The lifetimes of the first excited states of the N = 30 isotones 50Ca and 51Sc and the Z = 18 isotopes 44−46Ar isotopes have been determined using a novel technique that combines the Recoil Distance Doppler Shift method with the CLARA-PRISMA spectrometers in multinucleon transfer reactions. The results allow determinination of the effective charges above 48Ca and test the strength of the N = 28 magic number when moving away from the stability line.Gadea Raga, Andrés, Gadea.Andres@ific.uv.es ; Algora, Alejandro, Alejandro.Algora@ific.uv.es ; Rubio Barroso, Berta, Berta.Rubio@ific.uv.e

Algora, Alejandro

Angus, L.

Aydin, S.

Bazzacco, D.

Benzoni, G.

Bizzeti Sona, A. M.

Bizzeti, P.G.

Boutachkov, P.

Broda, R.

Corradi, L.

Crespi, F.

De Angelis, G.

Dewald, A.

Farnea, E.

Fioretto, E.

Gadea Raga, Andrés

Gottardo, A.

Grodner, E.

Górska, M.

Görgen, A.

Howard, A.

Królas, W.

Lenzi, S. M.

Leoni, S.

Lunardi, S.

Mason, P.

Menegazzo, R.

Mengoni, D.

Montagnoli, G.

Montanari, D.

Napoli, D. R.

Obertelli, A.

Pawłat, T.

Pissulla, T.

Recchia, F.

Rubio Barroso, Berta

Sahin, E.

Scarlassara, F.

Smith, J.F.

Stefanini, A. M.

Steppenbeck, D.

Szilner, S.

Ur, Calin A.

Valiente Dobón, J. J.

Wady, P. T.

Wrzesiński, J.

The lifetimes of the first excited states of the N = 30 isotones 50Ca
and 51Sc and the Z = 18 isotopes 44−46Ar isotopes have been determined
using a novel technique that combines the Recoil Distance Doppler Shift
method with the CLARA-PRISMA spectrometers in multinucleon transfer
reactions. The results allow determinination of the effective charges above
48Ca and test the strength of the N = 28 magic number when moving away
from the stability line

D. Mengoni

J. J. Valiente Dobón

A. Gadea

E. Farnea

S. M. Lenzi

S. Lunardi

A. Dewald

T. Pissulla

S. Szilner

A. M. Stefanini

R. Broda

F. Recchia

A. Algora

L. Angus

S. Aydin

D. Bazzacco

G. Benzoni

P. G. Bizzeti

A. M. Bizzeti Sona

P. Boutachkov

L. Corradi

G. de Angelis

E. Fioretto

A. Görgen

M. Gorska

A. Gottardo

E. Grodner

A. Howard

W. Królas

R. Menegazzo

G. Montagnoli

D. R. Napoli

A. Obertelli

T. Pawłat

B. Rubio

E. Sahin

F. Scarlassara

J. F. Smith

D. Steppenbeck

C. A. Ur

P. T. Wady

J. Wrzesiński

F. Crespi

S. Leoni

P. Mason

D. Montanari

AIR Universita degli studi di Milano

Lifetime measurements of excited states in neutron-rich nuclei around 48Ca

The lifetimes of the first excited states of the N = 30 isotones 50Ca and 51Sc and the Z = 18 isotopes 44−46Ar isotopes have been determined using a novel technique that combines the Recoil Distance Doppler Shift method with the CLARA-PRISMA spectrometers in multinucleon transfer reactions. The results allow determinination of the effective charges above 48Ca and test the strength of the N = 28 magic number when moving away from the stability line

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Vol. 40 (2009) ACTA PHYSICA POLONICA B No 3LIFETIME MEASUREMENTS OF EXCITED STATESIN NEUTRON-RICH NUCLEI AROUND 48Ca∗D. Mengonia, J.J. Valiente-Dobónb, A. Gadeac, E. FarneadS.M. Lenzia, S. Lunardia, A. Dewalde, T. Pissullae, S. SzilnerfA.M. Stefaninib, R. Brodag, F. Recchiab, A. Algorac, L. AngushS. Aydinb, D. Bazzaccob, G. Benzonii, P.G. BizzetijA.M. Bizzeti-Sonaj, P. Boutachkovk , L. Corradib, F. CrespiiG. de Angelisb, E. Fiorettob, A. Görgenl, M. GorskakA. Gottardob, E. Grodnerb, A. Howardm, W. KrólasgS. Leonii, P. Masonb, R. Menegazzob, D. MontanariiG. Montagnolia, D.R. Napolib, A. Obertellil, T. PawłatgB. Rubioc, E. S.ahinb, F. Scarlassaraa, J.F. SmithhD. Steppenbeckm, C.A. Urb, P.T. Wadyh, J. WrzesińskigaDipartimento di Fisica dell’Universitá and INFN Sezione di Padova, Pd, ItalybIstituto Nazionale di Fisica Nucleare, LNL, Legnaro, ItalycInstituto de Física Corpuscular, CSIC-Universidad de Valencia, Valencia, SpaindIstituto Nazionale di Fisica Nucleare, Sezione di Padova, Pd, ItalyeInstitut für Kernphysik der Universität zu Köln, Köln, GermanyfRuder Bošković Institute, Zagreb, CroatiagInstitute of Nuclear Physics, Polish Academy of Sciences, Kraków, PolandhSchool of Engineering and Science, University of Paisley, Paisley, Scotland, U.K.iDipartimento di Fisica dell’Universitá and INFN Sezione di Milano, Mi, ItalyjDipartimento di Fisica dell’Universitá and INFN Sezione di Firenze, Fi, ItalykGesellschaft für Schwerionenforschung (GSI), Darmstadt, GermanylCEA Saclay, IRFU/Service de Physique Nucléaire, Gif-sur-Yvette Cedex, FrancemSchuster Laboratory, University of Manchester, Manchester, U.K.(Received November 7, 2008; revised version received December 14, 2008)The lifetimes of the ﬁrst excited states of the N = 30 isotones 50Caand 51Sc and the Z = 18 isotopes 44−46Ar isotopes have been determinedusing a novel technique that combines the Recoil Distance Doppler Shiftmethod with the CLARA-PRISMA spectrometers in multinucleon transferreactions. The results allow determinination of the eﬀective charges above48Ca and test the strength of the N = 28 magic number when moving awayfrom the stability line.PACS numbers: 27.40.+z, 21.10.Tg, 23.20.–g, 21.60.Cs∗ Presented at the Zakopane Conference on Nuclear Physics, September 1–7, 2008,Zakopane, Poland.(485)486 D. Mengoni et al.1. IntroductionSpectroscopy of the moderately neutron-rich nuclei around the doubly-magic 48Ca has been possible only recently by means of deep inelastic stablebeam reactions and the use of radioactive beams.That region offers the unique opportunity to study the evolution of thenuclear properties when adding few nucleons to a doubly-closed configura-tion. Structural changes have been verified in the magic character of theN = 32 in Ca, Ti and Cr isotopes [1] and in the possible quenching of theN = 28 shell closure.Spectroscopic information are nevertheless limited and data on electro-magnetic transition probabilities are scarce or missing completely due to thedifficulty in populating such neutron-rich nuclei. Therefore an experimenthas been performed at Laboratori Nazionali di Legnaro, using the CLARA-PRISMA set-up [2, 3] in combination with the novel RDDS method [4]. Inthis contribution we report about the results obtained during such experi-ment.2. Experimental details and resultsA 48Ca beam at a bombarding energy of 310 MeV was delivered by theLNL Tandem-ALPI accelerator complex on a target consisting of 1.0 mg/cm2of enriched 208Pb evaporated onto a 1.0 mg/cm2 Ta support to accomplishthe stretching of the target. A thick 4 mg/cm2 Mg foil was used as an en-ergy degrader. Different target-degrader distances, ranging from 20 µm to3000 µm were employed during the experiment by arranging stack of vari-ous metallic rings. The target-degrader combination, which constitutes theplunger system, was placed at the centre of the reaction chamber of theCLARA array in order to measure the lifetimes of the excited states in theprojectile-like nuclei. More experimental details and a schematic view of theapparatus setup can be found in [5].Several nuclear species, ranging from Ti to Cl, have been populated.In this contribution we concentrate on Ca, Sc and Ar isotopes, which areisotopes abundantly populated and give the possibility to extract lifetimesfor the low lying excited states.In Fig. 1 (left) the 1027 keV 2+ → 0+ gamma transition in the 50Canucleus as a function of the target-to-degrader distance is shown. A lifetimeof 96±3 ps has been extracted by fitting the intensity ratio over the variousdistances. The lifetime of the state can be deduced when the recoil velocitybefore the degrader is provided and the intensity from the upper feedersproperly taken into account. The recoil velocity is calculated consideringthat the Doppler shift, between the shifted and unshifted peaks, is, for eachangle, a function of the velocity [5]. The feeding is taken into account usingthe total kinetic energy loss as described in [6]. The Q-value of the reactionis used to select an entry-point in the excitation mechanism.Lifetime Measurements of Excited States in Neutron-Rich Nuclei . . . 487 0 5 10 15 20 1500  1600Energy (keV)D = 30 µm  5 10 15 20Counts D = 100 µm  5 10 15 20D = 300 µm46Ar 0 80 160 240 900  950  1000  1050  1100Energy (keV)D = 300 µm  40 80 120 160Counts D = 1240 µm  20 40D = 2223 µm50CaFig. 1. Left part: 1027 keV 2+ → 0+ gamma transition in the 50Ca nucleus. Thetwo components, the shifted and unshifted one, are clearly distinguished along thevarious distances, due to the long lifetime of the state. Right part: 1553 keV2+ → 0+ gamma transition in the 46Ar nucleus. Only the shifted component isvisible due to the short lifetime of the level. For the distances, whose time of ﬂightis comparable with the time needed to overtake the degrader, the energy of theshifted peak continuously degrades into the unshifted one, as can be seen in thebottom panel on the right.The B(E2 ↓) reduced transition probability, which is 7.5±0.2 e2fm4, hasbeen derived from the lifetime and it is smaller than the corresponding valuein 48Ca. The energy and B(E2 ↓) of the 2+ state in 50Ca are well reproducedby these calculations. The excited level shows a single particle character,consisting of a quasi-pure (≈86%) ν(f87/2p23/2)configuration. The compar-ison with shell-model calculations allows to extract the effective charge ofthe neutron in the fp shell. The result seems to indicate a solely isoscalarexcitation of the core, Ref. [7].Concerning the Sc isotopic chain, a lifetime of 34±7 ps been derived forthe 1065 keV 11/2− → 7/2− transition in 51Sc. The B(E2) value derivedfrom the measurement is in agreement with the absence of isovector term inthe effective charges.For lifetimes shorter than few ps, as it is the case for the 2+ state in 46Ar,it is important to consider the slow-down of the recoils into the degrader.This leads to a continuous degradation of the shifted component into theunshifted one, which makes impossible to distinguish the two peaks, as canbe seen in Fig. 1 (right) for the 30 µm distance. To recover the degraded488 D. Mengoni et al.information, a realistic Monte Carlo simulation, which takes into accountthe energy and angular dispersion of the recoils in the degrader, has beenperformed. A detailed study of the simulation can be found in [8]. Theresult seems to confirm the weakening of the N = 28 magic number southof 48Ca.3. Summary and conclusionsThe analysis we performed on the RDDS data provided the lifetime of the50Ca, 51Sc nuclei and 44−46Ar nuclei. It has been possible to derive from ourresults, the neutron effective charges in the fp shell and extend the knowledgeon the shell evolution, through the B(E2 ↓) extraction, along the Ca and Scisotopic chain beyond the previous limits. Moreover the deduced effectivecharges seem to indicate a pure isoscalar excitation of the core. The lifetimefor 44−46Ar, as deduced from the simulations and models, seems to indicatea weakening of the N = 28 below 48Ca. In conclusion this experimentaltechnique, which uses the differential plunger in combination with a gammaand a magnetic spectrometer in multi-nucleon transfer reaction, has revealedas a reliable and promising method for lifetime measurements in view of theAGATA demonstrator experimental campaign.This work has been partially supported by the European Commissionwithin the sixth Framework Programme through I3-EURONS (con-tract No RII3-CT-2004-506065) and the DGF (Germany under contractNo DE1516/-1).REFERENCES[1] B. Fornal et al., Phys. Rev. C77, 014304 (2008).[2] A. Gadea et al., Eur. Phys. J. A20, 193 (2004).[3] A. Stefanini et al., Nucl. Phys. A701, 217c (2002).[4] A. Dewald et al., to be published.[5] D. Mengoni et al., LNL Annual Report 2007.[6] J.J. Valiente-Dobón, LNL Annual Report 2007.[7] J.J. Valiente-Dobón et al., submitted to Phys. Rev. C.[8] D. Mengoni et al., Eur. Phys. J. A, to be published.

Lifetime measurements of excited states in neutron-rich nuclei around 48 Ca

Lifetime measurements of excited states in neutron-rich nuclei around 48 Ca

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