The Energy Spectrum of Cosmic‐Ray Electrons from 10 to 100 GeV Observed with a Highly Granulated Imaging Calorimeter

Abstract

Cosmic-ray electrons1 have been observed in the energy range from 12 to D100 GeV with a new balloon-borne payload, the Balloon-borne Electron Telescope with Scintillating Fibers (BETS). This is the Ðrst publication of the absolute energy spectrum of electrons measured with a highly granulated Ðber calorimeter. The calorimeter makes it possible to select electrons against the background protons by detailed observation of both the longitudinal and the lateral shower development. The performance of the detector was calibrated by the CERN-SPS accelerator beams : electrons from 5 to 100 GeV, protons from 60 to 250 GeV. The balloon observations were carried out twice, in 1997 and 1998, at the Sanriku Balloon Center (Institute of Space and Astronautical Science) in Japan. The observation time was D13 hr in all at an altitude above 34 km. A total of 1349 electron candidates were collected, and the 628 events with energies above 12.5 GeV, well above the geomagnetic rigidity cuto † of D10 GV, have been used to compose a di †erential absolute energy spectrum at the top of the atmosphere. The energy spectrum is described by a power-law index of 3.00^0.09, and the absolute di †erential intensity at 10 GeV is 0.199^0.015 m~2 s~1 sr~1 GeV~1. The overall shape of the energy spectrum in 10 D 100 GeV can be explained by a di †usion model, in which we assume an energy-dependent di †usion coefficient (PE0.3) for an injection spectrum, E~2.4.

The Origin of Cosmic Ray Electrons and Positrons

“…For example, the PAMELA (Bonvicini et al 2001) and AMS 02 (Aguilar et al 2002) detectors will be magnetic spectrometers with proton-electron discrimination obtained using an electromagnetic calorimeter and a transition radiation detector. In addition, the proposed CALET experiment (Torii et al 2003) will adopt calorimetric techniques to measure the electron (plus positron) spectrum up to 10 TeV, allowing for the discovery of the maximum energy beyond which the spectrum will show a cutoff.…”

“…Many authors emphasized that the CR electron spectrum cannot be considered to be representative of a Galactic average, but it must reflect the recent history of the solar system neighborhood (see, e.g., Torii et al 2001;Müller 2001;Stephens 2001), especially in the highest energy range. However, the existing data on direct CR electron and positron measurements ( Fig.…”

Observation of Multi‐TeV Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array

“…They also argued that an electron injection index of 2.0 is within the expected Poisson fluctuations and is reasonable in the direction toward the Galactic center with the line of sight passing through the vicinity of many SNRs, taking into account the diffusion coefficient and the observed local electron index of around 3.0 below 1 TeV. Recently, the energy spectrum of local electrons has been obtained with balloon-borne instruments HEAT (Barwick et al 1998) and BETS (Torii et al 2001).…”

The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

“…The CR electron and positron data used are as follows: AMS01 (AMS Collaboration et al 2002), CAPRICE94 (Boezio et al 2000), HEAT (DuVernois et al 2001), SANRIKU (Kobayashi 1999), BETS (Torii et al 2001), PPT-BETS (Yoshida et al 2008), ATIC-1-2 (Chang et al 2008), H.E.S.S. (Aharonian et al 2008(Aharonian et al , 2009, Fermi-LAT (Abdo et al 2009b;Ackermann et al 2010), PAMELA (Adriani et al 2011).…”