Measuring up: The Compton Spectrometer and Imager

What are some of your responsibilities as professor of astronomy and astrophysics?
My primary responsibilities are research, teaching, and mentoring students. It is a particularly exciting time at UC San Diego as we just established a new department of Astronomy and Astrophysics in July 2023. As one of the founding faculty of the department, I have a lot of work to do in developing the overall curriculum and new courses for our students. It is exciting work though!

What led to your interest in astronomy and astrophysics?
Like many kids of my generation, I grew up fascinated by NASA’s space program and wanted to be an astronaut. I focused on science and math in high school, which is where my love for astrophysics really was ignited. I became more interested in the science than in pursuing my original interest in becoming an astronaut. I am fortunate that I was able to turn my hobby as a kid into my career!

You are also deputy principal investigator of the Compton Spectrometer and Imager (COSI) NASA Small Explorer mission. How did you come to work with COSI?
I developed the basic concept of the COSI telescope as a graduate student in the 1990s in response to the success of NASA’s Compton Gamma Ray Observatory (CGRO). The MeV Compton gamma-ray telescope (COMPTEL) instrument on CGRO was both scientifically a success and demonstrated the technical feasibility of the Compton imaging technique at MeV energies. Scientifically, the natural next step was to replicate the technique with high-spectral resolution detectors for detailed study of nuclear astrophysical processes, which are uniquely probed at MeV energies.

I have spent much of my research career developing the technologies needed for COSI and building a worldwide network of collaborative researchers who contribute to the instrument and the related science. Before this became a satellite mission, I led the development of COSI through NASA’s scientific-balloon program. Many young researchers established their careers through working on the COSI balloon program, and many of them are now members and leaders of the satellite mission.

The COSI balloon program demonstrated our novel compact Compton telescope design, utilizing cryogenic germanium detectors in a space environment. COSI was also designed to utilize NASA’s novel super-pressure balloons for extended scientific flights. The COSI balloon payload underwent a very successful 46-day flight from Wanaka, New Zealand, in 2016.

The 18.8 million-cubic-foot COSI Super Pressure Balloon prepares for lift-off from Wanaka Airport in 2016. Credit: NASA/Bill Rodman.

COSI is a gamma-ray telescope. How do gamma-ray telescopes differ from other types of telescopes?
We are unable to focus gamma rays, at least with current technologies. Hence our telescope is essentially a focal-plane camera without any focusing lenses or mirrors. This leads to a number of unique challenges for optimizing the telescope performance, such as how to image the sky without the help of optics, and how to measure small astrophysical gamma-ray signals when our detectors themselves “glow” in gamma-rays when exposed to cosmic radiation in space. Perhaps someone inspired by COSI’s science discoveries will develop the technology to focus gamma rays!

How will the COSI mission help us better understand the Universe?
While the science program for COSI is quite broad, it operates at gamma-ray energies that uniquely probe astrophysical processes occurring at the nuclear level. One of the unique science goals for COSI is to directly measure the creation and evolution of chemical elements across our galaxy. COSI allows us to directly see where new elements are being formed in our galaxy today. For example, COSI will be the first instrument capable of mapping the isoptope 60-Fe in our galaxy. This isotope is uniquely created in supernovae resulting from the core collapse and subsequent explosion of massive stars. Studying this isotope will allow us to directly map regions of supernova activity over the last few million years to discover where new iron-peak elements are being formed across our galaxy today.

Credit: Image by Jim Willis, courtesy of Northrop Grumman Space Systems; background image courtesy of the European Southern Observatory.

What do you see as the most important aspect of your work at this time?
As a senior researcher in my field, the most important aspect of my work at this time is supporting younger researchers in establishing their careers, both through their work with COSI and in terms of developing next-generation technologies.

What is most exciting or surprising about your work? What are some of the challenges?
The most inspiring aspect of my work at the moment is the community-wide excitement in seeing COSI succeed. More than once over my career, my collaborators and I were told that the COSI technologies and instrument design could never work in the space environment. Seeing the instrument come together as we finalize the mission design, with the support of the larger gamma-ray astrophysics community, is extremely rewarding.

What do you see as the future of missions such as COSI? What would you like to see?
COSI is already seen as a pathfinder mission for a larger, more sensitive next-generation observatory. There are many young researchers across the US who are developing novel technologies for a future mission. I am optimistic that COSI will have a very successful science return, but as with most new missions the most exciting discoveries will be the ones we are not currently anticipating. The results from COSI will likely have a huge impact on the scientific priorities of future gamma-ray missions.

What do you want attendees to learn from your talk at SPIE Optics + Photonics?
I hope attendees will come away with an appreciation for the COSI science goals as well as for the remarkable technology developments and decades of work that have gone into making this mission happen.