Milagrito

Milagrito was a prototype for the Milagro telescope. It had 228 photomultiplier tubes submerged under 1-2 meters of water. (The water height was varied to study the response of the detector as a function of depth.) Unlike Milagro there was only a single layer of PMTs, so no discrimination between gamma-ray induced air showers and hadronic air showers was possible. A schematic view of Milagrito is shown below.

Milagrito ran between February 1997 and April 1998. The trigger rate was between 150 and 400 events per second, depending upon the amount of snow/ice/water on top of the cover, the depth of water above the PMTs (but under the cover), and the trigger condition. The nominal trigger requirement was that at least 100 PMTs had to be struck by light within 300 nanoseconds (ns). (A nanosecond is a billionth of a second.)

Here is a visualization of an event observed by Milagrito. The read and blue lines represent the relative time that the PMT was hit. The red line means that the PMT was used in fitting the direction and the blue indicates that the PMT was not used in the fit. The plane of the pancake is readily evident.

Though Milagrito was a prototype it was a powerful instrument for studying the TeV sky. Milagrito observed the shadow of the Moon in cosmic rays, the active galaxy Mrk501, and analyzed data from 54 gamma-ray bursts seen by the BATSE instrument that were within our field of view.