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last update: 10/03/23
|New electronics tested for
Belle II central drift chamber
The final technical design for KEK’s Belle detector upgrade, called Belle II,
is now being finalized. This week features one of the central components
of the Belle II, the central drift chamber (CDC), which tracks charged particles resulting from particle interactions.
Dr. Shoji Uno of KEK has been working on drift chambers ever since he joined the Belle collaboration in 1990. Now joined by a postdoc, Dr. Nanae Taniguchi, the team of two is finalizing the design for the new central drift chamber (CDC) of the B Factory's upgrade, the Belle II.
The current detector, Belle, is located in the KEKB accelerator ring, at the point where the electron and positron beams intersect. The new Belle II detector will sit at the exact same position on the upgraded version of KEKB, the SuperKEKB. The CDC tracks the pathways of the charged particles that result from the interaction of the two high energy beams.
The CDC detects charged particles using wires. When particles pass through a helium based gas mixture inside the chamber, they ionize the gas molecules, knocking out electrons. An electric field is created by applying high voltage on ‘sense wires’ strung across the chamber. The field causes electrons to accelerate towards the wires. These electrons knock out additional electrons from the gas as they travel. A number of electrons and ions produced move speedily near the sense wire. The sense wire senses the motion and creates a pulse signal. The signal is then sent to the data acquisition (DAQ) system.
Same concept, robust structure
“The design of the new CDC does not change much from the original, because Belle’s CDC has worked very well throughout the past decade,” says Uno. “However, SuperKEKB will produce collisions at 40 times higher luminosity, so the new CDC must be able to work in conditions with 20 times higher background noise.” By adding more sense wires, Uno and Taniguchi can improve the signal-to-noise ratio.
“The key is to balance weight of the wires, the wire tension, and the performance deterioration due to gravitational sag of the wires,” explains Taniguchi. Since wires are strung horizontally, gravity makes the sense wires sag at the middle. The sagging can affect measurements. Taniguchi calculated the sag due to gravitation for a range of tensions, and simulated the effect on the performance. “If the influence of sagging on the performance of the detector is smaller than the influence of the detector’s inherent mechanical resolution, then we are good.”
The endplates for the new CDC will have to endure 4 tons of force. “The cross section of the new endplates will be a straight line rather than round.” The team calculated that this would be a balanced shape, providing a physically robust framework without having undue influence on the performance.
The super-layer scheme and small cells
Drift chambers are extremely complex systems, with many parameters to consider when designing. Using the accumulated expertise of chamber scientists, as well as past experimental data, the KEK team developed an optimal design. One widely accepted design, and the one used for the current Belle CDC, is called the super-layer structure. It consists of a combination of wires arranged in axial direction parallel to one another, and of wires arranged in varying angles—called stereo wires. Six layers of axial wires make one group and six layers of stereo wires make another. The Belle II’s CDC has nine groups of these.
New compact readout electronics
The CDC readout electronics will be completely replaced, owing to advancements in technology over the last two decades. The new CDC will employ an all-in-one technology that amplifies the signal, applies a pulse shaping, and then converts the analog signal into a digital signal.
The current Belle CDC sends signals all the way from the detector to an electronics hut 30 meters away via 30-meter cables to interpret the signals for data analysis. In the new CDC, all the electronics will be inside the Belle II detector, right beside the chamber itself. Digitalized data will be readily available through fiber optic connections, to be collected in the DAQ system.
Signals from wire chamber is very small, so they are amplified by preamplifier component of the electronics. Amplified signals are shortened to make sure sensitivity to higher frequency signals in Belle II. The electronics also need the ability to separate noise from signal. These functions, called shaping and discrimination, will be built into the new electronics. The electronics will use an application-specific integrated circuit (ASIC). An ASIC is a chip specially designed for a particular use. The new ASIC will process eight channels on a 4-millimeter by 4-millimeter chip for Belle II, whereas the previous chip processed one channel on a 1-centimeter by 3-centimeter chip for Belle.
In November of last year, Taniguchi successfully demonstrated the newly designed electronic device. She investigated optimal values for such parameters as gain, noise cancellation, and time resolution, using a hybrid board that contained all necessary parts.
“The new readout electronics with AISC are the result of many people’s efforts and many advanced technologies developed at KEK,” says Taniguchi. Their readout electronics with ASIC will be ready by this summer.
Thinking outside the box
Belle’s CDC is already an elaborate system that demonstrates superb craftsmanship. While the development of silicon detectors has become common and straightforward, drift chambers remain notorious for the incredibly large number of variables to be considered during design. “Because the system is too complex to be amenable to any meaningful computer optimization studies, a lot of the design choices are made based on our past experiences,” says Uno. “However, sometimes it’s necessary to go beyond experience and make educated guesses in chamber science.”
Drift chambers are packed with the very basics of particle physics experiment, and cultivate creativity and good intuitions. This is why many particle physicists think it profiting to work on them once in the course of their career. “CDC is such an important and profoundly elaborate detector component, which excites me greatly,” says Taniguchi. “There also is much work to be done in the future.” The team welcomes new collaborators to work with them towards the goal of the next generation B Factory detector.
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