NUCLEAR STRUCTURE - IPN Orsay
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NUCLEAR STRUCTURE
The recent activity of the IPN Nuclear Structure group has been mainly focused on the
study of the structure of exotic nuclei following three main research axes: 1) the study of
the evolution of spherical shell structure of nuclei far from stability; 2) the study of weakly
bound states and nuclear resonances; 3) the study of the properties of nuclei involved in
nucleosynthesis processes and more generally of astrophysical interest. During the last
few years the rapid evolution and major breakthroughs realized in these fields of research
were made possible thanks to the availability of new radioactive beams both in France
and abroad. Our group has a decisive influence on this evolution: indeed, some of the
members of our group are involved in the management and the exploitation of one of the
major projects of the IPN: ALTO (Accélérateur Linéaire et Tandem à Orsay) which allows
in particular the production of very neutron rich nuclei from photofission using ISOL
(Isotopic Separation On Line) technique. The study of rarer and shorter-lived nuclear
structures requires also a constant improvement in the quality (in terms of efficiency,
resolution, granularity, dynamic range etc) of the detectors installed at different facilities.
Some of the members of our group also contribute to this effort by being involved or
leaders in the upgrade of one of the most heavily used particle detectors at GANIL
namely the telescope array MUST, through the MUST2 project. MUST2 has now entered
an active exploitation phase and is delivering quantities of data.
STRUCTURE NUCLÉAIRE
L’activité récente du groupe de structure nucléaire de l’IPN s’est concentrée autour de
l’étude des noyaux exotiques suivant trois grands axes : 1) l’étude de l’évolution de la
structure en couches sphériques des noyaux loin de la stabilité ; 2) l’étude des états
faiblement liés et des résonances nucléaires ; 3) l’étude des propriétés de noyaux entrant
dans les modèles de nucléosynthèse stellaire et d’intérêt astrophysique de manière
générale. Les progrès rapides et les découvertes les plus importantes dans cette
discipline qui ont eu lieu durant les quelques dernières années ont été rendus possibles
grâce la disponibilité de nouveaux faisceaux radioactifs tant en France qu’ailleurs. Notre
groupe est particulièrement présent sur ce front : certains chercheurs de notre groupe se
sont en effet plus particulièrement investis dans la gestion et l’exploitation d’un des
projets phares de l’IPN qui consiste à produire des noyaux riches en neutrons par
photofission en technique ISOL (Isotopic Separation On Line) qu’est le projet ALTO
(Accélérateur Linéaire et Tandem à Orsay). Par ailleurs, l’étude de structures nucléaires
de plus en plus rares et éphémères nécessite un progrès constant dans la qualité
(efficacité, résolution, granularités, dynamique électronique etc) des détecteurs mis en
œuvre auprès des accélérateurs. Dans ce domaine également l’effort de notre groupe est
notable : certains membres du groupe ont consacré beaucoup d’efforts dans
l’amélioration et l’optimisation d’un des détecteurs à particules chargées les plus utilisés
auprès du GANIL qu’est l’ensemble de télescopes MUST au travers du projet MUST2,
qui est entré dans une phase d’exploitation très active et livre quantité de données.
8
8
Measurements of production yields, for mass
mass--separated fission
fragments, at ALTO
IPNO Participation: M. Lebois, M. Cheikh Mhamed, J. M. Curaudeau, M. Durcourtieux, S. Essabaa, S.
Franchoo, S. Galès, D. Guillemaud-Mueller, F. Ibrahim, C. Lau, J. Lesrel, A. Mueller, M. Raynaud, B.
Roussière, A. Said, D. Verney, C. Vogel and the ALTO collaboration
Mesure des taux de production des fragments de fissions séparés en masse auprès d’ALTO
Les taux de production d’isotopes riches en neutrons, produit par photofission, ont été mesurés auprès de
l’installation de type ISOL: ALTO (Accélérateur Linéaire et Tandem d’Orsay). La mesure a été réalisée en
étudiant les décroissances β des isotopes produits, puis extraits de la cible de carbure d’uranium (UCx),
puis ionisés et enfin séparés en masse par le séparateur isobarique PARRNe2. L’identification a été
obtenues par des coïncidences β−γ. Par la suite, nous nous contenterons de restreindre la présentation
des résultats à une chaîne isotopique d’intérêt. En parallèle, sont exposés les résultats issus d’une
estimation empirique des taux de production d’ALTO basée sur les données expérimentales obtenues
auprès de PARRNe, de même que les résultats issus d’une simulation FLUKA. L’analyse de ces données
nous a permis d’obtenir la confirmation des performances, attendues, de l’installation ALTO en terme de
taux de production (~1011fissions/s).
As a participation to the SPIRAL2 project, the ion source. The photons of the bremsstrahlung
Institute of Nuclear Physics of Orsay initiated the
construction of a new facility called ALTO
(Accélérateur Linéaire et Tandem d’Orsay). This
ISOL type facility has the particularity to use
electrons, as a primary beam, to induce the
photofission of uranium. In comparison with the
previous installation (1µA deuton beam at 26 MeV
provided by the Tandem at Orsay), the use of an
electron beam of 10µA at 50 MeV was expected to
improve the effective yields by a factor of hundred.
And casually, during the exploitation of the
deuteron based R&D PARRNe (Production
d’Atomes Radioactifs Riches en Neutrons) setup,
the production yields were already high enough to Fig. 1: Overview of the ALTO facility
perform β-decay experiments in order to explore
the neutron rich mass region around N=50 [1,2]. induce the fission of the uranium atoms located in
Obviously, with its hundred times better yields, the target. The target is heated to a temperature of
ALTO will, in the near future, open new 2200° to allow a faster release of the fission
perspectives to pursue this scientific program. But fragment in the UCx.
first, these better yields had to be confirmed. With the nominal energy and intensity for the
The ALTO Facility electron beam, we expect 1011 fissions per second.
The ALTO facility is based on the use of the former Fission fragments, after their diffusion/effusion in
first section of the LEP injector as an electron the target, are ionized and extracted with a 30kV
driver. This section can provide an electron beam voltage. Finally at the exit of the source the fission
at the energy of 50 MeV with a nominal intensity of fragment are magnetically mass separated and
10µA. The beam is focused sent to the measurement point.
and conducted tu a thick target The Production Yields
of uranium carbide (UCx). The Measurements
electrons are converted to During all the experiment, the
p h o t o n s t h r o u g h intensity has been limited to 100nA.
bremsstrahlung process With such conditions yields similar to
essentially in the first part (few what have been obtained with the
cm) of the target. The UCx 1µA deuteron beam during the
target can be associated with PARRNe experimental program
various types of ion source like were expected [6].
hot plasma, surface ionization The measurement itself consists in
[3,4] or laser. The the observation of the β-decay of all
measurements reported here Fig. 2: Overview of the experimental the elements coming from the mass
were performed with a MK5 setup used for the production yields separated beam. To do so, the beam
ISOLDE type [5] hot plasma 9 is collected on a mylar tape for a
9
definite time that we called collection time. This triggered by photons at the very entrance of the
collection point is surrounded with a 4πβ scintillator target. The consequence is the increase of release
and a germanium detector set in the closest time. And for the shortest half-life members of an
geometry. The isotopic chain of an
collection time is element with a high
optimized for each release time this
isobaric chain in means that all the
order to enhance isotopes that are
the activity of produced decay in
interest. The the target before
production yields at being released. To
masses 78 to 95, fully understand the
117 to 144 were different processes
measured. The that take place in
masses 96 to 116 the target, a
were not studied complete FLUKA
because they c a l c u l a t i o n
mainly correspond concerning all the
to “refractory isotopes would be
elements” i.e., necessary. Second,
e l e m e n t s Fig. 3: Production Yields at the ALTO facility like in the Tin
presenting either for the isotopic chain of Tin. isotopic chain, we
high boiling point, pointed out the
or a great chemical affinity with the target sensitivity of the production method to the details
compound. We used γ rays spectra and γ−β of the nuclear structure near the magical number
coincidences to determine the production yields for Z=50, and this sensitivity is missing in the different
ALTO. simulations. The main conclusion of these
measurements is, ALTO with 100 nA of electron
Experimental Results beam, is already able to provide similar yields as
The figure 3 presents some of the results obtained PARRNe (1µA deutons). And we can now affirm,
during the yields measurements. It gives the thanks to the empirical estimations, that the yields
values of the production yields measured for the for ALTO, with the nominal value of 50 MeV and
Tin isotopic chain. The production yields measured 10 µA for the electron beam, will be two orders of
with ALTO (filled square) is quite similar to those magnitude higher than those obtained at PARRNe.
previously obtained from neutron-induced fission
(triangle-up). On this figure are also presented the
results (empty circles) obtained from an empirical
estimation for the ALTO yields realized with the
11
hypothesis of 10 fissions and the use of References :
measured cross sections for neutrons of 14 MeV
[7]. The results of a FLUKA monte-carlo simulation [1] O. Perru et al., Eur. Phys. J. A 28 (2006)
[8] is also presented (triangle down). This 307.
simulation takes into account the physical process [2] D. Verney et al., Phys. Rev. C 76 (2007),
leading to the apparition of the two fission fragment 054312
in the target. One can notice that both of the [3] U. Köster et al., “Fission Yields
simulation or the estimation are quite accurate. But Measurements with the ISOL Method”,
FLUKA gives the best agreement with the Proceeding (2003)
measured values. However, a strong odd-even [4] C. Lau et al., Nucl. Instr. And Meth. B 204
staggering can be noticed. This could be due to (2003) 257
the fact that, even in the case of ISOL based [5] S. Sundell, H.L. Ravn and the ISOLDE
production technique, the effective yields could be collabotration, Nucl. Instr. And Meth. B 70
influenced by nuclear structure. It is expected (1992) 160
indeed that in the case of a cold fission process [6] C. Lau et al., Nucl. Instr. And Meth. B 204
the cross section follows closely the magic (2003) 246
number. If so, such a structure effect has been [7] B. Roussière, private communication.
clearly underestimated in the FLUKA simulation. [8] M. Cheikh Mhamed, Thesis, Université
d’Evry Val d’Essonne (2006) et references
Conclusions
incluses
For this brief report I had not the occasion to give
an exhaustive analysis of all the results gathered
during the yields measurement. But the same type
of study has been conducted for nuclei with
different chemical properties and the result leaded
to several conclusions. First, we manage to
demonstrate that the main part of the fissions were 10
10
First measurement of isoscalar giant
resonances in a short
short--lived nucleus : 56Ni
IPNO Participation: Monrozeau C., Khan E., Blumenfeld Y., Beaumel D., Ebran J.P., Frascaria N.,
Gupta D., Maréchal F., Scarpaci J-A.
Collaboration : Demonchy C.E., Gelin M., Mittig W., Roussel-Chomaz P. (GANIL), Caamaño M.,
Cortina-Gil D. (Univ. Santiago), Garg U. (Univ. Notre Dame), Gillibert A., Keeley N., Obertelli A.
(SphN)
Les résonances géantes monopolaires et quadrupolaires, de type isoscalaire, ont été mesurées pour la
première fois dans un noyau instable, le 56Ni. Celle nouvelle méthode utilise la cible active MAYA, qui
permet de concilier les contraintes dues aux faisceaux exotiques d’une part, et à la cinématique particulière
associée à la résonance géante monopolaire d’autre part.
Measurements of Giant resonances in unstable GMR and GQR are isoscalar probes such as
nuclei, although of major scientific importance, deuterons or alpha particles at energies between a
have up to now been very scarce due to a myriad few tens and a few hundreds AMeV. Essentially no
of experimental difficulties related in particular to work had been done for unstable nuclei due to the
low beam intensities and unfavourable conditions very unfavourable conditions in reverse
in inverse kinematics. The data available today kinematics. Indeed, the GMR cross section peaks
have been obtained in Coulomb Excitation at 0° in the centre of mass frame which gives rise
experiments at GSI and RIKEN and concern to very low recoil velocities for the light probe.
mainly E1 transitions in neutron-rich Oxygen, Neon
To measure the excitation energy range between 0
and Tin isotopes. The analysis demonstrates the
to 30 MeV in reverse kinematics, it is necessary to
presence of low-lying “pygmy” strength, but the
detect the recoiling particle (d or alpha) with
data on the “normal” IVGDR, if they exist at all,
energies ranging from a few hundreds keV to 2
have very low statistics due to the low number of
MeV at angles from 0 to 40° in the laboratory
frame. A standard set-up with a recoiling particle
telescope such as MUST2 would necessitate a
very thin target ( CD2 of 100μg/cm2) to minimize
straggling and thus require an intensity of over
107pps which is prohibitive for current radioactive
beam facilities. With respect to these experimental
constraints we have demonstrated that an active
target such as Maya [1] is the key to measuring the
GMR in unstable nuclei. An active target is a
detector in which the detector gas also acts as
target. Such a set-up has in principle an angular
coverage close to 4π and a large effective target
thickness. We have performed inelastic deuteron
scattering on the unstable 56Ni nuclei at the GANIL
facility [2].
The experiment was performed with a secondary
56
Ni beam at 50A MeV produced in SISSI. The
MAYA detector, filled with deuterium gas at 1050
mb pressure, was placed on the SPEG beamline.
A beam intensity of approximately 104 pps was
Fig 1: Excitation energy spectrum of 56Ni obtained
used during 15h of effective data taking. Figure 1
in the reaction 56Ni(d,d’)
shows the inelastic spectrum which exhibits a clear
giant resonance bump.
virtual photons around 20 MeV energy. In the case The bump was decomposed into two Gaussian
of the GMR and the GQR, there are also distributions which exhibit characteristic angular
predictions of soft modes, but no data has been distributions for L=0 and L=2 excitations, shown on
measured yet. Fig.2. The percentage of Energy Weighted Sum
Rule exhausted for each transition was found close
Extensive studies with stable nuclei have shown to 100%, in agreement with results for
that the best probes for the investigation of the 11 neighbouring stable 58Ni. These results validate
11
this original method, which is the first to allow the
measurement of isoscalar giant resonances in
unstable nuclei, and which can now be confidently
applied to cases farther from stability.
References
[1] C.E. Demonchy et al., NIM A573 145 (2007)
[2] C. Monrozeau, Thèse, Université Paris Sud
(2007); report IPNO-T-07-06 ; C. Monrozeau
et al., Phys. Rev. Lett. 100, 042501 (2008)
Fig 2: L=0 and L=2 angular distributions obtained
in the reaction 56Ni(d,d’)
12
12
Decay pattern of pygmy states in the neutron
neutron--rich nucleus 26Ne
IPNO Participation: J. Gibelin, D. Beaumel, Y. Blumenfeld, S. Fortier, N. Frascaria, V. Lima,
J. A. Scarpaci
Collaboration : Rikkyo University (Japan), RIKEN (Japan), University of Tokyo (Japan), ATOMKI
(Hungary), Tokyo Institute of Technology (Japan), Kyushu University (Japan)
Nous avons effectué l’excitation Coulombienne sur une cible de plomb, d’un faisceau exotique à 58 MeV/
nucléon de 26Ne, noyau riche en neutrons, afin d’étudier la possible existence de résonances dipolaires
pygmées au-dessus du seuil d’émission neutron. L’expérience a été conduite auprès de l’accélérateur de
l’institut RIKEN, à Tokyo (Japon) et incluait un détecteur de gammas, un hodoscope pour particules
chargées et un détecteur de neutrons.
A l’aide de la méthode de la masse invariante appliquée dans la voie de décroissance 25Ne+n et des
distributions angulaire de diffusion du 26Ne nous observons de la force de moment angulaire égal à un
entre les seuils d’émission un neutron et deux neutrons et extrayons sa valeur de probabilité de transition.
Notre méthode nous permet aussi d’accéder pour la première fois aux rapports d’embranchement neutron
de la décroissance d’une résonance pygmée.
How the Giant Dipole Resonance strength evolves provided a 3% (FWHM) resolution on the
when going from stable to weakly bound nuclei remaining energy (E). Unambiguous mass and
with extreme neutron-to-proton ratio is a topical charge identification of all projectile like fragments
question. Recent theoretical approaches based on was obtained using the E-ΔE method. In-beam
mean field calculation predict that in the neutron gamma rays were detected using a 4p gamma-
rich 26Ne almost 5% of the Thomas-Reiche-Kuhn detector, DALI2 [4], which consisted of 152 NaI
(TRK) energy weighted sum rule is exhausted by detectors placed around the target. For 1.3 MeV γ
strength centered around 8 MeV. This region of rays, its measured efficiency is approximately 15%
energy is located between the one neutron and the with an energy resolution of 7% (FWHM). The
two neutron emission thresholds. In order to Doppler corrected gamma energy distribution
investigate this prediction, we performed Coulomb obtained in coincidence with the 25Ne isotope
excitation of 26Ne at intermediate energies on a allows us to identify the gamma decay from the
lead target and used the invariant mass method to adopted 1702.7(7), 2030(50) and 3316.4(11) keV
reconstruct the B(E1) strength. excited states.
The hodoscope for neutron detection was an array
Experimental details
of 4 layers of 29 plastic rods each, placed 3.5 m
The experiment was performed at the RIKEN
Accelerator Research Facility through downstream of the target. Each layer was
fragmentation of a 95 MeV/nucleon 40Ar primary composed of 13 [2.1mx6x6cm2] and 16
[1.1mx6x6cm2] rods, arranged in a shape of a
beam on a 2-mm-thick 9Be target. The 26Ne was
separated by the RIKEN Projectile Fragment
Separator (RIPS) [2]. Particle identification was
unambiguously performed by means of the time-of-
flight (TOF) and the purity was 80%. The 26Ne
beam of intensity, 5.103 pps and incident energy
58 A.MeV, was tracked with two parallel-plate
avalanche counters providing incident angle and
hit position onto the target. It then impinged
alternatively on a 230 mg/cm2 natPb and a 130 mg/
cm2 natAl target. Data obtained with aluminium
target are used in the following to subtract, from
lead target data, components other than
E1excitation in the excitation energy spectrum.
Fig. 1: 26Ne excitation energy spectrum, on lead
The outgoing charged fragments were measured target.
using a set of telescopes placed at 1.2 m upstream
cross. Its total intrinsic efficiency for the detection
of the target. They consisted of two layers (X and
Y) of 500 um silicon strip detectors (SSD) with of one 60 MeV neutron was calculated to be 25%.
5 mm strips which provided an energy resolution of Finally 29 thin plastic rods covered the front face of
the wall in order to veto charged particles as well
2% (FWHM). The last layer used 3-mm-thick Si(Li)
from the charged-particle detector MUST [3], as to provide an active beam stopper. The neutron
13 position is determined with an error of ±3 cm and
13
the energy, from TOF information, with a 2.5 MeV Exp. branching ratios
Final 25Ne state
(FWHM) precision for the neutrons of interest. [%]
Pb
Results Energy Pb
Jπ Pb (L=2)
The excitation energy spectrum of 26Ne [MeV] (L=1)
=Al
reconstructed for the 25Ne+n decay channel
obtained with the natPb target is represented on 0.0 1/2+ 5+17-5 5+32-5 4+5-4
Fig. 1. The method used to perform this 1.7 & 2.0 5/2++3/2+ 66±15 42±30 95+5-15
reconstruction takes into account all possible 3.3 (3/2-) 35±9 60±17 5+6-5
excited states of the 25Ne and allows us to deduce
the corresponding decay branching ratios. Table 1: Experimental neutron branching ratios for
Between 8 and 10 MeV, a sizable amount of cross- the structure at Eכ9 MeV in 26Ne to the 25Ne
section is observed. Note that above S2n = states.
10 MeV, the decay of 26Ne is expected to occur
mainly by 2 neutron emission, which explained the the 25Ne ground state, which is in contradiction
abrupt decreasing. In intermediate energy inelastic with the predicted structure of the pygmy states.
scattering with a heavy target such as lead, the Indeed, it is established that the 25Ne ground state
Coulomb dominance of the E1 excitation is well- configuration mainly corresponds to a neutron in
known. the 2s1/2 orbit. If, as predicted by most of the
The contribution of possible E2 excitation to the calculation, the main configuration of the pygmy
spectrum obtained with the lead target is state were actually ν(2s−11/2 2p3/2) or ν(2s−11/22p1/2)
determined by means of angular distribution using a strong decay to the 25Ne ground-state should
data in lead target only. The result of the fit of this occur. This discrepancy indicates that the
distribution using ECIS 97 calculation is presented populated pygmy states are more mixed and/or
in Fig. 2. We deduce that 4.9±1.6% of the TRK is involve different transitions. Interestingly,
exhausted by our structure centred at E* = 9 MeV, calculations reported in [7] predict a dominant
contribution of the K=1− state, with nearly equal
weights of ν (2s −11/22p1/2) and ν (1d−15/21f7/2)
transitions which is in better qualitative agreement
with our data. Theoretical branching ratios,
presently not available, are highly desirable for a
more definite comparison. The extraction of such
branching ratios would be of great interest.
In summary the present study of the neutron rich
nucleus 26Ne using intermediate energy inelastic
scattering has shown the presence of pygmy
states located around 9 MeV excitation energy.
The contribution of E1 states corresponds to nearly
5% of the TRK sum rule. These global features are
in agreement with self consistent mean-field
calculations performed in various frameworks. The
Fig. 2: Angular distribution of 26Ne between one decay pattern of the observed pygmy states has
neutron and two neutrons emission threshold. been measured
which correspond to a B(E1) = 0.49±0.1 e2fm2, in for the first time, providing a stringent test of the
agreement with Cao and Ma calculations [1] in the microscopic models describing the wave function
framework of the QRRPA. Other models [5, 6, 7] of these states. The measured decay pattern is not
predict similar results for the existence of a low- consistent with models predicting a structure
lying state. However they are divergent on the corresponding to excitations of neutrons from the
collectivity and the configurations involved. Fermi surface.
It is well-known that the decay pattern of References
continuum states can give access to the [1] L.-G. Cao, and Z.-Y. Ma, Phys. Rev. C, 71,
components of the wave-function of these states. 034305 (2005).
The excitation energy reconstruction method used [2] T. Kubo et al., Nucl. Ins. and Meth. B, 70,
in the present experiment allows us to extract for 309-319 (1992).
the first time data on the decay of pygmy [3] Y. Blumenfeld et al., Nucl. Ins. and Meth. A,
resonances of neutron-rich nuclei. The 421, 471-491 (1999).
experimental branching ratios to bound states of [4] S. Takeuchi et al., RIKEN Accel. Prog. Rep.,
25
Ne are presented in Table I. For both Pb and Al 36, 148-149 (2002).
targets, the branching ratio for the decay to the [5] D. Arteaga and P. Ring, Prog. in Part. and
ground-state of 25Ne is compatible with zero. The Nucl. Phys. 59, 314 (2007), and Private
large difference between branching ratios obtained communication.
with the two targets proves that states of different [6] S. Peru et al., Nucl. Phys. A 788, 44 (2007).
nature have been excited. A striking feature of the [7] K. Yoshida and N. V. Giai, arXiv:nucl
observed decay pattern is the absence of decay to 14
14
Lifetime measurement of the six
six--quasiparticle isomer in 140Nd
and of the three
three--quasiparticle isomer in 139Nd
IPNO Participation: M. Ferraton, R. Bourgain, C.M. Petrache, D. Verney, F. Ibrahim, N. de Séréville,
S. Franchoo, M. Lebois, C. Phan Viet, L. Sagui, I. Stefan, J.F. Clavelin, M. Vilmay
Les temps de vie des isomères découverts récemment dans les noyaux 140Nd et 139Nd ont été mesurés en
utilisant la réaction de fusion-évaporation 126Te(18O, xn) et la technique du faisceau pulsé auprès du
Tandem de l’IPN Orsay. Le temps de demi-vie obtenu pour l’isomère du 140Nd est de 1.23 (7) µs, une
valeur en accord avec le spin-parité 20+ attribué à cet état, interprété comme une configuration sphérique
de six quasiparticules complètement alignées. Le temps de demi-vie déduit pour l’isomère du 139Nd est de
272 (4) ns. On attribue un spin-parité 23/2+ à cet état isomérique, qui est interprété comme une
configuration [ν d3/2 -1 h11/2-2]23/2+ complètement alignée.
The nuclei around the N = 82 shell closure whose Events were stored on hard disk using the
configurations can be considered relative to a NARVAL software. The data acquisition was based
146
Gd (Z = 64, N = 82) core are a fertile field of on COMET-6X cards, used as high resolution
spectroscopic investigations both at low and high ADCs. Each COMET-6X is made of a 40 MHz, 32
spins. At low spins the presence of isomers based bits DSP, and builds events consisting of the
on simple particle-hole excitations helps to absolute detection time coded on 47 bits with a
establish the active quasiparticle configuration in a time resolution of 400 ps, the measured energy
specific nucleus and test the suitability of various coded on 15 bits and the bit pattern of the
nuclear potentials, whereas at high spins, the detection channel. The coincidences between the
combined contribution of neutron holes in the N = input signals were then established off line.
82 core and neutron particles in the high-j orbitals The events were stored in γ-γ 2-dimensional
above the N = 82 gap drive the nuclear shape
matrices, as follows : γ-γ coincidences with a
toward a stable triaxial shape with γ ≈ + 30° [1,2]. prompt time gate, in order to study the cascades
Recently published experimental data on 140Nd,
above the isomers, γ-γ coincidences with a
which has two neutron holes in the N = 82 shell,
delayed time gate to study cascades below the
show several isomeric states, with configurations
isomers, and prompt-delayed coincidences to
of up to 6-quasiparticles [3]. In 139Nd, the existence
study connexions between the cascades above
of isomeric states was reported long time ago [4],
and below the isomers.
but their lifetimes and positions in the level scheme
To deduce the lifetimes of the isomers, we
could not be established. In the present
contribution we report results on the lifetime of the
isomeric states in 140Nd and 139Nd obtained in a
pulsed beam experiment performed at the Tandem
accelerator of IPN Orsay.
High-spin states in 140Nd and 139Nd were populated
in the 126Te(18O,xn) reaction, with a pulsed 18O
beam of 75 MeV. The 400 µg/cm² 126Te target was
deposited on a 10 mg/cm² gold backing. Double γ-
γ coincidences were detected using four Compton-
suppressed Ge detectors which were positioned in
the horizontal plane, around the reaction chamber,
at angles of ± 45° and ± 135° with respect to the
beam axis.
The pulsed beam was obtained using a chopper-
buncher system, providing Gaussian ion bunches
with a 1.8 ns FWHM, and a 5 ns FWTM. In order
to allow the measurement of lifetimes up to a few
microseconds, a repetition rate of 10 µs was Fig. 1. Zoom on the Eγ - tγ showing the tran-
chosen. We have measured the energy and sitions of interest to determine the lifetime
detection time of the emitted photons with respect of the 20+ isomer in 140Nd.
to the beam pulse. 15
15
incremented Eγ - tγ matrices (see Fig. 1), on which The 139Nd nucleus
we were able to select through contour lines the The transitions in 139Nd below the 19/2+ state have
transitions of interest and project them on the time a prompt as well as a delayed component, induced
axis. by an isomeric state. We did not observe
transitions directly populating the 19/2+ state,
which indicates that the excitation energy of the
isomer is less than 40 keV above the 19/2+ state at
Ex = 2572 keV.
By an exponential fit of the time spectrum of the
1071 keV (17/2- → 15/2-) transition, we obtained a
half life of 272 (4) ns (Fig. 3).
Comparing with the Weisskopf estimates for a 40
keV transition of various electromagnetic
characters, we deduced that the spin-parity of the
isomeric state must be 23/2+. Such a state can be
understood either as a h11/2-1 neutron-hole coupled
to the 7- isomer of 140Nd having a ν d3/2-1 ⊗ h11/2-1
configuration, or as neutron-hole d3/2-1, coupled to
the 10+ isomer of 138Nd, which has a ν h11/2-2
configuration.
Fig. 2. The summed time spectra of the 120,
182, 188 and 258 keV transitions from the two References
main cascades de-exciting the 20+ isomer in
140
Nd. [1] C.M. Petrache, et al., Phys. Rev. C61 (1999)
011305.
[2] C.M. Petrache, et al., Phys. Rev. C72 (2005)
140 064318.
The Nd nucleus [3] C.M. Petrache, et al., Phys. Rev. C74 (2006)
The lifetime we deduced for the 20+ isomer in 034304.
140
Nd is 1.23 (7) µs. It was obtained from the time [4] M. Müller-Veggian, et al., Nucl. Phys. A344
spectra of the clean 120, 182, 188 and 258 keV (1980) 89.
transitions from the two main cascades de-exciting
the isomer (see Fig. 2). This value is in good
agreement with the Weisskopf estimates of the
partial half-lives of the transitions de-exciting the
isomer, giving a strong support to the 20+ spin-
parity assignment. The existence of the 20+ isomer
strongly supports the spherical and triaxial shape
coexistence predicted by the cranked-Nilsson-
Strutinsky model for medium-spin states in nuclei
around to the N = 82 shell closure [2,3].
Fig. 3. Time spectrum of the 1071 keV transi-
tion used to determine the lifetime of the 23/2+
isomer in 139Nd.
16
16
Neutron correlations in 6He viewed through nuclear break
break--up
reactions
IPNO Participation: M. Assié, J.A. Scarpaci, D. Beaumel, Y. Blumenfeld, M. Chabot, H. Iwazaki, F.
Maréchal, C. Monrozeau, F. Skaza, T. Tuna
Collaboration : University of Camerino, GANIL, LPC, NSCL/MSU, SUBATECH, University of Surrey,
Uppsala university
Les corrélations dans le halo de l’6He ont été étudiées à travers le break-up nucléaire et la mesure des
neutrons de 0 a 110 degrés. Les deux neutrons ont été détectés en coïncidences avec les 4He. Le spectre
des angles relatifs entre les neutrons montre clairement une corrélation entre les neutrons compatible avec
ce qui est attendu du break-up d’un état di-neutron et cigare.
are close together (di-neutron) and one where the
Introduction
two neutrons are on opposite sides with respect to
Strong interest has developed in two-neutron halo
systems especially in their correlations [1]. In a the core (cigar)[4]. This work concluded for a large
re-
Fig.1: Sketch of the nuclear break-up for a di-neutron and a cigar configuration. Right
panel is the result in term of the relative angle between the two neutrons.
cent experiment on the break-up of 11Be,
we have studied large angle neutron
emission [2] which not only confirmed
that nuclear break-up mainly leads to
emission of particles at large angles but
also that nuclear break-up can be used
to extract spectroscopic information
through the so-called Towing Mode
mechanism. We want to extend the use
of this mechanism to the study of two
neutron halo the borromean nuclei. In-
deed, neutron correlations in these nu-
clei have been studied through interfer-
ometry [3] to try to access correlation
properties of these nucleons inside the
halo. In the ground state of 6He, the co-
Fig. 2 : Experimental set-up. The neutron wall is de-
existence of two configurations is pre-
picted in yellow and the ensemble EDEN is
dicted: one in which the two neutrons
17
shown in blue.
17distance between the two neutrons forming the gles. A calculation using the time dependent
halo of 6He, pointing towards a cigar pattern. Schrödinger equation and a two-neutron wave
In a naïve description of the nuclear break-up, we function is in progress. The evolution of this wave
expect, as illustrated in Fig.1, a small relative angle function as the break-up occurs will be calculated
between the two neutrons if they were initially in a for the two configurations (di-neutron and cigar)
di-neutron configuration. Contrarily, in the case of and the correlation function will be compared to the
a cigar configuration, whenever one of the neu- data from which the percentage of each configura-
trons feels the nuclear potential of the target the tion will be extracted.
other one does not feel it and should emitted at a
low angle giving rise to a large relative angle. This References :
nuclear break-up is sketched in Fig.1 and the ex- [1] H. Sagawa and K. Hagino, Proceedins of Inter-
pected relative angle is presented in the right national Symposium on Physics of Unstable Nuclei
panel. (Hoi An, Vietnam, 2007).
The experiment [2] V.Lima et al., Nucl. Phys. A795, (2007), p1-18.
In this new experiment performed at the GANIL [3] F.M.Marqués et al., Phys. Rev. C, 64 (2001)
facility, we have measured the two emitted neu- 061301(R).
trons of the break- [4] M.V. Zhukov et al, Phys. Rep. 231, 151 (1993).
up of 6He in a large
angular domain,
between 10 and
110 degrees, in co-
incidence with a 4He
ejectile detected in
a phoswhich detec-
tor composed of a
500 µm silicon
stripped detector
and a 3 mm thick Si
-Li (see set-up in
figure 2) and cover-
ing angles between
8 and 20 degrees.
Relative angles
The relative angle
between the two
detected neutrons is
plotted in Fig.3. The
uncorrelated distri-
bution was deduced Fig.3 : Relative angle between the two neutrons. Purple histogram is the
by mixing up events measured distribution whereas the red curve is the result after
(taking one neutron subtraction of the uncorrelated distribution of the mixed events
in a given event and
the second neutron in another
event) and is presented as a blue
histogram. It shows a big differ-
ence with the originally meas-
ured distribution. This evidences
a strong correlation between the
two neutrons that can be shown
either by subtracting the mixed
event distribution from the ex-
perimental distribution as shown
in red in Fig.3 or by constructing
the correlation function which is
the ratio of the two spectra (see
Fig.4). This latter spectrum
washes out the bias of the ex-
perimental set-up and is directly
comparable to the calculations.
The expected contributions of the Fig.4 : Correlation function. The low relative angles are what is
di-neutron gives rise to the low expected for a nuclear break-up of a di-neutron whereas
relative angles and the cigar con- the large relative angles would be the signature of the
figuration to the large relative an- cigar configuration
18 break-up.
18Indirect study of the astrophysical reaction
13C(
C(αα,n)16O via the transfer reaction13C(7Li,t)17O
IPNO Participation: F. Hammache, M. G. Pellegriti, P. Roussel, L. Audouin, D. Beaumel, S. Fortier,
J. Kiener, A, Lefebvre-Schuhl, V. Tatischeff, P. Descouvemont, L. Gaudefroy, M. Stanoiu, M.Vilmay
Collaboration : IPN, CSNSM, IAA-ULB, GANIL, GSI.
Dans les étoiles AGB (Asymptotic Giant Branch) de masses faibles (1 à 3 M~), la principale source des
neutrons nécessaires pour amorcer le processus de nucléosynthèse s est la réaction 13C(α,n)16O. La sec-
tion efficace de cette réaction est malheureusement mal connue aux énergies astrophysiques (E≤200 keV)
à cause de la grande incertitude liée à la contribution de l'état sous le seuil à 6.356 MeV de l'17O. En effet,
les résultats des études précédentes de cette contribution mènent à des conclusions très différentes. Dans
ce travail, nous avons déterminé le facteur astrophysique de cette réaction à travers la détermination du
facteur spectroscopique Sα de l'état à 6.356 MeV au moyen de la réaction de transfert 13C(7Li,t)17O. Nos
résultats confirment le caractère dominant de la contribution de l’état à 6.356 MeV de l’17O à la section effi-
cace de la réaction 13C(α,n)16O aux énergies astrophysiques.
Introduction Experiment description
Nearly half of the elements heavier than iron are The experiment was performed using a 7Li3+ beam
produced by a slow sequence of neutron capture provided by the Orsay TANDEM. Two self-
reactions, the so-called s-process. For the AGB supporting enriched 13C targets were used. A 12C
stars of 1-3 solar masses at low temperatures, the target was also used for calibration purposes and
13 background subtraction. The reaction products
C(α,n)16O reaction is considered as the main neu-
were analyzed with an Enge Split-pole magnetic
tron source for the s-process [1]. Hence, all the
spectrometer and detected at the focal plane by a
models describing the s-process nucleosynthesis in
50 cm long position-sensitive gas chamber and a
these AGB stars depend critically on the neutron
ΔE proportional gas-counter. The particle identifi-
flux from the 13C(α,n)16O reaction and so on the
cross section of this reaction which occurs in these cation was made unambiguously using ΔE versus
stars at temperatures around 108 K, i.e around the position measurements. The tritons were detected
Gamow peak Ecm~190 keV. at angles ranging from 0 to 31 degrees corre-
sponding to angles up to 43 degrees in the center
A direct measurement of 13C(α,n)16O reaction at
of mass frame. Due to the presence of 12C impuri-
this energy is extremely difficult because the corre- 13 7 17
sponding cross section is drastically low. Thus, di- ties, spectra coming from the C( Li,t) O reac-
rect measurements [2] have only been performed tion were contaminated by the excited levels of
16 12 7 16
down to 270 keV too far away from the energy O, produced via the reaction C( Li,t) O. So
range of interest around 190 keV. R-matrix extrapo- the (7Li,t) reaction was measured on both 13C and
12
lations of the cross sections measured at higher C targets at each angle with the same setup.
energies have then to be performed and have to Results
include the contribution of the 1/2+ state of 17O 13 7 17
The experimental C( Li,t) O differential cross
which lies at 6.356 MeV (3 keV below the α+13C sections measured for the 6.356, 3.055, 4.55 and
threshold). This contribution strongly depends on 7.38 MeV, at the two incident energies of 34 and
the α-spectroscopic factor Sα of this state. With val- 28 MeV, are displayed in Fig.1a and Fig.1b, re-
ues Sα~ 0.3-0.7 adopted in the NACRE compilation spectively. The data points displayed for 3.055
[3] and considered in the s-process modeling, a rise MeV in the 34 MeV left-column are Clark's meas-
of the astrophysical S-factor is expected when the urements [7] at 35.5 MeV. The accuracy assigned
energy decreases. This rise is compatible with the to our measured cross sections includes the uncer-
experimental data of ref. [2] but their error bars are tainties on the peak yield, the number of target at-
too large to derive definite conclusions and they are oms, the solid angle and the integrated charge.
compatible also with constant S-factor. Two experi-
ments [4,5] investigated the effect of the sub- Finite-range DWBA calculations, using the
threshold resonance on the astrophysical S-factor FRESCO code were performed. For the triton
through the α transfer reaction 13C(6Li,d)17O and the
channel, the optical potential parameters used
results of these studies [4,5,6] lead, however, to were taken from ref. [8]. Concerning the 7Li chan-
different and controversial conclusions. Therefore, nel, we used for the transfer data at 34 MeV, the
it appeared highly desirable to perform a new optical potential parameters of Schumacher et al.
determination of this alpha spectroscopic factor [9]. For the data at 28 MeV, we used both those
13 7
through another transfer reaction C( Li,t) O17 from ref. [9] as well as those provided from fitting
19 the elastic scattering cross sections we measured.
and an extended DWBA analysis.
19The optical potential parameters finally selected ror bar on γα2 (6.356 MeV). At the energy of inter-
are those giving the best fit for all the studied tran- est, Ecm~0.19 MeV, the contribution of the 1/2+ sub
7
sitions in the ( Li,t) reaction. -threshold state to the total S-factor is dominant
The calculated angular distributions normalized to (~70%).
the data are shown in Figure 1. For both incident Our calculated 13C(α,n)16O reaction rate,
energies, the calculated curves agree fairly well
with all the measured angular distributions of the
different populated states.
The α-spectroscopic factors were extracted from
the normalization of the finite-range DWBA curves
to the experimental data. The spectroscopic factor
for the overlap between α+t and 7Li was taken to
be 1.0 [10].
Fig 2: Astrophysical S factor (see text)
5.41±1.8 ×10-15cm3 mol-1 s-1, at temperature T=0.09
GK important for the s-process in low mass AGB
stars, is found to be 1.3 times less than adopted in
the NACRE compilation [3] but with a substantially
reduced range of allowed values.
In conclusion, with our measurement the dominant
character of the 1/2+ state at astrophysical ener-
Fig 1: Angular distributions (see text) gies is confirmed like in Keeley’s work [6] and con-
trary to Kubono’s [4] and Jonhson’s [5] results.
However, the differences with the ANC measure-
The good agreement between the DWBA calcula- ment of Johnson et al. [5] should be more thor-
tions and the measured differential cross sections oughly studied and understood.
of the different excited states of 17O at the two
bombarding energies of 28 MeV and 34 MeV
respectively, gives strong evidence of the direct References
nature of the (7Li,t) reaction populating these levels [1] R. Gallino et al., Astrophys. J. 497, 388 (1998)
and confidence in our DWBA analysis. An Sα mean [2] H. W. Drotleff et al., Astrophys. J. 414, 735
value of 0.29±0.11 is deduced for the state of inter- (1993)
est at 6.356 MeV of 17O, which is in good agree- [3] C. Angulo et al., Nucl. Phys. A 656, 3 (1999)
ment with that obtained by Keeley et al. [5] and [4] S. Kubono et al., Phys. Rev. lett. 90, 062501
those used earlier (Sα~0.3-0.7) in the s-process (2003)
models. [5] E. D. Johnson et al., Phys. Rev. Lett. 97,
The α-reduced width γα2 of about 13.5±4.3 keV for 192701 (2006)
the 6.356 MeV state was obtained by using the [6] N. Keeley et al., Nucl. Phys. A 726, 159 (2003)
expression from [10]. The contribution of the 1/2+ [7] M. E. Clark, K. W. Kemper, and J. D. Fox Phys.
state to the astrophysical S-factor when using our Rev. C 18, 1262 (1978)
deduced γα2 is shown in dashed curve in Figure 2. [8] J. D. Garrett et al., Nucl. Phys. A 212, 600
Its value at 190 keV is 9.6×105 MeV-b, which is (1973)
twenty-five times larger than in reference [4] and [9] P. Schumacher et al., Nucl. Phys. A 212, 573
five times larger than the asymptotical normalisa- (1973)
tion constant (ANC) measurement of reference [5]. [10] F. D. Becchetti et al., Nucl. Phys. A 305, 293
The present value of γα2 has been used to evaluate (1978)
the 13C(α,n)16O S-factor at astrophysical energies.
All the 17O resonances up to 7.38 MeV have been
included in an R-matrix calculation.
Our resulting S-factor is shown in Fig.2 as a solid
curve, with the uncertainty associated with the er- 20
20Missing
Missing--mass spectroscopy of the neutron deficient nucleus 12O
using the MUST2 array
IPNO Participation: D. Suzuki, H. Iwasaki, M. Assié, D. Beaumel, Y. Blumenfeld, N. De Séréville, S.
Franchoo, J. Guillot, F. Hammache, F. Maréchal, A. Ramus, J. A. Scarpaci, I. Stephan
Collaboration : Department of Physics, University of Tokyo (Japan), RIKEN (Japan), SPhN-CEA
Saclay (France), RCNP Osaka (Japan), GANIL (France), Universidad de Sevilla (Spain)
L’étude des états du noyau non-lié Z=8 d’ 12O a été entreprise au GANIL via la mesure de la réaction 14O
(p,t) en cinématique inverse à une énergie incidente de 50 MeV/A. La disparition du nombre magique N=8
ayant déjà été observée dans des noyaux légers tels que le 12Be, la question se pose concernant
notamment le noyaux miroir 12O. La localisation et l’identification de ses premiers états permet d’y
répondre. Dans cette optique, la section efficace de la réaction de transfert de deux neutrons 14O(p,t)
induite par un faisceau d’ 12O produit par fragmentation sur une cible cryogénique d’hydrogène a été
mesurée. Les tritons de recul étaient détectés à l’aide de l’ensemble MUST2. Les premières données
obtenues lors du run de calibration avec un faisceau stable de 16O sont présentées, montrant la pertinence
de l’approche choisie.
Low-lying states of the neutron-deficient isotope energy versus scattering angle of recoiling tritons
12
O have been studied via the 2-neutron transfer in the laboratory frame. Loci corresponding to the
reaction 14O(p,t) at 50 MeV/nucleon in inverse ground 0+ state and the proton-unbound 2+ states
kinematics. Recently, extensive studies have been at 6.6 MeV and 7.8 MeV in 14O are clearly seen in
performed to investigate the shell evolution in the figure. A typical resolution of 0.9 MeV in
exotic nuclei. In particular, the breakdown of the FWHM was obtained for the resultant excitation
N=8 shell closure has been demonstrated in light energy spectrum. The usefulness of the method
neutron-rich nuclei, such as 12Be, from the
experimental findings of the low-lying intruder
states [1-3]. A further interest arises in mirror
nuclei, addressing an open question about
persistence or disappearance of the proton
magicity Z=8 far from stability. In this view, the low-
lying level scheme of the neutron-deficient isotope
12
O, which has not yet been established, attracts a
great interest, since the effects of the shell
evolution are expected to appear in the low-lying
excitation properties.
In order to gain access to the proton-unbound
nucleus 12O, the missing-mass method was
applied to the (p,t) reaction. Due to the direct
nature of the reaction, one can not only identify
new states in 12O, but also obtain information on
relevant important properties, such as the spin- Fig. 1 Scatter plot of kinetic energy versus scattering
parity of the states or the spectroscopic amplitude, angle, in the laboratory frame, of recoil tritons
by applying the distorted-wave analysis to the detected by the MUST2 array in the 16O(p,t)
14
O reaction at 40 MeV/nucleon.
observed angular distribution.
The experiment was performed at the SPEG
facility in GANIL. The secondary 14O beam was has thus been demonstrated by the calibration run
produced by the SISSI device and directed onto with the 16O(p,t) reaction, and the spectroscopic
the 1mm-thick solid hydrogen target developed at information on 12O is to be obtained.
GANIL. Recoil particles were detected using 4
References
telescopes of the MUST2 array [4], each of which
[1] S. D. Pain et al, Phys. Rev. Lett. 96(2006)
was composed of the double-sided Si strip
032502.
detector and the 16-fold segmented CsI detector.
[2] A. Navin et al, Phys. Rev. Lett. 85(2000)266.
A large active area of the telescope with a high
[3] H. Iwasaki et al, Phys. Lett. B481(2000)7,
granularity facilitates efficient measurements with
[4] H. Iwasaki et al, Phys. Lett. B491(2000)8,
high experimental resolutions.
[5] S. Shimoura et al, Phys. Lett. B560(2003)
As calibration data, the (p,t) reaction was
31.
measured with the stable 16O beam at 40 MeV/
[6] E.Pollacco et al, Eur. Phys. J.A Supp.1
nucleon. Figure 1 shows the scatter plot of kinetic
21 (2005)287.
21Study of neutron
neutron--deficient tellurium isotopes by laser spectroscopy
IPNO Participation: B. Roussière, N. Barré, H. Croizet, M. Ducourtieux, S. Essabaa, C. Lau, F. Le
Blanc, A. Olivier, Y. Richard, J. Sauvage, R. Sifi
Collaboration : CERN (Switzerland), Laboratoire Aimé Cotton (Orsay), MacGill University (Canada),
LPSC (Grenoble), Mainz University (Germany), Tandar-Buenos Aires (Argentina)
Etude des noyaux de tellure déficients en neutron par spectroscopie laser
Les noyaux de tellure déficients en neutron ont été étudiés par spectroscopie laser en utilisant le dispositif
COMPLIS, installé sur l’une des lignes de faisceau d’ISOLDE (CERN). Les mesures de spectroscopie laser
sont effectuées par l’ionisation résonante en trois étapes des atomes de tellure désorbés par laser. Le dé-
placement isotopique, dont on extrait la variation de rayon carré moyen de charge, a été mesuré pour les
états fondamentaux de 116,118Te en étudiant la transition optique 5p4 3P2 → 5p3 6s 3S1 de 214,35 nm. La
comparaison des valeurs de rayon de charge avec les résultats de calculs théoriques prenant ou non en
compte les effets dynamiques montre que, dans la série isotopique des tellures, les effets dynamiques sont
importants, en particulier en milieu de couche neutron.
Laser spectroscopy measurements give access to results obtained in this work but also the neutron-
the nuclear (magnetic and spectroscopic quadru- rich and stable values previously measured
pole) moments and to the change in the mean [refs. 4,5] is presented in figure 1 and compared to
square charge radius (δ), providing direct infor- the results of microscopic static and dynamic cal-
mation on the shape and deformation of the nuclei. culations [ref. 6]. It appears obviously from figure 1
In the tin (Z = 50), indium (Z = 49) and cadmium (Z that, in the tellurium isotope series, the dynamical
= 48) isotope series, important dynamical effects effects are prevailing, especially near the neutron
associated with a parabolic behaviour of the δ mid-shell.
curve have been observed [refs. 1-3]. On the other
hand in the isotopic series with Z ≥ 54, the shape
of the δ curve indicates quadrupole deforma-
tion. Thus, in the tellurium (Z = 52) isotopes, it was
interesting to determine the stronger phenomenon
between quadrupole deformation and dynamical
effects. This requires the study of the neutron-
deficient tellurium isotopes.
The experiment has been performed at ISOLDE. In
order to produce the neutron-deficient Te isotopes,
a cerium oxide target associated with a hot plasma
ion source has been bombarded by the proton
beam delivered by the PS Booster. This target and
ion source system was used on-line for the first
time, and first of all we have performed a target Fig. 1: Experimental and theoretical charge radii.
test. For different target temperatures, yields for Experimental values have been obtained by
the 117 mass (which corresponds to the maximum calibrating the isotope shift values measured for
of the cross section for Te) have been measured. stable isotopes with the radius values known from
Unfortunately they have been found much lower muonic atom experiments [ref. 4]. Theoretical
values are the static (HFB) and dynamic (GCM-
than expected. We have looked for the release of GOA) results [ref. 6] obtained in the frame of a
tellurium as molecular compounds (TeO, TeO2, microscopic mixing configuration approach.
TeCO) but without success. In spite of the low
yields obtained (~106 atoms/μC for 117Te), we could References
measure for the first time the isotope shift of two [1] J. Eberz et al., Z. Phys. A326 (1987) 121
neutron-deficient tellurium isotopes: 118Te66 and [2] F. Le Blanc et al., Phys. Rev. C72 (2005)
116
Te64, crossing then the neutron mid-shell N = 66. 03430
Laser spectroscopy measurements have been per- [3] H.J. Kluge and W. Nörtershauser, Spectro-
formed, using the COMPLIS experimental set-up, chimica Acta B58 (2003) 1031
by three-step resonance ionization on laser- [4] G. Fricke et al., At. Data Nucl. Data Tables
desorbed tellurium atoms. The isotope shift has 60 ( 1995) 177
been obtained by scanning the first excitation step [5] B. Roussière et al, IPNO Activity Report
(the 5p4 3P2 → 5p3 6s 3S1 optical transition at 2004-2005 p. 18
214.35 nm) of the ionization process. [6] J. Libert, B. Roussière and J. Sauvage,
The Te charge radius curve, including not only the 22 Nucl. Phys. A786 (2007) 47
22β+/EC decay of 189m+gPb, identification using hyperfine spectra
IPNO Participation: J. Sauvage, B. Roussière, S. Franchoo.
Collaboration : LPSC, Grenoble, France; IKS, K.U. Leuven, Belgium; IPJGU, Mainz, Germany;
PNPI, Gatchina, Russia; ISOLDE, CERN, Genève, Switzerland; VSS, Gent, Belgium;
CUT, Göteborg, Sweden.
Désintégration β+/EC du 189m+gPb, identification à partir des spectres hyperfins
La désintégration β+/EC du noyau 189Pb a été étudiée par spectroscopie laser et nucléaire. Les spectres
hyperfins, HFS, ont été obtenus par la mesure de l’intensité des rayonnements γ émis par les noyaux des
atomes photo-ionisés en fonction de la fréquence du faisceau laser. Ils indiquent l’origine de l’alimentation
du niveau de 189Tl que le rayonnement γ désexcite. Le schéma de niveaux du 189Tl a été établi à partir de
mesures de coïncidences γ−γ−t, réalisées pour une fréquence du faisceau laser qui photo-ionise les deux
isomères du 189Pb. Les spectres hyperfins ont permis d’identifier les niveaux du 189Tl qui étaient alimentés
par l’isomère de haut spin et ceux qui étaient alimentés par l’isomère de bas spin du 189Pb. Ils ont donc
joué un rôle extrêmement important et décisif dans la construction du schéma de niveaux.
The neutron-deficient Pb isotopes were studied Fig.1d but none of them represents perfectly the
using in-source laser spectroscopy at ISOLDE [1]. experimental HFS. This is probably due to the un-
At the same time, a study of the β+/EC decay of the certainty on the HFS of the 228 keV gate (shown in
189
Pb isomers has been performed. The Pb atoms Fig.1d) used to represent the descendant. Howe-
were selectively ionized via a Resonant Ionization ver, the experimental HFS lies between the two
Spectroscopy (RIS) process in three steps. The calculated HFS. This allowed us to estimate a
photo-ions were then extracted by 60 kV, mass- contribution of the LS feeding of 28±10% for the
separated and guided towards a counting setup. To 318 keV HFS.
obtain information on the 189Pb decays, two types of
measurement were performed: one with a laser fre-
quency (λ) scanning to get λ−γ matrix, the other with
the laser tuned on a λ value allowing the ionization
of the two isomers to get singles γ spectra and γ−γ−t
coincidence matrix. The hyperfine spectra (HFS)
were obtained by measuring the intensity of the γ
rays emitted by the nuclei of the photo-ionized
atoms as a function of the frequency of the laser
beam inducing the first excitation step of the RIS
process. The COMET-NARVAL data acquisition
system [2] was used.
The HFS obtained for a gate set on a γ ray depends
upon the feeding origin of the level the γ ray de-
excites. The HFS obtained for the 386 keV gate
signs a pure feeding from the high-spin (HS) isomer
and the HFS of the 667 keV gate signs a pure fee-
dind from the low-spin (LS) isomer (shown in red
and blue in Fig. 1a, b). The HFS of the 422 keV gate
(in down triangles) indicates clearly a pure feeding
from the LS isomer (see Fig.1a) whereas the HFS of
the 821 and 865 keV gates (in triangles) correspond
to pure feedings from the HS isomer (Fig. 1b). The
HFS of the 464 and 318 keV gates, shown as histo-
grams in Fig. 1c, d, have different looks and corres-
pond to feeding admixtures. A HFS calculated assu-
ming a feeding of 75% from the HS isomer and of
25% from the LS isomer, shown in red dotted line in
Fig. 1c, is in perfect agreement with the experimen-
tal HFS of the 464 keV gate, suggesting that this γ Fig.1 Examples of HFS obtained for gates
line could be a doublet.The 318 keV γ line contains set on γ lines (see text). Frequency values
at least 3 γ rays, one of them belongs to the descen- in cm-1 correspond to the laser beam before
dant. Two HFS calculated for different admixtures doubling.
23
are displayed in green and red dotted lines in
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