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Decay study of neutron-rich At isotopes with a chemically



                                                           20 January 2005

          Letter of Intent to the ISOLDE and neutron Time-of- Flight
                             Experiments Committee

         Decay study of neutron-rich At isotopes with a chemically
                       selective negative ion source
              W. Kurcewicz1 and H. Mach2 for the IS322 and IS386 Collaborations
    Institute of Experimental Physics, Warsaw University, Poland
    Department of Radiation Sciences, Uppsala University, Sweden

                              Contactperson: L. M. Fraile

Abstract: Since a development of Negative Ion Source is being done at ISOLDE, we ask in
this Letter to extend the investigation of the properties of the Negative Ion Source to include
also the astatine isotopes in the off- and on-line test experiments at ISOLDE PSB. We intend
to search for octupole signatures in the neutron-rich Rn isotopes (-decay products of At),
which could be performed if there exists a target system, which provides satisfactory intensity
of At beams with high chemical purity. Studies on heavy Rn nuclei are strongly
complementary to the IS322 and IS386 projects. In fact, they were already approved under

The region of nuclei, which has shown the best evidence for a strong octupole instability in
the nuclear ground state is relatively small, and includes only a handful of cases near 225Ra.
The close lying g 9/2 and j 15/2 neutron orbitals and the f 7/2 and j 13/2 proton orbitals, in the
proximity of the Fermi surface for nuclei around A = 225, form basis for the existence of a
unique region of strong octupole deformation. The extensive experimental studies of isotonic
and isotopic sequences of nuclei from this region, performed at ISOLDE facility, were
focused on the signatures of an octupole shape, such as low-lying bands of opposite parity
(parity doublets in odd-A nuclei) connected by relatively strong E1 intra-band transitions (see
review [1] and refs.[2-4]). The aim of the IS322 experiment [5] was to investigate the limits
of the “island” of the octupole instability in the actinide region. It is of particular importance
to demonstrate experimentally the disappearance of the octupole deformation in the presence
of a well developed quadrupole field. The experiment was performed using the advanced
time-delayed (t) method, and the Tardis array of 12 Compton-suppressed Ge detectors and
a LEPS detector. It represented the first use of such complex techniques at ISOLDE.
Recently, the studies of electric dipole moments in the octupole collective region of heavy
radium with focuses on 226Ra and 229Th, were performed in the frame of the IS386 experiment
Presently, with the current development work on the Negative Ion Source at ISOLDE, there is
possibility to extend the IS322 and IS386 experiments and study the electric dipole moments
in the neutron-rich radon isotopes. Previous attempts to construct such a target system have
not been successful [5]. The negative surface ionizer gives pure At beams from targets of
ThO2 or a mixture of Th foils and Ta foils [7], but the intensities are lower by two orders of
magnitude compared to what could be expected. A target of thorium carbide equipped with a
hot plasma ion-source gives high yields of At isotopes (estimated to 8 x 105, 1 x 105 and 2 x
103 atoms/s of 219At, 221At and 223At, respectively), but with poor chemical separation. The
yields of contaminating isobaric activity from the thorium carbide target may be three orders
of magnitude higher than At yields. The present development of a negative surface ion-source
at ISOLDE [8], which shows good yields for 79,81Br and 127I isotopes, could open a new
opportunity for the decay studies of At isotopes.

In this letter we propose to extend the investigation of the properties of a negative ion source
to include also astatine isotopes for the off- and on-line experiments at ISOLDE facility. In
the off-line experiment the 211At source could be used to measure the astatine yield. The 211At
activity could be produced at the cyclotron in Oslo and transported to CERN [9]. The
prospects for spectroscopic study of neutron-rich radon nuclei at ISOLDE depend very much
on the negative ion source developments.


[1] W. Kurcewicz, Hyperfine Interactions 129 (2000) 175, and references quoted therein
[2] L. M. Fraile et al., Nucl. Phys. A686 (2001) 71
[3] M. J. G. Borge et al., Nucl. Phys. A690 (2001) 227c
[4] K. Gulda et al., Nucl. Phys. A703 (2002) 45
[5] A. J. Aas et al., ISOLDE IS322 experiment, spokesman W. Kurcewicz, CERN/ ISC 92-28
[6] P. Alexa et al., ISOLDE IS386 experiment, spokesman H. Mach, CERN/ INTC 2000-022
[7] D. G. Burke et al., Z. Phys. A333 (1989) 131
[8] R. Wilfinger et al., ISOLDE poster presented at the International Nuclear Physics
Conference, INPC2004, June 27 – July 2, 2004, Goeteborg, Sweden
[9] P. Hoff, private communication to W. Kurcewicz, October 2004

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