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Proposal for the continuation of experiment S104:
Investigation of Actinide Nuclei by Fragmentation of 238U PARTICIPANTS T. Aumanna), M. Begemann-Blaich b), J. Benlliurec), Ph. Dessagned), H. Emlingb), T. Enqvistb), F. Fargetb), J. Friesee), R. Gernhäusere), A. Heinzf), A. V. Ignatyukg), A. R. Junghansb), W. F. J. Müllerb), K.-H. Schmidtb), C. Schweitzerb), H. Simonh), K. Sümmerer b), J. Taieb i), L. Tassan-Got i), W. Trautmann b), S. Zhdanovj), a) NSCL MSU, b) GSI Darmstadt, c) Universidad de Santiago de Compostella, d) Université de Strasbourg, e) TU München, f) TU Darmstadt, g) IPPE Obninsk, h) CERN, i) IPN Orsay, j) Institute of Nuclear Physics, Alma Ata
INTRODUCTION The secondary-beam facility of GSI is a unique place world-wide for the production of relativistic secondary beams from fragmentation of 238U. In the experimental program S104, both the fragmentation of heavy nuclei including their fission characteristics ,,,, and dedicated fission experiments with secondary beams from 238U fragmentation ,,, have been performed. The investigations on fragmentation reactions now continue in experiments using a hydrogen target in a dedicated research program (S184). In the present proposal, we would like to concentrate on experiments on nuclear fission. Following the recommendation of the EA from June 1996 we propose to continue the research on nuclear fission with a more elaborate experimental equipment. The objectives of the experiment are very ambitious but, according to careful estimates, they are not impossible to reach. The necessary technical developments like the transport of heavy radioactive beams to CAVE B and a considerably improved mass resolution at ALADIN are in the scope of a general experimental progress. They will also provide improved conditions for other experiments.
PREVIOUS RESULTS
While conventional low-energy fission experiments are restricted to spontaneously fissioning nuclei and to nuclei in the vicinity of long-lived isotopes, used as target material (altogether about 80 nuclei), the novel experimental approach developed at GSI allowed for the first time to study the low-energy fission properties of more than 70 short-lived neutron-deficient actinides and preactinides. An outstanding feature of the secondary-beam experiment is the excellent Z resolution for the fission fragments. In addition, the average total kinetic energies as a function of charge split could be determined.
Fission was induced by electromagnetic excitations in a lead target. The inevitable nuclear-induced fission events were mostly suppressed on an event-by-event basis by a condition on the number of protons in the fission fragments. The remaining part was subtracted by a reference measurement with a low-Z target.
Figure 1: Measured fission-fragment nuclear-charge distributions from 221Ac to 234U are shown on a chart of the nuclides.
One of the most important results of the preceding experiments is the systematic coverage of the transition from single-humped (symmetric) fission near 208Pb to double-humped (asymmetric) fission near 234U (see refs. ,). A subset of the data in the particularly interesting zone around 226Th is shown in Figure 1. These data in combination with the total kinetic energies give unique information on nuclear-shell structure at extreme deformation far from stability. The data could successfully be described in terms of fission channels by fitting the basic parameters of the Brosa model. They represent a new challenge for a full theoretical description of the fission process which is not yet available. An even more exciting finding is the systematic appearance of an even-odd structure in the elemental yields for odd-Z fissioning nuclei, see Figure 2. It revealed that an even-odd structure also generally appears in the presence of unpaired nucleons. This is in contrast to the previous understanding of even-odd effects in fission. We observed an increase of the even-odd structure in both odd-Z and even-Z nuclei in the most asymmetric charge splits. This finding sheds new light on variations of the even-odd effect as a function of mass asymmetry found previously. The systematic increase of the even-odd effect at large asymmetry, previously attributed to particularly low dissipation, is now at least partly understood in terms of the phase space available for unpaired protons in the fissioning system: While the flow of the condensed Fermi liquid of fully paired nucleons in fission is governed by the chemical potential, the unpaired nucleons which carry the intrinsic excitation energy of the system feel the density of excited states. In view of these results, general conclusions on the viscosity of cold nuclear matter have to be reconsidered.
Figure 2: Nuclear-charge distribution of fission fragments from electromagnetic-induced fission of 220Ac (Z = 89). The amplified yields clearly show a strong even-odd structure in the wings of the distribution.
As a third field of interest, the fission probabilities of semimagic nuclei near N = 126 were measured in different excitation-energy regimes ,,. In sharp contrast to the expectation that the ground-state shell effect, known to be larger than 5 MeV, stabilises these nuclei against fission, no reduction of the fission probabilities for isotopes near N = 126 was seen. We interpret this behaviour as mainly being a consequence of the collectivity of nuclear excitations up to energies of several tens of MeV. This finding is highly relevant for the expectations on the production of spherical superheavy elements in heavy-ion fusion reactions. Most theoretical estimations performed up to now seem to be by far too optimistic.
NEW EXPERIMENTAL SET UP
The technical equipment proposed for the new experiment is a combination of several very elaborate devices, developed previously by different groups for various purposes. We combine our efforts to put together a powerful equipment for fission studies in inverse kinematics. The experimental set-up to be installed in CAVE B is shown in Figure 3. It is rather complex and has been optimised for this experiment in detailed model calculations. The different detectors and their tasks are given below in the sequence they will be mounted in the beamline:
Most of the detectors have already been used successfully in similar applications. Some development is necessary. E. g. the position resolution of the fibre detectors available up to now has to be improved. We hope to achieve this by using thinner fibres (diameter 0.5 mm, distance 0.167 mm) and by adding a second layer which is displaced by 0.167 mm. By analysing the hits in the two layers, a resolution of better than 0.2 mm should be obtained.
Another task is the transport of the secondary beam from the FRS to CAVE B. The transport of the heavy secondary beams with an emittance of about 30 p mm mrad measured, should be much less difficult than the transport of light fragmentation products with their much larger emittances.
Figure 3: Schematical drawing of the experimental set-up to be applied in the proposed secondary-beam fission experiment in CAVE B with part of the detectors depicted.
The dimensions and the acceptance ranges of the detectors are sufficient to detect almost all fission fragments over the full ranges in angle and velocity. The detector resolutions listed above are necessary to fully identify all fission fragments in A (A/D A » 250) and Z (Z/D Z » 120) and to give the necessary information on deexcitation by neutrons and gamma rays to reconstruct the TXE with a resolution of about 10 MeV.
IMPROVED EXPERIMENTAL CONDITIONS
The great progress brought about by the previous secondary-beam experiments concerning the rather free choice of the fissioning system to be investigated, is paid by a certain disadvantage: The excitation energy leading to fission is not sharp, because the only excitation mechanism which is able to cope with the low secondary-beam intensities and with the inverse kinematics is the electromagnetic excitation in a secondary high-Z target. It leads to fission from excitation energies around the fission barrier up to several MeV above the fission barrier (about 7 MeV FWHM).
The new proposed experiment is designed to improve the situation significantly: In addition to nuclear-charge yields and average total kinetic energies, also the mass number of the fission fragments, the multiplicity and the velocities of the emitted neutrons, and the gamma radiation will be measured. This aims towards a kinematically complete experiment which carries more stringent information about the fission process.
In principle, the excitation energy E* of the fissioning nucleus can directly be evaluated on an event-by-event basis as the sum of the Q value (since the fission fragments are identified in A and Z), the excitation energy of the fragments (from the neutrons and gamma rays registered) and the total kinetic energy (TKE). However, the TKE cannot be determined with the desired accuracy. The magnetic resolution of ALADIN and the angular straggling in the radiator of the Cerenkov counter already result in an uncertainty in TKE as large as 20 MeV FWHM, almost independent of the direction of fission. Therefore, it is not possible to determine the excitation energy of the fissioning system for single events with a resolution better than the theoretically known width of the excitation-energy distribution (about 7 MeV FWHM). The difficulty to reach a good TKE resolution is related to the high beam energy of about 150 GeV; the value of 20 MeV corresponds already to a relative accuracy of about 10-4! Disregarding other experimental problems, a magnetic resolution of Br /D Br @ 5000 would be required to reach an accuracy of 1 MeV in TKE for fission in beam direction.
Nonetheless, the experiment allows to determine the excitation energy (TXE) of the final products of the fission process from the number of emitted neutrons and the total gamma energy. The TXE is also a very interesting quantity of the fission process, and is comparable to the initial excitation energy E* of the fissioning system in its importance.
The measurement of the total kinetic energy with the above-mentioned resolution is still very important, since the mean value can be determined as a function of charge and mass split with high precision. It yields information on the compactness of the scission-point configurations of the different fission channels.
SCIENTIFIC AIMS
SHELL STRUCTURE IN FISSION
The temperature dependence of shell structure in fission yields and kinetic energy will be studied by the variation of fission channels as a function of TXE.
Compared to the previous experiments performed directly behind the FRS, the additional knowledge of the mass number of the fission fragments better defines the influence of shell structure in both neutron and proton number, in particular in charge polarisation (N/Z ratio of the fragments). The progress brought about by experiments in inverse kinematics is demonstrated by the analysis of electromagnetic-induced fission of 238U measured at the FRS by C. Donzaud et al.: The different degree of charge polarisation of the different fission channels was clearly demonstrated. The new experiment will be even more powerful since both fission fragments are registered in coincidence. Also, the studies on fluctuations in the charge polarisation, interpreted as an interplay of quantum oscillations and collective motion, can now be extended to lighter fissioning systems. New results on the dynamic evolution of the fissioning system from saddle to scission are expected in a particularly interesting region of multimodal fission.
NUCLEAR VISCOSITY
High-quality data on the proton even-odd effect only exist for thermal-neutron-induced fission (from conventional experiments) and for fission after electromagnetic excitation at relativistic beam energies (from the previous secondary-beam experiments). The variation of the even-odd effect as a function of TXE will give us information on the dying out of pairing correlations with increasing excitation energy. From these data one may also conclude on the variation of nuclear viscosity as a function of temperature.
FISSION CROSS SECTIONS
The high fission probability in magic nuclei near N = 126, found in the previous experiments, may be investigated as a function of TXE. This will answer the important question, from which excitation-energy range the observed surprisingly high fission rates of semimagic nuclei originate. This will give more detailed information on the systematics of level densities of spherical, transitional, and deformed nuclei.
BEAM-TIME REQUEST
Parasitic beam time
A parasitic beam time of about 20 days is requested
The total time needed for these preparative studies is difficult to estimate. It will depend on the progress obtained.
Main beam time
After these preparations, we would like to investigate first the fission properties of 238U with the new experimental set-up. This serves as a test case which can be compared to previous results at FRS and ALADIN. For the main experiment, we chose three nuclei, produced as secondary beams, 222Th 226Th and 229Th, in order to vary the strengths of symmetric and asymmetric fission channels. Besides the lead target, an aluminium target will be used to correct the data for fission after nuclear excitations as described in refs. ,.
Basic data to estimate the beam time required:
Beam-time estimate:
Total 19 days
Beam intensity
The following table gives the required primary-beam intensity for the different cases. (A loss of 90% due to secondary reactions and beam transport to CAVE B is assumed for the secondary beams.)
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