TARGETing the Highest Energy Physics

May 5, 2015

Some of the highest energy physical processes in the Universe emit powerful gamma-rays that can be detected when they hit our atmosphere with a flash. A group of KIPAC scientists have developed a new electronics chip that can sample 16 different signals at a billion or more times per second, in order to follow the extreme show.

Prototype camera module for the Cherenkov Telescope Array, including a multi-anode photomuliplier tube and TARGET digitzer chips

Gamma rays are blocked by the Earth’s atmosphere, which is why the Fermi Gamma-ray Space Telescope operates in orbit above the Earth. At very high energies, however, scientists can use the interaction of the gamma-rays in the atmosphere to their benefit: when the particles hit the atmosphere they generate a shower of subatomic particles which in turn emit a flash of Cherenkov light - a luminous analog of a sonic boom. By detecting the Cherenkov light with large reflecting telescopes, scientists can determine the energy and direction of the original gamma ray.
These Cherenkov telescopes have "cameras" - although one might not recognize them as such - they have more in common with the sophisticated instruments of particle physics experiments than with your handheld camera. Each pixel must be capable of detecting as little as a few photons of light and to do so very fast, in order to identify the faint flash of Cherenkov light, which lasts only a few billionths of a second, and discriminate it from the steady background light present in even the darkest night skies. A Cherenkov telescope camera is several feet across and collects light focused onto it from a telescope mirror that can be dozens of feet in diameter. In order to detect individual photons and resolve the few-nanosecond flashes, the camera pixels use photo-multiplier technology rather than CCDs as in more standard telescope cameras.
In order to follow the fleeting atmospheric light show with better precision, KIPAC researchers Stefan Funk, Hiro Tajima, Justin Vandenbroucke, and Akira Okumura, KIPAC graduate student Keith Bechtol, and colleague Gary Varner of the University of Hawaii have developed a new electronics chip that can sample 16 different signals at once, each at a billion or more times per second. The chip, named "TARGET," was designed in particular to capture the tenuous Cherenkov light signals for the under development Cherenkov Telescope Array (CTA).
The TARGET camera electronics system is designed to provide a high channel density in a modular system, in order to enable a large number of fine pixels at low cost, weight, and power consumption per pixel. Weight and power consumption are important considerations in the design of the Cherenkov telescopes because the camera must be supported on a very long boom in a stable position with respect to the telescope position, and if the power consumption is too large then the camera electronics generate more heat than can be easily dissipated. The modular design of the camera system makes it possible to construct, install, and maintain a large number of cameras reliably, which is important because CTA will feature dozens of individual telescopes.
CTA will detect very-high energy (> ~50 GeV) gamma-rays from the highest energy processes in the Universe where extreme physics beyond that accessible in terrestrial laboratories reigns - active galactic nuclei, supernova remnants, X-ray binaries, starburst galaxies, and possibly gamma-ray busts. A particularly exciting science goal for CTA is to detect and image regions in the cosmos where very-high-energy gamma rays are produced by dark matter particles colliding with one another and annihilating.
This work is described in a paper available from astro-ph at arXiv:1105.1832.
Science Contact:
Justin Vandenbrouke
Tidbit author: Justin Vandenbrouke and Jack Singal