Decay spectroscopy of suburanium isotopes following projectile fragmentation of 238U at 1 GeV/u

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Abstract

Charged-particle decay spectroscopy of heavy suburanium isotopes produced by projectile fragmentation of 1 GeV/u 238U was tested using a simple implantation–decay set-up equipped with fast-reset preamplifiers. The products were separated and identified with the GSI fragment separator (FRS) and implanted into a stack of Si detectors. Measurements were performed of implantation decay correlations of nuclei with half-lives in the range of 0.1–100 ms. Yields of very neutron-deficient protactinium, thorium and actinium isotopes were measured and yield extrapolations imply that with the availability of a beam intensity of 109 ions/s, projectile fragmentation of relativistic 238U can be an effective method to access new nuclear species including possible proton emitters in the suburanium region.

Introduction

Proton radioactivity studies have been conducted across a large swathe of the proton drip-line from Z=51 to 83 [1]. Such studies have revealed remarkably detailed information on the structure of these nuclei, and have been used to study the influence of nuclear shape and shell structure on proton tunnelling. A very important scientific goal is to extend these studies into the suburanium region where the presence of oblate deformed shapes [2], and intruder state configurations, will provide a new window on the phenomenon of proton decay.

So far our knowledge of the nuclear structure of neutron-deficient nuclei in this region has been achieved by means of fusion–evaporation reactions. Mass evaluations combining directly measured masses in the sub-lead region with Q-values in α decay chains have established that the proton drip-line has been crossed for odd-Z isotopes with 83Z91 (see Fig. 1 in Ref. [3]), specifically, for 189Bi, 195At, 201Fr, 207Ac and 213Pa. To date only one proton emitter, 185Bi [4], [5], [6], has been discovered above the Z=82 shell closure. Extrapolation of the mass measurements [3], [7] indicates that, for other heavier odd-Z suburanium elements, the nearest isotopes for which proton emission may occur with significant branches are 189At, 197Fr, 203Ac and 207Pa, 2–5 mass units from the lightest known isotopes. Further progress in this region is limited by the beam intensity available and the rapid increase of the fission probability, especially for products with higher atomic number Z.

In the last decade, projectile fragmentation reactions have proved to be very powerful for the study of light proton-rich nuclei. This technique has been successfully applied in mapping out the proton drip-line in light elements up to Z=50, discovering the most proton-rich doubly magic nuclei 100Sn [8] and 48Ni [9], and ground state two-proton radioactivity of 45Fe [10], [11]. Compared with fusion–evaporation reactions, projectile fragmentation reactions, especially at high energies have the advantages of allowing the use of a thick target, the unambiguous identification of very exotic nuclei on an event-by-event basis and very short flight times. In principle, as an alternative to fusion reactions, projectile fragmentation of 238U at relativistic energy can also be applied to the production and decay spectroscopy of very proton-rich suburanium isotopes. This method, however, has some drawbacks and has never been experimentally tried before. Previous studies show that the production cross-sections of near projectile fragments are much reduced compared to lighter beams, 208Pb for example, [12] due to the influence of fission. Secondly, because of the large charge exchange cross-sections for heavy elements [13], transmission efficiencies of heavy fragments are reduced due to the charge state distributions. This also causes ambiguities in the determination of nuclear charge state (Z). For charged-particle decay spectroscopy following projectile fragmentation, another challenge is the huge energy deposited by implants, which will saturate the decay channel preamplifier for milliseconds, preventing the detection of the decay of rare isotopes with shorter half-lives. In the case of light fragments, where the implantation energy is of the order of hundreds mega-electron volts, this problem has been overcome with specially designed fast-reset preamplifiers developed at GSI [14]. For heavy isotopes, this challenge will be more severe. In the present experiment, the energy deposited by suburanium isotopes is of the order of 10 GeV. This paper reports the first experiment on the α decay spectroscopy of suburanium isotopes using a simple implantation decay setup equipped with fast-reset preamplifier following projectile fragmentation of 1 GeV/u 238U. The experimental results indicate that with the advent of higher 238U beam intensities, the projectile fragmentation method is promising for charged-particle decay spectroscopy of new nuclear species in the suburanium region.

Section snippets

Production and identification of suburanium isotopes

The experiment was performed on the fragment separator (FRS) [15] at GSI. A schematic diagram of the FRS with the detectors used is shown in the upper part of Fig. 1. The 1 GeV/u 238U beam, provided by the heavy-ion synchrotron SIS impinged on a 1 g/cm2 beryllium target located at the entrance of the FRS. The beam was delivered in spills lasting 6 s with a repetition period of 10–18 s. The beam current was 107–5×108 ions/spill. The number of incoming projectiles was measured by a secondary electron

Results

In this experiment, the FRS was tuned for centering the fully stripped protactinium isotopes and the implantation of fully stripped protactinium and thorium isotopes. The transmission efficiencies for actinium isotopes were significant as well. The implantation decay correlations and yield measurements were performed starting with the FRS setting for 224Pa near the stability line, and moving towards the proton drip-line. Particle identification for heavy suburanium isotopes was achieved

Discussion and outlook

The α-decay spectroscopy of short-lived suburanium isotopes following the projectile fragmentation of 238U with an existing simple detection system has been proved feasible thanks to the application of the fast-reset preamplifiers. With the availability of 238U beam intensity 109ions/s, the yield rates for the new proton-rich thorium and actinium isotopes are expected to be high enough for charged-particle decay spectroscopy. The projectile fragmentation method will become an effective

Summary

Charged-particle decay spectroscopy of neutron-deficient suburanium isotopes following the projectile fragmentation of relativistic 238U has been proved feasible in a test experiment. With the coming of much higher beam intensities, projectile fragmentation of 238U therefore provides a viable alternative method for producing new very proton-rich heavy isotopes between lead and protactinium. Accompanying this it would be advantageous to develop a highly segmented, fast recovery DSSD system to

Acknowledgements

We are grateful to K.H. Behr, A. Brünle, K. Burkard, W. Hüller and M. Winkler for the technical support in the preparation for and during the experiment. Z.L., P.J.W., T.D. and A.R. would like to acknowledge funding from EPSRC. This work was partially supported by the EC under Contract HPRI-CT-1999-50017.

References (28)

  • Yu.N. Novikov

    Nucl. Phys. A

    (2002)
  • B. Blank

    Phys. Rev. Lett.

    (2000)
  • C. Scheidenberger

    Nucl. Instr. and Meth. B

    (1998)
  • M. Pfützner

    Nucl. Instr. and Meth. A

    (2002)
  • H. Geissel

    Nucl. Instr. and Meth. B

    (1992)
  • M. Pfützner

    Nucl. Instr. and Meth. B

    (1994)
  • T. Enqvist

    Nucl. Phys. A

    (2001)
  • I. Mukha, et al., GSI Science Report 2002, p....
  • H. Ikezoe

    Phys. Rev. C

    (1996)
  • P.J. Woods et al.

    Ann. Rev. Nucl. Part. Sci.

    (1997)
  • P. Möller

    At. Data Nucl. Data Tab.

    (1995)
  • C.N. Davids

    Phys. Rev. Lett.

    (1996)
  • G.L. Poli

    Phys. Rev. C

    (2001)
  • A.N. Andreyev

    Phys. Rev. C

    (2004)
  • Cited by (0)

    1

    Present address: Department of Physics, University of Surrey, Guildford GU2 7XH, UK.

    2

    Present address: Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA.

    3

    Present address: Instituut voor Kern-en Stralingsfysica, K.U. Leuven, B-3001 Leuven, Belgium.

    4

    Present address: Department of Mechanical Engineering, University of Applied Sciences, D-96450 Coburg, Germany.

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