1. Peripheral collisions as a means of attaining high excitation –Velocity dissipation is key quantity R. Yanez et al, PRC (in press) Proximity emission as a clock of the statistical emission time scale Outline Thanks to Indiana University:S. Hudan, R. Yanez, A.S. Botvina, B. Davin, R. Alfaro, H. Xu, Y. Larochelle, L. Beaulieu, T. Lefort, V.E. Viola Washington University, St. Louis:R.J. Charity, L.G. Sobotka Michigan State University: T.X. Liu, X.D. Liu, W.G. Lynch, R. Shomin, W.P. Tan, M.B. Tsang, A. Vander Molen, A. Wagner, H.F. Xi, C.K. Gelbke Decay of highly excited projectile-like fragments produced in dissipative peripheral collisions at intermediate energies. Thermodynamic properties of nuclear matter (esp. N/Z exotic) Decay properties of hot nuclei (finite, reaction dynamics, etc.) R.T. de Souza, Indiana UniversityHIC03, Montreal

2. Experimental details Ring Counter : Si (300  m) – CsI(Tl) (2cm) 2.1    lab  4.2  δZ/Z ~ 0.25 Mass deduced † Beam LASSA :  0.8  Mass resolution up to Z=9 7    lab  58  114 Cd + 92 Mo at 50 A.MeV  Detection of charged particles in 4  † EPAX K. Sümmerer et al., PRC 42, 2546 (1990) Projectile 48 B. Davin et al., NIM A473, 302 (2001)

3. 114 Cd 92 Mo Overlap zone is highly excited 1.PLF* and TLF* are relatively unexcited. 2. nearly unchanged from beam velocity. 3.Impact parameter is the key quantity in the reaction. PLF* TLF* Select PLF at very forward angles 2.1    lab  4.2  Participant- Spectator model L.F. Oliviera et al., PRC 19, 826 (1979) Z projectile

4. PLF * decay following a peripheral collision PLF* = good case: (as compared to central collisions)  System size (Z,A) is well -defined   Normal density  Large cross-section (high probability process)   0  0 Circular ridge  PLF* emission “Isotropic” component Projectile velocity Other emission (mid-rapidity,...) Examine emission forward of PLF* Select 15≤Z PLF ≤46 with 2.1    lab  4.2 

5. With decreasing V PLF*, the kinetic energy spectra have less steep exponentials  higher temperatures V beam -V PLF* B  Barrier parameter T  Temperature parameter D  Barrier diffuseness parameter Maxwell-Boltzmann J.P.Lestone, PRL 67, 1078 (1991). “pre-equilibrium” component  2% Forward of the PLF*

6. Evaporation and velocity damping v beam IMFs also well characterized by MBD, exhibit larger slope parameters  emission earlier in de-excitation cascade  Multiplicities increase with velocity damping  T slope increases with velocity damping  “Linear” trend for both observables

7.  (Linear) dependence of E* with velocity damping  High E* is reached (  6 MeV/n) Velocity damping and excitation energy Reconstruct excitation of PLF* by doing calorimetry: particle multiplicity, kinetic energies, and binding energies. D. Cussol et al., Nucl. Phys. A 541, 298 (1993)  Good agreement with GEMINI *  Some sensitivity of M  to J, level density * “Statistical model code” R.J. Charity et al., PRC63, 024611 (2001) Multiplicities, average emitted charge predicted by GEMINI support deduced excitation scale.

8. Select PLF* size by selecting residue Z. Select excitation by selecting V PLF* Vary N/Z by changing (N/Z) proj.,tgt. When selected on V PLF*, total excitation is independent of Z PLF. If Z PLF is related to the overlap of the projectile and target (impact parameter), this result says that has the same dependence on V PLF*, independent of overlap. 10 20 30 40 50

9. Statistical decay in an inhomogeneous external field vs. PLF* TLF* V PLF* TLF* V successive binary decays of PLF* as it moves away from TLF* with velocity V modified Weisskopf approach consider all binary partitions up to emission of 18 O -- both ground and particle-stable excited states. Starting at an initial distance D, the total decay width, Г, is calculated τ=ħ/Г and P(t) ~ exp(-t/ τ) PLF* Initial distance = 15 fm (Z,A) PLF* = 38, 90 ; based on experimental data Z TLF* = 42 ; taken as point source For a fixed PLF*-TLF* distance 2 j f 2 f j

10. de-excitation of isolated and proximity cases fairly similar as a function of time At E*/A = 2 MeV, proximity case de-excites slightly faster No difference is observed at E*/A = 4 MeV By 250 fm/c, most of rapid de-excitation has occurred. V=0.2728c  t=250 fm/cD=70 fm Distinguish: Early emissions: D ≤ 70 fm Late emissions: D > 70 fm

11. Distinguish: Early emissions: D ≤ 70 fm Late emissions: D > 70 fm Early emissions are backward peaked Late emissions have a symmetric angular distribution Angular distribution is peaked in direction of the TLF* with an enhancement by a factor of 3-7 as compared to cos(θ)=0. Asymmetry of the angular distribution can provide a “clock” of the statistical emission time scale. Towards TLF*Away from TLF*

12. As expected, early emissions populate the tail of the kinetic energy distribution. Coulomb proximity introduces a correlation between emission angle and time. As they occur on average earlier, backward emissions (towards the TLF*) are “hotter” and forward emissions are “colder”. Calorimetry based on forward emission that assumes isotropy under- predicts the initial excitation of the PLF*

13. Sensitivity of different emitted particles as a “clock” d, t, 3 He and in particular IMFs exhibit emission time distributions more sharply peaked at short times as compared to p and α. These particles are therefore preferentially emitted towards backward angles.

14. Selection of Experimental data: E α ≤ 22 MeV (α’s on ridge) ┴ 114 Cd + 92 Mo at 50 A.MeV

15. Both the asymmetry of the angular distribution and the kinetic energy spectra of forward emitted alpha particles can be explained by this schematic Coulomb proximity model.

16. Sensitivity of the “clock” Y backward /Y forward decreases with increasing initial distance (equivalent to increased pre-saddle time) For a fixed distance, Y backward /Y forward decreases with both increasing E* and J  decreased influence of barrier difference caused by external field. Alternatively, increasing the external field increases the asymmetry.

17. Conclusions  Highly excited PLF* formed in peripheral heavy-ion collisions at E/A = 50 MeV Excitation energy is connected with velocity dissipation Different overlaps have the same dependence of on velocity dissipation  Coulomb proximity decay provides a clock for the statistical emission time scale Examine dependence on E*, Z target, V PLF* to characterize emission.

18. Proximity Coulomb decay: A clock for measuring the statistical emission time scale Backward enhancement of alpha particles along Coulomb ridge. IMFs show a larger backward/forward enhancement than alpha particles IMFs preferentially sample the earlier portion of the de- excitation cascade. Previous work: D. Durand et al., Phys. Lett. B345, 397 (1995).