AbstractâThe proton Zero Degree Calorimeter (ZP) for the. ALICE experiment will measure the energy of the spectator protons in heavy ion collisions. The ZP is ...
The proton Zero Degree Calorimeter for the ALICE experiment R. Arnaldi, S. Basciu, E. Chiavassa, C. Cical`o, P. Cortese, A. De Falco, G. Dellacasa, N. De Marco, A. Ferretti, M. Floris, M. Gallio, R. Gemme, A. Masoni, P. Mereu, A. Musso, C. Oppedisano, A. Piccotti, F. Poggio, G. Puddu, E. Scomparin, S. Serci, E. Siddi, G. Travaglia, G. Usai, E. Vercellin and F. Yermia
Abstract— The proton Zero Degree Calorimeter (ZP) for the ALICE experiment will measure the energy of the spectator protons in heavy ion collisions. The ZP is a spaghetti calorimeter, which collects and measures the Cherenkov light produced by the shower particles in silica optical fibres embedded in a brass absorber. The details of its construction will be shown. The calorimeter has been tested at the CERN SPS in July 2004 with pion and electron beams of various momenta ranging from 50 to 200 GeV/c. The linearity of the response of the calorimeter and its energy resolution as a function of the beam energy have been measured. The preliminary results of the test are presented. Index Terms— ultrarelativistic heavy ion collisions, calorimeter, centrality, ALICE.
the Cherenkov light produced by the shower particles in silica optical fibers [3]. This technique offers the advantages of high radiation hardness, fast response and reduced lateral dimension of the detectable shower. In addition, quartz-fiber calorimeters are intrinsically insensitive to radio-activation background, which produces particles below the Cherenkov threshold. The ALICE ZDC should have an energy resolution comparable with the fluctuations on the number of spectator nucleons at fixed impact parameter, which range from ≈ 20% for central events to ≈ 5% for peripheral ones, according to simulations that use HIJING as event generator [4]. The linearity of the response as a function of the deposited energy is also an important issue.
I. I NTRODUCTION
T
HE ALICE experiment [1] at the LHC collider is dedicated to the study of ultrarelativistic heavy ion collisions. The primary aim of the experiment is to establish and analyse the formation of the Quark Gluon Plasma (QGP), i.e. the state of matter where quarks and gluons are deconfined. Many QGP signatures (e.g. production of charmonia and bottomonia states, strangeness) depend on the energy density, estimated through the centrality of the collision. Therefore a direct and model-independent measurement of the impact parameter of the collision is essential. In the ALICE experiment the centrality will be determined by measuring the energy carried away by the non-interacting nucleons (”spectators”). The spectator protons and neutrons will be separated from the ion beams, using the separator magnet (D1) of the LHC beam optics [2] and respectively detected by two sets of calorimeters, located at opposite sides with respect to the beam intersection point IP2. Each set of detectors consists of a proton (ZP) and a neutron (ZN) “Zero-degree Calorimeter” (ZDC); the devices will be placed in front of the D2 magnet, 116 meters away from IP2, as shown in fig. 1. The ZDCs are quartz-fiber spaghetti calorimeters that exploit
Manuscript received October 19, 2004. P. Cortese, G. Dellacasa are with Universit`a del Piemonte Orientale, Alessandria and INFN-Gruppo Collegato di Alessandria, Italy S. Basciu, C. Cical`o, A. De Falco, M. Floris, A. Masoni, G. Puddu, S. Serci, E. Siddi, G. Usai are with Universit`a and I.N.F.N. Sezione di Cagliari, Cittadella Universitaria di Monserrato, 09042 Monserrato (Ca), Italy R. Arnaldi, E. Chiavassa, N. De Marco, A. Ferretti, M. Gallio, R. Gemme, P. Mereu, A. Musso, C. Oppedisano, F. Poggio, A. Piccotti, E. Scomparin, G. Travaglia, E. Vercellin and F. Yermia are with Universit`a and I.N.F.N. Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy
II. D ETECTOR DESCRIPTION The size of the spot of the spectator protons where the ZP will be placed depends strongly on the currents in the magnetic elements of the beam line (see fig. 1). The dimensions of the calorimeter are chosen to intercept the maximum number of spectator protons and to obtain a shower containment and an energy resolution of the order of 10%, similar to that of the neutron calorimeter. The proton calorimeter uses brass as absorber material and its dimensions are 22.8x12x150 cm3 .
Fig. 1. Schematic representation of the beam line from the intersection point IP2 to D2 magnets.
The design of the proton calorimeter is conceptually similar to that of the NA50 ZDC [5]. The calorimeter consists of 30 grooved brass plates, each of them 4 mm thick (see fig. 2). The grooves run parallel to the beam axis and host 1680 quartz fibres with a pitch of 4 mm corresponding to a filling ratio, i.e. the ratio of active volume to the absorber volume, of 1/65. The fibres are of HCG-M-550-U type (manufactured by SpecTran Specialty Optics Company, USA) and have a pure silica core, silica fluorinated cladding and a hard polymer coat with a
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Fig. 4. Experimental setup of the test: S1, S2, S3, S4 (trigger scintillators), MU1, MU2 (scintillators for muon detection), MWPC defining beam impact point on ZP front face.
Fig. 2.
Photo of the ZP calorimeter at the end of the assembly phase.
diameter of 550, 600 and 630 µm, respectively. The numerical aperture is 0.22. The use of the quartz fibres is required by the high radiation level of the environment where the calorimeter will operate; the deposited dose in the calorimeter is estimated to be 104 Gy/day at the nominal LHC luminosity of 1027 cm−2 s−1 . The fibres come out from the rear face of the calorimeter and are read by 5 photomultipliers. One every two fibres is sent to a single photomultiplier (PMTc), while the remaining fibres are connected to four different photodetectors (PMT1 to PMT4), collecting the light from four towers: the arrangement can be seen in fig. 3. The five bunches of fibres are directly coupled to five Hamamatsu R329-02 photomultipliers.
Fig. 3.
each tower. A fine tuning of the ADC signals for each channel was made offline to compensate possible small differences. An example of the response of the calorimeter when a 120 GeV/c pion beam was hitting the center of its front face is shown in fig. 5. The pion ADC spectrum is not symmetric. To take into account this fact, we fit the spectrum with the following function: 2 f (x) = Ae
−(x−µ) 2σ(x)2
where: σ(x) = σ0 + σ1
,
(1)
x−µ . µ
(2)
The parameter µ gives the peak value and σ0 gives the width of the distribution. The response of ZP as a function of the beam energy (fig. 6) shows a linear behaviour, at least in this limited energy range for both electrons and pions, despite of the fact that the calorimeter is far from being compensated. Anyway, the proton calorimeter need not to be compensated, since it measures only spectator protons all having the same energy; in fact, the charged particles with a momentum lower than 1.6 TeV/c will be swept away by the separator magnet D1 and the energy due to neutral particles hitting the calorimeter is found to be negligible compared to the energy carried by the spectator protons. The ratio e/π depends on the beam energy and ranges from 2 at 50 GeV/c to 1.6 at 180 GeV/c. Finally, from the plot an energy threshold of 15 GeV can be noticed for the pions.
Schematic connections of the fibres to the PMTs.
sumtot
sum of all 5 PMTs
Counts
2
III. T EST RESULTS The performance of the proton calorimeter was studied in July 2004 at the H6 beam line of the CERN SPS. The aim of the test was to check the response of ZP to hadron and electron beams of various momenta ranging from 50 to 200 GeV/c, the latter being the highest beam energy available on this beam line.
×10
Entries Mean
90
RMS
120 GeV pions
80 70 60
199698 720.7 203.9
p0
9092 ± 26.6
p1
667.4 ± 0.6
p2
166.9 ± 0.4
p3
68.65 ± 0.66
50 40
The experimental setup, as shown in fig. 4, consisted of four plastic scintillators, used as trigger system, the calorimeter placed on a movable platform and a MWPC installed in front of it; two plastic scintillation counters beyond an iron wall detected muons. The analogic signals from the PMTs of the calorimeter were sent to a Lecroy 2249W ADC module. A first equalization of the PMT high voltages was carried out at the beginning of the test, sending the beam at the center of
30 20 10 0 0
Fig. 5.
2
5
10
15
20
25
ZP response to 120 GeV/c pion beam.
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×10 30 35 40 ADC channel
30
ADC channel derivative
250
25
200
20
150
15 FWHM = 14 mm
100
50
0 -40
Fig. 6.
ZP response as a function of the π − and e− beam energy.
derivative
ADC channel
300
10
5
-30
-20
0 -10 0 10 20 30 40 beam horizontal position (mm)
Fig. 8. Response of a single ZP tower as a function of the horizontal impact position of a 120 GeV/c pion beam.
of 14 mm (FWHM) and represents the ”visible” shower size. IV. C ONCLUSION The detector will be used in the ALICE experiment to trigger on the centrality of the collision. In the range of the measured energies the response of the calorimeter for hadrons appears to be linear, even if the calorimeter is not compensated. The measured resolution of the ZP calorimeter, extrapolated to the energy of the single spectator nucleon in ALICE (2.7 TeV) is of the order of 10%; from the simulations [6] it turns out that this value is well below the fluctuations on the number of the spectator nucleons for the central events. σ(E)
Fig. 7. ZP energy resolution ( E ) as a function of the π − and e− beam energy. The arrow indicates the energy of the single spectator nucleon in ALICE (2.7 TeV).
The energy resolution for pions and electrons is shown in E(GeV ). The points are fitted with fig. 7 as a function of 1/ the function a/ E(GeV ) + b. We find a = (201.42 ± 2.4)% and b = (6.6 ± 0.2)% for pions and a = (98.15 ± 1.6)% and b = (2.0±0.2)% for electrons. An extrapolation of the fit to the energy of spectator protons in the ALICE experiment gives an energy resolution of 10%, in agreement with the experiment requirements [6]. In order to estimate the hadronic shower’s transverse size, we studied the response of a single ZP tower as a function of the horizontal beam impact coordinate. The information about the beam position is given by the MWPC. In fig 8 we report the response of a single tower (blue squares) as a function of the horizontal beam impact position on the front face of the calorimeter; the blue line is an arctangent fit to the experimental points. The same figure shows the transverse shower profile (red circles) as the derivative of the data. The red line, which is the derivative of the arctangent function, shows a width of the order
ACKNOWLEDGMENT The authors would like to thank all the people who contributed to the design, construction and successful operation of the calorimeter, in particular M. Arba, L. La Delfa, M. Tuveri and D. Marras from INFN Cagliari and G. Alfarone and R. Farano from INFN Torino, who provided an essential technical support. The help and suggestions of I. Efthymiopoulos during the test were greatly appreciated. R EFERENCES [1] [2] [3] [4] [5] [6]
”ALICE Technical Proposal” CERN/LHCC 95-71 ”LHC Conceptual Design” CERN/AC/95-05 (LHC) P.Gorodetsky et al., NIM, A 361, 1995 161 X.N.Wang and M.Gyulassy, Phys. Rev., vol. D44, 1991 3501 R. Arnaldi et al., NIM A 411 (1998) 1 ”ZDC Technical Design Report” CERN/LHCC 99-5
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