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.
Outline

• 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

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
R.T. de Souza, Indiana University
HIC03, Montreal
2
Experimental details
114Cd 92Mo at 50 A.MeV
LASSA ???0.8? Mass resolution up to Z9 7? ?
?lab ? 58?
B. Davin et al., NIM A473, 302 (2001)
Ring Counter Si (300 ?m) CsI(Tl) (2cm) 2.1? ?
?lab ? 4.2? dZ/Z 0.25 Mass deduced
? Detection of charged particles in 4p
EPAX K. Sümmerer et al., PRC 42, 2546 (1990)
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Overlap zone is highly excited

• PLF and TLF are relatively unexcited.
• ltVPLFgt 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)
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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)
Select 15ZPLF46 with 2.1? ? ?lab ? 4.2?
Other emission (mid-rapidity, ...)
Circular ridge ? PLF emission Isotropic
component
Examine emission forward of PLF
Projectile velocity
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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.
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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 ltEgt 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.
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Statistical decay in an inhomogeneous external
field
For a fixed PLF-TLF distance
2
2
vs.
f
j
f
j

• 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
  18O
• -- both ground and particle-stable excited
  states.
• Starting at an initial distance D, the total
  decay width, ?, is calculated
• th/? and P(t) exp(-t/ t)
• PLF

Initial distance 15 fm (Z,A) PLF 38, 90
based on experimental data ZTLF 42 taken as
point source
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.

V0.2728c ?
Distinguish Early emissions D 70 fm Late
emissions D gt 70 fm
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Angular distribution is peaked in direction of
the TLF with an enhancement by a factor of 3-7
as compared to cos(?)0.
Distinguish Early emissions D 70 fm Late
emissions D gt 70 fm

• Early emissions are 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.
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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 a.
• These particles are therefore preferentially
  emitted towards backward angles.

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Selection of Experimental data Ea 22 MeV (as
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.
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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.
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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
  ltEgt 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
Proximity Coulomb decay A clock for measuring
the statistical emission time scale
Previous work D. Durand et al., Phys. Lett.
B345, 397 (1995).