Two-stage fragment separator may help find new, neutron-rich isotopes

Pushing the bounds of nuclear theory means producing and then measuring never-before-observed rare isotopes. One of the many experimental challenges is plucking the few desired particles from the enormous number of other isotopes and reaction products produced by high-energy reactions. It’s a task that just got easier at NSCL, which recently concluded one of the first nuclear physics experiments in the world to utilize a two-stage fragment separator.

The team was pursuing a fundamental question within nuclear science – how many neutrons can be incorporated into various atomic nuclei before these nuclei fall to pieces? More than a hundred years after the birth of nuclear physics, the answer is unknown except for a small handful of the lightest elements.

The plots above, which show the distributions of particles observed from the same nuclear reaction at NSCL using a one-stage separator (top) and a two-stage separator (bottom), show the benefits of the new technique. The total number of particles arriving at the end is approximately the same in both cases. However, the proportion of unusual nuclei is much larger in the second case. For example, the two-stage separator enabled the detection of many more neutron-rich isotopes, such as those circled in red -- oxygen-24 (8 protons, 16 neutrons), fluorine-27 (9 protons, 18 neutrons) and fluorine-29 (9 protons, 20 neutrons). more

For more than two decades, a group of NSCL scientists has been working on the problem of producing and identifying the most exotic nuclei. After pushing the laboratory’s A1900 fragment separator to its limits with the discovery in January 2007 of silicon-44, they conceived of a new experimental technique that combined the A1900 capabilities with those of the beam line leading to the S800 spectrograph. In effect, the team reasoned, the two devices could work together as a double filter to remove extraneous background particles and make it easier for detectors to hunt for faint, elusive isotopes.

Just as more powerful telescopes reveal previously unknown details in the cosmos, NSCL’s new ability to take a more selective look at subatomic particles should open up new vistas on the nuclear landscape.

“Historically, as nuclear science continues to look for the edge of the neutron drip line, which defines the limits of stability, we regularly turn up surprises,” said University Distinguished Professor David Morrissey, one of the lead researchers involved in NSCL’s search for the most exotic nuclei.

Success in implementing this two-step approach to sifting nuclear wheat from the chaff depended in part on improved operational efficiency at NSCL. With support from the National Science Foundation, the laboratory has instituted a program to aggressively monitor the performance and expected lifetimes of all critical equipment. Regularly scheduled maintenance shutdowns are used to replace components before they fail. The practice, in place for several years, contributes to the laboratory’s steadily improving levels of availability:

Year NSCL Beam availability
2004 85.06%
2005 89.68%
2006 91.03%

All along the beam line, several important technical advances have helped to boost beam intensity, also important when hunting for new, previously unobserved rare isotopes. Better oven technology has improved evaporation of particles from the metal isotopes used to produce the primary beam of calcium ions, an enhanced injection line has optimized the link between the ion source and the coupled cyclotrons, and new devices installed in the cyclotrons called scrapers now help to cut off at an early stage some of the partially accelerated beam that will not make it to the final desired energy and thus spare downstream components.

“Really, we’ve taken a bunch of little steps all in the right direction,” said Andreas Stolz, NSCL assistant professor and deputy department head of operations. “The result is that we have better beam intensity and an improved ability to produce and hunt for neutron-rich rare isotopes.”

Plans for most next-generation nuclear science facilities, including the proposed Isotope Science Facility at Michigan State University, call for two-stage fragment separators.

The experiment was supported by the National Science Foundation and Michigan State University.

NSCL is a world-leading laboratory for rare isotope research and nuclear science education.

- Geoff Koch, April 19, 2007