Commissioning and Calibration of the Zero Degree Calorimeters for the ALICE experiment

Transcription

1 Commissioning and Calibration of the Zero Degree Calorimeters for the ALICE experiment Roberto Gemme Università del Piemonte Orientale A. Avogadro (Alessandria) on behalf of the ALICE Collaboration

2 Outline Quartz fiber calorimetry Detector description & read-out segmentation Calibration and Monitoring systems Test beam calibration Laser system Cosmic rays Commissioning In-situ physical calibration In-situ tower inter-calibration strategy

3 Detection technique Quartz fiber calorimetry ZDCs are sampling calorimeters with silica optical fibers, as active material, embedded in a dense absorber. The principle of operation is based on the detection of the Cherenkov light produced in the fibers by charged particles of the shower, induced by the spectator nucleons. NA50 ZDC behavior in an environment 10 times harsher than the ALICE one This technique fulfils various ALICE requirements: high resistance to radiation reduced transverse size of the detectable shower signal width < 10 ns compact detectors fast response

4 ALICE ZDCs: detector description The Neutron Zero Degree Calorimeter (ZN) The Proton Zero Degree Calorimeter (ZP) Dimensions 7.2x7.2x100 cm3 Passive material: W-alloy (ρ=17.6 g/cm3), 44 grooved slabs 1.6 mm thick Active material: 1936 quartz fibers ( 365 mm) Fiber spacing 1.6 mm Filling ratio of 1/22 The fiber numerical aperture is 0.22 Dimensions 22.8x12x150 cm3 Passive material: brass (ρ=9 g/cm3) 30 grooved slabs 4 mm thick Active material: 1680 quartz fibers ( 550 mm) Fiber spacing 4 mm Filling ratio 1/65 The fiber numerical aperture is 0.22

5 ZDCs read-out segmentation The fibers are placed at 0 with respect to the beam axis and come out from the rear face of the calorimeter, bringing the light to the PMTs. One out of two fibers is sent to a photomultiplier (PMTc), while the remaining fibers are collected in bundles and sent to four different photomultipliers (PMT1 to PMT4), forming four independent towers. The chosen PMT is the Hamamatsu R329-02, with quantum efficiency around 25%. ZN ZP PMT 1 PMT 1 PMT 2 PMT 2 PMT 3 PMT 3 PMT 4 PMT 4 PMT c PMT c

6 Test beam calibration The ZN and ZP detectors have been tested in the H6 beam line of the CERN SPS with hadron and e-/e+ beams of various momenta ( GeV/c) to: intercalibrate the individual PMTs verify the uniformity of the response as a function of the particles impact point on the calorimeter front face measure the light yield (for hadron beam: 0.9 phe/gev for ZN, 1.1 phe/gev for ZP ) extrapolate from the measured data the detector energy 2.76 TeV σ (E) 11.4% E for ZN and σ (E) 13% for ZP E Test performed in the H8 beam line with an 115In beam of 158AGeV/c Chek the linearity of the response as a function of the number of the incoming nucleons

7 ZDCs on the movable platform ZN and ZP are placed on a movable platform controlled via ALICE DCS Scintillators Vertical position adjusted to follow crossing angle Garage position (20 cm lower than the beam plane) during injection Photomultiplier rear box Laser diode in sealed box Light pulser controller Light sources for monitoring purposes: Laser Diode Cosmic rays (coincidence of 2 scintillators)

8 ZDCs Stability Monitoring (I) HV=1320V σ/e~1% Pulsed light from a Laser Diode allows to monitor PMT + fibers radiation damage with an accuracy of 1% fibers Laser Diode (λ=405nm, pulse width=70ps) ZN ZP filter fissure fibers reference PMT

9 ZDCs Stability Monitoring (II) Absolute PMT gain measurement by means of single photoelectron signal disentangle PMT from fiber radiation damage µ1 µ2 single photoelectron with laser light attenuated by neutral filters (need access in the tunnel) signal I = I0 10-D, D=4 (Kodak Wratten Gelatin Filter) with cosmic ray very small light released trigger requires coinc. of the 2 scintillators (rates: 1.5 ev./s in ZN, 10 ev./s in ZP on surf.; ~30 times less in the tunnel) (μ2 μ1 ) 25fC/ch G= At e gain ADC (low range) e=1.6 * C At=cable attenuation

10 ZDC commissioning at IP2 on surface (I) Measurement of Absolute Gain at different HV for all PMTs Good agreement between cosmic rays, laser light measurements and previous PMT characterization in lab. PMT gain stability can be monitored during the experiment

11 ZDC commissioning at IP2 on surface (II) Extension of Absolute Gain Measurement at low voltage for all PMTs In Pb-Pb collisions PMTs will work at G~105. PMT response to laser light measured in the range ~ ~2300 V, using different neutral filters (I = I0 10-D). D=1 D=2 D=2.6 D=2.4

12 ZDC commissioning at IP2 on surface (III) Extension of Absolute Gain Measurement at low voltage for all PMTs The data have been normalized to absolute gain at HV~2000 V.

13 ZDCs installation in LHC tunnel In summer 2007 the 2 ZDC systems have been installed in the tunnel

14 ZDC commissioning in the LHC tunnel Linearity measurements for all channels 200 pc Runs standalone in the framework of ALICE DAQ system performed flashing all the PMTs with 2 different laser light intensities (trigger rate = 104). Higher and lower PMT charge ratios measured at different laser light intensities, inserting filters. Preliminary check of linearity (PMT + bleeder + electronics) for all channels done. limitation on the signal charge due to the max input amplitude of FIFO temporarily used (1.4 V).

15 In-situ physical calibration strategy During H.I. runs ZNs will measure LHC luminosity, by measuring the rate of mutual e.m. dissociation of colliding nuclei in the neutron channel. The single neutron spectrum will provide a physical calibration (and a stability monitoring). 1n LHC Prediction Neutron emission in e.m. dissociation Pb+Pb ATeV Neutron emission in e.m. dissociation of 30 AGeV Pb ions Signal in ZN 2n All emitted neutrons fall in the ZN acceptance (pt<250mev/c). 3n Energy resolution (11%) allows clean separation of 1n-2n-3n contribution.

16 In-situ tower inter-calibration strategy We implement an inter-calibration method based on the PMTc signal, which provides a complementary measurement of the shower energy. n=5000 Pb+Pb ATeV minimum bias events generated with fast simulation. The light in each tower multiplied by a factor coefi ranging from 0.9 to 1.1 -> 4 towers artificially not calibrated (10% at most) Minimization respect to pari n 1 ( EC Ec is the light in PMTc 4 pari coef i Ei ) 2 1 Ei is the light in the i-th tower (PMTi) the parameters pari, which re-equalize the response of the 4 towers, are obtained with an accuracy of 1%.

17 Conclusions The 2 ALICE ZDC systems are installed in the LHC tunnel Commissioning have been performed Laser system monitors the PMT+fibers response with 1% precision PMT gain stability can be monitored during the experiment Linearity of all channels checked In-situ physical calibration and tower inter-calibration strategy have been presented

18 Backup slides

19 ZDCs Stability Monitoring laser (λ=405nm, pulse width=70ps) fibers filter fissure ZN ZP filter fissure fibers reference PMT

20 Expected centroid resolution of the spectator neutrons spot GEANT-based simulation has been performed in order to evaluate the resolution on the centroid r 2.76 TeV neutron for different neutron multiplicity. he centroid of the spectator neutrons spot on the neutron calorimeter ZN front-face is stimated by means of the relations: 4 x = const t i x wi wi 4 y = const yit wi 1 4 wi = wi E α simulation 2.76 TeV neutrons i 1 where xit and yit are the coordinates of the centre of he i-th tower and Ei is the light in the i-th tower. α and const are free parameters introduced in order o get an accurate reconstructed impact coordinate. Thanks to this localizing capability ZN can be used: to monitor the beam crossing angle to estimate the reaction plane

21 ZN: spatial resolution 100 GeV/c positive hadrons Difference between the impact coordinate reconstructed by the ZN, by means of the relations 4 x= x q ADCiα i= 1 i 4 α ADC i i= 1 4 y= i= 1 4 y iq ADCiα α ADC i i= 1 and the one measured by the MWPC. - a = 0.5 optimized from simulation - Only particles hitting ZN in a region of 8 mm radius around the centre have been considered; the position reconstruction is accurate essentially near the intersection of the 4 towers -> Spatial resolution ~ 3 mm ZN in ALICE can measure, event by event, the centroid of the spectator neutrons -> can monitor the beam crossing angle at I.P. and estimate the reaction plane

22 Energy resolution The black line are the results of the fit: σ ( E) a = b E E For positrons(zn) /electrons(zp) ZN: a = (89.9 ± 0.7) % GeV1/2, b = (0.0±2.9) % ZP: a = (106.8 ± 0.9) % GeV1/2, b = (5.±0.3) % For positive hadrons(zn) /negative hadrons(zp) ZN: a = (256.6 ± 2.9) % GeV1/2, b = (10.3±0.6) % ZP: a = (237.0 ± 2.) % GeV1/2, b = (12.5±0.2) % 100 GeV ZN response to 158A GeV/c 115In beam: Extrapolation to 2.7 TeV hadrons (ALICE spectator nucleon energy in Pb-Pb interactions) σ (E) σ (E) ZN: ZP: 11.4% 13% E E comparable to the spectator energy fluctuations σ (E) to be compared to 2.8% E 1 σ (E) 115 E E = %

23 ZN response vs spectator nucleons number -2 Projectile 115 ZDC response to spectators is linear vs the number of nucleons entering the detector In+119Sn Target C 12 Projectile Target In beam 115 Al 27 The low energy end point of the ZDC spectrum corresponds to the most central collisions Espec = EZDC(end point) - NparEpar Cu 63 Epar is found assuming for central collisions 115In +119Sn Npar=115 and Nspec=0 -> Epar = EZDC(e.p.)/ Sn

24 ZN e/π and e/h ZP e The e/π ratio has been corrected for lateral leakage and fitted with where fπ 0is either Wigmans or Groom s fπ 0 = 0.11log( E[GeV ]) e h = e π 1 (1 h ) fπ 0 ( E ) fπ 0 = 1 ( E / Eο ) m 1 with E0 = 1 GeV and m = > The ZDC are non-compensating (e/h > 5 for ZN, e/h 3 for ZP) but in ALICE will be used to measure the number of the spectator nucleons