S. Buitink et al., "A large light-mass component of cosmic rays at 1017 - 1017.5 electronvolts from radio observations," Nature, V531, pp70-73, March 2016.
abstract:
Cosmic rays are the highest-energy particles found in
nature. Measurements of the mass composition of cosmic rays with
energies of 1017 - 1018 electronvolts are essential
to understanding whether they have galactic or extragalactic
sources. It has also been proposed that the astrophysical neutrino
signal comes from accelerators capable of producing cosmic rays of
these energies. Cosmic rays initiate air showers - cascades of
secondary particles in the atmosphere - and their masses can be
inferred from measurements of the atmospheric depth of the shower
maximum (Xmax; the depth of the air shower when it contains
the most particles) or of the composition of shower particles reaching
the ground. Current measurements have either high uncertainty, or a
low duty cycle and a high energy threshold. Radio detection of cosmic
rays is a rapidly developing technique for determining Xmax
with a duty cycle of, in principle, nearly 100 per cent. The radiation
is generated by the separation of relativistic electrons and positrons
in the geomagnetic field and a negative charge excess in the shower
front. Here we report radio measurements of Xmax with a
mean uncertainty of 16 grams per square centimetre for air showers
initiated by cosmic rays with energies of
1017 - 1017.5 electronvolts. This high resolution
in Xmax enables us to determine the mass spectrum of the
cosmic rays: we find a mixed composition, with a light-mass fraction
(protons and helium nuclei) of about 80 per cent. Unless, contrary to
current expectations, the extragalactic component of cosmic rays
contributes substantially to the total flux below 1017.5
electronvolts, our measurements indicate the existence of an
additional galactic component, to account for the light composition
that we measured in the 1017 - 1017.5 electronvolt
range.
back to publication
list
|