Search

The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This 'internal conversion' has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons. In the inverse process-nuclear excitation by electron capture (NEEC)-a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted in 1976 and has not hitherto been observed. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of 'isomer depletion', which has been investigated previously through other reactions, but is used here to obtain evidence for NEEC.

The irradiations of Ag to Sn targets by bremsstrahlung generated with 23-MeV electron beams are performed at the MT-25 microtron. Gamma spectra of the induced activities have been measured and the yields of all detected radionuclides and isomers are carefully measured and analyzed. A regular dependence of yields versus changed reaction threshold is confirmed. Many isomers are detected and the suppression of the production probability is observed with growing product spin. Special peculiarities for the isomer-to-ground state ratios were deduced for the 106mAg, 108mAg, 113mIn, 115mIn, and 123mSn isomers. The production of such nuclides as 108mAg, 115mIn, 117gIn, and 113mCd is of interest for applications, especially when economic methods are available.

In the present report, methods of radionuclide production for the nuclear-medicine purposes are described. In a budget approach, the application of low-energy accelerators is especially advantageous. Intense flux of bremsstrahlung at electron accelerators or high-current cyclotron beams of alpha particles must supply a great yield for many isotopes. The choice of a target material and of the projectile energy provides enough variation for concrete species formation. The innovating procedures are here proposed for optimizing of methods, for instance, application of the noble-gas target for production and transport of activities. The known and new variants of the “generator” scheme are discussed. Many isotopes are listed as promising in the context of the therapeutic and theragnostic applications. Among them are isotopes/isomers emitting soft radiation for the selective and careful body treatment, also the positron emitters for PET, and the halogen and alkali-metal species convenient for chemical separation.

Reactions induced by photons may be used as a probe for bonds and correlations of nucleons inside a heavy target nucleus. In contrast to strongly interacting particles, electromagnetic radiation perturbs nucleons only slightly. The yield of (γ, α) reactions could be used to test theoretical models that assume complete alpha-particle clustering or the presence of multiquark objects in heavy nuclei. The relative yields of (γ, n), (γ, p), and (γ, α) reactions were measured at the bremsstrahlung endpoint energy of 23 MeV for several targets. A much lower probability of (γ, α) in relation to (γ, p) reactions was proved despite very close threshold and spin factors for the two reaction types being considered. Therefore, alpha-particle clustering in heavy targets was not confirmed. The mechanism of particle release in nuclear reactions is discussed, and some details are clarified.

Among nuclides, 195mPt isomer is characterized by a number of properties that make it suitable for use in nuclear medicine. Effective ways of optimizing the Pt isomer yield at low impurity contents must be sought. The method based on double neutron capture by the 193Ir target nucleus with subsequent populating of 195mPt through β decay allows chemical isolation of the isomer. In this work, the problem is analyzed, test experiments on Ir activation with neutrons are conducted, and theoretical estimates of the 195mPt yield are presented. Full-scale model experiments on the IBR-2 reactor in Dubna should confirm the effectiveness of the method.

The high spin negative parity states of Ag have been investigated with the B+Mo reaction at 39 MeV beam energy using the INGA facility at TIFR, Mumbai. From the γ-γ coincidence analysis, an excited negative parity band has been established and found to be nearly degenerate with the ground state band. The spin and parity of the levels are assigned using angular correlation and polarization measurements. This pair of degenerate bands in Ag is studied using the recently developed microscopic triaxial projected shell model approach. The observed energy levels and the ratio of the electromagnetic transition probabilities of these bands in this isotope are well reproduced by the present model. Further, it is shown that the partner band has a different quasiparticle structure as compared to the yrast band. © 2013 Elsevier B.V.

Relative yields of photon-induced reactions are systematized in a function of the threshold parameter for moderately heavy targets at the bremsstrahlung end-point energy lower than 30 MeV. Regular dependence is established for the group of (γ, n), (γ, p), and (γ, d) reactions, while the yields of (γ, 2n) and (γ,α) reactions deviate from the regularity. Physical conclusions are discussed and possible application of this systematic for data processing is proposed. In particular, the constituent threshold and spin factors in the isomer-to-ground-state ratio could be isolated separately. For spin dependence of the yields, a new regularization parameter is introduced and previously hidden peculiarities are concluded.

In the present work, quantitative calculations were carried out for production and depletion of the 93m Mo isomer in a relatively simple experiment using 91Zr beam ions. Such studies could be arranged at existing and operating accelerator facilities, e.g. at GSI or in JINR. The 93m Mo nuclei produced in a He gas target due to the 4He(91Zr, 2n) reaction will recoil into a gas stopper with a high velocity, being then depleted due to NEEC in highly-ionized species.

Rate of fission for highly-excited nuclei is affected by the viscose character of the systemmotion in deformation coordinates as was reported for very heavy nuclei with Z C > 90. The long time-scale of fission can be described in a model of “fission by diffusion” that includes an assumption of the overdamped diabatic motion. The fission-to-spallation ratio at intermediate proton energy could be influenced by the viscosity, as well. Within a novel approach of the present work, the cross examination of the fission probability, time-scales, and pre-fission neutron multiplicities is resulted in the consistent interpretation of a whole set of the observables. Earlier, different aspects could be reproduced in partial simulations without careful coordination.

A well-known island of nuclear isomerism appears near A =175–180 due to the deformation alignment of single-particle orbits at high angular momentum. This sometimes results in the formation of multi-quasiparticle states with record spin that are long-lived because of “K-hindrance”, i.e. symmetry re-arrangement. Production methods and spectroscopic studies of these isomers remain a challenge for modern nuclear reaction and nuclear structure physics. In the present work, activities were produced by irradiation of 176 Yb (97.6% enriched) and nat Lu targets with 35 MeV 4 He ions from the internal beam of the U-200 cyclotron. Induced activities were analyzed by applying the methods of radiochemistry and gamma spectroscopy. Yields of the compound and nucleon-transfer reactions were measured and the isomer-to-ground state ratios were deduced. Calculated results were obtained using standard procedures to reproduce the (α, xn) cross-sections, and the systematic behavior of the nucleon-transfer yields was established. The isomer-to-ground-state ratios for direct reactions with 4 He ions were examined, resulting in a new characterization of the reaction mechanism.

A 985 MeV 178 Hf beam was Coulomb excited by a 208 Pb target at the ATLAS accelerator of Argonne National Laboratory. Gammasphere and the CHICO particle detector recorded particle-γ coincidence data. The aim was to populate and determine the mechanism of previously observed Coulomb excitation of the Kπ = 6+ (t1/2 = 77 ns ), 8- (4 s ) and 16+ (31 y ) isomer bands. New rotational bands were identified including an aligned band which appears to mix with the ground-state band (GSB) and the γ-vibrational band above ~ 12 ħ of angular momentum. Newly observed γ-decay transitions into the three isomer bands may elucidate the K-mixing which allows Coulomb excitation of these isomer bands, but direct decays from the GSB into the 16+ isomer band have not yet been confirmed.

For atomic-nuclear cooperative processes, the cross sections can be deduced in favorable cases from experiments either directly, or through the linked theoretical estimates. Probability of isomer transmutation in electron-assisted mechanisms must contain both nuclear and atomic components. The nuclear database is abundant and extensive, in particular due to the exploration of electromagnetic nuclear processes, like γ-emission, reactions induced by photons, Coulomb excitation and so on. In the present work, the nuclear excitation rate via NEET mechanism is calculated and compared to known experimental results. The NEEC probability is estimated schematically. Perspectives for NEEC detection within different experimental approaches are discussed. A new method is proposed to search for the isomer depletion under stopping of recoiling nuclei in a gas.

Electromagnetic transitions within nuclei reflect specific aspects of nuclear structure. This is particularly true for metastable excited states, or isomers, like 178 Hf m 2 ( T 1 / 2 = 31 years, excitation energy 2446 keV). The interaction of external radiation with isomers can be used to study atomic and nuclear properties and, perhaps, to induce a release of the stored energy. Some experiments indicated that low-energy photons near the L 3 edge (9.561 keV) of hafnium could cause this to occur for 178 Hf m 2 , but the lack of a viable physical model and null experiments by other groups have left these claims in doubt. The present work describes a new experiment to examine this process by closely duplicating the irradiation conditions in positive studies, but using a more advanced multi-detector γ array. No support for an induced depletion of 178 Hf m 2 by low-energy photons was obtained, with an upper limit for the integral cross section that is eight orders-of-magnitude below the reported value.

Fission of highly excited nuclei is affected by the viscous character of the system motion in deformation coordinates as is reported for very heavy nuclei with Z ⩾ 90. The long-time-scale fission was proved for such systems formed in heavy-ion-induced reactions. The overdamped diabatic motion may influence also fission of the spallation-residue products in reactions with protons at intermediate energy. In the present work, the experimental results on fission-to-spallation ratio are analyzed and the evidences for the long-time-scale fission are found in the fission excitation functions for medium-mass targets with Z = 70–75.

Production of Hf and Lu high-spin isomers has been experimentally studied in spallation reactions induced by intermediate energy protons. Targets of enriched 179Hf (91%) and natHf were bombarded with protons of energy in the range from 90 to 650 MeV provided by the internal beam of the Dubna Phasotron synchrocyclotron. The activation yields of the reaction products were measured by using the γ-ray spectroscopy and radiochemistry methods. The production cross-sections obtained for the 179m2Hf, 178m2Hf and 177mLu isomers are similar to the previously measured values from the spallation of Ta, Re and W targets. Therefore, the reactions involving emission of only a few nucleons, like (p,p′), (p,p′n) and (p,2pn), can transfer high enough angular momentum to the final residual nuclei with reasonable large cross-sections. A significant gain in the isomeric yields was obtained when enriched 179Hf targets were used. The mass distribution of the residual nuclei was measured over a wide range of masses and the fission-to-spallation ratio could be deduced as a function of the projectile energy. Features of the reaction mechanism are briefly discussed.

The state of experimental studies and promising proposals for the application of nuclear isomers presented as controlled energy or γ-ray sources are reviewed. The properties of isomeric states, methods of their production, and approaches to their efficient stimulation using various types of radiation are analyzed. The long-lived isomers, which can be accumulated in reactor irradiations or in other nuclear interactions with abundant yield, are listed. The isomers are estimated according to their specific energy accumulated per nucleus and the level of the cross section for their formation in reactions with neutrons. The nuclei are classified as promising either for obtaining controlled γ-ray pulses, for the enhanced release of the radioactive decay energy, or for experimental studies on detecting forbidden electromagnetic transitions from the ground to isomeric state. In all cases, the possibility of external-stimulus action on nuclear transitions has key significance, which should become the subject of investigations. The results of successful observation of stimulation of isomers are described at excitation energy E* > 1 MeV in the reactions with bremsstrahlung photons and Coulomb excitation in the ion beam. The essential increase in the K-hindered transitions with increasing energy and also the K-mixing at high rotational frequency for high-spin levels are discussed. The attention is focused on attempts to detect the triggering induced by the radiation in the x-ray range, in particular, that of the 178m2Hf isomer with the help of x-ray sources and the synchrotron radiation. Proposals for experiments with other isomers are considered. The possibility of affecting the nuclear states by means of ionization of electron shells of a corresponding atom is discussed as promising, and various schemes of similar experiments are proposed. The atomic cross sections are eight orders of magnitude higher than the nuclear ones; therefore, the stimulation of an isomer can occur even if the conversion from atomic excitations to nuclear ones has a low probability.

The 242m Am isomer, a well-known candidate for photodepopulation research, has been studied in this first ever Coulomb excitation of a nearly pure (≈98%) isomer target. Thirty new states, including a new rotational band built on a K π = 6− state, have been identified. Strong K-mixing results in nearly equal populations of the K π=5− and 6− states. Newly identified states have been assigned to the K π=3− rotational band, the lowest states of which are known to decay into the ground-state band. Implications regarding K-mixing and Coulomb excitation paths to the ground state are discussed.

Possible experiments are discussed in which Coulomb excitation of nuclear isomers would be followed by sequential energy release. The possibility of coherent Coulomb excitation of nuclei ensconced in a crystal by channeled relativistic heavy projectiles is considered. The phase shift between neighbor-nuclei excitations may be identical to the photon phase shift for emission in the forward direction. Thus, the elementary string of atoms may radiate coherently with emission of characteristics nuclear γ rays, and the intensity of the radiation would be increased due to the summation of amplitudes. Mossbauer conditions should be important for this new type of collective radiation, which could be promising in the context of the γ-lasing problem.

Fusion of two massive nuclei with formation of a superheavy compound nucleus is driven by the potential energy gradient, as follows from the analysis of nuclear reaction cross sections. The conservative energy of the system is deduced in a simple approximation using regularized nuclear mass and interaction barrier values. Different reactions for the synthesis of Z = 110−118 nuclei are compared and favorable conditions are found for fusion of the stable W-Pt isotopes with radioactive fission fragment projectiles, like 94Kr or 100Sr. Thus, the cold-fusion method can be extended for a synthesis of elements with Z > 113. Survival of the evaporation residue is defined by the neutron-to-fission probability ratio and by the successful emission of gammas at the final step of the reaction. Numerical estimates are presented. Fixation of evaporation residue products must correlate with longtime-scale fission, and available experimental results are discussed.

Plenary and semiplenary sessions

Reports that incident photons near 10 keV can induce the emission of gamma rays with concomitant energy release from the 31-year isomer of 178 Hf challenge established models of nuclear and atomic physics. In order to provide a direct and independent assessment of these claims, a multi-detector system was designed as a specialized research tool. The YSU miniball is unique in its combination of performance characteristics, compact size and portability, enabling it to be easily transported to and placed within the confines of beamline hutches at synchrotron radiation sources. Monochromatic synchrotron radiation was used in the most recent studies from which evidence of prompt triggering was claimed, suggesting similar sites for independent tests of these results. The miniball array consists of six high-efficiency BGO scintillators coupled with a single 65% Ge detector and provides time-resolved gamma-ray calorimetry rather than purely spectroscopic data. The need to record high detected folds from the array (up to seven-fold gamma coincidences) makes this system different in practice from standard spectroscopic arrays for which data is typically restricted to triples or lower folds. Here the system requirements and design are discussed, as well as system performance as characterized using the well-known natural decay cascades of 178m2 Hf. This serves as the foundation for subsequent high-sensitivity searches for low-energy triggering of gamma emission from this isomer.

Experiments on production of long-lived Hf 178 m 2 isomer in reactor irradiations are described. Properties of this nuclide are promising for its potential application as a relatively safe power source characterized by high density of accumulated energy. Metal natHf samples were activated in the Dubna IBR-2 reactor at positions corresponding to different neutron fluxes. Samples were bare or shielded by Cd and B4C layers. The gamma activity of the samples was analyzed with Ge gamma spectrometers during a two-year period following their irradiation. In the presence of dominant activation products 175Hf and 181Hf, the high-spin isomers Hf 178 m 2 and Hf 179 m 2 were also detected despite relatively low levels. The isomer-to-ground state ratios and cross-sections were determined from the measured yields. For Hf 178 m 2 , the cross-section for burnup (destruction) by neutron capture after its production was also estimated, clarifying the results from earlier experiments. In the context of suggestions for use of Hf 178 m 2 for applications, the results confirm that large-scale production of this isomer by reactor irradiations is not feasible. In contrast, the efficiency of production of Hf 179 m 2 is much higher and an amount of about 1016 atoms may be produced in standard reactor irradiations. For Hf 178 m 2 , more productive methods are known, in particular fast neutron irradiations at En ≥ 14 MeV and spallation reactions at intermediate energies. Neutron cross-sections for isomers may also be significant in astrophysics.

The long-lived high-spin 178m2Hf K isomer can be produced in nuclear reactions with different projectiles. The reaction yields and cross sections have been measured in a series of experiments and the results are now reviewed. The systematics of isomer-to-ground-state ratios are drawn and real production capabilities are estimated for the best reactions. Such a summary is relevant to the significance of isomer studies, both for nuclear-science knowledge and for possible applications. Potential isomer applications have been stressed previously in popular publications with probably overestimated expectations. The real possibilities are restricted in part by the production yield and by other shortcomings as well.

High-productivity methods are required for the accumulation of long-lived isomers in amounts that are sufficient for the creation of experimental targets. A tantalum sample was activated with the Yerevan synchrotron using 4.5-GeV bremsstrahlung and the presence of 178m2Hf was detected with good statistical accuracy by γ-activity measurements. The integrated and mean cross-section values were deduced from the experiment. The isomer-to-ground-state ratio was then estimated and compared with that known for the p+ Ta reaction studied at 660 MeV. In the present experiment, both converter and target were relatively thin for better definition of the experimental conditions. However, an assembly designed for high-productivity irradiations should be thick and then the converter can also serve as the target sample when irradiated with a high-energy electron beam. The optimization of the isomer production was solved analytically and the largest estimated yield was determined as calibrated to the experimental yield. The maximum yield of 178m2Hf was found to be of about 3×109 nuclei/s using an electron beam current of 100 μA. This is lower than the yield achieved with proton beams, although for a practical comparison the total cost and radiation safety conditions should be considered. The present results provide a basis for numerical estimations.

Yields and cross-sections of the radioactive nuclides produced after the irradiation of natural composition W and enriched 186 W targets at the Dubna synchrocyclotron were measured using the γ-ray spectroscopy methods with high-resolution Ge detectors. Among the detected nuclides we identified the spallation and fission products. High-spin isomeric states in the Hf and Lu nuclides were populated and the isomer-to-ground state ratios could be estimated. The nuclide yields were calculated using the LAHET code at six different values of the proton energy in the range from 100 to 800 MeV both for the nat W and enriched 186 W targets. The measured isotope yields are in general good agreement with the calculations. A shortcoming of the code is the inability to predict isomer-to-ground state ratios. The experimental data show that the 177m Lu, 178m2 Hf and 179m2 Hf high-spin isomers are produced with a 2.5 times higher yield in the 97% enriched 186 W target as compared to the nat W target under identical irradiation conditions. This makes significance for the creation of high-activity isomeric sources. The mass-distribution of the products and the fission-to-spallation ratio were also deduced and compared with theory prediction.

Experiments aimed at detecting and investigating neutron resonances in the $$^{178m_2 } Hf$$ isomer are described, and the results obtained in these experiments are presented. The investigations in question are of great interest since the structure of this isomer—it is interpreted as the (π7/2+, π9/2+, ν7/2+, ν9/2+) configuration—and its high spin of J=16 differ significantly from the structure and spin of nuclei studied previously. The experiments performed at the Kurchatov Institute employed a neutron source based on the FAKEL linear electron accelerator and a multisection detector from NaI(Tl) crystals that was able to ensure a 4π coverage. This equipment made it possible to study gamma-ray cascades in radiative neutron capture versus neutron energy. Despite an extremely small number of isomer nuclei, a low content of the isomer in the target used, and its high radioactivity, resonances were discovered that arise upon neutron capture by a high-spin $$^{178m_2 } Hf$$ nucleus. The parameters of these resonances were found. The mean spacing between the revealed resonances is about 1 eV, which is consistent with calculations based on the Fermi gas model. This indicates that the Fermi gas model describes well the density of both low-and high-spin levels. At the same time, the above agreement suggests that, upon the formation of a compound nucleus, the structure of the isomeric state is destroyed completely. On the other hand, glaring discrepancies between experimental data and the predictions of the statistical model were found: gamma transitions from high-spin resonances (J=31/2+, 33/2+) populate predominantly the low-spin ground state (J=9/2+) rather than the high-spin state of the $$^{178m_2 } Hf$$ isomer (J=25/2−); the radiative width is approximately one-third as great as that which is predicted by the statistical model; and the properties of gamma cascades are different for different resonances, this difference being beyond statistical fluctuations. The results of the present investigation make it possible to reveal special features in the behavior of the quantum number K at high excitation energies.

The productivity of the spallation reactions at proton energies of 100–660 MeV for accumulation of the radioactive isotopes and isomers has been studied experimentally. Spectra of Ta and Re targets activated at Dubna synchrocyclotron were measured using the methods of radiochemistry and gamma spectroscopy. Many radioactive products of the spallation and fission reactions are identified, and their yields are compared with the LAHET code simulations. Cross-sections σm and isomer-to-ground state ratios σm/σg are deduced for nuclear isomers, in particular, for high-spin isomers, as 178m2Hf, 179m2Hf and 177mLu. Spin distributions for the spallation-residue nuclei are estimated to understand the σm/σg ratios. Possibilities to optimize the methods for the long-lived isomers production are discussed, and it would be a necessary step on the way to accumulate such exotic radionuclides in milligram amount under reasonable cost and radiation safety conditions.

Nuclear isomers can be used for the storage and release of “clean” nuclear energy and several triggering schemes have been discussed. Here the possibility to utilize resonance between atomic and nuclear transitions in the form of a hybridization of atomic-nuclear excitation is considered. Several isotopes and specific nuclear levels are identified as candidates for triggering via atomic transitions. A variety of ionization states and atomic-shell configurations arises in a hot plasma generated by short high-power pulses of laser light. The non-radiative conversion of the ionization energy within an atom can be suppressed in the hot plasma surroundings. The time scales of different processes in nuclear, atomic and condensed-matter subsystems are compared and the fast ionization in a solid, X-ray radiance in a plasma, and sample melting and recrystallization may precede nuclear fluorescence. A time scale shorter than 0.1 ns makes this sequence promising for the collective excitation of short-lived modes in a nuclear subsystem.

Experiments with the long-lived, high-K isomer 178Hfm2 have been recognized as intriguing tests of multi-quasiparticle state structures and their interactions with external radiation. A triggered release of the energy stored by this isomer, 2.5 MeV per nucleus or 1.2 GJ/gram, in the form of a gamma-ray burst might prove valuable for numerous applications. The observation of “accelerated” decay of 178Hfm2 during irradiation by 90-keV bremsstrahlung has already been reported, but with poor statistical accuracy due to the experimental approach. That approach employed single Ge detectors to seek increases in the areas of peaks at energies corresponding to transitions in the spontaneous decay of the isomer. The need for better quality data to confirm those results has motivated the development of improved detection concepts. One such concept was utilized here to perform an initial search for low-energy (

The high spin structure of 108 Ag nucleus has been studied using the reaction 11 B + 100 Mo at 39 MeV with Indian National Gamma Array (INGA) at TIFR-BARC accelerator facility. From the two- and higher-fold coincidence analysis of the emitted -rays, the level structure of the nucleus is built, with addition of around ∼ 60 new transi- tions. A new positive parity dipole band has been observed and significant additions have been made in the low spin region. A pair of nearly degenerate, negative parity, dipole bands is established, which is studied using the triaxial projected shell model (TPSM).

X-ray-driven gamma emission describes processes that may release nuclear energy in a ‘clean’ way, as bursts of incoherent or coherent gamma rays without the production of radioactive by-products. Over the past decade, studies in this area, as a part of the larger field of quantum nucleonics, have gained tremendous momentum. Since 1987 it has been established that photons could trigger gamma emission from a long-lived metastable nuclear excited state of one nuclide and it appears likely that triggering in other isotopes will be demonstrated conclusively in the near future. With these experimental results have come new proposals for the creation of collective and avalanche-like incoherent gamma-ray bursts and even for the ultimate light source, a gamma-ray laser. Obviously, many applications would benefit from controlled bursts of gamma radiation, whether coherent or not. This paper reviews the experimental results and concepts for the production of gamma rays, driven by externally produced X-rays.

The time delays in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, $240\text{\ensuremath{-}}255$ MeV $^{48}\mathrm{Ti}$, $330\text{\ensuremath{-}}375$ MeV $^{58}\mathrm{Ni}$, and 390 MeV $^{74}\mathrm{Ge}$ have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order ${10}^{\ensuremath{-}18}$ s which depend weakly on the atomic number of the composite nucleus. This is inconsistent with the Bohr-Wheeler model of fission from a compound nucleus in statistical equilibrium at each stage in a neutron evaporation cascade and supports a picture of strongly damped quasifission. A simple diffusion model with one-body dissipation reproduces roughly the observed time scale and the exponential decay. It suggests that the outer fission barrier could play a significant role in the observed, very slow decays.

A sample containing 6.3×1014 nuclei of the 16+ isomer of 178Hf having a half-life of 31 yr and an excitation energy of 2.446 MeV was irradiated with x-ray pulses from a device operated at 15 mA to produce bremsstrahlung with an endpoint energy of 90 keV. The gamma spectra of the isomeric target were taken with a Ge detector. The intensity of the 325.5-keV (6+ → 4+) transition in the ground-state band of 178Hf was found to increase by about 2%. Such an enhanced decay of the 178Hf isomer is consistent with an integrated cross section value of 3×10−23 cm2 keV if resonance absorption occurs within energy ranges corresponding to the maxima of the x-ray flux, either near 20 keV or at the energies of the characteristic emission lines of W.

The yield of 24Na and 28Mg radionuclides was determined by their gamma activity with the aid of radiochemistry. The corresponding measurements were performed upon the activation of purified Th targets with bremsstrahlung photons having endpoint energies of 12, 16.5, and 24 MeV. Possible background sources are carefully analyzed, and it is concluded that the yield of the above light nuclei is about 10−6 per event of 232Th fission.

A sample of $6.3\ifmmode\times\else\texttimes\fi{}{10}^{14}$ nuclei of the 4-quasiparticle isomer of ${}^{178}\mathrm{Hf}$ having a half-life of 31 yr and excitation energy of 2.446 MeV was irradiated with x-ray pulses from a device typically used in dental medicine. It was operated at 15 mA to produce bremsstrahlung radiation with an end point energy set to be 70 or 90 keV. Spectra of the isomeric target were taken with a high purity Ge detector. Intensities of selected transitions in the normal decay cascade of the ${}^{178}\mathrm{Hf}$ isomer were found to increase by about $4%$. Such an accelerated decay is consistent with an integrated cross section of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\mathrm{cm}{}^{2}\mathrm{keV}$ for the resonant absorption of x rays to induce gamma decay.

Neutron activation analysis has been applied to determine the purity of super-enriched176Yb as well as a diagnostic to optimize yields from the P.A.R.I.S. mass separator at Orsay. In 25 mass separations carried out during 3 months, 910 mg of super-enriched176Yb2O3 were recovered from the original 7 g of176Yb2O3 (97.5% enrichment). Neutron activation analysis has shown that only two per thousand of the initially present174Yb remained in the super-enriched product. Using this176Yb super-enriched (99.998%) target, micro-quantities of the high spin (16+) isomer178m2Hf were produced by the (α,2n) reaction at the U-200 cyclotron in Dubna. Quantities of about 2·1012 atoms of this isomer have been irradiated by thermal neutron at the ORPHEE Reactor, Saclay. Only the neutron capture giving rise to the isomeric state 25/2− of179Hf has been measured, by detection γ-rays of this 25.1 days isomer. A cross section of 45±5 barns was obtained for178m2Hf(n,γ)179m2Hf.

Systematics for the appearance of K-mixing levels for the pumping or spontaneous decay of multi-quasiparticle isomers in Hf isotopes are detailed in this letter. The possible location of such a level in the nuclide 178Hf is discussed and an experiment is proposed to investigate its existence.

Isomer-to-ground-state ratios are measured for the group of photon- and α-induced nuclear reactions. The photonuclear reactions on high-spin isomeric targets - 178Hfm2 (16+) and 180Tam (9-) - were detected successfully. Results are analysed from the point of view of the K-selectivity manifestation for the level population. The yields comparison with the regular values confirms strong K-mixing at high excitations.

The 178m2Hf with its long-lived (T1/2=31 y), high-spin Iπ = 16+, isomeric state, is a challenge for new and exotic nuclear physics studies. The 178m2Hf isomer has been produced in microweight quantities using the 176Yb(α,2n) nuclear reaction, by irradiation with a high-intensity beam using the U-200 cyclotron in Dubna. Radiochemistry and mass separation methods have been developed, with the aim to separate and purify the produced Hf material. Thin targets of isomeric hafnium-178 on carbon backings have been prepared and used in experiments with neutron, proton and deuteron beams.

Nowadays large interest is attached to the production of exotic beams and targets for nuclear structure and reaction studies. The nucleus178Hf, with is long-lived (T1/2=31 years) high-spin isomeric statelπ=16+ at a relatively low-excitation energy (2.446 MeV), is indeed a unique probe for studying phenomena in a new way. A wide research program has been underway in the framework of the “Hafnium Collaboration” which now concems around 80 scientists. Various targets have been prepared and adapted to different types of experiments. Irradiation processes with high-intensity beams, high-purity chemistry methods and isotopic separations have been developed. The obtaining of appreciable amounts of very high purity178m2Hf by isotopic separation, has made it possible for us to carry out γ-ray measurements with good statistics and high energy resolution with a γ-spectrometer, in order to use this isomer as a calibration standard in the 200 to 600 ke V energy range. For example, relative to the main γ-rays, energies have been determined with an absolute uncertainty of about 2 e V and photon emission probabilities with a relative uncertainty better than 1%.

Reactions induced by photons may be used as a probe for bonds and correlations between nucleons inside a heavy target nucleus. Electromagnetic radiation perturbs the nucleons only slightly unlike the influence of strongly interacting particles. The yield of (γ, α) reactions could be used to test theoretical models assuming a complete α-clustering, or multi-quark objects in heavy nuclei. Relative yields of (γ, n), (γ, p), and (γ, α) reactions have been measured at the bremsstrahlung end-point energy of 23 MeV with several targets. Much lower probability of (γ, α) compared to (γ, p) reactions is proved despite practically similar threshold and spin factors for both types of reactions. Alpha clustering in heavy targets is not supported. The mechanism of particle release in nuclear reactions is discussed and some details are clarified.

Photonuclear reactions on isomeric and ground state Hf and Ta targets were studied systematically by activation technique. The reaction yields were measured and isomer-to-ground state ratiosY m /Y g were deduced. The systematics of theY m /Y g values versus spin difference of initial and final states in the photonuclear reaction shows that the structure selectivity for the population of the levels does not survive at high excitation energies.

A heavy-ion multiple Coulomb excitation experiment on a very exotic target containing microweight quantities of 178Hf in the Kπn = 16+ isomeric state has been performed at 4.77 MeV/u 208Pb beam energy. The first excited Iπ = 17+ state has been observed at an excitation energy of 357.4 ± 0.3 keV with respect to the isomeric state. The intrinsic electric quadrupole moment of Q0 = 8.2 ± 1.1 b has been derived from the experimental data within the rigid rotor model.

In inelastic scattering of 22 MeV deuterons from an isotope-separated target of $^{178}\mathrm{Hf}$ containing about 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ nuclei in the isomeric ${16}^{+}$ state we observed rotational excitation to the ${17}^{+}$ state at an excitation energy with respect to the isomeric state of 356.5\ifmmode\pm\else\textpm\fi{}0.4 keV and weak evidence for the ${18}^{+}$ state at 737\ifmmode\pm\else\textpm\fi{}2 keV. We compared the differential cross sections with coupled-channel calculations and with scattering from $^{178}\mathrm{Hf}$ (${0}^{+}$) and $^{177}\mathrm{Hf}$ (7/${2}^{\mathrm{\ensuremath{-}}}$). \textcopyright{} 1996 The American Physical Society.

Long-lived nuclear excited states (isomers) have proven important to understanding nuclear structure. With some isomers having half-lives of decades or longer, and intrinsic energy densities reaching 1012 J/kg, they have also been suggested for a wide range of applications. The ability to effectively transfer a population of nuclei from an isomer to shorter-lived levels will determine the feasibility of any applications. Here is described a first demonstration of the induced depletion of a population of the 438 year isomer of 108Ag to its 2.38 min ground state, using 6 MeV bremsstrahlung from a modified medical electron linac. The experiment suggests refinements to be implemented in the future and how a similar approach might be applied to study induced depletion of the 1200 year isomer of 166Ho.

There are much data on neutron cross sections over the chart of nuclides for stable isotopes and not as much for the radioactive ones. Double neutron capture experiments could be fruitful to provide more data. Time-integrated mean flux of slow neutrons reaches the value of 2.3-1012 n/cm2 s at the irradiation port near the active zone of the IBR-2 pulsed reactor of JINR. This is enough to detect the double neutron capture products by the activation method. A high capture cross section is obtained in the present experiment for intermediate radioactive 182Ta and 194Ir target nuclides. Together with the known data for 198Au, these values may prove an essential role of double neutron capture process for nucleosynthesis of 183W, 195Pt and 199Hg isotopes at stellar conditions.

Time delays in fission induced by bombardment of Mo with 170- and 180-MeV ${}^{32}$S, 225- and 240-MeV ${}^{48}$Ti, and 300-MeV ${}^{58}$Ni have been measured by observation of crystal blocking of fission fragments. In contrast to earlier measurements with a W target, the results are consistent with fission of a compound nucleus in competition with mainly neutron emission. Most of the fissions happen on a time scale much shorter than attoseconds but there is a significant component of fission with much longer lifetimes. The measurements are reproduced with a standard statistical model, including a Kramers correction to fission widths from the viscosity of hot nuclear matter. These new results support the interpretation of our earlier measurements with a W target, which indicate that there is a transition in heavy-ion-induced fission at large atomic number and mass, from multichance fission in the standard Bohr-Wheeler picture to fission without formation of a compound nucleus. The process is slowed down by nuclear viscosity, with measured delays of order attoseconds.

The scattering of thermal neutrons from isomeric nuclei may include events in which the outgoing neutrons have increased kinetic energy. This process has been called inelastic neutron acceleration, or INNA, and occurs when the final nucleus, after emission of the neutron, is left in a state with lower energy than that of the isomer. The result, therefore, is an induced depletion of the isomer to the ground state. A cascade of several $\ensuremath{\gamma}$'s must accompany the neutron emission to release the high angular momentum of the initial isomeric state. INNA was previously observed in a few cases, and the measured cross sections were only in modest agreement with theoretical estimates. The most recent measurement of an INNA cross section was ${\ensuremath{\sigma}}_{\mathrm{INNA}}=258\ifmmode\pm\else\textpm\fi{}58 \mathrm{b}$ for neutron scattering by $^{177}\mathrm{Lu}$${}^{m}$. In the present work, an INNA cross section of ${\ensuremath{\sigma}}_{\mathrm{INNA}}=168 \ifmmode\pm\else\textpm\fi{} 33 \mathrm{b}$ was deduced from measurements of the total burnup of the high-spin, four-quasiparticle isomer $^{178}\mathrm{Hf}$${}^{m2}$ during irradiation by thermal neutrons. Statistical estimates for the probability of different reaction channels past neutron absorption were used in the analysis, and the deduced ${\ensuremath{\sigma}}_{\mathrm{INNA}}$ was compared to the theoretically predicted cross section.