Observations of the chemical and isotopic composition of light cosmic-ray nuclei can be used to constrain the astrophysical models of cosmic-ray transport and interactions in the Galaxy. Nearly 200,000 light nuclei (Z>2) have been observed by AMS-01 during the 10-day flight STS-91 in June 1998. Using these data, we have measured the relative abundance of light nuclei Li, Be, B and C in the kinetic energy range 0.35 - 45 GeV/nucleon.
Cosmic rays detected on Earth with kinetic energies from 100 MeV to 100 GeV per nucleon are believed to be produced by particle acceleration mechanisms occurring in galactic sites such as supernova remnants. Most of our knowledge on the insterstellar propagation of the galactic cosmic rays comes from the study of secondary species, i.e. spallation products of C-N-O and Fe nuclei that are almost absent in the cosmic ray (CR) sources.
The relation between secondary CRs and their primary progenitors allows us to determine the propagation parameters, such as the diffusion coefficient and its energy dependence [1]. Among the ratios B/C and Sub-Fe/Fe, it is of great interest to determine the propagation history of the lighter nuclei Li, Be, and their isotopes, which are mostly of spallogenic origin. Their abundances depend not only on interactions of the primary species C, N and O, but also on tertiary contributions like Be→Li or B→Li. Therefore the Li/C and Be/C ratios may provide further restrictions on propagation models [2]. AMS-01 observed cosmic rays at ∼380 km of altitude during a period close to the solar minimum, providing data free from atmospheric-induced background and little influenced by the solar modulation. Using these data, we present a measurement of the ratio Li/C, Be/C and B/C.
The AMS-01 precursion mission of the Alpha Magnetic Spectrometer (AMS) project [3] operated successfully during the 10-day flight STS-91 onboard Discovery.
The AMS-01 spectrometer was composed of a cylindrical permanent magnet (analyzing power BL 2 = 0.14 Tm 2 ), a silicon microstrip tracker (six layers of double sided silicon sensors with resolution of 10 µm in the bending coordinate), four time of flight (TOF) scintillator planes (time resolution of ∼90 psec for Z > 1 ions) an aerogel Cerenkov counter (threshold velocity β=0.985) and anti-coincidence counters [4].
The detector response was simulated with GEANT3 [5]. The effects of energy loss, multiple scattering, nuclear interactions and decays were included in the code, as well as efficiencies, resolutions and reconstruction algorithms. Further simulations have been performed using GEANT4 [6] and FLUKA [7] within the framework of the Virtual MC tool-kit [8].
The identification of a cosmic-ray particle with the AMS-01 spectrometer was perfomed through the combination of independent measurements provided by the various sub-detectors. The particle rigidity R (momentum per unit charge p/Z) was provided by the deflection of the reconstructed particle trajectory. The velocity β = v/c was measured from the particle transit time between the four TOF planes along the track length. The particle charge magnitude |Z| was obtained by the analysis of the multiple measurement of energy deposition in the four TOF scintillators (up to Z = 2 [4]) and the six silicon layers (up to Z = 8 [9]). Figure 1 shows the charge histogram of CR particles with Z > 2 obtained with the silicon tracker. The particle mass was then determined from the resultant charge Z, velocity β and rigidity R:
The differential energy spectrum of the Z-charged particles measured by AMS-01 in the energy bin E of width ∆E is related to the measured counts ∆N Z by: where ∆T Z (E) is the effective exposure time and A Z (E) is the detector acceptance. In order to compute bin-to-bin ratios, all the elemental fluxes were determined in a common grid of kinetic energy per nucleon E, obtained by the rigidity measurements performed by the tracker. The relation between the measured energy of detected particles and their true energy were studied using unfolding techniques [10]. Most of systematic uncertainties arising from many steps of the analysis are suppressed in the ratios. Differences in the trigger efficiencies of the two species are present, as expected, from the charge dependence of delta ray production and fragmentation effects in the detector material. This study was performed through extensive simulations employing the transport codes GEANT3, GEANT4 and FLUKA, Uncertainties of 2-10 % were estimated for the trigger efficiency. The spill over from adjacent charges also produces net effects on the measurements, leading to errors of up to 5 %. The MC determination of the acceptance gave a statistical uncertainty of ∼1-3 %, increasing with energy.
Figure 2 shows the isotopic ratio ratios are presented in Figure 3 with the existing experimental data [11][12][13][14][15][16].
The error bars in the figure represent the sum in quadrature of statistical errors with the systematic uncertainties. Our B/C ratio measurement agrees well with the results from the first flight of CREAM in 2004 [15] and with the data collected by HEAO-3-C2 [14] from October 1979 and June 1980. The Be/C ratio is consistent, within errors, with the HEAO data, but not with balloon data [12]. Our Li/C data have unprecedented accuracy in a poorly explored energy region. In comparison with balloon data from Ref. [12], our data indicate a quite diff
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