## LINAC Target

The first step in making slow positrons is to make a shower by irradiating a high-Z converter target with 150 MeV electrons from the LINAC 13 . For this energy of primary electrons, the target should be about three radiation lengths of a material such as tungsten or a high-Z liquid metal. A relatively non-toxic alloy such as the binary eutectic 55.5 Bi-44.5 Pb with a melting point of 124 C might be a good choice for the latter if it is necessary. We assume a LINAC power of 10 kW and a pulse...

## Rearrangement Collisions

We consider the rearrangement process where NL and nl are the principal and angular momentum quantum numbers of the bound states of protonium and positronium respectively. Figure 1. Scattering thresholds with Pn principal quantum number N 22-24, and positronium principal quantum number n. The lines show the sum of protonium and positronium binding energies. Any final state with energy below -1 a.u., corresponding to zero collision energy, can be formed in zero-energy hydrogen-antihydrogen...

## Positronium Formation In The Alkali Atoms

As mentioned earlier, the formation of ground-state positronium in single ionization of the alkali atoms is an exothermic process, so that the parameter Er ( Eps) in equation 7 is negative. A naive assumption Figure 3. (a) Cross sections for atom formation in positronium collisions with singly ionized alkali atoms, (b) Ground state positronium formation cross sections in positron collisions with alkali atoms. The data are from 16 for Rb and Cs, 17 for Na, 18 for K and 19 for Li. < 7pg(ls) for...

## References

Bardsley, Phys. Rev. Lett. 28,127 (1972). 2 D.L. Morgan and Hughes, Phys. Rev. A 7, 1811, (1973) Phys. Rev. D 2, 1389 (1970). 3 W. Kolos, D.L. Morgan, D. M. Schrader and L. Wolniewicz, Phys. Rev. A11, 1792 (1975). 4 E.A.G. Armour and C. Chamberlain, this volume. 5 S. Jonsell, A. Saenz, P. Froelich, B. Zygelman and A. Dalgarno, Phys. Rev. A in press (2001). 6 P. Froelich, S. Jonsell, A. Saenz, B. Zygelman and A. Dalgarno, Phys. Rev. Lett. 84, 4777 (2000). 7 B. Zygelman,...

## Positronium Behavior In Porous Films

Positronium formation in porous materials. The shape of a typical positron implantation profile is depicted in the lower panel. The most important process a positron can undergo in an insulator is electron-capture to form the bound state of Ps. Ps formation in a porous insulator is depicted in Figure 3. The positron slows down through collisions in the material from its initial beam energy of several keV to several eV. It can either capture a bound molecular electron or recombine with...

## Conclusion And Outlook

The existence of positron-atom bound states and the ability to calculate their wave functions give unique insights into how positrons interact with complex electronic systems. For the last two decades the theoretical emphasis has been on scattering calculations. However, the theoretical uncertainties inherent in bound state calculations are smaller than those associated with scattering calculations and this allows for better understanding of the dynamics of positron-atom interactions. The...

## Positronium Formation In The Noble Gases

We became interested in correlations between ionization cross sections and ionization energies in the course of investigating positronium formation in the double ionization of the noble gases. Experimental studies by Bluhme et al. 4 and Moxom et al. 5 had revealed the almost complete absence of transfer ionization in the second Ore gap for helium and neon, but not for argon, krypton and xenon. These findings led us to develop a model of positronium formation, the basis of which is as follows....

## Antimatter Compounds

(a) Chemistry Department, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881, U.S.A., david.schrader marquette.edu (b) Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6142, U.S.A., moxomj ornl.gov Abstract This chapter is about chemical compounds containing both matter and antimatter, the latter in the form of positrons. Most of our knowledge of such compounds comes from quantum mechanical calculations. Our discussion includes some issues relating to calculating...

## Info

When the ionization potential of the neutral atom is less than 0.250 Hartree, the addition of the extra electron and the two positrons leads to formation of a Ps2 molecule which is then bound to the residual ion core. The Ps2 dominates because the two most weakly bound electrons are each more strongly attracted to a positron and therefore b 1. Similarities of the Ps2, Li+Ps2 and Na+Ps2 annihilation rates provides a good indicator of Ps2 cluster formation (inter-particle correlations also...

## Fundamental Studies Involving Positrons And Positronium

There continues to be interest in fundamental aspects of positronium physics, including lifetime measurements and spectroscopy. The recent observation of magnetized Rydberg positronium 28 has added a measure of excitement to the field. Many experiments of this type can be performed using a rapid cycle pulsed positron beam, as described above, coupled to a pulsed laser system. In addition, there continues to be interest in the production of systems containing more than one positron 29 , despite...

## Total Cross Section

The contribution of the interaction potential in positron and electron scattering by atoms and molecules is summarized in Table 1. This effect of the contribution is revealed in the total cross section (TCS). The total cross-section for positron scattering is given by Q+t Q+ela + Q+ex + Qps + Q+ion + Q+dis + Qa where Q+eu is the elastic scattering cross-section, Q+e, is the sum of all contributions from rotational, vibrational and electronic excitations, is the Ps Table . Positron and electron...

## Positron Accumulation And Lifetime

Because of method's simplicity, we initially attempted to load positrons by following, as much as possible, the method described in Ref. 17 of field ionizing high-Rydberg positronium. We summarize here the accumulation rate obtained with this method. We also plan to accumulate positrons by the method outlined in Ref. 2 , where positrons are loaded through Coulomb collisions with trapped 9Be+ ions. Results of this method will be discussed in a future publication. The basic idea from Gabrielse's...

## Calibration Of Pore Size And Open Pore Systems

It is critically important for PALS to calibrate Ps lifetimes with pore sizes. The 3y Ps annihilation parameter, which measures the convoluted effect of pore size and density, will also benefit from such calibration. In the very large pore (classical) regime (pores with mean free paths on the order of 100 nm), a calibration was performed using high porosity (90-98 ) silica powders 18 . In the other extreme (sub-nm pores), the quantum mechanical model 13 first developed by Tao and Eldrup has...

## Annihilation On Atoms And Molecules

The annihilation rate is a sensitive measure of short-range correlations between the positron and the bound electrons. We follow the convention of expressing the annihilation rate r in terms of the parameter Zeg, which is the annihilation rate relative to that for positrons in a gas of uncorrelated electrons (i.e., the Dirac annihilation rate). where r0 is the classical electron radius, c is the speed of light, and nm is the number density of atoms or molecules. For large molecules, it is well...

## Positron Annihilation

To conserve energy and momentum, electrons and positrons usually annihilate by a second order process in which two photons are emitted 3, 4 . The process is shown in Fig. 1. At the first vertex the electron emits a photon, at Figure 1. Feynman diagram for the electron positron annihilation. Figure 1. Feynman diagram for the electron positron annihilation. the second vertex it emits a second photon and jumps into a negative energy state (positron). This phenomenon is analogous to Compton...

## E H Ise H2s 2 p

Atomic hydrogen would clearly be an ideal candidate for such a search of positron scattering resonances, given the considerable amount of theoretical work which already exists. Similar resonances in electron scattering have been revealed in a beautiful series of measurements by Williams 86 . However, given the considerable experimental difficulties posed by atomic hydrogen sources, in combination with those posed by high resolution positron beams and scattering experiments, H may not be the...