Extended selection rules for allowed beta decays in actinides

Abstract

An exhaustive survey of the presently available experimental data on allowed beta decays with logft & 6.0 for A ) 228 nuclei results in identification of 30 transitions which are seen to obey the extended asymptotic quantum numbers selection rule A)V =0, bn3+b A=0,~h n3~& 2. All these transitions are seen to connect an up-spin (X = f ) proton in an N = n 3+4 = 6 Nilsson orbital and a down-spin (X= $) neutron, also in an N =n, +A=6 orbital. Two possible candidates for allowed unhindered decays are presented. Our analysis also reveals the first evidence for four-quasiparticle excitations in eveneven nuclei of the actinide region. PACS number(s): 23.40.Hc, 27.90.+b, 21.60.Ev Selection rules for the allowed unhindered (au) beta decays of strongly deformed odd-mass nuclei, derived by Alaga [1] in terms of the Nilsson model asymptotic quantum numbers Q [Nn3AX], require b,I =0, +1 (0~0) b, n. no; AQ=bI, AX=0, hn3=0, EA=O . The transitions, permitted by Eq. (1) but not by (2), are referred to as allowed hindered (ah). Mottelson and Nilsson [2] observed that the allowed transitions of odd-A rare earths separate clearly into these two groups

β decay ofRa228and possible level structures inAcSood1, Gizon2, Burke3 et al. 1995Phys. Rev. CSelf Cite70101View full textAdd to dashboardCite

“…Taking into account the presently known experimental information and the guidelines for 2qp configuration assignments in the actinides from P transition rates [13], a partial Ac level scheme shown in Fig. 2 is proposed.…”

“…The 2qp band numbers K are listed in each case with spin-parallel (KT ) on the left, and spin-antiparallel (Ks) on the right, with the calculated parameter A (in keV) listed in the middle. (c) A recent analysis of all the known allowed P transitions with log ft ( 6.2 in the actinide region [13] clearly established the rule that any P transition with log ft & 5.5 in this region connects only the orbitals having their asymptotic quantum numbers de6ned by % = ns + A = 6 with Z"=g and Z"=$. The proposed 13.52-keV p transition &om the 20.19 1 level to the 6.67 keV 1+ level involving simultaneous change of both the proton and neutron orbitals can be understood by considering Coriolis mixing of the K =0+ and K =1+ bands; as mentioned earlier, Coriolis mixing of the negative parity band is also indicated by the observed M2 transition between the 6.28 keV 1 0 and the 3+ 3 ground state.…”

“…The data obtained Rom conversion-electron and p-ray spectroscopy are analyzed in Sec. III, using more definitive guidelines now available for configuration assignments in the actinides from P-transition rates [13] and the modeling of level schemes for odd-odd deformed nuclei [14,15] using updated experimental information on neutron and proton single-particle orbitals in the odd-mass neighbors [3,5].…”

Intrinsically forbidden ‘allowed’ beta transitions in actinides

“…An illustration of this phenomenon is seen in figure 3(b) wherein 3.6 m 236 Am (I π K = 5 − 5) has 97% branch to the 1185.5 keV I π K = 5 − 5 level in 236 Pu. This K = 0, n: 5 2 [633] → p: 5 2 [642] transformation obeys the (n 3 + ) = 0 selection rule of Sood and Sheline [16] for the fast allowed transitions with very low (4.9) log ft value. However this 1185.5 keV 5 − 5 level decays by an 319.5 keV M1 to the 866.1 keV 5 − rotational level of the K π = 0 − octupole band, involving a K = 5 transition.…”

“…Operative selection rules for allowed fast (log ft 6.2) beta decays in actinides were suggested earlier by Sood and Sheline [16] as modification of Alaga rules. We have recently [17] carried out a similar exercise for the first-forbidden-fast (1ff) beta transitions, while pointing out that 'these parity changing '1f' transitions are far more likely than the allowed decays in the 'frontier' region of nuclei with A 250'.…”