Resonance conversion as a dominant decay mode for the 3.5-eV isomer in 229m Th

on the basis of calculations performed withi nthe relativistic multiconfiguration Dirac-Fock method, it is shown that the probability of the decay of the 3.5-eV nuclear level in the ^229m Th isomer via resonance conversion exceeds the probability of its direct radiative decay by at least a factor of about 600. The possibility of experimentally observing delayed soft photons or alpha particles in the decay of this isomer is discussed.     

229m Th(3/2+, 3.5 eV) and a check of the exponentiality of the decay law

A new object is proposed for checking the exponentiality of the decay law of an isolated metastable state at long times — the anomalously low-lying level 3/2⁺ (3.5±0.5 eV) in ²²⁹Th. Highly efficient excitation of this level by laser radiation in combination with optimal collection of optical photons emitted in an isomeric transition make it possible to achieve measurement times longer than 50T _1/2. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 521–525 (25 April 1998)     

Matrix element of the anomalously low-energy (3.5±0.5 eV) transition in 229Th and the isomer lifetime

The matrix element of the anomalously low-energy (3.5±0.5 eV) nuclear M1 transition between the first excited state and the ground state of the ²²⁹Th nucleus is determined with allowance for the Coriolis mixing of the rotational bands. The upper and lower limits on the lifetime of the level with respect to an isomeric transition are given. A method is proposed for measuring the half-life of the low-lying isomer ^229m Th directly in a ²³³U sample. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 233–238 (25 February 1998)     

Investigation of the low-lying isomer in 229Th by collinear laser spectroscopy

A new ion beam of ²²⁹Th is available at the Jyväsklyä IGISOL facility, produced from the α decay of ²³³U. A small branching ratio (≈ 2%) is believed to populate the inferred low-lying (5.5 eV) isomeric state in ²²⁹Th. A laser ionization scheme is currently being developed to improve the yield of ²²⁹Th from the source. The ion source uses a novel electric field configuration for fast and efficient extraction of α-recoils and is able to provide beams of short lived (τ≥ 30 ms) radioactive nuclei. Identification of the isomeric state by collinear laser spectroscopy will reduce the lower lifetime limit of the state and provide the first direct evidence for its existence.     

Wigner crystals of 229Th for optical excitation of the nuclear isomer

We have produced laser-cooled Wigner crystals of 229Th3+ in a linear Paul trap. The magnetic dipole (A) and electric quadrupole (B) hyperfine constants for four low-lying electronic levels and the relative isotope shifts with respect to 232Th3+ for three low-lying optical transitions are measured. Using the hyperfine B constants in conjunction with prior atomic structure calculations, a new value of the spectroscopic nuclear electric quadrupole moment Q=3.11(16) eb is deduced. These results are a step towards optical excitation of the low-lying isomer level in the 229Th nucleus.     

Studying the decay of thorium-229 isomer by means of conversion spectroscopy

The design and expected characteristics of an experimental setup for generating a beam of singly, doubly, and triply charged ions of ^229mTh isomer are considered. The scheme of ion transport, the preparation of the thin source, and the means of registering low-energy conversion electrons are described.     

Excitation of the isomeric 229mTh nuclear state via an electronic bridge process in 229Th+

We consider the excitation of the nuclear transition 229gTh-229mTh near 7.6 eV in singly ionized thorium via an electronic bridge process. The process relies on the excitation of the electron shell by two laser photons whose sum frequency is equal to the nuclear transition frequency. This scheme allows us to determine the nuclear transition frequency with high accuracy. Based on calculations of the electronic level structure of Th+ which combine the configuration-interaction method and many-body perturbation theory, we estimate that a nuclear excitation rate in the range of 10 s?1 can be obtained using conventional laser sources.     

Excitation and decay of low-lying nuclear states in a dense plasma produced by a subpicosecond laser pulse

The excitation of low-lying nuclear levels in a hot, dense plasma, produced by a subpicosecond pulse with intensity exceeding 10¹⁶ W/cm², is investigated theoretically and experimentally. The basic channels of electronic (inelastic scattering and inverse internal electron convergence) and photon (photoexcitation) excitations of such states as well as the influence of the broadening of a nuclear level on the excitation efficiency and the presence of hot electronic component are examined. The experimental data from measurements of the decay kinetics of the low-lying nuclear level 6.238 keV of the stable isotope ¹⁸¹Ta, which were obtained on two experimental laser systems, are presented.     

On the energy of the 3.5-eV level in <Superscript>229</Superscript>Th

Values determined in several studies for the first isomeric state of ²²⁹Th as 3.5(10), 3.4(18), 5.5(10), and 7.6(5) eV differ markedly from one another. A comparison of the first two studies that was performed in the present article indicates that there is a systematic error in the energies of the transitions in ²²⁹Th. It is shown that each of the first two values is dependent on the placement or energy of only one transition (or on both of these two factors). The third value, that of 5.5(10) eV, is not independent, since it is a result of rather complicated calculations for the scheme of energy levels in ²²⁹Th with allowance for the first two studies. It is shown that the accuracy in determining the error in 7.6(5) eV is likely to be exaggerated. It is concluded that the energy of the 3.5-eV state ranges between 0 and 15 eV.     

Resonance Conversion as the Predominant Decay Mode of ^{229m}Th

Calculations performed within the advanced MCDF method show that the probability of decay of the 3.5-eV level of $^{229m}$ Th through the resonance electronic bridges exceeds the direct radiative nuclear decay width by a factor of around $5\cdot 10^3$ . Experimental consequences for detection of the delayed soft photons or $\alpha$ particles from the decay of the isomer are discussed.     

Experimentally realized linewidths of narrow (10eV) nuclear gamma resonances

The experimental status of Mössbauer linewidths are reviewed and reported for the three narrowest gamma resonances: ⁷³Ge, ⁶⁷Zn, and ¹⁸¹Ta. The crucial role played by the solid-state host in achieving narrow emission or absorption linewidths, as well as high recoil-free fraction are emphasized. Factors contributing to line-broadening in experiments with ¹⁸¹Ta and ⁶⁷Zn gamma resonances are highlighted. Consequences of these observations on a choice of a suitable lasing medium for Mössbauer gamma-ray lasers is commented upon.     

Nuclear levels in 228Th populated in the decay of 228Pa (II)

The electron-capture decay of ²²⁸Pa to levels in ²²⁸Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited K ^π=0⁺ bands and two K ^π=2⁺ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that ²²⁸Th has less transitional character than hitherto assumed. Received: 30 March 1998     

Inhibition of the radioactive decay of the isomer 119m Sn with the use of Mössbauer backscattering

In research on the production of beams of coherent γ rays (γ-ray lasers), conditions under which a substantial change Δλ/λ=?(0.114 ±0.027) in the radioactive decay constant λ (the isomeric level 89.53 keV ^119m Sn, T _1/2=293 days) can occur have been found experimentally for the first time. This is made possible by coherent Mo ssbauer (23.87 keV) backscattering from a resonant screen located nearby. An interpretation of the effect observed is proposed on the basis of the idea of dynamic synchronization of oscillations between a nuclear level and a standing wave of Mössbauer radiation. Possibilities for further increasing Δλ/λ up to 0.5 are found.     

OnE0 transitions in the decay of152Eu m (9.3 h)

The conversion electron spectrum of the decay of¹⁵²Eu ^m (9.3 h) has been specially investigated forE0 transitions with the aid of a magnetic spectrometer and coincidence techniques. Besides the knownE0 transitions of 685 keV in¹⁵²Sm and 615 keV in¹⁵²Gd two further pureE0 transitions of 432 keV and 1048 keV have been observed. It is shown that theseE0 transitions are identical with those observed previously in the decay of¹⁵²Tb. NoE0 transitions could be found from a 0⁺ state of 1083 keV in¹⁵²Sm. The measured intensities together with the gamma spectrum measured by Barretteet al. give a consistent decay scheme of¹⁵²Eu ^m . Furthermore the half-life of the 0⁺ level at 615 keV in¹⁵²Gd was measured using the method of “delayed coincidences”. The result was (0.2≦t _1/2≦2.1)×10^?10 s. From this theE0 transition probability for the level was derived as (0.3≦W _K (E0)≦3.7)× 10⁹ s^?1 and theρ-value as 0.10≦¦ρ¦≦0.36. The results are discussed within the framework of the collective model.     

Energy splitting of the ground-state doublet in the nucleus 229Th

The energy splitting of the 229Th ground-state doublet is measured to be 7.6+/-0.5 eV, significantly greater than earlier measurements. Gamma rays produced following the alpha decay of 233U (105 muCi) were counted in the NASA/electron beam ion trap x-ray microcalorimeter spectrometer with an experimental energy resolution of 26 eV (FWHM). A difference technique was applied to the gamma-ray decay of the 71.82 keV level that populates both members of the doublet. A positive correction amounting to 0.6 eV was made for the unobserved interband decay of the 29.19 keV state (29.19-->0 keV).     

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.