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

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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

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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

Reaction Peak energy peak -lines

Li(p,)Be 440 keV 5 mb (17.6, 14.6) MeV

B(p,)C 163 keV 2 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

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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.

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• 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

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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

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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

Pchambers

10 min

2 Pa

Start-up Slow-down

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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

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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

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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) (rad)

“raw” spectrum Corrected applying rough equalization from -runs

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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

Ene

rgy

dep

osit

in X

EC

4.4 and 11.6 MeVCompton Edges

ttrigger (LXE - TC) in 10 ns bins

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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

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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 runsSee physics talks

“CW” lines

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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

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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