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THE AMS-02 EXPERIMENT
The Alpha Magnetic Spectrometer (AMS-02) is a high-precision particle detector
built to operate in space [1]. It was installed on the International Space Station
(ISS) in May 19th 2011 during the STS-134 NASA Endeavour Shuttle mission and
it started taking data few days later. AMS-02 is the successor of the AMS-01
detector [2] that flew in June 1998 on the Shuttle Discovery (NASA mission STS-91).
AMS-02 will run until to the decommissioning of the ISS, scheduled for 2024.
1. SUB-DETECTORS
AMS-02 has been designed as a scaled-down version of the detectors sitting inside particle
accelerators. At its core, a cylindrically shaped permanent magnet generates a magnetic
field of ~0.15 T which bends the trajectory of charged particles traversing it. A
multi-layer silicon tracker precisely measures the X and Y position and charge of the
particles: seven layers are inside the magnet bore, constituting the inner tracker,
while two are outside, increasing the lever arm from approximately 1 m to almost 3 m.
Together with the magnet, the tracker acts as a spectrometer, measuring the particle
rigidity in the range 1 GV to few TV. A Time of Flight (TOF) system composed of four
layers of segmented scintillators (two above and two below the magnet) provides the main
trigger to the experiment, measures the particle velocity and absolute charge
and discriminates between up-going and down-going particles.
The inner tracker is surrounded by an Anti-Coincidence Counter (ACC), a series of
scintillating paddles which veto in the trigger particles entering AMS-02 from the sides.
Three additional sub-detectors enrich the capabilities of AMS-02 to identify particles:
a Transition Radiation Detector (TRD), on top of the upper TOF layers, separates electrons
(e-) and positrons (e+) from hadrons and measures the absolute charge of nuclei; a Ring
Imaging Cherenkov (RICH), below the lower TOF layers, extends the velocity measurement
range of the TOF and provides isotopic separation; finally, the Electromagnetic Calorimeter
(ECAL), at the bottom of the detector, precisely measures the energy of e+, e- and photons
in the range 0.5 GeV to 1 TeV and increase the lepton-hadron discrimination power of AMS-02.
For more details, see [3-7] and references therein.
2. TRIGGER
The AMS-02 trigger recognizes charged particles by a logical combination of TOF, ACC and
ECAL responses, producing a fast trigger and a level 1 trigger (LVL1). The trigger system
is very flexible, using pre-programmed masks that can be changed from the ground.
The LVL1 is divided in seven sub-trigger signals:
(1) Single charge
All 4 TOF layers above a high threshold, corresponding to 50% of a proton MIP, with
zero ACC fired.
(2) Normal ions
All TOF layers above a super-high threshold, corresponding to 400% of a proton MIP,
with no requirements on the ACC.
(3) Slow ions
Same as (2), but in a longer time window.
(4) Electrons
Same as (1), plus energy deposited in the ECAL above threshold in both X and Y
projections.
(5) Photons
Same as (4), but no requirements on the TOF and the electromagnetic shower axis
must point inside the AMS field of view.
(6) TOF unbiased trigger
3 TOF layers above high threshold, prescaled by a factor 100.
(7) ECAL unbiased trigger
Energy deposited in the ECAL above threshold in any projection,
Prescaled by a factor 1000.
The status of the DAQ is sampled every 20 ns, thus counting the fraction of time in a
second in which the detector is not busy, i.e. the live-time. The AMS-02 trigger rate
and live-time vary along the orbits of the ISS: at the Equator it is 200 Hz, while it
reaches almost 2000 Hz at the magnetic poles and in the South Atlantic Anomaly (SAA).
The live-time is always greater than 90%, except at the magnetic poles and in the SAA,
where the rate is so high that the AMS-02 DAQ saturates and the live-time goes to zero.
For more details, see [8].
3. TEST BEAM
The fully assembled detector was tested at the SPS at CERN, by exposing it to the primary
proton beam (400 GeV/c), and to the secondary beams of positrons and electrons (10-290 GeV/c),
and charged pions (10–180 GeV/c). The test beams covered a variety of positions and angles
from the top, bottom and the sides, to realistically simulate the cosmic ray environment on
the ISS.
4. MONTE CARLO SIMULATION
The AMS-02 collaboration developed a Monte Carlo (MC) description of the entire apparatus,
based on the GEANT-4.10.1 package. The software simulates the electromagnetic and nuclear
interactions of particles with the passive and active material of the detector. All digitized
signals, including those of the trigger, are simulated according to the measured characteristics
of the electronics and then undergo the same reconstruction as used for real data. The MC samples
used in the analysis have sufficient statistics such that they do not contribute to the errors.
For more details, see [5-7,9,10].
5. DATA TAKING
AMS-02 data taking is divided in runs, usually lasting ~23 minutes.
Raw data consist of noise-subtracted digitized signals from the ~300000 analog channels for
every triggered event. Each channel is calibrated in flight every two runs (~46 minutes, i.e.
half ISS orbit). Housekeeping information (temperatures, gains, gas pressures, DAQ status) are
also collected each second. Every run stop, the memory of all digital signal processors is
checked with a CRC test: in case of failure, the node is rebooted.
Raw data are transferred from the ISS to the main computing center at CERN, where they are
corrected for additional offline calibrations and equalizations. Corrected data are then used
to build physical objects (track segments, RICH rings, ECAL showers, etc) for each triggered event.
These reconstructed data are finally available to all collaboration members for dedicated analysis.
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MONTHLY FLUXES ANALYSIS
Proton, helium, electron and positron fluxes have been measured at a time resolution of 27 days
(Bartels rotation) [13,14]. Events are binned in rigidity (p, He) or energy (e+, e-). The bin width
is chosen according to the rigidity or energy resolution.
The typical workflow of the analysis is the following:
(1) The desired species (p, He, e+ or e-) is selected among all the events by applying several
cuts on the charge measurements of the TOF and tracker layers and on the lepton-hadron
discriminators of the TRD and ECAL. Additional quality cuts (for example on the chi
squared of the fitted track) are used to further clean the sample. Deuterons are not
excluded from the proton sample, while the helium sample include both 3He and 4He
isotopes, see [4,5,6] for more details.
(2) The measured rigidity or energy is required to be greater than a factor of 1.2 times the
maximum geomagnetic cutoff within the AMS-02 field of view, ensuring that the particle
is a primary cosmic ray. See section 2 below for more details on the cutoff
determination.
(3) The effective acceptance (geometrical factor times selection efficiency is computed
by the MC simulation in each bin, following the same analysis as for real data.
(4) The selection efficiency of each cut is cross-checked between data and MC in each bin,
and the small differences (of the order of few percent) are taken into account to
correct the effective acceptance got from the MC.
(5) The trigger efficiency is measured directly from data, using the prescaled unbiased trigger
sample, and then cross-checked with MC.
(6) For p and He, the event counts in each bin is unfolded with at least two different unfolding
algorithms, to correct for the bin-to-bin migration due to the finite rigidity resolution of
the tracker. For e+ and e-, the binning is such that the bin-to-bin migration is negligible
above 2 GeV and no unfolding is needed.
(7) The flux F_i in the i-th bin is computed, under the assumption of isotropy in the incoming
direction of particles, as following:
F_i = N_i / (A_i * delta_i * eff_trg_i * T_i * W_i)
where N_i is the unfolded event counts, A_i the effective
acceptance,delta_i are the data/MC corrections to the selection
efficiencies, eff_trg_i the trigger efficiency, T_i the exposure
time (see section 2 below), and W_i is the bin width.
The assumption of an isotropic galactic cosmic ray flux outside the magnetosphere is reasonably
true since diffusion is the dominant process in the heliosphere above 400 MeV/n. The flux measured
by AMS-02 inside the magnetosphere consist of only primaries and all trajectories are allowed
because the upper geomagnetic cutoff in the AMS-02 field of view is used. In addition, since AMS-02
is orbiting on the ISS, its pointing direction is always changing during a Bartels rotation. For this
reason, local anisotropies related to the geomagnetic frame are washed out, as it has been verified
directly from data.
Solar energetic particles can be highly directional, especially in the early stages of a solar event,
but they are not included in the analysis (see section 1 below).
1. COLLECTION TIME
The collection time used in this analysis includes only those seconds during which the detector was in
normal operating conditions, AMS-02 was pointing within 40 deg of the local zenith, and the ISS was
outside of the SAA. In addition, those seconds when AMS-02 detects Solar Energetic Particles accelerated
by the Sun are excluded.
2. GEOMAGNETIC CUTOFF
The cutoff is calculated by backtracing particles from the top of AMS-02 out to 50 Earth’s radii [11]
using the most recent IGRF model [12]. The uncertainty on the cutoff determination is taken into account
in the systematic errors, as described in [4]. The geomagnetic cutoff factor is varied from 1.0 to 1.5
and the resulting fluxes show a systematic uncertainty of 2% at 1 GV and negligible above 2 GV. Using the
Tsyganenko 2005 model instead of the IGRF model does not introduce observable changes in the flux values
nor in the systematic errors. A safety factor of 1.2 is enough to account for the differences in the
geomagnetic cutoff computed with IGRF and Tsyganenko 2005 during disturbed periods (for example: March 7,
2012). However, the disturbed periods do not contribute much to the overall exposure time: the percent of
time with Kp>5 per Bartels rotation is on average less than 2%.
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DESCRIPTION OF DATA
The cadence of the data is 27 days, corresponding to a Bartels rotation (BR). Data period covered by this
release is from May 20, 2011 (BR 2426) to May 9, 2017 (BR 2506). BR 2426 starting date is May 15, 2011,
but days from May 15 to May 19, 2011 are not included because AMS-02 data taking started on May 20, 2011.
In BR 2459, days from October 22 to October 24, 2013 are not included because AMS-02 was performing detector
studies in that interval. In BR 2471, days from September 30 to October 7, 2014 are not included because
AMS-02 was performing detector studies in that interval. BR 2472 and BR 2473 (October 8, 2014 - November 30,
2014) are not provided because AMS-02 was performing detector studies in that interval. In BR 2504, days
from March 6 to March 8, 2015 are not included because AMS-02 was performing detector studies in that interval.
Data contained in this release consists of:
1) Proton differential flux, in units of 1/(m^2 sr s GeV/n) vs kinetic Energy per nucleon, in units of GeV/n,
in 45 bins from 0.433 to 59.37 GeV/n. The conversion from rigidity to kinetic energy per nucleon has
been done assuming that all particles are protons (mass = 0.9382720813 GeV/c^2). Statistical,
time-dependent, systematic and total uncertainties are also provided. Time-dependent uncertanties
include trigger and reconstruction efficiency uncertainties added in quadrature. These uncertainties
are different for each BR. Systematic uncertainties include acceptance, background contamination,
geomagnetic cutoff, event selection, rigidity resolution, unfolding, absolute rigidity scale and
time-dependent uncertainties added in quadrature. All systematic uncertainties, except the time-dependent
ones, are the same for all BRs. Total uncertainties are the statistical and systematic Uncertainties
added in quadrature.
2) Helium differential flux, in units of 1/(m^2 sr s GeV/n) vs kinetic
Energy per nucleon, in units of GeV/n, in 40 bins from 0.404 to 29.23 GeV/n. The conversion from rigidity
to kinetic energy per nucleon has been done assuming that all particles are 4He (mass = 4*0.9382720813
GeV/c^2). Statistical, time-dependent, systematic and total uncertainties are also provided. Time-dependent
uncertanties include trigger and reconstruction efficiency uncertainties added in quadrature. These
uncertainties are different for each BR. Systematic uncertainties include acceptance, background
contamination, geomagnetic cutoff, event selection, rigidity resolution, unfolding, absolute rigidity
scale and time-dependent uncertainties added in quadrature. All systematic uncertainties, except the
time-dependent ones,are the same for all BRs.Total uncertainties are the statistical and systematic
uncertainties added in quadrature.
3) Proton to helium differential flux ratio vs rigidity, in units of GV, in 40 bins from 1.92 to 60.30 GV.
The conversion from rigidity to kinetic energy per nucleon is not possible, since data are natively binned
in rigidity and thus the p and He bins in kinetic energy per nucleon do not match. Statistical,
time-dependent, systematic and total uncertainties are also provided.Statistical uncertainties are the p
and He statistical uncertainties added in quadrature. Time-dependent uncertainties are the p and He
time-dependent uncertainties added in quadrature. Systematic uncertainties are the p and He systematic
uncertainties added in quadrature, with correlations in rigidity resolution, unfolding, absolute rigidity
scale uncertainties taken into account.Total uncertainties are the statistical and systematic
uncertainties added in quadrature.
4) Electron differential flux, in units of 1/(m^2 sr s GeV) vs total energy, in units of GeV, in 49 bins
from 1.01 to 49.3 GeV. Statistical, systematic and total uncertainties are also provided. Systematic
uncertainties include acceptance and background contamination uncertainties added in quadrature. Total
uncertainties are the statistical and systematic uncertainties added in quadrature.
5) Positron differential flux, in units of 1/(m^2 sr s GeV) vs total energy, in units of GeV, in 49 bins
from 1.01 to 49.3 GeV. Statistical, systematic and total uncertainties are also provided.
Systematic uncertainties include acceptance and background contamination uncertainties added in
quadrature. Total uncertainties are the statistical and systematic uncertainties
added in quadrature.
6) Positron to electron differential flux ratio vs total energy, in units of GeV, in 49 bins from 1.01
to 49.3 GeV. Statistical, systematic and total uncertainties are also provided.Systematic
uncertainties include acceptance and background contamination uncertainties added in quadrature.
Total uncertainties are the statistical and systematic uncertainties added in quadrature.
The small differences between this ratio and manually computing the ratio directly from the
electron and positron fluxes are due to the independent optimizations of the flux and ratio analyses.
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REFERENCES:
[1] www.ams02.org
[2] M. Aguilar et al. (AMS-01 Collaboration)
The Alpha Magnetic Spectrometer (AMS) on the International Space Station:
Part I – results from the test flight on the space shuttle
Physics Reports 366(6):331, 2002
doi:10.1016/S0370-1573(02)00013-3
[3] M. Aguilar et al. (AMS-02 Collaboration)
First result from the Alpha Magnetic Spectrometer on the International Space Station:
precision measurement of the positron fraction in primary cosmic rays of 0.5–350 GeV
Physical Review Letters, 110(14):141102, 2013
doi:10.1103/PhysRevLett.110.141102
[4] M. Aguilar et al. (AMS-02 Collaboration)
Precision measurement of the proton flux in primary cosmic rays from rigidity 1 GV to 1.8 TV
with the Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 114(17):171103, 2015
doi:10.1103/PhysRevLett.114.171103
[5] M. Aguilar et al. (AMS-02 Collaboration)
Precision measurement of the helium flux in primary cosmic rays of rigidities 1.9 GV to 3 TV
with the Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 115(21):211101, 2015
doi:10.1103/PhysRevLett.115.211101
[6] M. Aguilar et al. (AMS-02 Collaboration)
Observation of the identical rigidity dependence of He, C, and O cosmic rays at high rigidities
by the Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 119(25):251101, 2017
doi:10.1103/PhysRevLett.119.251101
[7] M. Aguilar et al. (AMS-02 Collaboration)
Observation of new properties of secondary cosmic rays lithium, beryllium, and boron by the
Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 120(2):021101, 2018
doi:10.1103/PhysRevLett.120.021101
[8] C.H. Chung et al. (AMS-02 Collaboration)
AMS on ISS – Construction of a particle physics detector on the International Space Station, 2004
http://www-ekp.physik.uni-karlsruhe.de/~deboer/html/Forschung/AMS.pdf
[9] N. Tomassetti and A. Oliva
Identification of light cosmic-ray nuclei with AMS-02
33rd International Cosmic Ray Conference (Rio de Janeiro), 2013
http://www.cbpf.br/~icrc2013/papers/icrc2013-0896.pdf.
[10] M. Pohl
AMS tracking in-orbit performance
24th International Workshop on Vertex Detectors (Santa Fe)
Proceedings of Science, 2015
arXiv:1508.07759
[11] J. Alcaraz et al. (AMS-01 Collaboration)
Leptons in near earth orbit
Physics Letters B, 484(1–2):10, 2000
doi:10.1016/S0370-2693(00)00588-8
[12] E. Thébault et al.
International Geomagnetic Reference Field: the 12th generation
Earth, Planets and Space, 67:79, 2015
doi:10.1186/s40623-015-0228-9
[13] M. Aguilar et al. (AMS-02 Collaboration)
Observation of Fine Time Structures in the Cosmic Proton and Helium Fluxes
with the Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 121(5):051101, 2018
doi:10.1103/PhysRevLett.121.051101
[14] M. Aguilar et al. (AMS-02 Collaboration)
Observation of Complex Time Structures in the Cosmic-Ray Electron and Positron Fluxes
with the Alpha Magnetic Spectrometer on the International Space Station
Physical Review Letters, 121(5):051102, 2018
doi:10.1103/PhysRevLett.121.051102