NA62 Gigatracker sets new standards for silicon detectors

The NA62 experiment should start collecting its first data (technical run) in a little over one year. At the heart of the experiment is the Gigatracker, a newly conceived silicon pixel detector, whose job is to measure the arrival time and the position of the incoming beam particles. The demonstration detector has recently shown a time resolution of 175 picoseconds, an unprecedented record in the field of silicon pixel detectors.


The Gigatracker prototype.

A 115 metre long vacuum tank, a brand new set of detectors surrounding it and an extremely rare decay to study: this is the new NA62 detector, foreseen to be installed in the SPS North Area in 2012. “We will study a very rare decay of the K+. Such a decay is sensitive to contributions coming from new particles and therefore represents a powerful way of searching for new physics, complementary to the direct approach of the LHC detectors,” explains Augusto Ceccucci, NA62 spokesperson.

The particles from the SPS accelerator are sent to the experiment downstream at a very high rate and are not precisely pulsed; their arrival time in the experiment is therefore unknown. The scope of the Gigatracker is to provide a precise measurement of the arrival time of the particles before they enter the vacuum tank in which they fly and decay. “This information is used to associate the correct incoming kaon to the event observed downstream and to reconstruct its kinematics. We require a time resolution in the Gigatracker of less than 200 picoseconds, about 100 times more precise than the precision of the silicon trackers used by the LHC detectors. The recent tests carried out with the demonstration detector showed that we can reach a time resolution of 175 picoseconds,” says Alexander Kluge, coordinator of the CERN activities of the Gigatracker project.

After having hosted NA48, the cavern in the north area of the SPS prepares to accommodate NA62.

Three of these ultra precise devices will be installed upstream and, in its final configuration, they will be composed of a matrix of 200 columns by 90 rows of pixels. In order to affect the particles’ trajectory as little as possible, the material used to build the detector must be very thin. “The sensor will be 200 µm thick and the pixel chip will be 100 μm thick, about 50 μm thinner than the thinnest chips used in the LHC experiments. The Gigatracker will be placed in a vacuum, in order to minimize the interaction of beam particles with air. It will also be operated at a temperature of -20 degree Celsius to reduce radiation-induced performance degradation,” explains Alexander Kluge. The collaboration is developing new ways to ensure effective cooling, using light materials to minimize their effect on particle trajectory.

Besides being heavily used in high-energy physics experiments, silicon sensors are also exploited in imaging applications. In particular, the development of the Gigatracker silicon pixel detector stimulated interest in other fields of science where images with sub-nanosecond time measurement precision are used. Examples include medical imaging for positron emission tomography (PET); biomedical imaging based on the florescence lifetime imaging microscopy (FLIM), where fluorescent molecules are excited to produce light and the duration of the excitation – the life time – is measured; 3D time-of-flight imaging techniques used in high-tech cameras, where a light pulse is sent to the object and the reflected light is measured together with the time at which it arrives from the object. The NA62 Gigatracker is yet another example illustrating just how short the distance is between the conceptual developments done for particle physics and their application in fields that influence our everyday life.

The brief history of kaons

Kaons are particles made of quarks, of which one is the strange quark. There are charged (K+ and K-) and neutral kaons. The neutral kaons come in two types: a short-lived one (K0S) and a long-lived one (K0L). The CP (a product of Charge and Parity transformations) symmetry violation was discovered by James Christenson, James Cronin, Val Fitch and René Turlay in 1964 in the decay of neutral kaons. The discovery was awarded the 1980 Nobel Prize in physics.

The study of kaons at CERN is an established tradition: NA62’s predecessor is NA48 which is best known for establishing direct CP-violation in the two pion decays of the neutral kaons about a decade ago. A first extension (NA48/1) studied K0S rare decays while a second extension (NA48/2) focused on the search for CP-violation and the study of ππ scattering in charged kaon decays.

The new NA62 experiment should start operation in 2012 with a first technical run to measure the performance of the detectors. At full speed, the collaboration plans to study about 1013 kaon decays within 100 days of data taking.


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by CERN Bulletin