The Vasa

The Vasa, a Swedish vessel, is the world's only preserved 17th century ship. The Vasa sank during its maiden voyage on Aug. 10, 1628, and was largely forgotten. Three centuries later, she was located in a shipping lane just outside Stockholm harbor. Salvaged in 1961 with its hull largely intact, today the Vasa is among the most popular tourist attractions in Sweden.

This constellation symbolizes the contributions of Swedish astronomers at the Royal Institute of Technology in Stockholm who helped build Fermi's calorimeter, which measures the energies of particles moving through the Large Area Telescope (LAT).

The LAT consists of three elements. An "anticoincidence detector" determines whether a charged particle has entered the LAT so astronomers know which tracked events are caused by particles and which are caused by gamma rays. Next is the tracker, which consists of alternating layers of silicon and tungsten foil. When a gamma ray strikes a tungsten nucleus, it converts into two particles of matter — an electron and its antimatter equivalent, a positron. Silicon strips in the tracker keep tabs on the particles as they travel through the LAT, recording their paths so the instrument can determine where in the sky the original gamma ray came from.

Next up is the calorimeter. In order to determine the total energy of the incoming gamma ray, the particles it produced need to be stopped somehow. So the calorimeter is made of cesium iodide, a material with about 40 percent the density of lead that emits light whenever a charged particle passes through it.

In response to an incoming particle, the cesium iodide emits a specific wavelength of light that detectors pick up and convert to digital signals. By measuring the total light emitted as the particle slows, the calorimeter determines the total energy of the original particles, which is also the energy of the original gamma ray.

If this sounds complicated, well, it is. But by combining all of this information, Femi's LAT has produced the most sensitive and highest-resolution view of the gamma-ray sky ever made. It's a view that continues to improve thanks to its continued operation and to enhancements in the way LAT data are processed.

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