The antiparticles of electrons are called positrons. They can be made by hitting a heavy metal target such as tungsten with high current, high energy electrons. However, in addition to positrons, the target generates nearly equal amounts of electrons, which are captured simultaneously by electrical and magnetic forces in the positron capture portion that follows the target.

Electrons and photons are separated by magnetic force just after the capture phase. It is a challenge to simultaneously detect positrons and electrons in the capture part. Three factors make it difficult to see them clearly:

  • A radiation-hard environment.
  • Lack of space to place beam monitors.
  • The need to distinguish between positrons and electrons in a short time.

They are generated in large quantities at the “SuperKEKB B-Factory” (SuperKEKB), where they are crushed into electrons with a brightness that sets world records. Physicists investigate the mysteries of matter, antimatter imbalances and traces of other exotic particles beyond the Standard Model by examining the hundreds of decay patterns of B mesons and anti-B mesons in these encounters. One of the important components of this experiment is increasing the positron intensity to increase the collision velocity.

A team led by Prof. Tsuyoshi Suwada of KEK has successfully installed a new type of beam monitor in the SuperKEKB positron source.

Suwada said, “The idea is to use a wideband beam monitor with a simple rod antenna. This idea is well known in radio frequency wave sensing techniques. For the first time at KEK, successful experiments were carried out with the use of charged particle beams in high-energy accelerators, such as electron and positron beams. It turns out that an electron beam (or positron beam) precedes a positron beam (or electron beam) by a certain time interval in the time domain in the capturing section.”

“Interestingly, in experiments we found that the time interval between electrons and positrons varies intricately in the range from 20 to 280 ps on average, and their motion sequence is interchanged depending on the operating condition of the recording section. At the 0 degree capture phase, the electrons with negative signal polarity to the positrons with positive signal polarity, and the time interval is 137 ps.”

“At the 180 degree capture phase, the positrons with plus signal polarity precede the electrons with minus signal polarity, and the time interval is 140 ps. It turns out that the time interval between the electrons and positrons varies intricately in the time domain, and the sequence of motion is swapped at the trapping phases of 50 and 230 degrees.”

“Applied in the SuperKEKB, the improved capture efficiency of positrons helped the SuperKEKB improve its world-record brightness.”

“Useful information is about the radiation damage of the beam monitor system obtained at the injector linac during its long-term operation. This new beam monitor could be applied in next generation B plants and future e+ e linear colliders.”

Magazine reference:

  1. Suwada, T. Direct observation of the positron capture process at the positron source of the superKEKB B plant. Sci Rep 12, 18554 (2022). DOI: 10.1038/s41598-022-22030-5