Heavy Element Region Research Articles

The limits of the nuclear chart set by fission and alpha decay

I will review how our picture of heavy-element nuclear structure has evolved through remarkably simple ideas and related models. It is well known that the Bethe-Weizsäcker semi-empirical mass model had an important role in unraveling radioactive decay and element transmutation in the heavy-element region in the 1930s. A remarkable aspect is that this model could immediately after the discovery of fission be generalized to explain this phenomenon through the consideration of deformation of a charged liquid drop. Bethe and Bacher already raised the possibility that shell structure (by them calculated in terms of a single-particle oscillator potential) could give rise to noticeable deviations between results of the macroscopic mass model and experiment but limited data prevented firm conclusions. In the 1950s the single-particle models took a realistic form and also included deformation. The possibility of the existence of a relatively stable “island” of superheavy elements was raised already then. But it was not until the work by Strutinsky in the mid 1960s that a quantitative model for the nuclear potential-energy emerged in the form of the macroscopic-microscopic model. Although new elements have been discovered at an almost steady pace since 1940, theory indicates that we are close to the end of this era: repulsive Coulomb effects will set the limit of observable elements to near Z = 120.

Spectroscopy of the heaviest elements

The addition of modern arrays of silicon and germanium detectors at the target and focal plane positions of recoil separators has led to a wealth of new spectroscopic data concerning the structure of heavy elements. A particular region of interest has been that of the deformed nuclei close to the N=152 subshell gap. Both detailed decay and in-beam spectroscopic studies have provided complementary data on the location and ordering of single-particle states for proton number in the region of Z=100 and neutron number N=152. Instrumentation developments have allowed in-beam studies to be carried out at the unprecedented level of 20 nanobarns. The future prospects for such studies are also bright - new facilities employing high intensity stable beams are under construction and should yield lead to more significant results over the next decade. Additional long-term interest comes from the advent of next generation radioactive beam facilities which may allow limited studies in the heavy element region to be carried out.

Decay Studies of Neutron-deficient Am, Cm, and Bk Nuclei Using an On-line Isotope Separator

Spectroscopic studies of heavy and superheavy nuclei give usinformation on various nuclear properties in heavy element re-gion. Measured α-decay energies lead to the precise determina-tion of atomic masses which are one of the most essential quan-tities to define nuclear decay properties, fission and fusion prob-abilities, and shell stability in heavy nuclei. Gamma-ray spec-troscopy following α, β, and orbital-electron capture (EC) de-cays and fine structure of α-particle spectra reveal excited statesin daughter nuclei, single-particle energies, proton-neutron con-figurations, and nuclear deformation.Neutron-deficient Am, Cm, and Bk nuclei predominantly de-cay by EC, and their α-decay probabilities are very small. Thisproperty makes experimentalstudies for these nuclei difficult. Inorder to measure γ rays following the EC decay and very weakαtransitions, it is quite important to isolate the nuclei of interestfrom a large amount of other reaction products. The traditionalmethod to separate heavy nuclei is chemical separation owingto its high separation efficiency. Another efficient method is in-flight separation using recoil separators. However, these meth-ods are not so suitable to apply to the neutron-deficient Am,Cm, and Bk nuclei; their half-lives are in the order of a few min-utes, and their α-decay intensities are very small. In this work,we have studied EC and α decays of the neutron-deficient Amand Cm nuclei using the gas-jet coupled JAERI on-line isotopeseparator (ISOL)

Boiling points of the superheavy elements 117 and 118

It has been shown that the relativistic effect on the electrons reveal in the heavy element region.1–3 What kind of changes will appear in the heavy elements because of the relativistic effects? Can we observe the changes? We observed that the boiling points of astatine and radon are lower than that extrapolated values from lighter elements in the same groups.4,5 We examined the systematic behavior of the elements on the boiling point and have found a new method for the estimation of the boiling points of the superheavy elements in the halogen and rare gases. The estimated values of the elements 117 and 118 are 618 and 247 K, respectively which are considerably lower than those6 obtained until now. If these values are correct the production of the superheavy elements with heavy ions reaction may be affected. Further, the chemical properties may be fairly different from the lighter elements.

GSI experiments in the region of heaviest elements

In the course of the SHIP experiments aimed to synthesize new elements with atomic numbers , improved decay data on elements Z = (103-109) were also obtained. The presentation of recent results is the subject of this paper.

Stability and decay of nuclei at the end of the periodic system

Recent studies of nuclear mass models show that it is essential to account for the Coulomb redistribution energy when calculating the nuclear potential energy in the heavy-element region. Results obtained by use of a mass model that includes Coulomb redistribution effects are analyzed. Q-values for α- and β-decay are calculated. Half-lives for α-decay are estimated by use of the Viola-Seaborg systematics. For EC, β + decay and β − decay, half-lives are calculated in a microsciopic QRPA model. Calculated single-particle level structures in the heavy-element region are presented. These indicate possible regions of isomers that would be unusually stable with respect to spontaneous fission and α-decay. Finally, we discuss the implications of earlier extensive work on fission properties of nuclei in this region.

The coulomb redistribution energy as revealed by a refined study of nuclear masses

The macroscopic-microscopic approach to the calculation of nuclear masses has been further refined through the inclusion of ε 3 and ε 6 shape degrees of freedom. Most of the systematic discrepancies that previously existed for heavy nuclei have been eliminated. The agreement is so close that the effect of turning on and off the Coulomb redistribution terms in the finite-range droplet model can be clearly discerned, thus confirming the contribution of this physical effect to nuclear stability. Models without the higher-order terms of the finite-range droplet model, such as the finite-range liquid-drop model that we also investigate here, are clearly inferior in the heavy-element region.

Production of neutron-rich Bi isotopes by transfer reactions

Production of neutron-rich Bi isotopes was investigated by irradiating Hg, Tl, and Pb targets with $^{18}\mathrm{O}$ ions in the 5-10 MeV/nucleon range. Following irradiation, bismuth was chemically separated and the yields of $^{211\ensuremath{-}213}\mathrm{Bi}$ isotopes were determined via $\ensuremath{\alpha}$-emitting Po daughters. Effective residual transfers of $^{3\ensuremath{-}5}\mathrm{H}$ to $^{208}\mathrm{Pb}$, $^{7,8}\mathrm{He}$ to $^{205}\mathrm{Tl}$, and $^{8}\mathrm{Li}$ to $^{204}\mathrm{Hg}$ were observed. Cross sections generally peaked in the 7-8 MeV/nucleon region and ranged from a high of \ensuremath{\sim}2 mb for $^{3}\mathrm{He}$ absorption to \ensuremath{\sim}0.6 nb for $^{8}\mathrm{He}$ absorption. Total $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-decay branches of the ${J}^{\ensuremath{\pi}}={9}^{\ensuremath{-}},25$ min isomer of $^{212}\mathrm{Bi}$ were measured to be 67% and 33%, respectively, (3.2\ifmmode\pm\else\textpm\fi{}0.2)% of the decays being associated with $\ensuremath{\beta}$-delayed $\ensuremath{\alpha}$-particle emission. Production yields for the ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ ground state and ${9}^{\ensuremath{-}}$ and ${15}^{\ensuremath{-}}$ isomeric levels for $^{212}\mathrm{Bi}$ were extracted. The ratio of isomeric states to the ground state increased by more than two orders of magnitude over the energy range studied. However, the maximum value of the cross section ratio $\frac{\ensuremath{\sigma}({J}^{\ensuremath{\pi}}={15}^{\ensuremath{-}})}{\ensuremath{\sigma}({J}^{\ensuremath{\pi}}={9}^{\ensuremath{-}})}$ was only 0.04, implying a low angular momentum transfer. The reactions were analyzed using the Wilczy\ifmmode \acute{n}\else \'{n}\fi{}ski sum rule, which gave a rather poor fit to the results with respect to both the yield and the implied angular momentum transfer. The observed relatively high cross sections, modest angular momentum transfer, and broad excitation functions indicate that transfer processes provide a viable method for reaching neutron-rich nuclides in the heavy element region.

The Production of Neutron-Rich Nuclides in the Heavy-Element Region via 3He Induced Reactions

We have measured the production cross sections for 233Th and 231Th from the bombardment of 238U with 3He ions at 46-, 53-, and 60-MeV at the Brookhaven 60-in. isochronous cyclotron. We have also attempted to observe the decay of 233Ac produced via 238U(3He, 8B) or equivalent reactions using 61 MeV 3He ions by first separating thorium from actinium and then perf orming chemical purifications on the second thorium sample into which the actinium has decayed. In the four experiments we performed, three gave results consistent with the 3 half-life of 233Ac somewhat longer than 120 s and the production cross section from this target-projectile combination in the order of 1-2 ?b.

Nuclear Equipotential Surfaces in the Heavy Elements

The shifts of single-particle energy levels caused by distortions of nuclear equipotential surfaces is examined. Application of this approach is made to neutron single-particle states in the heavy-element region. Typical single-particle energy shifts are displayed in figures.The equipotential shapes studied in this paper are axially symmetric and were obtained by the replacement ${{\ensuremath{\omega}}_{0}}^{2}{\mathcal{r}}^{2}\ensuremath{\rightarrow}{{\ensuremath{\omega}}_{\ensuremath{\rho}}}^{2}{\ensuremath{\rho}}^{2}+{{\ensuremath{\omega}}_{z}}^{2}{z}^{2}+{{\ensuremath{\omega}}_{0}}^{2}({\ensuremath{\rho}}^{2}+{z}^{2})G({z}^{2})$ with $G({z}^{2})$ of the form $G({z}^{2})={e}^{\ensuremath{-}A{Z}^{2}}\ensuremath{\Sigma}\stackrel{}{n}{B}_{n}{({z}^{2t})}^{n}$A Woods-Saxon well was used for the central field, and eigenstates of the cylindrical harmonic oscillator ($|{n}_{\ensuremath{\perp}}, \ensuremath{\lambda}, {n}_{Z}〉$) were used as a basis set in these calculations. $^{235}\mathrm{U}$ is analyzed in detail.

Equilibrium deformations of heavy nuclei calculated with projected wave functions

The ground state equilibrium deformations, electric quadrupole moments and deformation energies are calculated for even nuclei in the heavy element region. Projected wave functions (PBCS) corresponding to a definite number of particles are used. It is found that the main effect of using the PBCS wave functions instead of the BCS ones appears at the beginning of the region and consists in the earlier appearance of the deformation and in its larger values.

14.8 MeV neutron activation cross-section measurements of a few tellurium isotopes

The activation cross-sections of the (n, 2n) and (n, α) products of different tellurium isotopes for 14.8 MeV neutrons have been measured. Enriched isotopes were used to determine the (n, 2n) cross-sections of the 9.36 h Te 127 and of the 67.5 min Te 129 and 33 d Te 129∗ activities relative to the known Al 27 (n, p)Mg 27 cross-section. By means of chemical separation, the 27 h Sn 121, 39.7 min Sn 123 and 2.45 h Sn 127 activities were compared amongst themselves in tellurium-free samples. The three tin cross-sections were finally obtained from the decay analyses of quickly separated Sn-products by comparing with the 67.5 min Te 129 residual activity as internal standard. The 9.7 min Sn 125 cross-section was also determined. A hitherto unreported activity ≈ 2 min was noticed. Analyses of the combined (n, α) data in the heavy element region suggest the presence of flat peaks in the observed cross-sections away from magic numbers. The simple evaporation theories do not predict such peaks.

Anomalous electric-dipole internal conversion

A group of highly forbidden electric-dipole transitions in the heavy-element region are known to exhibit anomalous internal conversion. These transitions are close to or below the threshold for K conversion. Their L and M subshell conversions show anomalies which exhibit a definite pattern: L I and L II subshell conversion is enhanced relative to the corresponding “book” values, while L III is approximately normal. This paper attempts to explain this pattern in terms of “penetration” nuclear matrix elements for internal conversion, which arise because of the penetration of the converting atomic electron within the nucleus. It is found that for E1 transitions there are two types of penetration matrix elements. One of these, ∝ j n · r ̂ v 2Y 1 ∗ , gives an important contribution only to s 1 2 ↔ p 1 2 electron transitions. Such electron transitions are present in L I and L II conversion, but are absent in L III conversion. Since calculations based on approximate nuclear models indicate that this matrix element is much larger than other penetration matrix elements, a ready qualitative explanation of the experimental results for the three L subshells is available. Similar results are expected for the M subshells, etc. Quantitative interpretation of the observed conversion anomalies evaluates the nuclear penetration matrix elements. These matrix elements contain information in addition to that contained in the gamma-ray matrix element.

Anomalous electric-dipole internal conversion

A group of highly forbidden electric-dipole transitions in the heavy-element region are known to exhibit anomalous internal conversion. These transitions are close to or below the threshold for K conversion. Their L and M subshell conversions show anomalies which exhibit a definite pattern: L I and L II subshell conversion is enhanced relative to the corresponding “book” values, while L III is approximately normal. This paper attempts to explain this pattern in terms of “penetration” nuclear matrix elements for internal conversion, which arise because of the penetration of the converting atomic electron within the nucleus. It is found that for E1 transitions there are two types of penetration matrix elements. One of these, ∫ j n · rr 2 Y 1 ∗ , gives an important contribution only to s 1 2 → p 1 2 electron transitions. Such electron transitions are present in L I and L II conversion, but are absent in L III conversion. Since calculations based on approximate nuclear models indicate that this matrix element is much larger than other penetration matrix elements, a ready qualitative explanation of the experimental results for the three L subshells is available. Similar results are expected for the M subshells, etc. Quantitative interpretation of the observed conversion anomalies evaluates the nuclear penetration matrix elements. These matrix elements contain information in addition to that contained in the gamma-ray matrix element.

Monte Carlo calculation of neutron evaporation; excitation energy dependence of nuclear level density

Experimental excitation function data in the heavy element region have been analyzed by a Monte Carlo technique to investigate the dependence of the nuclear level density on excitation energy. The analysis supports the prediction of the completely degenerate Fermi gas model that the nuclear temperature varies as the square root of the excitation energy. The value of the level density parameter a deduced from the experimental data is changed significantly when shell and pairing effects are taken into account by displacement of the characteristic energy surface from which the excitation energy is measured.

Neutron-Proton Pairing Interaction

The neutron-proton pairing interaction $\ensuremath{\lambda}$ between the last odd neutron and the last odd proton in the outermost neutron and proton shells of an odd-odd nucleus has been estimated from nuclear masses in the regions just beyond $Z=20$, $N=20$, and just beyond $Z=40$, $N=50$. Behavior of $\ensuremath{\lambda}$ in these two regions and in the heavy element region $Z>82$, $N>126$, as estimated by Ghoshal and Saxena, is discussed. It is found that the behavior of $\ensuremath{\lambda}$ may be understood in terms of a simple $\mathrm{jj}$-coupling shell model. According to this model, $\ensuremath{\lambda}$ arises from the spin-independent part of the two-body force and is proportional to $(2{j}_{1}+1\ensuremath{-}2z)(2{j}_{2}+1\ensuremath{-}2n)$, where $z$ is the odd number of protons in the outermost proton shell ${j}_{1}$, and $n$ is the odd number of neutrons in the outermost neutron shell ${j}_{2}$ of the odd-odd nucleus.

States of Even-Even Nuclei in the Near-Harmonic Region: Spectra ofRn218,Rn220, andRn222

Gamma-ray singles and coincidence spectra have been measured in the alpha decay of ${\mathrm{Ra}}^{222}$, and ${\mathrm{Ra}}^{224}$, and ${\mathrm{Ra}}^{226}$. Excluding the prominent transitions from the first excited states, the energies (and abundances relative to total $\ensuremath{\alpha}$ emission) of the observed radiations were: ${\mathrm{Ra}}^{222}$: 325 kev (8.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$), 475 kev (7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$), 525 kev (2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$), and 798 kev (2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$); ${\mathrm{Ra}}^{224}$: 290 kev (9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$), 410 kev (4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$), 650 kev (6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$); ${\mathrm{Ra}}^{226}$: 260 kev (0.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$), 420 kev (7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$, 450 kev (3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$), 610 kev (1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$). The observed gamma-gamma coincidences were ${\mathrm{Ra}}^{222}$: 325-325 kev, 325-475 kev, and 325-525 kev; ${\mathrm{Ra}}^{224}$: 241-290 kev and 241-410 kev; ${\mathrm{Ra}}^{226}$: 188-260 kev and 188-420 kev. It was not strictly determined that the ${\mathrm{Ra}}^{222}$ radiations were not due to other members of the ${\mathrm{Th}}^{226}$ family. These data have been used to deduce the following levels, spins, and parities in the daughter Rn nuclides: ${\mathrm{Rn}}^{218}$: 650 kev, 2+; 800 kev, 1-; and, possibly, 850 kev, 4+. ${\mathrm{Rn}}^{220}$: 530 kev, 2+; and 650 kev, 1-. ${\mathrm{Rn}}^{222}$: 448 kev 2+; and 610 kev, 1-. The 2+ states in ${\mathrm{Rn}}^{220}$ and ${\mathrm{Rn}}^{222}$ have been previously assigned by Scharff-Goldhaber. These results are incorporated in a general energy level systematics of even-even nuclei in the heavy element region.

0+ levels of even nuclei in the rare-earth region

The quadrupole vibrations of beta type of deformed nuclei have been found in only a few cases in the heavy element region. We started a search in order to find such levels also in the rare-earth region. A 0+ level of beta vibrational type at 685.0 keV was found in the deformed nucleus Sm152 by measuring conversion coefficients. Its corresponding 0+ level of two-phonon type was observed in Gd152 at 615.3 keV. The strongly deformed nucleus Er166 has a 0+ level at 1460.4 keV now confirmed by angular correlation measurements. The 1087 keV level in Os188 and the 1267 keV level in the spherical nucleus Pt194 were also measured by angular correlation to have zero spins, indicating the same trend of the 0+ levels as theoretically expected. (A second excited 0+ level was found in Os188 at 1766 keV and in Pt194 at 1480 keV.) When the beta and gamma vibrational bands are known, the theoretical and experimental rotational-vibrational coefficients can be compared: Btheory = kBexp. A k-value of 2–4 is found, indicating a weaker coupling than that theoretically expected. The experimental trend of levels in even nuclei with A = 50–250 is discussed, and some new spin assignments are proposed.

Astatine and francium

A discussion of At and Fr is presented with emphasis on their instability, chemical prcpenties in relation to position in the periodic chaat, aad the form in which they may be found in nature. lt is pointed out that elements 85 and 87 are found in nature only in uranium minerals as short-lived isotopes in the decay chains of Th/sup 232/ U/sup 235/, and U/sup 238/. An idealized sketch of the mass-energy surface for elements in the heavy element region is presented which illustrates the reason for the short half lives of elements 85 and 87. The nuclear properties of these elements are discussed and tabulated. Finally, the chemical properties of both elements are examined. (J.R.D.)

Classification of the Energy Levels of Odd-Mass Nuclei in the Heavy-Element Region

Most of the data available on energy levels in oddmass nuclei of the heaviest elements have been sum marized and evaluated. The observed levels were classified according to the level diagrams calculated by Nilsson for nuclei with prolate spheroidal deformations. Qualitatively, the agreement between the data and the Nilsson diagrams is very good. (auth)