1. Calibration and monitoring of the experiment using the Cockcroft-Walton accelerator G. Signorelli Sezione di Pisa MEG Review meeting - 20 Feb. 2008 On behalf of the CW group

2. 1 Disclaimer All the plots that I am going to show are to be considered as “online- plots” since we did not try to apply any calibration apart from gain equalization (trigger waveforms) Further work is needed to understand the calorimeter uniformity, extract the PMT quantum efficiencies and hence the resolutions

3. 2 Intro & reactions The Cockcroft-Walton accelerator was installed for monitoring and calibrating the MEG experiment Protons on Lithium or Boron –Li: high rate, higher energy photon –B: two (lower energy) time-coincident photons ReactionPeak energy  peak  -lines Li(p,  )Be 440 keV5 mb(17.6, 14.6) MeV B(p,  )C 163 keV2 10 -1 mb(4.4, 11.7, 16.1) MeV >16.1 MeV >11.7 MeV 4.4 MeV Lithium spectrum on NaI 17.6 MeV line 14.8 MeV broad resonance

4. 3 Installation issues Centering & Monitoring of the beam when the beam line is fully mounted –Pixel target –Movable crystal w/camera Target reliability and durability –Search for different target materials –Study of different targets Connection with the rest of the experiment –Insertion/extraction Connection with PSI –Integration of the safety system –Approval of Swiss Ministry of Health.

5. 4 A pixel target mounted, tested and used to center/measure the beam spot –A hybrid pixel-physics target is foreseen for the future The quartz crystal allows for the monitoring of the beam before the entrance into the bellows system –A MathLab program was developed Beam monitoring

6. 5 Target development Lithium –A LiF crystal target was tested and proved to be more reliable and durable: using one target for the full run Boron –Metallic Boron –B 4 C - Boron Carbide Hybrid target (Li 2 B 4 O 7 or LiB 3 O 5 ) –Possibility to use the same target and select the line by changing proton energy B lines appear increasing p energy B lines (coincidence) rate improves dramatically by increasing p energy

7. 6 Daily insertion of the p-target COBRA volume was sealed with the nitrogen bag to protect TC PMTs Insertion of CW pipe modifies volume –Control of the gas flow –Speed ~ 3.5 mm/sec < 10 minutes insertion/extraction  P COBRA < 2 Pa  P chambers not appreciable CW pipe locks the insertion of the muon target  P COBRA  P chambers 10 min 2 Pa Start-upSlow-down

8. 7 PIXE We tested the Proton-Induced X-ray emission from different materials –Possible to have an independent current normalization –Possible usage for DCH monitoring The energy of the X-ray can be easily chosen in a wide range by having a suitable target material  IXE could be used as a  rate measuring device. X-ray detector mylar window Cu target P-beam

9. 8 Physics: monitoring The main purpose of the CW is to monitor the stability of the xenon calorimeter Twice-a-week we had a 1-morning data taking –Gain familiarity with the apparatus –Learn the best way for implementing this calibration –Monitor liquid xenon during purification Clear 17.6 MeV peak on the 14.8 MeV broad resonance We could follow the improvement of light yield Correlation with absorption length measurement with  -sources Purification

10. 9 Uniformity and time scale Rate on calorimeter ~6 kHz By uniformly illuminating the calorimeter we can monitor the response of the detector at various positions. It is possible to perform the monitoring with a 30 min run –Suitable to follow the calorimeter day-by-day variations  (rad) “raw” spectrum Corrected applying rough equalization from  -runs

11. 10 Boron target The 2 simultaneous lines are useful to exploit the coincidence –Clean spectrum in the calorimeter by requiring a signal in the timing counter –Used at trigger level –Used for the initial set-up of the  e  trigger 4.4 MeV 11.6 MeV “Energy” deposit in TC Energy deposit in XEC 4.4 and 11.6 MeV Compton Edges  t trigger (LXE - TC) in 10 ns bins

12. 11 One more word on CW & TC The possibility of abundant and uniform gamma rays from Li and B is being exploited to –Equalize the TC bars –Measure TC bar parameters V eff, eff –Study the TC - LXE coincidence Timing synchronization and resolution, independent of the reconstruction of the positron track Again: calibrating the apparatus during beam-off periods

13. 12 Low energy vs high energy We can monitor on a day-by-day basis at an energy which is 1/3 of the working point of our detector; Thanks to the good linearity of our detector we can confidently extrapolate at higher energies 52.8 MeV Measured in  0 runs See physics talks “CW” lines

14. 13 Conclusion At mid October we were ready to deliver calibration photons at the center of the MEG detector Since 5 November we had a twice-a-week calibration and monitoring session for the experiment –XEC calibration and monitoring –TC calibration –Trigger set-up Some of these were unforeseen, the CW proved to be extremely useful The CW beam line was dismounted on Dec. 15 to install the liquid hydrogen target –Confirmed the energy scale and (dis)uniformity –We can confidently monitor the calorimeter in the 20 MeV range

15. 14 Todo’s The CW calibration has the advantage to allow the experiment set-up and calibration even during beam-off periods We are using this inter-run time to: –Implementing some new beam line elements Pneumatic Faraday cup Pneumatic quartz crystal Hybrid physics and pixel target –Studying timing calibration techniques We are studying a way of tagging in an independent way one of the two photons from boron, to give a T 0 to inter-calibrate XEC and TC