Recoil-Decay Tagging Studies at the University of Jyväskylä, Finland

As our knowledge of the nuclear chart is extended to more neutron deficient nuclei, the fusion-evaporation cross-sections for population of these interesting nuclei are greatly reduced. Since the cross-sections for competing reaction processes, such as fusion-fission, may be more than an order of magnitude larger, it becomes increasingly difficult to extract the γ rays of interest from the high background produced by the competing processes. Fortunately, many of the neutron-deficient nuclei above ¹⁰⁰Sn decay by emitting charged particles ( αs, β-delayed protons and direct protons) from their ground states. Observation of these characteristic decay products gives a signature that a certain nuclear species has been produced, and correlation with the recoils associated with a given decay allows extraction of the γ rays of interest. This technique is known as recoil-decay tagging.

The JUROSPHERE + RITU project was a joint European collaboration, located at the Accelerator Laboratory of the University of Jyväskylä, Finland, for recoil-decay tagging studies of neutron-deficient nuclei. The JUROSPHERE γ-ray array consisted of 10-15 Eurogam Phase I and 10 TESSA-type Compton-suppressed Ge detectors (photopeak efficiency ~1.5% at 1.3 MeV) and was coupled to the RITU gas-filled recoil separator. The RITU device magnetically separates fusion-evaporation residues from unwanted products such as fission and primary beam through their different magnetic rigidity. The transmission efficiency through RITU is reaction-dependent and ranges approximately from 10 to 50%. The system was commissioned on 11th March 1997, with experiments involving over 80 collaborators from 22 worldwide institutions running until 1st November 1997. A schematic and photograph are shown here.

Prompt γ rays produced at the target are collected by the JUROSPHERE germanium array, in coincidence with fusion-evaporation products implanted into the RITU focal plane Si-strip detector. Correlation of the implanted nuclear species to its subsequent decay allows extraction of the coincident γ rays of interest.

• ¹⁶⁸Pt and ¹⁷⁰Pt levels identified for the first time
• ¹⁷¹Ir levels identified
• ¹⁷⁶Hg levels observed up to 10⁺, showing prolate intruder structure above 6⁺
• ¹⁸⁴Pb excited states observed for first time beyond the mid-shell in the Pb isotopes
• ¹⁸⁷Pb γ rays identified
• ¹⁹⁸Rn levels (up to at least 6⁺) for first time, with production cross-section of approximately 200 nb
• ²⁰¹Rn levels identified for the first time
• ²⁰⁶Ra and ²⁰⁸Ra levels identified for first time
• ²²³Pa γ rays identified
• ²²⁶U level scheme established for first time, showing evidence for octupole deformation

Last modified: 6 February 1998