Nuclear gamma-ray lines from low-energy cosmic rays

High ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray (LECR) component. Such a hadronic component below a few hundred MeV per nucleon produces efficiently nuclear gamma-ray line emission in the 0.1 – 10 MeV range by interactions with the interstellar gas and dust. A detection of this emission would give the clearest evidence of LECRs. The objective is to detect the strongest lines, i.e. at 4.4 and 6.1 MeV from ambient 12C and 16O, respectively, and to map the total gamma-ray line emission from the inner Galaxy in the 0.1-10 MeV range.

Predicted gamma-ray emission from the inner galaxy (|l| < 60°, (|b| < 10°) compared to data from CGRO/COMPTEL and Fermi/LAT. The hadronic emission is produced by a wealth of nuclear de-excitation lines below 10 MeV and by pion decay at higher energies. Credit: J. Kiener et al.

Required instrument performances:
The diffuse emission is certainly concentrated in the Galactic plane with a longitudinal profile following roughly the H2 column density with eventual concentrations around acceleration sites. A large field-of-view is necessary for the observation of diffuse emission, ideally covering the inner Galaxy  (longitude |l| < 60°). The required energy resolution is of the order of the width of the strongest lines at 4.4 and 6.1 MeV, ΔE ~ 100 keV. The angular resolution should be of the order of the Galactic latitude profile of the emission, i.e. about 1o. The brightness of the 4.4-MeV narrow line from the inner Galaxy is expected to be in the range (0.2 – 2)
× 10-5 cm-2 s-1 sr-1 and the broadband emission (in the energy ranges 1 – 3 and 3 – 8 MeV) is about ten times brighter.

Performance parameter Goal value Remarks and notes
(FWHM, deg)
2 sr  
Angular resolution
(FWHM, deg)
Spectral resolution
(ΔE/E @ Energy)
0.02 @ 5 MeV  
Line sensitivity (@ Energy)
(cm-2 s-1, 3σ, 1 Ms)
~10-5 @ 5 MeV Diffuse emission. To detect the line from the inner Galaxy.
Continuum sensitivity (in which energy band?)
(cm-2 s-1 keV-1, ΔE=E, 3σ, 1 Ms)
2×10-8 @ 1-8 MeV Diffuse emission. To map the broadband emission from the inner Galaxy.
Timing performances -  
Polarimetric capability
(Minimum Polarization Fraction for a Crab source in 1 Ms)
Real-time data? no