Liquid Argon TPC for Next Generation of MeV Gamma-ray Satellite

Transcription

1 Liquid Argon TPC for Next Generation of MeV Gamma-ray Satellite Hiroyasu Tajima for LArGO team Institute for Space Earth Environmental Research Nagoya University April 22, 2017 Active medium TPC Workshop KEK

2 Fermi LAT (Large Area Telescope) Pair-conversion telescope Good background rejection due to clear gamma-ray signature Tracker (TKR): pair conversion, tracking Angular resolution is dominated by scattering below ~GeV Calorimeter: energy measurement 8.4 radiation length Use shower development to compensate for the leakage Anti-coincidence detector: Efficiency > 99.97% Energy band: 20 MeV to >300 GeV Effective area: > 8000 cm 2 (~6 EGRET) Field of view: > 2.4 sr (~5 EGRET) Angular resolution: Energy resolution: 5 10% Anti-coincidence Detector Segmented scintillator tiles 99.97% efficiency Gamma-ray Burst Monitor γ Si Tracker 70 m 2, 228 µm pitch ~0.9 million channels (Japanese contribution) e + e - Large Area Telescope (LAT) CsI Calorimeter 8.4 radiation length 2/11

3 MeV Gap We have large sensitivity gap between 0.1 MeV and 100 MeV > 10 MeV: Pair conversion is a dominant process Angular resolution limited by scattering in conversion material < 10 MeV: Compton scattering is a dominant process Incident direction is not well constrained Backgrounds are hard to reject (activation of detector materials) Sensitivity (erg cm -2 s -1 ) IBIS-ISGRI JEM-X IBIS-PICsIT SPI COMPTEL EGRET CTA North MAGIC Fermi-LAT VERITAS /HESS HiSCORE CTA South LHAASO Energy (MeV) 3/11

4 Improvements from Fermi LAT Trade off between pair-conversion efficiency (more material) vs. angular resolution (less material) Tungsten converter + Si tracking Angular resolution is determined by thickness of Tungsten converter (not active) - 3% X0 12 layers + 18% X0 3 layers - Total 76% X0 General approach to improve the situation Focus on 1 MeV 1 GeV energy region Remove tungsten converter to improve angular resolution Effective area may be reduced Employ double-sided silicon strip detector to minimize the detector thickness and obtain 2D position Tracking of recoil electrons due to Compton scattering 0.5% X0 56 layers 25% X0 4/11

5 56 layers of 0.5 mm thick DSSDs ( cm 2 ) (5 5) (2 2) Spacing: 10 mm ~700 W eastrogam CsI calorimeter (8 cm thick, 4.3 X0) ( cm 2 ) (8 8) (23 23) Sensitivity (erg cm -2 s -1 ) IBIS-PICsIT SPI COMPTEL EGRET IBIS-ISGRI Fermi-LAT MAGIC VERITAS JEM-X e-astrogam CTA South CTA North HiSCORE LHAASO Energy (MeV) 5/11

6 Alternative Approach Liquid Argon Time-Propagation Chamber Fully active tracking detector (and converter) 3% X0 (6.5 mm) 32 LAr-TPCs - X0 = K, 68.9 kpa ~100% X0 diluted in 1 m Inter-layer distance 2.5 cm Conversions close to the bottom of the TPC layer gives better angular resolution due to less material and longer lever arm 5 better angular resolution than Fermi LAT average aperture angle of e + e - pair 6/11

7 Compton Telescope Performance It also functions as Compton telescope Full tracking of recoil electrons provide better background rejection Si sensors can provide only 2 3 hits Compton telescope performance Energy resolution ~ 3% Angular resolution ~ 1.8 MeV Attenuation length ~ 18 1 MeV 7/11

8 LAr TPC Layer Multi-layer printed cross strips 100 µm pitch 45 with respect to z axis Length of the longest strip is ~ 1cm to minimize capacitance ~32,500 e h/layer (5,000 e ~70 kpa) 8/11

9 Scintillation light from LAr TPC Scintillation light 36,400 photons/layer (5,600 photons/mm) λ ~ 128 nm Primary function is to provide triggers Start time of TPC Constrain fiducial volume of the track Reduce effect of cosmic-ray backgrounds 0.4 particles expected during the drift time Time of flight to constrain the direction of tracks (~1 ns resolution required) SiPM readout PMT is too bulky and too much material between TPC modules Photon detection efficiency is low (~10%) at ~128 nm Tetraphenyl-butadiene (TPB) coating as wave length shifter to increase P.D.E. R&D required to optimize photon yield (energy resolution) and time resolution Fiducial volume SiPMs Scintillation light SiPMs Drifting e 9/11

10 Single Module Dewar is needed to keep the LAr temperature (~84 K) and pressure (~70 kpa) Passive radiators can dissipate ~1 W/m 80 K Dewar containing a tracker module Top and bottom windows Calorimeter 10/11

11 Satellite Design A total geometrical area of 4 m 2 and 1 m height ~6 ton Full FoV > FoV of each module The 8 modules are covered by an Anti Coincidence Detector > 5 effective area of Fermi LAT 215 cm 50 cm 100 cm Cathode Readout Dewar containing a tracker module 105 cm 55 cm 11/11