Search For Isomers Research Articles

Identification of a Shape Isomer inU235

The shape isomer in 235U has been searched for in a neutron-induced fission experiment on 234U, which was performed at the isomer spectrometer NEPTUNE of the EC-JRC IRMM. A neutron source, with a tunable pulse frequency in the Hz to kHz range and its individually adjustable neutron pulse width in connection with an appropriate detector system turned out to be the ideal instrument to perform an isomer search, when decay half-lives above 100 micros are expected. From the delayed fission events observed for two different NEPTUNE settings and at mean incident neutron energies En=0.95 and 1.27 MeV the isomeric fission half-life could be determined to be T1/2=(3.6+/-1.8) ms. The corresponding cross section was determined to sigmaif=(10+/-8) microb. With these results an experimental confirmation for the existence of a superdeformed shape isomer in odd-uranium isotopes is given for the first time.

Stochastic Search for Isomers of the sec-Butyl Cation.

A stochastic search procedure for locating energy minimum structures was applied to the sec-butyl cation. A previously unreported structure 3' with strong H-hyperconjugative stabilization of the carbocation center was found at several levels of theory (HF, B3LYP, and MP2). The theoretical equilibrium isotope effect (EIE) for the monodeutero isotopomer of 3' (Keq = 1.4; t = -130 °C) indicates that the new structure is likely to be largely responsible for the experimentally observed EIE.

Excitation energy and deformation of the1/2+[431] intruder band inTc107

The already detailed study of $^{107}\mathrm{Tc}$ nucleus was complemented by a search for microsecond isomers at very low energy. For this purpose, this neutron-rich nucleus was produced by thermal-neutron-induced fission of $^{241}\mathrm{Pu}$. We have found a new 30.1 keV microsecond isomeric state which deexcites to the ground state by a strongly-hindered $E1$ transition. This isomer was identified as the $3/{2}^{+}$ level of the $1/{2}^{+}$[431] intruder band in $^{107}\mathrm{Tc}$ and is also the lowest-lying member of the band. The very low energy of the band head suggests a large quadrupole deformation. From a comparison with $^{105}\mathrm{Tc}$, where more information is known about the intruder band, it is deduced that the $1/{2}^{+}$[431] band has a quadrupole deformation, ${\ensuremath{\epsilon}}_{2}\ensuremath{\geqslant}0.35$ and a possible triaxial shape, $\ensuremath{\gamma}\ensuremath{\approx}{20}^{\ifmmode^\circ\else\textdegree\fi{}}$.

Periodane: A wealth of structural possibilities revealed by the Kick procedure

AbstractTwenty seven new isomers, considerably lower in energy than the two previously reported structures of the novel molecule periodane, LiBeBCNOF, are found theoretically using a stochastic search methodology at the B3LYP/6–31G(d) level of theory. The global minimum is predicted to lie 122 kcal/mol lower in energy than the previously claimed lowest energy structure. The importance of an automated isomer search method to help find isomers on the potential energy surface of molecules in the gas phase is emphasized. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Triaxiality inMo105andMo107from the low to intermediate spin region

The nuclear structure of the odd $^{105}\mathrm{Mo}$ and $^{107}\mathrm{Mo}$ isotopes was reinvestigated in the present work. The excited levels of $^{105}\mathrm{Mo}$ were studied by observing prompt \ensuremath{\gamma} rays emitted after the spontaneous fission of $^{248}\mathrm{Cm}$ with the EUROGAM2 multidetector array. The already well-studied level structure of $^{107}\mathrm{Mo}$ was also completed by a search for microsecond isomers. For this purpose, this nucleus was produced by the thermal-neutron-induced fission reaction at the ILL reactor, in Grenoble. We have shown that the experimental level energies and the \ensuremath{\gamma}-decay patterns are well reproduced by simple particle-rotor calculations, assuming that these nuclei have an asymmetric shape. The shapes of the odd and even Mo are compared in the neutron range $N=62\text{\ensuremath{-}}66$.

Stable Highly Doped C60-mSim Heterofullerenes: A First Principles Study of C40Si20, C36Si24, and C30Si30

Geometry optimization within the framework of density functional theory provides clear evidence of stable fullerene-like cage structures for C(40)Si(20), C(36)Si(24), and C(30)Si(30). In the case of C(40)Si(20), an extensive isomer search shows that the most stable arrangements are those in which the Si atoms and the C atoms form two distinct homogeneous subnetworks. Any other configuration corresponding to spatially separated sets of Si atoms leads to a decrease of the binding energy. Due to charge transfer from Si to C atoms, opposite charges are found in neighboring Si and C sites. Structural stability is ensured via the predominant occurrence of 3-fold bonding for both species.

Stochastic search for isomers on a quantum mechanical surface.

In studying molecules with unusual bonding and structures, it is desirable to be able to find all the isomers that are minima on the energy surface. A stochastic search procedure is described for seeking all the isomers on a surface defined by quantum mechanical calculations involving random kicks followed by optimization. It has been applied to searching for singlet structures for C6 using the restricted Hartree-Fock/6-311G basis set. In addition to the linear chain and ring previously investigated, 11 additional structures (A-K) were located at this level. These provide a basis for discussing qualitative bonding motifs for this carbon cluster. The application of a similar idea to searching for transition states is discussed.

On the belt and moebius isomers of the coronene molecule.

The structure and stability of ca. 100 nonplanar constitutional isomers of the coronene molecule of general formula C(24)H(12) have been estimated by semiempirical and by ab initio calculations. The isomers studied are unbranched cyclic catacondensed structures and fall into two classes, viz., the belt (B) and the Moebius (M) type. The calculations have shown some of the M-isomers to be more stable than the corresponding B-isomers and thus to form a platform for the future synthesis of or search for such isomers in Nature.

Nuclear structure studies by means of short-lived isomers at intermediate energies

Short-lived (Tl,z about 0.1 to 100~s) isomeric states have been studied for over forty years. Among the methods used for their production thermal neutron induced fission played initially the most, important role (see for example [l]). However, the mass A and atomic number Z of excited fission fragments usually were not directly measured in these experiments limiting the possibility for a search of weakly produced isomers. Presently, the most important, source of spectroscopic information on the structure of short-lived isomers is in-beam spectroscopy applied to fusion-evaporation, deep-inelastic and fission reactions at several MeV per nucleon. Strongly decreasing production cross sections very far from the line of stability put severe constrains on the search for new isomers using in-beam spectroscopy methods. In many cases this difficulty can be overcome using fragmentation-like reactions at several tens or hundreds MeV per nucleon. The study of short-lived isomeric states at intermediate and relativistic energies combines an unambiguous and fast event-by-event identification of nuclei far from stability with

Structure and energetics of phenol(H2O)n, n  7: Quantum Monte Carlo calculations and double resonance experiments

Using a variety of methods phenol water clusters phenol(H2O)n, n⩽7, are investigated with a focus on phenol(H2O)5,6. A comprehensive search for low-energy isomers is conducted on a polarizable intermolecular potential energy surface. Zero-point energy contributions are calculated rigorously with the rigid-body quantum Monte Carlo method. The OH stretch vibrational spectra of the isomers are calculated using a local-mode model and compared with experimental isomer-selective IR–UV spectral hole burning (SHB) spectra. The topology of the clusters phenol(H2O)5,6 is shown in deviate from the corresponding pure water clusters.

Pion scalar density and chiral symmetry restoration at finite temperature and density

A search for new isomers of nanosecond lifetimes were carried out in 153Eu via the 150Nd(7Li,xnγ) reaction. The single particle angular momentum alignment and dynamical moment of inertia estimated from the experimental data indicate a configuration change at rotational energy ħω∼ 0.30 MeV. A decrease of B(E2) values is also observed at the same frequency. An isomeric level is identified at an excitation energy of 3100 keV (Jπ=35/2−) which corresponds to this frequency. The lifetime of the level is found to be 8.6 ± 1.3 nanosecond.

On the search for density isomers in interaction of relativistic12C nuclei with lead

A Pb target was bombarded with12C nuclei of an energy of 15 and 43 GeV. The target was periodically transported to the inside of the neutron multiplicity detector which allowed measuring time distributions for events of multiple emission of delayed neutrons. Abnormal neutron emitters have not been found. Upper limits of probabilities of their production were estimated for the half-life range 10–105 s and for different mean multiplicities of neutrons¯n. The values obtained are within the interval from 1.4·10−9 (for¯n=12)to 4·10−6(for¯n=2).

Initial stage in a computer search for structural isomers with given properties

1. A new approach is proposed for the organization of structural isomers with given properties, which permits the exclusion of clearly unsuitable isomers in the initial stage of the computer search. 2. The use of this approach is possible when the desired parameters are susceptible to additive claculation and, especially, when there is no information or insufficient information on the mandatory structural fragments.

Search for shape isomers in 56Ni.

An excitation function for the 40Ca(/sup 16/O,/sup 28/Si/sup */)/sup 28/Si/sup */ reaction has been measured between 74.925 less than or equal to E/sub lab/ less than or equal to 77.25 MeV in 75-keV steps for 30/sup 0/ less than or equal to theta/sub lab/ less than or equal to 60/sup 0/. $Q: -value and angular distributions were measured at each beam energy. Intermediate width (GAMMAapprox. =200 keV) structures are present in the excitation function at the 3--5% level of the cross section. An analysis of the present excitation function, together with one previously obtained for the inclusive elastic and inelastic /sup 28/Si+/sup 28/Si scattering yields, indicates that the structures in the two excitation functions are correlated at an 80% confidence level. This suggests that the process responsible for the resonances is of compound-nuclear origin.

High-spin isomers in W and Os nuclei; Competition betweenK-quantization and triaxiality

Results of a search for high-spin isomers in W and Os nuclei withN=104–107 are presented. Targets of48Ca and48, 50Ti are bombarded with a pulsed136Xe beam, and delayed gamma-radiation is detected by an array of NaI- and Ge-detectors. Several new isomers, with spin values up to ∼22h, are found in178,180W and180, 182, 183Os. Markedly different decay patterns and hindrance factors are found in the Os isotopes as compared to the expectations based on theK-selection rule valid for axially symmetric nuclei. The findings are ascribed to the onset of triaxial distortions.

Search for isomers in nuclei near N = 50

Targets of 58,60,61,62,64Ni. Co, and Cu have been bombarded with 42 MeV 16O beams. Several isomers were produced but no new isomer was found, in particular in 74Kr which is expected to be a good candidate for yrast traps in the N = 50 region.

Isomeric transitions inTc98

A search for short-life isomers in $^{98}\mathrm{Tc}$ has been made with the reaction $^{98}\mathrm{Mo}(p, n)^{98}\mathrm{Tc}$ and a pulsed proton beam. Three new isomeric transitions have been identified in addition to the known one of 14.6 ns half-life. The mean lives measured for the three $\ensuremath{\gamma}$ rays of 47.8, 57.0, and 65.2 keV come out to be ${\ensuremath{\tau}}_{\mathrm{m}}=(11.6\ifmmode\pm\else\textpm\fi{}0.3), (11.6\ifmmode\pm\else\textpm\fi{}0.9), \mathrm{and} (11.4\ifmmode\pm\else\textpm\fi{}2.8)$ (11.6 \ifmmode\pm\else\textpm\fi{} 0.9), and (11.4 \ifmmode\pm\else\textpm\fi{} 2.8) ns, respectively. These transitions have been attributed to the decay of the 138.6 keV level. Two new levels at 73.4 keV and at 90.8 keV have been identified and are suggested as possible candidates for the 14.6 \ensuremath{\mu}s isomer. An upper limit (${\ensuremath{\tau}}_{\mathrm{m}}<2$ ns) has also been obtained for the mean life of the levels at 65.4, 81.6, 152.0, 190.3, and 203.9 keV.NUCLEAR REACTIONS $^{98}\mathrm{Mo}(p, n\ensuremath{\gamma})$, ${E}_{p}=4\ensuremath{-}5$ MeV, pulsed beam; measured ${E}_{\ensuremath{\gamma}}$, delayed $\ensuremath{\gamma}$. $^{98}\mathrm{Tc}$ isomer observed, deduced lifetime. Enriched target.

Search for High-Spin Isomers in 150Dy

A search for yrast traps in 150Dy has been made by studying the γ-ray decay following the 144Nd (12C, 6n) 150Dy and 146Nd (12C, 8n) reactions. Although an average value of about 30 units of angular momentum is imparted to the final nucleus using these reactions, no evidence is found for the existence of isomers with half-lives longer than 10 ns.

Search for fissile isomers in the (n, 2n) reaction

Search for fissile isomers in the (n, 2n) reaction

Experiments with the Delayed Coincidence Method, including a Search for Short-lived Nuclear Isomers

The delayed coincidence method has been studied in some detail and employed in a search for short-lived isomers in the product nuclei of β-emitters. The influence of counter time-lag fluctuations, which determine the short half-life limit in this method, is discussed in relation to the procedure for recording the delayed coincidences. It is shown that the time lags exhibited by a Geiger-Muller counter are distributed approximately according to a Gaussian law. A differential delayed coincidence recorder has been constructed in which the overall time-lags in the two input channels due to rise-time effects have been stabilized. At the coincidence stage of this recorder the quadratic mean deviation in the time-lag difference between the two channels due to rise-time effects has been kept as low as similar 5 × 10−9 sec. Details are given of the experimental procedure used in searching for short-lived isomers. The range of possible half-lives most thoroughly investigated extends from about 3 × 10−7 sec. to about 10−3 sec. for isomeric transitions giving conversion electron energies greater than about 100 kev. Possible improvements in the range of the experiments are discussed. The short-lived isomers of 181Ta and 187Re have been confirmed. The half-life of the metastable state in 187Re has been found to be (5.26±0.12)×10−7 sec. Negative results are reported for 19 isotopes. In particular the metastable states reported by other authors in 198Hg, 141Pr, and 124Te are not confirmed. The half-life of ThC' has been found to be (3.04±0.04)×10−7 sec.