Alpha Magnetic Spectrometer

The Alpha Magnetic Spectrometer (AMS-02), to be realized by an international collaboration, leaded by the Nobel Prize Samuel Ting, is the first large acceptance (0.65 m²sr) particle detector designed to operate in space to measure, for many years, cosmic ray fluxes.  It will be installed, as part of the scientific program, on the International Space Station (ISS) Alpha, which will fly in a low (about 400 km) orbit around Earth, where it will acquire data for at lest three years. 
In its first configuration (known as AMS-01), it was installed on the shuttle Discovery, that during the STS-91 mission (2 - 12 June 1998) carried it along an orbit similar to that of the ISS.  During the 10 days of this trial mission, AMS acquired data corresponding to approximately 100 millions of triggers, letting us understand how well it will behave during the ISS. 
Following the precursor flight data analysis, a major upgrade of the detector has been proposed (called AMS-02 in the following), based on a superconducting magnet and on the addition of various detectors (Synchrotron radiation detector - SRD, Transition radiation detector - TRD, Ring Imaging Cherenkov detector RICH, Electromagnetic Calorimeter - ECAL).   The upgrade will allow AMS to 
extend the particle measurement and identification to very high energies. 
 

AMS-01

The AMS-01 detector is based on a permanent Ne-Fe-B magnet with an analyzing power of 0.15 T m2 that contains 4 of the 6 layers of the Silicon tracker and the scintillator counters of the anticoincidence system.   Each tracker plane gives a measure of 2 coordinates (x,y) with a resolution of 30 (x) and 10 (y) microns  and of energy deposition, thus giving the particle momentum and charge. 
The trigger is given by the time of flight (TOF) system, that in addition measures the velocity of the traversing particles and their charge.   Combined to the tracker measurements, this allows the determination of the particle mass. 
The detector is completed with a threshold Cherenkov counter, below the magnet, to improve the separation between electrons and protons up to 3.5 GV.

The AMS Bologna Group was responsible for the Time Of Flight (TOF) system of AMS-01.
This subdetector is used to trigger the data acquisition (DAQ) process, to take a precise (rms=120 ps) measurement of the time needed by a particle to traverse the AMS detector.

AMS-02

During the years spent on the International Space Station, AMS will have a different configuration from that of the STS-91 flight.  The magnet will be a superconducting one, and the tracker will have 8 Si planes, to achieve better rigidity resolution, there will be a RICH to extend to higher energies the TOF measurement range, and a TRD and an electromagnetic calorimeter will improve the capability to distinguish between different particles.  In addition, it is foreseen a SRD to analyze electromagnetic emissions from high energy electrons. 
If you want the most recent data available, go to the AMS home page.

The AMS Bologna Group is responsible for the Time Of Flight (TOF) system of AMS-02, and participates to the development of the proximity focusing Ring Imaging Cherenkov counter (RICH) of AMS-02.
 
 

AMS PHYSICS GOALS

The cosmic ray isotope distribution is currently well known only at low energies, below about 500 MeV/nucleon kinetic energy, whereas the element distribution is known up to about 35 GeV/c/nucleon and beyond for some element. In this context, AMS could extend the measurements up to 10 GeV/nucleon for light isotope identification. For the elements the upper momentum limit would be the maximum value measurable by the tracker with sufficient accuracy (about 1 TeV/nucleon).

Cosmic antimatter search

The main physics goal of AMS is the search for cosmic antimatter, that is for primary antinuclei.    During their propagation, cosmic rays interact with the interstellar medium and with the solar wind, producing a lot of different particles and antiparticles, for example antiprotons and antineutrons.   There is a small but finite probability to produce light antinuclei, like antideuteron, but this probability is exponentially decreasing with increasing nucleus mass.   There is a little probability that AMS will find few secondary antideuterons, but it's pratically impossible to reveal heavier antinuclei during its 3 years mission.    The detection of antihelium should then interpreted as the evidence of antistars.

The cosmic evolution, starting from the "big bang", could have produced two scenarios: a symmetric universe with equal amounts of particles and antiparticles, or a antisymmetric one with only particles.   In the first case, today we expect to see "islands" of matter and islands of antimatter separated by vacuum -- they cannot touch themselves -- but we don't know what is their separation: in principle it could be so big that the nearest antimatter island is outside our visible universe.

The light coming from stars and antistars is the same, so that we could test the existence of antimatter islands only by searching for antiparticles escaped from them.    Because the universe evolution should have been the same for matter and antimatter, we expect the same isotopic distribution for the cosmic antimatter: about 75% antiprotons, about 25% antihelium nuclei, few scars of other antinuclei.    This is the reason why AMS people is interested in antihelium nuclei: they are the only way to "see" antimatter islands.

For a short history of antimatter studies, click here.
 

Cosmic Rays propagation

The population of cosmic rays reaching the earth results from a highly complex succession or combination of physical processes including synthesis in the source of primary cosmic rays, ejection from the source, acceleration in supernovae originated shock fronts, transport in the galactic magnetic field and interstellar gas/plasma, spallation on interstellar matter giving rise to secondaries, interaction with the solar wind, stochastic reacceleration, etc...

The study of the cosmic ray abundances and energy distributions is an invaluable source of information on their transport history, and on the nature and distribution of their sources. In practice, the problem consists in solving a system of transport equations which incorporate terms arising from the various processes taking place during the propagation, Solving the system provides the source abundance of the particles (with parametrized momentum distributions), together with the parametrized escape length of the particles, halo distribution and matter density, depending on the particular model and on the nuclei studied.

Some specific information relevant to the propagation conditions can be extracted from some particular subsystems, like D/⁴He, ³He/⁴He, B/C or subFe/Fe. More generally, all purely secondary nuclei like Li, Be, B, ¹⁵N, ^17,18O, ¹⁹F, ²¹Ne, ⁵⁹Co for example are of particular interest for the comparison between primaries and secondaries originating from the former. The abundance of stable secondaries provide the path length distributions of cosmic rays through the interstellar matter, and their momentum dependence, the latter depending on the reacceleration processes.

Unstable nuclei play a very important role since a few of them having suitable mean lifetimes can provide unique information on their time confinement inside the galaxy. As an example, Beryllium is produced by fragmentation of heavier cosmic rays and is entirely secondary. ¹⁰Be with a half-life of 1.6 10⁶ years is the most important radioactive clock for measuring the age of the cosmic rays. The ratio of radioactive ¹⁰Be to the stable ⁹Be is sensitive to the propagation lifetime of the cosmic rays and not to the total amount of matter traversed. At relativistic energies, time-dilation extends the decay lifetime of the ¹⁰Be, allowing it to probe longer mean ages. By extending measurements of beryllium isotopes to high energies (2÷8 GeV/nucleon) AMS can distinguish between current models of cosmic ray transport.