Thermalization in nucleus-nucleus collisions

doi:10.1016/0370-2693(92)90642-H

Physics Letters B

Volume 282, Issues 3–4, 28 May 1992, Pages 299-304

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Abstract

Impact parameter dependent excited state populations of intermmediate mass fragments are investigated for ³⁶Ar induced reactions on ¹⁹⁷Au at E/A = 35 MeV. Population inversions, indicative of non-thermal excitation mechanisms, are observed in peripheral collisions characterized by low associated charged particle multiplicities. These population inversions disappear for collisions with larger associated charged particle multiplicities, consistent with a more complete thermalization for more complex final states. Discrepancies, observed in central collisions, suggest that the limit of local thermal equilibrium has not yet been observed.

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Recent studies have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. The probability of emitting n-fragments is found to be reducible to the probability of emitting a single fragment through the binomial expression. The resulting one fragment probability shows thermal scaling by producing linear Arrhenius plots. Similarly, the charge distributions associated with n-fragment emission are reducible to the one-fragment charge distribution. Thermal scaling is also observed. The reducibility equation contains a constant whose value, zero or positive, can be related to a univariant (two phase) or bivariant (one-phase) regime. The light fragment particle-particle angular correlations also show reducibility to the single-particle angular distributions as well as thermal scaling. A mass scaling associated with the angular correlations suggests emission from several small sources (A ≈ 20). The limits of applicability of scaling and reducibility are discussed as well as their implications for the mechanism of multifragmentation.
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Two-particle correlation functions were measured for the emission of protons, deuterons, tritons, α, and lithium fragment at small relative momenta in ⁴⁰Ar-induced reactions on ¹⁹⁷Au at 25 MeV/nucleon. Based on three-body trajectory calculations the emission time scales for the particles were extracted from p-d, d-d, t-t, and lithium-lithium correlation functions. The mean emission time was found to decrease with increasing sum of the kinetic energies of the particle in a correlated pairs, indicating the emission of more energetic particles at earlier stages of the reaction than at later stages. The time scale for the emission of lithium fragments emission suggests that sequential binary disassembly occurs in ⁴⁰Ar+¹⁹⁷Au reaction at MeV/nucleon. Mean emission temperature of 3.5_−0.8^+1.3 and 3.6±0.4 MeV were also extracted from the relative populations of the excited and ground states of the respective emitted nuclides ⁸Be and ⁴He.
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^☆: This paper is based upon work supported by the National Science Foundation under Grant number PHY-89-13815.

^∗: W.G.L. acknowledges the receipt of a US Presidential Young Investigator Award.

¹: Present address: Indiana University Cyclotron Facility, 2401 Milo B. Sampson Lane, Bloomington, IN 47405, USA.

²: Present address: Cyclotron Institute, Texas A&M University, College Station, TX 77843-4242, USA.

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Physics Letters B

Thermalization in nucleus-nucleus collisions☆

Abstract

Prog. Part. Nucl. Phys.

Prog. Part. Nucl. Phys.

Phys. Rep.

Phys. Lett. B

Nucl. Phys. A

Nucl. Phys. A

Nucl. Phys. A

Nucl. Phys. A

Nucl. Instrum. Methods A

Nucl. Instrum. Methods A

Phys. Rev. Lett.

Phys. Rev. C

Nucl. Phys. A

Phys. Lett. B

Energy levels of light nuclei A = 8,9,10

Statistical exploration of fragmentation phase space and source sizes in nuclear multifragmentation

Reducibility and thermal scaling in nuclear multifragmentation

The correlation functions and emission time scales for light particles and light fragments in <sup>40</sup>Ar+<sup>197</sup>Au reactions

Limiting temperatures and disappearance of fusionlike residues

Multifragment decay of highly excited nuclei