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last update: 10/06/23
|The TOP counter: a new method
for particle identification
The newly developed time-of-propagation (TOP) counter
is starting to gather attention from the experimental particle
physics community. The TOP counter is now of the baseline
detector for the barrel particle identification at the next-
generation B-Factory experiment, KEK’s Belle II. Read on to
learn more about the design of this TOP counter and the
international team involved in its development.
Detecting single photons to picosecond (a trillionth second) time resolution is something like hitting a 'hole in one' in golf at a distance greater than the distance from the Moon. Yet that is the type of precision required for the photon detectors to be used in the proposed SuperKEKB-BelleII experiment. The proposed KEKB-Belle upgrade, the SuperKEKB-Belle II experiment, will have 40 times the luminosity of its predecessor. These collisions will produce many particles of unknown quark composition and energy. Many of these particles decay while flying inside the Belle II detector. The job of this mammoth detector will be to measure the properties of these particles and determine their trajectories.
“One of the most critical things that we will be measuring is flavorful final states. That is, we want to know whether or not there are any strange quarks present in an event, in addition to the more common up and down quarks,” explains one of the two co-leaders of the Belle II barrel particle identification detector (PID) team, Prof. Gary Varner of the University of Hawaii. “The easiest way to determine that is by measuring kaons.”
The threshold method
The Belle PID has two components: a barrel and an endcap. In the original Belle detector, high-momentum particle identification in barrel region was based on a simple black-and-white method, the threshold method. When pions and kaons travel through an aerogel radiator, they emitting light, called Cerenkov light, when their velocity exceeds the speed of light in the radiator. The emission angle depends on the index of refraction of the aerogel and on the velocity of the particles. Namely, by choosing the index of refraction of the aerogel one can design a detector in which particles slower than the speed of light in that material index would not emit Cerenkov light at all. In this carefully chosen aerogel PID, pions would emit Cerenkov light, but kaons would not.
However, kaons can interact with other detector components, giving rise to unwanted signals. The most significant of which is the interaction with the face plate of the photo sensor. To improve the detector efficiency, something more than the simple threshold method was needed.
Time-of-propagation for precise identification
In the US, the BaBar detector at SLAC, a friendly rival to KEKB-Belle, had a more ambitious barrel PID design, the detector of internally reflected Cerenkov light (DIRC). The DIRC design uses measurements of the exact emission angle of Cerenkov light via two-dimensional hit positions on a photo sensor array. Since the Cerenkov light propagates at a speed slower than the particle itself in the quartz media, it forms a cone-shaped shock-front, just like a mach cone formed along the leading edges of a plane during supersonic flight. The Cerenkov cone then forms a ring image when projected onto an image plane. The DIRC counter projects the ring image, and by measuring the opening angle of the ring, particle types are identified.
Although the upgraded version of the DIRC, the focusing DIRC, has demonstrated excellent resolution for Belle II’s barrel PID, it is too large, and would not fit within the existing Belle detector. The Belle II barrel PID needs to be compact. The Cerenkov ring developed in the quartz radiator is propagated to the ends of the bar for readout. In the endcap region, the incident particles—such as pions and kaons—usually arrive perpendicular to the detector, and there is enough space to allow the image to expand so that the ring image can be directly recorded. In the barrel region, photons can hit at nearly any angle, from perpendicular to modestly shallow. Covering the large barrel with precision sensors is financially not feasible. In the Belle II barrel PID, radial space is also much more constrained, so that the direct measurement of the Cerenkov ring is not realistic.
Cerenkov photons emitted by particles as they travel through the quartz bar are confined within the bar by total internal reflection. They repeatedly reflect off the walls of the bar, following a zig-zag path, until they reach the end of the bar, where they are read out by photo sensors. The number of times that a Cerenkov photon is reflected before it reaches the detector is related to the angle at which the photon was emitted by the original particle. The number of reflections is, in turn, related to the time it takes for a Cerenkov photon to travel the length of the bar. This time is also known as the time of propagation. Therefore, by measuring the time of propagation, one can infer the angle of emission, and thus the type of particle.
The TOP counter was originally proposed by Nagoya University physicist Prof. Takayoshi Ohshima in 1998. A proof-of-concept TOP counter was built and tested also at Nagoya University in 2000-2001, using existing technologies. With a refined quartz bar and photo sensors, a prototype of the Belle II PID conceptual design has also been built and tested by Nagoya scientists in 2002.
Focusing rays to improve time resolution
In principle, the TOP method can identify particles with much higher precision than the original threshold method. Unfortunately, there was one critical issue with this method. Specifically, the issue is that the speed of propagation of light inside the quartz bar differs slightly depending on the wavelength of the light. As the result, the information physicists can induce about time smears out in space, worsening the time resolution. This effect, called chromatic dispersion, had been pointed out in another Cerenkov-type counter, the correlated Cerenkov timing detector, originally developed by Ohio State University.
“The difference in propagation speeds due to chromatic dispersion becomes more significant as photons propagate farther,” explains Inami. “If an incident particle hits the end of the bar that is farthest away from the photo sensors, the time resolution could be as much as 250 picoseconds. The introduction of the focusing mirror reduced this error by half.”
Expanding rays to get improved spatial information
Another beautiful idea developed by the PID team was to spread out the Cerenkov rays that reached at the end of bar. This improves Belle II's ability to measure the rays’ spatial distribution. A small expansion region, called the wedge, is attached to the bar just in front of the photo sensors. This wedge permits the Cerenkov photons to spread out across the sensors at the end of the bar. In this case two rows of photo sensors, instead of a single row, are attached to the end of each module.
The final TOP counter is a quartz bar, 2.5 meters in length, with a wedge attached to the photo sensor side, and a focusing mirror on the other. Two rows of 16 photo sensors are attached at the end of wedge, covering the full 45-centimeter width. There are 16 TOP modules to cover the barrel PID, which has more than 8,000 readout channels altogether.
Developing a new, high-performance photo detector
The TOP counter requires extremely high quality photo sensors. These sensors must have good timing and position resolution, high quantum efficiency, large effective area, high magnetic field tolerance, and a square shape. All previous high precision timing photo sensors had a circular shape, and none satisfied the stringent performance requirements.
The MCP-PMT uses plates instead of dynodes for the amplification process. Each plate has many circular pores around 10-micrometers in diameter, and 30- to 40-micrometers apart. When a photon arrives, the electrons leaving the photocathode are accelerated towards the plates by an applied voltage. Electrons that enter the pores bounce along the inside of the pores, each time producing more and more electrons.
The amplification is about the same order as that of regular PMTs. The difference is improved timing. “Because the electron amplification occurs in a very spatially confined region, the spread in transit time is very small,” explains Varner. “While the transit time of electrons in a regular photon detector can vary from 0.5 to 3 nanoseconds, the range in an MCP-PMT is just 50 picoseconds or less.”
Inami and his colleagues at Hamamatsu spent three years working out a solution. They changed the internal structure of the device, and developed a cleaning process to reduce the unwanted chemical reactions of photoelectrons. A successful prototype was finally tested and proved suitable in 2008.
A large and active international team
The barrel PID team is a large international collaboration of physicists and engineers, including 10 from Nagoya University, 11 from the University of Hawaii, 7 from the University of Cincinnati, and 12 from the University of Ljubljana. The PID group is the largest international group at Belle II.
“The collaboration has tackled many issues in an analytical and critical manner, and has done so in a way that is possible only in a group with broad photo sensor backgrounds,” says the barrel PID liaison and one of the co-leaders of the endcap PID group, Prof. Ichiro Adachi of KEK. “We are very fortunate to have been able to make our own decisions, without the need for external panels or committees.”
Meanwhile, Nagoya has been working on further methods to compensate for chromatic dispersion. Since the effects of dispersion are more significant at shorter wavelengths, the team plans to implement a filter to remove photons with those shorter, more problematic wavelengths. Optimization studies by simulation, and beam test verification of the final counter design are planned.
“From detailed performance studies to actual construction, there is much work to be done,” says Adachi. “We are expecting many more new researchers in our group.”
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