Velocity damping and fragmenta2on in non- central intermediate energy heavy- ion collisions

Transcription

1 Degree of equilibra2on depends on contact 2me and gradient of poten2al driving equilibra2on Velocity damping and fragmenta2on in non- central intermediate energy heavy- ion collisions 112,124 Sn + 112,124 Sn E/A = 50MeV Even for system with no ini%al driving force toward N/Z equilibra2on: Enhancement of MRM in N/Z at expense of PLF Normal density 64 Zn + 64 Zn E/A = 45MeV IU, Laval, TAMU Low density degree of N/Z equilibra2on M.B. Tsang et. al., PRL 92, (2004) D. Thériault et. al., PRC 74, (R) (2006)

2 Dynamical Breakup It s a rich environment Interplay of sta2s2cal and dynamical processes Backward emission strongly aligned B. Davin et. al., PRC 65, (2002) Hierarchy observed between fragment size and velocity J. Colin et. al., PRC 67, (2003) To what extent does the dynamics impact the extrac2on of the density dependence of the symmetry energy?

3 Reac2on Characteris2cs: 114 Cd + 92 Mo at E/A = 50 MeV Emifng Source color indicates yield in log-scale α-particles detected in LASSA PLF: 15<Z<46, 6.5<v<9.5 cm/ns; 2 θ 4 Sta2s2cal Emission Coulomb Ridge Yanez et.al., PRC 68, (R) (2003) Boltzmann Distribution Shape & Yield are Independent of Emission Angle Isotropic Emission

4 damping PLF * : velocity damping Slower PLF* are more excited and emit more particles with higher kinetic energies. R. Yanez et al., PRC68, (R) (2003) By selecting the velocity of the PLF* we can select how excited it is.

5 AMD: A microscopic picture System: 114 Cd + 92 Mo at 50 MeV/nucleon events accumulated ( years!) Mass, charge, energy exchange Binary nature of the collision Transiently deformed nuclei Early cluster production, t 90 fm/c Clusters are also statistically emitted as the excited reaction products de-excite! S. Hudan, R.T. de Souza and A. Ono, PRC (2005)

6 136,124 Xe + 112,124 Sn at E/A = 50 MeV Charged- Par2cle Setup Forward Indiana Ring Silicon Telescope Large Area Silicon Strip Array beam s eye view LASSA Si(IP) Si(IP) CsI(Tl) 65µm, 500µm, 6cm 36 51, Δθ=0.92 T1 (FIRST) Si(IP) Si(IP) CsI(Tl)/PD 273µm (S2), 977µm (S2), 3cm 2 7, Δθ=0.094 Detector protected inside of 3 for most of the experiment FIRST: T. Paduszynski et. al., NIM A 547, 464 (2005) LASSA: B. Davin et. al., NIM A 473, 302 (2001) A. Wagner et. Al., NIM A 456, 209 (2001) T2 (FIRST) Si(IP) CsI(Tl)/PD 500µm (S1), 3cm 7 14, Δθ=0.45 Si- CsI stack detectors & Si- Si- CsI stack detectors ΔE- E technique 30 cm High Segmenta2on High Angular Resolu2on (592 Si channels)

7 Par2cle Iden2fica2on in T1 Telescope Si Si CsI Si- Si Z=54 elastic Si- Si 12 C 16 O 22 Ne 25 Mg 30 Si Good Z resolu2on for Z C 16 O 22 Ne 25 Mg 30 Si Si- CsI Good A resolu2on for Z 14

8 Projec2le- Like Fragment _ PLF _ Heaviest par2cle detected in FIRST (3-14 ) 124 Xe+ 124 Sn 50 A MeV As Z decreases from Z beam, velocity ini2ally decreases (damping) As Z con2nues to decrease, velocity remains constant (apparent satura2on of damping)

9 Two fragments in 3 θ lab 7 Z H 21 Z L 4 Z H + Z L at least Z=25 (~ ½ Z projec2le ) For Z L 8, V parallel distribu2on for Z H is single peaked Associated distribu2on of V parallel for Z L has two peaks located at larger and smaller veloci2es For Z L = 14 the V parallel distribu2on for Z H shows two peaks Velocity of Z L is not peaked at the center- of- mass, par2cularly for heavy Z L Consistent with binary decay of a PLF* Z PLF* =Z H +Z L v PLF* = v cm

10 v rel α v cm V cm distribu2ons for V H > V L ( backward emission ) and V L > V H ( forward emission ) are similar, exhibi2ng damping from beam velocity. Backward emission has a slightly larger damping on average than forward emission. Backward emission has an addi2onal component of higher rela2ve veloci2es not observed for forward emission (not just mid- rapidity emission).

11 From elas2cally scayered beam, smaller Z PLF is associated with lower velocity (increased damping). For inclusive events, this reaches a minimum of 8.9 cm/ns at Z~35. Velocity damping Binary decay of the PLF* is associated with larger damping, however the dependence on the size of the PLF* is approximately the same as for inclusive events. For the smallest PLF* measured, a slight increase in the most probable velocity of the PLF* is observed.

12 For the least damped cases, no forward backward asymmetry is evident. With increasing damping, the preference for backward emission increases. Angular distribu2on of the binary decay is preferen2ally peaked for cos(α) > 0, backward emission Although asymmetry of forward and backward emission decreases with increasing Z L, it is s2ll evident for Z L = 18.

13 The difference distribu2ons reflect the short- lived decays of the PLF*. The angular distribu2ons associated with these decays become broader with increasing Z L. We associate the forward emission with the long- lived sta2s2cal emission of a hot, rota2ng PLF*. Using the observed yield in the forward direc2on as a reference and assuming isotropic emission, we calculate the backward emission, correc2ng for the detector acceptance (ROOT)

14 Arrows indicate the integra2on region used. The angular distribu2on of the excess yield in the backward direc2on was also extracted as a func2on of the center- of- mass velocity of the Z H - Z L pair.

15 The yield of the short- lived/ dynamical component first increases with increasing Z L, is peaked at Z L = 6 and then decreases smoothly. The distribu2on of V cm associated with this process is significantly damped from beam velocity. All major trends observed for the extracted yield (difference) are also observed for the total yield observed backward.

16 Langevin model: Modeling the Short-lived ( backward ) component 1) Z L - PLF interac2on 2) TLF*- PLF* system Coulomb interac2on While the TLF* and PLF* separate, they evolve smoothly on a classical trajectory. Observed angular asymmetry ini2al deforma2on towards the TLF* Persistence of the ini2al configura2on High fric2on Ini2al configura2on near barrier TLF* R TLF α x PLF Propaga2on in 2me of the system:, with β rela2ve to the fric2on, F force due to the poten2al, temperature T, fluctua2ng term k (thermal). As the TLF* and PLF* separate the barrier changes.

17 For a given value of β and spin of the PLF* (40 hbar), the angular distribu2on is calculated as a func2on of the ini2al posi2on of Z L on the poten2al. Comparison with the measured angular distribu2on with the model calcula2ons allows extrac2on of the average ini2al configura2on.

18 Distance from the top of the barrier increases linearly with Z L Fragments with Z L 9 are produced outside of the barrier.

19 Emission times for short-lived ( backward ) component From the model the average emission 2me can be extracted Emission 2mes increase smoothly with increasing Z L Even near symmetric splits of the PLF* exhibit short emission 2mes of <1.5 x s (<450 fm/c) These 2mes are shorter than those extracted in Mo + Mo at E/A = 19 MeV but are comparable to the integrated 2mes for Sn + Nb at ~30 MeV/A [Casini93] G. Casini et al., PRL (1993) [Piantelli02] S. Piantelli et al., PRL (2002)

20 Do we observe N/Z equilibra2on on the 2mescale of the rota2on? The composi2on of fragments changes as a func2on of rota2on angle. Light fragments Z L < 8 in par2cular exhibit this dependence. Lack of equilibra2on for Z L < 8 is consistent with produc2on of these fragments outside the barrier. Observa2on of the dis- equilibrium for Z<8 indicates that the 2mescale for N/Z equilibra2on is longer than the saddle- to- scission 2me for the Z H - Z L system.

21 From binary to ternary fragmentation of the PLF* Goal: Study density dependence of asymmetry term with minimum sensitivity to dynamical effects Approach: Select non-aligned breakups with high excitation Simplest interesting case of PLF* multifragmentation : ternary breakup 124 Xe Sn at E/A = 50 MeV 124 Xe 112 Sn N/Z

22 Data Charge Dalitz Three or more fragments with Z 5 in 3 θ lab 15 The largest yield is associated with one large Z fragment and two mid- size fragments with similar Z. Also observed is a non- negligible yield of equal size ternary breakup Symmetric ternary breakup is selected as shown by the dashed circle. The maximum charge difference (ΔZ) = 4 Require : Z total 27 in 3 θ lab 15

23 SMM Charge Dalitz Z = 45, A = E*/A 6 MeV 0 I 83 hbar ρ/ρ 0 = 1/6 (Markov) low E*/A, Low I evapora2on- like payern low E*/A, high I fission- like payern with increasing E*/A, increased symmetric ternary breakup Overall experimental charge correla2on is not reproduced by a single SMM calcula2on

24 Isotope distributions for symmetric PLF* fragmentation Isotope distribu2ons for 4 Z 14 reveals that the yield for neutron- rich fragments is under- predicted by SMM. This discrepancy is par2cularly notable for heavy fragments with Z 7 For Z 7, this under- predic2on on the neutron- rich side can be more than a factor of 10.

25 Isotope distribu2ons Reducing the symmetry energy parameter γ from 25 to 14 MeV provides a beyer descrip2on of the yield of neutron- rich fragments

26 <N>/Z for projectile fragmentation The discrepancy between SMM and the experimental data is also visible in <N>/Z. Reduc2on of the symmetry energy parameter from γ=25 to γ=14 MeV provides a beyer descrip2on of <N>/Z however significant discrepancies exist. Reduc2on of the symmetry energy parameter was also required to reproduce the <N>/Z of projec2le- like fragments in dissipa2ve collisions. Soulio2s et al., PRC (2007) and the isotope distribu2ons of light IMFs (Z<7) Iglio et al., PRC (2006).

27 Conclusions Aligned binary fragmenta2on of the PLF* is a fast process (<τ> < 1.5 x s), even for near symmetric splits. The 2mescale decreases with increasing asymmetry. For the most asymmetric splits the 2mescale is 0.25 x s (~100 fm/c). Z L < 9 fragments are produced at or beyond the barrier. For Z L < 8, the dependence of <N> - Z on the degree of alignment suggests that N/Z equilibra2on between the Z L and Z H fragments occurs on the 2mescale of the rota2on. The dis- equilibrium in <N> - Z for Z L < 8, which decreases with increasing Z L, supports the conclusion that these light fragments are produced at or beyond the barrier. Isotope distribu2ons of fragments, par2cularly those with Z>7, associated with ternary symmetric breakup of the PLF*, provide a sensi2ve probe of the symmetry energy. The measured yield of neutron- rich fragments requires reducing the symmetry energy parameter in SMM.

28 Acknowledgements Indiana University A.B. Mcintosh S. Hudan J. Black D. Mercier Z. Gosser C.J. Metelko M.J. Rudolph R. Yanez GANIL A. Chbihi GSI S. Bianchin C. Schwarz W. Trautmann Universite Laval M-O. Fregeau J. Gauthier J. Moisan R. Roy Western Michigan University M.B. Famiano University of Moscow A.S. Botvina This work was supported by: U.S. Dept. of Energy Grant No DEFG02-88ER