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Nuclear Fragmentation

#1096


Multifragmentation of Nuclei Induced by Relativistic Light Ions

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
3 Approved without Funding

Registration date
14.08.1997

Leading Institute
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna

Supporting institutes

  • Khlopin Radium Institute, Russia, St Petersburg

Collaborators

  • Universitat Copenhagen / Niels Bohr Institute, Denmark, Copenhagen\nTechnische Universität Darmstadt, Germany, Darmstadt\nMichigan State University, USA, MI, East Lansing\nGSI, Germany, Darmstadt\nUniversity of Iowa / Department of Physics and Astronomy, USA, IA, Iowa City

Project summary

Nuclear fragmentation was discovered 60 years ago as a puzzling phenomenon in cosmic rays studies, when collisions of relativistic protons with a target are followed by emission of nuclear fragments heavier than alpha particles, but lighter than fission fragments. They are now called intermediate mass fragments (IMF, 2 < Z < 20). Later on, in the 1950s, this phenomenon was observed in experiments with accelerators and, after that, it was studied leisurely for three decades. The situation changed dramatically after 1982, when B. Jacobsson, et al, discovered the multiple emission of IMF in an emulsion irradiated by 12C (1030 MeV) at the CERN synchrocyclotron. The experimental data stimulated development of a number of theoretical models, which related the copious production of IMF's to the liquid-gas phase transition in nuclear matter. In the nucleus, as in a usual liquid, a peculiar condition can be created (high temperature and reduced density), where the system enters the region of the phase instability (spinodal region). This state may disintegrate into an ensemble of small drops (IMF's), surrounded by a nuclear gas (nucleons and helium nuclei). Thus, according to these models, multifragmentation is a new (multibody) decay mode of highly excited nuclei. However, there is a trivial alternative for explaining the multiple emission of these fragments: successive and independent IMF evaporation by the very hot nucleus. In this case, multifragmentation is only a new manifestation of the well-known decay mode. As for which actually occurs, only experiments can give the answer.

The idea of getting these new insights into the problem of the nuclear equation of state stimulated great interest in the multifragmentation phenomenon in the mid-1980s. Around a dozen very complicated experimental devices were created to investigate this process using heavy ion beams, which are well suited for producing extremely hot nuclei. In this case, however heating the nuclei is accompanied by compression, strong rotation, and shape distortion, causing the so-called dynamic effects in nuclear decay. It is difficult to untangle all of these effects to get the information on the thermodynamic properties of a hot nuclear system. The picture becomes significantly clearer when light relativistic projectiles (protons, helium) are used. One should expect the dynamic effects to be negligible in that case. Another advantage is that all of the fragments are emitted by the only source -the target spectator. Its excitation energy is almost entirely thermal one. Thus, the use of light relativistic projectiles is one way to observe and study thermal multifragmentation related to the liquid-gas phase transition in nuclear matter.

For such purpose, the 4p - FASA setup has been created. A number of experiments have already been conducted with the proton and alpha beams of the JINR synchrophasotron. The main parts of device are: (i) a fragment multiplicity detector (FMD) consisting of 64 thin (20 mg/cm2) CsI(Tl) counters which cover 89% of the 4p: range; (ii) five DЕ (ion. cham.) ґ E. (Si) telescopes, that serve as a trigger for the read-out of the system allowing measurements of the charge and energy distributions of IMF's at different angles.

A study of target multirragmentation in p + Au (at 2 ё 8) GeV and 4He + Au (at 14.6 GeV) gives the following conclusion:


- Multifragment emission is the dominant decay mode of a highly excited (up to 500 ё 700 MeV) target spectator.
- The charge distribution of IMF's is described by the power law Y(z) » Z-a with "a" showing a minimum as a function of the IMF-multiplicity ("critical behavior"). This observation can be considered as an indication that the system is close to the critical point for the liquid-gas phase transition, which is characterized by the power law size distribution of the fragments.
- From the fragment energy spectra and relative velocity correlation's, it has been found that fragments are emitted when the nucleus has expanded and a density of r < 1/3 r0 is reached.
- The multiplicity, charge, and energy distributions of IMF's are well described in the combined model, which includes the intranuclear cascade approach for the fast stage of reaction and the statistical multifragmentation model.
- The mean lifetime of a fragmenting system is found to be < 75 fm/c = 2.3 ґ 10-22s. This value was found by analysis of the angular IMF-IMF correlation and is significantly smaller than the characteristic Coulomb time (» 250 fm/c), which is the mean time of fragment acceleration in a Coulomb field. The trivial mechanism of IMF emission (sequential and independent evaporation) should be definitely excluded. Thus, the data give evidence for a new decay mode of excited nuclei-thermal multifragmentation (in addition to the three previous modes: gamma decay, particle evaporation, and fission).

In this project, we plan a significant upgrading of the FAS A setup (FASA-2) which would provide more detailed information on the nuclear multifragmentation process:


- A light charged particle multiplicity detector will be developed. For that purpose, an array of 64 plastic scintillates will be inserted into the vacuum chamber of the FASA-setup (in the forward hemisphere). The LCP multiplicity detector gives the possibility of measuring the very important characteristic of collision - the impact parameter. In this way, we can separate the events according to the excitation energy of the fragmenting nucleus (target spectator).
- To measure the nuclear temperature of the hot target, spectator we plan to supply the FASA device with a so called "isotopic thermometer". This is an DE ґ E-spectrometer, which provides the resolution of the inpidual mass number for the light fragments. The temperature is estimated from the yield ratios of 3,4He and 6,7Li isotopes. Measuring the temperature of the fragmenting nucleus as a function of the excitation energy (caloric curve) is very important, as the phase transition can manifest itself by the appearance of a plateau (T = const) on the caloric curve.
- The second step of the FASA-setup upgrade, is developing a new triggering system. It consists of 31 DE (gas) ґ E (Si) telescopes, which are closely packed to cover the one of the main flanges of the setup (1/12 of the total solid angle). The geometry of the new trigger system gives a better possibility for measuring the IMF-IMF angular correlation in the range of small relative angles. This is very important for improving the accuracy of the estimated time scale of the multifragmentation process.

The data will be analyzed with proper theoretical models. Detailed information on the properties of the new decay mode of hot nuclei-thermal multifragmentation will be obtained. A deeper understanding of the nuclear equation of state is expected.

The project will be realized in collaboration with the following scientific teams:


- dr. H. Oeschler et al., Institute fur Kernfphysik, TH Durmstatdt, Germany;
- prof. E.Norbeck et al., Iowa University, Iowa City, USA;
- dr. K.Tanaka et. al., National Laboratory for High Energy Physics (KEK), Tsukuba, Japan;
- prof. J. Bondorf et. al., Niels Bohr Institute, Copenhagen, Denmark;
- prof. J. Richert, Centre de Recherches Nucleaires, 1N2P3 - CNRS, Strasbourg France;
- prof A. Budzanovski et.al., H. Niewodniczanski Institute of Nucl. Physics, Cracow, Poland.

A significant part of the project will be accomplished by scientists and technicians who were earlier involved in the design and development of nuclear weapon delivery means, so the project meets the ISTC aims to redirect their professional skills to peaceful activities in the field of the fundamental scientific research.

Description of Major Equipment:


- Existing: synchrophasotron, experimental pavilion, FASA device.
- New: light charged particle detector, precise telescope-spectrometer; telescope module, pion spectrometer.


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