Search

The paper considers issues related to the ultimate content of impurity elements in metals in the processes of their refining by evaporation and distillation in vacuum, zone recrystallization and electromigration. It is shown that at all refining methods, with an increase in the number of refining cycles and the duration of the process, a “saturation effect” arises, associated with the achievement of the ultimate concentrations of impurity elements in refined metals. The reasons for the impossibility to produce absolutely pure metals have been discussed.

The production of ultra-pure raw material is a crucial step to ensure the required background level in rare event searches. In this work, we establish an innovative technique developed to produce high-purity (99.999%) granular zinc. We demonstrate the effectiveness of the refining procedure by measuring the internal contaminations of the purified zinc with a high-purity germanium detector at the Laboratori Nazionali del Gran Sasso. The total activity of cosmogenic activated nuclides is measured at the level of a few mBq/kg, as well as limits on naturally occurring radionuclides are set to less than mBq/kg. The excellent radiopurity of the zinc sample allows us to search for electron capture with positron emission and neutrinoless double electron capture of $$^{64}$$ 64 Zn, setting the currently most stringent lower limits on their half-lives, $$T_{1/2}^{\varepsilon \beta ^+} > 2.7\times 10^{21}~\text {year}$$ T 1 / 2 ε β + > 2.7 × 10 21 year (90% CI), and $$T_{1/2}^{0\nu 2\varepsilon }> 2.6\times 10^{21}~\text {year}$$ T 1 / 2 0 ν 2 ε > 2.6 × 10 21 year (90% CI), respectively.

Double-beta processes in $^{184}$Os and $^{192}$Os were searched for over 15851 h at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. by using a 118 g ultra-pure osmium sample installed on the endcap of a 112 cm$^3$ ultra-low-background broad-energy germanium detector. New limits on double-electron capture and electron capture with positron emission in $^{184}$Os were set at the level of $\lim T_{1/2} \sim 10^{16}-10^{17}$ yr. In particular the $2\nu$2K and $2\nu$KL decays of $^{184}$Os to the ground state of $^{184}$W are restricted as $T_{1/2}\geq3.0\times 10^{16}$ yr and $T_{1/2}\geq2.0\times 10^{16}$ yr, respectively. A lower limit on the half-life for the double-beta decay of $^{192}$Os to the first excited level of $^{192}$Pt was set as $\lim T_{1/2}=2.0\times 10^{20}$ yr at 90\% C.L., Comment: 22 pages, 9 figures, 2 tables

A search for $\ensuremath{\alpha}$ decay of naturally occurring osmium isotopes to the lowest excited levels of daughter nuclei has been performed by using an ultra-low-background broad-energy germanium $\ensuremath{\gamma}$ detector with a volume of 112 ${\mathrm{cm}}^{3}$ and an ultrapure osmium sample with a mass of 118 g at the Gran Sasso National Laboratory of the INFN (Italy). The isotopic composition of the osmium sample has been measured with high precision using negative thermal ionization mass spectrometry. After 15851 h of data taking with the $\ensuremath{\gamma}$ detector, no effect has been detected, and lower limits on the $\ensuremath{\alpha}$ decays were set at level of $lim{T}_{1/2}\ensuremath{\approx}{10}^{15}--{10}^{19}$ yr. The limits for the $\ensuremath{\alpha}$ decays of $^{184}\mathrm{Os}$ and $^{186}\mathrm{Os}$ to the first excited levels of daughter nuclei, ${T}_{1/2}(^{184}\mathrm{Os})\ensuremath{\ge}6.8\ifmmode\times\else\texttimes\fi{}{10}^{15}$ yr and ${T}_{1/2}(^{186}\mathrm{Os})\ensuremath{\ge}3.3\ifmmode\times\else\texttimes\fi{}{10}^{17}$ yr (at 90% C.L.), exceed the present theoretical estimates of the decays' half-lives. For $^{189}\mathrm{Os}$ and $^{192}\mathrm{Os}$ also decays to the ground states of the daughter nuclei were searched for due to the instability of the daughter nuclides relative to $\ensuremath{\beta}$ decay.

The first ever search for $\alpha$ decays to the first excited state in Yb was performed for six isotopes of hafnium (174, 176, 177, 178, 179, 180) using a high purity Hf-sample of natural isotopic abundance with a mass of 179.8 g. For $^{179}$Hf, also $\alpha$ decay to the ground state of $^{175}$Yb was searched for thanks to the $\beta$-instability of the daughter nuclide $^{175}$Yb. The measurements were conducted using an ultra low-background HPGe-detector system located 225 m underground. After 75 d of data taking no decays were detected but lower bounds for the half-lives of the decays were derived on the level of $\lim T_{1/2}\sim 10^{15}-10^{18}$~a. The decay with the shortest half-life based on theoretical calculation is the decay of $^{174}$Hf to the first $2^+$ 84.3~keV excited level of $^{170}$Yb. The experimental lower bound was found to be $T_{1/2}\geq 3.3\times 10^{15}$ a., Comment: 19 pages, 7 figures, 7 tables

A cadmium tungstate crystal boule enriched in $^{116}$Cd to 82% with mass of 1868 g was grown by the low-thermal-gradient Czochralski technique. The isotopic composition of cadmium and the trace contamination of the crystal were estimated by High Resolution Inductively Coupled Plasma Mass-Spectrometry. The crystal scintillators produced from the boule were subjected to characterization that included measurements of transmittance and energy resolution. A low background scintillation detector with two $^{116}$CdWO$_4$ crystal scintillators (586 g and 589 g) was developed. The detector was running over 1727 h deep underground at the Gran Sasso National Laboratories of the INFN (Italy), which allowed to estimate the radioactive contamination of the enriched crystal scintillators. The radiopurity of a third $^{116}$CdWO$_4$ sample (326 g) was tested with the help of ultra-low background high purity germanium $\gamma$ detector. Monte Carlo simulations of double $\beta$ processes in $^{116}$Cd were used to estimate the sensitivity of an experiment to search for double $\beta$ decay of $^{116}$Cd., Comment: 24 pages, 13 figures, 3 tables, accepted for publication on Journal of Instrumentation