SIRI-2 To Validate Space Radiation Detection Technologies

Naval Research Laboratory scientists have launched SIRI-2 on STP Sat-6. SIRI-2, a gamma-ray spectrometer, will exhibit europium-doped strontium iodide gamma ray detection technology for DoD operational purposes.

FREMONT, CA: “The technology being demonstrated in SIRI-2 will need to detect small radiation signatures or signals in the highly variable background radiation fields found in space," states Les Mitchell, Ph.D., an NRL Research Physicist. U.S. Naval Research Laboratory scientists have deployed SIRI-2, the second Strontium Iodide Radiation Instrument, onboard Space Test Program satellite Sat-6. One of SIRI-2's objectives is to show the performance of europium-doped strontium iodide detection technology with sufficient active area for DoD operations.

“The instrument will also study transient phenomena, such as solar flares during the one-year mission,” adds Mitchell.

Launched on Dec. 3, 2018, the first SIRI mission on board STP Sat-5 was designed to explore how the detector responds to background radiation in low-Earth orbit for one year (LEO). The SIRI-2 instrument is in a geosynchronous orbit, with a much different radiation background.

The SIRI instrument series is intended to validate new gamma-ray scintillator materials and readout circuits for use in space.

A scintillator is a substance that exhibits luminescence when stimulated by ionizing radiation and is frequently used for radiation detection. When impacted by incoming particles, luminescent materials absorb their energy and radiate it through visible light.

Silicon Photomultiplier (SiPM) technology, which converts scintillation light to electronic signals and is projected to replace conventional photomultiplier tubes, will be evaluated using the instrument. These materials and electronics react differently to varying degrees of intense on-orbit background radiation.

“We hope to show this technology can be used in space since it can be difficult for some technologies developed for terrestrial applications to operate in the harsh space environment,” explains Mitchell.

The Department of Defense has utilized scintillation detectors in space since the Vela high-altitude nuclear detection program of the 1960s. The scientific community uses scintillator technology extensively in astrophysics, solar research, and earth science.

The Department of Defense has utilized scintillation detectors in space since the Vela high-altitude nuclear detection program of the 1960s. The scientific community uses scintillator technology extensively in astrophysics, solar research, and earth science.

Mitchell and his crew are enthusiastic about the increase in solar activity. This mission is well matched with the peak of Solar Cycle 25.

"While we reduced the cost, weight, and power for comparably sized instruments," comments Mitchell. These improvements led to greater sensitivity and, in turn, improved source detection and identification. SIRI-2 completed on-orbit checkout on Jan. 10. Mitchell said, So far, the instrument is performing well."

“While the peak of the solar cycle is expected to occur in 2025, it appears the Sun is showing significant activity earlier than expected,” Mitchell concludes. “Solar flare activity is most active at the peak of the solar cycle, so we hope to not only space-qualify new technology for the DoD but also make significant contributions to solar physics by studying gamma-rays emitted during solar flares.”

SIRI-3, the successor to SIRI-2, will utilize the knowledge learned from earlier flights to construct a big prototype instrument for a late 2025 launch.

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