PROBING EXCITED STATES IN NUCLEI AT THE LIMITS OF STABILITY

1 Introduction

The study of nuclei far from stability has received much attention recently. Indeed, the thrust of the next generation radioactive beam facilities will be to study nuclei which lie at or near the neutron drip line. On the other hand, nuclei which lie at the limits of proton excess have been produced and identified for elements as heavy as Bismuth (Z=83). These isotopes were initially characterized by their decay properties, and little, if anything, was known about their excited states. A  similar situation exists for the heaviest elements (Z>100). Recently, with the coupling of Gammasphere [LEE:90] to other detection systems, it  has become possible to identify gamma rays in nuclei lying at and beyond the proton drip line and in the heaviest elements which are often produced with cross sections as low as 100 nanobarn (nb). Consequentially, studies of excited states in nuclei lying the furthest from stability have begun.

2 Gammasphere at the FMA

Gammasphere  is the pre-eminent detector for gamma-ray spectroscopy studies in the world. The device consists of up to 110 Compton-Suppressed Ge detectors, and it was built by a collaboration of physicists from Argonne National Laboratory  (ANL) , Lawrence Berkeley National Laboratory   (LBNL) , Oak Ridge National Laboratory  (ORNL) , and a number of U.S. universities. The device offers excellent energy resolution (2.3 keV at 1 MeV) and an order of magnitude increase in photopeak efficiency over previous Ge-arrays (~10% at 1 MeV). Between 1993-1997, the device was sited at the  88" Cyclotron Laboratory  at LBNL where the  major thrust of the experimental program was directed at the study of high-spin states in nuclei. A booklet summarizing the highlights of this first campaign at Gammasphere can be found here.

In the fall of 1997, Gammasphere was moved to the  ATLAS accelerator at ANL and placed in front  of the  Fragment Mass Analyzer (FMA)  in order to shift the emphasis of the experimental program from high-spin to nuclei far from stability. The FMA is a high resolution mass spectrometer which transports reaction products produced at the target position and disperses them by their mass/charge (M/Q) ratio at the focal plane, 8.8 meters away [DAV:92]. Presently, two general techniques are used at Gammasphere to make isotopic identification of weak channels produced in fusion-evaporation reactions. The first involves the measurement of evaporated particles, i.e. neutrons, protons, and/or alpha particles. The number and type of particles measured give some degree of nuclide identification. The second technique directly detects the residue, and nuclide identification is made by measuring the properties of the residue, i.e. mass, energy, time of flight and/or decay products. Experiments with the FMA utilize this second technique.
 

Figure 1:  Schematic diagram of the Recoil Decay Tagging (RDT) Method as it is implemented at  ATLAS.  The method allows for clean isotopic identification of gamma rays detected at the target by correlating the implanted recoils with their characteristic charged-particle radioactivity.

 While the FMA coupled to Gammasphere allows for mass identification of gamma rays, this is usually not sufficient for identifying the most proton-rich isotopes. This is due to the fact that after the fusion of the projectile and target, proton evaporation dominates over neutron evaporation in proton-rich compound systems. Since neutron evaporation leads to the production of systems lying furthest from stability, gamma rays emitted from nuclides with the largest proton excess become completely obscured by the gamma rays from the other isotopes produced in the heavy-ion reaction. Consequently, isotopic identification becomes necessary for isolating gamma transitions in these nuclei. With the FMA, this is  achieved by placing ancillary detectors behind the focal plane.  For light and medium mass nuclei (Z<50), it is possible to obtain isotopic selection by using an ionization chamber.  In heavier nuclei,  isotopic identification of gamma rays is made by correlating the characteristic charged-particle radioactivity of an ion implanted in a pixel of a double-sided silicon strip detector (DSSD) with a previously implanted recoil. Figure 1 shows schematically how the technique works utilizing the DSSD setup at the FMA.  This technique has wide applicability due to the fact that above the closed proton shell at Z=50, many nuclei near the proton-drip line decay by the emission of an alpha particle, and  beyond the drip line, odd-Z nuclides are observed to decay by proton emission. This technique is referred to as Recoil Decay Tagging (RDT) [PAU:95].
 

Figure 2:  Known chart of the nuclides. Black squares represent stable nuclei. Black circles represent nuclei  which have been measured with Gammasphere at ATLAS between January, 1998 and September, 1998.

 Experiments with Gammasphere coupled to the FMA were started in January, 1998. Figure 2 gives a summary of  the Gammasphere experiments performed thus far at ATLAS.  As the figure clearly illustrates, most experiments have been directed at nuclei far from stability, i.e. along the proton-drip line or above Z=90.

3 Studies Beyond the Proton Drip Line

Above Z=50, the limits of proton excess are defined by the proton emitters which have been identified in odd-Z nuclei up to Bismuth (Z=83). These nuclei lie beyond the proton drip line (Qp > 0) and are kept bound by the Coulomb force.  A number of new proton emitters have been identified recently using the FMA [DAV:97], and the lifetimes in nearly all of these cases can be well reproduced by WKB calculations using spectroscopic factors derived from spherical shell model calculations  This allows for definitive single-particle assignments to be given to the proton-emitting states [DAV:97]. Recently, proton radioactivity has been observed in 141Ho and 131Eu. However, the decay lifetimes cannot be reproduced by WKB calculations , and this has been interpreted as evidence for deformed grounds states in these nuclei [DAV:98].  By studying excited states built on top of these proton emitters, one should be able to independently confirm both the deformation and single-particle parentage of the proton-emitting state. This is especially critical for the deformed emitters where there are only a few known cases.  Other questions which can be addressed by studying excited states in proton emitters are: Experimentally, such studies are difficult because the cross-sections for producing proton emitters are quite low (<100 microbarn). In addition, the gamma-ray spectrum for an odd-A or odd-odd nucleus is in general more complicated than that of even-even systems making gamma-ray coincidence information necessary when determining the placement of even the first few excited states in a proton emitter. With the coupling of Gammasphere to the FMA , a 100 fold gain in gamma-gamma-recoil efficiency has been realized over what was previously available. Thus, this opens up  the opportunity to perform gamma-ray coincidence measurements on proton-emitters. Several such experiments have been peformed, and two of them are described briefly below.

The two known alpha-emitting states in 167Ir were recently found to  proton decay [DAV:97]. Based on the partial half-lives, the ground state was given an s1/2 assignment while the isomeric state at 175 keV was assigned to the h11/2  configuration. The spectroscopic factors deduced for these states agree with the low-seniority shell model calculations which assume that all states are spherical.  In order to study excited states in 167Ir, gamma-recoil coincidences  were measured with Gammasphere and the FMA using a p,2n reaction [CAR:98].  Figure 3a shows the gamma-ray spectrum correlated  with the alpha decay of the isomeric level in 167Ir utilizing the RDT technique (fig. 2).  These gamma rays are unresolvable in the mass-gated spectrum due to the fact that the latter is dominated by 167Os (2p,n) and 167Re (3p) gamma rays. The inset  shows the partial level scheme deduced from the gamma-ray coincidence data (all spins are tentative).  A sequence of gamma rays (672, 801,  865 keV) is observed to feed the isomeric states. The pattern of excitation  energy as a function of spin is consistent with a spherical or weakly deformed structure, and is thus, in agreement with the interpretation of the proton-decay data.

proton emitters

Figure 3: (Top) Gamma-ray spectrum correlated with the alpha/proton decay of the h11/2 isomer state in 167Ir. The inset shows the partial level structure built on top of the isomer. (Bottom) Gamma-ray spectrum correlated with the ground-state proton decay of 141Ho.

 Recently,  an experiment to measure excited states in 141Ho was carried out with Gammasphere coupled to the FMA using the  92Mo(54Fe,p4n)  reaction [SEW:98]. Despite the fact that the production cross section for 141Ho is only 250 nb, gamma transitions in 141Ho have been identified using the RDT technique.  Fig. 3b shows the gamma-ray spectrum correlated with 141Ho proton decays. If one assumes that the transitions marked in the figure form a rotational band, the deformation extracted from the bands moment of inertia agrees well with that deduced from the proton-decay half-life  [DAV:98].  However, due to low statistics and lack of gamma-ray coincidence information, the grouping of these transitions into a rotational sequence is tentative. Nevertheless, the power of the technique to measure weakly populated channels is evident and limited only by statistics at the 100 nb level.

4 Studies of the Heaviest Elements

Identification of new elements is an ongoing activity in nuclear physics. Elements up to Z=112 have been identified by measuring the energy of the alpha particle emitted when the nucleus decays and by correlating it with the decay of a known daughter nucleus. Experimental quantities extracted from heavy-element decay studies, i.e. alpha-decay energies and alpha-decay lifetimes, can be reproduced by theoretical calculations by assuming that these nuclei are deformed. A more direct determination of the deformation can be obtained by measuring excited states in the nucleus of interest. For heavy nuclei,  this is extremely difficult due to the fact that the production cross sections for very are at most a few 100 nb and typically much less.  There is one exception to this: the reaction using a 48Ca beam to bombard a 208Pb target gives a maximum cross section of 3 microbarn for producing 254No (Z=102) via the 2n channel.

Recently, a measurement was performed with Gammasphere in order to identify excited states in 254No using this reaction [REI:99].  For this experiment, the FMA was essential to allow for the separation of  254No residues from  the fission background produced at a 104 higher rate.  Unambiguous identification of 254No comes from the alpha-decay spectrum measured in the DSSD. The gamma-ray spectrum obtained for 254No is shown in  Fig. 4. These transitions form a rotational band. Due to large conversion coefficients for low energy transitions, it is most likely that the level fed by the 159 keV transition is not the ground state.  In order to estimate the spins of the levels connected by the observed transitions and the energies of the missing transitions, the observed  gama rays have been fit  using a rotational model.  Following this prescription, it was deduced that the 159 keV transition feeds the 6+ level, and the two missing transitions have energies of 44 and 102 keV. The proposed level scheme with deduced spins is given in Fig. 4, and it is consistent with a  a rotational band with quadrupole deformation estimated at 0.27 which agrees with the theoretical predictions.  An article in the January issue of  Physical Review Focus gives a more detailed discussion of this experiment.

 
254No Spectrum and Level Scheme

Figure 4:  254No spectra and deduced level structure. Panel (a) shows the gamma-ray spectrum produced by gating on residues detected at the back of the FMA. Panel (b) shows the gamma-ray spectrum produced by correlating gamma rays with the alpha decay of 254No using Recoil Decay Tagging (RDT).  The RDT gated spectrum confirms the assignment of the gamma-ray observed in (a) to 254No.
 

5. Summary

In the preceding sections experiments directed at the study excited states in proton-emitters and heavy nuclei have been presented. A number of experiments have also been performed with Gammasphere on proton drip line nuclei for Z<50. In these medium mass nuclei between the closed proton shells at Z=28 and Z=50, the proton drip line is approximated by the N=Z line. Many of these experiments have utilized an array of CsI detectors known as Microball in order to make isotopic identification by detecting evaporated charged particles. In some instances, an array of neutron detectors was also used. Topics addressed in these experiments include the role of T=1 and T=0 neutron-proton pairing in N=Z systems, astrophysical issues related to the R-P process,  proton emission from excited non-isomeric states, and the study of excited states in nuclei around 100Sn.

Studies of excited states in nuclei at the limits of stability with a device such as Gammasphere are in their infancy. In many instances, only a limited number of excited states have been placed, however, with the refinement of techniques and the development of more efficient ancillary detectors, a more complete understanding of these nuclei should become available in the coming years.

References

[LEE:90] I.Y. Lee, Nucl. Phys. A520, 641 (1990).

[DAV:92] C.N. Davids et al., Nucl. Inst. Meth. Phys. Res. B70, 358 (1992).

[PAU:95] E.S. Paul et al., Phys. Rev. C 51, 78 (1995).

[CAR:98] M.P. Carpenter et al., in Proc. Conf. Nuclear Structure '98, Gatlinburg, Tennessee, 1998, Vol. I, p. 13.

[SEW:98] D. Seweryniak, Bull. Am. Phys. Soc. 43, 1545 (1998).

[DAV:97] C.N. Davids et al., Phys. Rev. C 55, 2255 (1997).

[DAV:98] C.N. Davids et al., Phys. Rev. Lett. 80, 1849 (1998).

[REI:99] P. Reiter et al., Phys. Rev. Lett. 82, 509 (1999).