Using reverse kinematics, we have studied the breakup of 1.0ital A GeV gold nuclei incident on a carbon target. The detector system permitted exclusive event reconstruction of nearly all charged ...reaction products. The moments of the resulting charged fragment distribution provide strong evidence that nuclear matter possesses a critical point observable in finite nuclei. We have determined values for the critical exponents gamma, beta, and tau. These values are close to those for liquid-gas systems and clearly different than those for 3D percolation and the liquid-gas mean field limit.
Rise and fall of multifragment emission Ogilvie, CA; Adloff, JC; Begemann-Blaich, M ...
Physical review letters,
09/1991, Letnik:
67, Številka:
10
Journal Article
Recenzirano
We have studied multifragment decays of Au projectiles after collisions with C, Al, and Cu targets at a bombarding energy of 600 MeV nucleon. We find that with increasing violence of the collision, ...measured via the multiplicity of light particles, the mean multiplicity of intermediate-mass fragments originating from the projectile first increases to a maximum {l angle}{ital M}{sub IMF}{r angle}{congruent}3 and then decreases again. Calculations using the Boltzmann-Uehling-Uhlenbeck model suggest that the fragmentation is governed by the energy {ital E}{sub dep} deposited into the projectile spectator and that {l angle}{ital M}{sub IMF}{r angle} reaches its maximum around {ital E}{sub dep}{congruent}8 MeV/nucleon.
We present microstructural data and lattice preferred orientations (LPOs) of omphacites from a suite of eclogites, from the Adula/Cima Lunga nappe (Central Alps). Our work shows a surprisingly strong ...correlation between the measured LPO and the ordering state of cations in omphacite. Estimates of deformation temperature from metamorphic petrology, together with measured omphacite compositions and LPOs, determine the field (ordering state), on the omphacite phase diagram, into which each sample falls. LPOs dominated by L-type and S-type signatures are developed in samples that fall in the P2/n field (ordered structure) and C2/c field (disordered structure), respectively.
Dislocations with
b=1/2〈−110〉 or
b=001 are observed in the transmission electron microscope (TEM) in all samples. The former change from a perfect dislocation in the C2/c structure to a partial in P2/n. Any movement of a partial dislocation requires the formation or growth of a stacking fault. Furthermore, in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation. The energy to form a constriction is high in omphacite due to the large separation width. Thus, the activity of the
b=1/2〈−110〉 dislocation is hindered in the P2/n structure relative to the C2/c structure, which change the balance between the two and might give rise to the different LPOs.
Energy spectra for intermediate mass fragments produced in central Au+Au collisions at E/A=100 MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion ...energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the cm system, but they decrease with Z sub , suggesting that all fragments do not participate equally in the collective expansion.
Exsolution and coarsening in Fe-free clinopyroxene of composition En
45.1Di
54.9 was studied at temperatures of 1300, 1200, and 1100°C, and annealing times between 10 min and 4320 h. Based on the ...wavelength of the exsolution lamellae, the exsolution process can be divided into exsolution sensu stricto and coarsening. During exsolution sensu stricto, the average wavelength of the “001” and “100” lamellae remains constant in contrast to the subsequent coarsening process. A progressive development of the microstructure is observed before coarsening which includes formation of island-like regions of dark contrast without phase separation, occurrence of pigeonite and diopside domains predominantly elongated along (100), formation of “100” exsolution lamellae, dissolution of the “100” lamellae, and predominant occurrence of “001” exsolution lamellae. All observations are in accordance with the exsolution mechanism of spinodal decomposition. After termination of the exsolution process, coarsening of the exsolution lamellae is observed. The “001” lamellae coarsen according to the rate law
λ
t
n−λ
0
n=n * k * exp(−ΔH/RT) * (t−t
0),
with λ
t
the average wavelength at time t, λ
0 the average wavelength at time
t
0, Δ
H an activation energy,
R the gas constant,
T the temperature K, and
n and
k empirical constants.
From a multiple regression analysis the following values of the constants (± standard error) are obtained:
n = 2.86 ± 0.15,
k = 1.29*10
14 ± 3.41*10
8 nm
2.86/
h, and Δ
H = 80.34 ± 1.76 kcal/mol. The initial wavelength λ
0 was found to increase with increasing temperature (11.0 nm at 1100°C; 17.5 nm at 1200°C; 20.7 nm at 1300°C). The time t
0 to complete exsolution decreases with increasing temperature (60 h at 1100°C; 18 h at 1200°C; 8 h at 1300°C). The exponent n is within the error limits equal to 3, indicating volume diffusion as rate-controlling process. Coarsening of the exsolution lamellae was found to be substantially faster than reported in previous studies.
Fragment distributions have been measured for Au+Au collisions at ital E/ital A=100 and 1000 MeV. A high detection efficiency for fragments was obtained by combining the ALADIN spectrometer and the ...MSU-Miniball/WU-Miniwall array. At both energies the maximum multiplicity of intermediate mass fragments (IMF) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at ital E/ital A=100 MeV may be related to a reduction of the density in momentum space which is caused by the collective expansion.
Kinetic energies of light fragments ( A</=10) from the decay of target spectators in 197Au+197Au collisions at 1000 MeV per nucleon have been measured with high-resolution telescopes at backward ...angles. Except for protons and apart from the observed evaporation components, the kinetic-energy spectra exhibit slope temperatures of about 17 MeV, independent of the particle species, but not corresponding to the thermal or chemical degrees of freedom at breakup. It is suggested that these slope temperatures may reflect the intrinsic Fermi motion and thus the bulk density of the spectator system at the instant of becoming unstable.
— The microstructure of Fe‐rich clinopyroxene from synthetic analogues of chondrules was studied by transmission electron microscopy. The samples were cooled at various rates from 1455 °C to the ...quench temperature of 1000 °C. Slow cooling at rates below approximately 50–60 °C/h leads to the development of coherent pigeonite/augite exsolution lamallae on (001). A final wavelength of 19.6 ± 1.1 nm was obtained at a cooling rate of 10 °C/h, and 17.4 ± 2.4 ran at a cooling rate of 50 °C/h. Faster cooling at rates between approximately 50 and 450 °C/h yields only modulated structures with a wavelength on the order of 17–19 nm for the (001) orientation.
Coherent exsolution lamellae on (001) in clinopyroxene occur in chondrules of H, L, LL, and CV chondrites, indicating that slow cooling of chondrules at subsolidus temperatures is a widespread phenomenon. The variation of the lamellar wavelength observed in natural chondrules corresponds to a variation of the subsolidus cooling rates between ∼0.1 and 50 °C/h. The low cooling rates at subsolidus temperatures deduced from the microstructure of Fe‐rich clinopyroxene point to nonlinear cooling, with cooling rates decreasing with decreasing temperature.
Attempts are made to quantify cooling rates of chondrules from the microstructure of clinopyroxene and Ca-rich plagioclase. Pigeonite/diopside exsolution lamellae on (001) are frequently observed in ...Allende granular olivine pyroxene chondrules. Cooling rates of 2–10°C/h are obtained for the observed wavelengths between 25 and 33 nm from isothermal time-temperature-transformation diagrams. Modelling of lamellae growth during cooling yields similar cooling rates between 7 and 25°C/h. Coarsening of pigeonite/diopside exsolution lamellae takes place in the small temperature interval from approximately 1350 to 1200°C. The occurrence of type b-antiphase domains in Ca-rich plagioclase provides independent constraints on chondrule cooling rates. Isothermal time-temperature-transformation diagrams yield cooling rates of approximately 3–30°C/h for the observed domain sizes between 10 and 30 nm. Cooling rates derived from antiphase domains in plagioclase are valid in approximately the same temperature range than those deduced from pigeonite/diopside exsolution lamellae. Our estimates must be regarded as upper limits, and they seem to be significantly lower than those derived from dynamic crystallization experiments (100–2000·gC/h). As dynamic crystallization experiments yield cooling rates at higher temperatures near the liquidus, this observation may suggest nonlinear cooling. The absence of orthoenstatite lamellae in clinoenstatite indicates high cooling rates on the order of 10
4°C/h at temperatures around 1000°C.
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