Tracking Extraordinary Particles in the ILC
With the recent release of the Reference Design Report, the International Linear Collider (ILC) is beginning to take shape. As the design progresses, researchers around the world are concocting new ways to detect exotic particles. Here at SLAC, a group of physicists are leading the design effort for the innovative Silicon Detector (SiD), one of four proposed particle hunters for the ILC.
"The ILC requires detectors unlike those of any previous experiment," said Physicist Tim Nelson, a collaborator on the SiD project. "To make a detector with extraordinary capabilities, we're trying really unconventional designs for the tracking system."
Tracking systems help researchers reconstruct exotic particles based on the showers of ordinary particles into which they decay. The SiD tracker must precisely measure the momenta of charged particles from the curvatures of their paths through a uniform magnetic field, which in turn can reveal the natures of their extraordinary parents.
All SiDs use nested layers of thin silicon sensors to precisely measure these particle trajectories. The precision, speed and durability of these devices make them ideal for tracking but this technology has previously required large amounts of material and therefore large detectors. Since the ILC must make very delicate and precise measurements, its SiD design will need to minimize building materials to avoid instrumental interference.
To streamline the SiD tracker, Nelson and his colleagues at SLAC are developing a compact readout chip called KPiX. The chip has two components: an amplifier to magnify the tiny charge produced when a particle crosses the silicon sensor, and circuitry to digitize that signal and send it out for storage and later analysis.
Instead of having this system run continuously, the KPiX design allows the power-guzzling amplifier to remain on only during the 1 millisecond period when collisions occur—without compromising data collection. This reduces power consumption by about a factor of 100 and eliminates the need for the liquid cooling standard in most large silicon trackers.
In the design, noisy digital circuitry turns off while the sensor is in use, reducing the background noise that would otherwise obscure data collection. Additionally, Nelson believes that a delicate process called "bump bonding" can be used to connect the KPiX chip and the silicon sensor, minimizing bulky components traditionally required to isolate the two. The benefit of all this material trimming? The tracker should be both lighter and simpler to build.
"One of the biggest challenges here is making sure it is feasible to build more than 10,000 of these things," Nelson said. "To do this, we're taking existing technologies and putting them together in a way that is unique."
—Alison Drain, February 22, 2007
Above image: In this SiD tracking chip, two KPiX chips (red) are bump-bonded to a sensor.