The mid-rapidity calorimeter for the relativistic heavy-ion experiment WA80 at CERN

Nuclear Instruments and Methods in Physics Research A279 (1989) 479-502 North-Holland, Amsterdam

479

THE MID-RAPIDITY CALORIMETER FOR THE RELATIVISTIC HEAVY-ION EXPERIMENT WA80 AT CERN T.C. AWES 1) , C. BAKTASH 1) , R.P. CUMBY 1), R.L . FERGUSON 1) , A. FRANZ 2) , T.A . GABRIEL'), H.A . GUSTAFSSON 3), H.H . GUTBROD 4), J.W . JOHNSON'), B.W . KOLB 4), I.Y . LEE 1), F.E . OBENSHAIN 1), A. OSKARSSON 5), 1. OTTERLUND 5), S. PERSSON 5), F. PLASIL'), A.M . POSKANZER 2), H.G . RITTER 2), H.R. SCHMIDT 4), S.P . SORENSEN 1,6) and G.R. YOUNG') Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA zl Lawrence Berkeley Laboratory Berkeley, California 94720, USA 31 University of Lund, S-223 62 Lund, Sweden and European Laboratory for Particle Physics, CH-1211 Geneva 23, Switzerland 41 Gesellschaft für Schwerionenforschung D-6100 Darmstadt, FRG 5) University of Lund, S-223 62 Lund, Sweden 61 University of Tennessee, Knoxville, Tennessee 37996, USA

Received 20 February 1989 A sampling calorimeter designed for use at mid-rapidity in the relativistic heavy-ion experiment WA80 at CERN is described. Calibration and performance results are presented. Over the energy range of 2 to 50 GeV, the response of the mid-rapidity calorimeter was linear, and its energy resolution a/E was found to be given by 0.014+0 .11/FE and 0.034+0 .34/rE for electromagnetic and hadronic showers, respectively . Signal ratios of 1.2 and 1.4 were obtained for the e/h ratio of the lead -scintillator electromagnetic section and the iron-scintillator hadronic section, respectively . The calorimeter provided an accurate transverse energy trigger. The response and resolution for high-energy heavy ions were slightly better than anticipated on the basis of the low-energy calibrations. 1. Introduction The mid-rapidity calorimeter described in this paper was designed and tested for use in experiment WA80 at the CERN Super Proton Synchrotron (SPS) to investigate nucleus-nucleus collisions . Two types of calorimeters (fig . 1) are employed in WA80 : a mid-rapidity calorimeter (MIRAC) fully covering the pseudorapidity interval of ,l = 2.4 to 5.5 (,q = -In[tan(0/2)]), with a central hole through which the beam passed, and a zero-degree calorimeter (ZDC) to cover the central hole of the MIRAC and measure, at the high rate of 10 6 per second, the energy of the beam particles or beam fragments for every incident beam particle . Both are finesampling sandwich-type calorimeters and make use of the wavelength shifter technique to read out individually the electromagnetic and hadronic sections . The MIRAC consists of an electromagnetic section having a lead-scintillator composition followed by an iron-scintillator hadronic section. The ZDC consists of uranium- scintillator cells for both the electromagnetic and the hadronic sections and is described elsewhere [1]. The extremely large multiplicities in central nucleus-nucleus collisions (up to 500 charged particles for 160 on Au at 200 GeV/nucleon) make the use of 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

calorimetry particularly desirable. Aside from the obvious advantage of circumventing the problem of the individual measurement of a large number of particles, this high multiplicity actually becomes an advantage in a calorimetric measurement since the large multiplicity in each event tends to average out shower fluctuations . This fact results in an accurate determination of the energy flow on an event-by-event basis, from which any observed structures (in rapidity or azimuthal angle, for example) may be attributed to real characteristics of the physical processes involved. In addition, calorimeters may be used to provide specialized triggers for rapid selection of interesting events . It is believed that sufficiently high energy densities may be attained in relativistic nucleus-nucleus collisions for the quarks and gluons within the nucleons to become free to move around within the nuclear volume, thus forming a quark-gluon plasma . Therefore, it is of interest to enhance the selection of those most violent collisions in which a plasma is most likely to be formed. The calorimeters of experiment WA80 have been used to provide two of the specialized triggers to select such violent collisions. The MIRAC is used to provide a transverse energy, ET, signal . By triggering on events with large transverse energy, one hopes to enhance the

480

T. C. Awes et al. / The mid-rapidity calorimeter for the WA80 experiment

60 cm

ë

N

11 .7 Target

Fig. 1 . Calorimeter geometry of CERN SPS experiment WA80 . sample of events with large thermalized energy expected in plasma formation, or of those events which show other interesting large-E T phenomena . The MIRAC consists of 30 individual calorimeter modules arranged in five groups of six stacks (six-packs) and deployed in experiment WA80 in a nonprojective geometry as shown in fig . 1 . The central hole, through which the beam passes, has a horizontal opening of 5 .5 cm and a vertical opening of 7 .5 cm . Because of the wavelength shifter readout on the sides of the stacks and because of the side plates (see section 3 .2), there is about a 1 cm dead region on the sides of each stack . Therefore, the size of the opening in the active material is 7 .5 cm by 7 .5 cm . The front face of the MIRAC is located 650 cm from the target . The height of each stack is 120 cm, and the width of each six-pack is approximately 132 cm . Thus the four six-packs located about the beam completely cover the pseudorapidity interval from rl = 2 .4 to 5 .5 . Because of the noncircular geometry of the setup the outer corners of the central four six-packs extend the region of partial pseudorapidity coverage down to r1 = 2 .0, and the inner edges extend to rl = 5 .8 . A circular calorimeter arrangement and hole with equivalent areas would cover the pseudorapidity interval of q = 2 .2 to 5 .7 . The fifth six-pack of the MIRAC is located with the inside edge of its front face meeting the outside edge of

the front face of one of the center four six-packs . It is rotated about this edge by 11 .7' with respect to the beam axis so that its inside face contains a radius from the target (fig. 1) . It samples the pseudorapidity interval from r1 = 1 .6 to 2 .4, covering 10% of the solid angle within this interval . Although it is not useful for event characterization on an event-by-event basis because of its azimuthal bias, it can provide a measure of average event behavior within this pseudorapidity interval . Note that none of the 30 calorimeter stacks are placed such that the cracks between them project back to the target (except for the crack between the fifth six-pack and the others) . Because of the short period of time between the approval of experiment WA80 and the first beam period (approximately two years), it was not possible to make thorough prototype studies . Instead, extensive Monte Carlo design studies were made with the CALOR codes [2] to optimize the composition of the sampling cells . For the mechanical design of the mid-rapidity calorimeter, a modified version of the modular uranium-copper calorimeter stacks of the AFS collaboration [3,4] was adopted. A description of the Monte Carlo design code is given in section 2 . The design and construction, performance and overall response of the MIRAC are presented in sections 3 to 5 . The operation of the midrapidity calorimeter in experiment WA80 is discussed in section 6 .

2 . Description of design codes The design calculations for the MIRAC and the ZDC were performed with the CALOR computer system by following approximately the procedures used in previous calculations [5-8] . A flow diagram of the codes in CALOR is given in fig . 2 . The three-dimensional, multimedia, high-energy nucleon-meson transport code (HETC) [91 was used, with modifications, to obtain a detailed description of the nucleon-meson cascade produced in the detector. This Monte Carlo code takes into account the slowing down of charged particles via the continuous slowing-down approximation, the decay of charged pions and muons, inelastic nucleon-nucleus and charged-pion-nucleus (excluding hydrogen) collisions through the use of the intermediate-energy intranuclear-cascade-evaporation (MECC) model (E < 3 GeV) and scaling model (E > 3 GeV), and inelastic nucleon-hydrogen and charged-pion-hydrogen collisions via the isobar model (E < 3 GeV) and phenomenological fits to experimental data (E > 3 GeV) . Also accounted for are elastic neutron-nucleus collisions (E < 100 MeV) and elastic nucleon and charged-pion collisions with hydrogen . Heavy-ion projectiles of mass number A were represented by selecting A independent incident protons, each at an energy of E/A .

T.C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

HETC

HIGH ENERGY HADRONIC TRANSPORT CODE

SPECT

HADRONIC ANALYSIS CODE

MORSE

LOW ENERGY NEUTRON TRANSPORT CODE

EGS

ELECTRON-POSITRON AND GAMMA-RAY TRANSPORT CODE

FINAL COMBINE AND ANALYZE RESULTS FROM TRANSPORT CODES

Fig . 2 . Organization of the CALOR computer codes for Monte Carlo simulation of calorimeter response.

The intranuclear-cascade-evaporation model as implemented by Bertini is the heart of the HETC code [10]. This model has been used for a variety of calculations and has been shown to agree quite well with many experimental results. Even when agreement is not very good, the results produced by this model can lead the user to make correct decisions. The underlying assumption of this model is that particle-nuclear interactions can be treated as a series of two-body collisions within the nucleus and that the location of the collision and resulting particles from the collision are governed by experimental and/or theoretical particle-particle total and differential cross section data. The types of particle collisions included in the calculations are elastic, inelastic and charge exchange. This model incorporates the diffuseness of the nuclear edge, the Fermi motion of the bound nucleons, the exclusion principle and a local potential for nucleons and pions . The densities of the neutrons and protons within the nucleus (which are used with the total cross section to determine interaction locations) are determined from the experimental data of Hofstadter [10] . A zero-temperature Fermi distribution is used to determine nuclear potentials from these density profiles . The total well depth is then defined as the Fermi energy plus 7 MeV, the average binding energy. Following the cascade part of the interaction, there is excitation energy left in the nucleus. An evaporation model which allows for the emission of protons, neutrons, 2 H, 3 H, 3 He and 4 He is used to treat this energy. Fission induced by high-energy particles is accounted for during this phase of the calculation by allowing it to compete with evaporation . Whether or not a detailed model is included has very little effect on the total number of secondary neutrons produced or on the energy spectra of these neutrons . The source distribution for the electromagnetic cascade calculation is provided by HETC ; it consists of photons from neutral pion decay, electrons and positrons

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from muon decay (although this is usually not of interest in calorimeter calculations because of the long muon lifetime), deexcitation gamma rays from inelastic nuclear collisions and fission gamma rays . Since the discrete decay energies of the deexcitation gammas are not provided by HETC and only the total energy is known, individual gamma energies are obtained by uniformly sampling from the available energy until it is completely depleted. The transport of the electrons, positrons and gammas from the above sources is carried out with the EGS system [111 . The MORSE [12,13] Monte Carlo transport code is used to transport neutrons which are produced with energies below 20 MeV . The neutron cross sections used by MORSE were obtained from ENDFB/IV . Gamma rays (including those from capture, fission, etc .) produced during this phase of the calculations are stored for transport by the EGS code. The MORSE code was developed for reactor application and can treat fissioning systems in detail . This ability is very important since most of the fission compensation results from neutrons with energies less than 20 MeV . Time dependence is included in MORSE, but since neither HETC nor EGS has a timing scheme incorporated, it has been assumed that no time passes for this phase of the particle cascade . Therefore, all neutrons below 20 MeV . General time cuts used in the are produced at t=0 MORSE code are 50 ns for scintillator material . The nonlinearity of the charge collected or the resulting light pulse (L) due to saturation effects is taken into account by the use of Birks' law [14], _dL _ dx

dE/dx dE/dx'

1+kB

where kB is the saturation constant. For the scintillators studied, kB = 0.01 g/cm2 MeV or 0 .02 g/cm2 MeV . For electrons at all energies, it is assumed that kB=0 .

The energy detected in each module was individually weighted to account for different sampling fractions in the various sections of the calorimeters by multiplying the effective energy deposited in the plastic by

dE/dx

I

P,_XP,

where dE/dx I im:~ is the minimum energy loss for medium i and is taken from the Particle Data Group tables [15] and X pT and Xi are the respective thicknesses of the active and inactive regions. Comparisons with the test beam data are given throughout the remaining text . In most cases, the results obtained experimentally indicated slightly better performance in, for example, energy resolution and in spatial containment than did the calculations.

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T. C. Awes et al / The mid-rapidity calorimeterfor the WA 80 experiment

3. Design 3.1 . Design considerations Because of the intrinsic dimensions of hadronic showers (one interaction length - for MIRAC, approximately 20 cm) and the extremely high multiplicity of particles produced in relativistic heavy-ion reactions, it is not economically feasible to achieve sufficient granularity to obtain single-particle resolution over a significant fraction of the solid angle region of interest . In fact, even at 650 cm from the target, one expects of the order of a dozen particles within a 20 X 20 cm2 region at the most forward angles for central 160 + An reactions at 200 GeV/nucleon. Therefore, it was decided to make the electromagnetic section 15 radiation lengths long so that electromagnetic and hadronic energy would be separated cleanly in spite of the expected large multiplicity of particles per calorimeter element . This ensures essentially complete containment of the electromagnetic energy in the electromagnetic section, with 97 .4% and 91 .0% containment calculated for 1 GeV and 30 GeV photons, respectively. Given such a deep electromagnetic section, hadrons will, on average, deposit a sizeable fraction of their energy in this section as well as in the hadronic section . Corrections for hadronic energy deposited in the electromagnetic section may be made on the basis of the amount of hadronic energy observed in the hadronic section . A length of 6 absorption lengths was chosen for the hadronic section so that the high-energy, leading baryons in nucleus-nucleus collisions would be well contained. For 50 GeV protons, this would be expected to provide 96% energy containment . Fulfilling the resolution requirements for the MIRAC is made easier by the fact, noted above, that the large multiplicity of produced particles gives an improved resolution for the calorimetric energy measurements used to characterize high-energy nucleus-nucleus events . Thus only modest energy resolution is required . Also, cost considerations dictated not using uranium for the MIRAC. Instead, lead was chosen for the absorbing material of the electromagnetic section and iron for the hadronic section. Extensive Monte Carlo calculations with the CALOR design codes [2] indicated that the optimum resolution for hadrons was obtained with 3 mm lead sheets for the electromagnetic section, 8 mm iron sheets for the hadronic section, and 3 mm scintillator sheets in each . For this composition, reduced proton of ao = 45% at 1 GeV and 49% resolution ((Yo = a/ at 5 GeV are calculated for the balanced sum of electromagnetic and hadronic signals. The. balanced sum is obtained by adding the hadronic signal multiplied by 1.86 to the signal of the electromagnetic section. The balance factor 1 .86 is the calculated ratio of the energy loss in a unit cell of the hadronic section to that in a

F)

Table 1 Calculated MIRAC resolution and a/ (E) [%] e/h ratio

e/h

ratio for protons

1 GeV

5 GeV

45 1 .15

49 1.24

unit cell of the electromagnetic section. The calculated ratio of the balanced electron-to-hadron response of the calorimeter, the so-called e/h ratio, is 1 .15 and 1 .24 at 1 and 5 GeV, respectively. As defined here, this ratio corresponds to (e/h) F, the ratio for the electromagnetic section (see section 4 .2) . These calculations are summarized in table 1. It is interesting to note that the 3 mm lead-3 mm scintillator sandwich and 8 mm iron-3 mm scintillator sandwich compositions chosen here are in good agreement with recent schematic calculations of Wigmans [16] for optimum hadron resolution and e/h ratio. These calculations highlight the importance of hydrogen in the readout material for providing sensitivity to the neutron component of the hadronic shower and the importance of tuning this sensitivity by adjusting the ratio of absorber to readout-layer thicknesses to bring the intrinsic e/h towards unity. The physical processes discussed in ref. [16] are treated in detail in CALOR [2]. 3.2 . Mechanical construction For the mechanical design, a version of the AFS modular uranium-copper calorimeter stacks [3,4] was adopted, with minor modifications related primarily to the increased length of the stacks . Each stack has a cross section of 22 cm X 120 cm and is divided into six individual 20 cm X 20 cm towers that have four optical readout channels per tower. As with the AFS stacks, an aluminum I-beam is used as a support structure for the stacked plates and the photomultiplier tubes (fig . 3) . The plates are stacked on five stainless steel support rods, which are attached to the I-beam at the center and near the four corners of the stack. The hollow center rod allows the introduction of calibration sources. The electromagnetic section consists of 27 lead absorber cells. Each 3 mm lead plate is epoxied between two aluminum sheets of 0.8 mm thickness for mechanical rigidity. The hadronic section includes 119 stainless steel absorber plates of 8 mm thickness . The absorber plates of the two sections are interleaved with 3 mm scintillator plates . The spacing of the absorber and scintillator plates is maintained by five stainless steel washers located in holes in the scintillator plate at the support rod locations. The scintillator plates are protected and optical uniformity is maintained by a sheet of white Mylar placed on each side of the scintillator . A completed stack has an electromagnetic section of 15 .6

T. C. Awes et al. / The mid-rapidity calorimeterfor the WA 80 experiment

LEAD ABSORBER PLATE (ELECTROMAGNETIC SECTION)

WAVELENGTH SHIFTER READOUT (ELECTROMAGNETIC SECTION)

483

STEEL ABSORBER PLATE (HADRONIC SECTION)

WAVELENGTH SHIFTER READOUT (HADRONIC SECTION)

LIGHT GUIDE -J

PHOTOMULTIPLIER AND BASE ASSEMBLY

Fig. 3. Isometric view of one of the mid-rapidity calorimeter stacks together with a cross-sectional view showing the lead-scintillator electromagnetic section, the iron-scintillator hadronic section and the layout of the optical readout system . radiation lengths and a hadronic section of 6.1 absorption lengths, which gives a total of 6.9 absorption

3.3. Optical readout and electronics

kg . The mechanical properties of the stacks are sum-

is attained by using wavelength shifter (WLS) readout of the scintillator [17] . The scintillator and wavelength

lengths. Each stack is 214 cm in length and weighs 2227 marized in table 2.

A compact optical readout with minimal dead region

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TC. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

Table 2 Mechanical properties of a MIRAC stack Electromagnetic front section Number of absorber plates Size of absorber plates Composition of absorber

Number of scintillator plates Size of scintillator plates Number of optical channels Depth of electromagnetic section Number of radiation lengths Hadronic section Number of steel plates Size of steel plates Number of scintillator plates Size of scintillator plates Depth of hadronic section Number of absorption lengths Total stack Overall length Weight Number of absorption lengths

27 1200 X 205 mm2 0.8 mm aluminium+ 3 mm lead +0 .8 mm aluminium sandwich 27 1199 x 206 x 3.2 mm3 12 (6 right, 6 left) 224 mm 15 .6 119 1200 x 198 X 8 mm3

119 1199 X 200 x 3 .2 mm 3 1390 mm 6.1

2145 mm 2385 kg 6 .9

shifter were chosen after light output and attenuation

were measured for various samples (see ref. [1]) . The scintillator chosen was PS-15A *, which has a composi-

tion of 15% naphthalene and 1% butyl-PBD in a poly-

methylmethacrylate (PMMA) base . The emission spec-

tra for PS-15A and PS-10A are shown in fig. 4. The

light attenuation length of the scintillator has been measured to be about 64 cm . The scintillator is read out in six independent towers through WLS on the left and right sides of the stack. Five laser cuts of 17 .0 cm length

at the tower boundaries in each scintillator (every 20 cm along the 120 cm length) provided optical isolation of

the towers [18] . The laser cutting results in slits of 0.5 mm width, with a polished surface * * . Thin aluminium strips were inserted into the laser cuts to improve the containment of the light within a tower. As a result,

400

ac nmi

500

600

Fig. 4. Emission spectra for Polycast PS-10A and PS-15A (b), compared with absorption spectra for the two kinds of wave length shifter, WLS-P (a) and BBQ (c). The PS-15A is a better match in both cases.

approximately 90% of the light produced at the center

of a tower remains within the tower, while only 5% of the light leaks into each of the two adjacent towers (see section 4.5).

The scintillation light from each tower is transmitted

out

via two BBQ (benzimidazo-benzisoquinoline-7one)-doped acrylic wavelength shifters on the sides of the stack (fig . 3) [19] . BBQ was chosen because its

ever, its fluorescence spectrum is peaked at 500 nm, which does not match well to standard photomultipliers . The BBQ concentration must be optimized for the competing requirements

of high absorption efficiency for the scintillator light, on the one hand, and

low absorption efficiency for the fluorescent light re-

absorption spectrum is well matched to the emission

emitted by the WLS, on the other. A concentration of

spectrum of the PS-15A scintillator (see fig. 4). How-

80 mg/l BBQ in PMMA was chosen, and a quantity of an ultraviolet absorbing (UVA) chemical having a cutoff

* Polycast, Stamford, Connecticut, USA. ** Work performed at Laboratori Nazionali di Frascati, Italy.

chemical is necessary to reduce the sensitivity of the

for ultraviolet light below 365 nm was added. The UVA

WLS to Cherenkov light resulting from shower particles

T C. Awes et al / The mid-rapidity calorimeterfor the WA80 experiment

traveling within the WLS. The increased light from this effect would degrade the uniformity of the calorimeter response for showers beginning near the edge of the calorimeter [19,20]. A WLS thickness of 2 mm was chosen for the electromagnetic section . For the hadronic section a WLS thickness of 3 mm was chosen as a compromise between the requirements that it be thin to minimize the effect of Cherenkov light from shower particles passing through it and that it be sufficiently thick to obtain a long attenuation length for the wavelength-shifted light. The attenuation of light along the WLS was reduced by coating the end opposite the phototube with a TiOZloaded epoxy. The WLS light is brought to the photomultiplier by a light guide of the lamella type . For the hadronic section, the WLS is directly coupled to the light guide; whereas, for the electromagnetic section, the WLS light is guided past the hadronic section by an attached coupling sheet of PMMA (3 mm thick), equal in size to the hadronic WLS, which is then attached to the light guide. The WLS plates run the length of the calorimeter and are separated from the edge of the scintillator plates by two strands of 1 mm diameter fishing line . A UVA chemical with a cutoff of 420 nm was introduced into the EM coupling sheet to reduce the transmission of spurious Cherenkov light in the plastic. This UVA material has a much higher cutoff than that used in the WLS since the shifted wavelength is larger . Cross talk between the electromagnetic and hadronic readouts was prevented by wrapping the coupling sheet in aluminized Mylar and the light guides in aluminum foil . The resulting attenuation in a hadronic section WLS readout is shown in fig. 5 . Although the attenuation is not strictly exponential, an effective attenuation length of 102 cm is observed. The photomultipliers are coupled to the light guides by a 3 mm thick disk of clear elastic silicone rubber . To further reduce the effect of Cherenkov light produced by particles traversing the WLS, the rubber is also doped with a UV-absorbing chemical . The electronic readout of the MIRAC is summarized in table 3. The photomultipliers are supplied high voltage by a linear voltage divider, with all components carried on a printed circuit board soldered to the socket . Coaxial cables (type RG-174) for the high voltage and for the anode and dynode signals from each of the 24 photomultipliers are brought to three metal junction boxes on the back of each stack. The use of 1 kQ resistors to separate the high-voltage grounds from one another served to reduce noise. The high voltage for each photomultiplier is brought to the calorimeter high-voltage box from computer-controlled power supplies located in the counting room . The photomultipliers for the MIRAC were assigned to towers according to their relative gains, with the lowest gain phototubes chosen for towers at the largest pseudorapidities. The relative gain of each of the photomultipliers was

48 5

Fig. 5. Attenuation curves for wavelength shifter samples supplied by Polycast . The yo-values were determined by least-squares fits to (Nve) = (Np, )o exp( - y/yo ). Since the number of photoelectrons measured depends on the physical coupling to the photomultiplier, only the relative values should be compared. Starting at the top, the symbols are as follows : Top section: A, 3 mm BBQ coupled to PS-15A ; 0, 2 mm BBQ coupled to PS-15A ; El, 2 mm BBQ coupled to PS-15A with white reflective paint on one end; o, 3 mm BBQ+UVA (365 nm cutoff) coupled to PS-15A . Bottom section: o, 6 mm WLSP (0 .01% POPOP) ; El, 2 mm WLSP (0 .01% POPOP) . determined in a bench test prior to calorimeter assembly. This was done by determining the voltage necessary to produce a specified output signal when the tube viewed a reference LED light source . The anode signals are brought from the anode box to the counting room on two bundles of twelve RG-58 cables of 70 m length . There the cables are soldered to a panel of printed-circuit cards to make a transition to coaxial ribbon cable for the ADCs . The dynode signals are extracted with the use of ferrite cores and are collected at the dynode box, where the left- and right-side signals are summed for the electromagnetic and hadronic sections separately . The left and right dynode signals are summed with equal weight by means of a balun transformer network. The six dynode sum Table 3 Electronic system of mid-rapidity calorimeter Photomultiplier type Cathode diameter Cathode type Multiplier Maximum peak current High-voltage supply ADC

Thorn-EMI 9903 l'-, in . Bialkah 10-stage linear focussing 200 mA do LRS 1440 LRS 2282A and LRS 2282B

48 6

T C. Awes et al. / The mid-rapidity calorimeter for the WA 80 experiment

Wavelength shifter PCS1000,20m Leser box

Prtmery olslributlon box

Fig . 6 . Laser calibration system for the calorimeters of WA80 .

signals for the electromagnetic section and for the hadronic section of each stack are brought over short cables (RG-174) to the transverse energy, ET , trigger modules located near the MIRAC. Custom-built NIM modules are use to obtain transverse energy trigger signals. The NIM modules form E T by producing the weighted sum of the dynode sum signals, Ei , from each tower i; Et = Ew i Ei . The dynode signals are attenuated by the weight factors w, with interchangeable pi-section resistor attenuators which ensure proper 50 2 input impedance. The weight factors are given by w i = N(a/d) i sin(Oi)/Gi , where Gi is the relative gain factor for each tower (the same for both photomultipliers of the same section of a tower), Oi is the effective angle of the tower, (a/d) i is the average anode-to-dynode ratio of the two dynode signals summed in the dynode box on the tower, and N is an overall normalization factor such that the maximum weight factor wmax = 1. The relative gain and effective angles of the towers were determined from simulated data obtained with the Monte Carlo, LUND-model code FRITIOF [21] . The calculations took into account the effects of the nonprojective experimental geometry (fig . 1) as well as the effects of the shower development and energy deposition within the MIRAC. The anode and dynode signals were measured simultaneously for each photomultiplier by using the laser pulser system (section 3 .4) at the voltages corresponding to the run settings . The anode-to-dynode ratio was found to be 19 .0, on average, but had a distribution with a standard deviation of 2.8 . It was found that the ratio changed by about 10% for a voltage change of 100 V. The NIM modules are used at several levels to form a total transverse energy trigger signal . At the first level, a total ET signal is formed for the electromagnetic section of each of the four central six-packs (fig . 1), and a total ET signal is formed for the hadronic section of

each of the four central six-packs. At the second level, the four electromagnetic ET signals are summed to give a total electromagnetic ET for the MIRAC, and the four hadronic ET signals are summed to form a total hadronic ET . Finally these two signals are summed to form a total ET in MIRAC. In addition, the transverse energy sums of the fifth six-pack are available at each level for trigger purposes but are not included in the total MIRAC ET sums to prevent the introduction of an azimuthal bias in the trigger (fig . 1) . The analog E T trigger signals are carried on fast cables to the counting room, where discriminator thresholds are set to produce the transverse energy trigger-logic signals. For experiment WA80, only a total MIRAC high-ET trigger was used . 3.4. Laser monitor system

The calorimeter stacks were equipped with a laser pulser system by means of which light pulses were injected into the calorimeter wavelength shifter, fully simulating both the pulse height and timing characteristics of the scintillation light produced by particles penetrating the calorimeter scintillator material . The laser pulser system has played a crucial role in the calorimeter calibration . It was used both to fix the energy calibration by calibrating the laser signals relative to those of actual particles and then to adjust the photomultiplier gains and to monitor the stability of the optical readout. The laser light distribution system is shown schematically in fig. 6. A pulsed nitrogen laser (PRA Laser, Inc., Model LN100) *, with an emission wavelength of * Photochemical Research Associates, London, Ontario, Canada .

T C. Awes et al. / The mid-rapidity calorimeter for the WA80 experiment

487

337 nm, was operated at a pulse rate of about 1 Hz . The pulse duration was less than 1 ns, with a peak power output of 250 kW. Ultraviolet-transparent optics were used to split off a portion of the light to go to the WA80 beam trigger detectors and also to focus the light onto the primary distribution fiber . The fiber lengths for the beam trigger detectors and for the calorimeters were chosen to compensate for the flight time of real particles . Therefore, the laser events simulated the logic and timing of real events. The main optical fiber is 1 nun thick, with a UV-transparent core of fused silica (PCS1000) * . This 20 m long fiber brings the light to the primary distribution box, which is located at the MIRAC . In this distribution box, the laser light excites a plastic scintillator plate which reemits the light as scintillation light, peaking at 420 nm and having the proper pulse shape . The light is diffused and homogenized by passing it through a Lucite rod . It is then distributed across an air gap into a number of polystyrene optical fibers (CH4001) §, which bring the light to the secondary distribution boxes . Here the light is further distributed through Lucite diffusers to the individual photomultipliers . In the primary distribution box, a reference photomultiplier views the laser pulse and also the light from a constant-intensity light source (a 24'Am-doped Nat crystal) . The pulse-to-pulse variation, as well as the long-term variation, of the laser light can be corrected for by normalizing to this reference photomultiplier . The laser is also viewed by a stable pin diode as a backup check of the laser and reference photomultiplier stability. The laser tight is distributed from the secondary distribution box on radiation-resistant polystyrene plastic fibers . The fibers feed the light into the front end of the wavelength shifters of the MIRAC and the ZDC . The laser thus monitors changes in the transmission of light in the WLS and changes in the coupling to the light guides and the photomultiplier . The fibers were permanently glued into a hole drilled in the WLS . Outside a stack, the light fibers are bundled, with the fibers of the hadronic section of two adjacent towers bundled together in one connector and, similarly, for the electromagnetic section . In total, there are then six such connectors for each stack. This bundling allows different light intensity to be distributed to different zones of the MIRAC and to the electromagnetic and hadronic sections separatedly . All dismountable connectors are pressed against the Lucite rods without optical grease. A connector can be reinserted in its original position and orientation with a reproducibility of better than 1% . The system was kept intact during the

data-taking periods, however . The light intensity can be controlled by inserting neutral density optical filters at several locations . The overall intensity can be controlled by inserting filters at the laser box . Since the gain of the photomultipliers is set differently according to angle, because of the variation in the expected particle energies, it was necessary to distribute different light intensities to different phototubes . This was done by inserting filters in the secondary distribution boxes, where all phototubes at the same gain could be given the same light intensity . The laser system has been used extensively at all stages of the experiment . During the calibration and setup stage, it was used to determine the voltage-gain dependence of each photomultiplier. It was also used later to determine the anode-to-dynode signal ratio for each phototube. The gain settings determined during the in-beam calibrations were preserved by using the laser peak positions normalized to the reference photomultiplier . These calibrated laser peak positions were then used to set new voltages for the desired data-taking gain settings. This was done by iteratively adjusting the voltages, with the guidance of the individual voltage-gain dependences, until the desired laser peak positions were obtained . During the data-taking stage, laser events were recorded on tape during each SPS spill-off period . The on-line data acquisition program automatically issued a warning if a large change occurred in the

* Fibres Optiques Industries, France. § ELPAC, Munich, FRG .

Fig. 7. Distribution of laser peak ratios, as observed with the laser (a) between adjacent runs and (b) after 24 h.

80 70 60 50 40

c J W

20 10

W

LASER PEAK RATIO

48 8

T C. Awes et al. / The mid-rapidity calorimeter for the WA80 experiment

laser peak position for any photomultiplier and, thereby, monitored the entire readout chain from the high-voltage supply to the ADC. During the data analysis stage, the laser peak positions recorded during the data taking were used to correct, in software, for gain changes. As an example, fig. 7 shows the distribution of gain drifts for all 720 photomultipliers of the MIRAC . Fig. 7a shows the distribution of gain drifts between adjacent runs . The gain drift distribution after 24 h is shown in fig. 7b . 4. Performance 4.1 . Test and calibration configurations

Initial tests and response measurements for the MIRAC were made at the T9 beam line of the CERN Proton Synchrotron (PS) . Measurements were made with mixed positive beams of positrons, muons, pions, and protons of 2 to 10 GeV/c. For these tests, the calorimeter modules under study were configured with five stacks arranged side by side, which was similar to the geometry used during the WA80 experiment (fig . 1) . The five stacks were placed on a moveable platform that allowed position scans in both directions across the faces of the towers . Behind these five stacks, an additional three stacks were used to measure possible hadronic shower leakage. Upstream in the beam line, two threshold Cherenkov counters were used, each with a different threshold, in order to separate positrons from mucus and pions and to separate pions from protons. Muons were separated from pions by requiring a signal in a plastic scintillator located behind the second three calorimeter stacks . The beam was tagged by two upstream scintillators for additional time-offlight information . The beam position was defined by a coincidence between two plastic scintillators, with a 1 x 1 cm2 overlap, located directly in front of the five stacks. Beam particles which began to shower in material upstream from the calorimeter were rejected when a signal was observed in any one of a set of plastic Cherenkov counters located in front of the calorimeter . They covered an area of 25 x 25 cm2 and had a 1 x 1 cmz hole aligned with the beam definition counters . These veto counters indicated that only about 1 in 10 3 identified positrons reached the calorimeter before beginning an electromagnetic shower . Such a poor beam quality was expected because of the large amount of material in the beam line, which was simultaneously set up for other detector tests. As a result, it was difficult to make reliable measurements of the energy resolution of the calorimeter under these conditions . The beam was used primarily to study the linearity and response of the calorimeter at lower energies for positrons, pions and protons. Generally, the beam quality degraded with

decreasing momenta such that at 1 GeV/c it was simply unusable. The position response and shower development within the calorimeter were studied in detail, with position scans made at 10 GeV/c for positrons and pions. The position scans were made with normal beam incidence and also with the front five stacks rotated by 8 ° about a vertical axis . This corresponds to the average angle of incidence for particles in the WA80 geometry. Further linearity and resolution measurements were made at the XI beam line of the SPS. Mixed negative beams of electrons, muons and pions of 4 to 50 GeV/c were used to study the linearity and resolution of the calorimeter . In addition, all 180 towers were put into the beam in order to set the gain of the photomultipliers and to calibrate the laser pulse signal relative to the actual particle signal for each photomultiplier. For these calibrations, each six-pack (fig . 1) was placed on a moveable platform, and the beam was put into the center of each tower. Electrons were identified in an upstream threshold Cherenkov counter, and muons were identified in a plastic scintillator located behind an iron absorber downstream of the calorimeter . 4.2. Gain matching and balancing

Because of the different behavior of electromagnetic and hadronic showers and because of the different composition and readout of the electromagnetic and hadronic sections, one expects different calibration coefficients for the two sections and for electrons and hadrons. (We will refer to electrons, but the following discussions apply to any other electromagnetic showering particle as well .) A relation for these coefficients can be written in the following schematic matrix form : CE

I,

(3)

CH ~ L SH where K e and K h are the incident electron or hadron energies, S E and SH are the observed signals in the Kh

electromagnetic, E, and hadronic, H, sections, and the c's are the corresponding calibration coefficients . This expression is schematic in the sense that the calculated energies, K e and Kh, are not correct simultaneously but are correct only for the corresponding incident electron or hadron . The relation implies a linear calibration with no energy offset . For the present discussion, energy-dependent calibration constants can be assumed. The different calibration coefficients are interrelated by the following relations: cF = cE(elh)E,

c H-cH(elh)H, c H ° CE(EhIHh) --_ CFIBh,

T.C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment where (e/h) E is the ratio of the signal produced by an electron to that produced by a hadron of the same energy, which, in both cases, is totally contained within the electromagnetic section. The ratio (e/h)H is similarly defined for the hadronic section . The balance factor Be = (HeIE') is the ratio of signals produced by an electron depositing all of its energy in the hadron~c section to one depositing all of its energy in the electromagnetic section . The balance factor Bh is similarly defined for hadrons . The e/h ratio primarily reflects the physically different light-producing processes which occur in electromagnetic-versus-hadronic showers . It may also reflect slight differences in the light collection efficiency caused by the different spatial distributions for the two shower types. Note that the e/h ratio discussed here is not the same as the intrinsic ratio (e/h)", which is the ratio of light produced by purely electromagnetic showers to that produced by purely hadronic showers having no electromagnetic component [16] . Obtaining the best possible resolution for hadronic showers rentr quires having (e/h)' = 1, in which case the fluctuating electromagnetic component of hadronic showers gives the same amount of light per unit of energy deposited as the purely hadronic component [16,22] . The balance factor B = (H/E) depends on the relative gain setting of the electromagnetic and hadronic photomultipliers, on the relative light collection efficiency of the electromagnetic and hadronic sections, and in the case of the MIRAC (which has different compositions for the electromagnetic and hadronic sections), on the differing amounts of light produced in the two sections by a particle of a given energy. In general, the balance factor will be different for electrons and hadrons, especially when the two sections have different compositions, because of the differences in the amount and spatial distribution of the light produced by hadrons as compared to electrons . From eq. (4) above, it is clear that the e/h and balance factors are related according to

Be1B

h= (elh)Hl(elh)E.

Eq. (3) can be rewritten in the following illustrative form : CE/Be K SE CE E

[ KH][ CE( elh)E

l

CE(elh)EJBh][SH~>

(6)

which shows how the different calibration constants are related to the calibration for electrons in the electromagnetic section . The determination of the four parameters necessary for the complete calibration is described in the following. Before the calibration procedure was begun, the signal pulse height and resolution were investigated as a function of the ADC gate length. A gate length of 350

48 9

ns was chosen because of the rather long decay time of the BBQ wavelength shifters . A long gate length is also desirable from the point of view that it increases the response to the thermal neutron component of the hadronic showers and, therefore, improves the e/h ratio and hence the hadronic resolution [16] . It was found that the signal pulse height increased by about 10% as the gate width was increased from 150 to 350 ns, with a similar improvement in the resolution for the hadronic section . The pulse height increased by less than 2% with an increase of gate width from 300 to 350 ns . The basis of the calibration procedure was to match the gains of all photomultipliers in the electromagnetic sections by using 10 GeV/c electrons incident on the center of each tower . The voltages on the two photomultipliers of the EM section of a tower being calibrated were adjusted for each tower in turn until the electron peak positions were in the same channel for all EM photomultipliers. This was facilitated by an automated voltage adjustment routine which used individual voltage-gain dependence parameters for each photomultiplier . The laser pulser system had been used earlier for determining these parameters . The gains of all photomultipliers in the hadronic sections were simultaneously matched in the same manner with 10 GeV/c pions . The signals observed in a single photomultiplier for incident electrons or pions are shown in figs. 8a and 9a, respectively. Once the gains of the photomultipliers have been matched in the electromagnetic and hadronic sections separately, the total electromagnetic and hadronic signal sums can be formed. These summed signals are shown for electrons in fig. 8b and for pions in fig. 9b . From the separate gain-matched sums of the electromagnetic and hadronic signals, the relative response of the electromagnetic and hadronic sections can be determined . This is seen in fig . 10, where the sum of the pulse heights of all electromagnetic photomultipliers is plotted against the sum of the pulse heights of all hadronic photomultipliers for 10 GeV/c pions, as measured in the PS calibration. The small peak at low pulse height in the two sections is due to muons which do not penetrate the back calorimeter stack and hence are not identified in the muon scintillator . From such a plot, one can extract the two quantities, E h and Hh, which are given by the intersections of the pion ridge with the electromagnetic and hadronic axes, respectively . These points correspond to the total signal for pion showers that are completely contained within the electromagnetic or the hadronic sections, respectively . Thus the ratio of these two quantities gives the hadronic balance factor Bh = Hh 1E h . From a similar plot for 10 GeV/c electrons, one can obtain the similar quantities Ee and He and, thereby, extract the electromagnetic balance factor Be. From these four quantities are also obtained the elh ratios for the two sections, (elh) E = Ee/E h

490

T. C. Awes et al. / The mid-rapidity calorimeterfor the WA 80 experiment 50 1

800 700 600

40

500 400 300 200 100 0

Q

350 300 y 250 1- 200 Z D 150

20

10

100 50 0

0

200 160

5

10

15 EM SUM

20

25

30

Fig. 10 . Contour plot of the sum signal of all electromagnetic photomultipliers versus the sum signal of all hadronic photomultipliers, for 10 GeV/c pions.

120 80 40 0

CHANNEL

Fig. 8. Response of Ml RAC to 10 GeV/c electrons at the SPS, as observed in (a) a single electromagnetic photomultiplier, (b) the sum of all electromagnetic signals and (c) the sum of all electromagnetic signals times the electron balance factor plus the sum of all hadronic signals . 400 350 300 250 200 150 100 50 0 200 160 N fZ 120

0 U

60 40 0 350 300 250 200 150 100 50 0

CHANNEL

Fig. 9. Response of MIRAC to 10 GeV/c pions at the SPS, as observed in (a) a single hadronic photomultiplier, (b) the sum of all hadronic signals and (c) the sum of all electromagnetic signals times the hadron balance factor plus the sum of all hadronic signals.

and (e/h)H = He/H h . It should be noted that, because of the large number of radiation lengths of the electromagnetic section, the electron energy deposited in the hadronic section is quite small, typically less than 5% of the total, on average (see section 4.6), making the extraction of He somewhat uncertain. This is reflected in an uncertainty in extracting a reliable value for (e/h) H and for the calibration constant cH of eq. (3), which, however, is of minor importance since so little electromagnetic energy is deposited in the hadronic section. An alternative approach to establishing an absolute energy calibration for all photomultipliers is to use measured muon signals as corresponding to minimum ionizing particles and then to calculate the expected minimum ionizing energy deposited in the two sections . From the ratio of the observed signals to the calculated energy losses, one then obtains the balance factor for the two sections for minimum ionizing particles. If the ratio of signals from electrons to those from minimum ionizing particles, e/mip, is the same as that for muons or is known for the two sections, the balance factor for electrons can be deduced. Similarly, the hadronic balance factor can be determined from the h/mip ratios . The absolute energy calibration can then be deduced from the balanced sum of the two sections when incident particles of known energy are used [4,191 . We have chosen to use the direct method of calibration described above, which avoids all considerations of the relative response of showering particles compared to minimum ionizing particles. The use of muons for calibration purposes is complicated [181 by the energy dependence of their energy loss, which typically is not exactly minimum ionizing, and by the composition dependence [161 of the e/mip and h/mip ratios . Furthermore, it

T. C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

was observed that the e/l ratio had a stack-to-stack variation with a sigma of about 8% . This made the use of muon signals alone inappropriate for calibration purposes. The reason for this variation is apparently due to the fact that, while minimum ionizing particles sample all scintillator sheets more or less equally, showering particles obviously do not . Therefore, variations in scintillator thicknesses, which were observed but not controlled during assembly of the MIRAC, or in light output [20] can explain the observed variations in the e/ju ratio.

3000 2500 2000

am c

1500

L U

1000 500 0

4.3 . Response to electrons

With the balance factor between electromagnetic and hadronic sections determined, as discussed in the previous section, the energy deposited in the two sections can be correctly summed to obtain the total balanced sum signal. This is shown for 10 GeV/c electrons in fig . 8c for the sum of the electromagnetic photomultiplier signals multiplied by the electron balance factor, B e , plus the sum of the hadronic signals. In this case, the balance factor is of minor importance because, on average, more than 95% of the energy is deposited in the electromagnetic section (see section 4.6) . The sum energy peak is observed to be symmetric and Gaussian. The linearity of the energy response to positrons with incident momenta of 2 to 10 GeV/c is shown in fig . 11 . The response is linear over the energy range plotted but has a small deviation at low energy because of the poor beam quality obtained during the PS calibration (section 4 .1). These data were acquired with the condition that no signal was observed in the veto counters, which ensured that the shower did not begin in the material located upstream .

5000

3000

Û

0

10

30 40 20 Energy (GeV)

50

60

0

10

20 30 40 Energy (GeV)

50

60

0 .30 0 .25

w C

0.20 0.15 0.10 0.05 0.00

Fig . 12. Calorimeter response in terms of (a) linearity and (b) reduced resolution (ap = a/Vl~) for electrons of incident momenta of 4 to 50 GeV/c, as measured at the CERN SPS.

The linearity of the energy response of the calorimeter to electrons with incident momenta of 4 to 50 GeV/c is shown in fig. 12a . The response is observed to be linear to within 2% over the energy range of 10 to 50 GeV. The reduced resolution, a/V.E__ , for electrons is shown in fig . 12b . It is observed to increase from 13 .9% at 4 GeV to 21 .6% at 50 GeV . This indicates the presence of a constant contribution to the resolution, a/E. A linear fit of the data shown in fig . 13, where a/E is plotted against 1/Vrk, gives an energy resolution of a/E = 0 .014 + 0.11/ for electrons . The contribution of sampling fluctuations to the electromagnetic resolution [23,24] is estimated to be about a`/E = 0 .09/F, thus dominating it . Since all 144 photomultipliers of a six-pack were summed to produce the data of fig . 12, one expects a nonnegligible contribution to the electromagnetic resolution from photomultiplier noise and pedestal resolution. These effects should add directly to a and, therefore, should contribute to the resolution scaled by 1/E [25] . This contribution is estimated to be about 5% at 1 GeV. The constant term of 1 .4% contributing to the electromagnetic energy resolution is consistent with previous observations [26] . A contribution of this magnitude might be expected to be due to light attenuation in the scintillator [27] .

r

4000

c c

49 1

2000 1000 0 Energy (GeV)

Fig . 11 . Linearity of the calorimeter response for positrons of incident momenta of 2 to 10 GeV/c, as measured at the CERN PS.

49 2

T C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

m

1NE

Fig. 13 . Calorimeter resolution (a/E) vs 1/VE for electrons of incident momenta of 4 to 50 GeV/c. The curve is a least-squares fit to the data.

0.6

b)

0.6

4.4 . Response to hadrons The total hadronic signal for 10 GeV/c pions is shown in fig. 9c . The signal is formed from the sum of the 72 electromagnetic photomultiplier signals multiplied by the hadronic balance factor, B h , plus the sum of the 72 hadronic signals of the entire six-pack . The sum energy peak is observed to be symmetric and Gaussian. The linearity of the response of the calorimeter to protons and pions with incident momenta of 2 to 10 GeV/c is shown in fig. 14, where the sum signal is plotted against the kinetic energy of the particle. The response is linear over this range of energies, with a similar response for pions and protons. At low kinetic energy, the response to protons is observed to be slightly larger than the response to pions, despite the fact that pions should also deposit a substantial fraction of their rest mass as detectable energy. It is also observed that

c m Û

Energy (GeV)

Fig. 14 . Linearity of the calorimeter response for protons and pions of incident momenta of 2 to 10 GeV/c, as measured at the CERN PS.

0.2 0.0

0

10

20 30 40 Energy (GeV)

50

60

Fig. 15 . Calorimeter response in terms of (a) linearity and (b) reduced resolution for (ao = (7/VE_ ) for pions of incident momenta of 4 to 50 GeV/c, as measured at the CERN SPS . protons begin to shower in the electromagnetic section more frequently than pions; this is due to the proton's larger interaction cross section. The energy-dependent hadronic calibration coefficient C h (Kh) is defined as the slope of a line from the origin to the point corresponding to the incident energy Kh on the calibration curve in fig. 14 . The linearity of the response of the calorimeter to pions with incident momenta of 4 to 50 GeV/c is shown in fig. 15a. The energy response is observed to be linear to within 7% over this range . The reduced resolution, a/F for hadrons is shown in fig. 15b. It is observed to increase from 42 .3% at 4 GeV to 60 .8% at 50 GeV, and this again indicates the presence of a nonstatistical contribution to the resolution, a/E. This is shown in fig. 16, where a/E is plotted against 1/FE . A linear fit of the data gives an energy resolution of a/E = 0.034 + 0.34/ F for hadrons. The contribution to the hadronic resolution due to sampling fluctuations [24] in the hadronic section can be estimated to be about as/E = 0.27/ F . For hadronic showers, the obtainable resolution is limited by the intrinsic resolution due to fluctuations in the portion of the shower energy which is not observed because of nuclear binding and neutron emission. For iron/scintillator calorimeters similar to the MIRAC, the intrinsic contribution has

T. C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

1NE Fig. 16 . Calorimeter resolution (a/E) vs 1/F for pions of incident momenta of 4 to 50 GeV/c. The curve is a least-squares fit to the data .

been calculated to be a` °"/E = 0 .20/ JË [16] . Thus the observed energy dependence (0 .34/FE) of the resolution is entirely explained as the sum in quadrature of the sampling and intrinsic resolution contributions . The constant term of 3 .4% contributing to the hadronic energy resolution is similar to that observed in previous work [26] . Such a constant contribution is expected for calorimeters which are not fully compensating, that is, for calorimeters which do not have an equal response to electromagnetic and hadronic particles such that intr = 1 . Considering the measured e/h ratio of (e/h ) hadronic the section of the MIRAC (see section 4 .6), the constant term contributing to the resolution is expected to be somewhat larger than that observed [16] . The fact that a portion of the hadronic energy is deposited in the rather deep electromagnetic section, which has a lower e/h ratio and, therefore, better hadronic resolution, may be partly responsible for the lower measured value . On the other hand, an additional constant contribution to the energy resolution is expected because of light attenuation in the rather long wavelength shifter of the hadronic section [27] . 4.5. Position response and uniformity

The uniformity of the response as a function of position of incidence across the faces of the towers was investigated during the PS calibrations with 10 GeV/c positrons and pions (section 4 .1) . The response was studied for normal incidence and for incidence in which the calorimeter stacks were rotated 8 ° about their vertical axis, which is about the average angle of incidence for particles entering the MIRAC in WA80 . The total measured pulse height as a function of horizontal impact position is shown in fig. 17 for positrons and pions at the two different angles of incidence . The points indicate the peak positions, and the bars indicate the

49 3

full width at half maximum . For normal incidence, the total signal for positrons shown in fig . 17a is observed to increase slowly as the position of incidence moves from the center toward the edge of the tower . In addition, a sharp rise in the signal is observed when the point of impact is very close to the edge of the tower because of the Cherenkov light created by shower particles crossing the wavelength shifter . For pions, the position dependence is much weaker because of the larger shower size (fig. 17b) . With the 8 ° angle of incidence, the signals are observed to be less dependent on position . Because of the position dependence of the observed signals within a tower and between adjacent towers, a horizontal position coordinate may be calculated as xcalc = Ex i Pi/EPi , where x, is the coordinate of photomultiplier i with signal Pi . A vertical position coordinate may similarly be calculated as yca,c = Ey i P;/EPi . The calculated relative horizontal position versus the actual position is shown in fig . 18 for positrons and pions . The bars in the figure indicate the full width at half maximum of the calculated position at each location . The width is a result of the position definition of the beam (1 X 1 cm2 ) and of fluctuations in the shower development . As expected, the width is larger for the hadronic showers . The calculated relative vertical coordinate versus the actual position is shown for positrons and pions in fig. 19, from which similar conclusions may be drawn . The effectiveness of the laser cuts in the scintillator in isolating the individual towers is demonstrated in fig . 20 . Here the total pulse height in three of the central towers within a stack is shown as the point of impact of the beam is moved from the lower tower through the middle tower to the upper tower . It is observed that light from the electromagnetic showers is nearly completely contained within the struck tower, while for hadronic showers, the light is shared additionally between the neighboring towers because of the greater lateral dimensions of the hadronic showers . Comparison of the signal in a given tower with the signal observed in neighboring towers of the adjacent stacks allows one to deduce that only 5% of the light produced within the struck tower leaks past the laser cut into each of the two adjacent towers of the stack. 4.6. Containment and e/h ratio

Obtaining an optimum separation between electromagnetic and hadronic energy in a high-multiplicity environment required that the MIRAC stacks be designed with an electromagnetic section deep enough to contain the electromagnetic energy nearly completely . The fraction of the total measured energy which is deposited in the electromagnetic section is shown in fig . 21 for electrons and also for pions, as measured during the X1 calibrations. For electrons, it is found that, on average, 98 .5% of the energy of 4 GeV electrons is

494

T. C. Awes et al. /The mid-rapidity calorimeter for the WA80 experiment 500

500

400 m â)

x

a w

400 oa

300 200

a.

100 0

w 0

10

x

20 (cm)

30

200 100 0

40

500

0

5

10

0

5

10

x

15 (cm)

20

25

30

x

15 (cm)

20

25

30

500

400 300 200 100 0

300

+~ aq

400

â

200 100

0

10

x

20 (cm)

30

40

0

Fig. 17 . Total pulse height as a function of position for 10 GeV/c positrons (a and c) and pions (b and d) at normal incidence (a and b) and with an 8° angle of incidence (c and d) . The points indicate the peak locations, and the bars indicate the full width at half-maximum. The center of the tower is located at x = 20 cm . contained within the electromagnetic section. The contained fraction falls slowly to 90 .4% for 50 GeV electrons. On the other hand, the average hadronic shower is observed to deposit 25% of the incident energy in the electromagnetic section at 4 GeV, with the fraction falling to 13 .5% at 50 GeV. Of course, there are large fluctuations, especially for hadronic showers, in the actual energy deposited in each section for any given shower (see fig. 10). As discussed in section 4.2, the ratio (e/h)' ntr , which is the relative response of the calorimeter to electromagnetic energy compared to its response to purely hadronic energy, plays a crucial role in understanding the resolution achieved for hadronic showers. In practice, it is not possible to measure the intrinsic e/h ratio directly because the hadronic showers typically contain an electromagnetic component within the shower. The measured e/h ratios for the MIRAC are shown as a function of energy in fig. 22 and tabulated in table 4. They are shown for the lead-scintillator electromagnetic section and for the iron-scintillator hadronic section. The

ratios are obtained from the observed or extrapolated signals in which the energy of the electron or pion is totally contained within the electromagnetic or hadronic section, as described in section 4.2 . The (elh)E ratio is easily extracted for the electromagnetic section, in which both electrons and pions leave sizeable signals, making the determination of Ee and E h straightforward . On the other hand, because electrons typically deposit very little energy in the hadronic section, as shown in fig. 21, the extrapolation to deduce He, the signal expected for an electron depositing all of its energy in the hadronic section, is rather uncertain. This leads to a consequent uncertainty in extracting the (e/h) H ratio for the hadronic section. It was deemed reliable to extract (e/h)H only for incident electron momenta of 20 GeV/c and above, where, on average, the incident electron deposits 5% or more of its energy in the hadronic section. The (elh)H ratio is found to be about 1.4 for the hadronic section, in agreement with expectations [16] for an 8 mm iron-3 mm scintillator sampling calorimeter . It is interesting to note that, although the

T. C. Awes et al. / The mid-rapidity calorimeterfor the WA80 experiment

plier's tower) for data taking with the 200 GeV/nucleon

60

U k b

t60 beam at the SPS. This was accomplished by using

w

required factor. Because of the very large extrapolation

~ynill~ll!lyyn

in the gain setting (a factor of 50 gain reduction) for the calibration of these towers during the heavy-ion run by

160 beam directly into

putting the 60 GeV/nucleon

1~1~yj~plyn

20 10

four innermost towers, it was deemed wise to verify the

+i

30

U

the laser calibration system to reduce the voltage on the photomultiplier until the laser peak was reduced by the

(a)

50 40

495

each tower. For this calibration run, the 16 0 beam was fragmented upstream to produce the full range of A =

2 Z products from deuterons through oxygen at the 0

10

x

20 (cm)

30

40

same momentum-to-charge ratio and at a reduced intensity of a few times 10 3 s-1 . Although the oxygen

beam had an incident momentum of 60 GeV/c per nucleon (which corresponds to a kinetic energy of 59.1

60

U

50

k b

40

GeV per nucleon), it was found that the kinetic energy

of the fragmented beam was 52 .6 GeV per nucleon, as

measured in the ZDC. The total energy signal of the

Ili'ill'

30

U b m

x

.II!

20 10

0

10

70

Ilil'~Illildl!~ill'Illf x

20 (cm)

30

60 a,

40

Fig. 18 . Calculated horizontal position as a function of actual position for 10 GeV/c (a) positrons and (b) pions. The calculated values are relative to arbitrarily defined zero position and direction of motion . Bars on the points indicate the full width at half-maximum of the calculated position . The center of the tower is located at x = 20 cm.

,a

U

m U w

(a)

50

30 20

,dill

III

40

0

10

y

20 (cm)

30

40

70

e/h ratio is rather large compared to other iron-scintil-

lator calorimeters [28], the resolution obtained is nearly optimal [16] because of the relatively small contribution

from sampling fluctuations . The electromagnetic section

(e/h) e ratio of about 1.20 is found to be in good

agreement with the design calculations of Gabriel (table 1) and the more recent estimates of Wigmans [16]. The

ratio is observed to have the expected energy dependence, decreasing at high energies because of the in-

creasing electromagnetic component of the hadromc

showers and at low energies because of the increasing contribution from ionization energy loss for the hadrons. 4.7. Operation with heavy-ion beams

FRITIOF model simulations showed that it would be necessary to reduce the photomultiplier gains from the calibration gains by factors of from 2.5 to 50 (depending on the pseudorapidity of the photomulti-

U T

v m

_U U b m

x

60

(b)

1

50

II'

40

III

30 20

0

10

y

20 (cm)

I

111

30

40

Fig. 19 . Calculated vertical position as a function of actual position for 10 GeV/c positrons. The calculated values are relative to arbitrarily defined zero position and direction of motion . Bars on the points indicate the full width at half-maximum of the calculated position .

496

T. C. Awes et at / The mid-rapidity calorimeterfor the WA80 experiment 2.0

300 m

250 °~ x

200

lao

W

50

N C 0

1.6

"

Fe/Scn HAD Pb/Scn EM

1.2

C O

Û N d 0

10

y

20 (cm)

30

1.0 0.8 0 .600

10'

40

10 ,

Momentum (GeV/c)

Fig. 22 . Ratio of electromagnetic energy response as a function of incident momentum for the lead -scintillator electromag netic section and the iron -scintillator hadronic sections of the mid-rapidity calorimeter.

120

Table 4 Momentum dependence of e/h ratio for electromagnetic and hadronic sections of the MIRAC

90 60 30 0

1 .8

t

150

a

C

0

10

y

20 (cm)

30

40

Fig. 20 . Total pulse height in each of three towers centered at y = 0, 20 and 40 cm as a function of vertical position . The total EM pulse height for 10 GeV/c positrons is shown in (a), and the total hadronic pulse height for 10 GeV/c pions is shown in (b).

Momentum (elh)E [GeV/c] PS calibration

Momentum (e/h)E [GeV/c] X1 calibration

2 3 4 5 6 7 8 10

4 10 20 30 40 50

1.14_+0.06 1 .20+_0 .06 1.30+_0.07 1.18+0.06 1 .31+0.07 1 .21±0.06 1 .16+_0 .06 1 .18+0.06

1 .26+_0.06 1 .12+_0 .06 1 .15+_0 .06 1 .19±0.06 1 .14+_0 .06 1 .09±0.05

(e/h)H

1.39+0.14 1.38±0.14 1 .55+0.16 1 .44±0.14

fragmented beam, as measured in the MIRAC with the beam incident on one of the four inner towers (fig. 1), is

shown in fig. 23 . Each of the A = 2Z elements is clearly 12C separated, with the peaks corresponding to 4 He, and 16 0 dominating the spectrum .

0 C m

100 80

(0

H C W

0 W N 1 F Q w

60

Electrons Pions

40 20

Energy

(GeV)

Fig. 21 . Average fraction of total measured energy deposited in electromagnetic section for electrons and pions.

Fig. 23 . Total energy signal measured in the MIRAC for the fragmented 16 0 beam of 52 .6 GeV/c per nucleon incident on the center of one of the four inner towers adjacent to the beam line (fig . 1) .

T C. Awes et al. /The mid-rapidity calorimeterfor the WA80 experiment

The 160 peak energy was measured with the ZDC to be 89% of the actual incident energy. It was observed that 2.3% of the incident energy leaked out the back of the MIRAC into the ZDC . This is estimated to be about half of the total back leakage at 52 .6 GeV/nucleon ; the additional back leakage is not observed in the ZDC because of its distance from the MIRAC . It is further estimated that about 5% additional energy leaks out of the side of the inner tower into the hole in the MIRAC and is not seen in the ZDC . Therefore, it was found that after a factor of 50 extrapolation in gain from the calibration gain settings, the calibration obtained was correct to within 5% . It is also interesting to point out that the resolution obtained in the MIRAC for the 4 He, 12C and 16 0 peaks, gated by identified charge in one of the WA80 beam counters, is a/E = 4 .4%, 3 .1% and 3 .2%, respectively . This is somewhat better than the hadron resolution values of a/E = 5 .7%, 4 .7% and 4 .6%, respectively, expected on the basis of calibration results that were obtained at lower incident energies (section 4.4) .

5 . Response function In high-energy heavy-ion experiments, calorimeters are used in a slightly different way than in experiments with hadron beams, where the particle multiplicities are much lower . In the latter experiments, hadronic calorimeters have often been used to search for jet structures [4,30], and, typically, the emphasis in the off-line analysis has been to identify one or more clusters of towers in which a large amount of energy has been deposited. With the high multiplicities of the heavy-ion experiments, the emphasis has moved to a more global analysis of the energy deposition, in which quantities like the transverse energy, flow tensors, etc . are investigated . It is, however, important to realize that the high multiplicities in many ways improve the performance of calorimeters. Since each tower will be sampling many showers, the fluctuations in the energy deposition will be reduced . This is consequence of the Central Limit Theorem, which can be illustrated by comparing two situations : (a) one 100 GeV hadron or (b) ten 10 GeV hadrons impinging on a calorimeter . The energy resolution in the two cases will be the same (ao/ 100 ), but the fluctuations in the origin, the length and the radial size of the showers will be 10 smaller in case (b) . This implies that a much better separation of the electromagnetic and hadronic energy can be obtained. For 200A GeV 160+197Au, simulations have shown that, on the average, 20% of the energy carried by hadrons is deposited in the electromagnetic section of the MIRAC . Instead of using the raw energy deposited in the electro-

49 7

magnetic section, one can obtain a better estimate of the energy carried by electrons and photons by subtracting 20%/80% = 1/4 of the energy deposited in the hadronic section from the raw energy deposited in the electromagnetic section . Such a correction would be very inaccurate in a low-multiplicity situation, where the fluctuations in the origin of the hadronic showers would dominate, but for high multiplicities it provides a much improved estimate of the energy carried by hadronic and electromagnetic particles . By similar arguments one can correct for the leakage of showers between towers. An event restoration algorithm has been developed on the basis of simulations and is described in this section . We first describe the single-particle response function of the MIRAC; this is a parameterization of the average energy deposition and light collection obtained for a hadron or electron with arbitrary energy and angle of incidence impinging on the MIRAC. Next the calculation of the total MIRAC response function, which describes the average response of the MIRAC as a whole in nucleus-nucleus collisions, is discussed . The MIRAC response function includes effects like the nonprojective geometry, the broadening of the measured signal as a function of the direction of the incident particle (caused by the finite sizes of the showers), differences in hadronic and electromagnetic shower characteristics, etc . Finally, the unfolding procedure for taking into account the MIRAC response is described . The following coordinate systems are used in the discussions below. L The right-handed laboratory coordinate system with origin at the target, z-axis along the beam direction, and y-axis pointing vertically upward . C A right- or left-handed six-pack coordinate system with z-axis in the depth direction, y-axis pointing vertically upward, and horizontal x-axis . The origin is situated at the front face corner of the particular six-pack closest to the beam line . S A right-handed shower coordinate system with origin at the point where the impinging particle enters the calorimeter . The z-axis is along the direction of motion of the particle . T A tower coordinate system used for the description of the light collection within the plastic scintillators . The directions of all axes are identical to C . The origin is placed at the center of the front face of the tower in consideration .

5.1 . Single-particle response function The average fraction of the incident energy E nc of a particle of type I (I = e for electromagnetic, I = h for hadronic) deposited at the point (x s , ys , z s ) in the

49 8

T C. Awes et al. / The mid-rapidity calorimeter for the WA 80 experiment

calorimeter section J (J = E for electromagnetic, J = H for hadronic) can be parametrized as follows: Edep(xS,

YS ,

Z S) -

longitudinal(ZS

~1, .I `~longitudinal(ZS ) `~lateral(

x S> YS ~~

=d~i exp -d ,

d = ßl zs/A L.I ,

Table 5 Response function parameters. The index I = e, h refers to electromagnetic and hadronic showers, respectively . The index J = E, H refers to the electromagnetic and hadronic sections of the calorimeter, respectively. Symbol Description

longitudinal shower maximum parameter

Parameter Value

a

a e.tl

longitudinal shower decay length parameter

ße,o

a I = a l,p + a l,l In Eine , ßt = ßt,o + PI. , In F lateral (XS ,

+

1

2 rli

Ys) =

Eta

st exp { - P/lS) 2mls

exp(-P/lL },

ß

is = (XI,o+XI,1 v~d )A1,r,

fl =XI exp{ -(Lx+XT)/4), fr=X1 exp{ -(Lx-xT)/d),

ße,l

.0 ßh /8h,l

8

2 2 P = V-~s+Ys >

IL=(1+ ~' I,2)ls . The parametrization assumes that the shower can be factorized into longitudinal and lateral factors with the same form for electromagnetic and hadronic showers. The parametrization of the longitudinal dependence is similar to that chosen by UA1 [29] . We have chosen to let the z-coordinate have its origin at the front face of the calorimeter and not at the beginning of the actual shower. The latter choice is easier to work with in a Monte Carlo calculation, whereas our choice is simpler in an analytical calculation of the average signal in each phototube. The shower parameters are listed in table 5. This parameterization was chosen by fitting to model shower results calculated by T. Gabriel with the Monte Carlo code HETC [2]. The lateral dependence composed of two exponential functions is inspired by these results. The exponential term with the short decay length, 1 s , is supposed to describe the intense electromagnetic part of the hadronic showers, whereas the term with the longer decay length, 1L , describes the hadronic component, especially the long-range neutron part of the shower . The light collection in a photomultiplier is dependent on the shower location within a tower (see section 4.5) partly because of the change of the effective solid angle to the readout side and because of light attenuation in the scintillator . The fraction fl or fr of the energy deposited at the point (X T , YT, Z T ) and seen as light by the left or right phototube, respectively, is parametrized as

ae .l a h .o a h,l

2 .52 0 .772 0 .552 0 .281 0 .438 0 .052 0 .654 0 .069

fraction of energy deposited se in lateral short-range shower 8 h

1.0

lateral shower decay length parameter

1.30

0 .45

X e,0

k_l

ße,2 h,0 h,l

8 h,2

0 .047 0 .00 0 .049 0 .030 1 .96

effective scintillator attenuation length parameter

co El

22 .7 cm 0 .23

K

cross talk between towers

K

0 .040

A

radiation length

A

absorption length

A e .L A e. H

L

tower half size

e

`, h,E `,

where the normalization constant from the condition 1=J LLxJ

h.H

Lx L~ .A'l

1 .4 cm 2.4 cm 29.6 cm 21 .4 cm 11 .0 cm

10.0 cm

is determined

L ,, (fl +fr) dYT dXT .

The signal received by photomultiplier j in the section J(= E,H) is given by the integral of fEâ"P over the volume of the particular tower section: Y.=J

m~xfj(XT,

Lc z J r',,JZ

YT, -T)

XEâeP(xs, Ys , zs) d- T dYT dX T .

(10)

The laser cuts in the scintillators do not completely suppress the cross talk between the towers within a stack. The effect of the cross talk is that a fraction K of the signal 9, will be observed in the neighboring tubes . If the index m runs over the six towers within a stack, the uncalibrated signal in the mth tower .Sôm can be expressed as Ym = K_';e,n -1 + (1 - 2K) ~'m

+ KY_,,

.

11

T C. Awes et al. / The mid-rapidity calorimeter for the WA 80 experiment The attenuation of the light in the wavelength shifter is not explicitly parametrized but is implicitly taken into account in the ß parameters, which describe the decay length of the showers . A short attenuation length in the wavelength shifter will effectively make the shower appear longer since more light will be collected from the rear of the tower . The uncalibrated signals .5ô are converted into calibrated signals by multiplying with the appropriate calibration coefficient c from formula (3) . See ref . [4] . The overall normalization constant, .N, of the single-particle response function [eq . (7)] is determined as a function of the impinging particle energy Ei!nc and particle type I (hadron or electron) with the following calculational procedure : (1) The depth of the hadronic section is increased so that the shower is completely contained . The depth of the electromagnetic section and the lateral dimensions of the towers are not changed. (2) The signal in each phototube is calculated under the assumption that a particle of type I with energy E nc impinges perpendicular to the calorimeter front face at the center of a tower in the middle of a MIRAC six-pack . (3) N' is determined from the requirement that the sum of all phototube signals is 1 . The single-particle response function parameters were determined by making a X 2 -fit to a large number of measurements of the experimental response of a six-pack with different particles at incident energies between 2 and 50 GeV, both at the PS and SPS accelerators at CERN . The final values are shown in table 5 . 5.2. Event response function In the previous section, the calculation of the calorimeter response to a single particle was described . Let Sj (E , c , x c , yc ) denote the signal in the jth phototube when a particle coming from the target with incident energy E,' e hits the calorimeter at the front face point (x c , yc ) . The index j runs over all Ntube photomultipliers (= 720 for the MIRAC) in such a way that phototubes collecting light from the electromagnetic sections are labelled by odd integers and the hadronic section phototubes are labelled by even integers . Furthermore, let the index i label all the Nube 20 x 11 cm2 front surface elements .Q?i in such a way that odd is correspond to electrons impinging on the surface element and even is similarly correspond to hadrons . It is emphasized that the calculated signals SJ are assumed to be signals averaged over a large number of events . In a nucleus-nucleus experiment, there will be a distribution of particles 9,(E ,e , q, (p) with different incident energies across the surface element ,Wi . I = e corresponds to particles starting electromagnetic showers (electrons and photons) and I = h corresponds

499

to hadrons . The average response in the j th phototube causes by particles hitting the jth surface element can now be expressed as

f

R' ;-

f~, dxc dycf

dERe 9,(Ene , r~, $) (12)

XSj (E- c , xc, yc ) .

The function 9, has been calculated for selected combinations of beam energy, projectile and target on the basis of the Monte Carlo code FRITIOF [21]. The calculation is simplified by evaluating the integral only for the average electromagnetic and hadronic energies entering a surface element . 5 .3. Unfolding of response The quantity R ji , defined in the previous section, can be interpreted as the fraction of the energy originally hitting element i which will show up in phototube j after the shower and light collection. Because of the large fluctuations from event to event, R~ i has to be calculated as the average over many events . If the measured signal in the jib element is Sj , then the unknown energy E i impinging on the element i can be estimated by solving the following set of linear equations : Si _

Nn,ne i=1

R ji E i ,

(13)

or in matrix form, S=

(14)

RE,

where a bold character denotes a matrix. The accuracy of this assumption depends on the magnitude of the fluctuations in the response of the detector . In a highmultiplicity situation, where the fluctuations, as argued above, are small, it is a reasonably good approximation . This linear system can not be solved simply by multiplying through with the inverse matrix of R since in that case the small fluctuations (statistical noise) would be amplified by the inverse matrix and the final energy distribution would be strongly fluctuating (for a more detailed discussion of this point see ref. [31]) . Instead, an iterative algorithm must be used . The following method has been found to be both fast and robust [32] . Initial guess :

Eo

nthiteration : rn

=0 ; =S -

(15) RE n ,

_ En+1-En+Xn

IIrn112 JI RTrn

Il Z

R rn .

Then,

0 (Éi)n+1

SilR ii

if (Ei )n+1 < 0, if 0 < ( Éi)n+1 < SilRii if SilR ii < (Éi)n+1,

500

T C. Awes et al. / The mid-rapidity calorimeter for the WA80 experiment

where ( Eiln+lY ( E k ) n+l \ E iln+l

/_

\

e

lEkJn+l ikl(ËÀ)n +i >0}

Fig. 24 . Demonstration of the MIRAC event restoration algorithm. The LEGO plots show, for each tower, the energy of the hadronic and electromagnetic sections for a single central event. The upper row shows the energy impinging on the face of the calorimeter, and the bottom row shows the energy distribution after the event restoration. Each square of the plot corresponds to a tower of the MIRAC . 160

,

3 ô

100

TRANSVERSE ENERGY (HARDWARE) (CHANNEL)

Fig. 25. Scatter plot of the transverse energy calculated in software from the digitized photomultiplier signals of the four central six-packs of the MIRAC, excluding the inner four towers, versus the transverse energy calculated in hardware for the trigger, for which the signals from the same towers are used . The correlation is shown for minimum bias events . The size of a square corresponds to the number of events in the bin for 200 GeV/c per nucleon 16 0 on Au .

and 11 11 denotes the Euclidean norm, RT is the transposed matrix of R, and Jt n is a relaxation parameter with values a n = 0.75, 0.50, 0.25, 0.15, 0.10, 0.06 for n = 1, 2, . . . , 6, respectively . Better accuracy can be obtained by using one of the many nonlinear algorithms developed for image restoration, such as the maximum entropy method [33] ; however, they are, in general, too slow . An example of the use of the event restoration is shown in fig. 24 . The upper row shows the incident hadronic and electromagnetic energy and constitutes the distribution e which the event restoration algorithm is supposed to re-create. The middle row shows the simulated energy deposition of the same event in each tower and section corresponding to the distribution S. Note how the electromagnetic section especially is distorted . Finally, the bottom row shows the energy distribution after application of the event restoration algorithm. Note how the energy deposition pattern has improved, especially in the electromagnetic section of the central towers . 6. MIRAC performance in WA80 6.1 . Trigger performance

The MIRAC provides an important component of the WA80 trigger system . Heavy-ion collision events in which the deposited energy has been highly thermalized are expected to be most suitable for quark-gluon plasma formation and should be characterized by large transverse energy. Therefore, a high-transverse-energy trigger is used in WA80 to further enhance the sample of events which might have large thermalized energy. The transverse-energy trigger is produced by a discriminator set on the analog total ET signal from the MIRAC. The analog ET is obtained from the four main six-packs of the MIRAC, excluding the four inner towers, and is generated as discussed in section 3.3 . In fig. 25, the final calculated and unfolded transverse energy is plotted against the digitized analog transverse-energy signal that was used on an event-by-event basis for the ET trigger. The comparison is made for minimum bias events over the same interval of 2.4 < ,q < 4.0. The hardware trigger signal is found to be very well correlated with the fully calibrated and unfolded transverse energy, as required for an accurate trigger . The dispersion in this distribution determines how sharply the true high-E T threshold is defined. An example of the high-E T trigger selection

50 1

T. C. Awes et at. / The mid-rapidity calorimeter for the WA80 experiment

is shown in fig. 26 for the O + Au reaction at 200 GeV/c per nucleon. Generally, the high-E T threshold is set differently for each target-projectile system so that the highest-ET events can be accepted at a rate which does not require prescaling .

s a

6543[ 2

To test the homogeneity of the MIRAC, one can calculate the "center of energy" in analogy to the "center of gravity" from mechanics by : ,~ymaxyE(y) %~xmax xE(x) dx J dy fx yand yc= fymax fxmaxE(x) xc dx E(Y) dy , JY m,. Jx m. 

1 0

The integration limits (xrnin, xmax, Ymin and ymax) are given by the required tl-range, here chosen to be 2.4 < ,q < 4.0, where the MIRAC covers the full polar angle ¢. When one considers separate towers in the MIRAC, the above formulas change to : N YC

N

Ei=1Ei

(17)

N Ei-,Ei

Here the index i runs over all towers (i, . . ., N) which lie in the required 11 interval ; xi and yi are the x and y position, and Ei is the energy content of the i th tower. Instead of using the energy E; per tower, the transverse energy ETi - Ei sin(14i) may alternatively be used . In this case, the contribution from the towers farther away from the beam becomes more important, so the distribution of x c and yc is expected to be wider. On an event-by-event basis, the distribution of xc and yc measures the homogeneity of the calorimeter because any misalignment, cracks or other asymmetries

100

1 -15 -10 -5

0

5

10

11 15 20

Y, Icml

Ys Icml

= 1.36 (f 0.01) cm o, =2 .55(f0 .01)cm a,> = 0.09 (f 0.01) cm a, =2 .44(f0 .01)cm <6,>

(16)

C

r,-r-1

7

6.2 . Center of energy measured by the MIRAC

N

3

-10

10

15

20

Fig. 27 . Contour plot of the x- and y-position calculated from the energy measured in MIRAC for 2.4 < iq < 4.0 . The result is shown for 3 .2 TeV 16 0+Au events in which more than 0.8 TeV was observed in MIRAC . The projections on the x- and y-axes are also shown . should influence the symmetry of the distribution. If all of these possible contributions to the asymmetry of the calorimeter response are eliminated, then the mean values of xc and yc , (x c ) and (yc), measure the alignment of the calorimeter with respect to the beam position . This is important for further data analyses which might be sensitive to this asymmetry. Fig. 27 shows a two-dimensional distribution of xc and yc projected onto the front of the MIRAC for 160 + Au events with an energy content of En-11RAc 0 .8 >TeV within the 11-interval specified above. The vertical lines in the projections on the xc- and yc-axes of fig. 27 indicate the size of the beam hole in the center of the MIRAC. By fitting Gaussian distributions to these projections (solid lines in fig. 27), one determines the averages and the standard deviations given in table 6.

W

Fa

Table 6 The average and standard deviation of the center of energy measured with respect to the beam axis using either the total energy or the transverse energy measured in MIRAC

1

0

25

50

75

100

125

150

175

Energy per tower

200

TRANSVERSE ENERGY (GEV)

Fig. 26 . Transverse energy calculated in software, as in fig. 25, but for events having large transverse energy, as determined by the hardware transverse-energy signal .

xc Yc

( ) [cm]

1 .36±0 .01 0.09±0 .01

a [cm]

2.55±0.01 2.44±0.01

ET per tower ( ) [cm] a [cm] 1 .58±0 .01 0 .30±0 .01

3 .49±0 .01 3 .30±0 .01

502

T. C. Awes et al. / The mid-rapidity calorimeter for the WA80 experiment

7. Summary The mid-rapidity sampling calorimeter for CERN experiment WA80 has been designed and constructed on the basis of Monte Carlo simulations with the CALOR calorimeter design codes [2]. The calorimeter consists of a lead -scintillator electromagnetic section with an iron- scintillator hadronic section. The performance of the calorimeter has been found to be in good

agreement with that expected from the calculations . The calorimeter response was found to be linear with an energy resolution, a/E, given by 0.014 + 0.11/F and .034+0 for electromagnetic and hadronic 0 .34/FE showers, respectively. The e/h ratio, or the ratio of the signal produced by an electromagnetic shower to the signal produced by a hadronic shower was found to be

1 .2 for the lead -scintillator electromagnetic section and 1.4 for the iron-scintillator hadronic section at 10 GeV/c incident momenta. The energy resolution of the mid-rapidity calorimeter for high-energy heavy ions was

slightly better than anticipated from the lower-energy hadron calibrations . In the WA80 setup, the mid-rapidity calorimeter provides a complete transverse energy measurement over the pseudorapidity region q = 2.4 to

5 .5, with partial coverage over the extended region rl = 2.0 to 5 .8 . It has been used to provide an effective transverse-energy trigger in experiment WA80 to enhance the selection of high transverse-energy events .

Acknowledgements We acknowledge extensive discussions concerning the design, construction and calibration of the R807 calorimeters with C. Fabjan, C. Woody and A. DiCiac-

cio. We thank J. Lee of Polycast Corp . for providing numerous samples of scintillator and wavelength shifter for testing. We are indebted to P. Giromini of the Laboratori Nazionali di Frascati for assistance in using

their facilities to perform the laser cutting of the MIRAC scintillator plates . Assistance with the assembly of the calorimeters was provided by L. Klingler, S. Lundborg, M. Marquardt, A. Przybyla and A. Schwinn. Electronics design for aspects of the trigger was done

by J.L .

Blankenship and for the bases by R. Albrecht . We acknowledge the considerable assistance provided by U. Gastaldi, R. Coccoli and M. Cataneo with the tests at

the CERN PS and by M. Hubert and his staff with the tests at the SPS. We thank the operating staffs of the CERN PS and SPS for reliable operation of their accelerators during the several months of testing. This work has been supported by the Swedish Council for Planning and Coordination of Research (FRN) and the Swedish Natural Science Research Council (NFR),

by the US Department of Energy under contract DEAC05-84OR21400 with Martin Marietta Energy Systems, Inc. (ORNL), and by the US Department of Energy under contract DE-AC03-76SF00098 (LBL).

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[31 H. Gordon et al ., Nucl . Instr. and Meth. 196 (1982) 303. [4] T. Akesson et al ., Nucl . Instr. and Meth. A241 (1985) 17 . [5] J.E . Brau and T.A. Gabriel, SLD-New Detector Note no . 119 (May 22, 1984). [6] J. Brau and T.A . Gabriel, Nucl. Instr. and Meth . A238 (1985) 489. T.A . Gabriel and W. Schmidt, Nucl . Instr. and Meth . 134 (1976) 271 . T.A . Gabriel, Nucl . Instr. and Meth . 150 (1978) 146 . T.A . Gabriel, ORNL/TM-9727, Oak Ridge National Laboratory (1985) . [10] H.W . Bertini, Phys. Rev. 188 (1969) 1711 . [11] R.L . Ford and W.R . Nelson, SLAC-0210 (1978) . [12] M.B. Emmett, ORNL-4972, Oak Ridge National Laboratory (1975) . [13] N.M . Greene et al ., ORNL/TM-3706, Oak Ridge National Laboratory (1973) . [14] J.B. Birks, Proc. Phys . Soc. A64 (1951) 874. [15] Particle Data Group, Rev. Mod. Phys . 56 (1984) Sl . [16] R. Wigmans, Nucl. Instr. and Meth . A259 (1987) 389. [17] G. Keil, Nucl. Instr. and Meth. 87 (1970) 111 . [18] T. Xkesson et al ., Nucl . Instr. and Meth. A262 (1987) 243. [19] O. Bouner et al., Nucl . Instr. and Meth . 179 (1981) 45 . [20] P. Rapp et al ., Nucl . Instr. and Meth . 188 (1981) 285. [21] B. Andersson et al ., Nucl . Phys . B281 (1987) 289; B. Nilsson-Almgvist and E. Stenlund, Comp . Phys . Commun. 43 (1987) 387. [22] C.W . Fabjan et al ., Nucl. Instr. and Meth . 141 (1977) 61 . [23] U. Amaldi, Phys . Scripta 23 (1981) 409. [24] C.W . Fabjan and T. Ludlam, Annu . Rev. Nucl . Part . Sci. 32 (1982) 335. [25] J. Engler, Nucl . Instr. and Meth. A235 (1985) 301. [26] C. De Marzo et al ., Nucl . Instr. and Meth . 217 (1983) 405 . [27] Y. Sirois and R. Wigmans, Nucl . Instr. and Meth . A240 (1985) 262. [28] H. Abramowicz et al ., Nucl . Instr. and Meth . 180 (1981) 429. [29] M. Della Negra, Phys. Scripta 23 (1981) 469. [30] O. Botmer, Thesis, Niels Bohr Institute, Copenhagen (1985) unpublished . [31] R.L . Parker, Annu . Rev. Earth Planet . Sci. 5 (1977) 35 . [32] N. Gastinel, Linear Numerical Analysis (Academic Press, New York, 1970) . [33] B.R . Frieden, Opt. Soc. Amer . 62 (1972) 511; S.F. Gull and G.J . Daniell, Nature 272 (1978) 686.