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Nuclear Instruments and Methods in Physics Research A 617 (2010) 126–128

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

E-m

journal homepage: www.elsevier.com/locate/nima

The ATLAS zero degree calorimeter

Sebastian White

Brookhaven National Laboratory, New York, USA

a r t i c l e i n f o

Available online 17 October 2009

Keywords:

Quartz fiber calorimeter

Zero degree calorimeter

Radiation hardness quartz

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2009.09.120

ail address: Sebastian.White@CERN.CH

a b s t r a c t

In May of 2009 the ATLAS zero degree calorimeter was installed in its initial configuration and

integrated into the ATLAS trigger/daq. The detector was designed to measure Global characteristics of

events—particulary in PbPb collisions—through the measurement of energy and position of very

forward neutral particles. Here we discuss the design and tests—particularly of radiation hardness.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The ZDC was proposed as a tool for Heavy Ions where itmeasures collision impact parameter, b, via the multiplicity of‘‘spectator’’ neutrons which lie outside of the overlap region inthese collisions. Their multiplicity is proportional to b. Theforward neutrons have a non-uniform angular distribution aboutthe beam direction (‘‘directed flow’’) which can be used todetermine the interaction plane [1].

These tools for event characterization are unique to Heavy Ionsand are not available in pp collisions. Nevertheless the ZDC couldhave some role in event characterization in pp collisions also. Forexample, simulations show that a tagged forward neutron in theZDC could be a useful tool to resolve diffractive events in ATLAS.Neutron tagged events such as Drell–Yan would be equivalent topþp collisions. The interaction plane measurement and thepossibility to reconstruct neutral decays led us to incorporatecoordinate readout in the ZDC.

2. Design and construction

The ZDC operates in an extremely difficult environment at theLHC. In this location the dose at design luminosity is 6 krad/s,probably higher than any other detector at the LHC. We plan toremove them when the LHC reaches 1

10 of design luminosity andreinsert them for Heavy Ion runs.

Each ZDC calorimeter (there is one in the sector 8-1 TAN absorberand one at sector 1-2) consists of four ZDC modules each 1.14 Lint

(29 X0) thick along the beam direction. We chose a samplingcalorimeter design with 11–1 cm thick Tungsten absorber plates/module and layers of 1.5 mm quartz ‘‘strips’’ for Cherenkov lightproduction and readout. The strips terminate on an air lightguide toa 3 in. PMT. Fused silica (GE-124) is chosen for radiation hardnessand a hybrid of strip layers and penetrating rods provide sufficientlight signal as well as spatial resolution (see Fig. 1).

ll rights reserved.

The active beam area on the front face of the ZDC is10 cm� 10 cm. To measure the impact position of particles theplates were penetrated by a 1 cm� 1 cm matrix of holes and1.0 mm diameter quartz ‘‘rods’’, which pass through the holesand are read out by 1 cm Hamamatsu (R1635) PMTs. The positionreadout principle is shown in Fig. 2. The coordinate of anelectromagnetic shower (Table 1) is calculated from the pulseheight in several rods using the (steep) dependence of this signalon distance from the shower. This measurement for electro-magnetic showers uses only the signal in the first ZDC modulesince it completely contains the showers. For hadron position weuse the same principle in module 2 but 4 rods are read out by eachtube since the intrinsic resolution is worse. Fig. 2b shows oursimulation of the reconstructed 2g mass distribution from 1 MPYTHIA pp events at 14 TeV center-of-mass energy [2].

The strong dependence of response on position together withthe large energy range determines our dynamic range require-ment of 14 bits.

We simulated the ZDC performance in response to high energyneutrons and photons. During pp running at LHC (14 TeV) theprimary species seen by the ZDC below 500 GeV is photonsaccording to PYTHIA. Above this energy neutrons rapidly becomedominant with a small admixture of lambda’s.

The calculated signal of 1000 photoelectrons/TeV was con-firmed in a CERN test beam in 2006. Based on this signal wecalculate the performance parameters given in Table 1.

The energy resolution for neutrons was a design goal since itmakes it possible to clearly resolve peaks in the neutron multi-plicity distribution for heavy ions. This is important both for eventcharacterization and gain monitoring. The photon energy andposition resolution is useful for reconstructing decayed lambdas,for example. This is important for our inclusive neutron analysissince it will be used to correct for un-decayed lambdas. Lambdareconstruction could also be important for energy calibrationsince it uses both the energy of the first (electromagnetic) moduleand the remaining hadronic sections.

Both time and spatial resolution are potentially useful aspart of the luminosity measurement and beam diagnostics

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S. White / Nuclear Instruments and Methods in Physics Research A 617 (2010) 126–128 127

since they are sensitive to the luminosity distribution andbeam angle.

3. Readout and calibration

The PMT signals are sent from the TAN location to the ATLAScounting room (USA15) over 320 m of 100 ohm differential line to40 MHz waveform digitizers built for the ATLAS Level-1 trigger.Each signal is digitized both at gain of 1 and 16 to extend theeffective precision from 10- to 14-bit. The large area PMTs areeffectively digitized at 80 MHz since a copy with 12.5 ns delay isalso digitized.

All PMTs in the ZDC are pulsed with led light flasherswhose purpose is to monitor the PMT gains. The ZDC fast dynodesignals are sent to USA15 over 180 m coaxial cables where signalsfrom each arm are summed and discriminated (originallycorresponding to xF ¼ 0:1). The signals from each ZDC are thenlatched and combined in various triggers. The two arm coin-cidence will also be one of the principle signals for luminositymonitoring.

Energy calibration of the ZDC is most straightforward in HeavyIon running. For example, at full LHC energy evaporation neutronswill have an energy of 2.75 TeV. In minimum bias data there willalso be two neutron and possibly three neutron peaks with alinewidth of sE=E¼ 18% determined by the ZDC resolution. It isalso straightforward to select electromagnetic interactions whichhave almost exclusively one neutron.

For pp running, which will dominate the first year, there isnothing so nice. The calibration will start with balancing the

Fig. 1. ZDC layout. One detector is located 140 m downstream on either side of IP1.

The front modules are instrumented with shower coordinate readout.

Fig. 2. ZDC coordinate reconstruction principle (left)

PMTs. In the case of the 3 in. PMTs the input charge perphotoelectron can be adjusted directly with the High Voltageusing photostatistics on the LED flasher. The adjustment is lessprecise for the 1 cm PMTs since each HV supplies 8PMTs. Nevertheless the gain can be measured using the sameprinciple.

The relative calibration of different modules is initially basedon the assumption that light collection is the same in all modulesbut can also be determined using longitudinal shower profile.The overall energy scale calibration depends somewhat onassumptions unless there is a sharp endpoint to the neutronspectrum.

In any case the final calibration will come with Heavy Ion data.

4. Radiation damage

The calculated radiation dose at the ZDC is so low duringHeavy Ion running (at 1027 s�1 cm�2) that it is not a concern.However, in pp running it reaches 4–5 orders of magnitude higherso we carried out radiation tests. These determine the ppluminosity level at which we can run. While SiO2, used here asa Cherenkov radiator and high light transmission readout, isknown to have excellent radiation hardness, the integrated dose inpp running at full luminosity to quartz rods in the ZDC is expectedto reach 10–15 Grad. At this point loss of transparency is aconcern. We exposed samples of GE124 quartz rods, used in theZDC, to integrated doses of up to 28 Grad using 200 MeV protonsat the Brookhaven Isotope Production Facility.

We measured light attenuation in 6 cm rods at400olo630 nm and find that signal loss becomes significantat 5 Grad. Fibers receiving the maximum dose showed signs ofsurface crystal damage and interior tracks due to heavily ionizingfragments [3].

The ZDCs will be removed for pp runs once the luminosityreaches 1033. Special remote handling robots have also beeninstalled to reduce personnel exposure to activated materialduring these removals.

. Simulated 2g reconstruction in 1 M pp events.

Table 1ZDC performance, 2 TeV showers.

sðEÞ=E (%) sðrÞ (mm) sðtÞ (ps)

Neutron 17 1.4 100

Photon 7 0.2 100

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References

[1] ATLAS Heavy Ion Physics Performance Report.[2] ATLAS ZDC Letter of Intent- CERN/LHCC/2007-001LHCC I-016.

[3] N. Simos, S. White, et al., Grad-level radiation damage of SiO2 Detectors, in:Proceedings of the 2009 Particle Accelerator Conference May 2009, Vancouver,Canada.