Correlations between neck fragments and Fission fragments AB

Correlations between neck fragments and Fission fragments AB no recoil AB with recoil Two principal momentum effects: 1. Initial Linear momentum of projectile transferred to target nucleus 2. Momentum transfer imparted by backward emitted IMF • For high energy/isotropically emitted fragments the folding angle decreases linearly with Z recoil effect • For low energy/NSE fragments the folding angle first decreases then increases with Z implying heavier middle fragments arise from more peripheral interactions. Romualdo de Souza

Deduce the average fractional linear momentum transfer, <FLMT> from the measured folding angle. • high energy/isotropically emitted IMFs are associated with a <FLMT> of 90 independent of the size of the fragment. • For heavy low energy/NSE fragments <FLMT> decreases with increasing fragment charge (mass). Heavy neck (NSE) fragments are associated with more peripheral collisions! Romualdo de Souza

What about incident energy dependence of the phenomena? The conclusion that heavy middle fragments are associated with more peripheral collisions is valid at both energies. Romualdo de Souza

Isotropic/ High energy component Observations • Relative yields of YZ/Y 3 increase with increasing E*/A • The increase has an exponential character • Over the measured excitation window, increases by factors of 3 -5 are observed. These facts are understandable in terms of an emission barrier which increases with increasing Z. SIMON semi-quantitatively reproduces the trend! Romualdo de Souza

Near Scission/Low Energy component • For Z=4 -7 the relative yield is flat with respect to excitation energy suggesting that either the barriers are the same as for Z=3 or the barriers are zero. • For Z 8, the relative yield decreases with increasing excitation energy. This does not make sense in a scenario where barriers either increase with Z or are constant. We must remember that the deduced E*/A scale comes from the LMT and is equally related to the impact parameter , b, or the angular momentum, . In fact, the deduced E*/A is at best the initial excitation of the fissioning system. Romualdo de Souza

Near Scission/Low Energy component For peripheral collisions ( large ) the probability of emitting a large middle fragment exceeds the probability of emitting a small middle fragment. For peripheral collisons we might expect an incomplete transfer of angular momentum into rotation resulting in a “stretching” of the nuclear matter. If this collective motion is not dissipated into heat, the dynamical model of fission predicts longer more stretched (skinny neck) scission configurations and larger middle fragments. (a. u. ) Romualdo de Souza

Comparison of NSE parameter with statistical expectations Use SIMON as one basis of comparison For all LMT/excitations considered (even complete fusion) the observed NSE parameter is substantially smaller. The parameter measured in NSE is at least 3 times smaller than the parameter measured for statistical emission from compound nuclei substantially different emission conditions or nonstatistical emission Romualdo de Souza

Comparison of NSE parameter to experimental statistical emission data The parameter measured in NSE is at least 3 times smaller than the parameter measured for statistical emission from compound nuclei substantially different emission conditions or nonstatistical emission M. A. Mc. Mahan et al. , Phys. Rev. C 54, 1995 (1985) Romualdo de Souza

Conclusions We have observed NSE of IMFs in hot ternary fission of 3, 4 He + 232 Th at E/A=50, 90 Me. V and 12 C + 232 Th at E/A=16, 22 Me. V. For the C induced reaction: Isotropic (early stage) emission has an excitation energy dependence consistent with statistical emission and emission barriers which increase with Z as expected. NSE/neck emission -Light middle fragments (Z=4 -6) equal or zero emission barriers Heavy middle fragments (Z>8) peripheral collisions preferred • source size effects for heavy fragments • limited excitation “cuts off” the availability of the statistical decay More central collisions (larger FLMT) More peripheral collisions (smaller FLMT) Smaller middle fragments Larger middle fragments More heat less , stretching Less heat larger , stretching Conclusion: Dynamics play a role in formation of the different scission configurations. Romualdo de Souza

Can all this neck emission can be explained as statistical emission? (as suggested by Moretto et al. ) PZ/P 3 = K(Z) exp(-(BZ-B 3)/T) Ln(PZ/P 3) = A(Z) - B/T = A(Z) – (a/E*)1/2 B B = BZ-B 3 B > 0 Barriers increase with increasing Z (a la LDM) B = 0 Barriers are the same B < 0 Barriers decrease with increasing Z heavy middle fragments arise from more stretched (long, thin necks) configurations than light middle fragments. Romualdo de Souza

Why the NSE process is dynamical (with statistics mixed in) Ternary fission can be thought of as occurring in two stages: 1. Formation of the scission configuration or configurations 2. Decay of these configurations There is no doubt that statistics is involved in the second stage! Moretto simple energy considerations Small middle fragments short, thick necks Large middle fragments long, skinny necks Carjan more complete dynamical calculations Important question: What led to the formation of the different scission configurations? You do not observe the long, skinny necks in SF/low energy or induced TF Entrance channel effect Romualdo de Souza

Moreover, The deduced E* which comes from the FLMT is at best the initial excitation of the system -- it makes sense early when the system is still compact. FLMT is also related to the angular momentum, of the collision smaller LMT (smaller deduced initial excitation) more peripheral collisions larger LMT (larger deduced initial excitation) more central collisions. Thus, larger middle fragments and smaller middle fragments arise from different initial angular momenta/impact parameters, and the parent distributions giving rise to the small and large neck fragments is different. Romualdo de Souza

Acknowledgements Indiana Univ. , Bloomington Simon Fraser Univ. T. A. Bredeweg R. G. Korteling S. L. Chen (General Electric, Medical Imaging) Michigan State Univ. Dr. D. E. Fields (Univ. of New Mexico) Dr. R. Lemmon (Surrey) Dr. E. Cornell (LBNL) Dr. R. Popescu (BNL) Dr. Y. Lou (LBNL) B. Davin R. Yanez (Univ. of Chile) K. Kwiatkowski (Los Alamos) Washington Univ. , St. Louis R. Charity L. G. Sobotka Vic Viola Argonne Nat’l Lab. Rd. S D. Hofmann Undergraduates: G. Wozneski and D. Hulbert Ball State Univ. J. L. Wile ( Pathologists Associated) Romualdo de Souza