Laser spectroscopy studies on nobelium
Michael Block
1
,
2
,
3
,
1
GSI Helmholtzzentrum für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany
2
Helmholtz Institut Mainz, Staudingerweg 18, 55128 Mainz, Germany
3
Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany
Abstract. Laser spectroscopy of the heaviest elements provides high-precision data on their atomic and nuclear
properties. For example, atomic level energies and ionization potentials allow us to probe the influence of
relativistic effects on their atomic structure and to benchmark state-of-the-art atomic structure calculations.
In addition, it offers an alternative route to determine nuclear properties like spins, magnetic moments and
quadrupole moments in a nuclear model-independent way. Recently, a sensitive method based on resonant laser
ionization has been applied to nobelium isotopes around N = 152 at GSI Darmstadt. In pioneering experiments,
several atomic states have been identified extending the reach of laser spectroscopy beyond fermium. In this
contribution, the main achievements and future perspectives are briefly summarized.
1 Introduction
The atomic and chemical properties of the heaviest ele-
ments (Z
100) are affected by strong relativistic effects
and quantum electrodynamics [1–3]. Relativistic effects
increase approximately with the square of the atomic num-
ber and are responsible for the distinct color of gold and
for the liquid state of mercury at room temperature. For
example, these effects stabilize s and p
1/2
orbitals, whereas
p
3/2
and d orbitals are destabilized in energy. This leads to
changes of the atomic ground state configurations com-
pared to the trend observed in the elements we find in
nature. In lawrencium (Z = 103), for example, a 7p
1/2
ground state is predicted instead of a 6d state [4]. At
some point, a deviation from the regular pattern that gov-
erns the ordering of elements in the periodic table is ex-
pected. Such deviations were predicted for the superheavy
elements copernicium (Z = 112) and flerovium (Z = 114)
in the mid 1970s [5]. According to Pitzer these elements
should already show properties of noble gases even though
the next nobel gas in a regular periodic table would be
oganesson (Z = 118).
Experimentally, atomic and chemical properties of the
heaviest elements have been mainly studied by chem-
istry techniques either in gas phase or in liquid phase [6–
8]. Copernicium was actually found to be a rather regu-
lar member of group twelve [9], while the properties of
flerovium are still a matter of ongoing research [10, 11].
An alternative approach to study atomic properties
of heavy elements is through laser spectroscopy. Well-
established techniques provide high-precision data on
atomic properties such as atomic level energies, lifetimes
and ionization potentials that allow us to benchmark theo-
e-mail: m.block@gsi.de
retical predictions and to probe the influence of relativis-
tic effects. Also, nuclear properties are reflected in the
atomic spectrum. This enables studies of the evolution
of nuclear structure features in a complementary way to
traditional nuclear spectroscopy. For example, hyperfine
spectroscopy give access to the nuclear spin and to nuclear
moments. The shift of an atomic transition in different iso-
topes allows determining changes in mean square charge
radii of nuclei and hence, the nuclear size and deformation
in a nuclear model-independent way. However, accurate
calculations of the hyperfine parameters A and B or the
field shift factor F are required inputs from atomic theory.
2 Resonant ionization laser spectroscopy
of heavy nuclides
The method of choice for optical spectroscopy of rare
isotopes is resonant laser ionization spectroscopy (RIS).
This technique features high sensitivity, in particular if the
laser-created ions are detected by their characteristic ra-
dioactive decay. The method can even be applied to atoms
where no experimental information on atomic transitions
exists, which had been illustrated in fermium [12] and,
more recently, in astatine [13], for example.
For more than a decade, fermium (Z = 100) was
the heaviest element that had been studied by laser spec-
troscopy [12]. A tiny quantity of 46 pg of the long-lived
isotope
255
Fm (T
1/2
≈ 20 h) was prepared in the high-
flux reactor at Oak Ridge National laboratory. This en-
abled offline experiments in which several atomic transi-
tions were identified. Good agreement with predictions by
multi-configuration Dirac-Fock (MCDF) calculations [14]
was observed. In addition, broadband spectroscopy of the
complex hyperfine spectrum of
255
Fm with a nuclear spin
EPJ Web of Conferences 163, 00006 (2017)
DOI: 10.1051/epjconf/201716300006
FUSION17
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).
of I = 7/2 was feasible and the hyperfine factors A and B
were obtained from a fit to the data [15].
The heavier members of the actinide series beyond fer-
mium have to be studied online. However, these elements
cannot be produced at isotope separator online (ISOL) fa-
cilities, but in fusion-evaporation reactions. This is exper-
imentally challenging due to low production rates on the
order of at most few particles per second. In addition, the
radionuclides have to be slowed down from tens of MeV to
rest and they have to be neutralized to perform RIS. This is
nowadays accomplished using gas cells filled with an inert
buffer gas like argon.
Si detector
Filament
Nobelium ions
Electrodes
Laser light
laser created ions
1
S
0
1
P
1
IP
l
1
l
2
Figure 1. Left: Schematic view of the buffer gas cell for laser
spectroscopy. Right: Simplified laser excitation scheme.
A dedicated RIS version, the so-called radia-
tion detected resonance ionization spectroscopy method
(RADRIS), has been developed for nobelium laser spec-
troscopy at GSI Darmstadt [16, 17]. The setup is schemat-
ically shown in 1. In the RADRIS method, nobelium ions
are separated from the primary beam by the separator for
heavy ion reaction products (SHIP [18, 19]) and slowed
down in 100 mbar ultra-pure argon gas. The fraction of
the stopped ions that remains in a charged state is accumu-
lated on a tantalum filament. Neutral nobelium atoms are
then evaporated by heating this filament. The atoms are
ionized in a two-step laser excitation scheme with pulsed
lasers. For the initial level search the first step is provided
by a tunable dye laser that is scanned across the spectral
region in which atomic states were predicted. The second
excitation step into the continuum is provided by a fixed-
frequency excimer laser (at 351 nm) featuring high power
to compensate the lower cross section of a non-resonant
excitation. The laser-created ions are detected by their
characteristic α decay registered by a silicon detector to
which the ions are transported by electric fields. Details on
the method and the employed setup have been described
elsewhere [20].
3 Scientific questions addressed by laser
spectroscopy
Detailed studies of the nuclear structure and its evolution
in regions of enhanced shell stabilization are of great im-
portance for a better understanding of the heaviest ele-
ments. Their very existence is intimately linked to nu-
clear shell effects. Ultimately, such investigations will
shed light on the nature of the underlying strong interac-
tion at one of the extremes of the nuclear chart. However,
the nuclides at the predicted spherical closed shells with
N = 184 and Z = 114, 120, or 126 [21] are experimen-
tally still inaccessible at present. The most neutron-rich
nuclide known to date contains only 177 neutrons [22]. In
addition, detailed studies of the elements with Z ≥ 110 are
hampered by limited statistics due to low cross sections on
a level of 1-10 picobarn.
Lighter nuclides in the region around Z = 100, N =
152 can be produced with higher rates that allow Penning
trap-mass spectrometry [23], laser spectroscopy [24] and
α
-γ spectroscopy [25]. Indeed, some of the single-particle
orbitals that are responsible for the spherical shell gap at
Z = 114 appear at low excitation energy in deformed nu-
clei around Z ≈ 100, N = 152. Thus, by fixing the position
of these orbitals at a given deformation allows discrimi-
nating models predicting the spherical gap. This region
also forms the backbone for spin assignments in heavier
nuclides in decay spectroscopy experiments based on sys-
tematics along isotones.
A complementary determination of nuclear spins by
laser spectroscopy is desirable. In addition, measure-
ments of dipole and quadrupole moments allow us to de-
termine single-particle configurations in specific nuclei.
This would, for example, provide an alternative to de-
termine the configuration of the long-lived K = 8
−
iso-
mer in
254
No for which different interpretations have been
put forward based on decay spectroscopy [26–30]. The
magnetic moment that could be measured by laser spec-
troscopy could answer the questions whether this isomer
is based on a quasi-neutron or quasi-proton configuration
that can both lead to K = 8
−
states at similar energy.
Nobelium atoms have a favorable atomic structure for
laser spectroscopy: the 5f shell is filled by 14 electrons
and the two valence electrons in the 7s orbital result in
a ground-state configuration [Rn] 5f
14
7s
2 1
S
0
. This fa-
cilitates accurate calculations of atomic properties. Mod-
ern calculations of the electronic structure in the heavi-
est elements require the explicit treatment of relativistic
and quantum electrodynamics effects. In addition, electron
correlations have to considered. This is accomplished em-
ploying many-body approaches like multi-configuration
Dirac-Fock or relativistic coupled cluster methods [1–
3, 14]. They can reach high accuracy with a typical pre-
cision of meV for level energies. The accuracy of such
calculations is often estimated by a comparison of cal-
culated properties to experimental data in the lanthanide
region since the available data for the heaviest elements
are limited. According to theoretical predictions nobelium
features a strong ground-state transition
1
S
0
-
1
P
1
around
30,000 cm
−1
[14, 31, 32].
4 Status and future perspectives
A practical advantage for studies of nobelium isotopes
around N = 152 is their rather high yield:
254
No can be
produced in the reaction
208
Pb(
48
Ca,2n) with a cross sec-
tion of about 2 µb corresponding to a yield of few particles
per second at an accelerator facility like GSI Darmstadt.
EPJ Web of Conferences 163, 00006 (2017)
DOI: 10.1051/epjconf/201716300006
FUSION17
2
of I = 7/2 was feasible and the hyperfine factors A and B
were obtained from a fit to the data [15].
The heavier members of the actinide series beyond fer-
mium have to be studied online. However, these elements
cannot be produced at isotope separator online (ISOL) fa-
cilities, but in fusion-evaporation reactions. This is exper-
imentally challenging due to low production rates on the
order of at most few particles per second. In addition, the
radionuclides have to be slowed down from tens of MeV to
rest and they have to be neutralized to perform RIS. This is
nowadays accomplished using gas cells filled with an inert
buffer gas like argon.
Si detector
Filament
Nobelium ions
Electrodes
Laser light
laser created ions
1
S
0
1
P
1
IP
l
1
l
2
Figure 1. Left: Schematic view of the buffer gas cell for laser
spectroscopy. Right: Simplified laser excitation scheme.
A dedicated RIS version, the so-called radia-
tion detected resonance ionization spectroscopy method
(RADRIS), has been developed for nobelium laser spec-
troscopy at GSI Darmstadt [16, 17]. The setup is schemat-
ically shown in 1. In the RADRIS method, nobelium ions
are separated from the primary beam by the separator for
heavy ion reaction products (SHIP [18, 19]) and slowed
down in 100 mbar ultra-pure argon gas. The fraction of
the stopped ions that remains in a charged state is accumu-
lated on a tantalum filament. Neutral nobelium atoms are
then evaporated by heating this filament. The atoms are
ionized in a two-step laser excitation scheme with pulsed
lasers. For the initial level search the first step is provided
by a tunable dye laser that is scanned across the spectral
region in which atomic states were predicted. The second
excitation step into the continuum is provided by a fixed-
frequency excimer laser (at 351 nm) featuring high power
to compensate the lower cross section of a non-resonant
excitation. The laser-created ions are detected by their
characteristic α decay registered by a silicon detector to
which the ions are transported by electric fields. Details on
the method and the employed setup have been described
elsewhere [20].
3 Scientific questions addressed by laser
spectroscopy
Detailed studies of the nuclear structure and its evolution
in regions of enhanced shell stabilization are of great im-
portance for a better understanding of the heaviest ele-
ments. Their very existence is intimately linked to nu-
clear shell effects. Ultimately, such investigations will
shed light on the nature of the underlying strong interac-
tion at one of the extremes of the nuclear chart. However,
the nuclides at the predicted spherical closed shells with
N = 184 and Z = 114, 120, or 126 [21] are experimen-
tally still inaccessible at present. The most neutron-rich
nuclide known to date contains only 177 neutrons [22]. In
addition, detailed studies of the elements with Z ≥ 110 are
hampered by limited statistics due to low cross sections on
a level of 1-10 picobarn.
Lighter nuclides in the region around Z = 100, N =
152 can be produced with higher rates that allow Penning
trap-mass spectrometry [23], laser spectroscopy [24] and
α
-γ spectroscopy [25]. Indeed, some of the single-particle
orbitals that are responsible for the spherical shell gap at
Z = 114 appear at low excitation energy in deformed nu-
clei around Z ≈ 100, N = 152. Thus, by fixing the position
of these orbitals at a given deformation allows discrimi-
nating models predicting the spherical gap. This region
also forms the backbone for spin assignments in heavier
nuclides in decay spectroscopy experiments based on sys-
tematics along isotones.
A complementary determination of nuclear spins by
laser spectroscopy is desirable. In addition, measure-
ments of dipole and quadrupole moments allow us to de-
termine single-particle configurations in specific nuclei.
This would, for example, provide an alternative to de-
termine the configuration of the long-lived K = 8
−
iso-
mer in
254
No for which different interpretations have been
put forward based on decay spectroscopy [26–30]. The
magnetic moment that could be measured by laser spec-
troscopy could answer the questions whether this isomer
is based on a quasi-neutron or quasi-proton configuration
that can both lead to K = 8
−
states at similar energy.
Nobelium atoms have a favorable atomic structure for
laser spectroscopy: the 5f shell is filled by 14 electrons
and the two valence electrons in the 7s orbital result in
a ground-state configuration [Rn] 5f
14
7s
2 1
S
0
. This fa-
cilitates accurate calculations of atomic properties. Mod-
ern calculations of the electronic structure in the heavi-
est elements require the explicit treatment of relativistic
and quantum electrodynamics effects. In addition, electron
correlations have to considered. This is accomplished em-
ploying many-body approaches like multi-configuration
Dirac-Fock or relativistic coupled cluster methods [1–
3, 14]. They can reach high accuracy with a typical pre-
cision of meV for level energies. The accuracy of such
calculations is often estimated by a comparison of cal-
culated properties to experimental data in the lanthanide
region since the available data for the heaviest elements
are limited. According to theoretical predictions nobelium
features a strong ground-state transition
1
S
0
-
1
P
1
around
30,000 cm
−1
[14, 31, 32].
4 Status and future perspectives
A practical advantage for studies of nobelium isotopes
around N = 152 is their rather high yield:
254
No can be
produced in the reaction
208
Pb(
48
Ca,2n) with a cross sec-
tion of about 2 µb corresponding to a yield of few particles
per second at an accelerator facility like GSI Darmstadt.
Applying the RADRIS method to the nobelium iso-
tope
254
No, the strong ground state transition
1
S
0
-
1
P
1
has
been searched for in a tedious effort [16, 17]. Such ex-
periments depend crucially on the guidance by theoretical
predictions. Different atomic transitions in the nobelium
atom have been predicted prior to the GSI experiment
[14, 31, 32]. The typical uncertainties and the scattering
of the predictions resulted in scan range on the order of
≥ 2, 000 cm
−1
that had to be covered. For a typical line
width of in-gas cell spectroscopy of about 0.5 cm
−1
[16]
this corresponds to a few thousand frequency steps for the
initial search. In the case of
254
No the cycle time for one
such step sums up to about five minutes equivalent to a
minimum search time of more than 200 hours to cover the
range of the predicted atomic transitions.
Nonetheless, the
1
S
0
-
1
P
1
transition as well as several
Rydberg states in the nobelium isotope
254
No have been
identified for the first time recently [33]. An overall ef-
ficiency about six percent was achieved using a two-step
laser excitation that could be boosted for a limited time by
conditioning the filament. Based on the observed Rydberg
series in
254
No the first ionization potential could already
be derived with rather high precision. However, quenching
collisions led to the population of a metastable state, most
likely the
3
D
1
state that is close to the
1
P
1
state that was ex-
cited by the first laser pulse. This behavior was explained
by a rate equation model that describes the experimental
data well and allows determining the energy of the
3
D
1
state indirectly [33, 34]. However, Rydberg states could
be excited by the second laser from either of the two states
resulting in different series. In 2016, different Rydberg
series were unambiguously identified, including a series
originating form the
1
P
1
state from which the first ioniza-
tion potential of nobelium could be determined with high
precision. The data analysis is close to completion and
the results will be published shortly. All in all, the results
for atomic properties of nobelium showed good agreement
with theoretical predictions [14, 31, 32].
The measurements were extended to the nobelium iso-
topes
252,253
No. The lowest yield was available for
252
No
with a cross section of about 400 nanobarn in the reac-
tion
208
Pb(
48
Ca,2n)
252
No. Decay losses on the filament
reduced efficiency to about three precent for this shorter-
lived nobelium isotope [33]. The isotope shift of the
1
S
0
-
1
P
1
transition between
252,253,254
No was measured and the
hyperfine splitting in
253
No was observed. The data analy-
sis is ongoing and the results will be subject of forthcom-
ing publications. The experiments will provide the change
of the nuclear charge radius between the isotopes
252,254
No
as well as the magnetic moment and the ground-state spin
assignment in
253
No.
Resonant laser ionization spectroscopy in a gas cell is
a powerful method to identify atomic transitions in rare
isotopes where little is known experimentally. However,
the line width that can be achieved with in-gas cell spec-
troscopy is limited by pressure and Doppler broadening
to typically about 5 GHz. In some cases higher resolution
spectroscopy is required, for example to resolve individual
hyperfine components for the determination of the mag-
netic moment. A higher resolution was recently demon-
strated with a new technique, the so-called gas-jet laser
spectroscopy [35]. The method was recently developed at
KU Leuven and was applied to actinium isotopes where a
gain in resolution by almost one order of magnitude com-
pared to in-gas cell spectroscopy without sacrificing the
high efficiency.
Future efforts will be dedicated to extend the reach of
laser spectroscopy even further towards heavier elements.
For a first foray into an unexplored area, a broadband level
search with high-power lasers in a gas cell, for example
with the RADRIS technique, will be favorable. Once suit-
able transitions have been identified, high-resolution spec-
troscopy can be performed by in-gas jet spectroscopy. In
any case the steeply dropping cross section for the produc-
tion of heavier elements are challenging. In this respect,
laser spectroscopy will profit from new powerful stable-
beam accelerators. Such machines that are anticipated to
deliver 10–100 times higher primary beam intensities are
presently planned or under construction in several facili-
ties worldwide [36–38].
For the first exploration of the element lawrencium,
the RADRIS method can be adapted in a straightforward
manner. A key question in Lr that can be answered by
laser spectroscopy concerns the atomic ground-state con-
figuration as discussed above. Theoretical models predict
a 7p
1/2
ground state, but the 6d state is rather close in en-
ergy [4]. Recent experiments performed in Tokai, Japan
used a surface ionization technique to determine the first
ionization potential of Lr [39]. Their result agrees with the
theoretical prediction well and confirms that Lr terminates
the actinide series. However, the uncertainty of the em-
ployed method does not allow distinguishing between the
two different ground state configurations unambiguously.
Thus, an experimental verification by laser spectroscopy is
still of interest.
Acknowledgements
The experiments on nobelium discussed in this contribu-
tion were performed by the RADRIS collaboration com-
prising scientists from GSI Darmstadt, Helmholtz Institute
Mainz, Mainz University, Technical University Darmstadt,
KU Leuven, University of Liverpool, and TRIUMF Van-
couver. Their contributions are gratefully acknowledged.
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