This award provides equipment funds and summer support for two undergraduate students from Saint Mary's University of Minnesota for ten weeks in each of the summers of 2010-2012, to be engaged (a) in testing a new approach to magnetic shielding for large (eight-inch) diameter photomultiplier tubes (PMTs) in the MicroBooNE (Fermilab E974) detector, a liquid argon time projection chamber (LArTPC) that will run in the Booster Neutrino Beam (BNB) line at Fermilab; and (b) in parameterizing shielded tube performance characteristics (timing, gain, angular response, etc.) prior to installation and in situ in the detector. The broader impact includes providing an extraordinary and well-focused physics research experience for undergraduate students to work as part of an ongoing experiment at a major national laboratory as well as enabling Professor Nienaber to work with the undergraduate students to prepare neutrino physics outreach talks that the students can deliver to members of the Saint Mary's University and wider Winona, MN communities. It is anticipated that this project would have direct impact on the strength of the undergraduate research component of Saint Mary's Physics Department's major programs, and would indirectly improve participating students' ability to contribute to department offerings in modern physics, quantum mechanics, and upper-division laboratory courses.
This project focused on an important subsystem of a neutrino detector that will run at the Fermi National Accelerator Laboratory near Chicago. The detector is the centerpiece of the MicroBooNE (Micro Booster Neutrino Experiment) project, an international effort encompassing the talents of more than 100 scientists, engineers, and students from 20 laboratories and universities. MicroBooNE’s core is a Liquid Argon Time Projection Chamber, which tracks the charged particle debris generated when neutrinos produced by the Fermilab Booster proton accelerator interact in the liquefied argon gas the chamber contains. In addition to producing electronic signals that can be read out as particle footprints, the liquid argon generates tiny flashes of ultraviolet light; picking up this extraordinarily faint light lets the software controlling the detector know that an interaction has occurred. Accurately "seeing" these scintillation light pulses depends on exquisitely sensitive light sensors called photomultiplier tubes (PMTs), and optimal PMT performance in the low-temperature (liquid argon) environment inside the detector tank is crucial to the experiment. PMT behavior is adversely affected by any stray magnetic fields, even ones as weak as the geomagnetic field of the Earth, so this project focused on blocking unwanted magnetic effects on the tubes. Since the photomultiplier tubes are to be immersed in the liquid argon, conventional shields cannot be used, since their ability to block magnetic fields diminishes significantly at cryogenic temperatures. The tubes are very sensitive to light, so any testing of their response must take place in a light-tight enclosure. They also need to be tested at cryogenic temperatures (liquid nitrogen [BP 77 K], less expensive than argon [84 K]); this necessitated the use of a vacuum-insulated immersion vessel. Testing PMT response to geomagnetic fields is accomplished by rotating the tube (figure 1: PMT) about a line perpendicular to its axis of symmetry – that is, the pitch (as opposed to roll or yaw) angle. Since the Earth’s field direction at a given latitude doesn’t change, rotating the tube reproducibly changes the magnetic field strength along the tube’s orientation (figure 2: Kendziora rotator in vessel). An optical fiber connected to a pulsed LED carries a controlled, extremely low-intensity pulse of light to the apex of the tube (figure 3: tube in rotator housing); the tube’s response is quantified by the average amount of electric charge produced by the tube. The shields were fabricated from two different alloys (Cryoperm and Amumetal 4K) by the Amuneal Corporation of Philadelphia.. These shields surrounded the tubes on the sides and on the base, leaving the top open to incoming photons (figure 4: shielded tube). We rotated an unshielded tube, and observed an expected variation with angle. We also found something unexpected: the shields designed to block magnetic effects at cryogenic temperatures also worked at room temperature. We then determined the shields worked effectively at liquid nitrogen temperatures: an unshielded tube showed a response that changed as the tube was rotated, and this variation was removed by either the Cryoperm or Amumetal 4K shields. Since the Amumetal 4K alloy is less expensive, it was chosen for the MicroBooNE shields. The original tests used shields that extended from the base of the tube to the level of its apex (see figure 4; the perforated ring is part of the rotator housing, absent in actual shield deployment). Concerns were raised that extending the shield that high would "shadow" the tube, limiting its access to light from the liquid argon. The rotator was modified so that the shield only extended to the PMT equator (figure 5: equator-height shielding; students Morgan Fournier (left) and Evan Shockley). This configuration diverged from the "conventional wisdom" having magnetic shields for PMTs extend above the apex of the tube; some wondered if the shielding effectiveness would be diminished. The rotation measurements were repeated, and (again, unexpectedly) the equatorial height shields eliminated the geomagnetic field effects observed with unshielded tubes. Shields at this height were purchased and are being installed in the MicroBooNE cryostat (figure 6). The impact of this project extended beyond the demonstration of the effectiveness of these cryogenic magnetic shields. Four undergraduate students (two for one summer, and two for two summers) worked at Fermilab and at Saint Mary’s on this project. They participated not only in making the measurements, but in writing and editing MicroBooNE education and outreach materials for the public. Two undergraduates presented contributed papers at American Physical Society meetings in the spring of 2011 and 2012; one of those two students, Evan Shockley, presented a paper at a regional undergraduate research symposium, gave another presentation at the Fermilab New Perspectives 2012 conference, and presented a science colloquium at Saint Mary’s. The undergraduates were also co-authors on an already-published (JINST, 1307, T07005 [2013]) paper on the PMT test stand, and are participating in the writing of an article on the cryogenic shields.
