I have always been interested in the properties of fundamental particles - the base constituents of the material universe. The understanding that the neutrino (an elementary fermion) is a massive particle - originally hypothesized to be massless - has spurred-on the field of neutrino physics.
Are neutrinos Dirac or Majorana type particles? These names refer to the theoretical physicists who developed mathematical formulations for the description of the neutrino. The neutrino, unlike all other elementary fermions, has no electric charge. This lack of charge allows for the two different formulations, attributed to the theorists mentioned above. Determining the Dirac or Majorana nature of the neutrino is a major goal of current neutrino research. One way to determine this is to look for an extremely rare nuclear decay that is only allowed if neutrinos are Majorana type particles. In this rare decay, a nucleus simultaneously emits two beta particles without emitting the two neutrinos that are required for normal nuclear beta decays. This process is called neutrinoless double beta decay and is depicted here with the lowest order Feynman diagram.
I am currently involved with the Majorana Project which is an experiment to search for this extremely rare nuclear decay that would be a signature of the Majorana nature of the neutrino. Another extremely important question in neutrino physics is, "What is the absolute mass of the neutrino?" The rate (half-life) of neutrinoless double beta decay is related to the absolute mass of the neutrino. The determination of these two properties of the neutrino is critical a more complete understanding of these elementary particles. These ideas are vindicated by the American Physical Society's Joint Study on the Future of Neutrino Physics: The Neutrino Matrix which recommended as a high priority a neutrinoless double beta decay experiment. The Majorana DEMONSTRATOR is currently underway at the Sanford Underground Research Facility (SURF). The Majorana DEMONSTRATOR apparatus is shown here without the external neutron moderation shield, revealing the lead gamma-ray shield and the interior copper cryostats which hold the germanium detectors.
I participated in an interview with South Dakota Public Broadcasting's Innovation at SURF in July 2011. Here is a YouTube video of the interview underground; I join the discussion about 30 minutes into the show and talk about neutrinoless double beta decay and the Majorana experiment.
A few years ago I got involved in a collaborative effort to measure the cross-section of the neutrino-nucleus interaction via coherent scattering. First off, measuring this process is interesting because it has never been done before and would help further map out the Standard Model's suite of fundamental interactions. Dr. Juan Collar (U. of Chicago) and others demonstrated a high purity germanium spectrometer design which may be able to make this measurement possible. See their paper arXiv:nucl-ex/0701012. If this process is measured it opens the door onto a new range of low energy neutrino measures including low energy solar neutrinos and the background of supernova relic neutrinos. And did I mention this detector design is also a dark matter detector candidate?
Interestingly the same technique used for measuring coherent neutrino scattering is also well suited for attempting direct detection searches for WIMP dark matter. In the case of dark matter one is looking for the interaction of a massive weakly interacting particle which is supposed to play an important role in the formation of galaxies. That is to say, the luminous matter seen in galaxies with telescopes does not have enough gravitational pull to keep them from flying apart. Hence there must be some "dark matter" which is providing the gravitational mass to hold galaxies together. Currently we have a detector located at Soudan Underground Laboratory attempting to look for very low energy nuclear recoils which would be the tell-tale sign of dark matter particles 'bouncing' off nuclei in our germanium gamma-ray spectrometer. I've posted more details on a seperate page here: CoGeNT Dark Matter Experiment.
In ultra-low background gamma-ray spectroscopy ubiquitous backgrounds are the primary enemy. However, once radio-isotope free materials are found and a radio-assay detector is constructed with the needed sensitivity, the new problem becomes sample associate backgrounds. A sample associated background is one or more gamma-rays present in the measurement sample which mask the gamma-ray of interest by producing a dominating Compton continuum.
This project is developing a maximal Compton suppression detector by directly immersing the high purity germanium (HPGe) detector in scintillating liquid argon. In all prior Compton suppression detector designs, there existed some material between the germanium crystal and the surrounding active veto system that would provide Compton continuum suppression. Directly immersing a germanium detector in a scintillating liquid noble gas minimizes the non-active material near the germanium crystal and therefore maximizes the Compton suppression capability. The two gamma-ray spectra shown here are from a bench-top apparatus we operated in conjunction with University of Washington (CENPA) researchers. The two spectra show good energy resolution of a HPGe spectrometer operated in liquid argon (left) and the Compton suppression ability provided by monitoring the liquid argon as a Compton veto (right). A paper documenting this work is available: arXiv:nucl-ex/0610018. Current work is pushing to increase the size of the liquid argon volume, there-by increasing the total Compton suppression factor.
Basic research will help us solve tomorrow's problems. Applied research solves the problems we are faced with today. I am lucky to be able to participate in both basic and applied research at the Pacific Northwest National Laboratory. Below are descriptions of a couple of the applied research projects I have worked on.
International treaty verification organizations that monitor the status of both nuclear capable and non-capable countries have a specific interest in the detection of fission products that would result from any explosive nuclear critically. We have developed an apparatus that is sensitive to the beta particles emitted from such fission product material. What makes this project note-worthy is the size and durability of the apparatus. This large suit case sized instrument uses scintillating optical fibers and position sensitive photomultiplier tubes to locate beta emitters with cm2 accuracy. Quickly locating the material of interest speeds the process of isotopic identification.
Current state-of-the-art gamma-ray detector technology relies upon liquid nitrogen to cool the sensitive detection elements. This cooling apparatus is bulkly and difficult to transport. Cadmium zinc telluride crystals provide the potential for portable, room-temperature gamma-ray detection with competitive energy resolution and sensitivity. The Pacific Northwest National Laboratory is over-seeing a multifaceted materials and radiation detection investigation of CZT. The goal of the program is to assist in bringing high-quality (defined as spectroscopic grade) CZT material to the level of a practical and, thus, profitable manufacturing status.
I did my doctoral work as a collaborator on the Sudbury Neutrino Observatory (SNO) experiment. Below I give a brief overview of the SNO experiment and the work I completed to earn a doctoral degree from the University of Washington.
The Sudbury Neutrino Observatory (SNO) experiment is credited with solving the "solar neutrino problem", a 40 year long apparent deficit of the measured flux of solar neutrinos, as measured here on Earth. The resolution of this problem was the determination that approximately two-thirds of the neutrinos produced by the Sun's internal fusion reactions, change flavor, from electron to muon or tau, by the time they arrive at the Earth. This figure shows the three different measurements that the SNO experiment can make. Previous experiments were only sensitive to the type of reactions represented by the red and green bands. The SNO experiment's ability to add this third type of measurement, the blue band, showed conclusively the flux of electron neutrinos (horizontal axis) and muon or tau neutrinos (vertical axis) matches the prediction of the Standard Solar Model (SSM), represented by the dashed, black lines. This result is only possible if neutrinos are massive particles, confirming previous observations of neutrino flavor change and the massive nature of neutrinos.
Once neutrinos are shown to be massive particles, a number of new phenomenon are possible. In my doctoral disseration (ps, pdf), I used the data from the Sudbury Neutrino Observatory to search of an electron antineutrino signal from the Sun. SNO is identifibly sensitive to electron antineutrinos through charged current interactions on deuterons, as shown in the reaction equation:
This would be an additional conversion mechanism for solar neutrinos. Alas, there is no experimental evidence (including my work) that shows there is a conversion of solar neutrinos into antineutrinos. However, such investigations place limits on allowable models of neutrino properties. For example, this conversion mechanism is the more likely if neutrinos are Majorana type particles, rather Dirac type particles.