Stored positrons for antihydrogen production

The production of antihydrogen is examined in the light of recent experimental results on a technique for the efficient accumulation, manipulation, and storage of positrons. From these data, we argue that this high-efficiency positron trapping technique could be adapted for the production of antihydrogen and would offer significant advantages over other positron trapping techniques currently being proposed for this purpose. This revised version was published online in August 2006 with corrections to the Cover Date.     

Extremely cold positrons for antihydrogen production

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10⁻¹⁷ Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi²²Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×10⁶e⁺/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.     

Trapped positrons for antihydrogen

Positrons from a 12 mCi²²Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e⁺/s is observed for 32 h in this initial demonstration. More than 1.6×10⁴ positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 10⁵ antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.     

Accumulation and storage of low energy positrons

We describe an experiment in progress which is designed to efficiently accumulate and store positrons from a radioactive source. The potential uses of such a collection of positrons is briefly discussed, as well as the limits of these accumulation and storage methods.     

The production oflow energy positrons and positronium

A summary is given of the techniques of positron and positronium production which may be of relevance to the production of antihydrogen atoms at low kinetic energies. Topics covered include positron beams from radioactive isotopes and via pair production at electron accelerators, methods for trapping and accumulating positrons and prospects for the use of slow positrons to create antihydrogen at CERN.     

Relativistic antihydrogen production

Antihydrogen has recently been produced in collisions of antiprotons with ions. While passing through the Coulomb field of a nucleus an antiproton will create an electron-positron pair. In rare cases the positron is bound by the antiproton and an antihydrogen atom produced. We calculate the production of relativistic antihydrogen atoms by bound-free pair production. The cross section is calculated in the semiclassical approximation (SCA), or equivalently in the plane wave Born approximation (PWBA) using exact Dirac-Coulomb wave functions. We compare our calculations to the equivalent photon approximation (EPA). Received: 19 December 1997 / Published online: 10 March 1998     

Extremely cold antiprotons for antihydrogen production

The possibility to produce, trap and study antihydrogen atoms rests upon the recent availability of extremely cold antiprotons in a Penning trap. Over the last five years, our TRAP Collaboration has slowed, cooled and stored antiprotons at energies 10¹⁰ lower than was previously possible. The storage time exceeds 3.4 months despite the extremely low energy, which corresponds to 4.2 K in temperature units. The first example of measurements which become possible with extremely cold antiprotons is a comparison of the antiproton inertial masses which shows they are the same to a fractional accuracy of 4×10⁻⁸. (This is 1000 times more accurate than previous comparisons and large additional increases in accuracy are anticipated.) To increase the number of trapped antiprotons available for antihydrogen production, we have demonstrated that we can accumulate or “stack” antiprotons cooled from successive pulsed injections into our trap.     

Trapping of positrons at vacancies in magnesium

Trapping of positrons at vacancy-type defects in magnesium was studied by positron lifetime and Doppler-broadening measurements. Vacancy defects were produced by quenching, electron irradiation and deformation at low temperatures as well as by thermal agitation at elevated temperatures. In the first three cases we observed trapping at multiple vacancies, which anneal out between 77...400 K. Thermal equilibrium measurements show S-shape behaviour originating from positron trapping at magnesium monovacancies. However, changes in the positron parameters were very small, which is due to the weakness of the positron-vacancy interaction. A detrapping analysis yielded a positron-vacancy binding energy of the order of 0.3...0.4 eV.     

Experiments at the Giessen source TEPOS with low energy positrons

The TEPOS facility at the Giessen LINAC delivers intense positron beams in the energy range between some eV and 6 keV; with postacceleration up to 80 keV. Results for remoderation and positron storage will be discussed. Further the energy-loss of positrons in thin aluminium foils at incident energies of 6–20 keV was measured. Cross sections for K- and L-shell ionization of thin silver and gold targets by positron and electron impact were determined at projectile energies of 30–70 keV. The experimental results are presented in detail; they are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and take into account the deceleration or acceleration of the incident projectile in the nuclear field of the target atom.     

Positron accumulation and storage for antihydrogen production

We propose a scheme to stack and accumulate positrons, emitted randomly from a radioactive source. The positrons are moderated and accumulated at low energy.     

High rate production of antihydrogen

We show that antihydrogen production is the dominant process when mixing antiprotons and positrons in the ATHENA apparatus, and that the initial production rate exceeds 300 Hz, decaying to 30 Hz within 10 s. A fraction of 65% of all observed annihilations is due to antihydrogen.     

Possible antihydrogen production using trapped plasmas

Since antiprotons have been captured in an ion trap, we consider the possibility of producing antihydrogen by merging cold trapped plasmas of antiprotons and positrons. The calculated, instantaneous rate for antihydrogen production by the 3-body recombination is much higher than for other proposed techniques, opening up intriguing experimental possibilities.     

Possible formation of antihydrogen atoms from metastable antiprotonic helium atoms and positrons/positroniums

The possible formation of antihydrogen atoms via the collision of metastable antiprotonic helium atoms with positrons and positroniums is discussed based on the known behavior of positrons in helium media.     

A single trapped antiproton and antiprotons for antihydrogen production

During the last several years, our TRAP collaboration has pioneered techniques for slowing, trapping, cooling and indefinitely storing antiprotons to energies more than 10¹⁰ times lower than previously possible. The radio signal from a single trapped antiproton is now being used for precision measurements. Many cold antiprotons are stacked as another important step toward the eventual production of antihydrogen, and positrons have been trapped in vacuum.     

Trapping of antiprotons — A first step on the way to antihydrogen

A first step towards producing and effectively utilizing antihydrogen atoms consists of trapping antiprotons. The immediate next step must then be to control, i.e. trap, the produced antihydrogen. The current state of the art in trapping antiprotons and positrons is reviewed, and the challenges in trapping the resulting neutral particles are discussed.     

Positron plasma diagnostics and temperature control for antihydrogen production

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends critically on parameters such as the plasma density and temperature. We discuss nondestructive measurements, based on a novel, real-time analysis of excited, low-order plasma modes, that provide comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The plasma length, radius, density, and total particle number are obtained. Measurement and control of plasma temperature variations, and the application to antihydrogen production experiments are discussed.     

Near-thermal energy width of moderated high energy positrons

More than 10^?7 of the positrons emitted by ¹¹C (produced by proton bombardment of boron) emerge from the room-temperature boron target with a low energy (less than 1 eV) and a measured energy width of (0.100+0.010) eV.     

Differential cross section surfaces for low energy scattering of electrons and positrons from rare gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (1/r ⁴) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.