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The energy calibration is performed with y rays of energy up to 9 MeV from the reaction Ni(n, y)Ni, with electrons from muon decays, and with the P decays from spallation products of cosmic-ray-muon interactions. From these calibrations, the absolute energy normalization is known to be better than 3%. The rms energy resolution for an electron is expressed by 22%/[E, /(10

Protons with energies up to ~ 10 15 eV are the main component 1 of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking 2 . Electrons are known to be accelerated to cosmic-ray energies in supernova remnants 3,4 , and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (π 0 ), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in γ-rays with a particular spectral-energy distribution 5,6 . Here we report the observation of cascade showers of optical photons resulting fromγ-rays at energies of ~ 10 12 eV hitting Earth's upper atmosphere, in the direction of the supernova remnant RX J1713.7-3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.

A new calculation of the Galactic diffuse gamma-ray spectrum from the decay of secondary particles produced by interactions of cosmic-ray protons with interstellar matter is presented. The calculation utilizes the modern Monte Carlo event generators, Hadrin, Fritiof and Pythia, which simulate high-energy proton-proton collisions and are widely used in studies of nuclear and particle physics, in addition to scaling calculation. This study is motivated by the result on the Galactic diffuse gamma-ray flux observed by the EGRET detector on the Compton Gamma-ray Observatory, which indicates an excess above about 1 GeV of the observed intensity compared with a model prediction. The prediction is based on cosmic-ray interactions with interstellar matter, in which secondary pion productions are treated by a simple model. With the improved interaction model used here, however, the diffuse gamma-ray flux agrees rather well with previous calculations within uncertainties, which mainly come from the unobservable demodulated cosmic-ray spectrum in interstellar space. As a possible solution to the excess flux, flatter spectra of cosmic-ray protons have been tested and we found that the power-law spectrum with an index of about −(2.4 ∼ 2.5) gives a better fit to the EGRET data, though the spectrum is not explained completely.Subject headings: cosmic rays -gamma rays -interstellar medium 1.Gamma-Ray Observatory, are described fairly well by a model based on dynamic balance and realistic interstellar matter and photon distributions (Hunter et al. 1996).However, the observed intensity exceeds the model prediction by as much as 60% for energies above about 1 GeV. One of the possible explanation of this discrepancy is that the theory of diffuse gamma-ray production may not be adequate at high energies: astrophysical gamma-rays above 1 GeV have never been measured before with high statistical accuracy (Hunter et al. 1996).There are three basic components for the production of Galactic diffuse gamma-rays: nuclear interactions between cosmic-rays and matter, bremsstrahlung collisions between electrons and matter, and inverse Compton scattering of electrons with low-energy photons. Above about 200 MeV, the first component, more specifically the gamma-ray production from the decay of neutral pions produced in cosmic-ray (protons and nuclei) collisions with interstellar matter, is known to be the dominant one. Previous works on this component used isobaric models and scaling models for the nuclear interaction: see Dermer (1986a) for detail. These studies, however, were focused on lower energies. This is natural since the gamma-ray spectrum from pion decay has a peak around 70 MeV and drops rapidly toward higher energies. In order to compare the observed high-energy diffuse gamma-ray emission with model predictions, however, it is necessary to use more detailed models which describe high-energy proton-proton (p-p) collisions more accurately. Here we use Monte Carlo event generators that are commonly used in the analysis of high-energy phys...