Calibration and monitoring of the experiment using the Cockcroft-Walton accelerator

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. 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. 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 >11.7 MeV 4.4 MeV >16.1 MeV Lithium spectrum on NaI 17.6 MeV line 14.8 MeV broad resonance

4. 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. Beam monitoring • 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

6. 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 • B4C - Boron Carbide • Hybrid target (Li2B4O7 or LiB3O5) • 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. 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 • PCOBRA < 2 Pa • Pchambers not appreciable • CW pipe locks the insertion of the muon target PCOBRA 2 Pa Start-up Slow-down 10 min Pchambers

8. 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. Cu target P-beam mylar window X-ray detector

9. 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. 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 Corrected applying rough equalization from -runs “raw” spectrum (rad) (rad)

11. 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 4.4 and 11.6 MeV Compton Edges Energy deposit in XEC 11.6 MeV “Energy” deposit in TC ttrigger (LXE - TC) in 10 ns bins

12. 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 • Veff,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. 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 “CW” lines 52.8 MeV Measured in 0 runs See physics talks

14. 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. 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 T0 to inter-calibrate XEC and TC