Is reducibility in nuclear multifragmentation related to thermal scaling?1

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Abstract

Thermal scaling (Arrhenius law for an “elementary” probability p of binomial function) and reducibility in intermediate mass fragments (IMF's) production are examined for data of the reaction 129Xe+ natSn at 50 MeV/u. The study of the longitudinal velocities and of the average transverse energies of the IMF's contradicts the assumption that the total transverse energy of all detected particles Et is related to a well defined temperature. The separation of Et into the total transverse energy of light charged particles (Z=1,2) and that of IMF's elucidates the algorithm which induces a linear behavior of log(1/p) versus 1/Et. Even in the case of a single thermalized source, calculations based on a sequential statistical model show that the Arrhenius law cannot be observed if Et is taken as an estimation of the thermal energy.

Section snippets

Introduction.

In heavy ion collisions at intermediate energy (> 20 MeV/u), the production of several Intermediate Mass Fragments (IMF's, usually defined as nuclei with atomic number 3≤Z≤20), becomes an important reaction channel. These IMF's are apparently produced over the whole range of impact parameters and, for the most violent collisions, their total mass can sum up to half the mass of the entire system and they exhaust a significant fraction of the available energy. This important decay mode is not yet

Experimental results and discussion.

The data used in the present analysis come from the study of the 129Xe+natSn system with the INDRA detector at GANIL for incident energies ranging from 25 to 50 MeV/u. These nuclei are heavy enough to provide a large number of IMF's but not so heavy that fission is an important channel. The description of the apparatus and the conditions in which the experiment was performed can be found in 1, 22. The main features of these reactions are described in 1, 10, 11, 12, 13, 14, 15, 16, 17, 18,

Simulation of a statistically decaying single source.

To test the hypothesis of thermal scaling in a system where thermal equilibrium is achieved, the following simulation was performed. A single source with Z=54, A=129 and thermal excitation energy E was subjected to sequential decay according to the transition state method (Z>2) and Hauser-Feschbach formalism (Z≤ 2) as implemented in the SIMON code [24]. The temperature was related to the excitation energy via the usual Fermi gas relationship:E= a.T2. The value of E was varied randomly from 1

Conclusion.

Both the experimental measurement of the IMF velocity (Fig. 2, [1]) and the observed trend of the transverse energy associated to Z=2 particles and to IMFs (Figs. 2 and 3) give evidence that the assumption that Et is proportional to a well determined temperature T of the whole system is not supported by experimental facts. The analysis of a thermal statistical model calculation shows clearly that the use of Et instead of T destroys the initial Arrhenius-type behavior : a binning of events

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  • Cited by (0)

    1

    Experiment performed at Ganil.

    2

    on leave of absence from Institute of Physics, Jagiellonian University, Reymonta 4, 30059 Kraków, Poland.

    3

    present address : CEA DAPNIA/SPhN, CE Saclay, 91191 Gif-sur-Yvette, France.

    4

    present address : GSI, Postfach 110552, 64220 Darmstadt, Germany.

    5

    present address : CEA, DRFC/STEP, CE Cadarache, 13108 Saint-Paul-lez-Durance, France.

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