1. The Electromagnetic calorimeter of the MEG Experiment
XIII International Conference on Calorimetry in High Energy Physics
Pavia Italy, May 2008 G. Gallucci, INFN Pisa

2. MEG Experiment sensitivity  10-13 @ 90% CL
Reserch of rare muon decay m -> e + g with lepton flavour violation at Paul Scherrer Institut (Villigen, Switzerland)
In the Standard Model
BR(m -> e + g ) = 0 Leptonic flavour and number conservation
Neutrino oscillation
SUSYGUT theories
SUSY SU(5) BR   10-13
SUSY SO(10) BR   10-11
BR (m -> e + g)  Impossible to measure
MEG Experiment sensitivity  @ 90% CL

3. Event Signature and background
Accidental contribution more important than correlated one
A very good electromagnetic calorimeter
y = Eg / mm
x = Ee/ mm
Bacc  Rm dx (dy)2 dw2eg dteg (ln(dy) )

4. Photon detector: calorimeter (1)
Operated for 2 Months in 2007

5. Photon detector: calorimeter (2)
Refrigerator
800 liters of Liquid Xenon
(the biggest Xenon calorimeter in the world)
External structure made in steel except front part in alluminum honeycomb and carbon fibers
Internal PMTs supported structure made in alluminum and plastic (peek) for inner face
846 PMTs installed with photocatodic coverage  30%
Solid angle coverage  10% HV
Signals
Cooling pipe
Vacuum for thermal insulation
Al Honeycomb
window

6. Scintillation process:
Liquid Xenon
Density g/cm3
Liquefaction temperature °K
Energy per scintillation photon eV, 23.6 eV
Radiative length cm
Decay time , 22, 45 nanosecond
Wavelenght of emission peak nanometer
Rayleigh diffusion  40 cm
Refractive index (on emission peak)
Scintillation process:
1) Xe* + Xe  Xe2*  2 Xe + hn ) Xe+ + Xe  Xe2+
Xe2+ + e  Xe + Xe**
Xe**  Xe* + heat  1)
Ultraviolet scintillation light
Absorpiotn lenght from H2O and O2
High number of scintillation photons ( 40k g/MeV)
Fast time response
High density, compact detector

7. Average Quantum efficiency
PMTs
R9869 Hamamatsu
Photocatodic surface
96% K - Cs – Sb ,
4% Al to reduce Photocatodic
impedence at low temperature
Compact structure with 12
amplification stages in order to
operate into a low magnetic field
Last two stages have a Zener
dyode to stabilyze Voltage
Average Quantum efficiency
15 %

8. Cryogenic tools Detector Gas phase-purifier High pressure storage
LXe storage tank
Liquid phase-purifier

9. Data Acquisition System
Each PMT read from 2 types of waveform digitizer to reject pile-up and to subtract pedestal
Trigger  100 MHz
DRS  2 GHz
Gamma Ray g g
Alpha particle a a

10. Runs with different Led amplitude to compute PMTs gains (Nphe~1/s2).
Leds and PMTs gains
In order to monitor the calorimeter stability and response, Pmts characteristics (gains and QE):
There are 36 leds mounted in 12 different positions with different attenuations.
Runs with different Led amplitude to compute PMTs gains (Nphe~1/s2).

11. Alpha, QE and Absorption lenght
25 alpha sources 241Am on 5 wires.
Absorption lenght evaluation
Ratio Data/MC vs distance fitted
with an exponential curve.
Quantum Efficiency
Evaluation in cold gas or Liquid
Wire thickness  50 mm
Alpha average path  40 mm
Shadow effect (Rings)
l > 3 % C.L.

12. Cockroft-Walton Accelerator
Reaction
Peak energy
s peak
g-lines
Li(p,)Be
440 keV
5 mb
(17.6, 14.6) MeV
>16.1 MeV
>11.7 MeV
4.4 MeV
sE = 3.6 %

13. Charge exchange reaction
p- + p  p0 + n
p0 Decay
p0  g + g
m(p0)  135 MeV/c2 p(p0)  2.9 MeV/c MeV < Eg < 83.9 MeV
A p- beam against a LH2 target
NaI Crystals “grid”
It is possible to move tha NaI in different positions around f angle

14. Spectra of photons from muon radiative decay
Red = computed from theory
Preliminary
Blue = measured from Michel runs
Pile-up
Preliminary
Integrated spectra

15. Energy Resolution and Linearity
Gamma Linearity
Energy resolution on 54.9 MeV
52.8 MeV CW
Measured in 0 runs
Risolutions (FWHM)
Gamma Energy (on 55 MeV)
4.8 %
Gamma Position (mm)
15.0
Gamma Time (nanosec)
0.15

16. Position and Time Resolution
s=64 psec
Position Resolution (FWHM)
Gp  15 millimeters
Using Pb collimator holes and edges

17. Conclusions
The Calorimeter performances are suitable for MEG experiment goals.
The detector is now in the purification phase till the first week of June.
After the purification, we will start to take physics data.