exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3
exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3 exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3
Experimental study of 84 Ga β decay at ALTO IPNO Participation: F. Azaiez, M. Cheikh Mhamed, E. Cottereau, P.V. Cuong, S. Essabaa, M. Ferraton, K. Flanagan, S. Franchoo, D; Guillemaud-Mueller, F. Hammache, C. Lau, F. Le Blanc, M. Lebois, J.-F. Le Du, J. Libert, B. Mouginot, C. Petrache, B. Roussière, L. Sagui, N. de Séréville, I. Stefan, B. Tastet, D. Verney and the Pôle Tandem/ALTO technical teams Au cours de la campagne de tests programmée dans le cadre du lancement d’ALTO, un faisceau de 84 Ga, sélectionné à l’aide du séparateur en ligne PARRNe, a été produit avec une intensité suffisante (~10/s) pour permettre une étude de décroissance radioactive. Les sources de 84 Ga, constituées par l’arrêt du faisceau sur la bande de Mylar du dérouleur dont est équipé la ligne étaient entourées d’un ensemble de détecteurs à scintillateur plastique (détection et germanium (détection ). L’analyse des intensités relatives des raies émises lors des décroissances et -n de 84 Ga et des ses descendants a permis de mettre en évidence pour la première fois l’existence d’un isomère dans ce noyau. Tous les éléments de l’analyse concordent à montrer que la décroissance de ces deux états alimentent les états 2 + 1 et 4 + 1 de 84 Ge, noyau pair-pair le plus exotique étudié à ce jour au-delà de la fermeture de couche N=50 au voisinage de 78 Ni. Introduction Considerable efforts have been recently deployed in order to experimentally reach the region in the immediate vicinity of 78 Ni to assess the doubly magic character of this very neutron rich nucleus. One of the most interesting questions related to the persistence of the Z=28 and N=50 shell gaps far from stability is whether 78 Ni can be considered as an inert core (a good core) for the nuclear shell model. This is not only crucial for the development of nuclear shell model calculations in this exotic region but also for the underlying physical picture this model carries: Doubly magic nuclei are considered literally as the supporting pillars of the common understanding of nuclear structure. 84 Ga(1+) beam production Test runs for mapping the effective dose rates in the Tandem building were performed with a primary electron beam intensity limited to 1 μA corresponding to ≃10 10 fissions/s inside the target container. Those tests were scheduled in the framework of the ultimate safety controls of the ALTO commissioning phase. An ensemble of a target containing 72 g of 238 U connected to a surface ion source made of a W tube heated up to ≃2200◦C was irradiated. Alkalies and elements with very low first ionization potentials such as Ga and In were selectively ionized. This particular configuration with Z-selection insured by the source, the intrinsic selectivity provided by the photo-nuclear mechanisms themselves and the A-selection achieved by the mass-separator (A/ A≃1500) allowed to obtain a pure beam of 84 Ga 1+ ions with an intensity of a few tens per second. These ions were collected onto a Al-coated Mylar tape to detect -rays following their and -n decays. Detection setup The -detection system consisted of one coaxial large volume HPGe detector (70% relative efficiency) of the non-tapered CHATO type with a Fig. 1 Upper part: -gated γ -spectrum recorded at mass 84 (0 keV to 1700 keV). Peaks marked with dots correspond to the decay of 89–96 Rb isotopes stopped within the mass separator (see text). Lower part: Zoom of the spectrum for the 600–750 keV and 1200–1350 keV energy ranges. resolution of 2.3 keV at 1.3 MeV and one small EXOGAM CLOVER detector from the prototype series (100% relative efficiency) with a typical resolution for the central signal of a single crystal of 2.0 keV at 1.3 MeV. These two detectors were placed in a 180 ◦ geometry close to the point where the beam was collected onto the tape. In this configuration a total photopeak efficiency of 3% at 1.3 MeV was achieved. This collection point was surrounded by a tube-shaped plastic scintillator for detection ( ≃30-50%). In an experiment with an isotopic and isobaric-pure beam, the main function of the tape cycling is to provide a complementary piece of information to help in the identification of unknown new lines which are expected to appear 11
in the spectra by analyzing their time dependence. The evacuation of the activities of the daughter nuclei was not an issue because the precursor population was very weak and because the difference between the half-life of 84 Ga and its and -n descendants was large enough. Then the tape sequence was formed by a 9 s build-up phase followed by a 1 s decay time, evacuation of the source and repetition ad libitum. -activity analysis The -gated -spectrum which was recorded with the PARRNe separator field set to A=84 is shown in Fig. 1. From that picture it is seen that -lines belonging to the decay of neutron rich Rb isotopes with mass ranging from 89 to 96 were present in the spectra (dots in Fig. 1). The Rb isotopes which are strongly favoured in both the fission process and the ionization mechanism ( i ≃80%) were stopped within the separator chamber which was intentionally not shielded to allow the simultaneous safety measurements with the required conditions. In spite of the fact that they were accumulated few meters downstream from the tape system and that the -spectra were conditioned by simultaneous detection of a signal in the plastic scintillator, the activity of the accumulated neutron-rich Rb isotopes was such that the associated -lines could appear as random coincidences in the - conditioned spectra. Despite this difficult experimental environment, the -lines characterizing the activity of the daughters : 84 Ge (T 1/2 = 947(11) ms) and 84 As (T 1/2 = 4.5(2)s) are clearly visible establishing that 84 Ga was successfully collected onto the tape. We confirm the existence of two - rays in the decay of 84 Ga, one at 624.3(7) keV and the other at 1046.1(7) keV (see Fig. 1), both already observed at ISOLDE having a fast decaying component and at the HRIBS Oak Ridge. The 624.3-keV and 1046.1-keV peaks are attributed to the 2 + 1 → 0 + gs and 4 + 1 → 2 + 1 transitions in 84 Ge, respectively. There exists a peak at 247.8(3) keV clearly visible in our spectra which half-life could be determined with T 1/2 =75±33 ms, compatible with the previously measured 84 Ga half-life T 1/2 =85±10 ms and was attributed to the transition 1/2 + 1 → 5/2 + gs in 83 Ge fed through the neutron delayed branch. It is interesting to note that this 248-keV line was not observed in our previous 83 Ga - decay experiment. This constitutes an interesting additional piece of evidence for the proposed I =5/2 + assignment of the 83 Ga ground state. The two -lines at 100.0 keV and 242.4 keV attributed to the decay of 84 Ge are also clearly observed in our spectra. They correspond to transitions between states of 84 As. There is however a clear unbalance between the -activities of the different members of the A=84 isobaric chain that even a quick inspection of Fig. 1 can reveal : The relatively high intensities of the 100.0-keV and 242.4- keV -rays is clearly not compatible with the very low one of that at 624.3 keV. A quantitative analysis of the -activity balance leads to the conclusion that the largest part of the activity observed in this experiment corresponds to the decay of a low spin state which feeds essentially directly the 84 Ge ground state through the -channel and the 83 Ge 1/2 + 1 state through the -n-channel. In addition to this analysis, comparison with similar measurements made at ISOLDE leads to the conclusion that there must exist two decaying states in 84 Ga: One with very low spin, dominant at ALTO, and a second, well produced at ISOLDE, of spin I ≥ 3 (see Fig. 2). Fig 2. Decay paths in the hypothesis of two - decaying states in 84 Ga. State 1 is assumed to be of low-spin value (I=0) and state 2 of “high”-spin value (I≥3). Nature of the two -decaying states in 84 Ga The coexistence of two close lying levels with spin difference I>∼3 in 84 Ga is easily accounted for by considering the natural valence space available for three neutrons and three protons above a 78 Ni core. The recent experimental breakthrough in close vicinity of 78 Ni provides indeed a rather clear picture of the single particle level ordering in this very neutron rich region: For protons, 1f 5/2 and 2p 3/2 are close to each other and in this order in energy while for neutrons, 2d 5/2 is the lowest closely followed by 3s 1/2 . Then the natural proton-neutron configuration for 84 Ga ground state is ( 1f 5/2 ) 3 ⊗( 2d 5/2 ) 3 . Use of the Paar model describing the proton and neutron coupling to the core vibrations shows that I =0 − is the favoured member of the proton-neutron multiplet which fits well with our experimental findings. The first excited configuration is either ( 1f 5/2 ) 3 ⊗[( 2d 5/2 ) 2 ( 3s 1/2 ) 1 ] or [( 1f 5/2 ) 2 ( 2p 3/2 ) 1 ]⊗( 2d 5/2 ) 3 . The first configuration gives rise to a doublet 2 − –3 − and the second produces a state I =4 − which is energetically favoured in Paar’s model. The two states with I =3 − originating from each of those two configurations are likely to be connected which could push the lowest below the 4 − state. The higher spin long-lived state is then either 3 − of mixed configuration or 4 − of clean configuration [( 1f 5/2 ) 2 ( 2p 3/2 ) 1 ]⊗( 2d 5/2 ) 3 . These attributions of spins and configurations, and the adequacy of the particle-core vibration coupling picture follow and complete the general trends of the nuclear structure emerging in the region close to 78 Ni in terms of dominant single particle states and dynamics. For further details and necessary references cf M. Lebois et al. PRC 80 (2009) 044308. 12
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Experimental study of 84 Ga β decay at ALTO<br />
<strong>IPN</strong>O Participation: F. Azaiez, M. Cheikh Mhamed, E. Cottereau, P.V. Cuong, S. Essabaa, M. Ferraton,<br />
K. Flanagan, S. Franchoo, D; Guillemaud-Mueller, F. Hammache, C. Lau, F. Le Blanc, M. Lebois,<br />
J.-F. Le Du, J. Libert, B. Mouginot, C. Petrache, B. Roussière, L. Sagui, N. de Séréville, I. Stefan, B.<br />
Tastet, D. Verney <strong>and</strong> the Pôle T<strong>and</strong>em/ALTO technical teams<br />
Au cours de la campagne de tests programmée dans le cadre du lancement d’ALTO, un faisceau de 84 Ga,<br />
sélectionné à l’aide du séparateur en ligne PARRNe, a été produit avec une intensité suffisante (~10/s)<br />
pour permettre une étude de décroissance radioactive. Les sources de 84 Ga, constituées par l’arrêt du faisceau<br />
sur la b<strong>and</strong>e de Mylar du dérouleur dont est équipé la ligne étaient entourées d’un ensemble de détecteurs<br />
à scintillateur plastique (détection et germanium (détection ). L’analyse des intensités relatives<br />
des raies émises lors des décroissances et -n de 84 Ga et des ses descendants a permis de mettre en<br />
évidence pour la première fois l’existence d’un isomère dans ce noyau. Tous les éléments de l’analyse<br />
concordent à montrer que la décroissance de ces deux états alimentent les états 2 + 1 et 4 + 1 de 84 Ge, noyau<br />
pair-pair le plus exotique étudié à ce jour au-delà de la fermeture de couche N=50 au voisinage de 78 Ni.<br />
Introduction<br />
Considerable efforts have been recently deployed<br />
in order to experimentally reach the region in the<br />
immediate vicinity of 78 Ni to assess the doubly<br />
magic character of this very neutron rich nucleus.<br />
One of the most interesting questions related to<br />
the persistence of the Z=28 <strong>and</strong> N=50 shell gaps<br />
far from stability is whether 78 Ni can be considered<br />
as an inert core (a good core) for the nuclear shell<br />
model. This is not only crucial for the development<br />
of nuclear shell model calculations in this <strong>exotic</strong><br />
region but also for the underlying physical picture<br />
this model carries: Doubly magic <strong>nuclei</strong> are considered<br />
literally as the supporting pillars of the common<br />
underst<strong>and</strong>ing of nuclear <strong>structure</strong>.<br />
84 Ga(1+) beam production<br />
Test runs for mapping the effective dose rates in<br />
the T<strong>and</strong>em building were performed with a primary<br />
electron beam intensity limited to 1 μA corresponding<br />
to ≃10 10 fissions/s inside the target container.<br />
Those tests were scheduled in the framework<br />
of the ultimate safety controls of the ALTO<br />
commissioning phase. An ensemble of a target<br />
containing 72 g of 238 U connected to a surface ion<br />
source made of a W tube heated up to ≃2200◦C<br />
was irradiated. Alkalies <strong>and</strong> elements with very low<br />
first ionization potentials such as Ga <strong>and</strong> In were<br />
selectively ionized. This particular configuration<br />
with Z-selection insured by the source, the intrinsic<br />
selectivity provided by the photo-nuclear mechanisms<br />
themselves <strong>and</strong> the A-selection achieved by<br />
the mass-separator (A/ A≃1500) allowed to obtain<br />
a pure beam of 84 Ga 1+ ions with an intensity of a<br />
few tens per second. These ions were collected<br />
onto a Al-coated Mylar tape to detect -rays following<br />
their <strong>and</strong> -n decays.<br />
Detection setup<br />
The -detection system consisted of one coaxial<br />
large volume HPGe detector (70% relative efficiency)<br />
of the non-tapered CHATO type with a<br />
Fig. 1 Upper part: -gated γ -spectrum recorded at<br />
mass 84 (0 keV to 1700 keV). Peaks marked with<br />
dots correspond to the decay of 89–96 Rb isotopes<br />
stopped within the mass separator (see text).<br />
Lower part: Zoom of the spectrum for the 600–750<br />
keV <strong>and</strong> 1200–1350 keV energy ranges.<br />
resolution of 2.3 keV at 1.3 MeV <strong>and</strong> one small<br />
EXOGAM CLOVER detector from the prototype<br />
series (100% relative efficiency) with a typical<br />
resolution for the central signal of a single crystal<br />
of 2.0 keV at 1.3 MeV. These two detectors were<br />
placed in a 180 ◦ geometry close to the point where<br />
the beam was collected onto the tape. In this configuration<br />
a total photopeak efficiency of 3% at 1.3<br />
MeV was achieved. This collection point was surrounded<br />
by a tube-shaped plastic scintillator for<br />
detection ( ≃30-50%). In an experiment with an<br />
isotopic <strong>and</strong> isobaric-pure beam, the main function<br />
of the tape cycling is to provide a complementary<br />
piece of information to help in the identification of<br />
unknown new lines which are expected to appear<br />
11
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