04.03.2011 -
The PAMELA experiment - an international scientific
enterprise coordinated by the National Institute for Nuclear
Physics (INFN - Istituto Nazionale di Fisica Nucleare) - has
discovered that the protons and helium nuclei in cosmic rays in the
Galaxy are not accelerated to high energies in the same way (in the
range of hundreds of GeV). This could mean that there are various
types of cosmic particle accelerators that act with different
mechanisms. In fact, until now, it was believed that this "job" was
carried out by the highly turbulent remains of large, exploded
stars (supernovae), in exactly the same way for all charged
particles. Now, however, the observations made by PAMELA show that
protons and helium may be accelerated by different
sources.
The study that explains this phenomenon will be
published online tomorrow in Science Express, an online
magazine providing electronic publication of selected papers from
the American magazine Science, in advance of full
print.
The data was collected by PAMELA between 2006 and
2008 and relates to the flow of protons and helium nuclei; in other
words almost all the cosmic radiation that we intercept. It has
been discovered that the spectrum of the protons and that of the
helium nuclei have different paths. This difference could therefore
be proof that something is causing them to accelerate in different
ways.
One theory could be the existence of an unknown
acceleration mechanism that acts differently for the various
particle species. A more traditional explanation suggests that
galactic cosmic rays are accelerated both by novae, enormous
nuclear explosions caused by hydrogen accumulation on the surface
of a white dwarf, and by a different kind of supernova. Protons are
supposedly accelerated by the explosion of smaller supernovae,
where the stellar atmosphere is mainly rich in protons, and the
helium nuclei in larger stars, where the stellar atmosphere is
richer in helium. These different conditions could produce the
diverse effects on their energy flows, as observed by
PAMELA.
"These results," states Piergiorgio Picozza, Principal
Investigator for PAMELA, "seriously call into
question the paradigm that holds that cosmic rays are accelerated
solely by the shock wave from supernova remains, then propagated
throughout the Galaxy. They require more complex acceleration
processes that will soon be the subject of an in-depth theoretical
study. This is an important advance in our understanding of cosmic
ray acceleration mechanisms in our Galaxy, one that complements the
recent AGILE and FERMI experiments."
PAMELA is an experiment on a Russian satellite that
orbits at a height of between 350 and 610 km and is the result of a
collaboration between the National Institute for Nuclear Physics,
the Russian Space Agency and Russian research institutes, with the
participation of the Italian Space Agency (ASI - Agenzia Spaziale
Italiana) and contributions from German and Swedish space agencies
and universities.
PAMELA (Payload for Antimatter Matter Exploration
and Light nuclei Astrophysics) is an experiment resulting from a
collaboration between the National Institute for Nuclear Physics
(INFN - Istituto Nazionale di Fisica Nucleare), the Russian Space
Agency and Russian research institutes, with the participation of
the Italian Space Agency and contributions from German and Swedish
space agencies and universities. The satellite equipment PAMELA was
launched in orbit from the Baikonur cosmodrome (Kazakhstan) on
15th June 2006 and since then has been collecting data
aboard the Russian satellite Resurs-DK1, that orbits the Earth at a
height of between 350 and 610 km.
The contribution of the Trieste division of
INFN in AREA Science Park
The primary purpose of the PAMELA experiment is to
study the different components that make up primary cosmic
radiation, with a precision and a range in terms of energy
intervals never before achieved. To achieve these objectives,
PAMELA uses various advanced, high performance particle detectors,
capable of identifying the different particles in cosmic rays by
the simultaneous measurement of their charge, their velocity and
their energy.
One of the main components of PAMELA has been
entirely designed and developed by researchers in INFN's Trieste
Division. This is the electromagnetic calorimeter, a sophisticated
detector capable of measuring the energy of the particles passing
across it and identifying, with extreme precision, the various
types, separating protons and nuclei from electrons and positrons.
The Trieste Division also coordinates all the data analysis work
from the experiment.