A Muon Veto for the Ultra Cold Neutron Asymmetry Experiment

1. Cosmic Ray Muon Beta detector Introduction Figure 4:Experimental Area B - The UCNA Experiment Cosmic Ray Muons Goals Muon Veto Paddle Beta spectrometer d d n u u p d u e- W - v Ultra Cold Neutrons electron antineutrino Active Veto Concept Figure 7: Plot of Efficiency versus Distance from Paddle End Figure 8: Testing Setup Progress and Future Work Acknowledgements Protons Electrons Pions Muons Figure 5: Schematic of Muon Veto LANL Distinguished Student Program, T. Ito, M. Makela, A. Saunders and the UCNA collaboration Figure 9: Constructing a new scintillator paddle Vince Bagnulo with Dr. Jeff Martin for the UCNA Collaboration A Muon Veto for the Ultra Cold Neutron Asymmetry Experiment Results In order to test the efficiency of the plastic scintillator, it was first necessary to develop a testing method. The method that was settled upon was using three smaller, trigger paddles and placing two above the large scintillator paddle and one below. The efficiency was thus the ratio between the triple coincidence rate (using just the test paddles) and the quadruple coincidence rate (using all four paddles). The testing method was applied to the paddle that had been already constructed and the results of this test are displayed in Figure 7. As can be seen from the graph, the efficiency of the large paddle falls below 50% at its far end. This is far below the desired efficiency of 90%. Thus, this paddle was inadequate. The above test was not fruitless, however, as a testing method had been developed which can henceforth be used on all future scintillator paddles. Also, it was known that the poor efficiency of the paddle was most likely due to a broken glue joint that developed during the construction process rather than faulty materials. The goal of the Ultra Cold Neutron Asymmetry (UCNA) experiment is to measure the A parameter of neutron decay to a precision of 0.2%. This will be done using trapped neutrons. The equation describing beta decay is displayed in Equation 1 and the corresponding Feynman diagram in Figure 1. This measurement could yield insight into physics beyond the Standard Model. beta-spectrometer magnet From UCN source neutron proton electron beta detector Equation 1: Neutron Beta Decay polarizer magnet Figure 1: Beta Decay Feynman Diagram “prepolarizer” magnet UCN guide path The distinctive characteristic of ultra-cold neutrons is that they reflect completely off of some surfaces. This allows them to be stored in containers for periods of time on the order of the neutron lifetime (approximately 15 minutes). In the UCNA experiment, neutrons are generated using a spallation source. The resulting high-energy neutrons are then cooled to ultra-cold temperatures and transmitted, through guide pipes, to a neutron trap which is inside a beta spectrometer. A schematic of the beta spectrometer is depicted in Figure 2, below. This spectrometer detects the electrons resulting from neutron beta decay. However, background particles are also detected and must be corrected for in order to obtain a high precision measurement. One source of background particles is cosmic ray muons. Cosmic ray muons are detected by the beta detectors resulting in unwanted background events. In order to obtain a high precision measurement, this background must be corrected for. This was achieved through the use of an active veto system. This system will detect any muons impinging on the beta detectors allowing these unwanted events to be cancelled out.. The particular veto system proposed for UCNA consists of surrounding the beta spectrometer with several large plastic scintillator detectors. Plastic scintillator detectors are large sheets of plastic doped with a fluorescent substance which reemits energy deposited by ionizing radiation as visible light. This light is then converted into an electronic signal using photomultiplier tubes (PMTs). The goal of the veto system is to detect any muons that would contact the beta detectors with an efficiency of at least 90%. A schematic of the proposed design is depicted in Figure 5. The PMTs were found to be quite susceptible to the ambient magnetic field. This problem was resolved by surrounding the PMT with a shield of mu-metal, an alloy with high magnetic permeability, and placing it within a soft iron cylinder. This shielding can be seen in Figure 6.This shielding was still found to be insufficient and so active shielding was used. The mu-metal shield was wrapped in a coil of wire through which current can be passed to generate a magnetic field to cancel out the ambient field. Repairs to the damaged scintillator paddle were attempted, however, it was found that is was easier to simply construct a new paddle. Construction is depicted in Figure 9, below. Once constructed, this new paddle must be tested, as previously described. If the efficiency is found to be sufficient, the remaining scintillator paddles will be constructed and placed onto the beta spectrometer. Alternative means for obtaining an active veto will be considered in parallel. Desiredprecision: ExpectedSignal Rate: 5-50 Hz Background rate: 3 Hz (predominantly cosmic ray muons) Electrons Figure 2: Beta spectrometer schematic Cosmic rays are particles, predominantly protons, which impinge on the earth’s atmosphere. These particles originate partially from the sun but mainly from sources outside of the solar system. These particles interact with the molecules in the earth’s atmosphere eventually resulting in muons which reach the earth’s surface. At the earth’s surface, cosmic ray muons have the following parameters: Depicted in Figure 6 is the state of the veto system as of the beginning of summer this year. Only one paddle had been assembled and placed on the beta spectrometer. The plastic scintillator that was used had been salvaged from a previous experiment and so its efficiency was unknown and needed to be tested. As seen in Figure 2, the PMTs are located in a magnetic field. This interferes with their operation. The significance of the interference needed to be determined and, if necessary, corrected for. Also, the remaining paddles needed to be constructed and put into place. energy  4 GeV flux  1 muon/cm2/min angular distribution  cos2  Figure 6: Veto System as of start of summer 2006 Figure 3: Cosmic Rays