Bingham, C R

Gaulard, C

Köster, U

Muto, S

Nishimura, K

Ohtsubo, T

Ohya, S

Risagari, L

Simpson, G

Stone, J R

Stone, N J

Veskovic, M

Walters, W B

CERN Document Server

CERN-INTC-2010-014/INTC-P-27911/01/2010EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH  PROPOSAL TO THE ISOLDE AND NEUTRON TIME-OF-FLIGHT COMMITTEE  Magnetic dipole moment of the doubly closed-shell plus one proton nucleus 49Sc.   T. Ohtsubo1, S. Ohya1, N.J. Stone2,3, J.R. Stone2,4, C.R. Bingham3, C, Gaulard5, U. Köster6,7, K. Nishimura8, S. Muto9, L. Risagari5, G. Simpson10, M. Veskovic11, and W.B. Walters4.   1 Department of Physics, Niigata University, Niigata 950-2181, Japan 2 Department of Physics, University of Oxford, OX1 3PU Oxford, UK 3 Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA 4 Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742 5 CSNSM, 941405 ORSAY, France 6 Institut Laue Langevin, F-38042 Grenoble Cedex 9 7 ISOLDE, CERN, CH1211 Geneva 23, Switzerland 8 Faculty of Engineering, Toyama University, Toyama 930, Japan 9 Neutron Science Laboratory, KEK, Japan 10LPSC, Grenoble Cedex 9 11Department of Physics, University of Novi Sad, 21 000 Novi Sad, Serbia  Abstract  It is proposed to measure the magnetic moment of 49Sc by the Nuclear Magnetic Resonance on Oriented Nuclei (NMR-ON) method using the NICOLE on-line nuclear orientation facility. 49Sc is the neutron rich, doubly closed-shell, nucleus 48Ca plus one proton. Results will be used to deduce the effective g-factors in the 48Ca region with reference to nuclear structure and meson exchange current effects.   Spokesperson: Takashi Ohtsubo   Contact person: Ulli Köster 1. Introduction The nuclear magnetic moment is a sensitive probe for the study of nuclear structure.  The electromagnetic properties of nuclei that are doubly closed-shell plus or minus one nucleon are expected to be most approachable by theory, are most approachable to theory. Experimental ground state magnetic moments of such configurations deviate from single-particle shell model estimates by often significant amounts arising from inaccuracy of the description of the single-particle nuclear state and of the one-body magnetic moment operator. Corrections for these inadequacies, mainly requiring rather involved modeling of second-order core polarization, isobar-current and meson-exchange current effects [1], are treated differently in different models leading to different predictions for magnetic moments of the doubly-closed-shell plus or minus one nucleon, as demonstrated in Fig.1  Honma et al. [2] used an effective GXPF1 interaction in their large-scale shell model calculation in the full pf shell.  This interaction contains empirical monopole and pairing corrections to the part of the interaction derived from the nucleon-nucleon potential, however these corrections proved insufficient to account for core excitations across the N or Z=28 shell gap.  Their calculation is exact for Sc isotopes (with no truncation of the model space). Both free-nucleon and effective single-particle g-factors were used in their magnetic-dipole moment operator which did not include specifically second-order correction terms.  The predictions for the heavy odd-A Ca, Sc and Ti nuclei in this model deviate from experiment by plus/minus 6% on average that is plus/minus 0.3 μN  in the estimate for 49Sc.  The work of Speidel et al. [3] describes shell model calculations with four effective interactions, FPD6, KB3, VHG and FPY. The configurations included where the full f7/2 sub-shell, allowing particle excitations to p3/2, f5/2 or p1/2. Core polarization effects were studied both in shell model diagonalisation and in first-order perturbation theory. These methods were used to interpret the trends in experimental g-factors of the first 2+ states in even-even Ti, Cr and Fe nuclei in this region. Using the results of this study, odd-A g-factors were deduced for 49Sc for the four interactions, with a range shown in Fig.1 [KB3 and VHG gave the same, central, result]. The uncertainty is of order 0.4 μN.  Speidel et al. also estimated the dependence upon the number of particles excited from the f7/2 subshell to the remainder of the fp shell. This effect, considered only for the FPD6 interaction, produced a smaller additional spread in moment predictions of plus/minus 0.07 μN for one, two and three particles excited outside the f7/2 subshell. A precise measurement of the magnetic moment of the ground state of 49Sc may well inspire a more microscopic calculation, applicable at present only to isotopes having single nucleons outside a double-magic core, of the effective g-factors in the fp shell in the spirit of Towner and Khanna [1], similar to that performed for 67Ni [4] as well as contribute to improvement of the large scale shell model calculation in the fp shell outlined above.   To add valuable new input near 48Ca we propose measurement of the magnetic moment of 49Sc (Iπ = 7/2−, T1/2 = 57 m) by the Nuclear Magnetic Resonance on Oriented Nuclei (NMR-ON) method using the NICOLE on-line nuclear orientation facility.   2. Experimental Background We propose to measure the magnetic moment of 49Sc with good precision. Table 1 gives a list of the known magnetic moments of odd-A Sc isotopes. Figure 1 shows these values with several theoretical shell model calculations of 49Sc [2,3].    Table 1: Magnetic moments of odd-A Sc isotope ground states – all single 7/2- proton.  A Neutrons T1/2 magnetic moment [μN] 41 (7/2-)0 596 ms 5.431(2) 43 (7/2-)2 3.891 h +4.62(4) 45 (7/2-)4 Stable +4.756487(2) 47 (7/2-)6  3.345 d 5.34(2) 49 (7/2-)8 57.2 m to be measured    Table 2: Magnetic moments of odd-Z, 7/2- ground states, N = 28 isotopes.  Isotope Protons:Iπ T1/2 magnetic moment [μN] 49Sc  (7/2-)1 7/2- 57.2 m to be measured 51V (7/2-)3 7/2- 3.891 h +5.1487057(2) 53Mn (7/2-)5 7/2- Stable +5.024(7) 55Co (7/2-)7 7/2- 3.345 d +4.882(3)     Table 2 shows the systematic variation of magnetic moments of other, f-subshell, 7/2- ground state magnetic moments. This valuable series will also be completed, with the inclusion of the (7/2-)1 element, by the measurement of 49Sc.  Recently we observed NMR-ON resonances for several Sc isotopes in iron [5,6]. The hyperfine field at Sc in iron was also measured. The observed hyperfine field for 47Sc is BBhf( ScFe) = −13.17(5) T [5]. This precise field value means that observation of NMR/ON in Sc will yield the moment with an accuracy of better than 1.0 % [any hyperfine anomaly between the two isotopes will be less than 0.1%]. The expected frequency, based on this field and the model moment estimates is about 156 MHz, well within the range accessible at NICOLE. 4749An essential of standard On-Line Nuclear Orientation experiments is that the half-life of the isotope under study is comparable with or longer than the spin-lattice relaxation time with which the nuclei cool to the iron lattice temperature. We observed the effective relaxation time for 47Sc which has the same spin and parity as 49Sc, to be T1'(47ScFe) = 350(70) s at 10 mK with the external magnetic field of BB0 = 0.2 T. For Sc, the relaxation time is expected to be similar to this value, since the magnetic moment of Sc is expected to be close to that of Sc and is thus adequately shorter than its half-life of 57.2 m.  4949 47            40 42 44 46 48 50AA=405.06.0μ [μN]A=48Schmidt valueHonmaSpeidelmagnetic moment of odd A Sc isotopes Figure 1: Magnetic moments of odd-A Sc isotopes and model predictions for 49Sc [2,3].  Production of Sc beams Table 2 shows the yields for the Sc isotopes from the ISOLDE yield database. However the numbers for the heavier isotopes are for a Ta target. There is no data for the pure beta emitter 49Sc because of the presence of many activities at the mass 49 position from multiply charged ion species from which any 49Sc component cannot be separated. Such a beam, with unknown, complex contamination, would not be acceptable for an NMR/ON experiment dependent upon beta detection. Instead we propose to use either a Ti foil target plus CF4 leak plus plasma source for which the estimated yield is 1 106/μC beam, or a standard UCx target with surface ionizer which produces 49K (T1/2 1.24 s) and 49Ca (T1/2 8.8 m) with estimated rates such as to produce ~ 5 105/μC of 49Sc by decay [7]. The beta activities from these parent isotopes will not orient as their lifetimes are shorter than their relaxation times and they will thus not contribute to the measured polarization or to resonance detection. Should the contaminant beta activity prove troublesome a source will be accumulated, then the beam closed off, whereupon the longer lived 49Sc rapidly dominates the detected beta decay, this cycle being repeated as necessary.  Table2: Yields of Sc isotopes at ISOLDE. Nucleus yield at ISOLDE  (ions/μC) target material3.0E+7 Ti 47Sc 2.5E+6 Ti 48Sc 3.0E+7 Ta 50Sc 3.6E+3 Ta  3. Nuclear Orientation Experiment  We propose to use the NMR-ON method, detecting resonant destruction of asymmetry of the β rays from 49Sc nuclei, implanted at low temperature in a pure iron ferromagnetic host. Figure 2 shows the decay scheme of 49Sc nucleus. Although the spins/parities of 49Sc and 49Ti are both 7/2-, their isospins are respectively 7/2 and 5/2. The allowed beta transition is dominated by the Gamow-Teller matrix element and is estimated to show a 0o/180o asymmetry of about 30% at 15 millikelvin, based on the known field and an estimated moment of 5.3 μNThe NICOLE 3He/4He on-line dilution refrigerator will be used with β-ray detectors inside the cryostat at 0o and 180o to the polarization axis, as was done for on-line NMR/ON of 67Ni at ISOLDE [4]. External plastic scintillators may alternatively, or in addition, be used for detection of the high energy betas which can penetrate the thin cryostat windows [8]. External HP Ge detectors at 0o and 90o to the polarization axis will be used for gamma detection. A modulated RF field produces resonant absorption between the hyperfine-split substates of the oriented nuclei. This resonance can be detected by reduction in the observed β-ray asymmetry. The temperature of the sample will be monitored using γ-transitions in an oriented 60CoFe sample soldered to the cold-finger of the dilution refrigerator.   4921Sc4922Ti7/2−7/2−99.94%β−Qβ− = 2006 keV57.2 m 00 Figure 2: Decay scheme of 49Sc.  4. Experimental Procedure and beam time request We need time to set up the beam and to observe the asymmetry as the dilution refrigerator is cooled from close to 1 K in order to get good base temperature knowledge of β-ray asymmetry from the somewhat mixed source. Once this is established, the resonance search can be initiated. A reasonable estimate for the experiment is 8 shifts. We propose to collect a source of 3.345d 47Sc, cold implanted into iron, for subsequent off-line NMR-ON measurement for careful calibration of the system. For this collection we request 4 shifts, giving a total of 12 shifts.   Support from CERN: For the experiments proposed here, we will need liquid nitrogen for the detectors and liquid nitrogen and liquid helium for the 3He/4He dilution refrigerator.    Reference 1. A. Arima et al., Adv. Nucl. Phys. 18, 1, (1987), I.S. Towner and F.C. Khanna, Nucl. Phys. A399, 334, 1983. 2. M. Honma et al., Phys. Rev. C69, 034335, (2004), and private communication. 3. K.-H. Speidel et al., Phys. Rev. C62, 031301(R), (2000). 4. J.Rikovska et al., Physical Review Letters 85, 1392 (2000). 5. S. Ohya et al., Hyperfine Interactions 180, 485, (2007). 6. T. Ohtsubo et al., Hyperfine Interactions 180, 509, (2007). 7. U. Köster, private communication. 8. P.F. Mantica Jr. et al., Hyperfine Interactions 75, 415 (1992). 

Magnetic dipole moment of the doubly closed-shell plus one proton nucleus $^{49}$Sc.

Magnetic dipole moment of the doubly closed-shell plus one proton nucleus $^{49}$ Sc.

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Magnetic dipole moment of the doubly closed-shell plus one proton nucleus 49^{49}49Sc.

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Magnetic dipole moment of the doubly closed-shell plus one proton nucleus $^{49}$ Sc.