In this thesis a limit is set only on the half-life of pure right-handed neutrinoless double beta decay. To avoid the fine tuning problem, several new Majoron models have been constructed. Here the term Majoron means light or massless boson with couplings to neutrinos. In this definition the Majoron is not constrained to be a Goldstone boson. There are also other Majoron models which predict double beta decay with emission of one or two Majorons. A model for neutrino masses is proposed in the context of large extra dimensions.
This leads to a bulk singlet Majoron which is observable in neutrinoless double beta decay. There are in total 10 Majoron models one bulk Majoron and 9 light or massless bosonic Majorons that are of interest in double beta decay experiments.
The half-life of Majoron-emitting double beta decay is expressed by. Accurate calculations of the NME are necessary in order to measure the effective neutrino mass with minimum uncertainty.
For medium to heavy double beta decay isotopes the shell model calculations are difficult to carry out. The QRPA can handle a great number of intermediate states. In double beta decay the initial nucleus decays to the final nucleus through virtual excitations of all states of the intermediate nucleus.
The intermediate excited states are obtained by solving the QRPA equations. The QRPA calculations contain two two-body interaction matrix elements: particle-particle pp which is correlated to the proton-proton interaction and particle-hole ph which is correlated to the proton-neutron interaction. The g p p constant, which has a large effect on NME and double beta decay, is a free parameter. Another important issue in QRPA is that the nuclei of experimental interest are assumed to have spherical symmetry.
This chapter presents the experimental criteria and status of double beta decay experiments. The double beta decay isotopes and the detector components must have high purity. This goal can be achieved via purification of double beta decay sources and constructing smaller and more granular detector components. This produces a good signal over background ratio. To reject background, event reconstruction and good particle identification is required.
The NME is understood for some isotopes more than others. However, it is not experimentally favourable and interesting to detect the decay in all these isotopes. As double beta decay is rare, it will be difficult to detect it above background. Currently there are nine double beta decay isotopes which are experimentally considered for neutrinoless double beta decay searches: 48 Ca, 76 Ge, 82 Se, 96 Zr, Mo, Cd, Te, Xe and Nd. This thesis is about the double beta decay study of Nd, thus the rest of this section describes the properties of this isotope.
The double beta decay of Nd to the ground state of Sm is accompanied by the simultaneous emission of two electrons. The observation of neutrinoless double beta decay can answer important questions in neutrino physics. The double beta detectors are divided into two main categories: homogeneous source equal to detector experiments in which the double beta decay source is part of the active detector and heterogeneous source not equal to detector experiments in which the source is separated from the detector.
The homogeneous detection technique gives good energy resolution. The heterogeneous experiments contain tracking detectors which give them a good particle identification. Semiconductor germanium Ge detectors are among the most popular double beta decay experiments. This is because of the cheap enrichment process for 76 Ge. The NME of this isotope is also relatively well known. The final design of the detector consisted of five individual sub-detectors with a total mass of The experiment ran between and It is believed that the background and the systematic uncertainties of this experiment are underestimated.
This sensitivity can rule out or confirm the Heidelberg-Moscow results. The final design of this experiment is still under development. This experiment uses bolometers containing double beta decay isotopes, running at extremely low temperatures order of mK. When a double beta decay event occurs, the electrons of the decay increase the temperature by depositing energy inside a crystal.
By measuring the temperature rise the deposited energy is measured. This detector has set a limit on the half-life of neutrinoless double beta decay of Te to be more than 3. It is expected to reach a half-life sensitivity of 2. CdZnTe contains nine double beta isotopes, with Te and Cd having the highest abundance in the detector. It is also a possibility that this experiment could load the scintillators with enriched Nd.
These type of experiments are divided into two categories: the time projection chambers TPCs , where the double beta decay isotope is a filling gas or liquid, and tracking-calorimeter experiments, where the double beta decay source is a solid foil. The experiment is developing a high resolution liquid xenon TPC with good tracking capability.
A high voltage cathode will be located in the middle of the TPC volume. At each end there will be horizontal and vertical anode wires. These wires will be for charge collection, allowing energy measurement and particle trajectory reconstruction. The liquid xenon is also an efficient scintillator.
The use of both collected charge and scintillator light improves the energy resolution. In the second phase the EXO detector is expected to trap and identify the Ba ions, the daughter isotope of Xe, by laser spectroscopy. The radioactive background events can be significantly reduced by Ba tagging.
This experiment will comprise 20 individual modules, each of which has a rectangular shape with the source foil in the centre surrounded by the tracker and the calorimeter.
DCBA will use natural Nd in its first phase of data taking and then enriched Nd in the second phase. The main objective of the experiment is to search for evidence of neutrinoless double beta decay in a variety of isotopes. Particle tracking and energy measurements are also performed independently, giving good particle identification and understanding of radioactive backgrounds.
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