A Comment on the Letter by W. Lichten and A. Robatino, Phys. Rev. Lett. 54, 781 (1985). See Also: W. Lichten and A. Robatino, New atomic mechanism for positron production in heavy-ion collisions, Phys. Rev. Lett. 54, 781 (1985). http://prola.aps.org/abstract/PRL/v54/i8/p781_

Greiner, Walter

Müller, Berndt

Reinhardt, Joachim

English

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VOLUME  55,  NUMBER  1  PHYSICAL REVIEW LETTERS  1 JULY  1985 Comment on "New Atomic Mechanism for Pos- itron Production in Heavy-Ion Collisions" In two recent experiments sharp lines have been ob- served in the spectra of  positrons created in collisions of  very heavy ions.'r2  According to theoretical expec- tations,  the combined  nuclear  charge  of  the systems studied is supercritical, i.e., the K shell of  the united atom is  unstable  against  spontaneous  positron  emis- sion.  Since the time scale of  ordinary collisions is too short to expect a clear signal from this rnechani~m,~  it has  been  suggested  that  the  observed  positron  lines originate  from  long-lived  nuclear  compound  sys- tem~.~?~  Although at present  there is no independent confirmation of  this model and though it does not in a natural way  predict the observed Z dependence of  the effect, no plausible alternative has yet been found. Here we want to argue that the mechanism proposed by  Lichten and ~obatino~  also is not well founded and should be dismissed as an explanation for the observed positron lines.  The argument of Ref. 6 is motivated by the  idea  (i)  that  multiple  excitations  have  not  been treated correctly up to now and  (ii)  that two-electron transitions play a key role in positron creation. Regarding  (i) we mention that Ref. 3 gives an exact description  of excitations  of the many-particle  systems as long  as  electron  correlation  interactions  are  neglected. The main  effect  of  the  electron-electron interaction may be included by  use of  a mean screening potential VS.' There is no indication that the explicit electron- electron two-body  interaction  (Hee -  I/,)  plays  a sub- stantial role in inner-shell processes.  In fact, the pro- posed two-electron transitions have been found to lead to an exceedingly  small  transition  ~idth.~  The claim that  the strength of  two-electron  transitions  is deter- mined  by  the  dynamical  coupling  matrix  elements, thus being very large,  is unfounded.  Dynamical exci- tations  are induced  by  the  one-body  operator  R a/aR (and  possibly  rotational  coupling) .  Multiply  excited configurations can be reached only by  the repeated ac- tion of  this operator.  This is fully accounted for in the coupled-channels calculations. The explicit calculation presented in Ref. 6 is based on the assumption that positron excitation is restricted predominantly  to a narrow  region around R = 500 fm internuclear distance, where the radial-coupling matrix element between the bound states 2p3/2u and  2p1/2u has a rnaxirn~m.~  For this postulate we can See no jus- tification. It  is  interesting  to  study  the  effect of  the  2p312u +  2pl12a  coupling  within  the  correct  coupled- channels  framework.  The  result  of  two  calculations for a central U+U collision at 5.9 MeV/u is shown in Fig.  1.  The dashed  line gives the spectrum of  posi- trons emitted out of  the 2pl12u  state only.  Holes are fed into this state via radial coupling from the 2p312u -- /  '  ,  p-states  ' /  \ \  i FIG. 1.  Energy spectra of  positrons emitted in head-on U +U collisions at bombarding energy of 5.9 MeV/nucleon. level,  which  is considered  to  be  initially  emply.  For comparison the figure  also  shows  our  earlier  results (solid  line)  including six  npl12u bound  states  (occu- pied  initially  up  to  4plI2u)  and  the  positive-energy contin~um.~ It is noticed that the effects of a hole present in the 2p312u level  before the collision  would  dominate the positron  spectrum.  This is due to the very large hole transfer  rate  into  the  2p1/2a  state,  which  in turn  is quite  strongly  coupled  to  the  positron  continuum. Although the shapes of  the two curves in the figure differ somewhat, obviously no oscillatory features are introduced.  Our  result  emphasizes the need  to  per- form  quantitative  calculations  based  on  concrete models as  demonstrated  for  the model  of  long-lived nuclear  c~m~osites,~~~  if  any  conclusions  are  to  be drawn with respect to the origin of  the positron  lines. In conclusion, we have shown that the mechanism ex- plicitly  considered  by  Lichten  and Robatino does not lead to narrow structures in the positron spectra. J. Reinhardt, B. Müller, and W. Greiner Institut für Theoretische Physik Johann-Wolfgang Goethe Universität 6000 Frankfurt, West Germany Received 5 February 1985 PACS numbers:  34.50.-s,  14.60.Cd, 34.90.  +q 'J.  Schweppe  et al.,  Phys.  Rev.  Lett.  51, 2261  (1983); T. Cowan et al., Phys. Rev. Lett. 54,  1761 (1985). 2M. Clemente et al., Phys. Lett. 137B,  41 (1984). 35. Reinhardt et al., Phys. Rev. A 24,  103-28  (1981). 4H.  U. Müller et al., Z. Phys. A 303,  173-88  (1981). 5~.  Heinz et al., Ann. Phys. (N.Y.) 158,  476 (1984). 6W.  Lichten  and A. Robatino,  Phys.  Rev. Lett. 54, 781 (1985). 7T. deReus et al., J. Phys. B 17, 615 (1984). *G. Soff et al., Phys. Rev. A 20, 169 (1979). 134  O  1985 The American Physical Society 

Comment on "New atomic mechanism for positron production in heavy-ion collisoins"

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