Heavy Element Isomer Spectroscopy
Taken From Nuclear Physics News, 2011 -- by Rod Clark
Where does the Periodic Table end? What is the mass of the heaviest atomic nucleus? Among the first to address these questions were Niels Bohr and John Wheeler who, in 1939, developed a macroscopic model of fission which treated the nucleus as a charged liquid drop . Story has it that Bohr was confined to his stateroom during a transatlantic voyage with seasickness and worked on the model to take his mind off his nausea; this was the same trip on which Bohr brought the news to America of the discovery of fission. Their model suggested that there is a limiting value of Z2/A, and nuclei like 238U (with 92 protons and 146 neutrons) lie just below this limit. Having only a few more protons would mean the nucleus is no longer stable with respect to deformation and it would split into pieces (there is no barrier to fission as illustrated in Figure 1). Clearly, if that was the whole story, readers of this article would be grateful for its brevity.
The missing ingredient involves the quantum level structure of nucleons in the nucleus. Additional binding is found for nuclei lying close to closed shells where large gaps in the nucleon level sequence occur. Combining the macroscopic picture with the microscopic structure results in the possibility of heavy nuclei, with many more than one-hundred protons, becoming “shell stabilized” with sizable barriers to fission and forming a “super-heavy island” of relative stability (see Figure 1). One can say that the existence of super-heavy nuclei is a direct consequence of the quantum physics. Most macroscopic-microscopic treatments predict the center of this island to lie near Z=114 . However, more sophisticated models using self-consistent microscopic approaches (in today’s parlance density functional theories using effective nucleon-nucleon interactions) suggest the center of the island may lie at even higher proton numbers near Z=120 or even Z=126 . The simple question of where the Periodic Table might end remains controversial and a decade ago it appeared we might never receive a satisfactory answer. But a new generation of experiments is now addressing the fundamental issue of the maximum limit of nuclear mass and charge. The renaissance in heavy element science came about when experimentalists contrived to take advantage of an unexpected gift from Mother Nature and by cleverly exploiting advanced technologies to broaden the explorations of the heaviest elements.
The unexpected gift concerns the reactions used to create heavy elements. Elements from Z=107 (Bohrium) through Z=112 (recently named Copernicium) were all discovered in the 1980’s and 1990’s at GSI in Germany . The German-led team used reactions bombarding intense beams of elements like Fe, Ni, and Zn on targets of Pb and Bi. The reaction cross sections (essentially a measure of the probability of a heavy element surviving its violent creation) fell gradually with increasing Z until they were very close to the picobarn level. In real terms, a picobarn cross section means that one can hope to find only one or two events over a month-long experiment under the best of circumstances. It seemed that to go further would require the most heroic efforts (exemplified by our colleagues in RIKEN, Japan, who used the Zn+Bi reaction to create just two events of element-113 in more than two-hundred days of beam on target ). However, over the last decade scientists working at the Flerov Laboratory in Dubna have announced the discovery of new isotopes of super-heavy elements created with unexpectedly large cross-sections (of the order of a few picobarns) using intense 48Ca beams on actinide targets. The reasons for these relatively large cross sections is still poorly understood but their occurrence has now allowed the Russian-led team to discover new elements up to Z=118 , thereby probing the limits of nuclear existence through direct measurement.
A second, more indirect, approach to exploring the mass limit of the nuclear chart is through detailed spectroscopy of the heaviest systems (for a recent review see ). Measuring the properties of excited levels in nuclei from Fm (Z=100) on up, one can learn about the single-particle structure, pairing, shapes, and excitation modes of the heaviest nuclei. These fundamental properties can then be used to test the current models, the same models which are used to make predictions of the location and extent of the true super-heavy nuclei discussed above. Reactions to make nuclei in the region from Fm (Z=100) to Sg (Z=106) have relatively large cross sections (of the order of a few nanobarns, or even above a microbarn in the most favorable cases). However, one still needs to select the nuclei of interest from an even more massive background of fission (the compound nucleus is highly excited and will most often break apart before it has a chance to equilibrate and de-excite) which are formed in reactions with cross sections of around a barn. This requires the advanced technologies mentioned above. Large magnetic separators are required to select the final recoil products of interest and many of these devices were developed for the super-heavy element search-and-discovery experiments discussed above. In addition to these powerful separators, highly sensitive detectors are needed to study the decays of the final nuclei. In particular, highly segmented Si detectors are used for studying particle decay, while high-resolution Ge detectors are used to study the gamma rays that are emitted from excited states.
A breakthrough for spectroscopy came from groups at Jyvaskyla, in Finland, and Argonne, in the United States, when they applied a newly conceived technique to perform gamma-ray spectroscopy following the decay of isomers (long-lived excited states) in trans-fermium nuclei [8, 9]. It is this latter approach which is the main topic for the remainder of this article. Isomers are excited metastable states of nuclei . A typical lifetime for a nuclear state may be a few picoseconds, but isomers can survive for many orders-of-magnitude longer with some isomers known to have half-lives of seconds, minutes, hours, or even years (the longest lived isomer currently known belongs to 180Ta and has a half-life of more than 1015 years!). This metastability arises because the isomer has some property which is very different to any of the states to which it might decay. In the case of nuclei near Z=100 and N=152 (252Fm) it is the orientation of the angular momentum that gives rise to isomers. These isomers involve configurations of nucleon orbitals which yield high K values, where K is the projection of the total angular momentum onto the axis of symmetry defined by the nuclear shape. Electromagnetic decays from these states may involve an energetically unfavorable re-orientation of nucleons to states with much lower K values and such transitions can become hindered, leading to the metastability. However, the long lifetimes can be used as a powerful filter for identifying these K isomers and by studying their decay to lower-lying states we are gaining a wealth of new information on the structure of the heaviest nuclei.
The method to search for isomers can be described as follows. First, the compound nucleus of interest is formed and recoils through the magnetic separator which selects it from the background of unwanted transfer and fission products and the un-reacted beam particles. The recoil ion is implanted in a pixel of a highly segmented double-sided-silicon-strip detector (DSSSD) at the focal plane of the separator. If the nucleus is in an isomeric state it will stay in the excited state for some time before decaying via an electromagnetic transition to a lower-lying state. The electromagnetic decay to the ground state will usually involve gamma-rays, conversion electrons, and X-rays. The individual conversion electrons may be of very low energy but since the recoil is implanted in the DSSSD they sum to give a clearly measurable energy signal in the detector pixel that indicates an isomer has decayed. The nucleus will then de-excite to its ground state which will then decay with its usual mode, typically via alpha particles or fission. It was the extremely cunning idea, proposed by Graham Jones of Liverpool University, of using the pixel of the DSSSD as a conversion-electron calorimeter that was the key to all the new measurements that have been performed subsequently . The net result is a very cleanly identifiable event sequence of recoil-implant, a burst of conversion electrons, followed by the ground-state decay, all in the same pixel of the DSSSD, which allows us to perform spectroscopy of the isomer decay.
Many institutions around the world are now attempting such experiments and the competing efforts are driving innovation and discovery. Groups following this line of research are using facilities at Dubna (the VASSILISA+GABRIELA collaboration, e.g. ), GSI (SHIP, e.g. , and TASISpec ), Jyvaskyla (RITU+GREAT, e.g. ), Argonne (FMA, e.g. ), and Berkeley. At Lawrence Berkeley National Laboratory, the 88-Inch Cyclotron provides intense beams (as of today the typical intensity of a 48Ca beam is a large fraction of a particle microamp, but with on-going upgrades the Cyclotron should soon be reliably accelerating beams with intensities exceeding two particle microamps). The centerpiece of the heavy-element program is the Berkeley Gas-Filled Separator (see Figure 2) which is arguably the best heavy-element separator in the world, with an efficiency approaching 70% for the reactions of interest. Its large bend angle (about 70°) ensures excellent separation of reaction products from un-reacted beam particles. For the last two or three years, we have performed a series of isomer decay studies using a single 1 mm thick 16 strip ×16 strip DSSSD, with an active area of 5×5 cm, as the focal plane implant detector, along with a standard clover Ge detector mounted close to the DSSSD to measure gamma rays. These focal-plane detectors can be dramatically improved upon and a new system, which will yield an order-of-magnitude improvement in detection sensitivity, will be in place at the focal plane of the BGS this summer. However, the current simple set-up has allowed us to obtain some remarkable new results on K-isomers, and their decay, in nuclei from Fm to Sg. The quality of the data is illustrated in Figure 3, which shows results for the N=152 isotones, 254No , 255Lr , and 256Rf .
From Figure 3, one can see the effect of decreasing cross sections with the increasing Z of the final nucleus. The nucleus with the highest Z and A for which we have managed to do any sort of electromagnetic decay spectroscopy of excited states is 261Sg (Z=106) . But remember, even simply identifying the presence of a single isomeric state, a state which must have some unusual structural property, is giving the first information on the structure of nuclei very close to the super-heavy region, and is therefore of great importance. For nuclei with lower Z (as in the case of 254No shown in Figure 3) we are able to perform much more detailed spectroscopy. These types of studies have yielded fascinating results including identification of individual nucleon orbits which lie close to the Fermi surface. As discussed above, this is an essential test of the various models and their nuclear potentials used in making predictions about the super-heavy region. We continue to find that the macroscopic-microscopic models, predicting Z=114 as the next proton shell gap, consistently provide the best description of the available data.
I hope that these isomer studies will steadily yield an even richer scientific bounty. The excitation energy of a K isomer immediately places bounds on the magnitudes of the proton or neutron pair gap, while systematic trends of the energy and configurations of K isomers may reveal the role of high-order multipole deformations. In the latter case, effects from, say, hexadecapole deformations may play a decidedly more important role in the heavy nuclei than is ever likely to be seen in lighter mass regions. Rotational bands, many of which have been found in these types of investigations, provide information on the pairing, single-particle structure, nuclear shape, and the robustness of fission barriers at high spin. The decay of the K-isomers themselves may also provide a new probe of fission barriers and the fission process. There are now a couple of examples of isomers that have a dominant decay branch involving fission or alpha decay (notably, a fissioning isomer in 250No  and a proposed alpha-decaying isomer in 270Ds ). Since K isomers are excited states with high spin, and involve broken pairs of nucleons, isomers decaying through the fission barrier provide a unique tool to examine the effect of decreased pairing, sizeable angular momentum, and increased excitation energy on the decay process. Indeed it has been suggested that these effects may combine to make some isomers more stable than their respective ground states and opens the possibility that we may more easily find new isotopes and elements by searching for their isomers.
So, where does the Periodic Table end? What is the mass of the heaviest atomic nucleus? We still don’t have clear answers, but following the breakthroughs over the last decade we have an excellent chance of answering them in the next.
 For example, H. Meldner, Ark. Phys. 36 (1967) 593 ; R. R. Chasman et al., Rev. Mod. Phys. 49 (1977) 833; S. Ćwiok et al., Nucl. Phys. A 573 (1994) 356.