Alpha-decay Rates of Yb and Gd in Solar Neutrino Detectors

The α-decay rates for the nuclides 168,170,171,172,173,174,176Yb and 148,150,152,154Gd have been estimated from transmission probabilities in a systematic α-nucleus potential and from an improved fit to α-decay rates in the rare-earth mass region. Whereas α-decay of Gd in natural gadolinium is a severe obstacle for the use of gadolinium as a low-energy solar-neutrino detector, we show that α-decay does not contribute significantly to the background in a ytterbium detector. An extremely long α-decay lifetime of Yb is obtained from calculation, which may be close to the sensitivity limit in a low-background solar neutrino detector. PACS numbers: 23.60.+e,26.65.+t,27.60+j,27.70.+q Typeset using REVTEX 1 The solar neutrino problem is an important current subject to be studied in relation to the fundamental physics of neutrino oscillations. After the successful measurements of the solar neutrinos by Super-Kamiokande [1], SAGE [2], GALLEX [3], and SNO [4], the problem appears to be solved best in terms of oscillation of νe into other neutrino flavors. To arrive at the final solution of the solar-ν problem, a real-time measurement of the νe-energy spectrum including pp-, Be-neutrinos is now of central interest [5]. The solar neutrino detection requires an extremely low-background measurement with new technologies. Raghavan [6] has suggested a flavor-specific detection scheme with low thresholds for the real-time detection of solar neutrinos. The neutrino captures νe+ Yb → e− + Lu∗ and νe + 160 Gd → e− + Tb∗ are based on charged current mediated GamowTeller transitions. For the identification of a neutrino capture event a delayed coincidence between a prompt e− and a γ-ray from the decay of a nanosecond isomer in Lu or Tb is used. For both suggested nuclei, Yb and Gd, one finds sufficient Gamow-Teller strength at low energies and a nanosecond isomer which is populated in the neutrino capture reaction. Typical detection rates of solar neutrinos are of the order of ten counts per year and per ton of detector material [7]. A new solar neutrino detector using Yb materials has been proposed in LGNS at Gran Sasso [6–8], and sources of background for such a detector are presently being studied [9]. The α-decay from the material in a solar-ν detector is a possible serious problem to pollute the spectrum consisting of rare neutrino events. For example, it has turned out that Gd material is not suitable for a solar-ν detector; the Gd isotope decays by α emission with a half-life of T1/2 = 1.08×1014 years, and is followed by the subsequent Sm α-decay with T1/2 = 7×1015 years. The number of Gd atoms included in the natural Gd metal with 20 tons amounts to 1.5×1026 atoms. The decay rate calculated by the well-known radioactive decay formula dN dt = −N0λ e (1) amounts to about 30,000 counts/sec for the 20 ton Gd material. This is a huge decay rate,