The article “HERA strikes it RICH” in the November 1998 issue described the first rings seen in the Ring Imaging Cherenkov (RICH) being commissioned for the HERA-B experiment at the HERA electronproton collider at DESY, Hamburg.
In those early days, moist Hamburg air served as the Cherenkov radiator, but over the end-year shutdown the 100 cubic-metre RICH vessel was filled with C4F10 gas, which is ten times as dense as air.
The greater density has three related consequences:
- Cherenkov rings from highly relativistic particles more than double in size;
- more than four times as many photons contribute to the characteristic rings (proportional to the square of the Cherenkov angle);
- the threshold momentum for producing Cherenkov light is reduced by the same factor of two.
The first events with the 920 GeV HERA proton beam in January dramatically confirmed these expectations. Because of the RICH optics design, some rings have photons detected on both the upper and the lower photomultiplier arrays. One of the rings is smaller than the others, showing that it comes from slower particles.
The design and construction of the gas circulation and purification system for the HERA-B RICH relied heavily on expertise from CERN. The groups responsible for the HERA-B RICH project consist of the University of Texas at Austin; the University of Barcelona; the University of Coimbra in Portugal; Northwestern University; the University of Houston (incorrectly indicated in the November article); the J Stefan Institute; and the University of Ljubijana, Slovenia. Support was provided by DESY and the University of Hamburg.
