R. Zei

DISCREPANT HARDENING OBSERVED IN COSMIC-RAY ELEMENTAL SPECTRA

The balloon-borne Cosmic Ray Energetics And Mass experiment launched five times from Antarctica has achieved a cumulative flight duration of about 156 days above 99.5% of the atmosphere. The instrument is configured with complementary and redundant particle detectors designed to extend direct measurements of cosmic-ray composition to the highest energies practical with balloon flights. All elements from protons to iron nuclei are separated with excellent charge resolution. Here, we report results from the first two flights of ~70 days, which indicate hardening of the elemental spectra above ~200 GeV/nucleon and a spectral difference between the two most abundant species, protons and helium nuclei. These results challenge the view that cosmic-ray spectra are simple power laws below the so-called knee at ~1015 eV. This discrepant hardening may result from a relatively nearby source, or it could represent spectral concavity caused by interactions of cosmic rays with the accelerating shock. Other possible explanations should also be investigated.

Cosmic-ray Proton and Helium Spectra from the First CREAM Flight

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004–2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of �38.5 km with an average atmospheric overburden of �3.9 g cm −2 . Individual elements are clearly separated with a charge resolution of �0.15 e (in charge units) and �0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of 2.66 ± 0.02 for protons from 2.5 TeV to 250 TeV and –2.58 ± 0.02 for helium nuclei from 630 GeV nucleon −1 to 63 TeV nucleon −1 . They are harder than previous measurements

ENERGY SPECTRA OF COSMIC-RAY NUCLEI AT HIGH ENERGIES

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic-Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charge identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to ~1014 eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an E –2.66 ± 0.04 power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/n energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be 0.080 ± 0.025 (stat.)±0.025 (sys.) at ~800 GeV/n, in good agreement with a recent result from the first CREAM flight.

MEASUREMENTS OF THE RELATIVE ABUNDANCES OF HIGH-ENERGY COSMIC-RAY NUCLEI IN THE TeV/NUCLEON REGION

We present measurements of the relative abundances of cosmic-ray nuclei in the energy range of 500-3980 GeV/nucleon from the second flight of the Cosmic Ray Energetics And Mass balloon-borne experiment. Particle energy was determined using a sampling tungsten/scintillating-fiber calorimeter, while particle charge was identified precisely with a dual-layer silicon charge detector installed for this flight. The resulting element ratios C/O, N/O, Ne/O, Mg/O, Si/O, and Fe/O at the top of atmosphere are 0.919 ? 0.123stat ? 0.030syst, 0.076 ? 0.019stat ? 0.013syst, 0.115 ? 0.031stat ? 0.004syst, 0.153 ? 0.039stat ? 0.005syst, 0.180 ? 0.045stat ? 0.006syst, and 0.139?? 0.043stat ? 0.005syst, respectively, which agree with measurements at lower energies. The source abundance of N/O is found to be 0.054 ? 0.013stat ? 0.009syst+0.010esc ?0.017. The cosmic-ray source abundances are compared to local Galactic (LG) abundances as a function of first ionization potential and as a function of condensation temperature. At high energies the trend that the cosmic-ray source abundances at large ionization potential or low condensation temperature are suppressed compared to their LG abundances continues. Therefore, the injection mechanism must be the same at TeV/nucleon energies as at the lower energies measured by HEAO-3, CRN, and TRACER. Furthermore, the cosmic-ray source abundances are compared to a mixture of 80% solar system abundances and 20% massive stellar outflow (MSO) as a function of atomic mass. The good agreement with TIGER measurements at lower energies confirms the existence of a substantial fraction of MSO material required in the ~TeV per nucleon region.

Performance of a Dual Layer Silicon Charge Detector During CREAM Balloon Flight

The balloon-borne cosmic-ray experiment CREAM (Cosmic Ray Energetics And Mass) has completed two flights in Antarctica, with a combined duration of 70 days. One of the detectors in the payload is the SCD (silicon charge detector) that measures the charge of high energy cosmic rays. The SCD was assembled with silicon sensors. A sensor is a 4 × 4 array of DC-coupled PIN diode pixels with the total active area of 21 × 16 mm2. The SCD used during the first flight (December 2004-January 2005) was a single layer device, then upgraded to a dual layer device for the second flight (December 2005-January 2006), covering the total sensitive area of 779 × 795 mm2. Flight data demonstrated that adding a second layer improved SCD performance, showing excellent particle charge resolution. With a total dissipation of 136 W for the dual layer system, special care was needed in designing thermal paths to keep the detector temperature within its operational range. As a consequence, flight temperatures of the SCD, even at diurnal maximum were kept below 38°C. The SCD mechanical structure was designed to minimize the possibility of damage to the sensors and electronics from the impacts of parachute deployment and landing. The detector was recovered successfully following the flight and is being refurbished for the next flight in 2007. Details of construction, operation, and performance are presented for the dual-layered SCD flown for the second CREAM flight.

A Silicon Array for Cosmic-Ray Composition Measurements in CALET

The CALorimetric Electron Telescope (CALET) mission is proposed for a long exposure observation of high energy cosmic rays and gamma radiation, taking advantage of the JEM-EF facility on the International Space Station. The instrument is optimized for the search of nearby sources of acceleration of cosmic ray electrons in the TeV energy range. Its large collection power also allows for precision studies of the elemental composition of VHE nuclei and of their spectral features. The charge identification of the incoming particle is performed by a double-layered array of pixelated silicon sensors, covering a seamless sensitive area of the order of 1 m 2 . The conceptual design of the array and its front-end electronics are presented.

The CREAM Calorimeter: Performance In Tests And Flights

The Cosmic Ray Energetics And Mass (CREAM) balloon‐borne experiment, designed to directly measure cosmic‐ray particle energies from ∼1011 to ∼1015 eV, had two successful flights since December 2004, with a total duration of 70 days. The CREAM calorimeter is comprised of 20 layers of 1 radiation length (X0) tungsten interleaved with 20 active layers each made up of fifty 1 cm wide scintillating fiber ribbons. The scintillation signals are read out with multi pixel Hybrid Photo Diodes (HPDs), VA32‐HDR2/TA32C ASICs and LTC1400 ADCs. During detector construction, various tests were carried out using radioactive sources, UV‐LEDs, and particle beams. We will present results from these tests and show preliminary results from the two flights.

The Cosmic Ray Energetics and Mass (CREAM) timing charge detector

H. S. AHN , P. ALLISON , M. G. BAGLIESI , J. J. BEATTY , G. BIGONGIARI , P. BOYLE , J. T. CHILDERS , N. B. CONKLIN , S. COUTU , M. A. DUVERNOIS , O. GANEL , J. H. HAN , J. A. JEON , K. C. KIM , J. K. LEE , M. H. LEE , L. LUTZ , P. MAESTRO , A. MALININE , P. S. MARROCCHESI , S. MINNICK , S. I. MOGNET , S. NAM , S. NUTTER , I. H. PARK , N. H. PARK , E. S. SEO , R. SINA , S. SWORDY , S. WAKELY , J. WU , J. YANG , Y. S. YOON , R. ZEI , S. Y. ZINN . Inst. for Phys. Sci. and Tech., University of Maryland, College Park, MD 20742 USA Dept. of Physics, Ohio State University, Columbus, Ohio 43210, USA Dept. of Physics, University of Siena and INFN, Via Roma 56, 53100 Siena, Italy Enrico Fermi Institute and Dept. of Physics, University of Chicago, Chicago, IL 60637, USA School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA Dept. of Physics, Penn State University, University Park, PA 16802, USA Dept. of Physics, Ewha Womans University, Seoul, 120-750, Republic of Korea Dept. of Physics, Kent State University Tuscarawas, New Philadelphia, OH 44663, USA Dept. of Physics and Geology, Northern Kentucky University, Highland Heights, KY 41099, USA Dept. of Physics, University of Maryland, College Park, MD 20742 USA hsahn@umd.edu

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.

Design and characterization of a double-layered silicon charge detector for cosmic ray measurements in CALET

CALET (CALorimetric Electron Telescope) is a space instrument to explore a new frontier at higher energies for the cosmic-rays (electrons, gamma rays and heavy nuclei) and to search for dark matter.1 This mission is designed for a long exposure observation on the external JEM-EF facility aboard the International Space Station. It is optimized for the search of nearby sources of acceleration of cosmic ray electrons in the TeV energy range and gamma rays up to several TeV range. It can also extend the available data on cosmic ray composition and on secondaryto-primary ratios to higher energies allowing to discriminate among different propagation models and to derive the acceleration spectra at the source. An accurate measurement of the charge of the incoming particle is performed in CALET by a double-layered array of pixelated silicon sensors (Silicon Array or SIA), covering a seamless sensitive area of the order of 0.3 m2 and providing single-element identification up to Fe and above. The design of SIA instrument and its characterization are presented.