Calorimetry in particle physics experiments

Transcription

1 Calorimetry in particle physics experiments Unit N. 9 The NA48 ECAL example (LKR) Roberta Arcidiacono R. Arcidiacono Calorimetry 1

2 Lecture overview The requirements Detector layout & construction Readout electronics Performance R. Arcidiacono Calorimetry 2

3 NA48 Physics goals Evaluate CP-Violation parameter Re ( / ) with an accuracy of 2 x 10-4 Measured via evaluation of decay widths of short and long lived neutral kaons into 2π Statistical error dominated by neutral K L decays BR K L x 10-3 K L x 10-3 BR K s % K s % R. Arcidiacono Calorimetry 3

4 NA48 Physics goals Need good identification and rejection of neutral backgrounds Need good identification of K s and K L events Analysis requires < 0.1% background leads to calorimeter requirements R. Arcidiacono Calorimetry 4

5 The requirements on ECAL Design energy resolution of 3.5%/ E with a constant term of 0.5% for a good background rejection Determination of the energy 10-3 level Time resolution of about 200 ps to correlate events with the Ks tagger and to separate quasi-in-time accidental events Good space resolution (~1 mm) to reduce ambiguity in pairing photons and for mass resolution Noise level on one cell less than 5 MeV Maximum energy expected in one cell 50 GeV R. Arcidiacono Calorimetry 5

6 The NA48 beams KS KL beams are distinguished by proton tagging upstream of the KS target Need good event time measurement Spill shape R. Arcidiacono Calorimetry 6

7 The NA48 Detector LKR- Homogeneous Calorimeter: ionization chamber filled with liquid krypton Magnetic Spectrometer consisting of a dipole magnet and four drift chambers Hodoscope, Vetocounter Muon Veto Hadron Calorimeter Stable, linear, Easy to calibrate (!) R. Arcidiacono Calorimetry 7

8 !Event display! R. Arcidiacono Calorimetry 8

9 LKR design (Quasi) homogeneous calorimeter with an active volume of 7 m³ (liquid krypton) Characteristics of Krypton Critical energy 21.5 MeV Radiation Length X0 = 8,5 g/cm 2 = 4.72 cm Moliere radius = 6.1 cm Length of calorimeter = 27 X0 Electromagnetic shower <100 GeV are completely contained 0.6 X0 of passive material in front of LKR R. Arcidiacono Calorimetry 9

10 LKR design (2) Geometry Length 1.27 m inner octagon radius 1.2 m x2cm2 cells ribbon accordion structure (tilted +- 48mrad) projective geometry Readout Detector signals are read-out using the Initial current read-out technique, with fast shaping high rate, good time resolution FE inside Krypton vessel The calibration system is located very close to the pre-amplifier. Signals similar to the ones of a uniformly ionizing particle are injected. R. Arcidiacono Calorimetry 10

11 during construction... R. Arcidiacono Calorimetry 11

12 LKR structure details Showers characteristics require to sum up more than 100 cells to measure the energy of the shower The cell to cell dimensional errors are less than 50 m R. Arcidiacono Calorimetry 12

13 Signal Processing R. Arcidiacono Calorimetry 13

14 DATA flow schema LKR cell R. Arcidiacono Calorimetry 14

15 Read-out card: CPD Calorimeter Pipeline Digitizer modules: perform the final shaping and the digitization of the calorimeter signal, store the data in circular buffers (200 s) 10 bits ADCs at 40 MHz with dynamic range expansion (dynamic range switching) in four gain ranges signal is differentiated with a time constant of 20 ns by a passive network; fed both to a Bessel filter and to three discriminators for the gain switching logic the Bessel filter produces a pulse delayed by 50 ns, with a 40 ns rise time, 73 ns FWHM and 160 ns at the baseline, followed by an undershoot at 3% of the pulse amplitude (for about 2.85 s). R. Arcidiacono Calorimetry 15

16 Read-out card: CPD Calorimeter Pipeline Digitizer modules: decision of which gain range to use is made by the three discriminators with different thresholds the output of the amplifier is sampled by a 10 bit 40 MHz FADC (Philips TDA8760) and the 2 bit information coming from the three discriminators is added to the samples Gain dispersion ~ 3%. The calibration signal is used for gain equalization and offset determination. Gain stability within 0.1% level. Offset variations monitored R. Arcidiacono Calorimetry 16

17 Data Concentrator To reduce the amount of data read out, a scheme of zero suppression is used. A simple suppression scheme based on comparison of each individual signal with a threshold would discard the cells covered by the tail of the shower (needed to reach the design accuracy). TZSA (Trigger and Zero Suppression ASIC) processor: it marks the channels with an energy peak and in a second phase it marks the channels up to a given radius from this center. To achieve two-dimensional zero suppression, a technique borrowed from image processing has been used, which performs an expansion algorithm around the outstanding peak R. Arcidiacono Calorimetry 17

18 reconstruction of 2pi0 R. Arcidiacono Calorimetry 18

19 Performances R. Arcidiacono Calorimetry 19

20 Cell-to-cell response The cell-to-cell response of the liquid krypton calorimeter, which is uniformly irradiated with electrons from Ke3 decays (K L en ). Precision (standard deviation) ~ 0.4% only with electronics calibration R. Arcidiacono Calorimetry 20

21 Na48 ECAL performances resolution linearity R. Arcidiacono Calorimetry 21

22 Energy resolution Measured in Ke3 decays, E/p of electrons Sampling Term Shower fluctuations 1/E Total shower noise Constant Term Inter-calibration between individual cells Variation in Energy Response Precision of pulse reconstruction R. Arcidiacono Calorimetry 22

23 Linearity Linearity Measured in Ke3 decays, electrons sample (E + 45 MeV)/p (45 MeV is the average energy lost in the material upstream the calorimeter) ~ 0.1% in range GeV R. Arcidiacono Calorimetry 23

24 1 mm for E > 25 GeV Position resolution measured with electron beam in exp area (at fixed target experiment it is possible to take advantage of monocromatic beams) R. Arcidiacono Calorimetry 24

25 Time resolution 500 ps for photons GeV 220 ps for reconstructed 0 0 Use K 3π 0 (n)γ e+e to check photon time measurement, by comparing photon time to the reconstructed time of e+e pairs (measured by scintillator counters) R. Arcidiacono Calorimetry 25

26 Energy scale In the neutral mode : Need good knowledge of Energy scale to define the decay region ( fiducial region) NB: the decay region definition should be the same for 0 0 and π+π decays Use known anti-ks counter position Accuracy of energy scale setting few 10 4 R. Arcidiacono Calorimetry 26