OBSERVATION OF DIFFUSE COSMIC AND ATMOSPHERIC GAMMA RAYS AT BALLOON ALTITUDES WITH AN ELECTRON-TRACKING COMPTON CAMERA

Abstract

We observed diffuse cosmic and atmospheric gamma rays at balloon altitudes\nwith the Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I (SMILE-I) as\nthe first step toward a future all-sky survey with a high sensitivity. SMILE-I\nemployed an electron-tracking Compton camera comprised of a gaseous electron\ntracker as a Compton-scattering target and a scintillation camera as an\nabsorber. The balloon carrying the SMILE-I detector was launched from the\nSanriku Balloon Center of the Institute of Space and Astronomical Science/Japan\nSpace Exploration Agency on September 1, 2006, and the flight lasted for 6.8\nhr, including level flight for 4.1 hr at an altitude of 32-35 km. During the\nlevel flight, we successfully detected 420 downward gamma rays between 100 keV\nand 1 MeV at zenith angles below 60 degrees. To obtain the flux of diffuse\ncosmic gamma rays, we first simulated their scattering in the atmosphere using\nGeant4, and for gamma rays detected at an atmospheric depth of 7.0 g cm-2, we\nfound that 50% and 21% of the gamma rays at energies of 150 keV and 1 MeV,\nrespectively, were scattered in the atmosphere prior to reaching the detector.\nMoreover, by using Geant4 simulations and the QinetiQ atmospheric radiation\nmodel, we estimated that the detected events consisted of diffuse cosmic and\natmospheric gamma rays (79%), secondary photons produced in the instrument\nthrough the interaction between cosmic rays and materials surrounding the\ndetector (19%), and other particles (2%). The obtained growth curve was\ncomparable to Ling's model, and the fluxes of diffuse cosmic and atmospheric\ngamma rays were consistent with the results of previous experiments. The\nexpected detection sensitivity of a future SMILE experiment measuring gamma\nrays between 150 keV and 20 MeV was estimated from our SMILE-I results and was\nfound to be ten times better than that of other experiments at around 1 MeV.\n