Doppler and sympathetic cooling for the investigation of short-lived radioactive ions

At radioactive ion beam (RIB) facilities, ions of short-lived radionuclides are cooled and bunched in buffer-gas-filled Paul traps to improve the ion-beam quality for subsequent experiments. To deliver even colder ions, beneficial to RIB experiments' sensitivity or accuracy, we employ Doppler and sympathetic cooling in a Paul trap cooler-buncher. The improved emittance of Mg+, K+, and O-2(+) ion beams is demonstrated by a reduced time-of-flight spread of the extracted ion bunches with respect to room-temperature buffer-gas cooling. Cooling externally-produced hot ions with energies of at least 7 eV down to a few Kelvin is achieved in a timescale of O(100 ms) by combining a low-pressure helium background gas with laser cooling. This is sufficiently short to cool short-lived radioactive ions. As an example of this technique's use for RIB research, the mass-resolving power in a multireflection time-of-flight mass spectrometer is shown to increase by up to a factor of 4.6 with respect to buffer-gas cooling. Simulations show good agreement with the experimental results and guide further improvements and applications. These results open a path to a significant emittance improvement and, thus, unprecedented ion-beam qualities at RIB facilities, achievable with standard equipment readily available. The same method provides opportunities for future high-precision experiments with radioactive cold trapped ions.

Automated summary

At radioactive ion beam (RIB) facilities, the ion-beam quality for subsequent experiments is improved by cooling and bunching ions of short-lived radionuclides in buffer-gas-filled Paul traps. Doppler and sympathetic cooling are employed in a Paul trap cooler-buncher to deliver even colder ions, beneficial to RIB experiments' sensitivity or accuracy. The improved ion-beam quality is demonstrated by reduced time-of-flight spread of the extracted ion bunches for Mg+, K+, and O-2(+) ion beams with respect to room-temperature buffer-gas cooling. Combining a low-pressure helium background gas with laser cooling enables the cooling of externally-produced hot ions down to a few Kelvin in a short timescale, making it suitable for cooling short-lived radioactive ions. The use of this technique in RIB research increases the mass-resolving power in a multireflection time-of-flight mass spectrometer, showing good agreement with simulations and guiding further improvements and applications. These results open a pathway to significant emittance improvement and unprecedented ion-beam qualities at RIB facilities using readily available standard equipment. This method also provides opportunities for future high-precision experiments with radioactive cold trapped ions.