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 with the Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I (SMILE-I) as the first step toward a future allsky survey with a high sensitivity. SMILE-I employed an electron-tracking Compton camera comprised of a gaseous electron tracker as a Compton-scattering target and a scintillation camera as an absorber. The balloon carrying the SMILE-I detector was launched from the Sanriku Balloon Center of the Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency (ISAS/JAXA) on September 1, 2006, and the flight lasted for 6.8 hr, including level flight for 4.1 hr at an altitude of 32-35 km. During the level flight, we successfully detected 420 downward gamma rays between 100 keV and 1 MeV at zenith angles below 60 degrees. To obtain the flux of diffuse cosmic gamma rays, we first simulated their scattering in the atmosphere using Geant4, and for gamma rays detected at an atmospheric depth of 7.0 g cm −2 , we found that 50% and 21% of the gamma rays at energies of 150 keV and 1 MeV, respectively, were scattered in the atmosphere prior to reaching the detector. Moreover, by using Geant4 simulations and the QinetiQ atmospheric radiation model, we estimated that the detected events consisted of diffuse cosmic and atmospheric gamma rays (79%), secondary photons produced in the instrument through the interaction between cosmic rays and materials surrounding the detector (19%), and other particles (2%). The obtained growth curve was comparable to Ling's model, and the fluxes of diffuse cosmic and atmospheric gamma rays were consistent with the results of previous experiments. The expected detection sensitivity of a future SMILE experiment measuring gamma rays between 150 keV and 20 MeV was estimated from our SMILE-I results and was found to be ten times better than that of other experiments at around 1 MeV.

New readout and data-acquisition system in an electron-tracking Compton camera for MeV gamma-ray astronomy (SMILE-II)

“…The detector for this mission comprised a small ETCC with a 10×10×15 cm 3 TPC and 33 pixel scintillator arrays (PSAs) [2112 Gd 2 SiO 5 :Ce (GSO) pixel scintillators]. This detector obtained good background noise rejection and observed the spectra of both diffuse cosmic and atmospheric gamma rays in the energy range from 100 keV to 1 MeV [19]. In addition, this detector provided good performances for medical applications [20].…”

“…Hence, we developed the medium-sized ETCC with a (30 cm) 3 TPC and 108 PSAs (6912 GSO pixel scintillators) for SMILE-II [22]. The SMILE-II ETCC also benefited from more efficient tracking of recoil electrons compared with that of SMILE-I: the ∼10% efficiency of SMILE-I [19] was improved to ∼100% efficiency by improving the readout electronics and the algorithm for finding tracks in the TPC [23]. Together, the larger detection volume and improved tracking method led to the significant increase in effective area, while keeping the weight and power consumption similar to those of SMILE-I.…”

The performance evaluation of the Electron Tracking Compton Camera

“…Therefore the incident gamma-ray direction has uncertainty. Whereas ETCC determines the direction and the energy of the incident gamma-ray by using those of the scattered gamma ray and the recoil electron [2] [3]. The Compton scattering occurs when the incident gamma ray is scattered about the orbital electron of a gas molecule.…”

Imaging Polarimeter for a Sub-MeV Gamma-Ray All-sky Survey Using an Electron-tracking Compton Camera

“…Details of its design and performances are described in Mizumura et al (2014), Matsuoka et al (2015), Mizumoto et al (2015), and Tanimori et al (2015); Tanimori et al (2017). Based on the current analysis, the Compton interaction point in the TPC is determined by a track fitting analysis with a spatial resolution of less than 1 cm (Takada 2007), and the absorption positions of scattered gamma-rays in the PSAs are determined within the size of the pixel scintillator (6×6×13 mm 3 ). In general, Compton cameras, including the ETCC, can be used as Compton polarimeters because the Compton camera determines the three-dimensional direction of the scattered gamma-ray as the direction from the interaction point to the absorption point and obtains the angular distribution of the scattered gamma-ray D(θ, φ) for each gammaray.…”