The High Resolution The High Resolution Fly’s EyeFly’s Eye

HiRes Cosmic Ray ExperimentHiRes Cosmic Ray Experiment

Betsy Maryott

Wayne High School

WHO:•The University of Utah Cosmic Ray Group

               George Cassiday

               Haven Bergeson

               Gene Loh

               Pierre Sokolsky

•First generation began observations in 1981 and operated until 1993•Second generation revised in 1994 and continues today

WHERE: FE1 and FE2: Located in the West Desert of Utah, within the United States Army Dugway Proving Ground (DPG), on top of Little Granite Mountain (160 km southwest of Salt Lake City).

                           

   

Map showing the location of DPG relative to Salt Lake City.

WHY HERE: Clean, High and Dry!!

•established infrastructure and security

•an altitude of ~4,500 feet or a vertical depth of ~860 g/cm2 (very thin air)

•nearest population centers are more than 30 miles away (light pollution is mostly blocked by the surrounding mountains)

•average annual rainfall and snowfall are 7.5 and 15.5 inches, respectively

•complete absence of surface water within the boundary of DPG, and the average humidity is 48%

•for 347 days per year, the visibility at DPG is better than 10 miles

•average wind velocity of 6 miles per hour

•its isolation from major urban centers means there is a minimum of aerosol contamination

WHAT: The Detector: a Multi-Faceted “Eye”

•67 Volcano Ranch prototype modules were constructed (first generation)

•each detector module contains a spherical mirror of 1.6 m diameter with a 1.5 m focal length

•each mirror is equipped with either 12 or 14 photomultiplier tubes (PMT) mounted at the focal surface

•in 1986, FE2 was added 3.4 km from FE1 for stereoscopic observations

•each PMT has a “Winston cone” to change the collection area from a circle to a hexagon

•the mirrors and the PMT's are oriented to divide the sky into 880 pixels, each covering 0.0066 steradians (~5 degrees by 5 degrees)

                           

   

Geometrical arrangement of PMT pixels in the sky.

The “Fly’s Eye”

WHAT WERE THEY LOOKING FOR: Air “Fluorescence”

•the passage of charged particles in an extensive air shower results in the ionization and excitation of the gas molecules (mostly nitrogen)

•some of this excitation energy is emitted in the form of visible and UV radiation

•actually a “luminescence” or “scintillation” process, not fluorescence

                            

  

Visible light: 400 – 700 nm

FE1 and FE2 look for UHE cosmic rays – the “ankle” and “toe”

FE1 and FE2: The New Generation, 1994 - present

•constructed 14-mirror array at Little Granite Mountain

•operated in conjunction with CASA-MIA from 1994 - 1997

•constructed 4-mirror prototype at Camel’s Back Ridge for more stereoptic investigations

•reconfigured in 1997 to 63 mirrors, at the focal plane of each mirror is a “camera” of 256 PMT’s

•FE1 amd FE2 can provide 360˙ azimuth between 3˙ – 31˙ of angle

•can reconstruct the geometry of the shower by looking at the EAS from two points

•atmospheric observation now being conducted using a flashing xenon laser (aerosol distribution)

RESULTS:

•UHE cosmic ray composition are protons, not Fe

•recorded an event of 3.2x1020 eV (beyond GKZ cut-off energy)

•weak anisotropy toward the galactic plane for energies 1-3x1018 eV

•since 1997, over 2880 events recorded of over 1x1015 eV, 1000 of which have been observed in stereo