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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.

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.

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.

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.

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.

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.

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 (

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 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.

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.

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%.

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.

Fission of highly excited nuclei is affected by the viscose 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 blocking lifetime experiments are analyzed together with the independent data on fission‐to‐spallation ratios for medium‐mass targets. Cross examination of the fission probability, time‐scale, and pre‐fission neutron multiplicity allows a new conclusion on fission mechanism. The novel approach is specified for estimate of the characteristic fission time and for the consistent interpretation of whole set of experimental results. The standard statistical approach fails for understanding of them without contradictions, though some partial data at restricted range ...

Fusion of two massive nuclei with formation of super‐heavy compound nucleus (CN) 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 simple approximation using regularized nuclear mass and interaction barrier values. Different reaction for the synthesis of Zc = (110–118) nuclei are compared and the favourable 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 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.

Fission-fragment mass distributions in the ${}^{232}\mathrm{Th}(\ensuremath{\gamma},f)$ reaction have been measured from the cumulative yields of radionuclides following activation at bremsstrahlung endpoint energies ${E}_{e}=7.5,$ 12.0, and 24.0 MeV. The yields of the light nuclei ${}^{24}\mathrm{Na}$ and ${}^{28}\mathrm{Mg}$ were detected following activation at ${E}_{e}=16.5$ and 24.0 MeV. Energy dependences of the symmetric mode and light nuclide yields are discussed. Fine-structure manifestations near asymmetric and symmetric masses were observed.

A Reply to the Comment by Silviu Olariu and Agata Olariu. Also D. P. McNabb, et al. also P. von Neumann-Cosel and A. Richter. (c) 2000 The American Physical Society.