Experimental study of vapor-cell magneto-optical traps for efficient trapping of radioactive atoms

TL;DR

This study investigates the efficiency of vapor-cell magneto-optical traps for radioactive atoms, demonstrating how cell size and surface treatments influence trapping success, leading to optimized designs for radioactive isotope experiments.

Contribution

It provides experimental insights into how cell size and surface passivation affect MOT efficiency, enabling improved design for trapping radioactive atoms.

Findings

Trapping efficiency weakly depends on cell size.

Smallest cell reduces efficiency by about 40%.

Surface passivation and long coated tubes improve atom retention.

Abstract

We have studied magneto-optical traps (MOTs) for efficient on-line trapping of radioactive atoms. After discussing a model of the trapping process in a vapor cell and its efficiency, we present the results of detailed experimental studies on Rb MOTs. Three spherical cells of different sizes were used. These cells can be easily replaced, while keeping the rest of the apparatus unchanged: atomic sources, vacuum conditions, magnetic field gradients, sizes and power of the laser beams, detection system. By direct comparison, we find that the trapping efficiency only weakly depends on the MOT cell size. It is also found that the trapping efficiency of the MOT with the smallest cell, whose diameter is equal to the diameter of the trapping beams, is about 40% smaller than the efficiency of larger cells. Furthermore, we also demonstrate the importance of two factors: a long coated tube at the entrance of the MOT cell, used instead of a diaphragm; and the passivation with an alkali vapor of the coating on the cell walls, in order to minimize the losses of trappable atoms. These results guided us in the construction of an efficient large-diameter cell, which has been successfully employed for on-line trapping of Fr isotopes at INFN's national laboratories in Legnaro, Italy.