Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope

The CAT imaging telescope, recently built on the site of the former solar plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from October 1996 to March 1997. This steady source, often considered as the standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam to probe the performances of the new telescope, particularly its energy threshold (220 GeV at 20 degrees zenith angle) and the stability of its response. Due to the fine-grain camera, an accurate analysis of the longitudinal profiles of shower images is performed, yielding the source position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba

Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope

The CAT Imaging Telescope has observed the BL Lac object Markarian 501 between March and August 1997. We report here on the variability over this time including several large flares. We present also preliminary spectra for all these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late

Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope

The CAT imaging telescope on the site on the former solar plant Themis has been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997. This source is shown to be highly variable and the light curve is presented. The detected gamma-ray rate for the most intense flare is in excess of 10 per minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba

Measurement of the Crab Flux Above 60 GeV with the CELESTE Cherenkov Telescope

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cherenkov detector called CELESTE, which records gamma rays above 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospheric Cherenkov telescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure a gamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be <12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by a Cherenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.Comment: 34 pages, accepted by the Astrophysical Journa

PARTICULAR MULTINUCLEON TRANSFER PROCESS IN THE 7Li + 197Au REACTION FOR GRAZING CONDITIONS

L'émission prépondérante de particules alpha et de tritons dans la réaction 7Li +197Au ne provient pas d'une dissociation directe du projectile. Les particules sont émises à partir d'un système en rotation quasi lié formé par la cible et la structure en agrégats la moins liée du projectile. Ce mécanisme se produit essentiellement lorsque le projectile est dans le voisinage de la cible avec une vitesse relative négligeable.The preponderant emission of alpha particles and tritons in 7Li + 197Au cannot be accounted for by a projectile break-up hypothesis. The particles are emitted from a quasi-bound rotating system formed by the target and the less bound cluster-structure of the projectile. This mechanism essentially occurs when the projectile is in the vicinity of the target with a negligible relative velocity

Quadrupole Moments of First Excited States in Si, S, and Ar

Static quadrupole moments of the first excited states of {sup 28}Si, {sup 32}S, and {sup 40}Ar have been measured using the reorientation effect in projectile Coulomb excitation. The results obtained are Q({sup 28}Si,2{sup +}) = +0.11 {+-} 0.05 b, Q({sup 32}S,2{sup +}) = -0.20 {+-} 0.06 b, and Q({sup 40}Ar, 2{sup +}) = +0.01 {+-} 0.04 b

DIRECT TRANSPER REACTIONS INDUCED BY 16O AT INCIDENT ENERGIES ABOVE THE COULOMB BARRIER

Les réactions de transfert induites par des ions 16O sur différents noyaux cibles de la couche 2p-1f présentent les caractéristiques générales d'une réaction directe de surface, pouvant être décrite comme un processus quasi élastique dans le cadre du modèle de diffraction de Frahn et Venter. La grande sélectivité de ces réactions directes de transfert peut être appliquée à des études de spectroscopieThe transfer reactions induced by 16O ions on various 1f-2p shell target nuclei, exhibit the general features of a surface direct reaction which can be described as a quasi-elastic process in the framework of the Frahn and Venter diffraction model. The great selectivity of these direct transfer reactions can be applied to spectroscopic studies