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Register a new userPrecise Measurements of Atmospheric Muon Fluxes with the BESS Spectrometer
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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 GV 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 less than 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.
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The cosmic-ray proton, helium, and muon spectra at small atmospheric depths of 4.5 -- 28 g/cm^2 were precisely measured during the slow descending period of the BESS-2001 balloon flight. The variation of atmospheric secondary particle fluxes as a function of atmospheric depth provides fundamental information to study hadronic interactions of the primary cosmic rays with the atmosphere.
Primary and atmospheric cosmic-ray spectra were precisely measured with the BESS-TeV spectrometer. The spectrometer was upgraded from BESS-98 to achieve seven times higher resolution in momentum measurement. We report absolute fluxes of primary protons and helium nuclei in the energy ranges, 1-540 GeV and 1-250 GeV/n, respectively, and absolute flux of atmospheric muons in the momentum range 0.6-400 GeV/c.
We report cosmic-ray proton and helium spectra in energy ranges of 1 to 120 GeV and 1 to 54 GeV/nucleon, respectively, measured by a balloon flight of the BESS spectrometer in 1998. The magnetic-rigidity of the cosmic-rays was reliably determined by highly precise measurement of the circular track in a uniform solenoidal magnetic field of 1 Tesla. Those spectra were determined within overall uncertainties of +-5 % for protons and +- 10 % for helium nuclei including statistical and systematic errors.
The fluxes of atmospheric muons and neutrinos are calculated by a three dimensional Monte Carlo simulation with the air shower code CORSIKA using the hadronic interaction models DPMJET, VENUS, GHEISHA, and UrQMD. For the simulation of low energy primary particles the original CORSIKA has been extended by a parametrization of the solar modulation and a microscopic calculation of the directional dependence of the geomagnetic cut-off functions. An accurate description for the geography of the Earth has been included by a digital elevation model, tables for the local magnetic field in the atmosphere, and various atmospheric models for different geographic latitudes and annual seasons. CORSIKA is used to calculate atmospheric muon fluxes for different locations and the neutrino fluxes for Kamioka. The results of CORSIKA for the muon fluxes are verified by an extensive comparison with recent measurements. The obtained neutrino fluxes are compared with other calculations and the influence of the hadronic interaction model, the geomagnetic cut-off and the local magnetic field on the neutrino fluxes is investigated.
In two long-duration balloon flights over Antarctica, the BESS-Polar collaboration has searched for antihelium in the cosmic radiation with higher sensitivity than any reported investigation. 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 8.4times 10^6 |Z| = 2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0times 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 of 6.9 times 10^-8 was determined by combining all the BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies from 1.6 to 14 GV, an upper limit of 1.0 times 10^-7 was determined for the combined BESS-Polar data. These are the most stringent limits obtained to date.
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