Greiner, Walter

Hamilton, Joseph H.

Poenaru, Dorin N.

Săndulescu, Aurel

English

Hochschulschriftenserver - Universität Frankfurt am Main

VOLUME  54,  NUMBER  5  PHYSICAL REVIEW LETTERS  4 FERKUARY  1985 Comment on "Exotic Nuclear Decay of  223Ra  by Emission of  14C Nuclei" In their recent Letter confirming the 14c  radioactivi- ty of 223~a,'  Gales et aL2 state, "Nearly  a century after Becquerel,  Rose and Jones  (RJ) from the University of  Oxford have recently reported a novel mode of ra- dioactive  decayl which  was  completely  unexpected." They  noted  that  because  this  "was  a  surprising discovery,"  it  was  "highly  desirable  to  confirm  this first result"  which they did.2 The purpose of  this Comment is to  point  out that the 14c  decay is but one example of  a broader range of heavy nuclear decays and should not have been unex- pected  as  seen  from  an  earlier  paper3  (unknown  in Refs. 1 and 2), which predicted, in the region of heavy nuclei with Z  88, "a  new type of decay which can be interpreted as highly mass-as.ymmetric~ssion  or as emis- sion  of  a  heavy cluster. " The usual argument against heavy-cluster  emission  is  the fact  that  the Coulomb barrier increases strongly with increasing charge of the emitted particle and, therefore, the barrier penetrabil- ity is very low.  However, it was noted that the influ- ence of shell effects for some particular two-body frag- mentations give rise to a barrier penetrability  compar- able with the penetrability in  de~a~.~ The penetrabilities for  various heavy  cluster emis- sions with Z =  4 to 30 were calculated for several Ra, Th, Pu, U, Cf, Cm, No, and Fm nuclei and found to be  comparable to  those of  a  decayS3  These calcula- tions did not take into account the probability  of  for- mation  of  such  clusters.  Noting  that  heavy-cluster emission had not been observed, the authors conclud- ed that the probability of formation of heavy clusters is many orders of  magnitude smaller than the probability of a-particle formation.  While they estimated that the emission of Si in the region of  238~  was the most prob- able emission to be observed, their Fig. 7 shows a very large penetrability  for carbon emission by  several Ra isotopes. That work was continued and extended in two ways. A  new  semiempirical  formula  was  obtained  for  the alpha-decay half-life4 and an analytical expression for the lifetime was  ~btained.~  These were  then used to make a systematic study of  various cluster emissions from the ground states or low excited states populated by  beta decay of a precursor  (ß-delayed  heavy-cluster radioactivity).  The 3-'0~e  clusters were  studied  first and 5~e  radioactivity and ß-delayed-5~e  radioactivity were  predi~ted.~.~  There, it  was  ~tressed~,~  that  the maximum probability  of  a heavy-cluster emission oc- curs  for  processes  which  lead  to  a  doubly  magic daughter.  While their more extensive calculations of higher  Z  clusters  were  being  ~ompleted,~  Rose  and ~ones'  reported  the 14c  radioactivity of  223~a  to give the first evidence for the earlier predicted3 new type of decay mode. In a subsequent first report of  the more extensive calculations, Poenaru et al.  found that while Rose and Jones'  indeed had found the best 14c  emitter relative to a decay in 223~a  there are a number of  other cases deserving of  attention  based  on their calculated half- lives compared to those for cr  decay, for example, 14c radioactivity of  226~a,  the 26~e  radioactivity of  232~h, 24,25Ne  radioactivity  of  233~,  and  30~g  emission  by 237~p.  The 14c  radioactivity  of  223~a  was  also  con- firmed by  Ogloblin  et aL8  Thus the new  radioactivity decay  mode  predicted  by  Sandulescu,  Poenaru,  and  reiner^  is well  established.  It  will  be  interesting to confirm  the  other  predicted  modes.  These  modes offer  new  ways  to  probe  nuclear  clustering and shell effects in nuclei. A. Sandulescu Joint Institute for Nuclear Research 141 980 Dubna, U.S.S.R. D. N. Poenaru Central Institute of  Physics R-76900 Bucharest, Romania W. Greiner J. H. Hamilton Institute for Theoretical Physics J. W. Goethe University D-6000 Frankfurt am Main, West Germany and Joint Institute for Heavy Ion Research Oak Ridge, Tennessee 37830 and Physics Department, Vanderbilt University Nashville, Tennessee 37235 Received 9 October 1984 PACS numbers: 23.90.  +W,  21.10.Dr, 27.90.  +  b 'H. J. Rose and G. A. Jones, Nature (London) 307, 245 (1984). 2S. Gales,  E.  Hourani,  M.  Hussonnois, J.  P.  Schapira, L. Stab, and M. Vergnes, Phys. Rev. Lett. 53, 759 (1984). 3A.  Sandulscu, D. Pi. Poenaru, and  W. Greiner, Sov. J. Part. Nucl. 11, 528 (1980). 4D. N. Poenaru and  M. Ivascu, J. Phys. (Paris) 44, 791 (1983). SD.  N. Poenaru and  M. Ivascu, Central Institute of  Phy- sics, Bucharest, Report No. NP-27, 1983 (unpublished). 6~.  N.  Poenaru and  M. Ivascu, J. Phys. (Paris) 45, 1099 (1984). 'D.  N.  Poenaru,  M.  Ivascu,  A.  Sandulescu,  and W. Greiner, J. Phys. G 10, L183 (1984). 8A. A. Ogloblin et al., private communication. 490  @ 1985 The American Physical Society 

Comment on "Exotic Nuclear Decay of 223Ra by Emission of 14C Nuclei"

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