Reexamination of the Optical Gamma Ray Decay inT229h

Abstract

Optical measurements of a clean, 2-mC 233 U sample were made to verify light emission from gamma ray decay of the first excited nuclear level in 229 Th. The results showed that the light observed in earlier studies was likely to be caused by alpha-particle induced fluorescence of air. In vacuo, no light emission was discernable. The 229 Th system, therefore, does not appear to provide the level of access for studying atomic-nuclear interactions suggested by the previous measurements. [ S0031-9007(98)08232-5] PACS numbers: 23.20.Lv, 27.90. + b, 32.30.Jc Interactions between the nuclear and atomic systems have attracted considerable attention because of the potential of stimulating nuclear transitions by laser excitation of atomic states. In the inelastic bridge mechanism, for example, nuclear deexcitation is coupled to atomic excitation. In particular, an electromagnetic nuclear transition couples to an atomic electron, which is elevated to an excited state, thereby reducing the energy of the final gamma ray [1-3]. The mechanism suggests that the inverse may be possible, i.e., that atomic deexcitation may induce nuclear excitations [4]. Nuclear excitation energies, however, are generally much larger than atomic excitation energies. The identification of cases where the energies are comparable is, therefore, of highest importance.The nuclear spectroscopy of high-energy gamma rays from 229 Th produced in the alpha decay of 233 U suggests that the ground state is a closely spaced doublet separated by 3.5 eV [5][6][7]. The energy of the first excited level, thus, is lower than any other known excited nuclear level. Recently, Irwin and Kim reported the detection of photon emission resulting from the deexcitation of the 229 Th isomer [8]. The energy of the observed photons was determined to be about 3.5 6 1.0 eV, in agreement with the value inferred indirectly from the nuclear spectroscopy of the high-energy gamma rays. Moreover, one of two samples of 233 U studied showed a second photon peak near 2.4 eV [8]. The observation of this peak was consistent with predictions that the inelastic bridge mechanism could cause excitation of the thorium atom from the 6d 3͞2 ground level to the 7p 1͞2 excited level. The Letter suggested that 229 Th provides a perfect isotope for future studies of low-energy nuclear-atomic interactions since the energy spacing is well within the realm of present-day laser capabilities.A subsequent study by Richardson et al.[9] also observed emission in the UV. Employing a higher resolution spectrograph than Irwin and Kim, they were able to discern three peaks near 3.5 eV. Their result, thus, appeared to confirm the measurement of Irwin and Kim.In the following we present experimental studies of the optical emission of a clean sample of 233 U decaying to 229 Th. We found no evidence for the optical emission attributable to the deexcitation of the predicted low-lying 229 Th isomer. We found light caused by alpha-particle induced fluorescence of air. In the absence of air, no light emission was di...

Radiative lifetime and energy of the low-energy isomeric level inTh229

“…III, the chemical environment can significantly affect the half-life of the isomeric state [3]. It most likely explains why, given the currently accepted value for the isomeric transition energy, that many previous experiments performed in powders, solids and solutions produced null results [52][53][54][55][56]; similarly, non-VUV sensitive measurements could have been affected by internal conversion [57,58]. The internal conversion process could also have strong implications for the experiments reported in Refs.…”

Nuclear clocks based on resonant excitation of γ-transitions

“…In early experiments, two false detections of the decay of the isomer following α-decay of 233 U were reported [12,13], but it was quickly clarified that the observed light was luminescence induced by the background of α-radiation [14,15]. All further attempts at a direct observation of the optical transition failed.…”

“…Recoil nuclei: The 229 Th isomer is populated with about 2% probability in the α-decay of 233 U and early attempts at an optical detection of the isomer decay have looked for light emission from 233 U sources [12,13], without success because of a strong background of radioluminescence [14,15]. This problem can be mitigated by using 229 Th recoil nuclei: If the α-decay of 233 U occurs close to the surface of the sample, the freshly produced 229 Th may be ejected and can be collected on an absorber (such as CaF 2 or MgF 2 , for instance) placed in front of the 233 U.…”

Coherent Control of Nuclei and X-Rays

“…Their observed value for the isomeric transition were 3.5 eV [159] and likely 4 eV [160], respectively. However, in 1999, the studies of three different groups indicated that these conclusions must be based on the incorrect interpretation of experimental data [161][162][163]. G. M. Irwin and K. H. Kim later 5.1 Introduction of the agreed that their observation might result from the α-particle induced fluorescence of air, actually N 2 , surrounding the sample [161,162,164].…”