New Scheme for Positron Accumulation in Ultrahigh Vacuum

A new positron accumulation scheme compatible with ultrahigh vacuum conditions has been developed, which is realized by preparing a high density electron plasma as high as $\approx {10}^{11}{\mathrm{c}\mathrm{m}}^{-3}$ and an ion cloud as energy absorbers. The present accumulation rate normalized by the intensity of ${}^{22}\mathrm{N}\mathrm{a}$ positron source is $(3.6\pm 0.3)\times {10}^2{e}^{+}/\mathrm{s}/\mathrm{m}\mathrm{C}\mathrm{i}$, which is more than one and a half orders of magnitude higher than other ultrahigh vacuum compatible schemes so far reported.

Figure 1

(a) A schematic diagram of the experimental setup consisting of an electron gun (EG), a multiring trap (MRT), a movable remoderator (RM), a Faraday cup (FC), and a NaI detector. (b) The electric potential configuration along the MRT axis in the electron-ion accumulation mode. (c) The same as (b) in the positron accumulation mode (see text).

Figure 2

The accumulation efficiency ${\epsilon }_{\mathrm{a}}$ of positrons in the PT as a function of the remoderator bias $V_{\mathrm{R}\mathrm{M}}$ for (a) $N_{\mathrm{e}}\approx 1.8\times {10}^{10}$ and (b)  $1.2\times {10}^{10}$. Here, ${\epsilon }_{\mathrm{a}}$ is defined as the ratio of the number of positrons accumulated in the PT to the number of positrons injected in the RM during accumulation.

Figure 3

(a) The optimized accumulation efficiency ${\epsilon }_{\mathrm{a}}^{\mathrm{m}\mathrm{a}\mathrm{x}}$ and (b) the corresponding remoderator bias $V_{\mathrm{R}\mathrm{M}}^{\mathrm{m}\mathrm{a}\mathrm{x}}$ as a function of $N_{\mathrm{e}}$. The right vertical scale of the panel (a) gives the accumulation rate normalized by the ${}^{22}\mathrm{N}\mathrm{a}$ positron source intensity. The vertical error bars reflect statistical errors, which do not include the systematic error of 3%. The horizontal error bars show reproducibility of $N_{\mathrm{e}}$.