K. Anraku

Precise Measurement of Cosmic-Ray Proton and Helium Spectra with the BESS Spectrometer

We report cosmic-ray proton and helium spectra in energy ranges of 1-120 GeV nucleon-1 and 1-54 GeV nucleon-1, respectively, measured by a flight of the Balloon-borne Experiment with Superconducting Spectrometer (BESS) in 1998. The magnetic rigidity of the cosmic ray was reliably determined by highly precise measurement of the circular track in a uniform solenoidal magnetic field of 1 T. Those spectra were determined within overall uncertainties of ±5% for protons and ±10% for helium nuclei including statistical and systematic errors.

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.

Measurements of cosmic-ray low-energy antiproton and proton spectra in a transient period of solar field reversal.

The energy spectra of cosmic-ray low-energy antiprotons ( *ps) and protons ( ps) have been measured by BESS in 1999 and 2000, during a period covering reversal at the solar magnetic field. Based on these measurements, a sudden increase of the *p/p flux ratio following the solar magnetic field reversal was observed, and it generally agrees with a drift model of the solar modulation.

Successive measurements of cosmic-ray antiproton spectrum in a positive phase of the solar cycle

Abstract The energy spectrum of cosmic-ray antiprotons ( p s) has been measured by BESS successively in 1993, 1995, 1997 and 1998. In total, 848 p s were clearly identified in energy range 0.18–4.20 GeV. From these successive measurements of the p spectrum at various solar activity, we discuss about the effect of the solar modulation and the origin of cosmic-ray p s. The p / p ratios showed no distinctive year-to-year variation during the positive Suns polarity phase.

Measurement of Cosmic-Ray Hydrogen and Helium and Their Isotopic Composition with the BESS Experiment

The cosmic-ray hydrogen and helium spectra have been measured by the Balloon Borne Experiment with a Superconducting Solenoid Spectrometer (BESS), which has been flown from Lynn Lake, Manitoba, Canada, annually since 1993. The BESS experiment provides excellent rigidity measurement and precise particle identification with a large geometric acceptance. We present here the hydrogen and helium nuclei energy spectra from 0.2 to 10 GeV nucleon-1 and their isotopic composition from 0.2 to about 1 GeV nucleon-1 for the first BESS flight. This provides the first simultaneous measurements of the cosmic-ray secondaries, deuterons, and 3He, with their primaries, protons, and 4He over this energy range in a period of solar minimum. In this paper, we have achieved significant improvements in data analysis in the following aspects. First, the latest available cross-section data and their parameterizations were utilized in the simulation code developed for this study. Second, a complete simulation was performed for both protons and heavy ions: the δ-ray effect was properly simulated and showed a large influence on the measurement of heavy ions at high energies. Third, the secondary particle correction, which dominates the systematic uncertainty at low energies for singly charged particles, protons and deuterons, was calculated iteratively with the simultaneously measured primary cosmic-ray spectra. In general, the results of this experiment are consistent with other recent measurements using balloon-borne or satellite experiments, but with better precision. The measured spectra of protons, deuterons, 3He, and 4He and their corresponding ratios are compared with different interstellar/heliospheric propagation calculations, which were derived to fit observations of heavy nuclei. The overall good agreement indicates that the propagation history for light cosmic-ray elements, protons, deuterons, and helium nuclei is similar to that of the heavy nuclei. The 2H/1H ratio is sensitive to the propagation models, and our results show a tendency of better agreement with the reacceleration model than the standard leaky-box model.

Cosmic-ray antiproton flux in the energy range from 200 to 600 MeV

We have studied the low-energy antiprotons in the cosmic rays by utilizing data obtained by the Balloon-borne Experiment with a Superconducting magnetic rigidity Spectrometer (BESS) flown in 1993 July from Lynn Lake, Manitoba, Canada. A detailed description of the event selection criteria and background corrections is given. Seven antiprotons are found that give an antiproton flux of 6.4 -->+ 5.5?3.5 ? 10-3(m2 sr s GeV)-1 and an antiproton/proton ratio of 5.2 -->+ 4.4?2.8 ? 10-6 in the 200-600 MeV energy range. These results are consistent with a secondary origin of low-energy cosmic-ray antiprotons within our uncertainties, but they still require the precise measurement of the spectrum shape below 500 MeV to clarify exactly the model of particle propagation and possible contributions from exotic sources.

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.

Balloon-borne experiment with a superconducting solenoidal magnet spectrometer

Abstract A balloon-borne superconducting solenoidal magnet spectrometer, BESS, has been designed and developed to investigate cosmic-rays and particle-astrophysics. Search for primordial antimatter in cosmic-rays and observation of gamma-rays are primary objectives to study early history of the Universe. The cylindrical spectrometer configuration with a thin superconducting solenoid magnet is optimized to provide a large geometrical acceptance of 0.5–1.0 m 2 sr for these scientific objectives. The spectrometer system has been completed and is planned to be launched for the first scientific flight from Lynn Lake, Canada, in 1993. This report describes the progress of the spectrometer development and discuss about its capability to search for primordial antimatter in cosmic rays.

Measurements of atmospheric muon spectra at mountain altitude

We report new measurements of absolute fluxes of atmospheric muons at mountain altitude. The measurements were carried out with the BESS detector at the top of Mt. Norikura, 2 770 m above sea level, in Japan. The overall errors were less than 10%. The measured results are discussed in comparison with theoretical calculations.

A NEW LIMIT ON THE FLUX OF COSMIC ANTIHELIUM

Abstract A very sensitive search for cosmic-ray antihelium was performed using data obtained from three scientific flights of the BESS magnetic rigidity spectrometer. We have not observed any antihelium; this places a model-independent upper limit (95% C.L.) on the antihelium flux of 6×10−4 m−2sr−1s−1 at the top of the atmosphere in the rigidity region 1 to 16 GV, after correcting for the estimated interaction loss of antihelium in the air and in the instrument. The corresponding upper limit on the He /He flux ratio is 3.1 ×10−6, 30 times more stringent than the limits obtained in similar rigidity regions with magnetic spectrometers previous to BESS.