- Email: [email protected]
Performances of zero degree calorimeters for the ALICE experiment
Nuclear Instruments and Methods in Physics Research A 456 (2001) 248}258
Performances of zero degree calorimeters for the ALICE experiment R. Arnaldi , E. Chiavassa , C. Cicalo`, P. Cortese , A. De Falco, G. Dellacasa, N. De Marco , A. Ferretti , M. Gallio , P. Macciotta, A. Masoni, A. Musso , C. Oppedisano *, A. Piccotti , G. Puddu, E. Scalas, E. Scomparin , S. Serci, E. Siddi, C. Soave , G. Usai, E. Vercellin Department of Physics, Universita% di Torino, INFN Sezione di Torino, Via Giuria 1, Torino 10125, Italy Universita% and INFN, Cagliari, Italy Universita% del Piemonte Orientale, Alessandria and INFN, Torino, Italy Received 12 April 2000; accepted 15 May 2000
Abstract Results on the performances of two hadron calorimeter prototypes for the ALICE experiment are presented. The two prototypes are made of brass and copper as passive material and PMMA "bres, arranged in a spaghetti calorimeter geometry, as active material. The calorimeters were tested at the CERN SPS, using hadron and positron beams of variable momentum in the range 50}200 GeV/c. The main features of the detectors, such as linearity, energy resolution and uniformity of response with respect to the beam impact point on their front face, are presented. The experimental results are compared to Monte Carlo simulations. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj; 42.81.Wg; 42.70.Ce; 42.88.#h Keywords: Sampling calorimeters; Quartz optical "bres; Radiation hardness
1. Introduction The aim of the ALICE experiment, planned at the forthcoming LHC collider at CERN, is to study the phase of matter in which quarks and gluons are decon"ned, the Quark}Gluon Plasma (QGP). Such a phase transition is predicted by non-perturbative lattice QCD calculations to occur when high values
* Corresponding author. Tel.: #39-011-670-7375; fax: #39011-670-7386. E-mail address: [email protected] (C. Oppedisano).
of temperature (¹'180 MeV) and energy density (e&2}3 GeV/fm) are reached. Central lead}lead ion collisions at a CM energy of (s"5.5 TeV per nucleon are expected to provide such conditions. The LHC collider will operate at a luminosity value of 10 cm\ s\. A rate of 10 minimum bias lead}lead ion collisions per second is expected. Among them only &10% will correspond to the more interesting central events. The centrality of the collision will be determined measuring the energy carried by non-interacting nucleons #ying at 03 with respect to the beam direction, by means of Zero Degree Calorimeters (ZDCs) [1]. Two sets of calorimeters will be located
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 5 8 6 - 6
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at &115 m from both sides of the beam interaction point (IP). The spectator protons and neutrons will be separated from the ion beams using the separator magnet D1 of the LHC beam optics [1] and respectively detected by proton (ZP) and neutron (ZN) calorimeters. The ALICE ZDCs should have an energy resolution comparable with the spectator energy #uctuations for a given impact parameter, which ranges from +20% for central events to +5% for peripheral ones (according to simulations using HIJING [2] as event generator). Therefore, the required resolution for an incident nucleon of 2.7 TeV should be of the order of 10%. The transverse size of both ZP and ZN calorimeters is severely constrained since they will be placed very close to the beam pipes, so that calorimeters with a limited shower transverse size are required. Moreover, the calorimeters will operate in a radiation compelling environment (&10 Gy/day will be released in the ZN at the foreseen luminosity), therefore they have to be radiation hard detectors. These constraints can be ful"lled by calorimeters which make use of quartz "bres as active material [3]. The charged particles from the shower generated in the absorber produce Cherenkov light in quartz "bres and the light is optically guided by the same "bres to the photodetectors. A quartz "bre calorimeter has been succesfully used in the heavy ion "xed target experiment NA50 at the CERN SPS [4], where the radiation damage is 10 times higher than the one expected at the LHC. Another advantage of this detection technique is the intrinsic speed of the Cherenkov e!ect which makes these detectors suitable for triggering purposes. In this paper, we report on the performances of two prototypes of the proton calorimeters (ZPs). The results of the experimental tests carried out at the CERN SPS will also be compared to Monte Carlo simulations.
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test was to compare various "lling ratios and "lling geometries by using two di!erent "bre diameters and "bre spacing. The prototypes have in addition di!erent absorber materials, brass and copper, which are very similar for what concerns their physical characteristics (density, interaction and radiation length). Both are easy to machine and relatively cheap and are characterized by a reduced neutron production due to the low Z [5]. The calorimeters, named ZPc and ZPb (c for copper and b for brass), were equipped with polymethylmethacrylate (PMMA) "bres, much less expensive than the quartz ones, since the radiation hardness of the active material was not a demanding task for these tests. The highest light yield is obtained with "bres oriented at almost 453 with respect to the beam direction. With "bres parallel to the beam axis the signal is reduced, but the energy resolution is not signi"cantly worse [6]; in this case it is much easier to couple the "bres to the photomultipliers. We adopted this solution for our prototypes. Four Philips XP2020 photomultipliers were used for ZPc and two for ZPb. Each PM collected the light from a subset of "bres uniformly distributed in the passive material (see Fig. 1); in this way, considering one or more PMs, we could obtain di!erent "lling ratios. The "bre spacing has to be smaller than the radiation length of the passive material in order to avoid the absorption of the shower electrons from the electromagnetic component before they reach the "bres.
2. Proton calorimeter prototypes Besides the usual measurements of linearity, energy resolution, shower transverse size and uniformity of the detector response, the purpose of the
Fig. 1. Fibre position on the front face of the ZPc and ZPb prototypes. The way the "bres are connected to the di!erent photomultipliers is also shown.
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The characteristics of the two prototypes are summarized in Table 1. The ZPc (ZPb) calorimeter was built by stacking 40 (26) grooved plates 4 mm (6 mm) thick of copper (brass) to form a parallelepiped of (16;16;150) cm. The length corresponds to about 10 interaction lengths. The "bres, placed in the grooves, are 250 cm long. The "rst 150 cm, inside the absorber matrix, represent the active part Table 1 Characteristics of the two calorimeter prototypes
Dimensions (h, w, l) Filling ratios Absorber Absorber density Absorber interaction length Absorber radiation length Number of plates PMMA "bres diameter Number of "bres Fibre spacing Number of PMs
ZPc
ZPb
(16;16;150) cm 1/325, 1/162, 1/80 Copper 8.96 g/cm 15.06 cm
(16;16;150) cm 1/170, 1/85 Brass 8.28 g/cm 18.4 cm
1.43 cm
1.5 cm
40 500 lm
26 750 lm
1600 4 mm 4
676 6 mm 2
of the calorimeter, the following 100 cm, bent at 90, act as light guides (see Fig. 2).
3. Experimental set-up The calorimeters have been tested at the H6 beam line of the CERN SPS. The two prototypes were irradiated with secondary and tertiary positron and hadron beams of momentum ranging from 50 to 180 GeV/c. A schematic layout of the test set-up is shown in Fig. 3. A system of scintillators S }S provided two di!erent triggers, selecting incident beams of 1;1 cm or 2;2 cm cross area. Two scintillator sticks D and D tagged the events inside an 4 & area of 1;1 mm. Each calorimeter was put on a X and Y movable platform. A MWPC, placed in front of them, was used to determine the beam impact point on the front face of the calorimeters with an accuracy of &200 lm. An iron wall beyond the calorimeters "ltered muons present in the beam, which were detected by MU and MU scintillators. The PM output signals were read out by LeCroy 2249 ADCs.
Fig. 2. Perspective view of the proton calorimeter prototype ZPc.
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Fig. 3. Schematic view of the experimental set-up used in the test. S }S : trigger scintillators. D , D : scintillator sticks. MU , 4 & MU : scintillators for muon detection.
4. Monte Carlo simulations The Monte Carlo simulations are performed with the GEANT 3.21 code, using the GHEISHA [7] package for the simulation of hadronic interactions. Cherenkov light production and transport along the "bres are evaluated using a separate program. It produces the distribution of Cherenkov light yield in optical "bres as a function of particle velocity, distance from the "bre axis and incident angle of the particle trajectory. Sets of data tables are produced taking into account the numerical aperture of the "bre, its attenuation length as a function of the light wavelength and the quantum e$ciency of the photocathode of the PM coupled to the "bres. A shower particle is tracked by GEANT in the absorber until it transverses a "bre, then the photon yield is directly read from the tables. The conversion factor between the Monte Carlo results and the experimental data was obtained both measuring the PM gain and by means of a direct measurement of the number of photoelectrons per ADC channel (see Section 5.1). The experimental results, whenever possible, will be compared with the simulation.
5. Detector calibration 5.1. Number of photoelectrons per ADC channel To achieve an absolute calibration of the detectors in terms of photoelectron produced per GeV of energy of the incident particle, we had to know the gain of the PMs. To do that we used the GEANT 3.21, CERN Program Library Long Writeup W5013.
Fig. 4. ADC spectrum with muon selection for a ZPc PM. The single photoelectron peak is evident.
events in which a single muon passes through the calorimeter. In this case, the amount of energy deposited is very small, giving rise to a low signal generated by a few photoelectrons. We determined the amplitude of the single photoelectron peak by means of these events. As an example in Fig. 4 the ADC spectrum of a ZPc PM for a muon event is shown, "tted with the function f(x)"P e\V\I N#P e\V\I N. The con version factor between ADC channels and photoelectrons is 7.4$0.5 ch/phe, for a ZPb PM we found a value of 6.8$0.5 ch/phe. The PM gains were also measured in laboratory, by use of a spark light source with a very stable intensity during the measurement period. The emitted light was "ltered and attenuated in order to reach the single photoelectron response condition. In this case we obtained a conversion factor for the ZPc PM, of 7.58 ch/phe; this value is in good agreement with the previous result. Using the linear correlation between ADC channels and energy deposited in the calorimeter, we
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"nd, for both calorimeters, a value of 0.8 photoelectrons produced per GeV for hadronic beams and 1.2 photoelectrons per GeV for positron beams. 5.2. PMs calibration The PM responses were "rst equalized on-line during the test by sending a hadron beam in the centre of the front face of the calorimeters. The small size of the electromagnetic shower prevents the use of positron beam. Since the "bres seen by each PM are uniformly distributed inside the whole calorimeter, the PMs high voltages were carefully tuned to obtain the same mean value of the ADC pulse height spectrum. A further "ne o!-line tuning was then performed to compensate possible small di!erences. The calibration was monitored through the complete run period and it turned out to be stable within 2}3%.
6. Experimental results 6.1. Analysis of the ADC spectra After pedestal subtraction [1] and rejection of muon events, the ADC spectra were "tted with the function f(x)"Ne\V\I NV, where the term p(x)"p #p (x!k)/k accounts for the asym metry of the distribution. The non-Gaussian shape of the spectra for hadrons is due to the fact that the
calorimeter is not compensating. Moreover it has been shown [8] that the response for pions and protons is slightly di!erent. It could be the cause of asymmetry in the ADC spectra. Fig. 5 shows the typical "t to the ADC spectra for hadrons and positrons. The positron beams at the highest energies used have a hadronic contamination. We thus "tted these distributions with a superposition of two asymmetric functions. It was possible to resolve the hadron and the positron peaks only when the sum of all the PM signals, corresponding to the highest "lling ratio of about 1/80, was considered. For lower "lling ratios, the two peaks are not resolved and therefore the measurement of the resolution for positrons at these energies is not reliable. 6.2. Linearity The linearity of the response as a function of the beam energy for the two prototypes was studied both with hadron and positron beams. The results are shown in Fig. 6. The data show a very good linearity. An energy threshold of few GeV, reproduced by simulations, can be noticed in the hadron data. 6.3. e/h ratio It is possible to determine the ratio e/h from the measured experimental data e/p through the
Fig. 5. Fit to the ADC spectra for 100 GeV hadrons and 75 GeV positrons (ZPc prototype).
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Fig. 6. Response as a function of the hadron (left) and positron (right) beam energy E, for the ZPc (top) and ZPb (bottom) prototypes. Data are "tted with straight lines.
relation e/h e (E)" p 1!f (1!e/h)
Table 2 The e/p ratios for di!erent beam energies for ZPc and ZPb prototypes
(1)
where f is the em fraction of the hadronic shower, which can be parametrized as a function of energy as f "0.12 ln E [5]. Two corrections to the experimental e/p data have to be applied. The "rst correction is due to the fact that the hadronic shower is not completely contained in the calorimeter, while it is for the electromagnetic one; from simulation we estimated this correction factor to be around 20%. The second correction takes into account the fact that electromagnetic showers develop before the hadronic ones, so the light produced by electron beams
Beam energy (GeV)
e/p for ZPc
e/p for ZPb
50 100 120 150
1.36 1.23 1.17 *
1.34 1.30 * 1.04
is more attenuated; this correction is very small (less than 5%). The estimated e/p ratios for 50, 100 and 120 GeV for both ZPc and ZPb are listed in Table 2. We found an experimental e/h value of 1.90 for ZPc and of 1.84 for ZPb.
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6.4. Energy resolution The energy resolution was determined, for all the hadron and positron beam energies, as the ratio R"p /k, p being the local width for x"k and k the peak value. The results are reported in Tables 3 and 4 and plotted in Fig. 7 for the two prototypes. In Fig. 8, the resolution plotted for di!erent "lling ratios is "tted with the simple formula a/(E(GeV). A possible (small) constant term in resolution cannot be constrained by the experimental points and therefore has been neglected. Table 3 Peak value, width and resolution of Cherenkov light distributions as a function of the hadron beam energy for the two prototypes p (phe)
Resolution (%)
ZPc hadrons } data 50 34.7$0.2 100 80.7$0.5 120 97.7$1.4 150 124.4$7.1 180 146.5$0.9
11.1$0.2 19.5$0.4 22.2$0.7 25.4$2.0 27.6$0.6
32.0$0.7 24.2$0.5 22.8$0.8 20.4$2.0 18.9$0.4
ZPb hadrons } data 50 38.0$0.7 100 83.3$6.6 150 130.3$0.8 180 150.7$0.9
14.1$0.6 20.3$1.4 26.1$0.6 27.6$0.7
37.2$1.7 24.3$2.5 20.0$0.5 18.3$0.5
E (GeV)
k (phe)
The simulations for hadrons underestimate the resolution by about 25%, while for positrons we have a better agreement (as can be seen in Fig. 9). The understanding of this disagreement is still under study. 6.5. Shower's transverse size In Fig. 10, we plot, the response of the ZPc prototype as a function of the beam impact point on its front face determined by the MWPC. The black circles are the experimental data, while the open circles are the results of the simulation. We
Table 4 Peak value, width and resolution of Cherenkov light distributions as a function of the positron beam energy for the two prototypes p (phe)
Resolution (%)
ZPc positrons } data 50 60.2$0.4 75 91.5$0.5 100 118.7$0.7 120 141.7$0.8
10.6$0.3 14.0$0.3 15.3$0.4 17.1$0.4
17.6$0.5 15.3$0.4 12.9$0.4 12.0$0.3
ZPb positrons } data 50 64.1$0.5 100 129.5$3.1 150 173.4$6.1
12.0$0.4 17.1$0.6 18.1$4.4
18.7$0.6 13.2$0.5 10.4$2.6
E (GeV)
k (phe)
Fig. 7. Resolution (p(E)/E) as a function of 1/(E for the ZPc and ZPb prototypes for hadrons (left) and positrons (right).
R. Arnaldi et al. / Nuclear Instruments and Methods in Physics Research A 456 (2001) 248}258
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Fig. 8. Top: resolution (p(E)/E) as a function of 1/(E for the ZPc prototype in hadrons (left) and positrons (right) considering di!erent "lling ratios. Bottom: same for ZPb.
also plot the data validated by the 1;1 mm trigger. The Monte Carlo and measured pro"les are in good agreement. In the lower side of the plot, the derivatives of the previous distributions are calculated and "tted with a superposition of two Gaussians whose relative widths and normalizations are estimated by the simulation. Due to the limited number of experimental points near the edge of the calorimeter, we "tted the derivative of the Monte Carlo distribution, since it is in good agreement with the experimental distribution. We obtained a transverse size of 2.2 cm (FWHM). Applying the same procedure to the positron beam sample, we were also able to obtain a direct measurement from the data; in this case the value is of 0.7 cm (FWHM).
6.6. Uniformity of the response We studied the uniformity of the calorimeters response, again using the information from the MWPC. In Fig. 11, we report the ratio of the light seen by two PMs over the sum of the four PMs for ZPc, and of one PM over the sum of two PMs for ZPb (corresponding in both cases to a "lling ratio of about 1/160), as a function of the beam impact point. The response for hadrons is almost uniform on the whole front face of both calorimeters. For positrons, for which the transverse shower dimensions are of the order of "bre spacing, the ZPc calorimeter shows a lower dependence on the positron beam impact point. Thus, for a "lling ratio of 1/160
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Fig. 9. Resolution (p(E)/E) for the ZPc (top) and ZPb (bottom) prototypes in hadrons (left) and positrons (right) compared with the Monte Carlo simulation.
the choice of a 4 mm "bre spacing guarantees a su$ciently uniform response. 7. Conclusions We designed and tested two prototypes of proton calorimeters for the ALICE experiment. The detectors have shown a good linearity, with a light yield for hadrons of 0.8 photoelectrons per GeV of incident energy. The experimental energy resolution for hadrons is well described by the simple formula a/(E(GeV) without a constant term. The response of the calorimeters is uniform with respect to the beam impact point on their front faces. From the experimental results we conclude that the two prototypes have very similar performances
for what concerns light yield and energy resolution; ZPc has shown a better uniformity, due to the smaller "bre spacing. For this reason, a calorimeter with the characteristics of the ZPc prototype seems a good candidate for the proton calorimeters of the ALICE experiment. Probably, since the brass is easier to machine than copper it will be chosen as passive material. Monte Carlo simulations reproduce very well the uniformity and also the transverse size of the hadronic showers but underestimate the energy resolution by about 25%. The ALICE proton calorimeters will be equipped with quartz "bres, which have a lower numerical aperture (0.22 compared to 0.5 for PMMA "bres), so the expected photoelectron yield will be about 0.3 photoelectrons per GeV. To have an
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Fig. 10. Top: Response of the ZPc prototype as a function of the horizontal beam coordinate on the front face of the detector for hadrons (left) and positrons (right). Bottom: derivative of the above distributions and "ts.
estimate of the energy resolution at LHC, we performed simulations of an ALICE ZP, with the same structure of ZPc, with hadrons up to 2760 GeV and the points obtained were "tted with the function a/(E(GeV)#b. The constant term is not anymore negligible at these high energies because #uctuations in the lateral energy loss are dominant compared to the statistical #uctuations in the number of photoelectrons. The resolution estimated from the simulations is &10% which qualitatively agrees with the energy resolution needed for ALICE.
Acknowledgements
Fig. 11. Light seen by two PMs over the sum of the four PMs for ZPc (left) and by one PM over the sum of two PMs for ZPb (right), as a function of the beam impact coordinates for hadrons and positrons (see text for details).
We wish to thank all the people who contributed to the design, construction and successful operation of the calorimeters. In particular, the technical support provided by M. Arba and M. Tuveri from INFN Cagliari and G. Alfarone from INFN
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Torino, and the help and suggestions of K. Elsener during the test were greatly appreciated.
References [1] Zero Degree Calorimeter Technical Design Report, CERN/LHCC, 1999.
[2] [3] [4] [5]
X.N. Wang, M. Gyulassy, Phys. Rev. D 44 (1991) 3501. P. Gorodetzky et al., Nucl. Instr. and Meth. A 361 (1995) 161. R. Arnaldi et al., Nucl. Instr. and Meth. A 411 (1998) 1. O. Ganel, R. Wigmans, Nucl. Instr. and Meth. A 365 (1995) 104. [6] G. Anzivino et al., Nucl. Instr. and Meth. A 357 (1995) 380. [7] H.C. Fesefeldt, Technical Report PITHA 85-02, III Physikalisches Institut, RWTH Aachen Physikzentrum. [8] N. Akchurin et al., Nucl. Instr. and Meth. A 399 (1997) 202.
Performances of zero degree calorimeters for the ALICE experiment R. Arnaldi , E. Chiavassa , C. Cicalo`, P. Cortese , A. De Falco, G. Dellacasa, N. De Marco , A. Ferretti , M. Gallio , P. Macciotta, A. Masoni, A. Musso , C. Oppedisano *, A. Piccotti , G. Puddu, E. Scalas, E. Scomparin , S. Serci, E. Siddi, C. Soave , G. Usai, E. Vercellin Department of Physics, Universita% di Torino, INFN Sezione di Torino, Via Giuria 1, Torino 10125, Italy Universita% and INFN, Cagliari, Italy Universita% del Piemonte Orientale, Alessandria and INFN, Torino, Italy Received 12 April 2000; accepted 15 May 2000
Abstract Results on the performances of two hadron calorimeter prototypes for the ALICE experiment are presented. The two prototypes are made of brass and copper as passive material and PMMA "bres, arranged in a spaghetti calorimeter geometry, as active material. The calorimeters were tested at the CERN SPS, using hadron and positron beams of variable momentum in the range 50}200 GeV/c. The main features of the detectors, such as linearity, energy resolution and uniformity of response with respect to the beam impact point on their front face, are presented. The experimental results are compared to Monte Carlo simulations. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Vj; 42.81.Wg; 42.70.Ce; 42.88.#h Keywords: Sampling calorimeters; Quartz optical "bres; Radiation hardness
1. Introduction The aim of the ALICE experiment, planned at the forthcoming LHC collider at CERN, is to study the phase of matter in which quarks and gluons are decon"ned, the Quark}Gluon Plasma (QGP). Such a phase transition is predicted by non-perturbative lattice QCD calculations to occur when high values
* Corresponding author. Tel.: #39-011-670-7375; fax: #39011-670-7386. E-mail address: [email protected] (C. Oppedisano).
of temperature (¹'180 MeV) and energy density (e&2}3 GeV/fm) are reached. Central lead}lead ion collisions at a CM energy of (s"5.5 TeV per nucleon are expected to provide such conditions. The LHC collider will operate at a luminosity value of 10 cm\ s\. A rate of 10 minimum bias lead}lead ion collisions per second is expected. Among them only &10% will correspond to the more interesting central events. The centrality of the collision will be determined measuring the energy carried by non-interacting nucleons #ying at 03 with respect to the beam direction, by means of Zero Degree Calorimeters (ZDCs) [1]. Two sets of calorimeters will be located
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 5 8 6 - 6
R. Arnaldi et al. / Nuclear Instruments and Methods in Physics Research A 456 (2001) 248}258
at &115 m from both sides of the beam interaction point (IP). The spectator protons and neutrons will be separated from the ion beams using the separator magnet D1 of the LHC beam optics [1] and respectively detected by proton (ZP) and neutron (ZN) calorimeters. The ALICE ZDCs should have an energy resolution comparable with the spectator energy #uctuations for a given impact parameter, which ranges from +20% for central events to +5% for peripheral ones (according to simulations using HIJING [2] as event generator). Therefore, the required resolution for an incident nucleon of 2.7 TeV should be of the order of 10%. The transverse size of both ZP and ZN calorimeters is severely constrained since they will be placed very close to the beam pipes, so that calorimeters with a limited shower transverse size are required. Moreover, the calorimeters will operate in a radiation compelling environment (&10 Gy/day will be released in the ZN at the foreseen luminosity), therefore they have to be radiation hard detectors. These constraints can be ful"lled by calorimeters which make use of quartz "bres as active material [3]. The charged particles from the shower generated in the absorber produce Cherenkov light in quartz "bres and the light is optically guided by the same "bres to the photodetectors. A quartz "bre calorimeter has been succesfully used in the heavy ion "xed target experiment NA50 at the CERN SPS [4], where the radiation damage is 10 times higher than the one expected at the LHC. Another advantage of this detection technique is the intrinsic speed of the Cherenkov e!ect which makes these detectors suitable for triggering purposes. In this paper, we report on the performances of two prototypes of the proton calorimeters (ZPs). The results of the experimental tests carried out at the CERN SPS will also be compared to Monte Carlo simulations.
249
test was to compare various "lling ratios and "lling geometries by using two di!erent "bre diameters and "bre spacing. The prototypes have in addition di!erent absorber materials, brass and copper, which are very similar for what concerns their physical characteristics (density, interaction and radiation length). Both are easy to machine and relatively cheap and are characterized by a reduced neutron production due to the low Z [5]. The calorimeters, named ZPc and ZPb (c for copper and b for brass), were equipped with polymethylmethacrylate (PMMA) "bres, much less expensive than the quartz ones, since the radiation hardness of the active material was not a demanding task for these tests. The highest light yield is obtained with "bres oriented at almost 453 with respect to the beam direction. With "bres parallel to the beam axis the signal is reduced, but the energy resolution is not signi"cantly worse [6]; in this case it is much easier to couple the "bres to the photomultipliers. We adopted this solution for our prototypes. Four Philips XP2020 photomultipliers were used for ZPc and two for ZPb. Each PM collected the light from a subset of "bres uniformly distributed in the passive material (see Fig. 1); in this way, considering one or more PMs, we could obtain di!erent "lling ratios. The "bre spacing has to be smaller than the radiation length of the passive material in order to avoid the absorption of the shower electrons from the electromagnetic component before they reach the "bres.
2. Proton calorimeter prototypes Besides the usual measurements of linearity, energy resolution, shower transverse size and uniformity of the detector response, the purpose of the
Fig. 1. Fibre position on the front face of the ZPc and ZPb prototypes. The way the "bres are connected to the di!erent photomultipliers is also shown.
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The characteristics of the two prototypes are summarized in Table 1. The ZPc (ZPb) calorimeter was built by stacking 40 (26) grooved plates 4 mm (6 mm) thick of copper (brass) to form a parallelepiped of (16;16;150) cm. The length corresponds to about 10 interaction lengths. The "bres, placed in the grooves, are 250 cm long. The "rst 150 cm, inside the absorber matrix, represent the active part Table 1 Characteristics of the two calorimeter prototypes
Dimensions (h, w, l) Filling ratios Absorber Absorber density Absorber interaction length Absorber radiation length Number of plates PMMA "bres diameter Number of "bres Fibre spacing Number of PMs
ZPc
ZPb
(16;16;150) cm 1/325, 1/162, 1/80 Copper 8.96 g/cm 15.06 cm
(16;16;150) cm 1/170, 1/85 Brass 8.28 g/cm 18.4 cm
1.43 cm
1.5 cm
40 500 lm
26 750 lm
1600 4 mm 4
676 6 mm 2
of the calorimeter, the following 100 cm, bent at 90, act as light guides (see Fig. 2).
3. Experimental set-up The calorimeters have been tested at the H6 beam line of the CERN SPS. The two prototypes were irradiated with secondary and tertiary positron and hadron beams of momentum ranging from 50 to 180 GeV/c. A schematic layout of the test set-up is shown in Fig. 3. A system of scintillators S }S provided two di!erent triggers, selecting incident beams of 1;1 cm or 2;2 cm cross area. Two scintillator sticks D and D tagged the events inside an 4 & area of 1;1 mm. Each calorimeter was put on a X and Y movable platform. A MWPC, placed in front of them, was used to determine the beam impact point on the front face of the calorimeters with an accuracy of &200 lm. An iron wall beyond the calorimeters "ltered muons present in the beam, which were detected by MU and MU scintillators. The PM output signals were read out by LeCroy 2249 ADCs.
Fig. 2. Perspective view of the proton calorimeter prototype ZPc.
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Fig. 3. Schematic view of the experimental set-up used in the test. S }S : trigger scintillators. D , D : scintillator sticks. MU , 4 & MU : scintillators for muon detection.
4. Monte Carlo simulations The Monte Carlo simulations are performed with the GEANT 3.21 code, using the GHEISHA [7] package for the simulation of hadronic interactions. Cherenkov light production and transport along the "bres are evaluated using a separate program. It produces the distribution of Cherenkov light yield in optical "bres as a function of particle velocity, distance from the "bre axis and incident angle of the particle trajectory. Sets of data tables are produced taking into account the numerical aperture of the "bre, its attenuation length as a function of the light wavelength and the quantum e$ciency of the photocathode of the PM coupled to the "bres. A shower particle is tracked by GEANT in the absorber until it transverses a "bre, then the photon yield is directly read from the tables. The conversion factor between the Monte Carlo results and the experimental data was obtained both measuring the PM gain and by means of a direct measurement of the number of photoelectrons per ADC channel (see Section 5.1). The experimental results, whenever possible, will be compared with the simulation.
5. Detector calibration 5.1. Number of photoelectrons per ADC channel To achieve an absolute calibration of the detectors in terms of photoelectron produced per GeV of energy of the incident particle, we had to know the gain of the PMs. To do that we used the GEANT 3.21, CERN Program Library Long Writeup W5013.
Fig. 4. ADC spectrum with muon selection for a ZPc PM. The single photoelectron peak is evident.
events in which a single muon passes through the calorimeter. In this case, the amount of energy deposited is very small, giving rise to a low signal generated by a few photoelectrons. We determined the amplitude of the single photoelectron peak by means of these events. As an example in Fig. 4 the ADC spectrum of a ZPc PM for a muon event is shown, "tted with the function f(x)"P e\V\I N#P e\V\I N. The con version factor between ADC channels and photoelectrons is 7.4$0.5 ch/phe, for a ZPb PM we found a value of 6.8$0.5 ch/phe. The PM gains were also measured in laboratory, by use of a spark light source with a very stable intensity during the measurement period. The emitted light was "ltered and attenuated in order to reach the single photoelectron response condition. In this case we obtained a conversion factor for the ZPc PM, of 7.58 ch/phe; this value is in good agreement with the previous result. Using the linear correlation between ADC channels and energy deposited in the calorimeter, we
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"nd, for both calorimeters, a value of 0.8 photoelectrons produced per GeV for hadronic beams and 1.2 photoelectrons per GeV for positron beams. 5.2. PMs calibration The PM responses were "rst equalized on-line during the test by sending a hadron beam in the centre of the front face of the calorimeters. The small size of the electromagnetic shower prevents the use of positron beam. Since the "bres seen by each PM are uniformly distributed inside the whole calorimeter, the PMs high voltages were carefully tuned to obtain the same mean value of the ADC pulse height spectrum. A further "ne o!-line tuning was then performed to compensate possible small di!erences. The calibration was monitored through the complete run period and it turned out to be stable within 2}3%.
6. Experimental results 6.1. Analysis of the ADC spectra After pedestal subtraction [1] and rejection of muon events, the ADC spectra were "tted with the function f(x)"Ne\V\I NV, where the term p(x)"p #p (x!k)/k accounts for the asym metry of the distribution. The non-Gaussian shape of the spectra for hadrons is due to the fact that the
calorimeter is not compensating. Moreover it has been shown [8] that the response for pions and protons is slightly di!erent. It could be the cause of asymmetry in the ADC spectra. Fig. 5 shows the typical "t to the ADC spectra for hadrons and positrons. The positron beams at the highest energies used have a hadronic contamination. We thus "tted these distributions with a superposition of two asymmetric functions. It was possible to resolve the hadron and the positron peaks only when the sum of all the PM signals, corresponding to the highest "lling ratio of about 1/80, was considered. For lower "lling ratios, the two peaks are not resolved and therefore the measurement of the resolution for positrons at these energies is not reliable. 6.2. Linearity The linearity of the response as a function of the beam energy for the two prototypes was studied both with hadron and positron beams. The results are shown in Fig. 6. The data show a very good linearity. An energy threshold of few GeV, reproduced by simulations, can be noticed in the hadron data. 6.3. e/h ratio It is possible to determine the ratio e/h from the measured experimental data e/p through the
Fig. 5. Fit to the ADC spectra for 100 GeV hadrons and 75 GeV positrons (ZPc prototype).
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Fig. 6. Response as a function of the hadron (left) and positron (right) beam energy E, for the ZPc (top) and ZPb (bottom) prototypes. Data are "tted with straight lines.
relation e/h e (E)" p 1!f (1!e/h)
Table 2 The e/p ratios for di!erent beam energies for ZPc and ZPb prototypes
(1)
where f is the em fraction of the hadronic shower, which can be parametrized as a function of energy as f "0.12 ln E [5]. Two corrections to the experimental e/p data have to be applied. The "rst correction is due to the fact that the hadronic shower is not completely contained in the calorimeter, while it is for the electromagnetic one; from simulation we estimated this correction factor to be around 20%. The second correction takes into account the fact that electromagnetic showers develop before the hadronic ones, so the light produced by electron beams
Beam energy (GeV)
e/p for ZPc
e/p for ZPb
50 100 120 150
1.36 1.23 1.17 *
1.34 1.30 * 1.04
is more attenuated; this correction is very small (less than 5%). The estimated e/p ratios for 50, 100 and 120 GeV for both ZPc and ZPb are listed in Table 2. We found an experimental e/h value of 1.90 for ZPc and of 1.84 for ZPb.
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6.4. Energy resolution The energy resolution was determined, for all the hadron and positron beam energies, as the ratio R"p /k, p being the local width for x"k and k the peak value. The results are reported in Tables 3 and 4 and plotted in Fig. 7 for the two prototypes. In Fig. 8, the resolution plotted for di!erent "lling ratios is "tted with the simple formula a/(E(GeV). A possible (small) constant term in resolution cannot be constrained by the experimental points and therefore has been neglected. Table 3 Peak value, width and resolution of Cherenkov light distributions as a function of the hadron beam energy for the two prototypes p (phe)
Resolution (%)
ZPc hadrons } data 50 34.7$0.2 100 80.7$0.5 120 97.7$1.4 150 124.4$7.1 180 146.5$0.9
11.1$0.2 19.5$0.4 22.2$0.7 25.4$2.0 27.6$0.6
32.0$0.7 24.2$0.5 22.8$0.8 20.4$2.0 18.9$0.4
ZPb hadrons } data 50 38.0$0.7 100 83.3$6.6 150 130.3$0.8 180 150.7$0.9
14.1$0.6 20.3$1.4 26.1$0.6 27.6$0.7
37.2$1.7 24.3$2.5 20.0$0.5 18.3$0.5
E (GeV)
k (phe)
The simulations for hadrons underestimate the resolution by about 25%, while for positrons we have a better agreement (as can be seen in Fig. 9). The understanding of this disagreement is still under study. 6.5. Shower's transverse size In Fig. 10, we plot, the response of the ZPc prototype as a function of the beam impact point on its front face determined by the MWPC. The black circles are the experimental data, while the open circles are the results of the simulation. We
Table 4 Peak value, width and resolution of Cherenkov light distributions as a function of the positron beam energy for the two prototypes p (phe)
Resolution (%)
ZPc positrons } data 50 60.2$0.4 75 91.5$0.5 100 118.7$0.7 120 141.7$0.8
10.6$0.3 14.0$0.3 15.3$0.4 17.1$0.4
17.6$0.5 15.3$0.4 12.9$0.4 12.0$0.3
ZPb positrons } data 50 64.1$0.5 100 129.5$3.1 150 173.4$6.1
12.0$0.4 17.1$0.6 18.1$4.4
18.7$0.6 13.2$0.5 10.4$2.6
E (GeV)
k (phe)
Fig. 7. Resolution (p(E)/E) as a function of 1/(E for the ZPc and ZPb prototypes for hadrons (left) and positrons (right).
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Fig. 8. Top: resolution (p(E)/E) as a function of 1/(E for the ZPc prototype in hadrons (left) and positrons (right) considering di!erent "lling ratios. Bottom: same for ZPb.
also plot the data validated by the 1;1 mm trigger. The Monte Carlo and measured pro"les are in good agreement. In the lower side of the plot, the derivatives of the previous distributions are calculated and "tted with a superposition of two Gaussians whose relative widths and normalizations are estimated by the simulation. Due to the limited number of experimental points near the edge of the calorimeter, we "tted the derivative of the Monte Carlo distribution, since it is in good agreement with the experimental distribution. We obtained a transverse size of 2.2 cm (FWHM). Applying the same procedure to the positron beam sample, we were also able to obtain a direct measurement from the data; in this case the value is of 0.7 cm (FWHM).
6.6. Uniformity of the response We studied the uniformity of the calorimeters response, again using the information from the MWPC. In Fig. 11, we report the ratio of the light seen by two PMs over the sum of the four PMs for ZPc, and of one PM over the sum of two PMs for ZPb (corresponding in both cases to a "lling ratio of about 1/160), as a function of the beam impact point. The response for hadrons is almost uniform on the whole front face of both calorimeters. For positrons, for which the transverse shower dimensions are of the order of "bre spacing, the ZPc calorimeter shows a lower dependence on the positron beam impact point. Thus, for a "lling ratio of 1/160
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Fig. 9. Resolution (p(E)/E) for the ZPc (top) and ZPb (bottom) prototypes in hadrons (left) and positrons (right) compared with the Monte Carlo simulation.
the choice of a 4 mm "bre spacing guarantees a su$ciently uniform response. 7. Conclusions We designed and tested two prototypes of proton calorimeters for the ALICE experiment. The detectors have shown a good linearity, with a light yield for hadrons of 0.8 photoelectrons per GeV of incident energy. The experimental energy resolution for hadrons is well described by the simple formula a/(E(GeV) without a constant term. The response of the calorimeters is uniform with respect to the beam impact point on their front faces. From the experimental results we conclude that the two prototypes have very similar performances
for what concerns light yield and energy resolution; ZPc has shown a better uniformity, due to the smaller "bre spacing. For this reason, a calorimeter with the characteristics of the ZPc prototype seems a good candidate for the proton calorimeters of the ALICE experiment. Probably, since the brass is easier to machine than copper it will be chosen as passive material. Monte Carlo simulations reproduce very well the uniformity and also the transverse size of the hadronic showers but underestimate the energy resolution by about 25%. The ALICE proton calorimeters will be equipped with quartz "bres, which have a lower numerical aperture (0.22 compared to 0.5 for PMMA "bres), so the expected photoelectron yield will be about 0.3 photoelectrons per GeV. To have an
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Fig. 10. Top: Response of the ZPc prototype as a function of the horizontal beam coordinate on the front face of the detector for hadrons (left) and positrons (right). Bottom: derivative of the above distributions and "ts.
estimate of the energy resolution at LHC, we performed simulations of an ALICE ZP, with the same structure of ZPc, with hadrons up to 2760 GeV and the points obtained were "tted with the function a/(E(GeV)#b. The constant term is not anymore negligible at these high energies because #uctuations in the lateral energy loss are dominant compared to the statistical #uctuations in the number of photoelectrons. The resolution estimated from the simulations is &10% which qualitatively agrees with the energy resolution needed for ALICE.
Acknowledgements
Fig. 11. Light seen by two PMs over the sum of the four PMs for ZPc (left) and by one PM over the sum of two PMs for ZPb (right), as a function of the beam impact coordinates for hadrons and positrons (see text for details).
We wish to thank all the people who contributed to the design, construction and successful operation of the calorimeters. In particular, the technical support provided by M. Arba and M. Tuveri from INFN Cagliari and G. Alfarone from INFN
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Torino, and the help and suggestions of K. Elsener during the test were greatly appreciated.
References [1] Zero Degree Calorimeter Technical Design Report, CERN/LHCC, 1999.
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X.N. Wang, M. Gyulassy, Phys. Rev. D 44 (1991) 3501. P. Gorodetzky et al., Nucl. Instr. and Meth. A 361 (1995) 161. R. Arnaldi et al., Nucl. Instr. and Meth. A 411 (1998) 1. O. Ganel, R. Wigmans, Nucl. Instr. and Meth. A 365 (1995) 104. [6] G. Anzivino et al., Nucl. Instr. and Meth. A 357 (1995) 380. [7] H.C. Fesefeldt, Technical Report PITHA 85-02, III Physikalisches Institut, RWTH Aachen Physikzentrum. [8] N. Akchurin et al., Nucl. Instr. and Meth. A 399 (1997) 202.