I am currently a Research Scientist at the Columbia Astrophysics Laboratory where I am interested in investigating a variety of astrophysical phenomena through the development of instrumentation for balloon-born and satellite missions.
Links and downloads of papers and talks, can be viewed at my online CV.
Grazing incident optics for focusing soft x-rays (~0.1-10 keV) have become well developed over the last 20 years starting with the Einstein observatory. Currently, the XMM-Newton, Chandra and Suzaku satellites are yielding fantastic science returns in this energy band.
I seek to explore the next energy decade (10-100 keV). My work on hard X-ray optics began over a decade ago beginning with the High Energy Focusing Telescope (HEFT) balloon program. This successful development led to the Nuclear Spectroscopic Telescope Array (NuSTAR), a NASA satellite mission that will launch in February 2012. NuSTAR will be the first highly-sensitive, focusing telescope to image the hard X-ray sky, opening a new window for observing the universe through a dramatic improvement in sensitivity and angular resolution previously unachievable by the past generation of background limited collimated and coded-aperture instruments.
Scientific American featured this article on NuSTAR:
"Thanks to amazing nested mirrors, NASA's NuSTAR telescope is set to reveal hidden phenomena in the cosmos,"
Along with the article, Scientific American provides a Slide Show of the NuSTAR instruments.
The hard X-ray band is a natural place to study black holes and neutron stars. The penetrating power of hard X-rays allows us to see deep into regions that are obscured to optical telescopes and often even soft X-ray telescopes. This new technology will allow us to take a census of black holes on all scales, achieved through deep, wide-field surveys of extragalactic fields and the Galactic Center. It will also open exploration of topics such as stellar nucleosynthesis through mapping radioactive Titanium emissions (at 68 and 78 keV) in supernova remnants, and the origin and acceleration of cosmic-rays through mapping the hard X-ray continuum produced in the same shocks that produce cosmic-rays.
The challenge for developing focusing optics for higher energies is two-fold:
1) The grazing angle necessary to reflect X-rays by total external reflection is inversely proportional to the X-ray energy,
2) X-ray source intensities typically drop off exponentially with energy.
Thus, very light telescopes are required with huge effective area.
We developed a method at Columbia to mass produce thousands of extremely thin glass mirrors and assemble them using an error-compensating assembly and alignment (EMAAL) procedure. Using this approach for the HEFT hard X-ray balloon mission, three, large area telescopes have been built with 1.3 arcminute angular resolution and good response up to 70 keV (compared with ~2 arcminute resolution for the recent Suzaku satellite mission for 0.4-12 keV).
Here is a picture of HEFT in preparation for a successful flight in the spring of 2005 from Ft Sumner, NM.
NuSTAR ups the ante by producing larger, lighter and higher performance (sub-arcminute) telescopes. The HEFT approach of slumping glass microsheets into standard quartz mandrels using commercial ovens to achieve 40" angular resolution after mounting was the original baseline for the NuSTAR mission, which was selected by NASA through a competitive process beginning in 2003 (for details see the PDF paper Hard X-ray optics: From HEFT to NuSTAR). Unfortunately, after over three years of detailed study, NASA abruptly cancelled NuSTAR early in 2006 without prejudice because of extreme budget cuts to the science program and I began focusing more on dark matter (see below). After receiving word on Sept 21, 2007 that NASA Restarts Telescope Mission to Detect Black Holes we were back in business building X-ray optics. At this time Goddard Space Flight Center (GSFC) joined the collaboration to build the flight mirrors and both the optics assembly facility and a dedicated calibration facility were located at Columbia. The GSFC approach is to form the substrates onto polished forming mandrels.
Each NuSTAR optic is composed of 133 concentric layers of conical-approximation Wolter I mirrors aligned and held together by epoxy and precisely machined graphite spacers that run along the optic axis. The optics were fabricated using custom designed machines at Columbia University's Nevis Laboratory in Irvington, NY. I describe in more detail the NuSTAR hard x-ray optics design and performance (PDF). We also built the Rainwater Memorial Calibration Facility (RaMCaF), a dedicated 163 m long X-ray calibration facility to determine the performance of the optics.
The X-ray ground calibration measurements for the FM1 and FM2 flight optics, including installation and alignment, were performed over a period of 18 consecutive days (March 5-22, 2011). I recently reported the First results from the ground calibration of the NuSTAR flight optics (PDF). From a preliminary analysis of the data, our current best determination of the overall HPD of both the FM1 and FM2 flight optics is 52", and nearly independent of energy. The statistical error is negligible, and a preliminary estimate of the systematic error is of order 4". The as-measured effective area and HPD meet the top- level NuSTAR mission sensitivity requirements. NuSTAR flight optics modules FM1 and FM2 have been installed into the observatory and are ready for flight as shown below.
I also seek to identify the mysterious nature of dark matter, which is thought to dominate ordinary matter in the universe. Evidence points to cold dark matter (CDM) and the best candidate is generally thought to be a weakly interacting massive particle (WIMP). Dozens of groups are attempting to detect dark matter through direct recoil interactions with target nuclei in deep underground laboratories. An alternate approach is to search for indirect signatures from dark matter annihilations into products such as gamma-rays, neutrinos, positrons and antiprotons.
My interest lies in searching for antideuterons as a signature of dark matter annihilation in addition to antiprotons using the General Antiparticle Spectrometer (GAPS). While antideuteron production is suppressed compared to antiproton production, the corresponding background is even further suppressed, especially at low energies. Thus, a search for primary antideuterons below ~0.5 GeV/n is essentially background free, and detection of a signal event would be a discovery of new physics. The GAPS method begins with a target material in which antiparticles are captured into excited states. The X-rays that are emitted as the antiparticle cascades to lower energy states (Rydberg levels) before it annihilates with the target nucleus. Pions emitted from this annihilation as well as the discrete transition X-rays serve as a fingerprint that uniquely identifies the mass of the captured antiparticle.
Dozens of papers over the last few years have analyzed constraints on dark matter models (e.g., SUSY, mSUGRA, Kaluza-Kein) from antideuteron searches as well as other indirect and direct methods. In some regions of these models, antideuterons are the only viable detection method, while in other regions antideuterons are competitive with direct detection or other indirect detection methods. Multiple detection methods are required to discover and ascertain the underlying physics origin of dark matter.
We have extensively tested the GAPS method in an accelerator environment. Based on these results and extensive design studies, we have designed a flight instrument capable of reaching deep into the parameter space of many CDM models. The heart of the instrument consists of layers of coarsely pixellated Si(Li) detectors that function as both a target in which exotic atoms are formed and a detector of the subsequent atomic transition X-rays. The Si(Li) detectors also serve as a particle tracker for the incoming antiparticle as well as the annihilation pions. An array of plastic scintillators surrounding the Si(Li) detectors serves as a time of flight (TOF) trigger, providing a measurement of the incoming antiparticle velocity (and energy after subsequent mass identification).
The GAPS collaboration is growing (UCLA, Berkeley, Oak Ridge, ISAS/JAXA Japan) and we have assembled and are currently testing a prototype ballon experiment in preparation for a flight in 2012. The potential for a detectable signal from primary antideuterons from a full scale GAPS balloon mission in 2015 presents a potential breakthrough in the search for dark matter.
GAPS prototype balloon payload
I received my PhD in particle physics at the University of Virginia. My thesis research was primarily conducted the Paul Scherrer Institute (PSI) in Switzerland. The main projects I was involved in at PSI were my thesis project, a Search for a Neutral Particle of Mass 33.9 MeV in Pion Decay, the Pion Beta Decay Experiment, and measurements involving Pionic Atoms .
Feel free to visit these sites and learn about the experiments.
I received my undergraduate degree in Physics at Northwest Nazarene University.
