V. N. Pavlov

A cryogenic source of slow monochromatic positrons

A cryogenic source of slow monochromatic positrons based on 22Na radioactive isotope has been developed and produced at the Joint Institute for Nuclear Research. A monochromatic beam is formed from a continuous β+ spectrum with energies of 0–0.5 MeV using a solid neon moderator frozen onto a copper substrate that is cooled to temperatures of 5–7 K. The efficiency of condensation of neon onto the substrate is >99.8%. A slow positron beam with an intensity of 5.8 × 103 particles/s and a mean energy of 1.2 eV at a spectrum width of 1 eV has been extracted from a 22Na-based test source. The fraction of decelerated positrons is 1% of the total flux.

Development of the Low Energy Particle Toroidal Accumulator project

The Low Energy Particle Toroidal Accumulator (LEPTA), a positron storage ring with electron cooling, was constructed and put in operation at the Joint Institute for Nuclear Research (Dubna). The storage ring is a generator of directed beams of ortho-positronium (o-Ps) produced upon the recombination of the beam of positrons circulating in the storage ring with a single-pass electron beam. In 2004 the storage ring was put in operation with the circulating electron beam. The source of positrons of the positron injector was tested with a new radioactive source delivered from South Africa. The positron trap was put in operation for electrons. The electron cooling system was tested with a pulsed electron beam. The progress in commissioning LEPTA is described in this paper.

Testing of a cryogenic source of slow monochromatic positrons

The results of testing a cryogenic source of slow monochromatic positrons based on the 22Na isotope, designed and constructed at JINR, are presented. A solid neon moderator was used to generate a monochromatic beam from the continuous β+ spectrum of 0–0.5 MeV, with solid neon being frozen to the base layer, which is cooled to the temperature of 5–7 K. Test source of the 22Na isotope allows the forming of a slow positron beam of intensity 5.8 × 103 part./s and the average energy 1.2 eV at a spectrum width of 1 eV. The efficiency of moderation is 1% of the total positron flux.

Storage ring with longitudinal magnetic field LEPTA (Stellatron)

The project of the Low Energy Particle Toroidal Accumulator (LEPTA) is dedicated to the construction of a positron storage ring with electron cooling of positrons circulating in the ring. Such a specific feature of LEPTA automatically enables it to be a generator of positronium (Ps) atoms, which appear in the recombination of positrons with cooling electrons inside the cooling section of the ring. The project has the following goals: particle dynamics in the modified betatron, electron cooling with a circulating beam, positronium generation in flight, positronium physics, and feasibility of antihydrogen generation in flight. All key elements of the ring—the kicker, electron beam injection system, helical quadrupole, septum magnet—have been tested, and the expected design parameters have been achieved for these elements. The construction of LEPTA has been completed, and the circulating electron beam has been achieved. This paper discusses the issues of particle dynamics in such an accelerator, the results of numerical simulation and experimental findings of the research into beam dynamics, measurement of betatron number and beam lifetime.

Positron injector for LEPTA

The low energy positron injector for the Low Energy Particle Toroidal Accumulator (LEPTA) accumulator was assembled at the Joint Institute for Nuclear Research (JINR). Key elements of the injector have been tested. The cryogenic source of slow positrons was tested with a test isotope 22Na of the initial activity of 0.8 MBk. A continuous slow positron beam intensity of 5.8 × 103 particle per second with an average energy of 1.2 eV and a spectrum width of 1 eV has been obtained. The achieved moderator efficiency is about 1%. The accumulation process in the positron trap was investigated with electron flux. The lifetime of the electrons in the trap, τlife ≥ 80 s and capture efficiency ɛ ∼ 0.4, were obtained. The maximum number of accumulated particles was N exper = 2 × 108 at the initial flux of 5 × 106 electrons s−1.

The pulse injector of the low energy positrons

The pulsed injector of the low energy positrons for positron accumulator LEPTA has been constructed and tested at JINR. The injector is based on 22Na radioactive source. Positrons from the source are moderated in the solid neon and injected into positron trap, where they are accumulated during about 80 seconds. For injection the positrons are extracted by the pulsed electric field and accelerated up to the required energy. The injector will generate positrons of the energy of up to 10 keV at relative energy spread of 2•10-3, intensity of 108 - 109 particles per pulse and at injection pulse duration of 300 nsec. The cryogenic source of slow positrons has been tested with a test isotope 22Na of the initial activity of 80 MBk. The continuous slow positron beam with average energy spread of 1.2 eV, width of a spectrum 1 eV has been obtained. The achieved moderator efficiency is about 1%. The accumulation process in the positron trap was investigated with electron flux. The life time of the electrons in the trap, τlife ≥ 80 s and capture efficiency ε ~ 0.4 have been obtained. The maximum number of the accumulated particle was Nexper = 2*108 at the initial flux of 5•106 electrons per sec. The dynamics of slow positrons (electrons) in the injector and physics of the particle storage process are discussed in the report. The work is supported by RFBR grant No. 05-02-16320, the president of Russia Federation grant (MK-3948.2007.2) for supporting of young scientists and leading sceintific schools.

A positron injector for the LEPTA accumulator

An injector of monochromatic positrons for the low-energy positron accumulator (LEPTA) is being tested at the Joint Institute for Nuclear Research. The source of positrons is the radioactive source 22Na. At the output of the source, positrons are slowed down in a solid target. Frozen neon is used as a moderator. For this purpose, a system of cryocooling of the source and the neon supply line have been assembled. A method of detection of slow positrons has been developed and tuned. The first experiments with the frozen moderator have been performed. A continuous beam of slow positrons with an average energy of 1.2 eV and spectrum width of 1 eV has been obtained.

Low-energy positron injector

The LEPTA low-energy positron accumulator, which is to be used for producing directed fluxes of positronium and antihydrogen atoms, is under development at the Joint Institute of Nuclear Research. The monochromatic positron injector, operating in the pulsed mode, in the accumulator must generate a positron beam with intensity 108–109 particles in a pulse with duration less than 300 nsec, the positron energy is 10 keV, the relative energy spread in the beam is less than 2·10–3, and the beam radius is 0.5 cm.Radioactive 22Na serves as a positron source. The positrons at the exit from the source are decelerated in a solid target and enter the magnetic trap. There they are once again decelerated in a gas to thermal velocity and accumulate in ~100 sec. For injection into the accumulator, the positrons are pulled out of the trap by a pulsed electric field and acclerated up to the required energy.