The Low Energy Beam and Ion Trap

The low energy beam and ion trap (LEBIT) takes the fast rare isotope beams delivered by the A1900 fragment separator and carefully slows the isotopes down to low velocities. The rare isotopes can even be brought to rest and captured in special traps for ions. LEBIT makes a new class of precision experiments possible at the NSCL— highly accurate weighing of single rare isotopes, looking at the shape of a nucleus by laser spectroscopy, and precise observation of the decay of a nucleus—and more.

Expanded Description

The controlled slowing down of an ion beam that has a speed approaching that of light is a challenging task. The ions delivered by the A 1900 fragment separator are sent through a massive piece of matter, called a degrader, to absorb most of their energy. Through a thin window they enter a chamber filled with 1 atm Helium. Here, they lose their remaining energy and come practically to rest. They remain charged which allows them to be guided by electric fields to a very thin nozzle where the gas flow takes them into a vacuum chamber. Radio-frequency devices called ion guides are used for a loss-free low-velocity transport of the ions into ultrahigh vacuum. There they are modestly accelerated for their further transport. The ions receive their last treatment in a gas filled ion trap (the so-called “Paul trap”) where they are captured, cooled and finally released as short, high-quality ion bunches.

These bunches can then be used for experiments, or example, for high-accuracy atomic mass measurements. For this purpose the ions are captured in an ion trap (called a “Penning trap”) placed in a 9 Tesla magnetic field. A precise observation of the ion’s motion in this strong field allows its mass to be determined with a precision of 100 parts per billion and better.

Importance of LEBIT

LEBIT adds a new facet to the NSCL research program: precision experiments with low-energy rare isotope beams. The goal of physics is to better understand the basic properties of rare isotopes. One important kind of study to be performed with LEBIT is to find out how strongly neutrons and protons are bound together to form an atomic nucleus. This information is not only decisive for learning more about why matter exists and how it has been created in the universe, but also for understanding the inner ‘life’ of an atomic nucleus or for testing the most fundamental of physics laws.

The employment of atomic physics techniques like ion trapping or laser spectroscopy offers the opportunity to perform unique precision experiments on rare isotopes. Penning traps are ideal devices that allow the mass of single atoms to be determined with very high precision. The mass of an atom is smaller than the mass of all the electrons, neutrons, and protons the atom is made of. Einstein found that this missing mass corresponds to the energy you need to break an atom completely apart. Hence, by measuring the mass of a nucleus it can be understood how strongly the constituents of an atom are bound together.

With LEBIT, it is possible to make this study not only with stable atoms but with atoms that have many more neutrons than protons and vice versa. A systematic study of the binding of these nuclei gives insight into how the properties of these very exotic atoms change and whether our theoretical models are able to describe them. Furthermore, the knowledge of the mass of most of these exotic atoms is important for understanding how the elements in our universe have been synthesized.

The low-energy beams of LEBIT are also ideal for laser spectroscopy experiments. Tickling the atoms with laser light and looking at their response can tell something about the shape (pancake or cigar) of the nucleus inside the atom or about how ‘fast’ the nucleus spins.
Finally, rare isotopes can be captured in ion traps. They sit freely floating in space and can be observed decaying with appropriate detectors. These ‘ideal’ decay studies can answer very basic questions about known fundamental forces.

Technical Information

Status: Operational

Location: N4/N5 vaults

Contact Person: Georg Bollen

Funding Acknowledgement: The construction of LEBIT (downstream of the gas cell) was funded by Michigan State University, and the gas cell was funded by DOE under Contract DEFG02-00ER41144.

References:

The low-energy-beam and ion-trap facility at NSCL/MSU; S. Schwarz, G. Bollen, D. Lawton, P. Lofy, D. J. Morrissey, J. Ottarson, R. Ringle, P. Schury, T. Sun, V. Varentsov, and L. Weissman, EMIS-14 conference proceedings, Victoria, BC, Canada,May 6-10, 2002, Nucl. Instrum. Meth. B 204 (2003) 507.
doi: 10.1016/S0168-583X(02)02122-5