facilities world-wide. High-resolution in-source laser spectro-
scopy of isotope shifts and hyperﬁne structures
as well as an
extension of the study of b-delayed ﬁssion
to the At isotopes is
in preparation. Two theoretical methods are applied to calculate
the IP(At). The higher accuracy of the CCSD(T) result, compared
with the MCDF value is due to better treatment of dynamic
correlation within the former approach. The CCSD(T) approach
is particularly suitable in this case, as the ground states of both At
may be approximated by single determinants. The
dynamic correlation, which is handled better by CCSD(T), is the
In-source laser spectroscopy. Astatine isotopes were produced at the CERN
ISOLDE radioactive ion beam facility (see ref. 20) by directing a 1.4-GeV pulsed
proton beam of up to 2 mA from the CERN proton synchrotron booster (PSB) onto
a thick target of uranium carbide (UC
) or thorium dioxide (ThO
). Figure 4
illustrates the in-source laser spectroscopy method. The products of the proton-
induced nuclear reactions diffuse into a hot (E2,000 °C) metal tubular cavity
within which the neutral atoms are selectively photo-ionized by spatially over-
lapped beams of the RILIS lasers, wavelength tuned to the corresponding transi-
tions of a photoionization sc heme. Ions are extracted and accelerated by an
electrostatic potential of up to 60 kV. The isotope of interest is selected by the
ISOLDE mass separator dipole magnets and transmitted to the detection setups.
The photo-ion signal is recorded as a function of laser frequency.
RILIS laser setup. The RILIS laser system comprises tunable nanosecond dye (type
Sirah Credo) and Ti:Sa lasers, pumped by the second harm onic outp ut of Nd:YAG
lasers at a pulse repetition rate of 10 kHz. The wavelengths required for excitation
and ionization of the atoms were provided by the fundamental output of the lasers
or by the generation of their higher harmonics. A detailed description of the RILIS
laser system can be found in Fedosseev et al.
, Rothe et al.
and references therein.
Spectroscopy using the Windmill detector. For the initial laser spectroscopy
using the inefﬁcient two-colour scheme (cf. Fig. 1a,b), a sensitive a-decay spec-
troscopy setup (Windmill detector), as described in Andreyev et al.
, was used to
detect photo-ion rates in the range of 0.1–1,000 s
. The Windmill detector was
installed at the end point of one of the beam-lines of the ISOLDE general purpose
separator. The ion beam was implanted into one of ten carbon foils (20 mgcm
which are mounted on a rotating wheel. The carbon foil is surrounded by two
Si-detectors to acquire the a-decay spectrum of the implanted sample. The isotope
At was chosen for its suitable half-life of 7.2 s and a well-separated a-decay
energy of 6,643 keV. The number of 6,634 keV a-decays was counted for every
wavelength combination of the RILIS dye lasers. This measurement sequence
was synchronized to the PSB supercycle (E60 s), ensuring steady conditions.
Rydberg spectroscopy using FC detection. For the laser spectroscopy of Rydberg
states, the Ti:Sa lasers were used to generate wavelengths required for the ﬁrst and
second-step transition. The laser powers measured on the laser table were 33 mW
(fourth harmonic) for the ﬁrst step and 2 W (fundamental) for the second-step
transition. The dye laser (Sirah Credo with DCM dye dissolved in ethanol) was
scanned in the range of 16,599–15,523 cm
in two sections at a speed of
and 0.132 cm
. The laser wavelengths were continuously
measured with a wavelength metre (HighFinesse-Ångstrom WS/7), calibrated with
a frequency stabilized HeNe laser. The three-colour ionization scheme has a higher
efﬁciency and the photo-ion signal was obtained from a direct ion current mea-
surement with a FC installed in the focal plane of the separator magnet. The best
signal to noise ratio was obtained with the separator magnet set to transmit mass
A ¼ 205. This is because of the relatively high production cross section of the
At, its slow component of release from the ISOLDE target and relatively
long half-life (26.2 min), which ensures quasi-independence from the PSB super-
cycle sequence. The continuous scanning method requires a correction for the
integration time of the FC and potential delays in the data acquisition. The cor-
rection factor was determined separately by spectroscopy of stable manganese
isotopes under the same conditions.
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We thank the ISOLDE Collaboration for ﬂexible allocation of beam time. We
acknowledge the support from TRIUMF, which receives federal funding via a con-
tribution agreement with the National Research Council of Canada and support through
an NSERC discovery grant, as well as beam time allocation for experiment S1237 at
TRIUMF. We thank the GSI Target Group for manufacturing the carbon foils.
We acknowledge support by the Wolfgang-Gentner-Programme of the Bundesminis-
terium fu¨r Bildung und Forschung (BMBF, Germany), by FWO-Vlaanderen (Belgium),
by GOA/2010/010 (BOF-KU Leuven), by the IUAP- Belgian Science Policy Ofﬁce
(BriX network P7/12), by a grant from the European Research Council (ERC-2011-AdG-
291561-HELIOS), by the United Kingdom Science and Technology Facilities Council
(STFC), by the European Union Seventh Framework through ENSAR (contract no.
262010), by the Slovak Research and Development Agency (contract No. APVV-0105-10
and APVV-0177-11) and by the Reimei Foundation of JAEA. We acknowledge the Knut
and Alice Wallenberg Foundation (grant KAW 2005-0121) for funding the RILIS laser
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