Ultraviolet Photon Emission Observed in the Search for the Decay of the229ThIsomer

Abstract

Ultraviolet spectroscopy of 233 U and 232 U, in the form of uranium oxide dissolved in 3M nitric acid, has revealed a number of spectral lines with widths of less than 6 nm from the 233 U source but no detectable narrow lines from the 232 U source. These lines could possibly be attributed to the decay of the 229 Th isomeric state at an energy of around 4.0 eV. [S0031-9007(98) PACS numbers: 23.20.Lv, 27.90. + b, 32.30.Jc In 1994 Helmer and Reich [1] determined the separation of the ground and first excited nuclear states in 229 Th to be 3.5 61.0 eV. Recently the direct decay from this state may have been observed at higher resolution [2]. This is then the lowest excitation energy for a nuclear first excited state yet found and the first to be on a similar energy scale to atomic valence electrons. Such a small energy separation provides the potential to examine a regime of nucleus-atom interactions not previously observable. The value for the excitation energy was obtained by applying the Ritz combination principle to groups of g-ray energies of several tens to hundreds of keV. Using energies of 4-5 orders of magnitude larger than the energy under investigation makes it difficult to increase the accuracy of the measured energy separation by this method.The spins and parities of the ground and first excited states together with the small excitation energy suggest that the excited state should be a long lived isomeric state [3]. A photon emitted by the direct decay of the first excited state to the ground state would have a wavelength in the blue-ultraviolet region of the electromagnetic spectrum, i.e., between 270 and 500 nm, with 3.5 eV corresponding to 350 nm. Thus conventional UV spectroscopy is an ideal tool for increasing the accuracy to which the separation is known. A recent experiment [2] used UV spectroscopy of two solid sources of 233 U to look for the 229 Th-isomer deexcitation following the a decay of the 233 U. The results show a broad spectral feature at ϳ2.5 eV, with a set of spectral lines centered at approximately 3.5 eV.In the current work a 233 U source was also used; however, the radioactive isotope was in the form of uranium oxide dissolved in 3M nitric acid, producing an equilibrium in which uranyl nitrate ions dominate (UO 2 NO 1 3 ). The source activity was 4 MBq and its volume of 0.25 ml was contained in a 6 mm internal diameter quartz tube. 233 U has an a-decay half-life of 1.7 3 10 5 years, which ensures the activity will be essentially constant throughout the course of the experiment. It has been estimated that about (1-2)% of the total a decays of the uranium feed into the 229 Th first excited state [4,5]. Because both 233 U and 229 Th have long half-lives it was not considered necessary to remove the daughters produced since the production of the 233 U source in 1992.A control source, with 232 U as the main active element, was also made. The half life of 232 U is 72 yr, which, while adequate for the duration of the measurements means that a substantially smaller quantity of the isot...

Coherent Control of Nuclei and X-Rays

“…A similar conclusion was also claimed by D. S. Richardson et al with a liquid sample in 1998 [160]. 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 claimed in 1997 that they directly observed the ultraviolet photon from the deexcitation of the 229m Th with two different solid 233 U samples [159]. A similar conclusion was also claimed by D. S. Richardson et al with a liquid sample in 1998 [160]. Their observed value for the isomeric transition were 3.5 eV [159] and likely 4 eV [160], respectively.…”

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.…”

Magnetic hyperfine structure of the ground-state doublet in highly charged ionsTh89+,87+229and the Bohr-Weisskopf effect

“…The low lying 3/2 + (7.8 ± 0.5 eV) state [1,2] of the 229 Th nucleus has been the subject of intense experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and theoretical research [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] in the past decades. The interest is caused by new possibilities which are emerging in the study of such unusual nuclear level.…”