A campaign with EUROBALL detectors at GSI

The GSI facilities open unique opportunities for gamma-spectroscopy employing very heavy UNILAC beams or, in particular, radioactive ion beams (RIB) of reasonable intensity from SIS/FRS. Therefore it is worthwhile to consider experimental campaigns with Euroball detectors which will for the first time enable investigations at medium to high multiplicities not attackable with VEGA. On this site we present helpful information on different aspects of a EUROBALL detector campaign:

Motivation

Experimental requirements

FRS beams

Technical considerations

1. Motivation

The UNILAC is the only accelerator in Europe providing all stable isotopes up to 238U for near barrier nuclear strucuture experiments with the EUROBALL IV array, thus providing access to new areas of the nuclear chart using classical reactions. In addition, the velocity filter SHIP combined with EUROBALL IV can be employed for recoil tagging experiments with up to 5 times higher efficiency as compared to all previous set-ups.

The SIS / FRS facility, however, provides even more challenging new possibilities. By fragmentation or fission of relativistic heavy ions several 100 unstable rare isotopes can be generated with sufficient intensity for in-beam spectroscopy. Secondary beams can even be produced in high spin isomeric states.

Fig.1: Basic reactions at relativistic energies are single step Coulomb excitation leading to collective low and high energy states at low spin and fragmentation populating medium to high spin states similar to classical compound reactions.

A few physics examples:

Coulomb excitation at relativistic energies (50-200 MeV/u)

Shape evolution of N=Z nuclei
Sudden shape changes and shape coexistence in isotope chains
Probing of reflection asymmetric states

Primary and secondary fragmentation

Structure of exotic nuclei at medium to high spin
Nuclear structure at and beyond the p-dripline

Slowing down of the FRS fragment beam by a degrader to near barrier energies enables classical reaction types to be used for isotopes a few neutrons away from the stable ions.

Fig.1: Additional compound systems accessible by fusion of 132Sn beam nuclei obtainable with sufficient intensity and stable target nuclei.

A few physics examples:

High spin structure of 176Yb-178Yb
Few n-transfer with n-rich projectiles
Probing the collectivity of metastable high spin states by Coulex

2. Experimental requirements

Why EUROBALL detectors?

EUROBALL with its 239 crystals is optimized for large efficiency at high multiplicity. For classical nuclear structure experiments either at UNILAC or with slowed down fragment beams the value of an array similar to EUROBALL IV is therefore obvious.

In-beam experiments at relativistic energies are generally dealing with limited beam intensities, strong Doppler effects and are often also associated with high multiplicities from:

huge atomic (Bremsstrahlung) background -> M_x £ 10
severe (n, p, HI) particle background -> M_p £ 5
medium to high spin gamma decays -> Mg £ 25

Therefore it is necessary to use as many of the composite (Clover, Cluster) EUROBALL detectors as possible. Detector configurations:

Depending on the velocity of the used beam two general detector arrangements have to be foreseen. Suggestions for possible lay-outs are given below:

Slowed down and stopped FRS beams (also applicable for heavy Unilac beams)

Decay, Coulex, transfer, fusion Þ M_x < 1, M_p £ 5, Mg £ 25

Symmetrical array: 8 Cluster backward, 26 Clover central ring, 7 Cluster forward

Þ P_ph (M=1) » 10%, P_ph (M=30) » 7%

Soccer ball: 15 Cluster and 15 Clover in 30 hexagonal and pentagonal faces of a 32 sided polyhedron (J. Simpson, P. Nolan; Euroball IV '96)

Þ P_ph (M=1) » 10.4%, P_ph (M=30) » 6.5%

Relativistic FRS beams

Coulex, direct fragm., secondary fragm. Þ M_x £ 10, M_p £ 5, Mg £ 15

Ring array: 26 Clover backward rings, 15 Cluster + 4 Vega forward rings Þ P_ph (b =50%) » 4%

Required beam intensities

In the following figure rate estimates for typical experimental situations are given.

Fig.1: Beam intensity required for one photo peak count per hour. Optimized detector arrangements are assumed.

Fusion reactions with slowed down fragment beams as well as high energy secondary fragmentation reactions (probing the most exotic nuclei) can only be done within a reasonable measurement time if maximal Ge efficiency can be obtained. This requires that all the composite EUROBALL detectors are available.

3. FRS beams

Several hundred radioactive isotopes can be produced at the FRS by fragmentation or fission with sufficient intensity for in-beam spectroscopy. The intensity depends on the primary beam intensity and on the production cross section. Based on the EPAX parameterization a calculation of the production cross section for a particular isotope can be performed. Please note that the minimum primary beam energy for reliable EPAX calculations is 250-300 MeV/u. Keep also in mind that the transmission losses can be HUGE, especially for high-Z nuclei that suffer from broad charge state distributions. EPAX calculates fragmentation cross sections but not high energy induced fission.

4. Technical considerations