Isotope identification and fluxes with the AMS detector
IDPASC Portugal - PHD Programme 2015
2015 / 2016
Experimental Particle Physics | Astroparticle Physics
Instituto Superior Técnico
AMS (Alpha Magnetic Spectrometer) is a broad international collaboration involving around 500 researchers from 56 institutes in 16 countries that operates a cosmic ray observatory installed on the International Space Station (ISS). ESA (European Space Agency) and NASA (National Aeronautics and Space Administration), in the USA, are two of the main supporting organizations. AMS-02 is a state-of-the-art particle detector designed to directly detect cosmic ray particles in Earth's vicinity coming from the Universe. The main goals consist of performing a detailed measurement of the cosmic ray spectrum, searching for cosmological antimatter and searching for dark matter. Its whole set of subdetectors allows for a large particle spectrum, up to the iron element. The long exposure time together with its large acceptance (~0.5 m^2 sr) of AMS allows for an unprecedented collection of statistics, acquiring more than 60 000 million events, by the end of 2014. Observations of light isotopes provides information on the origin of cosmic rays and propagation in the Galaxy. Moreover, the study of unstable isotopes, like 10Be, is essential to disentangle the \ size of the galaxy halo from the diffusion coefficient, translating the force of the diffusion process cosmic rays undergo, two of the ingredient parameters of cosmic rays propagation. Some isotope\ s are of primary origin while others are produced by collisions of cosmic primaries with the interstellar matter. The LIP group has developed in the past expertise on isotopic separation with AMS a\ nd in particular using the RICH detector. Studies were performed for different isotopes: D/p, 3He/4He and 10Be/9Be [LArrudaMaster2003]. The separation of deuterons from protons, for instance, remai\ ns a hard task taking into account the percent fraction of secondary deuterons on cosmic rays. The higher fractions of He3/He4 and Li6/Li7 will make, hopefully, the task easier. Isotope identification requires a detector providing a good mass separation. The ability to separate masses relies on a good measurement of both velocity and momentum. The passage of the AMS superconducting magnet to a permanent magnet (B~0.15 T) implied a revision of the AMS detector capabilities to separate isotopes. The particle momentum is now measured with a resolution of ~10%. The new magnetic field characteristics make the classical separation method, based on the direct mass measurement, harder to explore. A new and very promising separation method was studied and reported to ICRC 2011 conference [F Barao, one of the authors]. It uses the Earth magnetic field together with the high accuracy on measuring particle's velocity of the AMS experiment. The method is very promising but requires validation. Moreover, it covers the energy region of interest (up to ~10-15 GeV) for deriving the galaxy halo size, so important for dark matter searches. The basic idea of the proposed new method is that associated to every detector position along its orbit (altitude and geomagnetic latitude), as well as to every particle arrival direction, there is a minimal rigidity value for a primary charged particle to be detected - the geomagnetic cutoff rigidity. This implies a different cutoff velocity value for different mass particles. Therefore, there will be three different velocity regions: a first one forbidden to primaries, followed by a second one where only the heavier isotope can be of primary origin and a third one where both isotopes are primaries. The velocity measurement with a precision of one per mil made by the RICH shall allow to distinguish the different particle regions.