Peripheral collisions as a means of attaining high excitation

1. 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.) 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 • 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 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 Michigan State University: HIC03, Montreal R.T. de Souza, Indiana University

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

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

4. PLF* decay following a peripheral collision r r0 PLF* = good case: (as compared to central collisions) System size (Z,A) is well -defined  Normal density Large cross-section (high probability process) Select 15≤ZPLF≤46 with 2.1lab 4.2 Other emission (mid-rapidity, ...) Circular ridge  PLF* emission “Isotropic” component Examine emission forward of PLF* Projectile velocity

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

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

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

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

9. Statistical decay in an inhomogeneous external field PLF* PLF* V V TLF* TLF* For a fixed PLF*-TLF* distance 2 2 vs. j f f j • successive binarydecays of PLF* as it moves away from TLF* with velocity V • modified Weisskopf approach • consider all binary partitions up to emission of 18O • -- 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 ZTLF* = 42 ; taken as point source

10. t=250 fm/c D=70 fm • 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  Distinguish: Early emissions: D ≤ 70 fm Late emissions: D > 70 fm

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

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” andforwardemissions 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, 3He 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: Eα ≤ 22 MeV (α’s on ridge) 114Cd + 92Mo 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” • Ybackward/Yforward decreases with increasing initial distance (equivalent to increased pre-saddle time) • For a fixed distance, Ybackward/Yforward 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 <E*> on velocity dissipation • Coulomb proximity decay provides a clock for the statistical emission time scale • Examine dependence on E*, Ztarget, VPLF* to characterize emission.

18. 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. Proximity Coulomb decay: A clock for measuring the statistical emission time scale Previous work: D. Durand et al., Phys. Lett. B345, 397 (1995).