A ground state atomic nucleus can be something of a black box, masking subtle details about its structure behind the aggregate interplay of its protons and neutrons. This is one reason nuclear scientists are so keenly interested in isomers -- relatively long-lived excited-state nuclei that more easily give up their structural secrets to experimentalists.
For years, gamma ray spectroscopy has been one of the only reliable means of studying isomers. But now scientists have a new tool at their disposal. In a paper that will be published in Physical Review Letters, researchers at Michigan State University's National Superconducting Laboratory (NSCL) report the first ever discovery of a nuclear isomer by Penning trap mass spectrometry.
The concept of excitation applies across physics and chemistry to everything from molecules to atoms to nuclei. Consider the basic physics behind a neon light. When voltage is applied across a tube filled with neon gas, the electrons orbiting the neon nuclei briefly are excited to higher energy levels before they come crashing back down to their ground states, releasing visible light.
Nucleons, the protons and neutrons that comprise atomic nuclei, can similarly be raised to higher energy levels. Most resulting excited-state nuclei exist on the briefest of timescales, with lifetimes often measured in trillionths of a second, before the nucleons decay to lower energy states, releasing various forms of radiation. However, some of these excited-state nuclei are quite stable and can exist for much longer periods of time, from fractions of seconds to millions of years.
These relatively long-lived nuclei are called isomers, which are the focus of intense scrutiny by nuclear scientists. Among the open questions about isomers: For which combinations of protons and neutrons can they exist? What are their properties? How long do they live? And what is their excitation energy (the energy required to raise their nucleons to higher energy levels)?
The discovery of the new iron isomer came while using NSCL's Low-Energy Beam and Ion Trap (LEBIT) device to make precision measurements of rare isotopes that are close, in terms of numbers of protons and neutrons, to nickel-68, a particularly enigmatic isotope.
With 28 protons and 40 neutrons, nickel-68 displays some of the characteristics of doubly magic nuclei, so named because they have just the right number of protons and neutrons to completely fill all the energy states, or shells, they occupy. (According to the nuclear shell model, protons and neutrons in most nuclei occupy different energetic shells, completely filling the low-lying states and only partially filling higher states; in doubly magic nuclei, all occupied shells are filled.) However, nuclei with slightly fewer protons and neutrons than nickel-68 reveal pronounced changes in structure, which generally is not the case for isotopes nearby other doubly magic nuclei.
"We have no good idea what is happening in this nuclear region, so more measurements are needed," said Georg Bollen, NSCL professor and co-author of the paper.
The experiment was conducted at NSCL's Coupled Cyclotron Facility, which produced various neutron-rich isotopes of iron and cobalt, including iron-65, with 26 protons and 39 neutrons. (The most abundant stable iron isotope on Earth has 26 protons and 30 neutrons.) These isotopes, produced by smashing beams of germanium nuclei traveling at half the speed of light into thin target material, were brought nearly to rest in a helium gas cell.
Next, the isotopes were guided by a series of electric fields into two ion traps. One was a Penning trap, a device commonly used in atomic and nuclear physics to precisely measure mass. A Penning trap catches and retains charged particles in a strong magnetic field. Responding to this field, captured particles move in what's known as a cyclotron motion, the frequency of which is directly related to the mass of the particle.
During the experiment, Bollen and his collaborators observed two distinct frequencies associated with the trapped iron-65 particles. They concluded that the heavier of the two was a previously unknown isomer of iron-65.
NSCL is the first laboratory in the world to stop fast beams of nuclei such that they can be trapped in space and studied with high precision. Bollen, one of the experts in this discipline at the interface between atomic and nuclear physics, helped to design and build ISOLTRAP, the first Penning trap spectrometer for the study of the mass of short-lived nuclei at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland.
"The nuclear region we looked at still has lots of uncertainty, but we were successful in adding an intriguing new piece of information," said Bollen. "And we did so by going beyond gamma ray spectroscopy, the classical means of studying isomers; finding isomers by weighing nuclei with very high precision bears interesting prospects for future studies."
Posted March 24th,2008