151
THE CLEO ELECTROMAGNETIC CALORIMETER
P. Skubic
Department of Physics and Astronomy University of Oklahoma Norman, Oklahoma 73019
CLEO is an all purpose magnetic detector presently in operation at the
Cornell Electron Storage Ring. The CLEO detector, shown in Fig. 1, has been 1 described in detail elsewhere. The electromagnetic calorimeter will be described here and results from early running of CLEO will be presented.
Detector Design and Construction
The primary electromagnetic detector covers 50% of 4n solid angle and is segmented into octants which form a barrel located at a radius of 2 meters from the beam line outside the 0.7 radiation length thick solenoid coil. Each octant is composed of 44 layers of proportional tubes interleaved with 0.2 r.l. thick lead, giving a total thickness of 11.3 r.l. (see Fig. 2). Alternating tube layers were crossed at 90 degrees and were epoxied together two layers at a time, and cured under a compression of 10 tons in order to maintain flatness of the tubes. Each octant forms a rigid, monolithic, self-supporting structure weighing approximately 10 tons. The signal wires from each octant are ganged in 774 groups of from 2 to 20 tubes as shown in Fig. 3. The width of the gangs corresponds roughly to the variation in average shower size with depth. The operating voltage is typically 1520 volts.
The signal grounds are tied to a support box which completely surrounds the detector body. The 0.5 ~f capacitance between the box and body provides an 152
AC ground as well as a convenient way to apply a uniform calibration pulse which is used to correct for variation in amplifier gain.
The input gas composition of 91% argon and 9% methane is trimmed to approximately 2% of the minor component and is constantly monitored by a binary gas analyzer. Bhabha events in the detector are used to eliminate long term drifts and provide an absolute energy calibration.
The solid angle (10% of 4n) near the beam line is covered by two pole tip shower counters as shown in Fig. 1. The mechanical design and construction is similar to that of the primary detector (see Fig. 4). Each module consists of 21 layers of tubes interleaved with 2 mID thick lead, giving a total thickness of 10 r.l.
At the ends of each of the eight dEdx particle identification modules, there are octant end shower counters. These detectors (see Fig. 5) each consist of 16 layers of tubes interleaved with 2 mrn lead and cover a combined solid angle of 12% of 4TI. A summary of the shower counter design parameters is given in Table I.
Electron Test Beam Results
The first completed octant was tested in an electron beam at Wilson
Synchrotron Laboratory. Shown in Fig. 6 is the response of the detector to normally incident electrons as a function of beam momentum. To determine the rms energy resolution, the data at each energy were fit to a Gaussian function.
Figure 7 shows the data from one 0.42 GeV run where the curve is the result of the fit which gave an rms width of 0.096 ± 0.003 with a x2 per degree of freedom of 0.91. 0 IE is plotted versus lIfE in Fig. 8 where the error bars 153
include statistical and systematic errors. The systematic errors were estimated from the variation in fit parameters for runs taken under similar conditions. The data are reasonably well represented by a straight line with an rms resolution of 15%/1E'.
Results from Electron-Positron Data
Figure 9 shows typical u+~- and e+e- events for electron-positron collisions with a center of mass energy of 10 GeV. Clear minimum ionizing tracks are observed in the shower counter for the ~+w- event indicating good efficiency. Cosmic ray studies indicate that the efficiency for minimum ionizing tracks is greater than 90% for each 2-tube layer. Two back-to-back showers are observed in the e+e- event. The total energy observed in the octant shower cOtmter for e+e----...e+e- and e+e-~'l events is shown in Fig. 10.
The distribution of the sum of azimuthal angles measured for 11 events is shown in Fig. 11. The distribution is centered at 0 as expected with a width of about 9 mrad. The peaks at 104 mrad are due to Bhabha events in which the electron and positron are bent by the magnetic field.
Figure 12 shows the angular distribution of Bhabha events in the octant shower counters where the curve is the expected theoretical distribution. The octant and pole tip counters provide the best measurement of the absolute luminosity for the CLEO detector. The ratio of the luminosity independently measured by the two detectors is plotted in Fig. 13 for a set of runs. Each point corresponds to 20,000 events in the pole tip counter and 1600 events in the octant counter. The average ratio is 0.976 ± O.OOS and is constant within errors.
The octant shower counter has been used to reconstruct nO's. In Fig. 14 the Y- J' mass distribution is shown separately for photon energies greater than
200 MeV and 600 MeV. In both cases a clear peak is observed at the ~o mass 154
although the combinatorial background is large for the more numerous lower energy photons.
Electrons with energies above 1 GeV have been identified using the 2 octant shower counter. Using simple cuts on the average number of hits per layer and the ratio of shower chamber energy and momentum measured by the inner drift chamber, a rejection factor of 20 was obtained with an efficiency of -80% for tracks within the octant counter solid angle. When used in conjunction with the dE/dx chamber, a rejection factor >100·was obtained with an efficiency of 50% for tracks within the fiducial volume.
In summary, the CLEO electromagnetic calorimeter has provided: 1) measurement of photon position and energy over 70% of 4n solid angle, 2) electron identification with dE/dx counters, 3) measurement of charged particle positions, 4) a neutral energy trigger, and 5) measurement of storage ring luminosity using Bhabha scattering. The detector has met design goals with stable operation and convenient monitoring. 155
Acknowledgments
The octant shower detectors were constructed by our group at Rutgers including: J. J. Mueller, D. Bechis, G. K. Chang, R. Imlay, D. Potter,
F. Sannes, P. Skubic, and R. Stone.
The octant end and pole tip detectors were constructed at Harvard by: c. Sebek, J. Haggerty, M. Hempstead, J. M. Izen, R. Kline, W. A. Loomis, W.
W. MacKay, F. M. Pipkin, J. Rohlf, W. Tanenbaum, and Richard Wilson.
References
D. Andrews, et al.,- The CLEO Detector, Laboratory of Nuclear Studies,
CLNS 82/538 (1982).
C. Bebek, et al., Evidence for New-Flavor Production at the y
(45), Phys. Rev. Lett. ~~, 84 (1981). TABLE I
-_._-_.Shower Counter Parameters
Octant Octant End Pole Tip
Anode Wires 25~ 25~ dia. materi a1 50~ gold plated tungsten gold plated tungsten qold plated tungsten number layers, groups, tota 1 44. 774. 3826 16, 196. 880 21.711.2352 alignment azimuthal and z azimuthal 0 +22 1/2 • +120° . -22 1/2 0 tc radlal -120 0 - to vertlcal ganging 12 planes 2 deep x 1 wide 4 planes. 1 deep x 2 wide 6 planes 1 deep x 1 w;de 12 planes 3 deep x 2 wide 4 planes; 2 deep x 4 wide 6 planes 2 deep x 2 wide 20 planes 5 deep x 4 wide 8 planes; 2 deep x B wide 9 planes 3 deep x 4 wide
Tube size 31.7 mmx 12.7 mm 19 mn x 9.5 "'" 19 (JJf1 x 9.5 l11li 1.24 IJIT1 wall 1.60 rrm wall 160 ITIl1 wall Lead Sheet 1.27 "'" 1.98 ron 1.98 nm {Thickness} Total 12 r.l. 11 r. 1. 10 r.l. D;mens;ons Active length 3.70 m Active width 1.54 m inside active radius 27.9 em max. active width 2.23 "' active height .83 m outside active radius 81.3 em distance to interaction min. distance to beam distance to interaction point point 2.36 m to 2.97 m line 1.32 m 1.19 m to 1.50 m Solid Angle Coverage
0 0 0 55 • 125 40°,51° 13°. 29 Emin , £max n/41f 0.47 o. 12 0.10 Gas 91% argon, 9% methane 91% argon. 9% methane 91% argon. 9% methane
J-I U1 0'1 dE/dX PROPORTIONAL CHAMBERS
OCTANT SHOWER U) SYNC. BEAM DETECTOR LINE ~ TIME OF --r- FLIIHT-- OUTER Z DRIFT COUNTERS CHAMBERS ELECTRONIC CIRCUITS
.,.- .....~.-.-. o I I t 234I , , SCALE IN METERS ~ Fig. lea) In '-oJ 158
MUON CHAW8E.-S MOUNTED ON EXTERIOR OF MAGNE
SUPERCONDUCTING SOLENOID
., .
lCALE IN METDtS
Fig. l(b)
0710212 159
ONE MODULE OF THE OCTANT SHOWER COUNTER ,,~-- 240 em ---7-----..;1 7-'-'-/---- /' / CJ~ , o~ J 0' I I I I I I ,I I I I I I \ I
\ I Y \ I / \ e I ~ / \ U I (l,/ \ N I / I / \ \ ",0 I \ I ,/ • 0 \45 ,'/
\\ 1','';'/ I / , '//
Fig. 2 160
OD50' thck leoo ___-~_r_...... y_I"~-r------'0.50 II
1-= Fig. 3 1.61 .------1.82 m------a~ ORIENTATION ORIENTATION OF THIRD OF SECOND ES PLANE ES PLANE A SINGLE PLANE OF THE POLE SHOWER COUNTERS Fig. 4 0110382 162 1.77m ~1------2.02m-- 1.91 em ORiENTATION ORIENTATION OF THIRD OF SECOND PLANE PLANE A SINGLE PLANE OF THE OCTANT END SHOWER COUNTERS. Fi g. 5 0710382 163 5.000 ,I. 000 -.... > eLLI >- 3.000 Co:) ~ UJ Z LaJ ~ U.J 3 2.000 C :t: en 1.000 0.000 0.000 1.000 2.000 3.000 ij.OOO 5.000 BEAM MOMENTUM (GEV Ie ) Fig. 6 164 100.000 eo.ooo (f) ~ Z LU > 60.000 UJ Yo.ooo 20.000 0.000 0.000 0.080 0.160 O.2YO 0.320 O.ijOO o.Yeo 0.560 0.640 0.720 0.800 SHOWER ENERGY Fig. 7 165 0.300 0.250 0.200 0.150 UJ a:'" %: Co:) U"J 0.100 o.oso 0.000 0.000 0.500 1.000 1.500 2.000 2 1/SQRT (E) ( ( Ge V) -1/ ) Fig. 8 OVERLAPPING TRACKS IN OX CLEAN TRACKS IN OX ...... OCTANTSHOWER COUNTER ...... OCTANT SHOWERCOUNTER . . ~~- . 11 TF I DX DK OCt... 4 ~- - . I ""-'-rv ~~~ Ii ::JiI ...... I - t!£/dX.... BhoIN dE/a"...... ~~ L 1 ~ , I . ~. ~. I . I I - J I .£ ...... I .. O'IOZII ~ <'" O"l Fig. 9 167 TOTAL SHOWER ENERGY > ~ 10 •••• ~ 77 It)" 80 0 70 (/)'" t 60 Z !to ""> 40 ""~ 30 0 ~ 20 w m D 2 ~ 2.0 &.0 14.0 aI.O GeV Z [YI + £72 > ~ ~ It) .-.-- 1-'· 0 300 en '"t- auZ > 200 &U L\. 0 ct 100 &AI CD 2 ~ Z t.O &.0 100 M.O •.0 GIV [,- +E I .....MI Fig. 10 168 900 8HABHA + Y Y 800 ~ 700 E CD yr " 600 (/)..... ~ 500 > LaJ U. o 400 0: W CD ~ 300 :::> z 200 100 -240 -160 -80 0 80 160 240 M (c#>1+q,2) mrod Fig. 11 0710'12 169 T BHABHA ANGULAR DIST. 900 IN OCTANT SHOWER COUNTERS 800 700 I U')... z I ~ 600 > I L&J ~ 1L 0 500 ct: L&J ;{ m ~ 400 :) , z 300 1+ +, ~V . 200 f+1 ~ 100 ,. ,..~...,...... -.56 -.28 .00 .28 .56 COSINE OF POLAR ANGLE Fig. 12 0710112 R RATIO OF LUMINOSITY MEASUREMENTS IN OCTANT AND POLE SHOWER COUNTERS LOO I I I I I f I I I I I iii f I I I f I oao 11&4&511 .. 704 7J2 7~O 7&1 711 104 124 157 173 III 127 141 157 .11 114 12004 017 .'t"I' RUN NUMBER Fig. 13 . ~ ~ o , ~ , T , -. , -- - , - I , 4100'" II 25 Data (0) .. 80 ... 25 Ooto (b) ~ E > 600MeV > [Ey >200MeV > 7 • eu 2 ~ o 300' I II III .. N - 0601N " I I I II en'" t :? I IIIII ll II z IlI W > 40 t ~ IlI - W ~~II ~ l o lIU11 G: 1 W tOO t: - ; 20 I II I I ~ tl =» z ~ III filnI.I IIIIIIIIIIIImIlIII I 1 1 I I I II, , I , , , o 120 240 360 480 600 720 o 110 220 330 4«) ~ 660 Y-7 Moss ( MeV) 7-' Moss (MeV) em.. ' Fi g. 14 ...... ~ .....