arXiv subjects
Publications and source records attributed to K. Zuber.
The physics potential of neutrinoless double beta decay is discussed. Furthermore, experimental considerations are presented as well as the current status of experiments. Finally an outlook towards the future, work on nuclear matrix elements and alternative processes is given.
The 110Pd double-beta decay Q-value was measured with the Penning-trap mass spectrometer ISOLTRAP to be Q = 2017.85(64) keV. This value shifted by 14 keV compared to the literature value and is 17 times more precise, resulting in new phase-space factors for the two-neutrino and neutrinoless decay modes. In addition a new set of the relevant matrix elements has been calculated. The expected half-life of the two-neutrino mode was reevaluated as 1.5(6) E20 yr. With its high natural abundance, the new results reveal 110Pd to be an excellent candidate for double-beta decay studies.
The mass difference of the nuclides 180W and 180Hf has been measured with the Penning-trap mass spectrometer SHIPTRAP to investigate 180W as a possible candidate for the search for neutrinoless doubleelectron capture. The Q-value was measured to 143.20(27)keV. This value in combination with the calculations of the atomic electron wave functions and other parameters results in a half-life of the 0+ \rightarrow 0+ ground-state to ground-state double-electron capture transition of approximately 5\cdot10E27 years/ ^2.
The ALEPH detector at LEP has been used to measure the momentum spectrum and charge ratio of vertical cosmic ray muons underground. The sea-level cosmic ray muon spectrum for momenta up to 2.5 TeV/c has been obtained by correcting for the overburden of 320 meter water equivalent (mwe). The results are compared with Monte Carlo models for air shower development in the atmosphere. From the analysis of the spectrum the total flux and the spectral index of the cosmic ray primaries is inferred. The charge ratio suggests a dominantly light composition of cosmic ray primaries with energies up to 10^15 eV.
The background on double beta decay searches due to elastic electron scattering of solar neutrinos of all double beta emitters with Q-value larger than 2 MeV is calculated, taking into account survival probability and flux uncertainties of solar neutrinos. This work determines the background level to be [1-2]E-7 counts /keV/kg/yr, depending on the precise Q-value of the double beta emitter. It is also shown that the background level increases dramatically if going to lower Q-values. Furthermore, studies are done for various detector systems under consideration for next generation experiments. It was found that experiments based on loaded liquid scintillator have to expect a higher background. Within the given nuclear matrix element uncertainties any approach exploring the normal hierarchy has to face this irreducible background, which is a limitation on the minimal achievable background for purely calorimetric approaches. Large scale liquid scintillator experiments might encounter this problem already while exploring the inverted hierarchy. Potential caveats by using more sophisticated experimental setups are also discussed.
The potential real-time spectroscopy of solar pp neutrinos using Nd-150 as target is investigated. The threshold of 196 keV would be the lowest of all solar neutrino experiments running so far. Experimental rates and parameters are discussed, about 580 SNU can be expected from pp-neutrinos and another 367 SNU from Be-7. Furthermore, it is investigated whether charged current reactions might cause a new background component for future double beta decay experiments based on a large amount of Nd-150.
A sensitive search of the 4-fold forbidden non-unique beta decay of V-50 has been performed. A total exposure of 185.8 kg x d has been accumulated. A reliable half-life value with the highest precision so far of $(2.29 \pm 0.25) \cdot 10^{17}$ years of the electron capture decay of V-50 into the first excited state of Ti-50 could be obtained. A photon emission line following the 4-fold forbidden beta decay into the first excited state of Cr-50 could not be observed, resulting in a lower limit on the half-life of the beta decay branch of $1.7 \cdot 10^{18}$ years. This is barely in agreement with a claimed observation of this decay branch.
In rather general terms the long term perspective of double beta decay is discussed. All important experimental parameters are investigated as well as the status of nuclear matrix element issues. The link with other neutrino physics results and options to disentangle the underlying physics process are presented.
An array of sixteen 1 cm^3 CdZnTe semiconductor detectors was operated at the Gran Sasso Underground Laboratory (LNGS) to further investigate the feasibility of double-beta decay searches with such devices. As one of the double-beta decay experiments with the highest granularity the 4 x 4 array accumulated an overall exposure of 18 kg days. The setup and performance of the array is described. Half-life limits for various double-beta decay modes of Cd, Zn and Te isotopes are obtained. No signal has been found, but several limits beyond 10^20 years have been performed. They are an order of magnitude better than those obtained with this technology before and comparable to most other experimental approaches for the isotopes under investigation. An improved limit for the beta^+/EC decay of Te 120 is given.
We present 11 independent measurements of the half-life and spectral shape of the 4-fold forbidden beta decay of 113-Cd using CdZnTe semiconductors with a total combined lifetime of 6.58 kg days. Our overall result gives a half-life of (8.00 +/- 0.11(stat) +/- 0.24(sys)) x 10^15 years and a Q value of 322.2 +/- 0.3(stat) +/- 0.9(sys) keV. For the first time half-lives well beyond 10^10 years have been deduced with a statistically representative sample of independent measurements.
Theoretical predictions suggest the presence of tetrahedral symmetry as an explanation for the vanishing intra-band E2-transitions at the bottom of the odd-spin negative parity band in $^{156}Gd$. The present study reports on experiment performed to address this phenomenon. It allowed to determine the intra-band E2 transitions and branching ratios B(E2)/B(E1) of two of the negative-parity bands in $^{156}Gd$.
The effect of moderate cooling on CdZnTe semiconductor detectors has been studied for the COBRA experiment. Improvements in energy resolution and low energy threshold were observed and quantified as a function of temperature. Leakage currents are found to contribute typically $\sim$5 keV to the widths of photopeaks.
A search for the various double beta decay modes of 124Sn and 112Sn has been performed on 75 kg.days of data. New half-life limits for excited states in 124Sn have been obtained including a lower limit for the decay into the first excited 2+ state of 124Te of T_half > 0.87e20 yrs (90% CL) and into the first excited 0+ state of T_half > 1.08e20 yrs (90% CL). Ground state and excited state transitions of 112Sn have also been experimentally explored. A limit for the 2 neutrino double electron capture of T_half > 1.8e19 yrs (90% CL) is obtained. The non-observation of de-excitation gammas from the 0+ at 1888.5keV results in a lower half-life limit on the 0 neutrino double electron capture decay of 112Sn of T_half > 0.8e19 yrs (90% CL), despite a possible resonant enhancement of the decay rate due to degenerated states.
The gamma-decay associated with the warm rotation of the superdeformed (SD) nuclei 151Tb and 196Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the SD well. A Monte Carlo simulation of the gamma-decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(E1) strength at low excitation energy, which may be related to the evidence for octupole vibrations in both mass regions.
For the first time an extensive search for various double beta decay modes of 124Sn and 112Sn has been performed. A total exposure of 43.29 kg.days has been accumulated. New half-life limits of 124Sn into excited states of 124Te have been obtained; the lower half-life limit for the first excited 2+ state at 602.7keV is T_1/2 > 3.1 *10^{18}yrs (90 % CL) and for the first excited 0+ state T_1/2 > 7.7 *10^{18}yrs (90 % CL). For the very first time, ground state and excited state transitions of 112Sn have been experimentally explored. The obtained half-life limits for ECEC and beta+/EC into the first excited 2+ state of 112Cd are both T_1/2 >1.4 *10^{18}yrs (90 % CL). A resonance enhancement in the decay rate for 0nuECEC might be expected for the 0+-state at 1870.9keV due to degeneracy with the 112Sn ground state. No signal was found resulting in a lower half-life limit of T_1/2 > 1.6 *10^{18} yrs (90 % CL) for this decay. As all the excited state searches are based on gamma-lines, all half-life limits apply for both neutrino and neutrino-less modes. Neutrinoless ground state transitions were searched for in the ECEC and beta+/EC mode and a limit of T_1/2 > 1.5 *10^{18} yrs (90 % CL) was obtained for ECEC decays of 112Sn, whilst the beta+/EC mode results are inconclusive.
Four 1cm^3 CdZnTe semiconductor detectors were operated in the Gran Sasso National Laboratory to explore the feasibility of such devices for double beta decay searches as proposed for the COBRA experiment. The research involved background studies accompanied by measurements of energy resolution performed at the surface. Energy resolutions sufficient to reduce the contribution of two-neutrino double beta decay events to a negligible level for a large scale experiment have already been achieved and further improvements are expected. Using activity measurements of contaminants in all construction materials a background model was developed with the help of Monte Carlo simulations and major background sources were identified. A total exposure of 4.34 kg.days of underground data has been accumulated allowing a search for neutrinoless double beta decay modes of seven isotopes found in CdZnTe. Half-life limits (90% C.L.) are presented for decays to ground and excited states. Four improved lower limits have been obtained, including zero neutrino double electron capture transitions of Zn64 and Te120 to the ground state, which are 1.19*10^{17} years and 2.68*10^{15} years respectively.
Exotic-deformation effects in 46Ti nucleus were investigated by analysing the high-energy gamma-ray and the alpha-particle energy spectra. One of the experiments was performed using the charged-particle multi-detector array ICARE together with a large volume (4"x4") BGO detector. The study focused on simultaneous measurement of light charged particles and gamma-rays in coincidence with the evaporation residues. The experimental data show a signature of very large deformations of the compound nucleus in the Jacobi transition region at the highest spins. These results are compared to data from previous experiments performed with the HECTOR array coupled to the EUROBALL array, where it was found that the GDR strength function is highly fragmented, strongly indicating a presence of nuclei with very large deformation.
The study of neutrinoless double beta decay is of outmost importance for neutrino physics. It is considered to be the gold plated channel to probe the fundamental character of neutrinos and to determine the neutrino mass. From the experimental point about nine different isotopes are explored for the search. After a general introduction follows a short discussion on nuclear matrix element calculations and supportive measurements. The current experimental status of double beta searches is presented followed by a short discussion of the ideas and proposals for large scale experiments.