arXiv:0907.0126v1 [nucl-ex] 1 Jul 2009 APS/123-QED Long-lived isomeric states in neutron-deficient thorium isotopes? J. Lachner, ∗ I. Dillmann, † T. Faestermann, G. Korschinek, M. Poutivtsev, † and G. Rugel Physik Department E12 and E15, Technische Universit¨at M¨ unchen, D-85748 Garching, Germany (Dated: July 1, 2009) The discovery of naturally occurring long-lived isomeric states (t 1/2 > 10 8 yr) in the neutron- deficient isotopes 211,213,217,218 Th [Marinov et al., Phys. Rev. C 76, 021303(R) (2007)] was re- examined using accelerator mass spectrometry (AMS). As AMS does not suffer from molecular isobaric background in the detection system, it is an extremely sensitive technique. In spite of our up to two orders of magnitude higher sensitivity we cannot confirm the discoveries of neutron- deficient thorium isotopes and provide upper limits for their abundances. PACS numbers: 27.80.+w,82.80.Rt,98.80.Ft I. INTRODUCTION Recently the discovery of ”high spin super- and hyper- deformed” isomeric states in neutron-deficient thorium isotopes was claimed [1]. The authors investigated so- lutions of natural thorium and announced the discovery of long-lived isomers in 211,213,217,218 Th. They conclude that the lower limit of the half lives must be 10 8 years and assume that these might belong to ”high spin K-type isomers”, which were ”preferentially produced by heavy ion reactions”. These observations are extremely surprising for sev- eral reasons. The ground states of the thorium isotopes in question are all α emitters with half lives ranging from 144 ms down to 117 ns for the N =128 nucleus 218 Th which has one of the shortest half lives ever measured for a nuclear ground state. In Table I we list the available information on these thorium isotopes. Even if the elec- tromagnetic decay for the isomers claimed by [1] would be so slow due to K-hindrance, also the α-decay would have to be slowed down due to angular momentum hin- drance by at least 16 to 22 orders of magnitude to explain the long half lives required for their natural occurence. TABLE I: Decay properties of 211,213,217,218 Th and known isomers, with excitation energy, isospin and parity. Isotope I π Eex [keV] t 1/2 Ref. 211 Th ? 0 (40 +3 -1 ) ms [2] 213 Th (5/2) - 0 (144 ±21) ms [3] 213 Th m (13/2) + 1180 (1.4 ±0.4) μs [4] 217 Th (9/2 + ) 0 (241 ±5) μs [5] 217 Th m (25/2 + ) ”2252+X” (67 +17 -11 ) μs [6] 218 Th 0 + 0 (117 ±9) ns [7] Furthermore there is no way within the current under- standing of nucleosynthesis how these states could have * Electronic address: johannes.lachner@physik.tu-muenchen.de † Excellence Cluster ”Origin and Structure of the Universe”, Tech- nische Universit¨ at M¨ unchen, D-85748, Garching, Germany been produced before the formation of the solar system. The stellar nucleosynthesis of isotopes heavier than iron occurs predominantly via neutron capture reactions in equal shares in the so-called slow and rapid processes (”s process” and ”r process”) [8]. The neutron density in the s process is in the range N n ≈10 8 cm −3 , forcing the reaction path to follow the line of β stability. The s process nucleosynthesis terminates in the reaction cycle 209 Bi(n, γ ) 210 Bi(β − ) 210 Po(α) 206 Pb. In order to populate thorium isotopes in the s process, a bridge of nuclear states with half lives in the order of years between 209 Bi and these thorium isotopes would be necessary. The long-lived primordial actinides 232 Th, 235 U and 238 U are produced in the high neutron density scenario (N n ≫10 20 cm −3 ) of the r process. The exact stellar production site is still under discussion, most probable scenarios are core-collapse supernovae or neutron star mergers. The starting seed for both, the r and s pro- cesses, are iron peak nuclei produced by previous fusion processes (”burning stages”) in the star. Due to its large neutron density the r-process reaction path is driven far away from stability close to the region of the neutron drip-line. By this mechanism the r process is able to climb within a few hundred milliseconds up to its termination point around A ≈260 (Z =94, Pu) where ”fission recycling” by spontaneous and β-delayed fission sets in and transfers some material back into the mass region A ≈130. When the neutron production ends and/or the temperature de- creases (”freeze-out”) the short-lived radioactive nuclei decay back to stability via long β-decay chains. This pro- duces the well-known observed solar r-abundance peaks at A ≈80, 130, and 195 [9], corresponding to the very neutron-rich r-process progenitor nuclei with N =50, 82, and 126. Beyond A=210 these long decay chains will reach α- emitters which are (almost) stable against β-decays. The transmutation of these radioactive isotopes will then pro- ceed via α-decay chains, ending either in bismuth and the lead isotopes or in the long-lived actinides 232 Th, 234 U, 235 U and 238 U. In any case the nuclides finally populated in the r process have a considerably higher A/Z ratio than the values A/Z ≤218/90 for the thorium isotopes in question.