity higher t h a n 100 ~ t m / n s within the small tubes u n d e r an operating voltage. In this section we present some tests p e r f o r m e d using 2 m m radius tubes. The tube wall is m a d e of 0.025 m m thick mylar coated with 2 0 0 - 3 0 0 A thick a l u m i n u m [8]. Several single straw tube prototype c h a m b e r s were constructed with 25 ~tm diameter gold plated tungsten
80 O a k X
Fig. 16. The shape of the averaged signal using 9°Sr into a 50 [2 resistor. 1000 signals are averaged. HV=2200 V, gas= methane. The vertical scale is 0.5 mV/div and the horizontal scale is 20 ns/div.
wire u n d e r 50 g tension. We were able to pressurize the c h a m b e r to 3 a t m for a m o n t h without any problem. Pressurizing tubes not only keeps them r o u n d a n d stiff but also could result in a b e t t e r resolution. In fig. 15, plateau curves o b t a i n e d with a 9°Sr source at several different pressures with pure m e t h a n e are plotted. W e observe a very nice plateau for all pressures. It is interesting that the knee of the plateau changes as a f u n c t i o n of pressure. Fig. 16 shows an averaged signal using 9°Sr source. T h e gas mixture is pure m e t h a n e at 1 a t m a n d the high voltage is set at 2200 V. The peak is a b o u t 2 m V into a 50 [2 resistor. The signal rises to the m a x i m u m within 3 ns a n d falls to a b o u t 10% of the peak after 30 ns. Using 4 m m d i a m e t e r straw tubes, we have constructed a 60 channel, 2.7 m long prototype. It has eight layers of tubes. Some preliminary results are already reported [9]. The detailed c o n s t r u c t i o n a n d p e r f o r m a n c e test will be reported in the near future.
O
8•
60
°•
0 0 0 0 O
O
References
40
O
20
O X
0
1,~00
i
i
1800
2200
•
14 PSI
* •
19 P S I 24 PSI
i
2600
Voltage(V) Fig. 15. Plateau curves obtained using small straw tubes (2 mm radius) filled with CH 4 under different pressures.
[1] C. Broil et al., Nucl. Instr. and Meth. 206 (1983) 385; P. Baringer et al., Nucl. Instr. and Meth. A254 (1987)542. [2] T. Alexopoulos et al., Phys. Rev. Lett. 60 (1988) 1622; S. Banerjee et al., Phys. Rev. Lett. 62 (1989) 12: T. Alexopoulos et al., Phys. Rev. Lett. 64 (1990) 991. [3] Stone Industrial Division, College Park 20704, MD, USA. [4] C. Lindsey et al., Nucl. Instr. and Meth. A254 (1987) 212. [5] M. Turala and J.C. Vermeulen, Nucl. Instr. and Meth. 205 (1983) 141. [6] F. Sauli, CERN preprint 77-09; B. Jean-Marie et al., Nucl. Instr. and Meth. 159 (1979) 213; L. Christophorous et al., Nucl. Instr. and Meth. 163 (1979) 141;
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S.H. Oh et al. / A straw tube drift chamber
T. Yamashita et al., K E K preprint 88-133; there are some disagreements between published drift velocity measurements, for example, in CH 4. [7] SSC subsystem proposals, submitted to SSC Lab, 1989, PC-024 (G. Hanson et al.), PC-035 (A.T. Goshaw et al.).
[8] The small radius tubes were wound by Precision Tubing, Wheeling, IL, USA. [9] S.H. Oh et al., Proc. Int. Workshop on Solenoidal Detectors for the SSC, KEK, Japan, eds. F. Abe and R. Hasegawa (1990) p. 279.
277
Design and performance of a straw tube drift chamber S.H. Oh, D.K. Wesson, J. Cooke, A.T. Goshaw, W.J. Robertson and W.D. Walker Department of Physics, Duke University, Durham, NC 27706, USA Received 19 November 1990
The design and performance of the straw drift chambers used in E735 is reported. The chambers are constructed from 2.5 cm radius aluminized mylar straw tubes with wall thickness less than 0.2 mm. Also, presented are the results of tests with 2 mm radius straw tubes. The small tube has a direct detector application at the Superconducting Super Collider.
1. Introduction
2. Design and construction of chambers
Drift c h a m b e r s are now used in almost every high energy physics experiment. There are m a n y different designs of drift c h a m b e r s a n d one of t h e m uses straw tubes as detector elements [1]. In this article, we report the design, construction, a n d p e r f o r m a n c e of large straw tube drift c h a m b e r s used in the spectrometer a r m of E735 at the F e r m i l a b collider. E735 is an experiment to study low transverse m o m e n t u m particles to search for a p h a s e transition of h a d r o n gas to q u a r k - g l u o n p l a s m a from the highest center of mass energy ~ p interactions [2]. We also present preliminary results of a study d o n e with 2 m m radius tubes which have a direct detector application at the Superconducting Super Collider (SSC). Using straw tubes has several advantages. First, since the cells are isolated from one another, cross talk is minimized. If a sense wire is broken, then the c h a n n e l can be easily removed without turning off all channels. Second, straw tubes can be pressurized for better resolution, a n d for mechanical rigidity. Third, the resolution of tracks is i n d e p e n d e n t of a particle's incident angle, so one does not have to incorporate angular correction factors when the drift distance is calculated from the drift time, as with typical drift chambers. This was the m a i n reason we chose straw drift c h a m b e r s since tracks in the spectrometer arm have large slopes. This article is organized as follows. In section 2, we discuss the design a n d construction of chambers, a n d in section 3, testing a n d p e r f o r m a n c e of the c h a m b e r s are discussed. In section 4 we describe a m e t h o d to measure the drift velocity a n d resolution. Section 5 includes the track reconstruction in the chambers, a n d section 6 contains a study d o n e with 2 m m radius tubes, which m a y have a detector application for the Superconducting Super Collider.
T h e basic ingredient of the c h a m b e r is a 2.5 cm radius tube m a d e of aluminized mylar. The wall of the tube consists of 0.018 m m of a l u m i n u m , 0.075 m m of p a p e r a n d 0.075 m m of mylar, a thickness of a b o u t 0.04% of a r a d i a t i o n length. The tubes were w o u n d by Stone Industrial [3] with length of a b o u t 110 cm. The tubes are strong e n o u g h to s u p p o r t a b o u t 10 kg of weight placed o n one end, a n d an internal pressure of at least 3 atm. The tubes are positioned between two a l u m i n u m plates as s h o w n in fig. 1. T w o a l u m i n u m plates on b o t h the top a n d the b o t t o m provide protection for the tube ends, feed-throughs a n d electronics. The size of the holes a n d the distance between holes were milled to be b e t t e r t h a n 0.1 mm. There are a b o u t 60 to 100 holes in each c h a m b e r . T h e holes in each c h a m b e r are in two staggered rows. After the holes were made, the center of each hole was m e a s u r e d with respect to a survey point to d e t e r m i n e the exact position of the sense wire. As shown in fig. 2, each tube is c o n n e c t e d to the plates b y m e a n s of a l u m i n u m end-pieces. A hose clamp is used to ensure the mechanical a n d electrical connection between the a l u m i n u m end-piece a n d the tube. Each a l u m i n u m piece is g r o u n d e d to the frame by a g r o u n d cable. A n end-piece m a d e of delrin a n d a feedt h r o u g h are inserted into the a l u m i n u m end-piece. 100 ~ m d i a m e t e r gold plated C u - B e wire is strung between feed-throughs u n d e r 250 g of tension. T h e elastic limit of the wire is a b o u t 1000 g. C u - B e wire is used because its t h e r m a l e x p a n s i o n coefficient is close to that of the a l u m i n u m frame. A larger d i a m e t e r wire is chosen to provide a higher electric field near the outer wall of the tube. T h e drift velocity saturates if the electric field is high enough. G a s leakage, especially a r o u n d the ends of the tubes,
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
278
S.H, Oh et al. / A straw tube drift chamber
amplifier-discriminator through a 0.001 rtF capacitor. The amplifier-discriminators are mounted on the top part of the frames of the chambers near the ends of the tubes. The amplifier-discriminator is designed using MC10116 and its performance is quite comparable with LeCroy 2735DC whose minimum threshold is 2 ~tA. 130 ft of twisted pair ribbon cables connect the output of the amplifier-discriminator to LeCroy TDCs. We have used 4299B with 1 ns time resolution. There are seven chambers with two planes of tubes in each chamber. The size varies from 75 cm (height) × 220 cm (length) to 110 cm x 340 cm. Four out of seven chambers have tubes positioned vertically to measure the horizontal position of tracks. The strong component of the magnetic field is along the vertical direction. The tubes in the other three chambers are slanted by 4 ° for stereo reconstruction. When in place, chambers with slanted tubes alternate between chambers with vertical tubes. There are about 700 channels in the detector. Fig. 1. The aluminum frame structure of a straw drift tube chamber for E735. A few tubes between the two aluminum plates are also shown.
was a major problem during construction. The gas seal is accomplished by pouring low viscosity RTV-615 around the tubes. RTV-112 is used to provide the gas seal around the feed-throughs and around the gas tubing. Gas is provided by 0.6 cm diameter plastic tubing at the top of the aluminum end-piece. 16 tubes are connected in series and an independent gas line is provided for each group of 16. This is to minimize contamination to other tubes in a chamber in case a tube develops a leak. When data is being taken, each chamber receives gas at a rate of about 1 ft3/h. The output from the sense wire is connected to an
3. Testing and performance of chambers In order to find the operating voltage for the chambers, the singles rate as a function of voltage was studied. Fig. 3 shows a typical plateau taken with cosmic rays, and 137Cs source with Ar-ethane (50%-50% mixture). The operating voltage is selected at the end of the plateau curve as shown in the figure. After chambers were constructed, we measured the resolution by using cosmic rays. A chamber is sandwiched between two scintillators. A trigger is generated from the coincidence signal from the two scintillators and used to start the TDC. The measured time is converted to distance from the sense wire. For the 10000
Feed-through
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s
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~
supply top p l a t e
=
5000
o
~
s
e
clamp
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2500
J
• 3%'00
100 g m Au-Cu wire Fig. 2. A detailed view of a tube end including end pieces.
32'50
Cs137
35'00
Voltage
37'50
4000
(V)
Fig. 3. Two plateau curves obtained using cosmic rays and 137Cs. The gas used is Ar-ethane (50%-50% mixture).
279
S.H. Oh et al. / A straw tube drift chamber
conversion, the drift time as a function of distance is measured (section 4) a n d plotted. This curve was fitted with a polynomial a n d used to convert the m e a s u r e d time to distance. Sometimes it was necessary to divide the data into two regions to get a good overall fit. After the conversion of time to distance, tracks were reconstructed (section 5) a n d the residual was calculated. W e have tried two different gas mixtures, A r - e t h a n e (50%-50%) a n d P10 ( A r - m e t h a n e , 90%-10%). The A r - e t h a n e mixture p r o d u c e d slightly better resolution t h a n A r - m e t h a n e mixture. Fig. 4 shows the residuals for the two gases, a n d the o is less t h a n 200 itm. The residual is calculated from tracks with more t h a n 6 hits. A n earlier study [4] showed that the radiation level from the accelerator at the c h a m b e r position was expected to be high. Therefore a radiation test was performed on a prototype consisting of a single tube. The tube was exposed to a n intense source (9°Sr) such that the sense wire drew a b o u t 1 ~tA/cm. This corresponds to a b o u t 0.09 C cm - l d -1. We used the A r - e t h a n e (50%-50%) mixture for the test because of the higher h y d r o c a r b o n content. H y d r o c a r b o n is a k n o w n aging agent. The exposure lasted for four m o n t h s with periodic checks m a d e of the current draw, single c o u n t a n d the shape of plateau. The results of the current draw a n d the single c o u n t as a function of integrated c h a r g e / c m on the sense wire is s h o w n in fig. 5. They remained c o n s t a n t during the period. After the test, the tube was o p e n e d a n d e x a m i n e d for radiation damage. N o signs of whisker growth or other deterioration were found. It was reported that some c h a m b e r s using the
500 a) Ar-Methane (90 10)
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500 b) A r - E t h a n e (50-50)
400 300 200 100 .
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.
.
.
.
.
.
~
.
-1.0
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.
.
.
.
1.0
.
.
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2.0
o
N
1.9 i
i
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b) ,n 24000 o
o
25000
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Fig. 5. Results of an aging test using 9°Sr source. (a) Shows the current draw by the sense wire as a function of total charge deposited on the sense wire. (b) Shows the single count as a function of total deposited charge.
same gas b e c a m e inoperative after m u c h less dosage [5]. W e believe that this is due to the lower electric field at the c a t h o d e a n d a m u c h larger c a t h o d e area.
4. Drift velocity measurement
W e have used a simple m e t h o d to measure the relationship b e t w e e n the drift time a n d the distance from the sense wire. A 1 m C i 9°Sr source is placed in front of a slit followed by a c h a m b e r a n d a n o t h e r slit as s h o w n in fig. 6. The two slits (1 m m wide a n d 1 c m in height) d e t e r m i n e the b e a m position. A scintillator is placed b e h i n d the second slit a n d used to start a T D C a n d the signal from the tube is used to stop the TDC. Since the energy of electrons from the 9°Sr source is low, we did not use the coincidence of two scintillators. W e used one small scintillator large enough to cover the slit to reduce the r a n d o m b a c k g r o u n d start. This m e t h o d provides a fast way to measure the drift velocity and resolution of different gases u n d e r operating conditions as s h o w n below. T h e time d i s t r i b u t i o n for a given slit position is plotted a n d fitted with a G a u s s i a n to determine the m e a n time a n d o. T h e m e a n time a n d o are plotted as a function of the slit position. In fig. 7 the m e a n time is plotted as a function of distance for methane. The position of the sense wire is the intersection point between two curves which fit the data points in the left a n d right side of the sense wire. By differentiating the fitted curve, the drift velocity can be calculated as a
280
S.H. Oh et al. / A straw tube drift chamber
+
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s,i,
I
lil
I
TDC ( ~
Straw Tube - - - ~ ~ ~
s,i,
I
lil
I
~ 1 -
Sr
90
AmplifierDiscriminator
Source
Fig. 6. Schematic to measure drift velocity and dispersion using a 9°Sr source. The size of the slit is 1 mm (width) by 1 cm (height). See text for details.
function of the distance from the sense wire or the electric field since the electric field is easily calculable for the cylindrical geometry. In fig. 8, the measured drift velocity as a function of electric field is plotted for several gases. Fig. 8a shows the electron drift velocity for A r - e t h a n e (50%-50%) and P10 (90% Ar-10% methane). Fig. 8b shows the same for pure methane and methane with 5% ethyl-alcohol. This mixture is tried because mixing alcohols increases the lifetime of chambers. Mixing alcohol changes the characteristics of the drift velocity as a function of the electric field. In fig. 9, the drift velocity using mixtures of C F 4 and C H 4 is presented. The mixtures are tested because of a possible application to an SSC detector. For an SSC drift chamber, gas with fast electron drift velocity (larger than 100 ~ m / n s ) is desirable since the time between bunches is 16 ns (see section 4). As seen from the figure, pure CF 4 as well as the mixtures of C F 4 and C H 4 have
I
1
I
i
I
drift velocities larger than 100 ~tm/ns. Although not plotted, some of our data points are compared with the existing data [6] and they are in fair agreement. The o of the time distribution is related to the resolution as a function of drift distance. We convert the o, measured in time to distance using the time to distance conversion plot, i.e., t _+ o,, in time to d + o d in distance. We point out that the od does not measure the absolute resolution directly because of the slit size and the multiple Coulomb scattering of electrons. However od provides a good way to compare the relative resolution as a function of the drift distance between different gases. In fig. 10a the o d (dispersion) is plotted as a function of drift distance. For P10 and A r - e t h a n e , the dispersion is fairly constant as a function of distance. In fig. 10b, the same is plotted for methane and methane with 5% ethanol. It is interesting that the dispersion rises steeply
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, 50
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Fig. 7. A typical mean drift time plotted as a function of distance. The sense wire is at the symmetry axis. The data is obtained with CH 4.
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r
b)
'/
CI~*
5% Ethanol
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Fig. 8. Drift velocity as a function of electric field as measured using the setup in fig. 6 for P10, Ar-ethane (50%-50%), CH 4, and CH 4 + 5% ethanol.
281
S.H. Oh et al. / A straw tube drift chamber
120 "g I00 8o
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at a c e r t a i n d i s t a n c e f r o m t h e s e n s e wire w h e n t h e a l c o h o l is m i x e d . T h i s m a y b e d u e to t h e e l e c t r o n a t t a c h m e n t to a l c o h o l m o l e c u l e s . W e h a v e tried several m i x t u r e s of d i f f e r e n t g a s e s w i t h d i f f e r e n t k i n d o f alc o h o l s , a n d we f o u n d t h a t m i x i n g a l c o h o l a l w a y s inc r e a s e s t h e d i s p e r s i o n at a l a r g e d i s t a n c e ( - 1 c m f r o m t h e s e n s e wire). In fig. 11, t h e d i s p e r s i o n is p l o t t e d for t h e m i x t u r e s o f C F 4 a n d C H 4 as a f u n c t i o n of d i s t a n c e . It w a s s h o w n earlier t h a t t h e s e m i x t u r e s p r o d u c e d f a s t e l e c t r o n drift velocity as r e q u i r e d for a n S S C d e t e c t o r a p p l i c a t i o n . Fig. 11 s h o w s t h a t C F 4 a l o n e is n o t a s u i t a b l e g a s for a d r i f t c h a m b e r d u e to a l a r g e d i s p e r s i o n . A s will b e s h o w n later, p u r e C F 4 n o t o n l y r e s u l t s in a large d i s p e r sion, b u t also p r o d u c e s a l o n g tail in t h e t i m e d i s t r i b u tion. T h e d i s p e r s i o n is r e d u c e d w h e n C H 4 is m i x e d . T h e r e d u c t i o n i n c r e a s e s as t h e f r a c t i o n of C H 4 i n c r e a s e s .
I 1.5
I 2.0
D (cm)
Fig. 11. Dispersion as a function of drift distance for different mixtures of C F 4 and C H 4.
M i x i n g o t h e r h y d r o c a r b o n gases, s u c h as e t h a n e p r o d u c e s t h e s a m e effect. It s e e m s to u s t h a t a m i x t u r e w i t h m o r e t h a n 50% o f C H 4 m a y be a c c e p t a b l e in t e r m s of dispersion. I n fig. 12, t h e c o n v e r t e d t i m e d i s t r i b u t i o n (to dist a n c e ) is p l o t t e d w h e n t h e slit is a b o u t 2 m m a w a y f r o m t h e s e n s e wire for d i f f e r e n t m i x t u r e s o f C F 4 a n d C H 4. A s p o i n t e d o u t earlier, p u r e C F 4 p r o d u c e d a l o n g tail.
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a)
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0 CH4 •
ii
Z
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,
.2
.4
.6
.8 t (crn)
,
.2
q
,
.4
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i
.6
i
.8
Fig. 12. Converted time (to distance) distribution when the slit is about 2 m m away from the sense wire for different mixtures of C F 4 and CH 4. The width of the slit is 1 mm.
S.H. Oh et al. / A strawtube drift chamber
282
The dispersion and the tail get smallei ~ as the C H 4 content increases.
5. Track reconstruction
Fig. 13 shows some tracks passing through the chambers. Since a drift time represents the distance from the sense wire, a hit is represented by a circle a r o u n d the sense wire. Tracks are reconstructed in the c h a m b e r s with vertical tubes a n d then the c h a m b e r s with slanted tubes are used for the stereo reconstruction. The first step to reconstruct a track is to calculate the four possible tangential lines between two circles as shown in fig. 14. The two circles are chosen in the furthest c h a m b e r from the interaction region to avoid any field from the analysis magnet. The magnetic field strength is a G a u s s i a n with m a x i m u m strength of 3 kG. The field strength drops to a b o u t 700 G at the first c h a m b e r (20 cm from the center) a n d to a b o u t 10 G at
radius r 1
~
~z
/
;
x
radius r 2
Fig. 14. Tracks which are tangential to two circles (hits). The number 1, 2, 3 and 4 and symbols correspond to the same in eq. (1)
the last c h a m b e r location (100 cm from the center of the magnet). The lines satisfy the following equations (see fig. 14 for the definition of variables):
s i n ( t ) ( x 2 - xl) - c o s ( t ) ( z 2 - z]) + (r2- rl) =O, (1) sin(t)(x 2-x~)
- c o s ( t ) ( z 2 - z,) - ( ~ - r,) = 0 ,
sin(t)(x2-xl) -cos(t)(z2-z])sin(t)(x2
--
1112 3 4
m
,5 6 7
JJ"~J
! \
O* °O
--1..._
Fig. 13. Tracks passing through the E735 spectrometer arm. E735 is one of the collider experiments at Fermilab. Hits in the straw chambers are represented by a circle with a radius corresponding the converted drift time to distance from the sense wire. The tubes in the chambers with even numbers are tilted by 4 o. The directions of protons and antiprotons are also shown.
-Xl)
- cos(,t)(z2 -Zl)
(r2+q) ~- (F 2 + r l )
(2)
=0,
(3)
:0.
(4)
After t is f o u n d from each equation, the slope ( t a n ( t ) ) a n d intercept are calculated for each line. Once the e q u a t i o n of a line is found, the line is projected to the next c h a m b e r (toward the interaction region) a n d the predicted position is c o m p a r e d with the measured position. If the difference is within a given limit, the hit is tagged to belonging to the line. A hit can be tagged by more t h a n one line. T h e search continues until there are five good points or the search reaches c h a m b e r s with appreciable magnetic field (first chamber). W h e n the search is over, those tracks with more than four points are fitted with a straight line. If only one track is reconstructed a n d its xZ/df is less than 2, then the track is accepted for further processing. If more than two tracks with x2/df less t h a n 2 resulted from the four tangential lines, the track with the most points is selected. If the n u m b e r of points is the same, then the line with best x2/df is chosen. Those hits belonging to good tracks are deleted from the data b a n k a n d not used further. This process is c o n t i n u e d until all hits are tried. Using the slope a n d intercept of the selected track, the a p p r o x i m a t e m o m e n t u m of the track is calculated using the interaction vertex a n d the integrated magnetic field. Using this m o m e n t u m , the track is swum through the magnetic field to the r e m a i n i n g c h a m b e r s a n d the nearest hits within a given limit are selected. After all hits belonging to a track are found, the track is finally fitted with a second order polynomial.
S.H. Oh et al. / A straw tube drift chamber
283
After the horizontal position of a track is found, the hits in the slanted tubes along the track are collected. F r o m the hits and k n o w n horizontal positions of the track, the c o r r e s p o n d i n g vertical position of each hit in the slanted tubes is calculated. A linear fit is p e r f o r m e d with the hits and the track with most hits (with good x 2 / d f ) is chosen. W e f o u n d this procedure reconstructs tracks correctly with better t h a n 95% efficiency.
6. Application for
the Superconducting
Super Collider
The concept of using straw tubes can be easily extended to a detector for the Superconducting Super Collider. As discussed earlier, there are several advantages of using tubes for a drift chamber. The chamber could be either a cylindrical central c h a m b e r or just a rectangular type tracking c h a m b e r [7]. The upper limit of the tube radius is set by the b u n c h spacing of the SSC. Since electrons p r o d u c e d from tracks of an interaction should have reached the sense wire before the next b u n c h arrives, the radius of the tubes should be a b o u t 1.5 m m if a fast gas with electron drift velocity 100 ~ t m / n s is used. As s h o w n previously, there are gases with drift velocity higher t h a n 100 ~ t m / n s within the small tubes u n d e r an operating voltage. In this section we present some tests p e r f o r m e d using 2 m m radius tubes. The tube wall is m a d e of 0.025 m m thick mylar coated with 2 0 0 - 3 0 0 A thick a l u m i n u m [8]. Several single straw tube prototype c h a m b e r s were constructed with 25 ~tm diameter gold plated tungsten
80 O a k X
Fig. 16. The shape of the averaged signal using 9°Sr into a 50 [2 resistor. 1000 signals are averaged. HV=2200 V, gas= methane. The vertical scale is 0.5 mV/div and the horizontal scale is 20 ns/div.
wire u n d e r 50 g tension. We were able to pressurize the c h a m b e r to 3 a t m for a m o n t h without any problem. Pressurizing tubes not only keeps them r o u n d a n d stiff but also could result in a b e t t e r resolution. In fig. 15, plateau curves o b t a i n e d with a 9°Sr source at several different pressures with pure m e t h a n e are plotted. W e observe a very nice plateau for all pressures. It is interesting that the knee of the plateau changes as a f u n c t i o n of pressure. Fig. 16 shows an averaged signal using 9°Sr source. T h e gas mixture is pure m e t h a n e at 1 a t m a n d the high voltage is set at 2200 V. The peak is a b o u t 2 m V into a 50 [2 resistor. The signal rises to the m a x i m u m within 3 ns a n d falls to a b o u t 10% of the peak after 30 ns. Using 4 m m d i a m e t e r straw tubes, we have constructed a 60 channel, 2.7 m long prototype. It has eight layers of tubes. Some preliminary results are already reported [9]. The detailed c o n s t r u c t i o n a n d p e r f o r m a n c e test will be reported in the near future.
O
8•
60
°•
0 0 0 0 O
O
References
40
O
20
O X
0
1,~00
i
i
1800
2200
•
14 PSI
* •
19 P S I 24 PSI
i
2600
Voltage(V) Fig. 15. Plateau curves obtained using small straw tubes (2 mm radius) filled with CH 4 under different pressures.
[1] C. Broil et al., Nucl. Instr. and Meth. 206 (1983) 385; P. Baringer et al., Nucl. Instr. and Meth. A254 (1987)542. [2] T. Alexopoulos et al., Phys. Rev. Lett. 60 (1988) 1622; S. Banerjee et al., Phys. Rev. Lett. 62 (1989) 12: T. Alexopoulos et al., Phys. Rev. Lett. 64 (1990) 991. [3] Stone Industrial Division, College Park 20704, MD, USA. [4] C. Lindsey et al., Nucl. Instr. and Meth. A254 (1987) 212. [5] M. Turala and J.C. Vermeulen, Nucl. Instr. and Meth. 205 (1983) 141. [6] F. Sauli, CERN preprint 77-09; B. Jean-Marie et al., Nucl. Instr. and Meth. 159 (1979) 213; L. Christophorous et al., Nucl. Instr. and Meth. 163 (1979) 141;
284
S.H. Oh et al. / A straw tube drift chamber
T. Yamashita et al., K E K preprint 88-133; there are some disagreements between published drift velocity measurements, for example, in CH 4. [7] SSC subsystem proposals, submitted to SSC Lab, 1989, PC-024 (G. Hanson et al.), PC-035 (A.T. Goshaw et al.).
[8] The small radius tubes were wound by Precision Tubing, Wheeling, IL, USA. [9] S.H. Oh et al., Proc. Int. Workshop on Solenoidal Detectors for the SSC, KEK, Japan, eds. F. Abe and R. Hasegawa (1990) p. 279.