1. The Hall C Heavy Gas Cerenkov
SHMS
HGC
ELECTRON BEAM
TARGET
HMS
Highly energetic electrons scatter from a liquid hydrogen target in Hall C producing an array of secondary particles which travel into two spectrometers (HMS and SHMS).
The Heavy Gas Cerenkov (HGC) is one of several devices used by the SHMS to gather data on secondary particles.
Cerenkov radiation is the optical equivalent of a sonic boom. In the diagram, an incoming pion travels at nearly the speed of light in vacuum. Upon entering the gas, it travels faster than the speed of light in that medium. As a result, the pion briefly polarizes nearby gas molecules which de-excite to form a cone of (Cerenkov) radiation.
DYNODES
ELECTRON
The HGC will discriminate pions from kaons, two types of subatomic particles. In passing through the heavy gas (octofluorotetrahydrofuran – C4F8O) in the tank, pions and lighter particles like electrons will emit Cerenkov radiation (light) while kaons and heavier particles like protons will not. By using curved mirrors to focus the light onto photomultiplier tubes (PMT’s) and observing the readout, pions and lighter particles can be identified. This information, along with data from other detectors is used in our experiment data analysis.
PHOTON
PHOTO-CATHODE
Photomultipliers are extremely sensitive detectors of light. In the figure, an incoming photon (light) is absorbed by an atom in the photo-cathode causing the emission of an electron via the photo-electric effect. This electron is attracted by the field of the first dynode. The increasingly positive voltage of each successive dynode releases an avalanche of freed electrons. The result is an amplified electrical signal proportional to the intensity of light incident on the photocathode.
Positively charged pion (left) consisting of an up and anti-down quark. Positive kaon (right) consisting of an up and anti-strange quark.