T. Hara

Energy spectrum of primary cosmic rays above 1017.0 eV determined from extensive air shower experiments at Akeno

The energy spectrum of primary cosmic rays above 1017.0 eV has been updated from data collected with two extensive air shower arrays operating at Akeno, one with area 1 km2 and the other with area 20 km2. Along with the authors previous results in the lower energy region, the energy spectrum has been determined over about five decades of energy from 1014.5 eV to 1019.8 eV. A change in the index of the power-law energy spectrum is observed around 1017.8 eV, as well as the usual features, namely the knee around 1015.7 eV and the ankle around 1019.0 eV. The indices of the differential power-law energy spectrum are: (2.62+or-0.12) below 1015.7 eV, (3.02+or-0.03) for 1015.7 approximately 1017.8 eV and (3.16+or-0.08) for 1017.8 approximately 1018.8 eV. There is an indication of a flattening of the spectrum above approximately 1018.8 eV with an index of (2.8+or-0.3). The flux above 1018 eV is (1.5 approximately 2.4)*10-12 m-2s-1sr-1 and is in good agreement with other experiments. The number of showers above 1019.5 eV is seven for an exposure of 80 km2 yr sr and further investigation by a new Akeno giant air shower array (AGASA), whose operation has started, is necessary to determine a cutoff energy in the spectrum, if any.

Energy Spectrum of Primary Cosmic Rays Between 10**14.5-ev and 10**18-ev

Size spectra of electrons (Ne) and muons (Nmu ) are obtained from the Akeno extensive air-shower experiment. The primary spectrum estimated from each spectrum is the same and is expressed by J(E0)dE0(4.0-5.0)*10-23(E0/1015.67)- gamma dE0 m-2 s-1 sr-1 where gamma =2.62+or-0.12 below and gamma =3.02+or-0.05 above 1015.67 eV. There is no other significant change of slope in either the electron or the muon size spectrum beyond the corresponding energy 1015.67 eV.

An upper limit on the muon flux at energies above 100 TeV determined from horizontal air showers observed at Akeno

Determination of the cosmic-ray muon energy spectrum above 100 TeV by observing extensive air showers from the horizontal direction (HAS) has been carried out at Akeno for four years. No definite shower of muon origin with size above 105 has been observed at zenith angles above 60 degrees . The upper limit of HAS intensity is 5*10-12 m-2 s-1 sr-1 (90% confidence level) for sizes greater than 105. This value indicates that the upper limit of the muon flux above 100 TeV is about 1.3*10-8 m-2 s-1 sr-1, which is in agreement with that expected from the primary spectrum with a knee, assuming scaling in the fragmentation region and 40% protons in the primary beam. It also implies that the critical energy at which a muon flux from prompt processes (decay of charmed particles) takes over from that due to conventional process (decay of pions and kaons) is higher than 100 TeV in the horizontal direction.

Anisotropy of the arrival direction of extensive air showers observed at Akeno

The anisotropy of the arrival direction of the primary cosmic ray of energy 1015 to 1018 eV is studied using approximately 300000 showers observed in 1981 and 1982 at the EAS array at Akeno. The anisotropy of the second harmonics appears to be (1.1+or-0.4)% in the region 1016 to 1017 eV. The showers with rich muon content seem to come preferentially from a direction of about 230 degrees in the right ascension. No statistically meaningful anisotropy is found for muon-poor showers.

Expanded Array for Giant Air Shower Observation at Akeno

Abstract As the first stage of a future huge air shower array, the Akeno array was expanded to about 20 km2 by adding 19 scintillation counters of 2.25 m2 area outside the present 1 km2 array and installing a new data collection system. These detectors are connected successively by two optical fiber cables. The total number of electrons and the arrival direction of extensive air showers of 1010 GeV can be determined with accuracies of 25% and 3° respectively with this array. The present recording system is applicable to other experiments in which many sensors are connected in sequence.

The Lateral Distribution of Electrons of Extensive Air Shower Observed at Akeno (920 g/cm2)

Lateral distribution of electrons is determined with scintillators of 50 and 3 mm thickness from the core to 500 m in the Akeno array. Lateral distribution can not be represented by the NKG function with a single age parameter. From size and depth dependence of age parameter and its fluctuation at fixed muon size, elongation rate and fluctuations in the depth of shower maximum are estimated and discussed.

Optical performance of the UT-FEL at first lasing

Abstract First laser oscillation has been obtained at a wavelength of 42.8 μm on June 25, 1993, using a 15-MeV S-band linac, in collaboration with the University of Tokyo. The optical resonator with a 40 period vertical undulator ( K = 0.95) is 3.78 m long and uses Au coated Cu mirrors. The optical axis of the mirrors is adjusted by CPU controlled actuators in order to coincide with the electron beam, which has been adjusted to pass through the center of the magnetic field as close as possible in advance. The power is about 10 4 times higher than that of the spontaneous emission. The FWHM of the FEL spectrum is less than 0.51 μm, which corresponds to Δλ / λ = 1.2%. The tuning range of the cavity is about ΔZ = 20 μ m.

Size dependence of frequency attenuation length of extensive air showers observed at Akeno

The size dependence of the frequency attenuation length ( Lambda ) of extensive air showers has been studied at Akeno for electron sizes between 105 and 108.5. Lambda decreases with increasing size up to 106 and increases slowly above 107. The value between 107 and 108 is 99+or-7 g cm-2. This indicates that the depth of the shower maximum above a few 1016 eV is shallower than that obtained by optical methods and is consistent with that from the equiintensity cut method. The result supports the view that heavy enriched primary particles with a conservative interaction model or proton-dominated primaries with interactions of high multiplicity are required.

Status report of the UT-FEL project

Abstract We present the status of UT-FEL program, a compact FEL utilizing an existing 15 MeV S-band rf linac. Some improvements of the accelerator to enlarge the beam brightness and assembling of a wiggler were completed last year. Preparations for an FEL oscillation experiment are now being carried out in collaboration with FEL Research Institute, Inc. (FELI). An electron and optical beam measurement system, an IR detector system and an optical resonator are under development. The oscillation experiment will be held at the end of 1992.

Photoproduced muons in EAS and the search for gamma-ray initiated showers beyond 1016 eV

The contribution of photoproduced muons to the total number of low-energy muons in extensive air showers is discussed. The content of photoproduced muons (>0.5 GeV) is estimated to be less than 60% at a primary energy of 1019 eV and 40% at 1017 eV from the Akeno experiments. Using this value, the authors set an upper limit for the gamma -ray flux from the direction of Cyg X-3 of 3.1*10-16 cm-2 s-1 for >or=1.5*1016 eV and 1.3*10-16 cm-2 s-1 for >or=3.5*1016 eV (90% confidence level). The excess of gamma -ray showers from galactic latitude mod b mod <10 degrees is less than 0.1% of the total cosmic-ray flux around 1.5*1016 eV.