Rare-Ion Beams Brought to Rest

A team at the NSCL has succeeded in bring beams of rare, short-lived isotopes traveling at approximately one-half the speed of light (330 million mph) to rest. Once captured, actually thermalized in a buffer gas, the buffer gas was stripped off and the ions were sent on to an ion trap for mass measurements carried out with extreme precision.

All of the isotopes produced at the NSCL including those with half-lives as short a few milliseconds, can be captured using a technique called “range compression.” The ions are first slowed down by letting them pass through precisely polished pieces glass plates until they have a speed as low as possible but without being stopped in the glass. The materials used to slow down the ions have to uniform at the level of a few microns, a size that is much smaller than the thickness of a human hair (40 – 120 microns). The isotopes then enter a cylindrical stopping chamber filled with ultra pure helium gas (impurities below the parts-per-billion level) at atmospheric pressure where they are brought to thermal velocities. At this point the next step is bring the isotopes into high vacuum. This is achieved by using electric forces that guide the ions to a hole in the stopping chamber. The helium flow through a supersonic nozzle flushes the isotopes into a vacuum chamber connected to three huge vacuum pumps. The ions are kept under control with electric forces while the helium coming out of the stopping cell is pumped away. Once they have reached a chamber with an excellent vacuum the ions are reaccelerated to reach a “low” velocity of only about 350,000 mph and they are sent in a pipe through the shielding wall to an experimental area.

A major difficulty that was overcome during the development was achieving the cleanliness of the helium gas and the vacuum chambers. Without such cleanliness many unwanted stable molecular ions are produced and the precious rare isotopes tend to undergo chemical reactions with the molecules and tend to be lost for the experiment. Assembling the equipment with clean room techniques and mass filtering of the products solved this problem. A number of studies of the efficiency of the stopping and extraction process have been completed including studies with different rare isotopes. The figure shows the measured efficiency for collecting 38Ca ions after passing through the mass filter as a function of the thickness of one of the degraders (in microns). The different curves indicate the effect of sending the ions into the helium at different rates.

Figure 1: The thin black curve shows the fraction (divided by ten) of rare ions that were found to stop in the gas as a function of the thickness of the glass (in micrometers) used to slow them down. The points show the fractions of the incident rare ions that could be extracted from the gas cell versus glass thickness; each set of points was obtained using a different beam intensity as indicated in the legend. more

David Morrissey
Morrissey at nscl.msu.edu 517-333-6321