Laser Spectroscopy of Short Lived Rare Earth Isotopes
H. A. Schuessler*, F. Buchinger† , and H. Iimura ¶
*Texas A & M University, College Station, USA, †McGill University, Montreal, Canada
¶JAERI, Tokai Mura, Japan
Abstract. The ISAC facility at TRIUMF is able to produce intense beams of mass-separated radioactive isotopes of
the alkali and the rare earth elements with unprecedented intensities. This will allow our laser spectroscopy collaboration
to extend the studies of exotic nuclei for these elements further away from the valley of stability into regions so far
inaccessible at other facilities, thus allowing a more stringent test of the descriptive and predictive power of global
nuclear models.
Our effort, which involves groups and researchers from the U.S. (TAMU), Japan (JAERI), and Canada
(TRIUMF,McGill and Calgary Universities) is based on collinear laser spectroscopy in a fast beam. In this method,
atoms or ions are excited by laser light in collinear geometry. In this way measurements of the optical isotope shift yield
the nuclear charge radii and measurements of the hyperfine structure the nuclear moments. The excitation can be done
by cw-lasers or alternatively by pulsed lasers without a loss in sensitivity, if bunched radioactive ion beams are available.
In its basic version, the resonances of the spectral lines are observed optically at right angles. However, the sensitivity
will be increased through modifications using particle or nuclear radiation detection. The progress and the plans of our
collaboration will be described.
Figure 1 displays the <r 2> values in the rare earth
region, taken from the compilation of Goriely, Tondeur
and Pearson [3]. The latter values were calculated
using the Hartree-Fock-BCS method. In the same
region we also calculated <r2> values with the Finite
Range Droplet Model(FRDM)[4].
For these
calculations the deformation parameters were taken
from [5]. Both models give similar results and aim at
the predictions of nuclear ground state properties, such
as masses, charge radii and deformation parameters
over the complete nuclear chart. This offers an
internally self-consistent set of data, which then can be
used as an input for calculations involving isotopes
very far from stability, for instance as in the analysis of
isotopic r-process abundance [6] and the study of
proton radioactivity close to the drip-line.
In both calculations the development of a sudden
jump into deformation is observed around N=74 when
going from Z=56 (Ba) to Z=63 (Eu). The larger is the
Z of the rare earth element, the more pronounced is the
deformation. Even though Ba is not a rare earth
element, it was included as a reference element from a
neighboring region. This sudden onset of deformation
continues to be present in the predictions from the HFBCS calculations for higher Z as well. It disappears in
the Droplet predictions for Z=64, is again present for
Z=65 and disappears completely for higher Z’s. Recent
data on nuclear masses in the rare earth region obtained
by ISOLTRAP [7] show some indication of a nuclear
structure effect for Z=61 and N around 77. A
correlation of this effect with the expected onset of
strong deformation might be possible.
INTRODUCTION
Laser spectroscopy experiments on short-lived
isotopes have provided a wealth of information on
changes in the mean square charge radii and nuclear
moments [1]. Much of this information is used for
testing and improving nuclear models, which are used
to understand processes involving exotic nuclei or
exotic nuclear matter. We present here initial results
and plans of our laser spectroscopy collaboration at
ISAC (Isotope Separator and Accelerator) in TRIUMF.
COMPARISON OF THEORY
AND EXPERIMENT
Nuclear charge radii: The systematics of nuclear
charge radii reveal significant aspects of nuclear
structure and have been the main source of information
concerning the density of nuclear matter. Since charge
radii yield charge density distributions, they give
information on the Coulomb energy of nuclei and in
this way also on nuclear mass formulas [2]. Even
though not conceived for it, mass formulas take into
account shell structure, pairing effects and
deformability. In addition the systematics of charge
radii, when taken for long chains of isotopes reveal
also shell closures and shape transitions.
The
experimental data provided by on-line laser
spectroscopy can then test how well nuclear theories
will reproduce these observations.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
223
29
28
26
Z=66
65
64
63
62
61
2
2
R (fm )
27
25
24
Ba
La
Ce
Pr
Nd
Pm
60
Sm
59
Eu
Gd
23
58
Tb
Dy
22
57
56
50
N=82
60
70
80
90
100
110
120
N
FIGURE 1. Charge radii from HF-BCS calculations
In the calculations, the change of deformation on
the neutron deficient side is much more pronounced
than a similar change of deformation for the neutron
rich isotopes. For the latter region, both models prefer
a smoother development of deformation than expressed
in the experimental data, with some discontinuities
observed around N=85.
A considerable amount of experimental information
on changes in mean square charge radii <r2> is already
available for elements in the region between Z=56 (Ba)
and Z=71 (Lu) and is shown in Fig. 2. Many of the
measurements cross the neutron shell closure at N=82.
Others do not reach this shell closure. In the Z
direction the range of elements covers the proton
subshell closure at Z=64. Thus the available rare earth
element laser spectroscopy data is able to partly
document the influence of the interaction of two shell
closures on nuclear ground state properties.
For those elements where data is available across
the N=82 shell closure, the influence of the shell
closure on δ<r2> is well pronounced through a kink in
the slope of the mean square charge radii graph. The
slope decreases below N=82 with increasing Z,
reaching a roughly zero slope at Z=58. For Z=62 and
Z=60 a decreasing <r 2> is observed for increasing N.
Above N=82 the overall slope of <r 2> is roughly
constant for all elements with Z between 54 and 71.
224
31.5
30.5
29.5
Z=71
Ba
La
28.5
70
Nd
27.5
Eu
Gd
Dy
68
2
2
R (fm )
Sm
69
Ho
67
26.5
Er
66
25.5
63
62
24.5
Tm
65
Yb
64
Ce
Lu
Tb
60
58
57
23.5
56
N=82
22.5
60
70
80
90
100
110
120
N
FIGURE 2. Charge radii derived from the measured changes in charge radii in the region between Z=56 (Ba) and Z=71
(Lu). Isotope chains are offset for the sake of clarity
When the data is interpreted in the frame work of
the FRDM model, which decomposes the δ<r2> values
between isotopes into a volume (δ<r2>v ) and
deformation (δ<r2>d ) dependent part
the neutron rich side some abrupt changes are seen.
These abrupt jumps into deformation are observed
between N=88 and N=90 for elements with proton
numbers close to Z=64. In Figure 2 it is seen that the
nearer the proton number is to Z=64, the sharper is the
onset of deformation. This effect, which can be
attributed to the stabilizing influence on nuclear ground
state shapes from the Z=64 subshell closure is more
pronounced in the odd Z elements indicating an
additional influence of the odd proton configuration on
nuclear shapes.
δ<r2> = δ<r2>v + δ<r2>d
the deviation from a constant increase in δ<r2> with
N can be attributed to a change in deformation of the
nucleus. Whereas this deviation from spherical to
deformed shape occurs in a smooth way for N<82, on
225
test of these nuclear models for the neutron deficient
La isotopes as well as for the other isotopes in this
region.
Concerning nuclear deformation, the nuclear
quadrupole moments will provide an additional source
of information. The rapid changes of <r 2> in isotopic
chains should also be reflected in a change of the
quadrupole moments. However, in a recent laser
spectroscopy experiment a prominent breakdown of
this correlation was observed. An unexpected rapid
increase in <r2> was reported very close to the magic
N=82 [11]. In Yb (Z=70) the change of <r 2> between
N=82 and N=84 is much more pronounced than in the
elements between Z=56 (Ba) and Z=68 (Er). Through
the measurement of the quadrupole moment of 155 Yb
and the isotope shift between this isotope and 154 Yb it
was shown that this rapid increase is unlikely to be a
deformation effect. No alternative explanation was
proposed in this work, but these measurements clearly
indicate the importance of simultaneous measurements
of moments and isotope shifts for the characterization
of nuclear ground state properties. For the specific
case of Yb, it would be desirable to determine the
moments and change in mean square charge radii also
for 151-153Yb to see if the data is consistent with the
155
Yb measurements, and to collect additional clues for
finding an explanation for this highly unusual effect.
Also octopole effects are unlikely for these magic
(N=82) and near magic(N=81,83) nuclei.
Whereas the development of deformation on the
neutron rich side is fairly well documented by
experimental data and some trends predicted by the
models can be qualitatively recognized, for the light
rare earth elements more studies have to be carried out.
As can be seen, none of the measurements on the rare
earth elements crosses into the region of expected high
deformation below N=74, Nd being the element where
laser spectroscopy comes closest. For La, Ce, Pr and
Pm, no extended measurements were carried out so far.
In addition for Z>63 measurements beyond the neutron
shell closure at N=82 are not available, with the
exception of Z= 66 (Dy) where only one isotope (
146
Dy) beyond N=82 has been studied. For Z>68 this
shell closure is not even reached. For the element of
substantial interest to us, namely lanthanum, only three
isotopes have been studied, all of them in our recent
and in some earlier work [8]. However for this and the
lightest elements in the rare earth region one actually
has the best chance for reaching the unexplored
deformation region at neutron numbers below N=74.
We intend to exploit the capacity of ISAC for initiating
laser spectroscopy studies in this region. At the same
time an extension of the measurements across the
N=82 shell closure will also be possible for many of
the heavier rare earth elements.
Concerning the sharp onset of deformation around
N=74, some information is available from the reduced
electric quadrupole transition probability B(E2) of the
even Z elements [9]. The most complete data set can
be found for Ce, where B(E2) values are available for
all even neutron deficient isotopes between N=66 and
N=82. An abrupt increase in the otherwise smoothly
changing deformation from ß=0.195 (at N=76) to
ß=0.264 (at N=74) is found. For the heavier elements
Nd (Z=60) and Sm (Z=62)only a limited set of data
with no B(E2) values in the region close to N=74 is
available and no definite conclusion about the
character of the onset of deformation can be reached.
Nuclear moments: The triaxial degree of freedom
is not included in the FRDM and HF-BCS calculations.
However it might play an important role in the nuclei
of the low Z elements in the rare earth region around
A=130. For example, in 129 La, evidence for shape
coexistence and γ-softness has been gathered through
fusion-evaporation heavy-ion induced reactions and
beta-decay experiments. Such characteristics in this
region have been described successfully within the
interacting boson fermion model as well as in the rigid
triaxial rotor plus particle (RTRP) model [10]. For
these types of models the nuclear magnetic dipole and
electric quadrupole moments are commonly used as a
testing ground. The ground state and the isomeric
states of the La nuclei have not been measured so far,
and the lack of such measurements prevents a detailed
METHODOLOGY
Experimental details: We have chosen La in the
first round of experiments at ISAC, because we have
already done extensive spectroscopy on the stable 139
La and the long lived 138La, 137 La isotopes and recently
also on 135 La. Our aim is to extend these measurements
down to 128La. The radioactive lanthanum ions will be
formed in a tantalum target in a surface ionization
source. After mass separation the radioactive
lanthanum ions will enter our collinear fast ion-laser
apparatus at ISAC.
Fluorescence detection: We will perform
resonance excitation of La isotopes in a collinear
geometry with a dye laser beam, and fluorescence
detection at right angles using a photo multiplier. For a
measurement, we can either sweep the laser through
resonance at a fixed ion beam energy, or preferably we
keep the laser frequency fixed and Doppler-tune the
ion beam through resonance. The latter technique is
advantageous when multiple sweeps through resonance
are needed to improve the signal to noise ratio. For
Doppler-tuning, acceleration of the beam by about 4
keV will be effected immediately before the
fluorescence region.
226
La II
3
D
1eV
1
3
538nm
D
P
1
3
3
1
D
F
S
OPTICAL DETECTION
(metastable ionic states
thermally populated)
CHARGE
EXCHANGE
D
0eV
La +(5d2 F)
La I
FIELD IONIZATION
(even Rydberg states)
589nm
COLLISIONAL IONIZATION
(odd intermediate states)
357nm
(metastable atomic
states)
–5.58eV 2
Na
(∆E = 0.44eV)
Sr
(∆E = –0.12eV)
La(5d6s D)
FIGURE 3. cw laser detection schemes of short- lived La isotopes, (upper part) the fluorescence detection realized for La + ions.
(lower part) the ultra sensitive Stepwise excitation field ionization method proposed for rare La atoms
gated. The detection channel is only open when the
ions are in the detection region, resulting in an
important suppression of background. First
experiments of this type were recently carried out by
Billows[15] using the classical optical detection
scheme. But gated detection can also be used with any
other previously applied detection scheme, thus adding
to the versatility of collinear spectroscopy without
introducing restrictions. However, we expect that the
matching of pulsed ion beams with pulsed laser
excitation and particle detection schemes will lead to
an even more impressive gain in sensitivity. Resonance
ionization spectroscopy from the ground state or
metastable states populated in the charge exchange is a
universal excitation scheme which should be applicable
to the vast majority of elements. The schemes can
include a high resolution step using narrow bandwidth
pulsed excitation and might ionize the atom using laser
light exclusively or use field ionization of a laser
populated Rydberg state. In conclusion, we expect that
the adaptation of pulsed lasers for collinear laser
spectroscopy on pulsed beams with low energy spread
will allow a gain in sensitivity comparable to that
realized before by the introduction of particle detection
with CW-lasers. This additional feature of pulsed
spectroscopy on pulsed beams is non-restrictive. It can
be combined in a modular way with existing detection
schemes and will allow the addition of new ones.
Particle detection with cw-lasers: We have
developed a novel version of collinear fast beam laser
spectroscopy [12].
After near resonant charge
exchange it utilizes cascade two-step excitation by
narrow band cw-lasers to pump ground and /or
metastable atoms to a high lying Rydberg level, from
where the atoms are field ionized with near unity
efficiency. This scheme is applicable to all rare earth
elements using different cw-laser frequencies and is
depicted in Fig.3 for La as an example. It was
demonstrated for 85Kr, that ion currents of only about
103 ion/s gave good signals and that it has a selectivity
?m/m=10 -10.
Future particle detection with pulsed-and cwlasers: While all the above mentioned experiments
were carried out on dc-beams of radioactive isotopes,
the possibility of producing pulsed fast ion beams with
low energy spread will enable another important gain
in sensitivity of collinear laser experiments. The
production of such beams becomes possible by
implementing a buffer gas filled rf-quadrupole coolerbuncher, or an ion funnel, or an ion carpet in the
collinear fast beam laser spectroscopy set-up. These
devices, which meanwhile exist in several nuclear
spectroscopy laboratories were applied in combination
with fast collinear beams recently at JYFL(Jyvaskyla
Accelerator Facility)[13,14]. An rf- buncher is also
being built in connection with TITAN( TRIUMF's Ion
Trap Facility for Atomic and Nuclear Science) at
ISAC. An obvious advantage of using pulsed beams
compared to dc-beams is that the detection can be
ACKNOWLEDGEMENT
227
12. X. Li, J. Lassen, and H. A. Schuessler, “Highefficiency ultra trace detection of 85Kr,” in Methods
for Ultrasensitive Detection II (J.W. Wilkerson,
editor) Proceedings of SPIE Vol. 4634, 23-33
(2002).
13. M. Wada, Y. Ishida, T. Nakamura, Y. Yamazaki, T.
Kambara, H. Ohyama, Y Kanai, T. M. Kojima, Y.
Nakai, N. Oshima, A. Yoshida, Y. Matsuo, T.
Kuboy, and Y. Fukuyama, “Slow and trapped RIbeams from projectile fragment separators,” NIM,
accepted for publication 2002.
14. A. Niemen, J. Uikari, A. Jokinen, J. Aysto, P.
Cambell, E.C.A. Cochrane, “Beam cooler for low
energy radioactive ions,” Nucl. Instr. Meth. A469,
(2001).
15. J. Billows “Laser spectroscopy of radio isotopes
and isomers,” Nucl. Phys. A682 206C, (2001).
This work is supported by NSERC of Canada.
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