Production of a Fully Spin-Polarized Ensemble of Positronium Atoms

Long-lived $|m|=1$ positronium (Ps) atoms are produced in vacuum when high density bursts of positrons with net polarization $p_0$ are implanted into a porous silica film in a 2.3 T magnetic field. We observe a decrease in the $|m|=1$ population as the density of the incident positron beam is increased due to quenching interactions between oppositely polarized Ps atoms within the target. Saturation of this density dependent quenching indicates that the initial positron spin polarization $p_0=28\pm 1\%$, and demonstrates the long term (${10}^2\mathrm{s}$) survival of positron polarization in a Surko-type buffer gas trap. We conclude that, at high Ps densities, the minority spin component is essentially eliminated and the remaining Ps is almost entirely ($\sim 96\%$) polarized, as required for the formation of a Ps Bose-Einstein condensate.

Figure 1

The delayed fraction measured at low beam density and impact energy for different target magnetic fields, $B_T$. The magnetic quenching curve was fitted to the standard theory [16] assuming zero polarization and a vacuum decay rate of $130{\mathrm{ns}}^{-1}$.

Figure 2

Delayed fraction (▲) and beam areal density on target (□) as a function of the RW compression frequency for positron implantation energy $K=6.2\mathrm{keV}$ and magnetic field $B_T=2.3\mathrm{T}$.

Figure 3

Delayed fraction data of Fig. 2 normalized and given in terms of the beam areal density $n_{2D}$. The solid line is a fit using the function defined in Eq. (7) including a normalization coefficient ($0.091\pm 0.004$), the polarization $p_0$, and a constant $\alpha$ as free parameters, where $\alpha \equiv \zeta /n_{2D}$. Data points obviously associated with the ZFM resonances (see Fig. 2) have been excluded from the fit.