Lens Array Increases Photodetection Efficiency

Daniel C. McCarthy

A two-lens array developed by physicists contributing to the HERA-B detector facility at the Deutsches Elektronen-Synchrotron facility shows that some of the most advanced experimentation in pure physics can produce very simple and practical solutions. Researchers at HERA-B study differences in the behavior of matter and antimatter by directing a beam of protons to collide with a wire target, thus producing a stream of quantum particles in the detector. A 3-m-long part of the detector is filled with C₄F₁₀ gas, where charged particles with velocity greater than the speed of light radiate Cerenkov light, which is reflected off a spherical mirror to form a ring pattern on the focal surface. This pattern is detected by a photomultiplier and digitally recorded for further analysis, once every 96 ns.

The lens array developed at the HERA-B proton-electron collider boosted detection efficiency of the photomultiplier from 25 to 65 percent. Courtesy of the Deutsches Elektronen-Synchrotron facility.

"The radius of the ring tells us the velocity of the particles," explained Reinhard Eckmann, a coordinator at HERA-B's ring-imaging Cerenkov facility. "Together with a momentum measurement using the bend of the particle in our magnetic field, we can then calculate the mass of the particles."

The team found, however, that even with the tightest possible arrangement of the 16-channel photomultiplier, only 25 percent of the functional detection surface was being used effectively. To boost the photon-detection efficiency, the group designed a relatively simple lens array that redirects the light onto the photomultipliers.

Each element of the array is a telescope made out of two aspherical lenses molded using UV transparent acrylic. The transmission of the acrylic starts at 300 nm, fitting perfectly to the spectral range of the photomultipliers.

The telescope can collect light with incident angles smaller than 140 mrad, introducing only small distortions of the image. "The array not only increases the efficiency of detection to 65 percent, but it also doesn't disturb our ability to determine the position of incident photons," said Eckmann.