John W. Mitchell

Precision Measurement of Cosmic-Ray Antiproton Spectrum

The energy spectrum of cosmic-ray antiprotons ( &pmacr;s) has been measured in the range 0.18-3.56 GeV, based on 458 &pmacr;s collected by BESS in a recent solar-minimum period. We have detected for the first time a characteristic peak at 2 GeV of &pmacr;s originating from cosmic-ray interactions with the interstellar gas. The peak spectrum is reproduced by theoretical calculations, implying that the propagation models are basically correct and that different cosmic-ray species undergo a universal propagation. Future BESS data with still higher statistics will allow us to study the solar modulation and the propagation in detail and to search for primary &pmacr; components.

Measurement of the cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

Abstract The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1–4.2 GeV. In this Letter, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiproton data to probe the effect of charge-sign-dependent drift in the solar modulation.

Precise measurements of atmospheric muon fluxes with the BESS spectrometer

The vertical absolute fluxes of atmospheric muons and muon charge ratio have been measured precisely at different geomagnetic locations by using the BESS spectrometer. The observations had been performed at sea level (30 m above sea level) in Tsukuba, Japan, and at 360 m above sea level in Lynn Lake, Canada. The vertical cutoff rigidities in Tsukuba (36.2°N, 140.1°E) and in Lynn Lake (56.5°N, 101.0°W) are 11.4 and 0.4 GV, respectively. We have obtained vertical fluxes of positive and negative muons in a momentum range from 0.6 to 20 GeV/c with systematic errors <3% in both measurements. By comparing the data collected at two different geomagnetic latitudes, we have seen an effect of cutoff rigidity. The dependence on the atmospheric pressure and temperature, and the solar modulation effect have been also clearly observed. We also clearly observed the decrease of charge ratio of muons at low momentum side with at higher cutoff rigidity region.

Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) collaboration has searched for antihelium in the cosmic radiation with the highest sensitivity reported. BESS-Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among 8.4×10(6) |Z|=2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0×10(7) |Z|=2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit to the possible abundance of antihelium relative to helium of 6.9×10(-8)} was determined combining all BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies for each flight, an upper limit of 1.0×10(-7) from 1.6 to 14 GV was determined for the combined BESS-Polar data. Under both antihelium spectral assumptions, these are the lowest limits obtained to date.

Measurements of atmospheric antiprotons

We measured atmospheric antiproton spectra in the energy range 0.2 to 3.4 GeV, at sea level and at balloon altitude in the atmospheric depth range 4.5 to 26 g/cm 2 . The observed energy spectra, including our previous measurements at mountain altitude, were compared with estimated spectra calculated on various assumptions regarding the energy distribution of antiprotons that interacted with air nuclei.

The BESS-Polar Ultra-Thin Superconducting Solenoid Magnet and Its Operational Characteristics During Long-Duration Scientific Ballooning Over Antarctica

An ultra-thin superconducting solenoid has been developed for a cosmic-ray spectrometer ballooning over Antarctica, which is named BESS-Polar II. The coil with a diameter of 0.9 m, a length of 1.4 m and a thickness of 3.5 mm, is wound with high-strength aluminum stabilized superconductor and provides 0.8 T in the spectrometer. Based on the experience at the BESS-Polar-I solenoid flight for nine days in 2004, the BESS-Polar-II solenoid, which was cryogenically improved, realized a persistent current mode operation for 25 days in the second flight campaign in December 2007 though January 2008. It has contributed to accumulate the cosmic-ray observation data with 4700 million events and 16 terabyte in a hard disk unit. This report will describe the second solenoid performance during the flight.

Measurements of Compton scattered transition radiation at high Lorentz factors

Abstract X-ray transition radiation can be used to measure the Lorentz factor of relativistic particles. Standard transition radiation detectors (TRDs) typically incorporate thin plastic foil, foam, or fiber radiators and gas-filled X-ray detectors, and are sensitive up to γ ∼10 4 . To reach Lorentz factors up to γ ∼10 5 , thicker, denser radiators can be used, which consequently produce X-rays of harder energies (≳100 keV ) . At these energies, scintillator detectors are more efficient in detecting the hard X-rays, and Compton scattering of the X-rays out of the path of the particle becomes important. The Compton scattering can be utilized to separate the transition radiation from the ionization background spatially. The use of conducting metal foils is predicted to yield enhanced signals compared to standard nonconducting plastic foils of the same dimensions. We have designed and built an inorganic scintillator-based Compton Scatter TRD optimized for high Lorentz factors and exposed it to high-energy electrons at the CERN SPS. We present the results of the accelerator tests and comparisons to simulations, demonstrating (1) the effectiveness of the Compton Scatter TRD approach; (2) the performance of conducting aluminum foils; and (3) the ability of a TRD to measure energies approximately an order of magnitude higher than previously used in very high-energy cosmic ray studies.

POEMMA: Probe Of Extreme Multi-Messenger Astrophysics

The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) mission is being designed to establish charged-particle astronomy with ultra-high energy cosmic rays (UHECRs) and to observe cosmogenic tau neutrinos (CTNs). The study of UHECRs and CTNs from space will yield orders-of-magnitude increase in statistics of observed UHECRs at the highest energies, and the observation of the cosmogenic flux of neutrinos for a range of UHECR models. These observations should solve the long-standing puzzle of the origin of the highest energy particles ever observed, providing a new window onto the most energetic environments and events in the Universe, while studying particle interactions well beyond accelerator energies. The discovery of CTNs will help solve the puzzle of the origin of UHECRs and begin a new field of Astroparticle Physics with the study of neutrino properties at ultra-high energies.

Elemental Abundances of Ultra-Heavy GCRs measured by SuperTIGER and ACE-CRIS and the Origin of Galactic Cosmic Rays

The Super Trans-Iron Galactic Element Recorder (SuperTIGER) long-duration balloon instrument and the Cosmic Ray Isotope Spectrometer (CRIS) on the NASA Advanced Composition Explorer (ACE) satellite have measured the abundances of galactic cosmic-ray elements from _(10)Ne to _(40)Zr with high statistics and single-element resolution. SuperTIGER launched from Williams Field, McMurdo Station, Antarctica, on December 8, 2012, flying for a record 55 days. During that flight we detected ∼1,300 nuclei with atomic number Z ≥ 30. The resolution in charge (Z) of SuperTIGER is excellent, with σ_Z ≈ 0.16 c.u. at _(26)Fe. SuperTIGER is sensitive to nuclei with energy at the top of the atmosphere of E > 0.8 GeV/nucleon. The instrument has now been recovered and preparations are underway for its next flight. ACE/CRIS has been taking data in space for more than 17 years since launch in 1997, has collected ∼625 nuclei with atomic number Z ≥ 30, and shows excellent resolution with clear separation between the charges for 30 ≤ Z ≤ 40. ACE/CRIS is sensitive to nuclei in the energy range 150 ≤ E ≤ 600 MeV/nucleon. Preliminary results from the balloon-borne SuperTIGER show good agreement with ACE measurements in space, validating our corrections to SuperTIGER abundances for nuclear interactions in the atmosphere. The results from these experiments will be discussed in the context of the OB association model for the origin of galactic cosmic rays. Future missions to measure elemental abundances to higher Z, the SuperTIGER-II LDB instrument and the orbiting Heavy Nuclei eXplorer (HNX) mission, will also be discussed.

Abundances of Ultra-Heavy Galactic Cosmic Rays from the SuperTIGER Instrument

R. P. Murphy∗1, W. R. Binns1, R. G. Bose1, T. J. Brandt2, P. F. Dowkontt1, T. Hams 2,6, M. H. Israel1, A. W. Labrador3, J. T. Link2,6, R. A. Mewaldt3, J. W. Mitchell2, B. F. Rauch1, K. Sakai2,6, M. Sasaki2,6, E. C. Stone3, C. J. Waddington4, J. E. Ward1,‡, and M. E. Wiedenbeck5† 1 Washington University in St. Louis, St. Louis MO 63130 USA 2 NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA 3 California Institute of Technology, Pasadena, CA 91125 USA 4 University of Minnesota, Minneapolis, MN 55455 USA 5 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA 6 Center for Research and Exploration in Space Science and Technology (CRESST) ‡ Now at Institut de Fisica d’Altes Energies (IFAE), Bellaterra, Spain