Mapping the N=152 Deformed Shell Closure with High-Precision Penning-Trap Mass Measurements of Transuranium Nuclides at TRIGA-TRAP

摘要

Superheavy elements with Z ≥ 104 owe their existence to shell effects, which stabilize their nuclei against immediate spontaneous fission caused by the large number of protons. These effects, without which such elements would not exist, lead to enhanced nuclear binding. Direct access to the binding energy can be obtained through high precision mass measurements, exploiting Einstein's relation between (nuclear) mass and (nuclear binding) energy. High-precision mass measurements with single ions can be performed with Penning-trap mass spectrometers. TRIGA-TRAP is such an experiment, installed at the research reactor TRIGA Mainz. It is optimized for measurements of neutron-rich nuclides produced by neutron-induced fission of ~(235)U or ~(249)Cf inside the reactor, and also of long-lived transuranium isotopes, thus reaching into the realm of the heaviest elements, where direct experimental values are rare because of the low availability of the isotopes of interest. The latest mass measurements of transuranium nuclides (e.g. Pu-244, Am-241/243, Cf-249) will be presented. Microgram amounts of the isotopes of interest were deposited on suitable substrates. The ions for the measurements were produced by a non-resonant laser ion source combined with a new miniature radio-frequency quadrupole device. The performed measurements, near the deformed shell closure at N=152, provide benchmark data for testing nuclear models used to predict properties of superheavy elements and provide anchor points of a-decay chains, which are used to identify SHE isotopes produced in heavy ion fusion reactions.