Detection and measurement of gamma rays with the AMS02 detector

1
Detection and measurement of gamma rays with the
AMS-02 detector

• Mariusz Sapinski, INFN, Roma1, on behalf of the
  AMS-02 collaboration

Astrophysical gamma sources
The Alpha Magnetic Spectrometer (AMS-02) detector
will collect data on the International Space
Station (ISS) for at least three years. The gamma
rays can be measured through gamma conversion
into ee- pair, before reaching the Silicon
Tracker or by measurement of a photon hitting
directly the Electromagnetic Calorimeter (ECAL).
AMS-02 will provide precise gamma measurements in
the GeV range, which are particularly relevant
for Dark Matter searches. In addition, the good
angular resolution and identification
capabilities of the detector will allow clean
studies of the main galactic and extra-galactic
sources, diffuse gamma background and Gamma Ray
Bursts.
The 3rd EGRET catalog contains 271 sources among
which 170 are unidentified. AMS-02 will allow to
collect more photons from these sources and
localize them more precisely. Discovery of new
sources is also expected.

Detector performance
Present status of gamma ray measurements
There are two complementary modes of gamma
detection. In so called conversion mode photons
are converted into electron-positron pairs in the
Transition Radiation Detector (TRD) material
(about 0.25X0) before reaching the first
Time-of-Flight (TOF) layer. The electron-positron
pair is triggered by the TOF system. In this mode
the viewing angle is about 42.
EGRET measurements of gamma ray fluxes performed
in 1990s are the most precise data available
nowadays in GeV energy range.
The precise data will allow, for example,
distinguish between two modes of pulsar emission
(Outer gap and Polar cup). Example for Vela
pulsar.
g
g
3.8 meters
TRD, 0.3 X0
Many recent theoretical works show need for more
precise measurements. The main fields where more
data are requested are emission from gamma
sources (pulsars, blazars, AGNs), diffuse gamma
background emission, Dark Matter searches and
Gamma Ray Bursts (GRB).
e
e-
Diffuse gamma background
2.5 meters
The photons which pass through AMS without
interaction can be measured only in ECAL. To
trigger for such photons a special ECAL-based
trigger has been developed. The viewing angle in
this, so called ECAL mode is about 23.
The galactic gamma ray diffuse background is
believed to be produced in the interstellar
medium by p0 decay, bremsstrahlung and inverse
Compton scattering.
uncovered energy range
The observation area
AMS-02 will be rigidly attached to the ISS. The
amount of time spend on observations of different
regions of the sky is determined. For instance
AMS-02 will see Galactic Center for about 400
hours per year.
Angular and Energy resolution for gs
The diffuse gamma ray spectrum from the Galactic
Center. EGRET measurements present controversial
excess with respect to modelized fluxes. AMS-02
will contribute to the understanding of the
diffuse gamma spectrum in the GeV region.
Gamma Ray Bursts
Large acceptance and large field of view of
AMS-02 will allow the observation of a few GRBs
during more than 3 years of mission. These
measurements, despite of its limited number, will
be very interesting due to the fact that they
will cover an unexplored energy range. At the
time of AMS-02 mission other dedicated GRB
observatories will be on orbit. Synchronization
of observations and the good time resolution of
AMS-02 will provide new results on GRBs.
The main background for gamma detection in both
modes are protons, due to their high abundance in
cosmic rays. Simulations show that the background
rejection factor at the level of O(106) can be
achieved. This will allow to keep
background-to-signal ratio at the level of a few
percent.