[en] A knowledge of the absolute efficiency of the photomultiplier tubes (PMTs) used in the Sudbury Neutrino Observatory (SNO) is important for modelling the behaviour of the detector. While the quantum efficiencies for the PMTs are known, the efficiency in single-photon counting mode (which includes collection efficiency and counting efficiency) is of more relevance. A calibration of the 8 in. Hamamatsu R1408 photomultipliers in this mode requires a known flux of incident light at the very low level of 10^-16 W cm^-2 (250 photons cm^-2 s^-1). A method of performing such measurements, utilising the extreme linearity obtainable with silicon photodiodes, is presented in this paper. The results of efficiency measurements on four PMTs are given and a comparison is made with previous results obtained using a Cherenkov source of known intensity. (author)

[en] The attenuation length of electrons drifting in liquid argon is examined as a function of electric drift field from 0.5 to 3.0 kV cm^-1, and of impurity level for low concentrations of oxygen ≅ ppb) and nitrogen (≅ ppm). Deviations from the 'standard' formula are noted, possible explanations are discussed, and a new, semiempirical formula is proposed. (orig.)

[en] A purification system based on reduced copper and molecular sieve has been built for a 7 m³ liquid argon time projection chamber and it has been successfully tested. After the initial purification an attenuation length of 0.7 m at 0.5 kV cm^-1 was measured by drifting electrons over distances up to 28 cm. Further purification yielded attenuation lengths ≥ 3.7 m at 0.5 kV cm^-1 which corresponds to an impurity level of ≤ 0.2 ppb oxygen equivalent. (orig.)

[en] With the next generation of Atmospheric Cerenkov Telescopes (ACT) well on their way, effort has being shifted now towards a further improvement of the capabilities of these instruments. These next generation experiments have improved up to an order of magnitude their sensitivity over their first generation counterparts, and have extended the energy coverage down to about 50 GeV. However, it is not clear whether the same approach is optimal for energies above 1 TeV, and with clear evidence of high energy emission above this energy from several astrophysical sources, we believe that this energy range could be further exploited. Many of our physics goals, such as, limits on quantum gravity or spectral variability and features of the high energy emission from AGNs, would benefit from an increase of sensitivity in this direction. Here, we investigate a possible way of increasing the collection area of ACTs above 1 TeV by using a wide-field-of-view (∼ 10 deg.) camera. Both, the scientific motivation and preliminary Monte Carlo studies of the performance of such a detector, are discussed and presented here

[en] This paper reports on the CYGNUS experiment which is composed of an air-shower array and muon detectors, located in Los Alamos, NM, and operating at energies above about 50 TeV. Recent results include a search for emission from Cygnus X-3 during the radio outbursts of June and July, 1989, preliminary results from a search for diffuse emission from the galactic plane, and preliminary results from a search for emission from possible northern hemisphere point sources, both known and unknown

[en] A unique feature of Cygnus X-3 is that occasionally it has large radio outbursts that begin very abruptly and last for several days. Several experiments in the past have claimed to observe signals above 1 TeV correlated with these radio bursts; the most recent bursts occurred in June and July 1989. No significant signal was observed by the CYGNUS experiment over time scales longer than a day during this time; a 90%-confidence-level limit of 3.0x10^-13 cm^-2 s^-1 is placed on the flux above 50 TeV during the period from 15 May to 31 July 1989

[en] The 11 ''in-phase'' source events from the 1986 muon-rich bursts associated with Hercules X-1 (previously reported by this group) have been studied for indications of further anomalous behavior. The most significant effect observed resulted from an analysis of the showerfront time-structures of these events. This analysis was then applied a priori to the rest of the source day, where an additional ∼9 signal events are expected to remain. The same effect was observed at a chance probability level of ∼0.1%. 1 ref., 7 figs

[en] Data from an extensive air shower detector of ultrahigh-energy cosmic rays shows shadowing of the cosmic-ray flux by the Moon and the Sun with significance of 4.9 standard deviations. This is the first observation of such shadowing. The effect has been used to determine that the angular resolution of the detector is 0.75 degree _-0.90degree^+0.13degree

[en] The CYGNUS extensive air-shower experiment is described. The design criteria, construction and operation details, and performance characteristics are presented. A discussion of the data analysis techniques is given. Finally, several enhancements and improvements in the apparatus are described. (orig.)

[en] We report here the preliminary results of our observations of the sporadic x-ray binary system 4U0115 + 63. The CYGNUS air shower array has been collecting data since April 1986. No significant excess is seen from the direction of this source, nor any correlation with its 24-day orbital period. A 90% confidence-level upper limit on the flux from 4U0115 + 63 is 2.8 x 10^-13 cm^-2 s^-1 above 50 TeV. This flux limit is considerably lower than those reported by other UHE experiments. Search for periodicity at the neutron star frequency is in progress. 12 refs., 3 figs., 1 tab