Results from a search for the $0\hspace{0.5em}\nu \beta \beta$-decay of ${}^{130}\mathrm{Te}$

A detailed description of the CUORICINO ${}^{130}\mathrm{Te}$ neutrinoless double-beta $(0\hspace{0.5em}\nu \beta \beta )$ decay experiment is given and recent results are reported. CUORICINO is an array of 62 tellurium oxide (${\mathrm{TeO}}_2$) bolometers with an active mass of 40.7 kg. It is cooled to $~8\text{−}10$ mK by a dilution refrigerator shielded from environmental radioactivity and energetic neutrons. It is running in the Laboratori Nazionali del Gran Sasso (LNGS) in Assergi, Italy. These data represent an exposure of 11.83 kg yr or 91 mole-years of ${}^{130}\mathrm{Te}$. No evidence for $0\hspace{0.5em}\nu \beta \beta$-decay was observed and a limit of $T_{1/2}^{0\hspace{0.5em}\nu }({}^{130}\mathrm{Te})⩾3.0\times {10}^{24}\hspace{0.5em}\mathrm{y}$ (90% CL) is set. This corresponds to an upper limit on the effective mass, $\langle m_{\nu }\rangle$, between 0.19 and $0.68$ eV when analyzed with the many published nuclear structure calculations. In the context of these nuclear models, the values fall within the range corresponding to the claim of evidence of $0\hspace{0.5em}\nu \beta \beta$-decay by H. V. Klapdor-Kleingrothaus et al. The experiment continues to acquire data.

Figure 1

The Tower of CUORICINO and individual 4 and 9 detector modules.

Figure 2

A sketch of the CUORICINO assembly showing the tower hanging from the mixing chamber and the various heat shields and the external shielding.

Figure 3

Typical calibration spectra of the CUORICINO array with a ${}^{232}\mathrm{Th}$ source: $5\times 5\times 5$ ${\mathrm{cm}}^3$ crystals upper frame, $3\times 3\times 6$ ${\mathrm{cm}}^3$ crystals lower frame.

Figure 4

The sum spectrum of the background from the $5\times 5\times 5$ ${\mathrm{cm}}^3$ detectors, from both runs, to search for $0\hspace{0.5em}\nu \beta \beta$ decay.

Figure 5

The summed background spectrum in the $~400$ keV region of interest, which includes the $0\hspace{0.5em}\nu \beta \beta$-decay energy 2530.3 ± 2.0 keV.

Figure 6

The total background spectrum from 2470 to 2590 keV. Clearly visible is the sum peak at 2505.74-keV due to the sum of the 1173.24 and 1332.50-keV $\gamma$-ray cascade in the decay of ${}^{60}\mathrm{Co}$. This activity is attributed to the ${}^{60}\mathrm{Co}$ in the copper frames generated by cosmic ray neutrons while the frames were above ground. The solid lines are the best fit to the region for fits using polynomials of order 0 to 2. The three lines in the region of interest are for bounds (68% and 90%) CL on the number of candidate $\beta \beta$-decay events.

Figure 7

Diagram showing the scheme on which Eq. (A2) is derived.