~3He/~4He Measurement With The Alpha Magnetic Spectrometer

Cosmic Rays(CRs),the high energy radiations from the outer space,consist of charged and neutral particles,of the charged ones 90%are protons,9%helium,and 1%heavier nuclei.From their discovery by Victor Hess in the early 20th century,the consequent discovery of new subatomic particles changed physics.Today,CRs are still a source of knowledge for the fundamental particle physics.An important direction in CR study is to search for exotic sources,such as the dark matter and the rare anti-matter components in cosmic rays.The search of these faint signals can be interpreted only with an improved knowledge of the cosmic rays standard model that includes the understanding of the origin,acceleration and subsequent propagation of the most abundant species that can produce,from their interaction in the Galaxy,the background for the rare species search.AMS-02 is an experiment launched in May 2011 and working continuously on the International Space Station since then.By the combined use of 5 independent detectors,AMS-02 is able to separate electrons,positrons,protons,anti-protons,and nuclei(chem-ical and isotopic composition)with great accuracy in the rigidity range between fraction of the GV to TV region.Thanks to the great accuracy of the measurement of the pri-mary most abundant species,such as electron,proton and helium fluxes,and the accurate measurement of nuclei spectra,such as B/C,that gives indication about the amount of material traversed by cosmic rays,the models prediction for the secondary population in anti-proton and position can be largely improved,allowing a more constrained research of exotic components in those fluxes.In this work,we explore the measurement of AMS-02 of the isotopes of helium(3He and 4He).3He in cosmic rays is essentially produced by the 4He interaction with the interstellar matters.Therefore the ratio ~3He/~4He contains information about the amount of material traversed by 4He in the Galaxy,and more in general,represents a useful constraint to cosmic rays propagation models.The measurement of this ratio in cosmic rays is bounded to large uncertainties(of the order of 10)in a kinetic range per nucleon that extends up to 5 GeV/n considering the experiments that give more than one measurement point.The calculation of the ~3He/~4He fraction in AMS-02 relies on the selection of helium particles using charge measured along the particle trajectory inside AMS.This charge selection is obtained with very high efficiency and very low contamination giving the accuracy of AZ/Z of 0.07.Then the combined use of the measurement of momentum with the silicon tracker,and velocity using the time-of-flight or the ring imaging Cherenkov counter,allows the measurement of mass.The mass can be reliably reconstructed in the energy range 0.5-10.0 GeV/n with a resolution from 10%to 18%.Monte Carlo(MC)template fit method is used to fit 1/m2 distribution of AMS-02 flight data to measure the ~3He/~4He ratio.Upon request of this method,the consistence between MC template and flight data must be guaranteed.MC template is firstly reweighed using the helium spectrum measured by AMS-02 and the solar modulation based on the Parker model,then the reconstructed β in MC is shifted back to its generated value to correct bias during reconstructions.In order to eliminate the discrepancy between data and the MC simulation,the distributions of β and rigidity in MC are studied and corrected in accordance with data.The template fitting is based on RooFit,and two additional parameters,beside ~3He/~4He,are imported to minimize the χ3/d.f.Three main detector effects are taken into consideration for rectifying the directly fitted ~3He/~4He ratio to the real ratio in primary cosmic rays,the different acceptances for 4He and 3He,the fragmentation from 4He to 3He inside detector and the energy losses in AMS materials.For the uncertainties of the measurement,in this thesis,following main aspects are treated,the β correction,fitting method,additional free parameters,energy loss correction and the acceptance corrections.With the precise measurement from AMS-02,the ~3He/~4He ratio is used to compare with GALPROP prediction based on the B/C and p/p measurements.Analysis shows that the diffusion coefficient for different species of cosmic rays are totally different,which indicates that different nuclei originate from different sources and experience diverse propagation processes during their traveling to the earth.Therefore,the tradi-tional constraint on the propagation parameters using B/C measurement solely is not suitable for the exact construction of cosmic ray models any more.Precise measurement of ~3He/~4He ratio will bring independent constraints which are competitive with those obtained from the B/C analysis.