US Naval Research Laboratory scientists released the second Strontium iodide radiation instrument (SIRI-2) instrument in December 2021 on board Space Test Program (STP) Sat-6. SIRI-2, a gamma-ray spectrometer, will demonstrate the effectiveness of europium-doped strontium iodide gamma-ray detection technology with sufficient active area for Department of Defense (DoD) operational needs.
The first SIRI mission launched on December 3, 2018 aboard STP Sat-5 on a one-year mission to study the detector’s response to background radiation in low Earth orbit (LEO). The much larger SIRI-2 instrument operates in geosynchronous orbit, where the radiation background is significantly different in composition.
“The technology being demonstrated at SIRI-2 will need to detect small radiation signatures, or signals, in the highly variable background radiation fields found in space,” said Lee Mitchell, Ph.D., NRL research physicist. “The instrument will also study transient phenomena such as solar flares during the year-long mission.”
The SIRI line of instruments is designed to space-qualify new gamma-ray scintillator materials and sensing electronics.
A scintillator is a material that exhibits the property of luminescence when excited by ionizing radiation and is commonly used for radiation detection. Luminescent materials, when struck by incoming particles, absorb their energy and re-emit the absorbed energy as visible light.
The instrument will also test a new silicon photomultiplier (SiPM) technology that converts scintillation light into electronic signals and is expected to replace conventional photomultiplier tubes. These materials and electronics react differently to the intense background radiation of orbit to varying degrees.
“We hope to show that this technology can be used in space, as it can be difficult for some technologies developed for terrestrial applications to work in the harsh space environment,” Mitchell said.
The Department of Defense has been using scintillation detectors in space since the 1960s Vela High Altitude Nuclear Detection Program. Scintillator technology is widely used in the scientific community in fields such as astrophysics and solar and earth science.
“While we’ve reduced the cost, weight and power for comparably sized tools,” Mitchell said. “These improvements have resulted in greater sensitivity and in turn improved source detection and identification.”
SIRI-2 completed the in-orbit check on January 10. Mitchell said, “So far the tool is performing well.”
One thing that excites Mitchell and his team is watching the solar activity increase. The solar cycle is an 11-year change in the Sun’s activity, measured in terms of variations in the number of observed sunspots on the Sun’s surface, and the mission is well-aligned with the peak of Solar cycle 25.
“Although the peak of the solar cycle is expected to occur in 2025, the Sun appears to be showing significant activity earlier than expected,” Mitchell said. “Solar flare activity is most active at the peak of the solar cycle, so we hope not only to qualify new technology for space for the Department of Defense, but also to make a significant contribution to solar physics by studying gamma rays emitted by time of solar flares.’
Following SIRI-2, SIRI-3 will use the knowledge gained from previous missions to develop a large prototype instrument that is expected to be launched in late 2025.
About the US Naval Research Laboratory
NRL is a science and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seabed to space and the information domain. NRL is located in Washington, DC, with major field sites at Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.
For more information, contact NRL Corporate Communications at (202) 480-3746 or [email protected]l.