A new concept of gamma-ray telescope. LArGO: Liquid Argon Gamma-ray Observatory
The completely anew and original idea of exploiting a liquid Argon Time Projection Chamber (LAr-TPC) as tracker-converter for a g-ray telescope, is presented by A.G. Caliandro (Institut de Ciències de l’Espai) and B. Rossi (Università di Napoli Federico II).
This interesting project, named LArGO (Liquid Argon Gamma-ray Observatory), has recently been submitted for FIRB funding as a Collaboration between Trieste and Naples Universities and INFN, coordinated by A.G. Caliandro.
The physical case is essentially twofold: on one side the study of sources in a broad 100 keV – 100 GeV energy range, aimed at covering the present observational gap between hard X-rays and gamma-rays, and overlapping at high energy the range declared by CTA (>50 GeV); on the other side the detection of the polarization of g-ray emission from astrophysical sources.
But where does the idea of using a LAr-TPC come from?
Let’s briefly recall the operation principle of a gamma-ray telescope. It is mainly composed by three sub-detectors: the principal one is a tracker-converter, where the photon converts and the pair is tracked, and whose spatial resolution drives the capability of reconstructing the incident photon direction; an anti coincidence detector covers the tracker to veto charged particles; finally, a calorimeter below the tracker measures the energy of the pair.
The solution adopted for the tracker converter by present gamma-ray telescopes (LAT, AGILE), i.e. several planes of high-Z material (tungsten) interleaved with active layers of silicon strip detectors (SSD), suffers from the conversion of the photon into an e+e– pair happening in a passive material.
Right about here come into play the main advantages brought by the LAr-TPC technique, broadly demonstrated by the successful operations of ICARUS detector at LNGS Gran Sasso Laboratories: since the conversion happens in LAr itself, which is fully active, particle tracking can efficiently start since the primary photon vertex. Indeed, being the radiation length in LAr X0 = 14 cm, a compact detector 20-30 cm deep has conversion efficiency 76%-88%, respectively (to be compared with the 63% of Fermi-LAT); furthermore, given the dE/dx = 2 MeV/cm stopping power, even a very low energy 10 MeV electron can be tracked for a few centimeters. Finally, LAr-TPCs can also be used as photon detector via Compton scattering (a la COMPTEL).
Summarizing, a LAr-TPC used as tracker-converter in a gamma-ray telescope can lead to an unprecedented angular resolution for a gamma-ray telescope, disclosing the possibility to detect the polarization of the gamma-ray emission. Even beyond, its capability of efficiently work in Compton regime as well as in pair conversion, would lead to a telescope with a broad energy range 100 keV – 100 GeV if coupled with a calorimeter 10 X0 deep.