Interacting boson model calculation of octupole states in deformed nuclei

Two new absolute transition rates are reported for the nucleus 144Sm following an (e,e') Coulomb excitation study. They are B(E3;3---+0+)=(38• and B(E1; 3---+2+)=(2.8__+0.4)x 10 .3 W.u. This large E1 matrix element, along with the previously known B(E1; 1----0 +) value support the interpretation of the 1-state in this nucleus as 2-phonon 2 + x 3-excitation. In the frame of the IBM-1 +f-boson model we show the need for a two-body term in the E1 transition operator. Estimates for the strengths of the one and two-body parts of the E 1 transition operator are obtained from these experimental data.PACS: 21.60.Ev; 23.20.CK; 27.60. +j Low-lying 1-states in nuclei in the N=82 region have been of interest for many years because of the large E1 strength seen in elastic photon scattering [1 3] and inelastic proton scattering [4]. The energies of these 1-states are slightly below the sum of the excitation energies of the lowest 2 + and 3-states, leading to their interpretation as two-phonon 2 + x 3-states [1,5]. Strong E1 transitions in the actinides have been explained variously in terms of stable octupole deformation [6] and collective dipole modes [7,8]. Such mechanisms might play a role in lighter nuclei also. In order to clarify the origin of the E 1 transitions in the N=82 region, it is of interest to look for other strong E 1 data in these nuclei.The 1448m nucleus offers a good possibility of obtaining the E 1 matrix element between the 2 + and 3-states [9]. The energy of the 3-state is 150 keV above that of the 2 + state (Fig. 1) and a branched decay of the 3-state to the 2 + and 0 + states has been reported [10,11]. In order to obtain the E1 transition rate we have measured an absolute B(E3) matrix element from the 3-state to the gs by (~, e') Coulomb excitation of a thin (1 mg/cm2), 89% en- where B(EL)) are measured in (e fmC) 2.From the partial level scheme shown in Fig. i, one can see that the above excitation ratio must be the same as the decay expression 1/1'= (I(3 -~ 0 +) + 1(3 ----, 2 +))/ (I(2 + ~ 0 +) --1(3 -~ 2+))where I's are the relative intensities of the corresponding 7-rays. Our measured value for this ratio is 0.22+0.02 (including the internal conversion coefficient ~=0.09 for the 3----r2 + transition [13]). By using the value B(E2; 0 + ~ 2+) =(2.66_+ 0.08)

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“…The Hamiltonian can be found in [18]. Within this model, negative parity states have an "octupole" or "quadrupole-octupole" nature.…”

mentioning

confidence: 97%

“…Collective negative parity states can be described in the IBM-1 formalism by the addition of a single f boson (with L = 3 and intrinsic negative parity) to the usual s--d model space [17] and [18] (sdf-IBM-1-model). The Hamiltonian can be found in [18].…”

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confidence: 99%

Evidence for the two-body nature of theE1 transition operator in the sdf-interacting boson model

Barfield

Brentano

Dewald

et al. 1989

Z. Physik A - Atomic Nuclei

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“…In deformed nuclei, low-lying negative-parity states are generally associated with octupole vibrations, and a simple pattern of K π = 0 − , 1 − , 2 − , and 3 − bands is frequently seen. In pioneering work, Neergård and Vogel [48] described the octupole states in deformed nuclei microscopically, and Barfield, Wood, and Barrett [49] treated the detailed spectra and E3 transition rates of nuclei in the rare earth region within the interacting boson model. Modern approaches to the problem, like the ones used in this paper are required for a more robust prediction of transition strengths and excitation energies of the negative parity states.…”

Section: Discussionmentioning

confidence: 99%

Lifetime measurements of low-spin negative-parity levels in Gd160

et al. 2017

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160 Gd(n, n γ) experiments were performed with accelerator-produced monoenergetic neutrons. Excitation functions at neutron energies from 1.5 to 2.8 MeV aided in the placement of γ rays in the level scheme and angular distributions at three neutron energies resulted in the determination of twenty eight excited-level lifetimes or limits in 160 Gd, including the lifetimes of several negativeparity levels attributed to octupole vibrations.

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“…In the boson model, negative parity states are described by the spdf interacting boson model [10][11][12]14,13,16,17,[19][20][21]28] or the sdf-IBM [22][23][24][25][26]. Otsuka [14] showed microscopically that p and f bosons are important in the low-lying negative parity states of even-even nuclei.…”

Section: Introductionmentioning

confidence: 99%

“…There have been intensive interests in the studies of octupole degree of freedom in nuclear structure recently both experimentally [1][2][3][4][5][6][7][8][9] and theoretically [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”

Section: Introductionmentioning

confidence: 99%