Cosmic Ray Shielding at NOvA Travis Olson KrissieNosbisch
Abstract We used a muon telescope to calculate the cosmic ray background at the NOvA site and determine how effective the barite and other rock around the detector were at shielding. The cosmic ray flux was found to be 0.0053 ± 7.1x10-5muons cm-2sr-1s-1 under the barite which is 29 ± 1.4% less than the flux outside. The rate of electromagnetic showers is 0.0028 ± .0006 showers per second which is 77 ± 5.3% less than the rate outside.
Top view of the box showing the garbage bags and duct tape for light-proofing. High voltage and signal cables Photomultiplier tube with mirrored base Plastic scintillator White reflective cloth Muon Telescope Design 72.7cm 56.6 cm • Muon telescope pieces (left), schematic (center) and full stack (right)
Muon Telescope Specifications • The telescope has an acceptance of 88.4 ± .36 cm2sr when the boxes are stacked on top of each other. • The stack of boxes has a column depth of 4.8 g/cm2 which makes an energy threshold of 35.05 MeV. • The plastic scintillator has dimensions of 45.72 ± .05cm x 45.72 ± .05cm x 1.91 ± .05cm and was originally purchased for the Soudan1 experiment. • The photomultiplier tubes are 5” PMTs that were originally bought for the HPW experiment.
DAQ Card • We used a Quarknet board version 2.0 to record the data.
DAQ Card Settings1 • For flux measurements the coincidence window was set to 144 ns and data was recorded for 240ns after the first event in a coincidence. • For the electromagnetic shower measurement the coincidence window was set to 1.2 μs and data was recorded for 3.6 μs after the first event. • Both measurements required a four way coincidence. • Times were chosen to allow coincidences to occur even if there were long cable delays, and as noted later the rate of accidental coincidences is small enough that it won’t play a role even with larger windows.
Optimization tests • Tests were done to find the optimum operating and threshold voltages.
Conclusions • North and middle flux reduced by 29 ± 1.4%. • South flux reduced by 67 ± .8%. • Electromagnetic showers reduced by 77 ± 5.3%.
References • Hansen, S.; Jordan, T.; Kiper, T.; Claes, D.; Snow, G.; Berns, H.; Burnett, T.H.; Gran, R.; Wilkes, R.J.; , 2“ Low-cost data acquisition card for school-network cosmic ray detectors," Nuclear Science, IEEE Transactions on , vol.51, no.3, pp. 926- 930, June 2004doi: 10.1109/TNS.2004.829447URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1311993&isnumber=29128 • Rossi, Bruno. "Interpretation of Cosmic Ray Phenomena." Reviews of Modern Physics, vol.20, no.3 1948 doi: 10.1103/RevModPhys.20.537 URL: http://rmp.aps.org/abstract/RMP/v20/i3/p537_1