Linear Gas Stoppers
Linear gas cells stop fast exotic beams in helium gas after appropriate momentum compression and slowing down in solid degraders. Different concepts are applied for the ion extraction. In the case of low-pressure gas cells (< 300 mbar He) a combination of electrostatic and RF potentials is used. The linear gas cell [1] used at the NSCL is operated at high pressure (1 bar He). Here, static electric fields guide the ions to an extraction nozzle where the force provided by the gas flow transports them out of the gas cell.
The NSCL gas cell, which is part of the LEBIT [2] facility, was the world’s first system to provide thermalized ions from fast exotic beams for precision experiments. In the mean time, a number of high-precision mass measurements have been performed with LEBIT, for example on 38Ca [3], 66As, 64Ge, 69Se, and 40,42S.
Figure 1: Schematic illustration of a linear gas cell. Fast exotic beams enter from the left after slowing down in solid degraders. The beams are stopped in a volume of helium gas and guided towards the nozzle of the cell on the right, where they emerge.
Figure 2: Photograph of the assembly providing a static electric field in the NSCL linear gas cell.
References
[1] L. Weissman et al., Nucl. Phys. A 746 (2004) 655c.
[2] R. Ringle et al., Int. Journal Mass Spectr. 251 (2006) 300.
[3] G. Bollen et al., Phys. Rev. Lett. 96 (2006) 152501.
Operation of NSCL as a national user facility is supported by the Experimental Nuclear Physics Program of the U.S. National Science Foundation.