g-factor of the 7 − isomeric state in 128 Ba

The time di fferential perturbed angular distribution technique ( T DPAD) has been used to measure the g-factor of the 2396 keV, 7 − isomeric state in128Ba. The measured value of g(7) is about 80% higher than the value for the expected configuration of the state.


Introduction
The even-even nuclei in the A ∼ 130 mass region exhibit interesting nuclear structure characteristics e.g.γ-softness, triaxiality etc.The high spin structure is dominated by rotational bands: an important feature here is the observation of two nearly degenerate s-bands crossing the ground state band [1], which shows prolate character for protons and an oblate character for neutrons.The aligned quasiparticles influencing the shape of the nucleus results in coexisting shapes for different intrinsic excitations.The neutron deficient even-even Ba isotopes exhibit collective properties which are good examples of the O(6) symmetry of IBA.Further, the presence of high-Ω orbital near the Fermi level favours the occurrence of K-isomeric states [2].The electromagnetic transitions from higher K to lower K are forbidden and the high K state becomes isomeric.The observation of 'K-forbidden' transitions provide the testing ground for the various mechanisms of approximate conservation of K quantum number [3].Very few nuclear moment measurements have been carried out in this mass region and this is the first time that the g-factor of a 7 − isomeric state in N = 72 isotonic chain has been measured in any investigation.
The K π = 7 − , 2396 keV isomeric state in 128 Ba has tentatively been assigned to a two quasineutron configuration through in-beam γ-ray spectroscopy.The half life, T 1/2 = 6.1 (2) ns, of the 7 − state is in a range that allows the observation of the precession pattern employing the T DPAD technique.The excited 128 Ba nuclei were recoil implanted into a Fe foil to observe the perturbation of the angular distribution pattern of the de-exciting γ-rays due to magnetic interaction.The internal magnetic field, −6.0 (2) T, at Ba in iron was calibrated [5] with respect to the g-factor, −0.159 (5), of the 10 + isomeric state in 132 Ba [4].

Experimental details
The partial level scheme showing the decay of the 7 − isomeric state is illustrated in Fig.
1 [6].The state of interest in 128 Ba is populated and aligned through the 12 C( 120 Sn, 4n) reaction using a 62 MeV, 12 C pulsed beam at the 10UD Pelletron accelerator facility, IUAC (New Delhi).The pulse repetition period was 250 ns.The target consisted of 500 µg/cm 2 120 Sn evaporated on a 1 mg/cm 2 thick Fe backing to stop the recoiling Ba nuclei.After rolling it to the desired thickness, the 99.99% pure iron foil was annealed [7], to get the saturated internal magnetic field at the recoil implanted Ba ions.Part of the same foil was used for the calibration of the internal magnetic field at Ba in Fe [5].The target was placed between the pole tips of an electromagnet to polarize the iron foil by a 0.2 T external magnetic field perpendicular to the detector plane.The γ-rays were detected by two high-purity germanium (HPGe) detectors.The detectors were positioned at ±45 • with respect to the beam direction at 25 cm distance from the target in the horizontal plane.The data was collected in list mode recording the energy and the signal w.r.t the beam pulse from a time-to-amplitude converter (TAC) for each HPGe detector.

DATA ANALYSIS
For the off-line analysis of the list-mode data, two-dimensional matrices of energy versus time were formed for each detector.From these matrices time-gated energy spectra and energy-gated time spectra were formed.The perturbed angular distribution pattern results in a time dependent modulation of γ-ray intensities.From the measured intensities at the respective angles, the normalized ratio factor, was formed and least squares fitted to the theoretical function EPJ Web of Conferences to extract the Larmor precession frequency, ω L .The anisotropy of the γ-ray angular distribution, A 2 , the phase, φ, for the bending of beam and the damping constant, λ = ( 1 τ rel ) were kept free parameters.The experimental ratio function R(t) and the fitted curve are shown in Fig. 2 for 989 keV delayed transition.

Results and Discussion
The g-factor of the isomeric state is extracted by taking the ratio of the precession frequency of the 7 − state with respect to 10 + state in 132 Ba, which is free from any uncertainty of the magnetic field value at Ba in Fe.K. Schiffer et.al have adopted the νh 11/2 g 7/2 (g = − 0.03) configuration for intra-band γ-ray transition analysis and compared the branching ratio with the rigid rotor calculation for the band built on the K = 7 − isomeric state.The measured value is ∼ 80% higher than the adopted one and also different in sign.This clearly shows that the configuration of the state does not correspond to the pure two quasineutron configuration.The possibility of K-mixing in this isomeric state has been pointed out by in-band analysis [6], suggesting an admixture of the low-K configuration, πh 11/2 d 5/2 into the νh 11/2 g 7/2 configuration.
a e-mail: meetu_a@yahoo.co.inDOI: 10.1051/ C Owned by the authors, published by EDP Sciences,

Figure 2 .
Figure 2. Spin rotation spectra of the 7 − isomeric state in 128 Ba.