The current study on a-tiec:lys is revicwed and discussed. Erriplia recc:iitly-discovered mifavored wdecays of the iiucleus '"'13i a i d of N= 127 isotoiic:s. Tlic uiifayored a-decay half-livcs of 'O'Bi and of N=127 isotones are iiivestigatetl mitli the cu-cluster model. The cluster mode1 can reproduce the experimental half-lives of '"'Bi i~iid of 1;'=127 isotories where the iiuclear structure effect of tlie preformatioii factor ol tlie tr-cluster in the ground state of iiuclei is takeii into account. This is an interesting gencralizatjion of' the cluster model of the favored d e c a y s to the uiifavored a-decays.
1. INTRODUCTlON
Recently there is sigiiificant progress on the experimental studies of the cu-decays oi' nuclei. ?daiiy new a-emitters a.re observed aiid their half-lives are measured [l-G]. Tliesc iiew resu1t)s are iiseful for our uiiderstaiiding of the mecliaiiisni of tlic decays i ~i i d for t h e f u r h r developments of a-decay models. Usually the a-decays occiirrctl betwceii thc: groiiiid states of c:veii-even iiuclei aiid between the states of odd-even niiclci arid odd-odd iiuclei with same angular niorneiituin and parity (61. This kind of decay is called as tlic favored a-decay in testhooks because there is 110 change of tlie angular rrioineiitiiin and parity for tlie pareill and daughter nuclei. If tlie state of the parent iiucleus has diff'ereiit, niigiilar inoineiituin and/or parity from that of tlie daughter iiucleus, tlie dectLy is calletl n s the iiiif'avored a-decay. There iue ina.iiy calculations on favored a-decays [7-111 mid the esperiinental half-lives are well reproduced by various models.
[l] observed that ''"Bi has ail a-decay eiicrg,y QcL = 3.137 j , 0.002 hleV aiid a half-life TI/' = (1.9 z t 0.2) x 10" years. Tliis is differelit froiii the traditional view in textbooks that '"'Bi is a stable nucleus. The lialf-life of 20g13i is tlie longest, iii all kiiowvli a-decays. Because the ground state spill and parity of '"'Bi ;].re (9/2)-aiid those ol 'O'T1 are (1/2)+, this decay is an extraordiii;iry process where i,he cliaiigc of tlic: aiigular inoirieiituiii is as liigli as 5 anti that of iiarity is iiiimis. The ci-dccays of N=127 isotoiies are also siiiiilar to that of 20gBi [6]. They forms a IICW class of n-decays n-here the mgular iiioineiitum and parity of a-particle is 5-. There are rare studies on this class of decays and we iiivestigate tlieni in this article.
Very recently llarcillac ei ul. ~ This \vorli is supported by the Sational Natural Scieiicc Fouiiddtioii of Chiiia under coiltract KO. 1012552 I, by thc fiiiid of Education hiillistry of Cliiiia undcr contract No. 20010284036. by tile Statc: Basic Rrscmcli Dcvelopinent Program in Chiiia iindcr contract No. G2000077400, hy the CAS laomledge Iiiiiovatioii Project Xo, KJCX2-SWN02.