The Voyager 1 spacecraft and instruments. Voyager 1 and 2 are now in the outermost layer of the heliosphere where the solar wind is slowed by the pressure of interstellar gas. Both spacecraft are still sending scientific information about their surroundings through the Deep Space Network (DSN).
Credit: NASA/JPL

The solid-state detectors we used on Voyager were similar to those that were starting to be used for gamma-ray astronomy. This was new development in the early 1980s and I thought, “This is great! A new field, using these detectors that I really know well.” It turned out to be a great idea to explore the gamma-ray sky with these devices, as others were also finding out, because they were more sensitive and better for spectroscopy than the instruments that were used before.

Since the 1960s, there were ideas that some stars and galaxies, and especially high-energy phenomena like supernovae, would make a lot of gamma-ray light, but the early instruments detected nothing. That’s because, first of all, these things don’t make a lot of gamma-ray light. The other problem is that our atmosphere glows bright in gamma-rays. When particles from the Sun and cosmic rays hit the Earth’s upper atmosphere, they create gamma radiation. So space has a lot of background noise in the form of high-energy particles that don’t allow you to see very clearly when gamma-rays are coming from astronomical sources.

[Left] A NASA high-altitude research balloon climbing to study the composition of the Arctic stratosphere from the Esrange Balloon Launch Facility near Kiruna, Sweden.
Credit: NASA/JPL-Caltech

[Right] Schematic of the Gamma-Ray Imaging Spectrometer (GRIS), a balloon-based instrument capable of high resolution gamma-ray spectroscopy.
Credit: NASA/Goddard Space Flight Center

We had an instrument called the Gamma-Ray Imaging Spectrometer (GRIS), which was a balloon-based instrument capable of high resolution gamma-ray spectroscopy. The instrument had seven of these detectors in a big shield, and they were mounted on a gondola the size of a table. All of this would go up on the balloon. There was also a pointing system – a way of aiming and steadying a telescope – just like there is on a telescope on a mountain, or in space.

The solid-state detectors we used then were made from solid germanium crystals, which are dense and good at stopping gamma-rays. We had to keep the germanium cold using liquid nitrogen. When germanium gets cold, its background noise from thermal fluctuations goes down and it can be used for detecting gamma-rays. We were developing this new technology along with groups at Bell Labs and the NASA Jet Propulsion Laboratory.