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 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 (s, -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 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 -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 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 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 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 rays identified
• ²²⁶U level scheme established for first time, showing evidence for octupole deformation

Back to Highlights Page

Last modified: 6 February 1998