Measuring the Cosmic Ray Composition at the Knee with BLANCA

BLANCA is a new, large non-imaging Cerenkov detector located at the CASA-MIA experiment it Dugway, Utah. Using 144 individual Cerenkov detectors, BLANCA covers an area of approximately 0.2 km and has an energy threshold of 300 TeV. The lateral density function (LDF) of the Cerenkov light in air showers is measured. Since the LDF depends on the primary particle species, a change in composition as a function of energy can then be determined. We report results obtained on data taken with the CASA-BLANCA detector since January. 1997. INTRODUCTION Composition measurements above ~ 10eV are extremely difficult, because direct detection via satellite and balloon experiments is not feasible due to low flux levels. In addition, this energy ( ~ 3 x 10 eV is where a break in the cosmic ray energy spectrum occurs (the knee). It is commonly believed that the knee is a spectral feature which can he explained by models of cosmic ray acceleration and propagation. The favored acceleration model. involving shock waves produced by supemovae, can produce cosmic rays up to ~ few x 10eV (Drury et al., 1994), but is unable to explain particles above this energy. These first order Fermi-shock acceleration models also give rise naturally to a power law E with index á ~ 2. while the measured spectral index for energies below the knee is á ~ 2.7. The difference in the power law predicted at the source of cosmic rays and that measured below the knee can be understood by invoking a propagation model that rclies on rigidity-dependent galactic diffusion (the "leaky box" model). This model assumes that the source spectrum is modified by an energy dependent escape rate froth the galaxy of E which, when combined with the predicted power law from the acceleration mechanism, reproduces the slope of the cosmic ray spectrum below the knee. This value of the escape rate has been indirectly determined from the abundance ratio of secondary to primary cosmic ray nuclei (Gaisser et al., 1990. Above the knee, the slope of the all particle spectrum steepens to á ~ 3.15, and this change is poorly understood. It is possible that the change in slope occurs because the acceleration mechanism turns off at energies near the knee or because particles are escaping from the magnetic confinement of the Galaxy. Both possibilities lead to the prediction that the composition of cosmic rays should grow heavier through the knee and above. On the other hand, it has also been speculated that cosmic rays above the knee come from a source completely external to our Galaxy, perhaps from energetic Active Galactic Nuclei (Protheroe and Szabo, 1992). In this case, the composition of cosmic rays would be expected to grow lighter through the knee. Reliable measurements of the composition are clearly needed at these energies to understand the process of acceleration and propagation of cosmic rays. EXPERIMENTAL TECHNIQUE To measure the cosmic ray composition with a ground array, one makes use of the properties of the produced air shower that are most closely related to the primary interaction: namely, the height in the atmosphere of shower maximum (Xmax) and the number of muons in the shower (N ì). The Xmax value