| Aerospace | Photonics/Optics | Sensors/Data Acquisition

Dr. Keith Gendreau, Principal Investigator, Goddard Space Flight Center, Greenbelt, MD

Dr. Keith Gendreau is the principal investigator of the upcoming Neutron star Interior Composition ExploreR (NICER) mission. He was the 2011 Innovator of the Year at Goddard Space Flight Center, and he has been developing X-ray detectors, optics, and other instrumentation to support a number of NASA missions.

NASA Tech Briefs: You have a webinar in July about Goddard's Modulated X-Ray Source. What is the Modulated X-Ray Source?

Dr. Keith Gendreau: It's an X-ray source that we've developed at Goddard to support a number of different missions and technologies that we are working on here. It's an X-ray source that we can actually arbitrarily adjust the output of, with sub-nanosecond time resolution. Originally, we developed it as an in-flight calibration source to provide calibration data to some of our high-resolution X-ray spectrometers. Since then, it's come to be useful for a number of other things. For example, we use it simulate the X-ray output from pulsars, which is one of the objects that we want to look at with the Neutron star Interior Composition ExploreR (NICER) mission on the Space Station; that will launch in August 2016.

Our Modulated X-ray Source (MXS) can be programmed to produce X-Rays that follow a prescribed light curve, and we can put into that light curve all sorts of interesting temporal data that is reflective of the orbit that the Space Station or any future spacecraft would be in, and we could actually try to extract that data by analyzing the X-ray timing data we get from our overall system. The MXS was used to make this simulated pulsar.

We've also done other things with it. We've developed the world's first communications system that uses X-rays as a medium to transmit data. X-rays have a very short wavelength. If you use the very short wavelength of X-rays to your advantage, then you actually modulate the X-rays, and it offers unique technical capabilities in terms of communication. For example, the diffraction limit of optics goes like the wavelength lambda (λ) divided by the diameter of the optic that is collimating the beam d. So λ/d is roughly how the beam divergence goes, and X-rays have a very small λ, which means the divergence is extremely small. In fact, it's about 1000 times smaller than what you would get from a comparable laser communications system. For future deep-space communication needs, an X-ray communication system enabled by our Modulated X-ray Source would allow much higher data rates for lower power over interplanetary distances.

In addition, you could tune the X-ray source to work at higher energies, in which case the penetrating capabilities of X-rays offer unique communication advantages. If you've ever seen the movie Apollo 13: They're coming to the end of their mission, Apollo 13 is reentering, and there is a couple-minute blackout where [the mission control team] doesn't know if the vehicle burnt up in the atmosphere or skipped off the Earth's atmosphere. That's because the vehicle was going extremely fast, heating the atmosphere into a plasma, which rejected radio communication with the Apollo 13 capsule. X-rays at high-enough energy can penetrate that plasma. An X-ray communication system tuned for high energies, enabled by the Modulated X-ray Source, would allow communication with hypersonic vehicles in that type of situation and others.

We're opening up the time domain in X-Rays with the Modulated X-ray Source, and there's a number of really interesting things we can do. For example: very precise dose control for medical X-ray imaging; customizable X-ray doses for when you go to the dentist, and you need to see if you have a cavity. You don't need to have the dose that you might normally get at a dentist X-ray machine. With a Modulated X-ray Source, it could be customized in a very intelligent way. This could have long-term medical benefits for folks.

There are a number of other interesting X-ray sources that you can imagine for medical applications, because of the unique triggering and modulation capabilities. The Modulated X-ray Source could provide higher spatial resolution and timing resolution for a number of situations. We're at the tip of the iceberg really. It is a new capability that people haven't used before. There are a number of other applications for the Modulated X-ray Source.

NTB: Are there possibilities as well for material and chemical identification?

Dr. Gendreau: Absolutely. We've actually made a new type of X-ray fluorescence (XRF) instrument that is driven by a Modulated X-ray Source, which allows a unique capability for XRF at a distance. There are other things that you can do with this as well. X-rays are also used to measure the spacing between atoms, in a process called X-ray diffractometry. If you can imagine having X-rays that modulate, you've essentially made an X-ray strobe light for measuring the spacing between atoms. If you can imagine taking a tuning fork and whacking it on a table and hearing it hum: At the atomic level, those atoms must be peeling back and forth. If you had a Modulated X-ray Source that was strobing X-rays at the appropriate acoustic frequency, you could use MXS to basically get strobe-light, freeze-frame pictures of the atoms peeling back and forth. It's another example of something that's enabled by time modulation in the X-ray domain of materials science.

NTB: Have the major limitations been with the inability to modulate? In what other ways has the MXS improved upon the standard sources?

Dr. Gendreau: The unique capability is the modulation. At the end of the day, it is very similar to other X-ray sources, but we can turn it on and off basically as fast as you can turn off an LED. That's really unique. If you go to a dentist's office to get an X-ray, basically there's mechanical shutter, and you can't open and close that shutter with nanosecond precision, and certainly not arbitrarily. The unique capability of the Modulated X-Ray Source is the ability to control its temporal output.

NTB: What is the status now of the NICER mission that you're currently working on?

Dr. Gendreau: The NICER mission, the Neutron star Interior Composition ExploreR, is an X-Ray timing instrument. It's an X-ray telescope that has very good timing resolution, in fact an order of magnitude better than previously flown. It will go on the International Space Station in August 2016, and its baseline mission for 18 months is to observe neutron stars and pulsars, in particular to do high-precision X-ray timing of these pulsars. We are about 24 months away from delivering our instrument to the launch site.

NTB: What's your role in developing that technology?

Dr. Gendreau: I'm the principal investigator for NICER, so I'm involved in pretty much all aspects of that mission. I worked on the X-ray optics, I worked on the X-ray detectors and architecture, I participate in all the other sub-systems, and the Modulated X-ray source work was used to actually test how NICER would work under situations we expect to see in space flight.

NICER is actually more than just doing timing of pulsars for the sake of understanding neutron stars. We also want to demonstrate what we call pulsar navigation, where we look at the pulsations from neutron stars as basically ticks of an extremely accurate clock that is more stable than atomic clocks on long time scales. We want to use these pulsars just like we use the atomic clocks that are inside the GPS satellites that are around the Earth to do high-precision navigation.

Basically, you look at when the time that pulses arrive from different atomic clocks and different GPS satellites to figure out where you are. Well, we might want to do the same thing, only using these pulsars. These pulsars are distributed on a galactic scale, as opposed to just medium-Earth orbit, which means that we would have a capability to navigate throughout the solar system and beyond, using this natural infrastructure in a very GPS-like way. Part of my job is to merge this technology demonstration of pulsar navigation along with our baseline science mission to understand the extreme physics of neutron stars.

NTB: Will X-ray communication tests be a part of the NICER mission?

Dr. Gendreau: With NICER, we're also pursuing an extension of our mission to include an X-ray communication demo in space, where NICER would be the receiver for the first space-based X-ray communication demonstration. We'd put one of our Modulated X-ray Sources on another spacecraft that's approaching the ISS and do — at, say, 100 kilometers away — a test of X-rays as a communication media using NICER.

NTB: How are X-ray detectors being used in other NASA missions?

Dr. Gendreau: There are a number of other missions that have X-ray instruments. There are X-ray CCDs; that's actually what I cut my teeth on in graduate school many years ago. They do imaging and spectroscopy of objects in the sky and soft X-ray band, where you see bright characteristic lines from the most abundant elements in the universe. This gives you a way to do a lot of interesting astrophysics.

We also at Goddard work on other types of X-rays detectors, including micro-calorimeters, which are spectrometers but extremely precise spectrometers. They measure the energy of x-ray photons by determining the temperature change that occurs when one X-ray photon interacts in a detector. These detectors are held at about 70 millikelvins above absolute zero, and they measure the temperature rise to something like a microkelvin precision to get really the highest energy resolution possible. There are other types of X-ray detectors that are developed here at Goddard: more traditional, proportional counters that are used for X-ray timing. NICER will have a solid-state detector that we're flying that does timing and spectroscopy; it doesn't do imaging, however.

NTB: What are your biggest technical challenges when working with the NICER technology?

Dr. Gendreau: Actually, it has nothing to do with the X-ray detectors or the optics; it's the fact that we are trying to point a telescope from the International Space Station as a platform. We have to worry about all the vibration and jitter of the Space Station, as well as how our instrument induces its own vibration as it's tracking and looking at pulsars. However, we've spent a lot of effort, and we feel very comfortable with our design. In the end, I think this is going to be one of the most precise pointing platforms that will be on the International Space Station. We're pushing back the frontier there.

NTB: Are you doing a bunch of tests to simulate that currently?

Dr. Gendreau: Yes, we have most of the components. We have engineering testing actuators, and what we do is we can check various parts of the models of how our pointing system works at a component level. In software and simulations, we can see how the system comes together. Occasionally, we merge all of these in a sort of system-wide test. It's still not as good as being on the Space Station, but we get a lot of good data to improve our confidence that we're going to be able to do this.

NTB: What do you think is most exciting about the development of these kinds of X-ray detectors, optics, and instrumentation?

Dr. Gendreau: We're going to be doing a technology demonstration with NICER to demonstrate that pulsars can be used as a way to navigate on not only an interplanetary scale, but an interstellar scale. I think, in a few hundred years, when mankind leaves the solar system, it's going to be accomplished using pulsars, and the way that we're demonstrating them on this mission, as navigation aids. In 500 years or so, they can look back at this experiment as the one that proved that this is the way to navigate to the other stars. I think that's really fantastic.

Want to learn more about the Modulated X-Ray Source? Register for a live Q&A with Dr. Gendreau. Sign up at www.techbriefs.com/webcasts. To learn more about the MXS, read a full transcript, or listen to a downloadable podcasts, visit www.techbriefs.com/podcast.