A new result from the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University yields clues to a new region of deformation in the nuclear chart. The NSCL provides beams of rare and exotic nuclei that have not been studied before. While the vast majority of nuclei are spherical, it is important to determine the exact shape of new, exotic nuclei. Anomalies in the nuclear structure of nuclei with unusual proton-to-neutron ratios sometimes leads to usual shapes which then offers important challenges to our understanding of atomic nuclei.
The NSCL segmented Ge detector array (shown at the left) surrounds the vacuum pipe that contains the NSCL/KÖln plunger device (shown at the right), which allows precision level lifetime measurements of the excited states produced in fast, rare isotope beams.
Researchers from the University of Cologne and MSU have developed a novel technique to measure lifetimes of the first excited states of exotic nuclei. The lifetime of a nuclear energy level provides a sensitive measure of deformation of the state. The plunger device, developed to measure picosecond (10-12) lifetimes, holds two thin metal foils in the beam path and is able to separate the foils by very precise distances. A nuclear excited state is produced in the first foil (the target) and decays in flight traveling a distance related to its lifetime. If the decay occurs after the nucleus passes through the second foil (the degrader) the nucleus will be traveling significantly slower. The photons emitted during the decay are detected in state-of-the-art segmented Germanium detectors that surround the plunger foils. The energies of the photons are Doppler-shifted according to the velocity of the nuclei and the lifetime can be obtained from measurements with different target-degrader distances. The researchers applied this technique to a series of neutron-rich 62,64,66Fe isotopes and discovered that the most exotic nucleus, 66Fe, has a very large deformation. This nucleus has ten more neutrons than the most abundant stable iron isotope.
In analogy to atomic structure, nuclei have a shell structure with enhanced stability of closed-shell configurations. The number 28 gives a closed-shell for both neutrons and protons in a spherical nuclear potential while the number 40 gives a closed-shell in the simpler harmonic oscillator potential. The neutron-rich isotope 68Ni40, proton magic number 28 and neutron number 40, is believed to be a doubly magic nucleus. Recent nuclear theories predict that a yet undiscovered region of deformed nuclei will be centered at around the even more exotic 62Ti40 nucleus, having six protons less than 68Ni and four protons less than 66Fe. The new data on 66Fe indicate that the deformed region extends further than expected, providing important information to refine the theoretical models.