HARPO: Measurement of polarised gamma rays (1.7 to 72MeV) with the HARPO TPC

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1 HARPO: Measurement of polarised gamma rays (1.7 to 72MeV) with the HARPO TPC Philippe Gros for the HARPO collaboration 1/39

2 Gamma Astronomy HARPO project Setup at the NewSUBARU photon beam Gas monitoring Trigger with micromegas signal Other experiences Conclusions 2/39

3 Milky Way / 天の河 /39

4 Sensitivity gap low sensitivity region 100kg liquid 10kg 10bar 10kg 1bar potential of an Argon TPC /39

5 Gamma Astrophysics non thermal phenomena /39

6 Need for polarimetry Separating processes Leptonic synchrotron self Compton (SSC) Hadronic proton synchrotron 1MeV 1GeV Polarisation can give the answer no difference in X visible in gamma H. Zhang and M. Böttcher, A.P. J. 774, 18 (2013) 6/39

7 HARPO Demonstrator scintillators micromegas +GEM readout strips field cage 7/39

8 TPC: photon conversion photon e+ e- The incoming photon interacts with the gas and decays into an electron-positron pair 8/39

9 TPC: Gas ionisation Ar++e- The electron and positron travel through the gas (mostly Argon) and ionises it, freeing many electrons and positive ions This takes a few nanoseconds 9/39

10 TPC: Drift and Readout The electrons drift along the electric field in a few microseconds. The are amplified and read out on the readout plane 10/39

11 Polarisation measurement φ The azimuthal angle φ is related to the polarisation direction expected distribution 11/39 φ [rad]

12 HARPO Demonstrator Purpose Assess challenges Demonstrate performance in test beam Realisation 30cm cubic TPC Ar/iC4H10 95/5 up to 5bar micromegas+2gem amplification 2x288 strips readout (x&y), 1mm pitch AFTER readout electronics, 511 time bins, up to 50MHz (33 used) trigger: 6 scintillators 12/39

13 Going to Japan 13/39

14 NewSUBARU SPring8 NewSUBARU XFEL Linac Synchrotron Storage Ring 14/39 14

15 NewSUBARU photon beam Polarised gamma ray beam Inverse Compton electron beam 0.6, 1., 1.2 or 1.5 GeV laser Nd (1ω or 2ω), Er or CO2 => polarised photons 1.71 to 72.3MeV Pulsed mode Nd: 20kHz, Er:200kHz, CO2: not 15/39

16 Gamma beam tro c e el eam b n gamma beam HARPO Laser 16/39

17 Laser beam 17/39

18 HARPO in beam gamma 18/39

19 Rotation 19/39

20 Beam campaign ~20 days of data taking 13 gamma energies, beam polarised or not 4 TPC orientation for angle systematics >60Mevents, >1TB of data probably >20% gamma converted in gas Angles #events Philippe Gros, LLR Egamma [MeV] 20/39 100

21 Monitoring 21/39

22 Monitoring 22/39

23 Monitoring 23/39

24 Monitoring 24/39

25 Monitoring 25/39

26 HARPO gas system Gas quality and stability are crucial Sealed vessel No air in for several month Leaks minimised in vacuum with He system 26/39

27 Gas monitoring No direct measurement of the gas Monitoring with track data High momentum traversing tracks in Z ~uniform average energy deposition (de/dx) along Z (if corrected for angle) Simple access to gain, drift velocity and absorption Dedicated trigger line => ~2% of the events 27/39

28 Cosmic rays RUN 1237 Gain Vdrift RUN 1569 absorption 28/39

29 Time evolution GAIN T [oc] Vdrift Temperature Pressure 29/39

30 Gas stability 23 days in beam with same gas Almost no leak (no noticable pressure loss) Slight decrease of Vdrift Stable gain Not much contamination (absorption). Longer analysis (with more statistics) necessary. 30/39

31 Gamma trigger Trigger on conversion in the gas volume at least one scintillator (=> timing + trigger information) if available, laser trigger signal mesh signal >1us ( 3cm) later t0 >1µs tmesh t0 31/39

32 Micromegas trigger signal Laser trigger VGEMtt VGEMtb GEMt Vmesh GEMb PM data PM PM PM VGEMbt VGEMbb tlaser PMm2 FPGA 5nF Ampli ORTEC142b CFD tmesh Trigger signal tmesh-tlaser>1µs µm (8nF) 32/39

33 Trigger performance (?) Conversion outside of the gas volume background 33/39

34 Trigger performance after cut before cut 34/39

35 Trigger performance Further analysis of course needed By eye it worked well Most events originate on the beam 35/39

36 A puzzling effect 36/39

37 Space charge? Ballistic deficit? Observed signal loss at the center of the detector (beam axis) Only at high rate space charge? ballistic deficit? other? Space charge positive ions accumulating in front of the readout plane Ballistic deficit shaping 116ns tracks in Z direction => several µs signals Improved by misaligning the detector 37/39

38 Conclusion and outlook Successful beam campaign with gamma rays Good gas stability over more than 20 days even more encouraging result now... Good trigger performance Most issues related to high rate in beam Lots of work necessary for quantitative results... Further studies for space trigger, higher pressure,... 38/39

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40 Backup 40/39

41 Pressure scan One day dedicated to pressure scan Clean gas at 1, 1.5, 2, 3 and 4 bar Signal amplitude (de/dx x gain) ~fixed (Adjusted on the fly) At high pressure, cathode voltage was limited Good running condition at all pressures Increasing micromegas current spikes at high pressure matching GEM over current => physics? 41/39

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