30TH INTERNATIONAL COSMIC RAY CONFERENCE MINOS Observations of Shadowing in the Muon Flux Underground

A high significance observation of two muon signals, the shadow of the sun and moon, have been seen by the 5.4 kt MINOS Far Detector, at a depth of 2070 mwe. The distribution of angular separation of muons near the moon was well described by a Gaussian, which was used to determine the angular resolution (0.34◦ ± 0.07◦) and pointing (0.3◦ ± 0.05◦) of the detector. Introduction & Motivation The MINOS Far Detector is a magnetized scintillator and steel calorimeter, located in the Soudan Mine in northern Minnesota, USA, at a depth of 720 m. While primary function of the Far Detector is to detect neutrinos from Fermilab’s νμ beam, the great depth and wide acceptance of the detector combined with the flat overburden of the Soudan site allow it to serve as a cosmic-ray muon detector as well. The detector is composed of 486 8 m octagonal planes 2.54 cm thick, spaced 5.96 cm. This 5.4 kt detector is 30 m long and has a total aperture of 6.94 × 10 cm sr. MINOS is the first underground experiment able to discriminate positively charged particles from those that are negatively charged for the purpose of CPT violation investigations, but this feature also allows independent study of positively and negatively charged cosmic rays. Optical telescopes use a standard catalogue of stars to establish the resolution and pointing reliability of a new instrument. This is not possible for a cosmic ray detector, as there are no cosmic ray sources available for calibration. There are two well observed phenomena in the otherwise isotropic cosmic ray sky, though they are deficits, not sources. The sun and the moon provide a means to study the resolution and pointing of a cosmic ray detector because they absorb incident cosmic rays, causing deficits from their respective location. These signals allow a measure of phenomena associated with cosmic ray propagation and interaction resulting from geomagnetic fields, interplanetary magnetic fields, multiple Coulomb scattering, etc. These extra-terrestrial objects have the same 0.5◦ diameter as viewed from Earth, though the sun shadow is more difficult to observe because is much farther away and has its own magnetic field that deflects the charged cosmic rays. The cosmic ray shadow of the moon has been measured by air shower arrays (CYGNUS [1], CASA [2] , Tibet [3] ), as well as underground detectors (Soudan 2 [4], MACRO [5, 6], L3+C [7]).