Excitation of the 229m Th nuclear isomer via resonance conversion in ionized atoms
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CLEI Theory
Excitation of the
229m
Th Nuclear Isomer via Resonance Conversion in Ionized Atoms
F. F. Karpeshin1)* and M. B. Trzhaskovskaya2) Received December 10, 2014; in final form, March 6, 2015
Abstract—Pressing problems concerning the optical pumping of the 7.6-eV 229m Th nuclear isomer, which is a candidate for a new nuclear optical reference point for frequencies, are examined. Physics behind the mechanism of the two-photon optical pumping of the isomer is considered. It is shown that, irrespective of the pumping scheme, a dominant contribution comes, in accord with what was proven earlier for the 3.5-eV isomer, from the resonance 8s−7s transition. Details of an optimum experimental scheme are discussed. It is shown that, after isomer excitation, the atom involved remains with a high probability in an excited state at an energy of about 0.5 eV rather than in the ground state, the required energy of the two photons being equal to the energy of the nuclear level plus the energy of the lowest 7s state of the atom. The estimated pumping time is about 1.5 s in the case where the field strength of each laser is 1 V/cm. DOI: 10.1134/S1063778815060125
1. INTRODUCTION 229m Th
isomer has aroused In recent years, the considerable interest as a candidate for a new nuclear optical reference point for frequencies. This nucleus has a uniquely low isomeric level whose energy ωn does not exceed 10 eV. The presence of this level in the nucleus is beyond any doubt because one can clearly identify it in interpreting nuclear reactions. However, a direct detection of this level is difficult because of its energy astonishingly small for nuclear physics (see, for example, [1]). On the basis of a compilation of experimental data on transition energies in this nucleus, it had been assumed for a long time that the isomer energy is ωn = 3.5 ± 1 eV [2]. Some authors used a value of 5.5 ± 1 eV [3]. The most recent experiment leds to a value of ωn = 7.6 ± 0.5 eV [4]. The interest in question is caused by the fact that the nuclear isomer line lies in the optical region accessible to lasers. For example, the rate of isomer decay can be sharply increased via resonance conversion stimulated by laser radiation, as was first proposed in [5] for 235 U and in [6] for 229m Th. The hope for creating a nuclear optical reference unit of time with a potential relative error of about 10−21 is also pinned on 229m Th [7]. Therefore, the problem of prime importance is to develop a regular means for the production of an isomeric source containing the 1)
D.I. Mendeleev Institute for Metrology (VNIIM), Moskovskii pr. 19, St. Petersburg, 190005 Russia. 2) Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, Gatchina, 188300 Russia. * E-mail: [email protected]
required number of atoms and to refine the isomer energy. In view of the foregoing, the development of advanced nuclear laser technologies would pave the most natural way toward this goal. First, such technologies would permit determining the isomer energy to the req
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