It has been known for many years that the first excited state of Th lies close to the ground state. Originally this energy was given as (0.1 keV; later, the authors reported a value of-1 + 4 eV. In an attempt to improve the value for this level energy, we have remeasured the energies of a number of p rays from U whose positions in the Th level scheme can be used to establish it. Compared with our earlier study, we have considered more p rays in Th, used more well-measured energy calibration and reference lines, used more detectors, used detectors with better low-energy resolution, more closely matched the counting rates in the pray peaks whose relative energy is measured, and specifically considered certain systematic errors. More than 111 pray spectra have been measured. From this large set of measurements we have deduced a value of 3.5 6 1.0 eV for the energy of this level.

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It has been known for some time that the intrinsic state labeled by the asymptotic quantum numbers j + [631] lies quite close ( < 0.1 keV) to the f + [633] ground state of 229 Th. Using the energies of selected y rays emitted following the a decay of 233 U, we have obtained a value of 1 ± 4 eV for the energy separation of these two intrinsic states.PACS numbers: 23.20.Lv, 27.90.+bIn a study 1 of the rotational-band structure of 229 Th, populated in the a decay of 233 U, states interpreted as the spin y through y-members of a y + [631] band were reported. The bandhead (/*-y + ) of this band, however, was not directly observed. From indirect, but convincing, evidence it was concluded 1 that this j + state must be quite close to (within 0.1 keV of) the 229 Th ground state (y + [633]) and that the reason for its nonobservation was that the energy separation of these two states was too small to be determined from the existing data. This inference was supported, essentially concurrently, by data from a study 2 of the 230 ThG/,/) reaction, in which various members of the y + [631] band, and, in particular, the bandhead itself, were observably populated. Although the ground state was not observed in this reaction, the location of the peak assigned as y, y [631] relative to the other band members indicated that it should lie close to the ground state.In a subsequent study 3 of the 233 U decay for the purpose of obtaining accurate values of the emission probabilities of a number of the more intense y rays, the energies of many of the y rays were determined with precisions of a few eV. This has made it possible, in turn, to obtain a more precise estimate of the energy separation of these two states. In this paper, we present the results of our redetermination of this energy separation. The portion of the low-energy level scheme of 229 Th which is pertinent to the present discussion is shown in Fig. 1. From this scheme the following three energy combinations can be used to determine the energy A of the y + bandhead:

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