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New Instrument to Detect Gamma-Ray Emissions from Astrophysical Objects

Collaborative research performed by The US Naval Research Laboratory (NRL) and NASA Goddard Space Flight Center (GSFC) will begin ComPair aboard a high-altitude balloon from Fort Sumner, New Mexico, August 10th, 2023.

New Instrument to Detect Gamma-Ray Emissions from Astrophysical Objects

During the thermal vacuum (TVAC) test campaign at NASA’s Goddard Space Flight Center, Greenbelt, Maryland in June, Dr. Daniel Shy, a Co-Investigator on the ComPair Program, inspects the NRL-led CsI calorimeter box. Image Credit: U. S Naval Research Laboratory

The ComPair mission instrument quantifies and detects gamma-ray emissions from astrophysical objects. The NRL instrument is considered to be one of the four subsystem instruments headed by NASA GSFC. The mission name–ComPair–has been motivated by the mechanisms by which gamma rays interact with matter.

They do so via three dominant processes –photoelectric effect, Compton scattering, and pair production. These interaction mechanisms are dependent on energy, where photoelectric occurs at the lowest energies and pair production at the highest.

Richard S. Woolf, Ph.D., Space Science Division Research Physicist, The U.S. Naval Research Laboratory

Woolf added, “The design of the ComPair instrument employs technology to measure gamma rays from both the Compton and the pair production regimes, hence Compton-Pair or ComPair.”

Woolf continued, “Gamma rays allow us to study the highest-energy processes in the universe: ranging from nucleosynthesis in supernovae; jets from supermassive black holes; and gamma-ray bursts from the merger of astrophysical objects with extreme densities, like neutron stars and black holes.”

Woolf concluded, “The atmosphere mostly blocks astrophysical sources of gamma rays from reaching the ground at sea level, to measure the rays, the instrument needs to reach above the majority of the atmosphere up to 130,000 ft.”

In the development of this instrument, the aim is to increase the technical readiness level (TRL) of this gamma-ray technology. One of the last steps in progressing the TRL for gamma-ray instruments is to fly on a high-altitude balloon.

The hope is that what we learn during this phase will be applied to a larger, more sensitive mission that goes into orbit for a future NASA-led mission known as the All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X). AMEGO-X is a medium-class mission that will be proposed to the NASA Explorers program announcement of opportunity, expected in 2026.

Richard S. Woolf, Ph.D., Space Science Division Research Physicist, The US Naval Research Laboratory

Following the launch of the Fermi Gamma-ray Space Telescope in June 2008, the gamma-ray community began to take into account where they must focus their attention next. The concept was to utilize a similar design to Fermi but with sensitivity in the much lower-energy range of the gamma-ray spectrum.

The ComPair instrument design is based on a previous NRL-led instrument, the Fermi Large Area Telescope (LAT) calorimeter, containing scintillation crystals, meaning they emit light or scintillates when ionizing radiation passes through it.

Richard S. Woolf, Ph.D., Space Science Division Research Physicist, The US Naval Research Laboratory

Woolf stated, “ComPair expands on the technology of the Fermi LAT calorimeter by using a novel scintillation light readout device known as a silicon photomultiplier (SiPM) coupled to the crystals that comprise the calorimeter. These SiPMs are more sensitive for reading out scintillation light, enabling gamma-ray detection in the Compton region.”

Scintillation is the physical process where a material, known as a scintillator, liberates visible or ultraviolet light under excitation from high-energy photons: such as gamma rays, X-rays, or energetic particles.

The NRL Space Science Division progresses technical capabilities in the radio communications, orbital tracking, and navigation realms impacting the operation of ships and aircraft, application of the near-space and space environment of the Earth, and the basic knowledge of geophysical phenomena and natural radiation.

Woolf stated, “NASA GSFC and NRL, have a history with this next-generation instrument from the start. NRL’s initial funding from NASA commenced in early 2015 and has continued with multiple rounds of funding since, allowing us to build a proof-of-concept prototype, test the prototype at several accelerator beam tests, and now fly it on a high-altitude balloon.”

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