J. J. Carroll
Department of Physics and Astronomy, Youngstown State University,
Youngstown, Ohio 44555 USA

ABSTRACT

The study of triggered depopulation of nuclear isomers, with accompanying gamma emission, is a rapidly-changing field which is only now attaining some degree of maturity. Because isomer decays via electromagnetic transitions are strongly inhibited, the interaction of these levels with externally-produced photons provides an important probe of nuclear structure. Also, since some isomers may store large amounts of energy for long times, a number of applications have been proposed, including the creation of a gamma-ray laser. Early experiments conclusively demonstrated triggering of the 10¹⁵-year isomer of ¹⁸⁰Ta, but even a partial correlation of the effect with known levels could only be obtained recently. Investigations of triggering for the 31-year isomer of ¹⁷⁸Hf were initially guided just by systematics and experiments are characterized by considerable controversy. Against this background, the field of triggered gamma emission is entering a new phase in which improved level data allow targeting of specific potentially-useful transitions. This paper summarizes the current state-of-the-art and discusses the changing nature of the field.
 

"Possible ways for triggering the ^179m2Hf isomer"
 

S. A. Karamian¹, J. J. Carroll², L. A. Rivlin³, A. A. Zadernovsky³ and F. J. Agee⁴

1   Joint Institute for Nuclear Research, Dubna, 141980, Russia
2   Department of Physics and Astronomy, Youngstown State University, Ohio, 44555, USA
3   MIREA Technical University, 78 Vernadsky Prospekt, Moscow 117454, Russia
4   Air Force Office of Scientific Research, NE, 4015 Wilson Blvd., Arlington, VA 22203, USA

ABSTRACT

Realistic approaches to triggering of the ^179m2Hf isomer are discussed. The nuclear level scheme of ¹⁷⁹Hf suggests three promising ways for triggering, but two of them are seemingly closed by the high multipolarities of the required electromagnetic transitions. New possibilities are described how to overcome such restrictions by utilizing a model of atomic-nuclear compound states. A new and productive method of observing triggering would base an experiment on the use of an electron cyclotron resonance ion trap, ECRIT. Some quantitative details of the behavior of ^179m2Hf and ^242mAm atomic-nuclear systems in an ECRIT environment are examined. Many important parameters are as yet unknown and they can be estimated only after the new type of experiments proposed here in which high rates of triggering are possible.
 

"Formation of the high-spin ^179m2Hf isomer in reactor irradiations"

S. A. Karamian¹, J. J. Carroll², J. Adam¹, E. N. Kulagin¹ and E. P. Shabalin¹

1   Joint Institute for Nuclear Research, Dubna, 141980, Russia
2   Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio, 44555, USA

ABSTRACT

The second isomer (T_1/2 = 25.1 d) in ¹⁷⁹Hf is one of a number of nuclear states that is interesting from the point of view of triggering a release of "clean" nuclear energy because it stores a specific energy of about 0.5 MJ/mg. The yield of this isomer in nuclear reactions is restricted due to its high spin, I = 25/2 ^- . The productivity of previously-known methods was enough for the creation of experimental amounts but not for potential applications. In this paper we show that irradiations in a reactor by the fast neutron flux within the fission spectrum are useful for the accumulation of ^179m2Hf in an amount of 10¹⁶ nuclei. In an experiment performed in the Dubna IBR-2 reactor, the yield, cross-section s_m and isomer-to-ground state ratio s_m / s_g were measured for the ¹⁷⁹Hf(n, n' g)^179m2Hf reaction. The systematics of the s_m / s_g values deduced from the experimental data available for this isomer are discussed.
 

“Forced-Gamma Emission” Studies Involving Nuclear Isomers Using Fast Neutrons and Bremsstrahlung X Rays"

N. A. Guardala⁺, J. L. Price⁺, J. H. Barkyoumb⁺, R. J. Abbundi⁺, G. Merkel^* and J. J. Carroll^#

+   NSWC/Carderock Division, 9500 MacArthur Blvd, W. Bethesda, MD 20817-5700
*   Army Reserch Lab/Adelphi, 2800 Powder Mill Road, Adelphi MD
#  Dept. Of Physics and Astronomy, Youngstown State Univ., Youngstow, Ohio
 

ABSTRACT

We propose to perform a series of experiments involving nuclear isomers which will investigate the probabilities and mechanisms for de-exciting the isomeric level down to the ground state upon exposure to external radiation in the form of fast neutrons and bremsstrhalung x rays. The isomers have half-lives on the order of 1 hr to 10 days which is a convenient time scale to measure statistically meaningful changes in the specific activities of the isomeric state. Furthermore, the selected isomers are relatively easy to produce in our laboratory in sufficient quantities so that they can be made in a reasonable time frame and without recourse to any exotic means of production, handling or preparation and without the need for high-purity separated isotopes as the feedstock. We believe that studies undertaken in this fashion will produce fundamentally valuable information on the factors which govern and influence “forced gamma emission” in nuclear isomers This type of information will potentially be very useful in similar studies involving longer-lived isomers such as:^178m2Hf, ^242mAm and ^108mAg which have the potential to be used in various emerging new technologies in the later part of the 21st Century.

"g rays emitted in the decay of 31-yr ¹⁷⁸Hf^m2"

M. B. Smith,¹ P. M. Walker,^1,2 G. C. Ball,¹ J. J. Carroll,³ P. E. Garrett,⁴ G. Hackman,¹ R. Propri,³ F. Sarazin,¹ and H. C. Scraggs¹
 

1   TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3
2   Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
3   Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio 44555, USA
4   Lawrence Livermore National Laboratory, Livermore, California 94551, USA
 

The spontaneous decay of the K^p = 16⁺, 31-yr ¹⁷⁸Hf^m2 isomer has been investigated with a 15-kBq source placed at the center of a 20-element g-ray spectrometer. High-multipolarity M4 and E5 transitions, which represent the first definitive observation of direct g-ray emission from the isomer, have been identified, together with other low-intensity transitions. Branching ratios for these other transitions have elucidated the spin dependence of the mixing between the two known K^p = 8 ^- bands. The M4 and E5 g-ray decays are the first strongly K-forbidden transitions to be identified with such high multipolarities, and demonstrate a consistent extension of K-hindrance systematics, with an inhibition factor of approximately 100 per degree of K forbiddenness. Some unplaced transitions are also reported.

"Hybridization of atomic-nuclear excitations and pumping of nuclear levels"

S. A. Karamian^* and J.J.Carroll^**

*   Joint Institute for Nuclear Research, Dubna, Moscow region, 141980 Russia
** Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio, 44555 USA

ABSTRACT

Gamma emission driven by external electromagnetic radiation describes processes that may release isomeric nuclear energy in a "clean" way, as bursts of incoherent or coherent gamma-rays without the production of radioactive by-products. Different schemes and best candidates for the isomer de-excitation via pumping of a triggering level are considered. Short, high-power pulses of laser light must be the most economic source for nuclear isomer triggering either directly by strong electromagnetic field, or by dense radiation. The processes of deep ionization of the atomic shells with following radiative and non-radiative decay of the vacancies are of special interest. The time scales of different processes in nuclear, atomic and condensed-matter subsystems are compared.
 

"Photon scattering experiments off ¹⁷⁶Hf and the systematics of low-lying dipole modes in the stable even-even Hf isotopes ^176,178,180Hf"
 

M. Scheck,¹ D. Belic,¹ P. von Brentano,² J. J. Carroll,³ C. Fransen,² A. Gade,² H. von Garrel,¹ U. Kneissl,¹ C. Kohstall,¹ A. Linnemann,² N. Pietralla,² H. H. Pitz,¹ F. Stedile,¹ R. Toman,³ and V. Werner²
 

1   Institut fu¨r Strahlenphysik, Universita¨t Stuttgart, D-70569 Stuttgart, Germany
2   Institut fu¨r Kernphysik, Universita¨t zu Ko ln, D-50937 Ko¨ln, Germany
3   Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555, USA

ABSTRACT

The low-lying dipole strength distribution in the rare isotope ¹⁷⁶Hf was studied in nuclear resonance fluorescence experiments performed at the Stuttgart Dynamitron facility using bremsstrahlung beams with endpoint energies of 4.1 and 2.4 MeV. In total, about 55 excited spin-1 states, unknown so far, were observed in the excitation energy range up to 4 MeV. Detailed spectroscopic information has been obtained on excitation energies, spins, decay widths, decay branchings, and transition probabilities. Ascribing a positive parity to all observed K = 1 states, the detected total B(M1)­ strength in the energy range of the scissors mode amounts to 2.56(6) m_N², nearly as much as for well-deformed midshell rare-earth nuclei. The total strength is higher than in the heavier Hf isotopes ^178,180Hf, but fits well into the systematics. The observed low-lying DK = 0 transitions (with probable E1 character) lie in the energy range around 2 MeV, as expected from the systematics. The excitation probabilities correspond to values which are characteristic for nuclei in the transitional region from deformed rotors to more g-soft nuclei.

"Initial search for triggered gamma emission from ¹⁷⁸Hf^m2 Using the YSU miniball array"

J. J. Carroll¹, J. Burnett¹, T. Drummond¹, J. Lepak¹, R.Propri¹, D. Smith¹, S. A. Karamian², J.Adam², F. Stedile³ and F. J. Agee⁴
 

1   Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio 44555, USA
2   Joint Institute for Nuclear Research, Dubna, Moscow Region 141980, Russia
3   Institut für Strahlenphysik, Universität Stuttgart, D-70569 Stuttgart, Germany
4   Air Force Office of Scientific Research, AFOSR/NE, 4015 Wilson Blvd., Arlington, VA 22203, USA
 

ABSTRACT

Experiments with the long-lived, high-K isomer ¹⁷⁸Hf^m2 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 ¹⁷⁸Hf^m2 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 (<20 keV) triggered gamma emission from ¹⁷⁸Hf^m2 using the YSU miniball detector array.

"Proposal for observation of a hidden nuclear population inversion"
 

F. J. Agee¹, J. J. Carroll², L. A. Rivlin³ and V. Vuletic⁴
 

1   Air Force Office of Scientific Research, NE, 801 North Randolph Street, Arlington, Virginia 22203, USA
2   Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio 44555, USA
3   MIREA Technical University, 78 Vernadsky Ave., Moscow 117454, Russia; Moscow State Institute for Radioengineering, Electronics and Automation (Technical University)
4   Stanford University, Varian Building, 382 Via Pueblo Mall, Stanford, California 94305, USA
 

ABSTRACT

Observation of a hidden nuclear population inversion, as a precursive experimental proof of the concept of recoil-assisted gamma-ray lasing, can be accomplished by pure optical methods
due to spectral shift of atomic transitions. This paper presents numerical estimates and a proposal for such a proof-of-principle experiment.

"Beam-Based Production of ^178m2Hf"
 

J. Paul Farrell¹, V. Dudnikov¹, J. J. Carroll² and G. Merkel³
 

1   Brookhaven Technology Group, Inc., Setauket, NY, USA
2   Youngstown State University, Youngstown, OH, USA
3   Army Research Lab, Adelphi, MD, USA
 

ABSTRACT

In this study, the production yield for the reaction ¹⁷⁶Yb(⁹Be, a3n)¹⁷⁸Hf was explored using the FN tandem injected superconducting LINAC at SUNY at Stony Brook at a ⁹Be energy of
65 MeV. By comparing the experimental yield of ¹⁷⁸Hf ground state g rays with those of ¹⁸⁰W as a function of energy, the cross section for production of the incomplete fusion g rays in ¹⁷⁸Hf was evaluated. Coincidence measurements were made to get information about the population strength of the high spin states in ¹⁷⁸Hf. From these measurements, the maximum cross section for the reaction ¹⁷⁶Yb(⁹Be, a3n)^178m2Hf is estimated to be no larger than 5 mb.

"Possibility of combining nuclear level pumping in a plasma with lasing in a solid"

S. A. Karamian¹ and J. J. Carroll²
 

1   Joint Institute for Nuclear Research, 141980 Dubna, Russia
2   Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, 44555, Ohio, USA

ABSTRACT

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.
 

"Gamma spectroscopy of Hf-178m2 using synchrotron x-rays"

H. E. Roberts, M. Helba
System Technology Group, SRS Technologies, Huntsville, Alabama 35806

J. J. Carroll, J. Burnett, T. Drummond, J. Lepak and R. Propri
Center for Photon Induced Processes, Youngstown State University, Youngstown, Ohio, 44555

Z. Zhong
National Synchrotron Light Source, Brookhaven National Laboratory, NY

F. J. Agee
Office of Scientific Research, United States Air Force, Arlington, VA 22203-1977

ABSTRACT

Preliminary survey experiments have been performed to examine the triggering of gamma emission from the 31-year Hf-178m2 isomer using intense monochromatic synchrotron radiation from the X15A beamline at the National Synchrotron Light Source at Brookhaven National Laboratory. Initial studies were performed to probe incident photon energies over the L₁, L₂, and L₃ X-ray edges of Hf and the 12-13 keV range. Resonances larger than the experimental minimum detectable level of 10-25 cm² keV were not observed.
 

"Non-radiative triggering of long-lived nuclear isomers"

A. A. Zadernovsky¹ and J. J. Carroll²

1   Applied Physics Laboratory, Quantum Nucleonics Section, R&D Department, Moscow Institute of Radioengineering, Electronics and Automation, Moscow 117454, Russia
2   Center for Photon-Induced Processes, Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555 USA

ABSTRACT
 

The triggering of long-lived isomeric nuclei by non-radiative excitation to a relatively short-lived mediating state is considered. Coulomb triggering in inelastic scattering of heavy ions, a transfer of triggering energy from resonant electron transitions of atomic shell (NEET) and triggering by capture of a free electron into a bound atomic state (NEEC) are discussed. Cross sections for the above processes of non-radiative triggering are presented and the relative efficiencies of these different triggering mechanisms are discussed. Numerical estimates are presented for the selected isomers.
 

"Photo-induced depopulation of the ^180mTa isomer via low-lying intermediate states:  Structure and astrophysical implications"

D. Belic¹, C. Arlandini,² J. Besserer,³ J. de Boer,³ J. J. Carroll,⁴ J. Enders,⁵ T. Hartmann,⁵ F. Kappeler,² H. Kaiser,⁵ U. Kneissl,¹ E. Kolbe,⁶ K. Langanke,⁷ M. Loewe,³ H. J. Maier,³ H. Maser,¹ P. Mohr,⁵ P. von Neumann-Cosel,⁵ A. Nord,¹ H. H. Pitz,¹ A. Richter,⁵ M. Schumann,² S. Volz,⁵ F.-K. Thielemann,⁶ and A. Zilges⁵
 

1   Institut fur Strahlenphysik, Universitat Stuttgart, D-70569 Stuttgart, Germany
2   Institut fur Kernphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany
3   Sektion Physik, Ludwig-Maximilians Universitat Munchen, D-85748 Garching, Germany
4   Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555
5   Institut fur Kernphysik, Technische Universitat Darmstadt, D-64289 Darmstadt, Germany
6   Institut fur Physik, Universitat Basel, CH-4056 Basel, Switzerland
7   Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark

ABSTRACT

The photo-induced depopulation of the quasistable isomer (t_{1/2  ³} 1.2 ´ 10¹⁵ yr) in ¹⁸⁰Ta with J^p = 9 ^- at E_x = 75 keV was studied at the new bremsstrahlung irradiation facility installed at the Stuttgart 4.3 MV Dynamitron accelerator in the energy range of bremsstrahlung endpoint energies between E₀ = 0.8 - 3.1 MeV. The onset of the isomer depopulation could be observed starting at an endpoint energy of E₀ approx 1 MeV. Higher-lying intermediate states were found at 1.22, 1.43, 1.55, 1.85, 2.17, 2.40, 2.64, and 2.80 MeV. The extracted integrated cross sections show a remarkably strong depopulation of the ^180mTa isomer by photoexcitation.  The results are compared with previous experiments and recent Quasiparticle-Phonon Model calculations. Implications of the results for a possible nucleosynthesis of ¹⁸⁰Ta in the s-process and the neutrino process are discussed.
 

"On the way to x-ray driven triggering of nuclear isomers"

S. A. Karamian¹ and J. J. Carroll²

1  Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia
2  Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio, 44555 USA
 

ABSTRACT

Different aspects relevant to the triggering of isomers are discussed in order to summarize the progress in the controlled energy-release problem.  Probabilities of the isomer excitation and triggering depopulation follow definite regularities as is established after recent experiments.  The values of the integrated cross sections can be now reliably predicated, and they appear to be typically low.  The possibilities to enhance the amplitude of process are discussed, in particular, the collective triggering of nuclei involved into monochrystalline medium.  For some isotope, a nuclear transition may have an energy that is close to the characteristic X-ray energy of the same element.  In the hot ionized plasma of such material, nuclear level may be excited or triggered by X-ray line.  Examples of that are proposed.

"X-ray driven gamma emission"

J. J. Carroll,¹ S. A. Karamian,² L. A. Rivlin,³ and A. A. Zadernovsky³
 

1   Center for Photon-Induced Processes, Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555 USA
2   Joint Institute for Nuclear Research, Dubna, 141980 Moscow Region, Russia
3   Moscow Institute of Radioengineering, Electronics and Automation, 78 Vernadsky Ave., 117454 Moscow, Russia

"Interpretation of the excitation and decay of ¹⁸⁰Ta^m through a K = 5⁺ band"

P. M. Walker,¹ G. D. Dracoulis,² and J. J. Carroll³
 

1   Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom
2   Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia
3   Department of Physics and Astronomy, Center for Photon-Induced Processes, Youngstown State University, Youngstown, Ohio 44555
 

ABSTRACT

The production and survival of ¹⁸⁰Ta in stars presents a nuclear structure and astrophysics puzzle. From earlier work in the laboratory, photon scattering from the 9 ^- , 75 keV isomeric state has been shown to result in eventual de-excitation to the relatively short-lived 1 ⁺ ground state, through intermediate states whose character is uncertain. It is suggested here that the lowest observed photon resonances match the energies of known states in the rotational band based on the recently assigned K = 5 ⁺ state at 592 keV, states which are accessible by E1 excitation from the 9 ^– isomer. Analysis of the previously observed resonance strengths and the known in-band properties implies -ray E1 transition probabilities which are stronger than expected given the nominal K forbiddenness.  Predicted widths for "back"-decay to the 9 ^- isomer and its rotational band are calculated and shown to be significant. Their observation would have implications for the proposed associations and mixing effects above the yrast line.

"The new photoactivation facility at the 4.3 MV Stuttgart DYNAMITRON: setup, performance, and first applications"

D. Belic¹, J. Besserer,² C. Arlandini,³J. de Boer,² J. J. Carroll,⁴ J. Enders,⁵ T. Hartmann,⁵ F. Kappeler,³ H. Kaiser,⁵ U. Kneissl,¹ M. Loewe,² H. Maser,¹ P. Mohr,⁵ P. von Neumann-Cosel,⁵ A. Nord,¹ H. H. Pitz,¹ A. Richter,⁵ M. Schumann,³ S. Volz,⁵ and A. Zilges⁵
 

1   Institut fur Strahlenphysik, Universitat Stuttgart, D-70569 Stuttgart, Germany
2   Sektion Physik, Ludwig-Maximilians Universitat Munchen, D-85748 Garching, Germany
3   Institut fur Kernphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany
4   Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555
5   Institut fur Kernphysik, Technische Universitat Darmstadt, D-64289 Darmstadt, Germany

At the Stuttgart 4.3 MV DYNAMITRON accelerator a new bremsstrahlung irradiation facility was installed for photoactivation experiments to complement the well-established arrangement for systematic photon-scattering studies taking advantage of the high photon fluxes achievable at this high-current accelerator. The population or depopulation of isomeric states in low-energy photon-induced reactions (E_gamma = 0.5 - 4.0 MeV) can be studied with sensitivities far beyond previous limits. The setup of this
facility, its performance, and the absolute calibrations are described in detail. First exemplary results for a test case, the reaction In-115( g, g' )In-115m, are presented and analyzed in combination with data from tow-energy photon-scattering experiments (In-115( g, g' )In-115). The quality of the results demonstrates the powerful capability of this new installation.

“Photon cross sections for resonant de-excitation of nuclear isomers as a precursor to a gamma- ray laser”
 

S. A. Karamian¹ and J. J. Carroll²
 

1   Joint Institute for Nuclear Research, Dubna, 141980 Moscow Region, Russia
2   Center for Photon-Induced Processes, Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555 USA

Integrated cross sections (ICS) for triggered decay of nuclear isomers have recently become accessible in experimental studies that were motivated both by general physical interest and by applications, such as a possible gamma-ray laser. Theoretical predictions for ICS are reviewed here and an equation is derived based on the Blatt-Weisskopf probability for an electromagnetic transition between two nuclear levels. This formula is effectively equivalent to the recognized Breit-Wigner expression despite their development from essentially different approaches. The results of a recent high-sensitivity experiment on the Ta^180m isomer de-excitation induced by bremsstrahlung are analyzed in this context. An important parameter of the de-excitation branch probability is determined now over a wide energy range from 1 to 10 MeV, including previously reported results.

“Improved method for detection of x-ray triggered decay of the 178Hfm2 isomer using a multidetector array”

J. J. Carroll,¹ S. A. Karamian,² M. K. Boyle,¹ Y. Kaneko,¹ R. Toman,¹ D. Fowler,¹ M. Helba,³ H. Roberts³ and F. J. Agee⁴
 

1   Center for Photon-Induced Processes, Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555 USA
2   Joint Institute for Nuclear Research, Dubna, 141980 Moscow Region, Russia
3   SRS Technologies Inc, Huntsville, AL 35806 USA
4   USAF, Office of Scientific Research, AFOSR, NE, Arlington, VA 22203 USA

Experiments conducted during the 1990s with the high-spin, 31-year-lived nuclear isomer Hf-178(m2) have been recognized as intriguing rests of multi-quasiparticle state structures and their interactions with external radiation. In addition, this isomer holds considerable promise for applications due to the large amount of energy that it stores, 2.5 MeV per nucleus, or 1.2 GJ/gram.  Should a triggered release of this stored energy be practical, this might lead to the creation of a pulsed gamma -ray source controlled by a soft X-ray device, and perhaps even a gamma-ray laser. Recently the observation of "accelerated'' decay of the Hf-178(m2) metastable level under irradiation by 90-keV bremsstrahlung was reported. Unfortunately, in those experiments it was impossible to identify, the branch of the gamma cascade triggered by external radiation in the presence of a high-intensity background. That background was created hy the spontaneous decay of Hf-178(m2) itself, as well as by contaminating Hf-172 activity present in the source. Thus, only a small increase in the intensity of the spontaneous decay gamma lines could be deduced and the details of the physical process remain unclear. The development of new, more sophisticated, experiments is therefore extremely important. One such experimental concept is discussed here.
 

“Light-mass yields and fine structure of mass distributions in ²³²Th photofission”
 

S. A. Karamian,¹ J. Adam,¹ A. G. Belov,¹ J. J. Carroll,² Yu. V. Norseev,¹ V. I. Stegailov¹ and P. Chaloun¹
 

1   Joint Institute for Nuclear Research, Dubna, 141980 Moscow Region, Russia
2   Center for Photon-Induced Processes, Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555 USA

Fission-fragment mass distributions in the Th-232(g, 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 Na-24 and Mg-28 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.
 

“Photoactivation of ¹⁸⁰Ta^m and its implications for the nucleosynthesis of nature’s rarest naturally occurring isotope”
 

D. Belic¹, C. Arlandini,² J. Besserer,³ J. de Boer,³ J. J. Carroll,⁴ J. Enders,⁵ T. Hartmann,⁵ F. Kappeler,² H. Kaiser,⁵ U. Kneissl,¹ M. Loewe,³ H. J. Maier,³ H. Maser,¹ P. Mohr,⁵ P. von Neumann-Cosel,⁵ A. Nord,¹ H. H. Pitz,¹ A. Richter,⁵ M. Schumann,² S. Volz,⁵ and A. Zilges⁵
 

1   Institut fur Strahlenphysik, Universitat Stuttgart, D-70569 Stuttgart, Germany
2   Institut fur Kernphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany
3   Sektion Physik, Ludwig-Maximilians Universitat Munchen, D-85748 Garching, Germany
4   Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555
5   Institut fur Kernphysik, Technische Universitat Darmstadt, D-64289 Darmstadt, Germany

The depopulation of the quasistable isomer in Ta-180 with J^p = 9 ^- at E_x = 75 keV by resonant photoabsorption was investigated with intense bremsstrahlung. The results indicate a dramatic acceleration of the isomer decay to the short-lived J^p = 1 ⁺ ground state under stellar s-process conditions. The consequences for a possible nucleosynthesis of nature's rarest isotope Ta-180. within the s process are discussed.