Beam-Based Production of 178m2Hf

In this study, the production yield for the reaction ¹⁷⁶Yb(⁹Be, α3n)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 γ rays with those of ¹⁸⁰W as a function of energy, the cross section for production of the incomplete fusion γ 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, α3n)^178m2Hf is estimated to be no larger than 5 mb. This revised version was published online in August 2006 with corrections to the Cover Date.     

Formation of the <Superscript>178<Emphasis Type="Italic">m</Emphasis>2</Superscript>Hf isomer in a natural mixture of hafnium isotopes irradiated by an intense γ quantum beam (experiment)

The results of an experiment on the production of the ^178m2Hf isotope by exposing a natural mixture of hafnium isotopes to an intense bremsstrahlung beam are reported. The sample under investigation was exposed to an electron beam with an energy of 30 MeV and current 200 μ A for 500 h. The spectrometric analysis of the irradiated sample using radiochemical methods for the separation of isotopes did not reveal ^178m2 Hf lines.     

Preparation of the 178m2Hf isomer used in the induced gamma decay experiment by X-ray from synchrotron radiation facility

It is widely acknowledged that the ^178m2Hf nuclide is the most suitable substance to study the decay characteristic of the isomer induced by low-energy X-ray. In order to conduct the experiment on the induced gamma emission, the research group has started producing the ^178m2Hf nuclide based on the ¹⁷⁶Yb(??, 2n)^178m2Hf reaction. After the chemical purification is conducted, the sample is prepared and used in Shanghai Synchrotron Radiation Facility. During the production of isomer, the natural metal Yb target is got through magnetron sputtering. Bombarded by ?? particles about 27 MeV, the ^178m2Hf nuclide reaches about 10¹². Yb target prepared in this way is most suitable for the production of ^178m2Hf nuclide in the CS30 cyclotron. There are various nuclides in the irradiated target and the main long-lived nuclides are ¹⁷³Lu, ¹⁷²Lu, ¹⁷⁵Hf, ¹⁷²Hf and ⁶⁵Zn. The chemical separation of ^178m2Hf is studied and its process is monitored by radioactive tracer. The above result shows that decontamination factors of Zn and Lu are 10⁵ and 10³, respectively, and the yield of hafnium is 69%. Under the protection of vacuum filtration technology, the purified ^178m2Hf isomers are entirely transferred to the surface of filter paper, in order to form the sample which satisfies requirements of X-ray triggering the ^178m2Hf isomer decay experiment in Shanghai Synchrotron Radiation Facility in the future.     

Experimental consequences ofSU(3) symmetry in ans d g Boson Model

Energies of collective levels in¹⁷⁸Hf and²³⁴U are compared with predictions of theSU (3) limit of thesdg Interacting Boson Model. All known positive parity states of¹⁷⁸Hf below 1.8 MeV (with the exception of a 0⁺ band) have been satisfactorily reproduced. Most of the bands in²³⁴U are also described by the model. However, a few predicted states have no experimental counterpart. The introduction of theg-bosons strongly reduces the previously observed discrepancies between experimentalB(E2)'s in²³⁸U and thesd-IBM calculation.     

Deuteron-induced reaction studies of the nuclear structure of178Hf

The level structure of¹⁷⁸Hf is interpreted on the basis of the population of the states following¹⁷⁸Hf (d, d′),¹⁷⁷Hf (d, p) and¹⁷⁹Hf (d, t) reactions. Evidence for quadrupole and octupole vibrational bands and unmixed and intermixed two-quasiparticle configurations is presented.     

Production, chemical and isotopic separation of the178m2Hf high-spin isomer

The ^178m2Hf with its long-lived (T_1/2=31 y), high-spin I^π = 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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Gamma-Ray Transitions Induced in Nuclear Spin Isomers by X-Rays

Because of the high density of energy storage and the large cross section for its release, nuclear spin isomers have attracted considerable recent interest. The triggering of induced gamma emission from them has encouraged efforts to develop intense sources of short-wavelength radiation. One of the more interesting examples is the 16⁺ 4-qp isomer of ¹⁷⁸Hf which stores 2.445 MeV for a half-life of 31 years meaning that as a material, such isomeric ¹⁷⁸Hf would store 1.3 GJ/g. Recently, a sample containing 6.3×10¹⁴ nuclei of the isomer of ¹⁷⁸Hf was irradiated with X-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with end point energies set to values between 60 and 90 keV. Emission of gamma radiation from the sample was increased by 1–2% above the quiescent value of spontaneous emission. Such an accelerated decay of the ¹⁷⁸Hf isomer is consistent with an integrated cross section of 2.2×10⁻²² cm² keV if the resonant absorption of the X-rays takes place below 20 keV as indicated by the use of selective absorbing filters in the irradiating beam. The work reported here describes the current experimental focus and results recently obtained with the use of coincident detection of emitted gamma photons by several detectors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of the gamma emission from the 31-yr isomer of 178Hf induced by x-ray irradiation

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.     

Optical isotope shift of172Hf (T 1/2=1.87 a)

In the course of preparing experiments with^178m2Hf, optical isotope shifts for the stable Hf isotopes and for¹⁷²Hf were measured. For the rms nuclear charge radii we obtain r²¹⁷² -(r²)¹⁷⁸=–0.242(16) fm².     

Activation cross sections and isomeric ratios in reactions induced by 14.5 MeV neutrons on152Sm,154Sm and178Hf

Cross sections for the reactions¹⁵²Sm(n, p)^152g,m1,m2 Pm,¹⁵⁴Sm(n, p)^154g,m Pm,¹⁷⁸Hf(n, p)^178m,g Lu,¹⁵⁴Sm(n, d)¹⁵³Pm and¹⁵²Sm(n, α)¹⁴⁹Nd were measured at 14.5 MeV neutron energy by the activation method. On the basis of these cross sections, the associated isomeric ratios in¹⁵⁴Pm,¹⁵²Pm,¹⁷⁸Lu and the comparison with the predictions of different compound and precompound models, conclusions are drawn about the role of the preequilibrium processes in 14.5 MeV neutron induced reactions. Calculations for equal angular momentum removal by equilibrium and preequilibrium emitted particles better reproduced the experimental isomeric ratios, than for higher angular momentum removal in the preequilibrium phase. The isomeric ratios may be used as a source of additional information about the spin of the isomeric states in¹⁵²Pm and¹⁵⁴Pm for which the spectroscopic information is uncertain.     

Brute-force orientation of178m 2Hf and magnetic moments of178m 1Hf and178m 2Hf

Gamma-ray anisotropy measurements have been carried out with brute-force oriented^178m ₂Hf (16⁺) to derive its magnetic moment 7.26 (±0.16; +0.2/−0) nm and E2/M1 mixing ratios of some γ-transitions observed in the decay of this isomer. This magnetic moment is in agreement with an earlier proposed four-quasi-particle configuration. The magnetic moment of the 8⁻ isomer at 1147.7 keV (4.0 s) is estimated to be between 1.4 and 5.3 nm, which confirms mixing of the {p[514]9/2 +p[404]7/2}8⁻ and {n[514]7/2+n[624]9/2}8⁻ configurations, which summed together give the 16⁺ isomer.     

Perturbed Angular Correlation of the stretched cascade in the decay of 180mHf using a digital spectrometer

We report on the measurement of the nuclear quadrupole interaction (NQI) at Hf sites using the nuclear probe ^180mHf in HfF₄·HF·2H₂O at 300 K by exploiting all possible start quanta in the stretched cascade with a digital Time Differential Perturbed Angular Correlation (TDPAC) spectrometer. With conventional spectrometers, multiple prompt start signals would paralyze the router. The gain in coincidence rate is about a factor of 5 compared to a conventional spectrometer using a single start only. With multiple starts ^180mHf is a promising new isomeric nuclear probe in TDPAC experiments. As an additional feature we implemented the possibility to measure up to four cascades simultaneously in order to save data collection time or to measure isobaric contaminations like ^111mCd and ¹¹¹In.     

Calculations of electric quadrupole moments and charge radii for high-K isomers

Configuration-constrained potential-energy-surface calculations are performed to investigate high-K isomers in ⁹⁷Y, ¹³⁰Ba, ¹⁷⁶Yb, ¹⁷⁷Lu, and ¹⁷⁸Hf that were observed to have increased electric quadrupole moments but decreased charge radii relative to the states on which they are built. Taking into account the effects of deformation change and unpaired protons, our calculations can reproduce the enhancement of electric quadrupole moments for the isomers in ⁹⁷Y, ¹³⁰Ba, ¹⁷⁶Yb, ¹⁷⁷Lu and the K ^π = 8^? isomer in ¹⁷⁸Hf, and can reproduce the reduction of charge radii for the K ^π = 27/2^? isomer in ⁹⁷Y and the K ^π = 16⁺ isomer in ¹⁷⁸Hf.     

Measurement of theβ-decay branch in180Lu to180m Hf for Investigation of the r-process nucleosynthesis of180m Ta

Theβ-decay branch of¹⁸⁰Lu to^180m Hf is measured by an integral method tracing the amount of^180m Hf daughter activity, to be 0.46 (15)%. Samples of¹⁸⁰Lu are prepared by on-line mass separation of¹³⁶Xe-on-tungsten transfer reaction products. The amount of isobaric^180m Hf in the samples is strongly suppressed by the employed separator ion source; its directly produced fraction is derived from comparison with^177m Hf activity. The properties of lutetium and hafnium release from the ion source are separately determined. With this measurementr-process nucleosynthesis is estimated to account for at most 22% of the solar^180m Ta abundance.     

Investigation of γ radiation from 178Hf in the respective (n, n′γ) reaction

The spectra and angular distributions of gamma rays were measured in the reaction ¹⁷⁸Hf(n, n???) induced by a beam of fast reactor neutrons. Data onmultipole mixtures in gamma transitions and a lot of new information about gamma transitions of energy 1.5 to 3.0 MeV were obtained. A comparison of these results with information known from the respective (n, ??) reaction made it possible to refine the schemes of deexcitation of ¹⁷⁸Hf levels at energies above 1.5 MeV, to determine more precisely features of these levels, and to introduce new levels and rotation bands at excitation energies of about 2MeV.     

Evidence for K mixing in 178Hf

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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparative analysis of the <Superscript>178<Emphasis Type="Italic">m</Emphasis>2</Superscript>Hf yield at reactions with different projectiles

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.     

Triggering the <Superscript>178<Emphasis Type="Italic">m</Emphasis>2</Superscript>Hf isomer via resonance conversion

Experiments on photoinduced deexcitation of ^178m2Hf are reanalyzed. A mechanismof triggering the 12.7-keV E3 transition, based on resonance conversion, is considered. That rises the efficiency by an order of magnitude. The main obstacle for enhancing the efficiency is a high internal conversion rate. For this reason, isomers with low multipole order (E1, M1) with high enough energy of the triggering transition are more suitable for triggering. Another way is offered by partial ionization of the outer electrons for decreasing the conversion rate. The results are applied to experiments in laser-produced plasma. The text was submitted by the authors in English.     

Application of exact angular momentum projection to excited bands in strongly deformed nuclei: (I). Negative parity bands

The method of exact angular momentum and particle number projection for two-quasiparticle excited states is applied to negative parity excited bands in ¹⁶²Er and ¹⁷²Hf. Quantitative results of theoretical calculations are very reasonable in comparison to experiments. Further improvement of the method for the formulation of a theory of excited bands is suggested.     

ISOL beams of hafnium isotopes and isomers

The production of ISOL beams of hafnium is described. Radioactive Hf isotopes were produced at ISOLDE by 1.4 GeV proton-induced spallation in Ta and W foils. Chemical evaporation in form of HfF₄ and mass separation in the molecular sideband HfF₃⁺ after electron impact ionization provided intense and pure beams. Beams of ^158-185Hf and short-lived isomers down to 1.1 s ^177mHf were observed, but the method could be extended to reach even more exotic isotopes: down to about ¹⁵⁴Hf (N=82) on the neutron-deficient side and up to neutron-rich ¹⁸⁸Hf.