The “IRES: U.S.-European International Research Experience-Particle Astrophysics for Undergraduates” program, funded by NSF and led by the University of Wisconsin–River Falls, brought us to Johannes Gutenburg University in Mainz, Germany, this summer to work with Professor Lutz Köpke and Professor Sebastian Böser.
Professors Köpke and Böser work with PINGU, the Precision Next Generation IceCube Upgrade, as part of the IceCube Collaboration’s effort to build a next-generation IceCube detector. PINGU is an infill array that will lower IceCube’s energy threshold to 1 GeV, from its current lower limit of 10 GeV.
The primary science objective of PINGU is to resolve the neutrino mass hierarchy question. Researchers have detected three types (or flavors) of neutrinos—electron, muon and tau neutrinos—and theory tells us that these are a mixture of three quantum states, called mass eigenstates. We know the relative mass difference between the first and second neutrino eigenstates, and we know that the third neutrino is either much less or much greater than the first two. If the third mass eigenstate is much greater, we call this scenario the normal mass hierarchy (NMH). If the third mass eigenstate is much less, we call this scenario the inverted mass hierarchy (IMH). Several detectors around the world have been designed to resolve this mystery, and PINGU is a strong competitor.
Resolving the mass hierarchy requires complex simulations and several years of data. Theory predicts that some of the atmospheric muon neutrinos produced by the interaction of cosmic rays with the atmosphere will oscillate into other flavors while traveling through the Earth to reach IceCube. The simulation produces a flux map, including flux expectations for all flavors and their interaction types. A time scale is applied to give an event rate per flavor and interaction type. This event rate represents a perfect reconstruction of neutrino interactions inside the PINGU sensor array.
Due primarily to scattering of photons in ice, our reconstructions will never be perfect. The scattering of photons in ice does happen in a predictable way, however, and we can “smear” the true event rates to account for various uncertainties in the detection process. I spent the summer working with Professor Böser to improve this “smearing” reconstruction phase.
We used a statistics package to produce simulated neutrino events, which are trained by real IceCube data and scaled to the energies we are most interested in. The idea is that the best way to predict smearing effects is to base smearing predictions on previous data. Unfortunately, it takes about 15 minutes of computing time to produce a single event. I developed a statistics tool over the course of my IRES summer internship to determine how many events are required to achieve a useful smearing prediction.
PINGU will require both hardware and software upgrades to the IceCube Neutrino Observatory. The optical sensors are deployed in vertical holes with 60 sensors attached to each cable, collectively referred to as a string. The outermost shell is used to reject non-target events, like cosmic ray muon tracks. The next shell is used to detect the highest energy cosmic ray events. The next shell, called DeepCore, is used to detect events down to 10 GeV.
The effects of neutrino oscillations at the South Pole are most pronounced between 1 and 5 GeV. However, in that energy range, we need a new detector, PINGU, in order to achieve the directional sensitivity required to identify these events. Placing sensors closer together gives improved directional sensitivity, but a more compact sensor design is required to achieve the desired spacing. Wavelength shifting optical modules (WOMs) are being designed as an upgrade to the current IceCube digital optical modules (DOMs) to address the need to observe lower energy photons.
Focusing on hardware, I sought the source of noise created somewhere in the newly designed WOMs. Observations made by Professor Köpke show that the photomultiplier tube (PMT) in the WOM detects a signal, even without a neutrino event. Above 0 °C, the noise is due to thermionic activity. However, as the temperature decreases, one would expect the noise to decrease. Unfortunately, Professor Köpke observed the opposite effect. One hypothesis was that this noise pattern is radioluminescence, light produced from decay of a radioactive source, like the potassium in the borosilicate WOM glass. These emissions from radioluminescence act like the photons of a real event and create a false signal in the PMT.
To test the theory of radioluminescence and observe the effect in action, I imitated a radioactive source using two types of glass—borosilicate glass, containing the radioactively decaying potassium, and the potassium-free quartz glass. My observations revealed that the borosilicate glass did produce a higher number of photons at colder temperatures, while the luminescence in the potassium-free quartz glass remained unchanged with temperature. Comparing the noise rate in the PMTs with the number of observed photons shows a consistent but not statistically significant trend of rate increasing as temperatures decrease. The data collected will be used for comparison to repeated trials in other labs to find the noise culprit and develop the best possible detectors for the experiment.
I was led to this opportunity in international research from previous experiences with IceCube. Ever since I attended a field trip to IceCube’s headquarters in Madison, WI, at the age of 14, I knew I wanted to be a physicist. I was fortunate to work with IceCube researchers at WIPAC in high school through their internship program, which led to a research position with the Askaryan Radio Array also at UW–Madison.
The research experience this summer has increased my understanding of running experiments and my sense of independence in scientific explorations. Professor Böser encouraged me to think for myself, letting me design hardware and my own experimental set-up. I learned just how much detail and consideration go into designing an experiment. Even the tiniest element can greatly affect an experiment, down to the type of connections used between two wires. I appreciate the importance of attention to detail, precision, and being able to accurately replicate findings much more now.
The opportunity provided by NSF has furthered my passion for physics. I learned to work better in a team setting and found constant inspiration from my lab mates. It has reaffirmed in me the quest for pure knowledge that I first felt when I encountered the IceCube experiment six years ago.
For more information on PINGU please read IceCube looks to the future with PINGU.