Elsevier

Physics Letters B

Volume 691, Issue 5, 9 August 2010, Pages 234-237
Physics Letters B

Discovery and investigation of heavy neutron-rich isotopes with time-resolved Schottky spectrometry in the element range from thallium to actinium

https://doi.org/10.1016/j.physletb.2010.05.078Get rights and content

Abstract

238U projectile fragments have been created at the entrance of the fragment separator FRS, spatially separated in flight within 0.45 μs and injected into the storage-cooler ring ESR at 7.9 Tm corresponding to about 70% light velocity. Accurate new mass values and lifetime information of the stored exotic nuclei in the element range from platinum to uranium have been obtained with single-particle Schottky spectrometry. In this experiment the new isotopes of 236Ac, 224At, 221Po, 222Po, and 213Tl were discovered. The isotopes were unambiguously identified and their masses measured. In addition, the time-correlated data have provided information on the lifetime of the new nuclides. The discovery of isotopes along with accurate mass measurement has been achieved for the first time at the FRS-ESR facility. The results will contribute to the knowledge of the decay products from the r-process nuclei and enable a crucial test of the predictive power of modern nuclear mass and half-life models.

Introduction

A basic goal of nuclear structure physics is to extend the knowledge on nuclear properties beyond the present limit of existence towards the driplines. New physical phenomena, like unexpected shell evolution at extreme neutron–proton asymmetry [1] and new decay modes [2], [3] have been discovered for the most exotic species. Presently, our knowledge is still rather scarce for neutron-rich nuclides above lead because their production cross sections are very small and the separation and particle-identification techniques for these heavy ions are very difficult and complex due to the atomic interaction, energy straggling and charge-changing collisions in conventional detector systems (e.g. tracking, time-of-flight, energy-deposition, and kinetic energy detectors). A breakthrough has been recently achieved in fragmentation studies with 208Pb and 238U projectiles at relativistic energies [4], [5]. In the latter experiment the fragments have been produced at 1000 MeV/u at the entrance of the in-flight separator FRS [6]. The magnetic system of the FRS can spatially separate the fragments of interest applying the Bρ–ΔE–Bρ method [6]. Redundance and verification are conventionally achieved in combination with particle identification detectors measuring the time-of-flight and the energy deposition to determine the mass number (A) and the proton number (Z). Here, we report on studies of stored highly-charged exotic nuclei applying a new combination of experimental separation tools for the discovery of isotopes with the ultimate sensitivity down to single ions.

Section snippets

Production and in-flight separation of stored uranium fragments

A 670 MeV/u 238U projectile beam extracted from the heavy-ion synchrotron SIS [7] with an intensity of 1 × 109/spill was focused on a 4 g/cm2 beryllium production target placed at the entrance of the fragment separator FRS in an experiment performed in 2004. Fast extraction was used with a spill length of 300 ns and a typical repetition rate of 0.2 per minute determined by the chosen spectrometry cycle. The fragments of interest were separated in flight with the FRS applying the Bρ–ΔE–Bρ method

Identification and investigation of new isotopes with time-resolved SMS

The standard criteria for the scientific approval of the discovery of a new isotope are that A and Z are unambiguously identified and that a physical property of the new nuclide has been measured in the same experiment. In such experiments dedicated to explore the limits of presently known nuclei the full particle identification and often the production cross section have been published [16], [17]. More information is gained, if first decay properties are deduced as well. With the described

Summary

The results of this experiment clearly demonstrate the discovery potential of the unique combination of FRS and ESR for the investigation of new rare isotopes. For the elements Ac, At, Po, and Tl the most neutron-rich isotopes have been discovered in the present experiment. The importance of cold fragmentation and nuclear charge-changing reactions is manifested by the observation and investigation of the so far most neutron-rich isotopes in the region of lead and uranium. Time-resolved Schottky

Acknowledgements

It is a pleasure to thank the technical staffs of the accelerators, the FRS, and the target laboratory for their valuable contribution to the beam quality and experimental setups. The authors gratefully acknowledge fruitful discussions with K. Blaum, I. Borzov, and R.F. Casten. We thank very much the Helmholtz Association of German Research Centers for the support VH-NG-033 which was a basis for the strong collaboration of the university Gießen and the research center GSI.

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  • Cited by (0)

    1

    Present address: Cyclotron Institute, Texas A & M University, TX 77843, USA.

    2

    Present address: Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany.

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