Atomic Collision Experiments with Ultra-Low-Energy Antiprotons
Hiroyuki A. Torii, Y. Nagata, H. Imao, Victor L. Varentsov, N. Kuroda, Masahiro Shibata, Koremitsu Ogata, Hiroshi Toyoda, T. Shimoyama, Y. Enomoto, H. Higaki, Yasuyuki Kanai, Akihiro Mohri, Y. Yamazaki, Yasuyuki Kanai, Y. Yamazaki
We have developed techniques to decelerate, cool and confine antiprotons in vacuo with an electromagnetic trap, for production of a Monoenergetic Ultra‐Slow Antiproton Source for High‐precision Investigation (MUSASHI) over the last several years. The ultra‐slow antiproton beam which can now be extracted stably has opened up the possibility to study ionization and atomic capture processes between an antiproton and an atom at an unprecedented low energy under the single‐collision condition for the first time. The collision energy can be tuned from 10 eV to 1 keV either by varying the beam transport energy or by biasing the voltages at the collision region.Since the number of available antiprotons is very much limited, the reaction probability must be maximized in order to make best use of them. We have prepared a powerful supersonic helium gas jet with a density of 3×1012 atoms/cm3 to be crossed with the antiproton beam. For rigorous identification of particles (e−, p̄ and (p̄A+)0) needed for reduction of huge background signals, we developed a detection system with two microchannel plates each with a delay‐line two‐dimensional position sensitive detector, and a box of scintillator plates. A set of electrodes and coils were placed near the collision point to guide electromagnetically the electrons perpendicular to the antiproton beam. The reaction events will be recognized by an electron signal followed by an antiproton annihilation with an appropriate time of flight. Our design and strategy of the experiment are discussed.
We have developed techniques to decelerate, cool and confine antiprotons in vacuo with an electromagnetic trap, for production of a Monoenergetic Ultra‐Slow Antiproton Source for High‐precision Investigation (MUSASHI) over the last several years. The ultra‐slow antiproton beam which can now be extracted stably has opened up the possibility to study ionization and atomic capture processes between an antiproton and an atom at an unprecedented low energy under the single‐collision condition for the first time. The collision energy can be tuned from 10 eV to 1 keV either by varying the beam transport energy or by biasing the voltages at the collision region.Since the number of available antiprotons is very much limited, the reaction probability must be maximized in order to make best use of them. We have prepared a powerful supersonic helium gas jet with a density of 3×1012 atoms/cm3 to be crossed with the antiproton beam. For rigorous identification of particles (e−, p̄ and (p̄A+)0) needed for reduction of huge background signals, we developed a detection system with two microchannel plates each with a delay‐line two‐dimensional position sensitive detector, and a box of scintillator plates. A set of electrodes and coils were placed near the collision point to guide electromagnetically the electrons perpendicular to the antiproton beam. The reaction events will be recognized by an electron signal followed by an antiproton annihilation with an appropriate time of flight. Our design and strategy of the experiment are discussed.
Antiproton, the antiparticle of proton, is a unique projectile in the study of atomic collision physics, which can be treated theoretically either as a `negative proton' or a `heavy electron'. Atomic capture of an antiproton will result in formation of a highly excited exotic atom. Antiprotonic helium atom has been studied intensively by means of precision laser spectroscopy, which has led to a stringent determination of antiproton mass and charge to a level of ppb. Comparison of these values with those of proton gives one of the best tests of CPT invariance, the most fundamental symmetry in physics. However, the dynamic processes of antiproton capture remain unclarified, except for some indirect information given by those optical and X-ray observations. With an aim to produce an antiproton beam at atomic-physics energies for `pure' collision experiments, we have so far developed techniques to decelerate, cool and confine antiprotons in vacuo, using a sequential combination of the Antiproton Decelerator (AD) at CERN, a Radio-Frequency Quadrupole Decelerator (RFQD), and an electromagnetic trap. Our recent success in stable extraction of monoenergetic ultra-slow antiprotons, about 3×105 in number available every 5 minutes, has opened up the possibility to study ionization and atomic capture processes between an antiproton thus provided as a beam and an atom prepared in the form of a supersonic gas-jet target, at an unprecedented low energy from 10 eV to 1 keV under the single-collision condition. Our design and strategy of the cross-beam experiments are discussed.
We have recently succeeded in decelerating and confining millions of antiprotons, 50 times more efficiently than conventional methods, in an electromagnetic trap. These antiprotons were cooled by preloaded electron plasma to an energy below an electronvolt. They were then extracted out of the magnetic field of 2.5 T and transported typically at 250 eV along a beamline, designed for efficient transport at 10–1000 eV. This unique beam from our apparatus named MUSASHI opens up a new field of atomic and nuclear physics probed by ultra‐slow antiprotons.In this paper, the whole experimental setup and procedure will be overviewed: deceleration, capture, cooling and extraction of antiprotons will be discussed in detail, including technical description of diagnostic devices.
Recently, slow and ultra‐slow antiprotons are now available, which will open a new field of research in atomic physics as well as in related fields. In realizing this, the combination of an RFQD (Radio Frequency Quadrupole Decelerator) and a large multi‐ring trap (MRT) installed in a super‐conducting solenoid has been employed. Several million antiprotons have already been accumulated, and a mono‐energetic antiproton beam of 10 eV has been extracted and transported through a specially designed beam line. A couple of basic experiments which get feasible by the developments of ultra slow antiproton beam are discussed, which include ionization and antiprotonic atom formation processes and also to study spectroscopic nature of various meta‐stable antiprotonic atoms under single collision conditions. A so‐called cusp trap configuration is also discussed, which could for the first time synthesize intense spin‐polarized antihydrogen beams. At the same time, it could trap antihydrogen atoms for a macroscopic time.
Ultra slow mono-energetic antiproton beams are under preparation combining novel techniques to decelerate antiprotons with an RFQD (Radio Frequency Quadrupole Decelerator), to trap, cool, and compress in an electromagnetic trap, and finally to extract as an ultra slow mono-energetic beam. This unique beam will make it possible to study the ionization processes around adiabatic energy regions, channeling, stopping power, antiprotonic atom formation processes under single collision conditions, to prepare metastable antiprotonic atoms such as p̄p, p̄He+, p̄He++, p̄Li+ in vacuum, and to make high precision laser spectroscopy of them. This project named ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) is expected to open a new physics regime in antimatter science under well controlled conditions.
Recently, a new project, ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons), has started, which aims at studying collision dynamics with slow antiprotons and high precision spectroscopy of antiprotonic atoms. It is expected that the ultra slow monoenergetic antiproton beams will open a new physics regime relating to antimatter science under very well controlled conditions. In the present report, several research plans, which includes ionization, excitation, stopping power, channeling, antiprotonic atom formation, high resolution antiprotonic atom spectroscopy, etc. are discussed. The way to prepare ultra slow antiprotons is also presented.