Dr. DeYoung and Dr.Peaslee (Nuclear Group)
Supported by the NSF-RUI (Nuclear Physics Division)
Previous studies have devised a method to determine the temperature of a nuclear system by using ratios of emitted particles from the excited nucleus [S. Albergo et al., Nuovo Cimento 89, 1 (1985)]. Basically, this method relates the nuclear temperature to the probability of emission of less stable isotopes. This method has been successfully used to observe a first-order liquid-gas phase transition at a temperature of approximately 5 MeV [J. Pochodzalla et al. (ALADIN), PRL 75, 1040 (1995)]. Further studies are actively seeking a phase transition to the quark-gluon plasma phase.
A problem with this temperature model is that there is no angular dependence because the literature suggests statistical emission dominates over any direct processes. We suggest pre-equilibrium scattering, a non-statistical process which dominates at forward angles, also needs to be taken into account. Pre-equilibrium scattering can cause a decrease in temperature at forward angles because the less stable isotopes are less likely to be emitted than in statistical emission.
In the isotopic yield ratio method, nuclear temperature is deduced by
T = B / ln (a ·
R). The value a = 1.59 is a constant derived from spin factors and kinematics.
B = Eb(4He) - E
This experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. A 84Kr + 197Au Þ 281X reaction was produced with beam energies of 35, 55, 70 MeV/A. The Miniball detector array was used for particle detection where CsI detectors covered 89% of the solid angle. Data were analyzed at four theta angles; 10o, 16.25o, 27o, 45o. Particle identification gating was completed using visual band gaps in spectra for the 70 MeV/A runs. Gain shifts in the electronics were minimal, so minor linear gain shift correction were implemented as necessary. There were no gain shifts between the 55 MeV/A and 35 MeV/A runs. Multiplicity gating was also done to determine if impact parameter had an effect on temperature. After the yields were obtained and temperatures deduced, modeling with Modgan was performed. Although Modgan is not optimized to reproduce this relatively high energy system, correction factors were added to deduce approximate results.
The results show that there is an angular dependence on temperature in the lab frame. At forward angles, the temperature has a steep positive slope to a peak temperature at approximately 30o in the lab frame. Because of the angular dependence of pre-equilibrium scattering, the diminished temperature can be accounted for by statistical emission not being the dominant decay process in the forward direction. This conclusion is also supported by multiplicity results. Multiplicity results show a high temperature for central collisions and low temperatures for peripheral collisions at forward angles. If central collision pre-equilibrium scattering is shadowed by the target nucleus, dampening its effect on temperature, then central collisions could be described by just statistical emission. Compared to the Modgan model, which models only statistical emission and central collisions, we have a good match between the model and the data. An unexplained phenomenon in the data is a slight decrease in temperature at backward angles beyond 30o in the lab frame. Modeling is being done and kinematic factors are being considered for this effect.