Title: Title
Author: Philippe CHOMAZ
Last saved by: Greenlees, Paul
Company: GANIL

In-beam spectroscopy of heavy elements

Abstract  
The high-spin structure of heavy nuclei will be investigated using state-of-the-art in-beam spectroscopic techniques. The data obtained will be used to constrain current theoretical nuclear models, and will provide a stringent test of their predictive power for the heaviest nuclei on the nuclear chart.

Keywords 
Heavy Elements, In-beam spectroscopy, gamma-decay, internal conversion, nuclear structure

Physics case

 High-spin studies of heavy nuclei provide data which is essential to test modern nuclear theories. The combination of efficient recoil separators with gamma-ray and conversion electron spectrometers has led to a wealth of new nuclear structure data for deformed nuclei in the region of ^254No. This high-spin data has allowed systematic studies of the moments of inertia, multi-quasiparticle configurations and rotational bands based upon single-particle states in odd-mass nuclei. An experimental determination of such parameters allows a critical assessment of the predictions of nuclear models to be made. One example is the prediction of the excitation energies of multi-quasiparticle configurations which are sensitive to the ordering and energies of the underlying single-particle states. 

Observable

A state-of-the-art detector array will be built around the target position to detect prompt radiation emitted in the decay of high-spin states in heavy nuclei. For low-energy transitions in high-Z nuclei, internal conversion is the dominant decay mode. The system should therefore be capable of simultaneous detection of gamma rays and conversion electrons. The system will be coupled to the recoil separator and focal plane spectrometer to be used for synthesis and decay experiments. The observables to be obtained are then: transition multipolarities, excited state spins and excitation energies, internal conversion coefficients, isomeric ratios and g-factors (through decay intensity ratios).
 
Proposed experiment
Tagging techniques (recoil, recoil-decay, isomer) will be used to extract the prompt radiation of interest from the background due to fission. So far, in the region of ^254No, such studies have been confined to cold fusion reactions using ^208Pb or ^209Bi targets. The advent of radioactive beams will allow a wider range of nuclei to be studied, and will also allow higher spin and excitation energy to probed through use of more symmetric reactions. One example would be to probe high-spin states in ^254No using the ^92Kr + ^164Dy reaction. A simultaneous alignment of i_13/2 protons and j_15/2 neutrons is predicted to occur at around 30 hbar in ^254No, beyond the current spectroscopic limit. Further interesting predictions are that nuclei such as ^260Rf should be gamma-soft, with a very low-lying 2_2^+ state. A systematic study of Rf isotopes would be possible using reactions such as ^132Sn + ^130-136Xe. Another approach would be to use, for example, more neutron-rich Ca isotopes in cold fusion reactions with targets such as ^204Hg, ^205Tl, ^208Pb and ^209Bi, in order to systematically study the nuclei close to those already investigated. It should be noted that for in-beam studies the required beam intensities may be below 100 pnA depending on the case in question.

 Example Reactions:

^92Kr + ^164Dy → ^256No^*
^132Sn + ^130-136Xe → ^262-268Rf^*
^50Ca + ^204Hg → ^254Fm^*
^50Ca + ^205Tl → ^255Md^*
^50Ca + ^208Pb → ^258No^*
^50Ca + ^209Bi → ^259Lr^*

Requirements:
Beam properties

The beam properties required are much the same as for the synthesis and decay experiments. As the expected excitation functions are narrow, the energy resolution should be better than 1% and emittance should be no worse than with presently available beams (5π mm mrad or better). This aids beam suppression in the separator devices.  

Some stable beam operation will be necessary to perform calibration of the various detection systems with known activities. A large variety of stable ion beams with reasonable (10-100 pnA) intensity will be required. 

Detection

The array surrounding the target will be coupled to the separator and focal plane spectrometer to be used for synthesis and decay measurements.
 Gamma-ray spectrometer

The ideal gamma-ray spectrometer for use with radioactive beams should be highly efficient, have high count rate capabilities and have full parallel readout. The high counting rate capability is necessary in order to allow the maximum beam intensities to be used. It goes without saying that the array should have excellent energy resolution and high granularity to minimize Doppler broadening for the symmetric reactions envisaged.
The AGATA (Advanced Gamma Tracking Array) array, currently under development, fulfills all the aforementioned requirements. As it is envisaged that the AGATA array will be used in a number of different institutions, (conventional?) alternatives could also be investigated for use at EURISOL.
Conversion-electron spectrometer

In high-Z nuclei, the decay of low energy transitions proceeds predominantly by internal conversion. It is therefore of importance to employ electron spectroscopic techniques in the study of the heaviest elements. The spectrometer should have a high (>10%) efficiency over a broad range of electron energy up to around 500 keV. In order to resolve L and M conversion electrons, the intrinsic energy resolution should preferably be around 1 keV. The device should work in conjunction with the recoil separator or spectrometer, and be capable of measuring high counting rates. 

Electron-gamma coincidence measurements are essential in the study of heavy nuclei.

Theoretical support

Again, the data obtained in these measurements will provide essential input to constrain nuclear models, and to enhance their predictive power. Theory should also guide the experimentalist to the nuclei where new phenomena in the region are most likely to be found.