Study of the HARPO TPC for a high angular resolution g-ray polarimeter in the MeV-GeV energy range David Attié (CEA/Irfu)
Outline Motivation of an MeV-GeV polarimeter Scientific case and expected performance HARPO (Hermetic Argon Polarimeter) Gamma-ray data taking in Japan Data analysis: event reconstruction Polarisation measurement Comparison with simulations Study of gas stability 2
Motivation for an Mev-GeV Polarimeter Polarisation 2-10 kev 30-200 MeV γ-rays origins: violent phenomena known in the Universe (AGN, pulsars, GRB..) Polarisation P is a key parameter in the models to understand the mechanics involved: Relativistic jets of matter: P = 0 Radiative processes (synchrotron, inverse Compton) : P 70 % Only Space Telescope possible in this energy range N H A R P O Active Galaxy Nucleus model Zhang & Bötter, ApJ 774 (2013)18 3
Scientific Case and Expected Performance High-Energy pressure Astrophysics: gaseous detector: converter and tracker Conversion Need to improve efficiency angular Z² resolution Sensitivity Better sensitivity:1 Z MeV < E g <100 MeV (Compton and pair creation telescopes) Angular resolution decrease with Z No polarimeter in space above E g =1 MeV HARPO Takahashi (2012) arxiv:1205.2423 D. Bernard, NIM A 729 (2013) 765 4
HARPO: Prototype of Time Projection Chamber Time Projection Chamber (TPC), sealed vessel, 30x30x30 cm 3 Gas: Argon-Isobutane 95/5, up to 5 bar PMTs 1 scintillator per side Micromegas+GEM Amplification by MICRO-PATTERN GAS DETECTORS Micromegas + 2 GEM Trigger used by 6 scintillators + 2 PMT for each Readout by strips: x & y: 1 mm pitch 2 288 channels (x, y) g beam Field cage T2K-TPC electronics: AFTER chips One FEC by coordinate read out by FEMINOS Synchronised using TCM (trigger clock module) X-Y (2n) strips instead of n² pads 5
Event Readout on X & Y Strips The incoming photon interacts with the gas and decays into an electron-positron pair photon e + e - The electron azimuthal and angle positron ω is related travel through to the polarisation the gas (mostly direction Argon) ω 0 and ionises it, freeing many electrons and positive ions This takes a A: few polarisation microsecondss asymmetry P: polarisation fraction 6
Gamma-rays Data Taking in Spring8/NewSubaru (Japan) g-rays from inverse Compton scattering 15 energies (1,7 72 MeV) Polarised and non-polarised beam Rate ~2 khz Photons to readout plane X-Y along Z-axis g interaction probability (~ 0,24 à 0,5 %) 1 week for installation and commissioning 20 days of data taking Events >60 10 6, >1To of data g 7
Trigger Definition and Performance Main Trigger is the combination of multiple components Trigger efficiency: Selection: ~ 50% > 95% of the noise rejected & Gas volume e + e - 0 3 6 9 12 µs Laser: Scintillators: Micromegas Mesh: >1 µs Main Trigger: 8
Pair Creation Events vs. E g Geant4 simulation + Garfield parameters for gas diffusion/transport and signal: Data Simulation 9
Data Analysis: Event Reconstruction Data taken without zero suppression with common noise Pedestal subtraction Strip hit selection: Q strip V pedestal > 50 ADC Clustering: cluster position, T cluster and C cluster Matching x(t) and y(t) + POCA reconstruction of the event in 3D 10
Reconstruction by Matching x(t) and y(t) Planes No tracking: local vertices Find region of interest Find peaks in polar distribution around point Match 2D vertices to get 3D picture φ>π Reconstruction efficiency: ~90% (preliminary) φ<π Tracking in progress using Kalman filter, but not easy in x(t) and y(t) projections 11
Polarisation Modulation Polarisation modulation of the azimuthal angle ω: Non-polarised beam Polarised beam 12
Extraction of the Polarisation Modulation 13
Polarisation Modulation: Beam Data Number of events: 10 6 14
Polarisation Modulation: Monte-Carlo Simulation Number of events: 4 10 5 15
Gas Stability During Data Taking Gain X & Y V drift X & Y Temperature Pressure 16
Gas Stability Study Charge Q vs. drift time, normalized with respect to the track angle The most probable value (MPV) is obtained from Landau fit Drift velocity can also be extracted T max = L TPC /V drift 17
Gas Stability Over 5 Months In The Lab Weekly data taking of ~1.5h, for 6 months Gas circulation and purification using an Oxisorb filter to remove H 2 O and O 2 Pressure Temperature Gain Attenuation Drift velocity arxiv:1512.03248 Purification 18
Conclusions and future plans A demonstrator has been successfully operated in a gamma-ray beam from 1.74-74 MeV and show the ability to measure polarisation modulation Monte-Carlo simulations are in good agreement with the data High pressure TPC is a good candidate of polarimeter for the next generation of high energy space telescope: Gas and detector stability Performance close to the expectation First hybrid TPC (Micromegas + GEM) working at high pressure Development of tracking for event reconstruction is in progress Trigger configuration in space need to be studied Funding needed for a balloon flight 19
Thank You 20
Trigger on Micromegas Signal V V tt GEMtb V GEM bt GEMbb V GEM t GEM b GEM 5nF Laser trigger PM PM PM t laser PMm2 FPGA PM data Trigger signal V mesh Ampli ORTEC142B CFD t mesh t mesh -t laser >1µs µm (8nF) 21
Trigger Performance for E g = 72,3 MeV Run 1276 run 1287 E e- = 1.5 GeV, laser Nd2w E g = 72,3 MeV Conversion rate : 50 Hz 99,2 % prompts rejected Late events: 51 % 22
Gas Circulation/Purification 23
Vertex Fitting Polar charge distribution around vertex Clean up: keep only straight lines delta electron Simple Robust: ignores obvious scattering and background potential for small opening angle Potential for improvement better peak finding use of distance info (focus on short distance for large opening angle, long distance for small) 24
Vertex Matching Assign signal to track As before: compare charge profile 1: match vertexes if there are several with same Z position 2: match the tracks in the vertex (simple: only 2 possibilities) Compare profiles: X(1,2) Y(1,2) same Compare profiles: X(1,2) Y(2,1) switch 25
Angular Resolution 26
Angular Resolution Agreement with theoretical prediction relatively small contribution of tracking Excellent agreement with simulation effect of saturation dominates at high energy Potential for improvement: estimation of track momentum even 100% should significantly improve 27
Efficiency 28
Polarisation Asymmetry 29
Sketch of HARPO-Like Telescope 1,30 1,30 1,30 m 3 30
The HARPO Team 31