Abstract
The shielding of sensitive experiments, such as NOvA, from cosmic rays is usually accomplished with lead shielding. However, lead plating is heavy, expensive, and inneffective. This experiment was designed to test if the electromagnetic B-field generated by passing current through a Faraday cage would slow cosmic ray muons from relativistic speeds to a speed at which they would decay within a short distance. Calculations suggested that 2.169 amperes of current would generate a strong enough magnetic field to slow the muons so they decay before they reach the bottom of the detector. After experimentation with the cage powered at 3.0 amperes , it was found that this method deflects between 62% - 92% of muons away from the center of the cage, but not the bottom plate as was expected. This result may be because the coiled magnetic B-fields do not always apply force in the direction opposite the motion of the muon,altering their trajectory such that they do not pass through the center of the cage statistically as often but are bounced back into the lower plates by the magnetic field of the sides and bottom.
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Introduction
Beta radiation, composed of free electrons, can be completely absorbed by a single sheet of aluminum foil. However muons, which travel at 99.4% the speed of light and weigh 200 times more than an electron, are a bit more difficult to stop. Lead shielding is always an option, but as Nate Bramham, Jesse Honig, and Maceo Hastings Porro discovered in 2011, 20cm of plating (which to cover our detector would weigh nearly 142kg) only reduces the coincidence rate by 17%. This inneffectivity is what led me to wonder if I could take advantage of electromagnetism in the form of a Faraday cage to stop the muons.
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Procedures
The purpose of the cage is to apply a significant impulse to the muon to slow it down from relativistic speeds (at which it actually experiences time dilation) to a velocity such that it will decay into electrons and neutrinos before it reaches the bottom plate and will therefore not be detected. See Figure 1and 2 for the calculations that suggested the feasibility of the shield. The cage was constructed from a sheet of 4 foot by 5 foot galvanized steel wire fencing. I had originally intended to power the circut with a 9 volt battery and a 1 ohm resistor, but that setup proved to be too short term and potentially to dangerous as both the battery and resistor generated large ammounts of heat. Instead, a DC laboratory power supply was used and proved to be a much more reliable, prescice, and safe means of conducting the experiment. Flux was measured with the plates stacked inside the cage as seen in Figure 3 for 30 minutes with the power on at 3.0 amperes and 0.1 volts, and then 30 minutes with the power off to give a control as to what the flux was at that time.
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Results
After the first test as shown in Figures 4 - 6, the electromagnetic field reduced the muon flux in plate 4 from 2500 events/m^2/60s to 950; 62% of muons were deflected. However, plate 4 was not at the bottom of the detector: it was at the top of the stack, in the center of the faraday cage. As well, all other plates (in the order from top to bottom 4, 3, 2, 1)measured slightly lower fluxes, but the most significant decrease was in plate 4. This measurment, though statistically significant, did not align with how the cage was designed so the plates were shuffled and the experiment repeated. This was to confirm if the measurment was in fact the result of deflection as opposed to a problem with the plate. When tested again, even with the plates in the new order 2, 1, 3, 4, the top plate registered the lowest muon flux. Even more signigicantly, the measured difference shown in Figures 7 - 9 was between 1500 events/m^2/60s and 120 events/m^2/60s; a change of 92%. These repeated results suggest that the cage works in a way that was not anticipated or that I fully understand.
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Discussions & Conclusions
This method of shielding proved to be highly effective against muons. Where lead has been tested to only reduce flux by 17%, the Faraday cage blocked between 62% - 92% of muons from reaching the detector. However, the shield did not work entirely as anticipated; rather measuring the lowest flux at the bottom of the cage, that point in fact lied in the center. I do not entirely understand why this is true, but it could be because the coiled magnetic fields of the wires deflected muons away from the center rather than outright stopping them. I feel that because of this phenomena, the powered Faraday cage warrants further study as a means of shielding from cosmic rays, as well as other trends that I noticed while conducting overnight measurments.
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