CERN. Geneva. ISOLDE and Neutron Time-of-Flight Experiments Committee

INTC

CERN Document Server

CERN-INTC-2008-019/INTC-SR-01020/04/2008Status report for IS427 CERN, INTC, May, 2008Charge radii of magnesium isotopes and transition to a deformedconfiguration towards N=201Institut fu¨r Physik, Universita¨t Mainz, D-55099 Mainz, Germany2Institut fu¨r Kernchemie, Universita¨t Mainz, D-55099 Mainz, Germany3Instituut voor Kern- en Stralingsfysica,Katholieke Universiteit Leuven, B-3001 Leuven, Belgium4Laboratorium voor Vaste-Stoffysica en Magnetisme,Katholieke Universiteit Leuven, B-3001 Leuven, Belgium5CERN, Physics Department, CH-1211 Geneva 23, SwitzerlandWe present the status of the IS427 experiment and justify the continuation ofthe studies in accordance with the complete scientific program laid down in theaddendum [1] to our original proposal [2]. Thus, we aim to detect the transi-tion to a deformed configuration in magnesium isotopes by laser-spectroscopymeasurements of the mean-square charge radii within the entire sd shell and be-yond. Observing a common parameter for the even-even and odd-mass isotopeswould yield a continuous picture of the nuclear-structure evolution throughoutthe chain and over the borderline of the island of inversion. We will combinefor the first time the sensitive β-decay detection with traditional fluorescencespectroscopy for isotope-shift measurements. These studies have the potentialto shed additional light on our understanding of nuclear structure away fromstability not only for the magnesium species, but also in general.keywords: COLLAPS, Collinear laser spectroscopy, charge radii, Mgcontact person: D. T. Yordanov, email: Deyan.Yordanov@cern.chMOTIVATION OF IS427The nuclear shell model has been successful in reproducing the magic numbers and pre-dicting spins of odd-A nuclei, especially for light and intermediate masses. However, heaviernuclei in several regions had to be approached differently, due to collective phenomena as-sociated with nuclear shapes deviating from spherical. The deformed shell model or Nilssonmodel [3] has been successfully applied in these regions. A natural question is: Does defor-mation play a role in light nuclear systems? A positive answer has been acquired with theearly investigation of the stable 20Ne and 24Mg. The sd shell for these nuclei is occupied byonly a few protons and neutrons, which induce strong α-type correlations [4] and cause welldefined prolate shapes. Advances in the methods for production and extraction of short-lived nuclear species in the last three decades enabled the studies of systems with a filledneutron sd shell (N=20) while having the proton sd shell almost unoccupied. The onsetof deformation in these nuclei was discovered via the extra binding energy and anomalousspin of 31Na [5, 6], which gave reasons to discuss a “collapse of the conventional shell-modelordering in the very neutron-rich isotopes” [7]. Studies of the neighboring N=20 isotones30Ne [8] and 32Mg [9] also revealed deformed shapes. In terms of the spherical shell modelthis phenomenon can be understood by taking into account the lower orbitals of the pf shell,which get populated before the sd shell is fully occupied, and thus indicating the disappear-ance of the N=20 shell closure. Several mechanisms have been proposed to contribute tothis inversion of the “normal” 0 particle - 0 hole (0p-0h) and “intruder” np-nh states: (i) a2 Status report on IS427β2-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-12-8-402081d5/22s 1/21d3/21f 7/22p3/25/2[202]3/2[211]1/2[220]1/2[211]1/2[200]3/2[202] 1/2[330]3/2[321]5/2[312]7/2[303]3/2[312]1/2[321]1/2[440]E    (MeV)sp(a)(b)FIG. 1: Nilsson diagram in the (sd-1f7/2-2p3/2) configuration space. (a), (b) Odd-neutron occu-pation in the ground states of 33Mg and 31Mg, respectively.reduction of the shell gap [10]; (ii) an increase in the neutron-neutron and proton-neutroninteractions [10]; (iii) the monopole effect of the tensor force [11]. In order to understandthe phenomena taking place in this “island of inversion” one has to provide experimentalinput for further theoretical studies.Achievements of the IS427 experimentNeutron-rich isotopesExperiment IS427 has been very successful in providing information on the nuclear struc-ture of magnesium isotopes and unraveling the causes for the existence of the island ofinversion. The spin and magnetic-moment measurement of 31Mg [12]? became a referencepoint for the continuation of the studies in this region. Calculations using the most recentat that time Hamiltonians for the sd-pf shells could not reproduce the experimentally ob-served level ordering. The measured spin and magnetic moment correspond to a 2p-2h state,which occurred in all calculations at a considerable excitation energy. With its nearly pureintruder ground state 31Mg is the nucleus in the magnesium chain at which the transitionto the island of inversion occurs. This transition is rather unique in the sense that adding asingle neutron to 30Mg, whose ground state is well described within the sd shell [13], resultsin a 2p-2h excitation to the pf shell. Furthermore, mixing with 0p-0h configurations is notnecessary to explain the measured quantities. For comparison, the transition to intruderground states in neon, sodium and aluminum occurs smoothly, with a number of isotopeshaving mixed wave functions [14–17].Our measured values of the magnetic moments of 27Mg and 29Mg [18]? are well reproducedby calculations within the sd shell, meaning that the presence of 2p-2h excitations in theirground states can be neglected.The studies within IS427 continued with the first measurement of the ground-state spinI=3/2 and magnetic moment of a nucleus beyond N=20 in the middle of the island of? References with asterisk denote publications derived from IS427.Charge radii of Mg isotopes 3inversion [19]?. The 33Mg ground state was shown to have a nearly pure 2p-2h intrudernature, contrary to the earlier suggested 1p-1h configuration and in agreement with thestructure of the other isotopes in this region.The results on 31Mg and 33Mg [12, 18–21]? are also indirectly linked to nuclear deforma-tion. Due to the high density of orbitals in the Nilsson diagram (Fig. 1) the Fermi levels for19 and 21 neutrons are highly segmented, with each segment belonging to a different Nilssonorbital with a different spin and parity. Thus, based on our spin and parity assignments, itwas possible to suggest considerable prolate deformations for both nuclei in accordance withthe segments (a) and (b) in Fig. 1. This idea has been investigated further theoretically [22].Extended target test on neutron-deficient isotopes. Spin and magnetic moment of 21MgIn our addendum [1] we requested 24 shifts of radioactive beam for measuring the isotopeshifts of magnesium over the entire sd shell and the nuclear moments of the neutron-deficientisotopes. For the latter we requested an on-line test with a silicon carbide (SiC) target inorder to evaluate the yields and investigate the possibilities for suppression of the strongisobaric contamination from sodium. Initially 14 shifts were allocated, 8 for the isotope-shifts measurements on the neutron-rich isotopes and 6 shifts for the target test. A reporton the results from the test has been requested for further evaluation of the IS427 demandsfor radioactive beam.The test run was performed with a silicon carbide target, number 353, in combinationwith the resonant-ionization laser ion source (RILIS) and the high-resolution mass separator(HRS). Yield measurements after irradiating the target showed an overwhelming contamina-tion of sodium at mass number A=21 (Tab. I), which prevented determining the productionof 21Mg. Nevertheless, the estimated yields of 22Mg and 23Mg (Tab. I) indicated a targetbehavior in accordance with the expectations.0ΔE = |A| . (I+1/2)βasymmetry (1% per division)21HFS     Mg IINMRΔν (a.u.)σ -σ+(a)(b)(3s 2S1/2 → 3p 2P3/2)FIG. 2: Hyperfine structure of 21Mg in the D2 line (3s2S1/2 → 3p 2P3/2). Nuclear orientation isproduced with circularly polarized laser light σ±, (a) and (b) respectively.4 Status report on IS427βasymmetry (0.2% per division)21NMR     Mgνrf (a.u.)FIG. 3: Nuclear magnetic resonance (NMR) of 21Mg measured with a large frequency modulation.After the yield measurements, the A=21 beam was guided into the collinear laser spec-troscopy setup where the lifetime of the β+ activity was recorded for different positionsof the HRS slits. As expected, at this low mass the resolution of HRS was sufficient forsuppressing the sodium contamination to a certain degree. The beam-gate settings wereadjusted to match the release of 21Mg, while suppressing the longer-lived 21Na. Finally, aset of degraders was used in front of the β detectors to completely stop the lower-energyparticles from the sodium decay. We detected about 1400 particles per proton pulse, whichsets a lower limit for the 21Mg yield (Tab. I), since the effect of the slits and our degraderson magnesium can not be precisely estimated. With these improvements, which took about3 shifts, we established the necessary conditions for meaningful measurements and physicsresults. In the remaining time the hyperfine splitting in the ground state of the 21Mg ion(Fig. 2) and the nuclear g factor (Fig. 3) were measured for the first time, thus also fixing thenuclear ground-state spin to I=5/2. The spin and magnetic moment of 21Mg are importantparameters for studying the mirror symmetry in atomic nuclei, since the properties of itsisospin partner 21F (T=3/2) are known. Our preliminary result enables the extraction ofthe isoscalar magnetic moment, which surprisingly lies outside the empirical limits given bythe Schmidt moments. The data analysis and interpretation are in progress.In order to finalize the measurements on 21Mg we had to calibrate the magnetic field byperforming a magnetic resonance on 31Mg. For this purpose we used the first part of our runon the neutron-rich isotopes, which was scheduled in sequence with the test. The remainingbeam time was then additionally shortened to about four shifts due to several interventionsin the HRS target area and problems with the beam line. Thus, we mainly tried to reachoptimized and stable measuring conditions and collected only fragmented data of limitedsignificance. This means we have to recover 4 shifts of our used beam time.TABLE I: Test yields of proton-rich magnesium and sodium nuclei. The measurements have beentaken in May 2007 with silicon carbide target 353 at 490 A and line at 250 A.21Mg 21Na 22Mg 23Mgτ1/2 123(3) ms 22.49(4) s 3.857(9) s 11.317(11) sions/µC > 3× 103 ≈ 4× 108 ≈ 1× 106 ≈ 1.5× 107Charge radii of Mg isotopes 5Charge radii across the borderline of the island of inversionThe initial proposal [2] which developed into the IS427 experiment suggested the mea-surement of quadrupole moments of the neutron-rich odd-mass magnesium nuclei in orderto obtain a quantitative measure for the nuclear deformation and follow the evolution ofthe nuclear shape from the sd shell through the N = 20 shell closure and beyond. How-ever, the results obtained so far revealed a number of special features in the magnesiumspecies, which prevented the implementation of this program. Our first discovery, the spinI = 1/2 of 31Mg [12]?, meant that this odd-mass isotope has no spectroscopic quadrupolemoment. Furthermore, the NMR amplitudes of 29Mg [19]? and 33Mg [18]? prooved to beconsiderably smaller than the produced asymmetries, likely due to implantation defects inthe host crystal. Optical detection of the quadrupole splitting also had to be ruled out dueto the unresolved hyperfine structure in the excited atomic state. Thus, the transition to adeformed configuration in the magnesium nuclei is still not well understood.We propose to detect this onset of deformation in neutron-rich magnesium isotopes andbring IS427 to a natural conclusion by performing laser-spectroscopy measurements of themean-square charge radii covering essentially all sd-shell isotopes, in particular the transi-tion to intruder ground state of 31Mg. Observing a common parameter for the even-evenand odd-mass isotopes would yield a continuous picture of the nuclear-structure evolutionthroughout the chain and over the borderline of the island of inversion. Only fragments ofthis picture are now available through the Coulomb excitation studies of the even isotopesand the considerations within the Nilsson model for the odd ones. The studies proposed inthis addendum have the potential to shed additional light on our understanding of nuclearstructure away from stability not only for the magnesium species, but also in general.NEW EXPERIMENTAL TECHNIQUEWe are aiming to combine for the first time the sensitive β-decay detection with traditionalfluorescence spectroscopy for meaningful isotope-shift measurements. This idea naturallyevolved from the success in simulating quantitatively [15] the nuclear polarization producedby optical pumping as a function of the laser frequency. While these simulations onlyqualitatively reproduced the hyperfine structure of atomic sodium, our studies of singlyionized magnesium (Mg ii) showed a remarkable agreement between theory and experiment[18]?. We are now confident that our knowledge is sufficient for extracting the small fieldeffect in the D1 line (3s 2S1/2 → 3p 2P1/2) and therefore the changes in the root mean-squarecharge radii from β-detection spectra even in the light magnesium species.BEAM-TIME REQUESTThe new technique will allow for measuring the charge radius of 31Mg, whose yield(Tab. II) is below the present limit for classical fluorescence spectroscopy. The even-even32Mg can not be polarized and hence the sensitive β-detection method can not be applied,but we will explore the conditions for increasing the optical sensitivity by using the pulsedbeam structure. This needs a normalization for fluctuating beam intensities, which can beprovided by ion counting. In the case of 33Mg, we have successfully applied the β-detectionmethod [19]?, but the precision required for extracting the field shift would demand a dedi-cated experiment. Thus, we aim to measure the isotope shifts of magnesium nuclei at least6 Status report on IS427in the mass range 22 ≤ A ≤ 31 (with some technical improvements there are good chancesto get beyond this range). For this research we request 20 shifts of radioactive beam, whichwill be used as follows:• 12 shifts for the neutron-rich isotopes, using a UCx target, RILIS and GPS;• 8 shifts for the neutron-deficient isotopes, using a SiC target, RILIS and GPS (possiblyHRS);These 20 shifts include the 16 shifts already requested in our addendum [1] and 4 shifts tobe recovered from our last run (see above). Herewith, we assume that ISOLDE will receiveat least half of the pulses in the proton cycle at the full intensity. If the number of availablepulses is lower, one has to consider proportionally higher number of shifts to complete thestudies.TABLE II: Maximum ISOLDE yields of neutron-rich magnesium isotopes recorded at COLLAPS.The yields of 27, 28Mg, given here for completeness, are taken from the ISOLDE database [23]. Themeasurements of 29, ..., 32Mg have been taken in May 2004 with uranium carbide target 267 at 730 Aand line at 350 A. In later experiments the yields have been typically a factor of up to 3 lower.27Mg 28Mg 29Mg 30Mg 31Mg 32Mg 33Mgτ1/2 9.458(12) m 29.91(3) h 1.30(12) s 335(17) ms 233(16) ms 120(20) ms 90.5(16) msions/µC ≈ 1.5× 107 ≈ 6× 106 ≈ 1.2× 106 ≈ 4.6× 105 ≈ 1.5× 105 ≈ 4.2× 104 ≈ 5.3× 103[1] INTC-P-183, Add. 1, CERN INTC-2006-025 (24 Apr 2006).[2] INTC-P-183, CERN INTC-2003-038 (24 Oct 2003).[3] S. G. Nilsson, Kgl. Dan. Viden. Selsk. Mat. Fys. Medd. 29, No. 16 (1955).[4] A. Bohr and B. R. Mottelson, Nuclear Structure, vol. 2 (W. A. Benjamin, Inc., 1975).[5] C. Thibault et al., Phys. Rev. C 12, 644 (1975).[6] G. Huber et al., Phys. Rev. C 18, 2342 (1978).[7] B. H. Wildenthal and W. Chung, Phys. Rev. C 22, 2260 (1980).[8] Y. Yanagisawa et al., Phys. Lett. B 566, 84 (2003).[9] T. Motobayashi et al., Phys. Lett. B 346, 9 (1995).[10] E. K. Warburton, J. A. Becker, and B. A. Brown, Phys. Rev. C 41, 1147 (1990).[11] T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005).[12] G. Neyens et al., Phys. Rev. Lett. 94, 022501 (2005).[13] O. Niedermaier et al., Phys. Rev. Lett. 94, 172501 (2005).[14] H. Iwasaki et al., Phys. Lett. B 620, 118 (2005).[15] M. Keim et al., Eur. Phys. J. A 8, 31 (2000).[16] P. Himpe et al., Phys. Lett. B 643, 257 (2006).[17] P. Himpe et al., Phys. Lett. B 658, 203 (2008).[18] M. Kowalska et al., Phys. Rev. C 77, 034307 (2008).[19] D. Yordanov et al., Phys. Rev. Lett. 99, 212501 (2007).[20] M. Kowalska et al., Eur. Phys. J. A 25, s01, 193 (2005).[21] D. T. Yordanov et al., Balk. Phys. Lett. p. 358 (2004), ISSN 1301 - 8329.[22] I. Hamamoto et al., Phys. Rev. C 76, 054319 (2007).[23] http://isolde.web.cern.ch.Publications [12, 18–21] descend directly from IS427.

Charge radii of magnesium isotopes and transition to a deformed configuration towards N=20

Abstract

Similar works

Full text

Available Versions

CERN Document Server