Y. Makida

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.

Search for Cosmic-Ray Antideuterons

We performed a search for cosmic-ray antideuterons using data collected during four BESS balloon flights from 1997 to 2000. No candidate was found. We derived, for the first time, an upper limit of 1.9 x 10(-4) (m2s sr GeV/nucleon)(-1) for the differential flux of cosmic-ray antideuterons, at the 95% confidence level, between 0.17 and 1.15 GeV/nucleon at the top of the atmosphere.

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.

A thin superconducting solenoid magnet for particle astrophysics

An extremely thin superconducting solenoid magnet is being developed to investigate cosmic-ray antiparticles in the Universe. The uniform solenoidal field is provided in a particle detector system to analyze the particle momentum. The solenoid coil is wound with advanced aluminum stabilized superconductor recently developed by using micro-alloying with Ni, followed by cold-work mechanical hardening. It is designed with a central magnetic field of 1.2 T in a volume of 0.9 m in diameter and 1.4 m in length. The radiation thickness of the coil is to be 0.056 X/sub 0/ with a physical coil thickness of 3.4 mm. This paper describes the conceptual design and progress of basic development work.

Performance of an ultra-thin superconducting solenoid for particle astrophysics

An extremely thin superconducting solenoid with a main diameter of 0.9 m and a length of 1.4 m has been fabricated for a balloon borne experiment in Antarctica to study anti-particles in cosmic rays. The solenoid has a 0.8 m diameter warm bore where a magnetic field of 0.8-1.0 T is induced. The coil was wound with mechanically advanced aluminum stabilized superconductor recently developed by using micro-alloying Ni into a pure aluminum base and by cold-work hardening, and consequently the electromagnetic force may be fully supported by the coil itself without any additional support structure. The solenoid was successfully charged up to 1.05 T without any premature quenches. Despite measured strains were beyond 1500 micro-strain, the coil behaved elastically. Because of relatively small RRR of 110, the quench energy is distributed rather unevenly, and a temperature difference of over 100 K was observed. Nevertheless, it was found to be safe to quench the magnet.

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.

Ballooning of an Ultra-Thin Superconducting Solenoid for Particle Astrophysics in Antarctica

An ultra-thin superconducting solenoid has been developed to provide a magnetic field of 0.8 T in a balloon-borne spectrometer for cosmic ray research, which is named BESS-Polar. The coil with a diameter of 0.9 m, a length of 1.4 m and a thickness of 3.5 mm was fabricated by using a mechanically strengthened aluminum stabilized superconductor. The coil winding is strong enough to eliminate the outer support cylinder which is necessary in the former thin solenoid type coils. Consequently the coil weight and material thickness are 40 kg, and 2.52 g/cm2, respectively. The BESS-Polar was launched near the US McMurdo Station in Antarctica on December 13th 2004, floated at an altitude of 37000 m around the South Pole for nine days. The solenoid was charged up on the ground and kept the field in a persistent current mode during launch and floating. This report describes the flight performance of the solenoid