The High Threshold Cherenkov Counter (HTCC) detects fast-moving charged particles in the CLAS12 detector using the Cherenkov effect. Charged
particles moving faster than the local speed of light c/n in a medium with index of refraction n (where c is the speed of light in vacuum, the
ultimate speed limit of the universe) emit light in a cone with an opening angle determined by the particle's velocity and the index of refraction
of the medium:
This process is similar to the "sonic boom" created by a jet moving faster than the speed of sound in air. In the case of the HTCC, the medium is CO2 gas at room temperature and atmospheric pressure, which has an index of refraction of 1.00045. In order to emit Cherenkov light in this gas, the minimum velocity is 99.96% of the speed of light! The energy of a particle of mass m and velocity v moving relativistically is given by:
Scattered electrons in the CLAS12 detector have energies in the range of 1-10.5 GeV (1 GeV = 109 eV, eV = electron-volt). Since the electron is very light, with a mass of only 0.511 MeV = 0.000511 GeV, the minimum speed of electrons in CLAS12 is about 99.999987% of the speed of light in vacuum, and therefore all electrons in CLAS12 emit Cherenkov light in HTCC at an opening angle of about 1.7 degrees. The next lightest type of particle commonly produced in CLAS12 experiments is a pion, with a mass of 139 MeV (=0.139 GeV) or nearly 300 times the mass of the electron. In order for a pion to move fast enough to emit Cherenkov light, it must have a minimum energy of about 4.6 GeV, which means that for all particles with energies below this threshold, the detection of light in the HTCC indicates that the particle passing through CLAS12 was in fact an electron and not a pion. At higher energies, the CLAS12 calorimeter system can differentiate between pions and electrons due to the different signal characteristics of electrons and pions at high energies.
The Cherenkov light emitted by fast-moving charged particles is collected by a series of 48 ellipsoidal mirror sections, each designed with a curved surface obtained by revolving an ellipse about its major axis, with one focus of the ellipse located at the interaction point where collisions occur, and the second focus located away from the beamline, out of the path of scattered particles of interest. Photomultiplier tubes (PMTs) are then placed at the location of the second focus of each ellipsoidal mirror section to detect the Cherenkov light. The mirror segments are designed so that they completely surround the beamline for particles scattered through angles from 5-35 degrees, with no gaps or shadowing between adjacent mirrors. There is a one-to-one correspondence between mirrors and PMTs in this design, providing for a coarse scattering angle determination based on the location of PMTs with signals. This novel optical design results in highly efficient light collection with a modest number (48) of detector channels.
Given the available space in CLAS12 and the other components of the spectrometer, the HTCC has to be located between the target and the forward drift chambers that measure the trajectories and momentum of charged particles. Because scattered particles pass through the HTCC before they pass through the tracking detectors, the materials of the HTCC in the path of scattered particles must be as lightweight as possible to avoid significant disruption of the particle trajectories before they are measured. For this reason, the mirrors are assembled from extremely low-density materials--precisely machined substrates of 100% closed-cell foam with a density of 31 mg/cc are laminated from both sides with acrylic to form a rigid substrate with a smooth optical-quality surface. These substrates are then sent to an external vendor for vacuum deposition of a reflective coating of Aluminum and a protective overcoat of Magnesium Fluoride or similar material to prevent oxidation of the metallic reflector coating. Because the spectrum of Cherenkov light emission is predominantly UV, high reflectivity of the combined coating deep into the UV (down to 200 nm wavelength) is required in order to meet the performance specifications for CLAS12. After each mirror is coated, its reflectance is measured at JLab before acceptance for use in CLAS12.
The PMTs used to detect the Cherenkov light have 5" diameter quartz windows for high quantum efficiency in the UV. The size of the PMTs was chosen based on realistic simulations of the distribution of Cherenkov photons reflected toward the PMTs by the ellipsoidal mirrors for the full range of electron trajectories in CLAS12, including the effect of the CLAS12 magnetic field. Even with a 5"-diameter light collection area, some of the Cherenkov photons fail to strike the active area of the photocathode. To collect these additional photons, parabolic reflectors called Winston Cones are installed in front of the PMTs, increasing the light collection efficiency from about 80% to nearly 100%.
Cherenkov photons that strike the PMT photocathode excite electrons via the photoelectric effect, which are then accelerated by an applied electric field through a series of 14 stages called "dynodes". The applied high voltage is divided approximately evenly between each of the dynodes. The charge of the signal is amplified at each stage by the liberation of additional electrons. The total gain of the PMT, defined as the ratio of the output charge to the number of photoelectrons liberated at the cathode, is 106-107 depending on the applied high voltage. The PMT signals are very fast, with a rise time and full-width at half-maximum of roughly 3 and 6 ns, respectively, and are therefore used as part of the level-1 trigger for CLAS12. The quantum efficiency, defined as the probability that an electron is liberated from the photocathode by a single photon of a given wavelength, typically ranges from 20-30% for wavelengths from 200-600 nm. The signal strength in the HTCC is characterized by the number of photoelectrons detected. Detailed simulations of the HTCC calibrated to the operational experience with the existing CLAS Cherenkov Counter indicate that electrons of interest in CLAS12 will produce an average of 20 photoelectrons, which enables highly efficient electron detection even at a relatively high threshold, allowing for efficient rejection of pions both in the level-1 trigger and in the offline analysis of the data.The image above shows a simulated electron track going through the HTCC, illustrating its principle of operation. The blue wireframe shows the approximate geometry of the CO2 gas volume defined by the HTCC containment vessel. As the electron track (cyan line) traverses the gas volume, Cherenkov photons (white lines) are emitted in a cone at a 1.7 degree opening angle. The photons are then reflected by the ellipsoidal mirror facets toward the PMTs. In this image, each mirror is color-coded to its associated PMT, shown as a circular disc representing the photocathode and a parabolic Winston Cone to increase the light collection efficiency. The green dots represent the position of "hits" in the PMTs--in this example, the Cherenkov light is shared between two adjacent mirrors, causing signals in two adjacent PMTs.
Last modified: May 6, 2013
Daniel S. Carman