1.3 GeV Booster Strage ring, (Experimental area #2)
The ST Ring is a type of electron circular accelerator known as a synchrotron. It accelerates electron beams, which are injected from the linear accelerator at around 90 MeV, up to a maximum of 1.3 GeV, storing them within the ring. The energy required for beam acceleration and storage is provided by high-frequency accelerating cavities with a resonance frequency of 500 MHz. By inserting very thin carbon fibers along the orbit of the circulating electron beam, it is possible to generate high-energy gamma rays through synchrotron radiation. Two such beamlines are installed for this purpose. In a typical operating pattern, the injected beam is accelerated in about 2 seconds, followed by a gradual generation of gamma rays over a period of about 10 seconds from 10 to 30 mA electrons. Then, in around 5 seconds, the next beam is injected.
The generation of neutral K mesons and Λ particles through reactions involving γ-rays (photons) and neutrons is characterized by the fact that these particles do not carry electric charge before and after the reaction. These particles, once generated, decay into other particles within a few tens to a few hundred picoseconds. This characteristic makes measurements challenging, and our Tohoku University group was the first in the world to measure them. Building on this success, we have designed and conducted experiments with the NKS2 measurement apparatus, which offers a wider and more precise measurement range.
NKS2 consists of detectors for measuring the momentum of charged particles, known as electromagnetic calorimeters and drift chambers, as well as a time-of-flight detector for particle identification. It also includes a system with liquid hydrogen and heavy water targets. Inside the BST Ring, it generates real photon beams through synchrotron radiation from circulating electrons and is equipped with a device for tagging real photons, determining their energy and time of generation.