During its second Antarctic flight, the CREAM (Cosmic Ray Energetics And Mass) balloon experiment collected data for 28 days, measuring the charge and the energy of cosmic rays (CR) with a redundant system of particle identification and an imaging thin ionization calorimeter. Preliminary direct measurements of the absolute intensities of individual CR nuclei are reported in the elemental range from carbon to iron at very high energy.
Deep Dive into Measurements of cosmic-ray energy spectra with the 2nd CREAM flight.
During its second Antarctic flight, the CREAM (Cosmic Ray Energetics And Mass) balloon experiment collected data for 28 days, measuring the charge and the energy of cosmic rays (CR) with a redundant system of particle identification and an imaging thin ionization calorimeter. Preliminary direct measurements of the absolute intensities of individual CR nuclei are reported in the elemental range from carbon to iron at very high energy.
The CREAM experiment was designed to measure the composition and energy spectra of cosmic rays approaching energies up to 10 15 eV. Since December 2004, four instruments were successfully flown on balloons over Antarctica where they collected several million CR events in the elemental range from hydrogen to iron, and with total particle energies reaching the 100 TeV scale and above. The final goal of the experiment is to provide a deeper understanding of the acceleration mechanism of cosmic rays and to test the validity of the astrophysical models describing their propagation in the Galaxy [1].
In this paper, we present preliminary energy spectra of the even-charged, abundant nuclei from carbon to iron as measured by the instrument during its second flight (CREAM-II).
The instrument for the second flight included: a redundant system for particle identification, consisting (from top to bottom) of a timingcharge detector (TCD), a Cherenkov detector (CD), a pixelated silicon charge detector (SCD), and a sampling imaging calorimeter (CAL) designed to provide a measurement of the energy of primary nuclei in the multi-TeV region. The TCD is comprised of two planes (120 × 120 cm 2 ) of four 5 mm-thick plastic scintillator paddles, read out by fast timing photomultiplier tubes (PMT). The CD is a 1 cm-thick plastic radiator, with 1 m 2 surface area, read out by eight PMTs via wavelength shifting bars. The SCD is a dual layer made of 312 silicon sensors, each segmented as an array of 4×4 pixels, which covers an effective area of 0.52 m 2 with no dead regions. The CAL is a stack of 20 tungsten plates (50×50 cm 2 , each 1 X 0 thick) with interleaved active layers of 1 cm-wide scintillating fiber ribbons, read out by 40 hybrid photodiodes (HPD). It is preceded by a 0.47 λ int -thick graphite target to induce hadronic interactions of CR nuclei. The second CREAM payload was launched on December 16 th 2005 from McMurdo and flew over Antarctica until January 13 th 2006, at a balloon float altitude between 35 and 40 km. A detailed description of the instrument and its flight performance can be found in [2].
A data set collected in the period Dec. 19 th -Jan. 12 th , under stable instrument conditions, was used in this analysis. The first step of the procedure consists of selecting events with an accurate trajectory reconstruction of the primary particle, and measuring its charge and energy. The direction of the particle is given by the axis of the shower reconstructed in the imaging calorimeter. This is obtained by a χ 2 fit of the candidate track points sampled along the longitudinal development of the shower. On each CAL plane, they are defined as the center of gravity of the cluster formed by the cell with maximum signal and its two neighbours. The fitted shower axis is back-projected to the target and its intersections with the two SCD layers define the impact points of the primary particle, with a spatial resolution better than 1 cm rms. In each SCD plane, the pixel with the highest signal is located inside a circle of confusion with a 3 cm radius centered on the impact point.
If the signals of the matched hits in the upper and lower SCD planes are consistent within 30%, they are selected as two independent samples of the specific ionization dE/dx and corrected for the pathlength traversed by the particle in the silicon sensors. A charge Z is assigned to the particle by averaging the two measured values of the specific ionization and taking into account its Z 2 dependence. The charge distribution reconstructed by the SCD is shown in figure 1; by fitting each peak to a gaussian, a charge resolution σ is estimated (in units of the electron charge e) as: 0.2 for C, N, O; ∼ 0.23 for Ne, Mg, Si; ∼ 0.5 for Fe. A 2σ cut around the mean charge value is applied to select samples of each element, while for iron a 1σ cut is used. In this way, 1288 O are retained, as well as 456 Si and 409 Fe candidates. The total energy (E d ), deposited in the calorimeter by an interacting nucleus, is measured by summing up the calibrated signals of its cells. In order to infer the primary particle energy E from E d , an unfolding procedure is applied. In fact, due to the finite energy resolution of the detector, the measured counts in a given energy interval must be corrected for overlap with the neighbouring bins. This can be done by inverting the matrix equation: M i = j A ij N j where N j and M i are the “true” and measured counts in each energy bin, respectively. A generic element of the mixing matrix A ij represents the probability that a CR particle, carrying an energy corresponding to a given energy bin j, produces an energy deposit in the calorimeter falling in the package [3], was developed to estimate the unfolding matrix. Sets of nuclei, generated isotropically and with energies chosen according to a powerlaw spectrum, are analyzed with the same procedure as that used for the flight data. Each matrix element A ij is calculated by
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