Slow positrons from a magnetic bottle

A magnetic bottle device with optimized sizes for high-efficiency positron moderation was manufactured and attached to a positron detection system based on two gamma detectors operating in coincidence to detect positron annihilation events. The detection system’s performance was tested with a 22Na calibrated source. Titanium foils with a natural isotope mixture that were 2-μm-thick were proton irradiated to produce 48V. Experiments with a stack of up to four 48V foils showed that the magnetic bottle trap achieved a 10-fold increase in the standard positron moderator efficiency of tungsten in transmission mode. However, the measured moderation efficiency of ∼0.4% was found to be in huge discrepancy with the value of 1.8% obtained by Gerchow et al. for a non-optimized trap. One possible reason for this discrepancy was pointed out, namely that Gerchow et al mainly detected moderated positrons but almost no fast/unmoderated positrons, which escape the magnetic bottle trap and should also reach the positron detector used. Our experimental and simulation results showed that the ratio of moderated to fast/unmoderated positrons that reach our detection system is about 1:7.