Pulse-shape studies with coplanar grid CdZnTe detectors and searches for rare nuclear decays with the COBRA experiment

The inference of massive neutrino states through the observation of flavor oscillations boosted the importance of direct and indirect mass searches, including the search for the hypothesized neutrinoless double beta decay (0νββ-decay). Nowadays, the search for this ultra-rare nuclear transition is one of the most active research fields at the intersection of nuclear, particle and astroparticle physics. Its main and evident feature is the explicit violation of the total lepton number, which is an accidentally conserved quantity in the Standard Model of particle physics, and would prove the Majorana nature of neutrinos. This, in turn, would support the theoretical explanation of the origin of the observed baryon asymmetry in the universe through the process of leptogenisis and could shed light on the role of neutrinos in the early universe’s structure formation. For the theoretical description of the 0νββ-decay, nuclear structure effects play an important role as they may affect considerably the decay rate. These nuclear effects are summarized as the nuclear matrix elements (NMEs), containing information about the initial and final states of the involved atomic nuclei and the decay mechanism. Under the assumption of light Majorana neutrino exchange, the inverse half-life is proportional to the effective Majorana neutrino mass, a kinematic phase-space factor, the involved NMEs and the fourth power of the weak axial-vector coupling gA. The search for the 0νββ-decay is driven by experiments and an accurate description of the nuclear structure effects is essential to estimate the required sensitivity to cover a certain mass range. In order to match theoretical calculations and the results of β-decay and ββ-decay studies, there is a scientific discussion regarding quenching effects of gA in nuclear media. Different methods are being investigated to determine an effective gA at the energy scale of nuclear transitions. One of those recently proposed methods exploits the dependency of the spectrum-shape of highly forbidden β-decays on gA. An ideal candidate for such an investigation is the fourfold forbidden non-unique β-decay of Cd-113, which is the most prominent signal in the current stage of the COBRA experiment searching for 0νββ-decays with cadmium zinc telluride (CZT) solid state detectors. The detector material CZT acts as a semiconductor at room temperature and contains intrinsically several candidates for rare nuclear transitions. The experiment is located at the LNGS underground facility in Italy, which is shielded against cosmic rays by a mean rock coverage of about 1400 m. In the present demonstrator phase, it consists of 64 coplanar grid (CPG) detectors that are arranged in four layers of 4 x 4 crystals. In the scope of this thesis, conventional and novel prototype CPG-CZT detectors, which are the basis for an anticipated large-scale experiment, are characterized by evaluating homogeneous and localized γ-ray irradiation measurements. Moreover, a novel pulse-shape discrimination (PSD) technique is established, optimized and applied in the analysis of the physics data obtained with the demonstrator array. The PSD optimization is complemented by dedicated laboratory measurements with the aim to create a pulseshape library of signal-like single-site events and high-energy cosmic muon interactions for which an analytic reconstruction model has been developed. For the first time, the COBRA demonstrator’s full exposure from Sept.’11 to Nov.’19 is analyzed, including a detailed background characterization as well as an automatized data partitioning to identify periods with increased backgrounds. The main subject is the study of the Cd-113 β-decay’s spectrum-shape to address the quenching of gA in lowmomentum exchange nuclear processes. The analysis of the experimental data is carried out in the context of three nuclear frameworks and confirms the idea of a significantly quenched gA. Furthermore, the data are analyzed with respect to the 2νββ-decay of Cd-116 and the long-lived α-decay of Pt-190 as a localized contaminant in the CZT detectors’ electrode metalization. Finally, the prospects of a search for excited state transitions of the ββ-nuclides Cd-116 and Te-130 are studied with elaborate Monte-Carlo simulations. The analysis section is concluded with an estimate of the achievable 0νββ-decay half-life sensitivity for multiple ββ-nuclides given the full exposure of the COBRA demonstrator and the recently achieved upgrade to the COBRA eXtended DEMonstrator (XDEM)

Modeling of GERDA Phase II data

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0νββ) decay of 76Ge. The technological challenge of Gerda is to operate in a “background-free” regime in the region of interest (ROI) after analysis cuts for the full 100 kg·yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Qββ for the 0νββ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2νββ) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of 16.04+0.78−0.85⋅10−3 cts/(keV·kg·yr) for the enriched BEGe data set and 14.68+0.47−0.52⋅10−3 cts/(keV·kg·yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components

Modeling of GERDA Phase II data

The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta ($0\nu\beta\beta$) decay of $^{76}$Ge. The technological challenge of GERDA is to operate in a "background-free" regime in the region of interest (ROI) after analysis cuts for the full 100$\,$kg$\cdot$yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around $Q_{\beta\beta}$ for the $0\nu\beta\beta$ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos ($2\nu\beta\beta$) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for GERDA Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of $16.04^{+0.78}_{-0.85} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched BEGe data set and $14.68^{+0.47}_{-0.52} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched coaxial data set. These values are similar to the one of Gerda Phase I despite a much larger number of detectors and hence radioactive hardware components

Pulse-shape studies with coplanar grid CdZnTe detectors and searches for rare nuclear decays with the COBRA experiment

The inference of massive neutrino states through the observation of flavor oscillations boosted the importance of direct and indirect mass searches, including the search for the hypothesized neutrinoless double beta decay (0νββ-decay). Nowadays, the search for this ultra-rare nuclear transition is one of the most active research fields at the intersection of nuclear, particle and astroparticle physics. Its main and evident feature is the explicit violation of the total lepton number, which is an accidentally conserved quantity in the Standard Model of particle physics, and would prove the Majorana nature of neutrinos. This, in turn, would support the theoretical explanation of the origin of the observed baryon asymmetry in the universe through the process of leptogenisis and could shed light on the role of neutrinos in the early universe’s structure formation. For the theoretical description of the 0νββ-decay, nuclear structure effects play an important role as they may affect considerably the decay rate. These nuclear effects are summarized as the nuclear matrix elements (NMEs), containing information about the initial and final states of the involved atomic nuclei and the decay mechanism. Under the assumption of light Majorana neutrino exchange, the inverse half-life is proportional to the effective Majorana neutrino mass, a kinematic phase-space factor, the involved NMEs and the fourth power of the weak axial-vector coupling gA. The search for the 0νββ-decay is driven by experiments and an accurate description of the nuclear structure effects is essential to estimate the required sensitivity to cover a certain mass range. In order to match theoretical calculations and the results of β-decay and ββ-decay studies, there is a scientific discussion regarding quenching effects of gA in nuclear media. Different methods are being investigated to determine an effective gA at the energy scale of nuclear transitions. One of those recently proposed methods exploits the dependency of the spectrum-shape of highly forbidden β-decays on gA. An ideal candidate for such an investigation is the fourfold forbidden non-unique β-decay of Cd-113, which is the most prominent signal in the current stage of the COBRA experiment searching for 0νββ-decays with cadmium zinc telluride (CZT) solid state detectors. The detector material CZT acts as a semiconductor at room temperature and contains intrinsically several candidates for rare nuclear transitions. The experiment is located at the LNGS underground facility in Italy, which is shielded against cosmic rays by a mean rock coverage of about 1400 m. In the present demonstrator phase, it consists of 64 coplanar grid (CPG) detectors that are arranged in four layers of 4 x 4 crystals. In the scope of this thesis, conventional and novel prototype CPG-CZT detectors, which are the basis for an anticipated large-scale experiment, are characterized by evaluating homogeneous and localized γ-ray irradiation measurements. Moreover, a novel pulse-shape discrimination (PSD) technique is established, optimized and applied in the analysis of the physics data obtained with the demonstrator array. The PSD optimization is complemented by dedicated laboratory measurements with the aim to create a pulseshape library of signal-like single-site events and high-energy cosmic muon interactions for which an analytic reconstruction model has been developed. For the first time, the COBRA demonstrator’s full exposure from Sept.’11 to Nov.’19 is analyzed, including a detailed background characterization as well as an automatized data partitioning to identify periods with increased backgrounds. The main subject is the study of the Cd-113 β-decay’s spectrum-shape to address the quenching of gA in lowmomentum exchange nuclear processes. The analysis of the experimental data is carried out in the context of three nuclear frameworks and confirms the idea of a significantly quenched gA. Furthermore, the data are analyzed with respect to the 2νββ-decay of Cd-116 and the long-lived α-decay of Pt-190 as a localized contaminant in the CZT detectors’ electrode metalization. Finally, the prospects of a search for excited state transitions of the ββ-nuclides Cd-116 and Te-130 are studied with elaborate Monte-Carlo simulations. The analysis section is concluded with an estimate of the achievable 0νββ-decay half-life sensitivity for multiple ββ-nuclides given the full exposure of the COBRA demonstrator and the recently achieved upgrade to the COBRA eXtended DEMonstrator (XDEM)

Pulse-shape studies with coplanar grid CdZnTe detectors and searches for rare nuclear decays with the COBRA experiment

The inference of massive neutrino states through the observation of flavor oscillations boosted the importance of direct and indirect mass searches, including the search for the hypothesized neutrinoless double beta decay (0νββ-decay). Nowadays, the search for this ultra-rare nuclear transition is one of the most active research fields at the intersection of nuclear, particle and astroparticle physics. Its main and evident feature is the explicit violation of the total lepton number, which is an accidentally conserved quantity in the Standard Model of particle physics, and would prove the Majorana nature of neutrinos. This, in turn, would support the theoretical explanation of the origin of the observed baryon asymmetry in the universe through the process of leptogenisis and could shed light on the role of neutrinos in the early universe’s structure formation. For the theoretical description of the 0νββ-decay, nuclear structure effects play an important role as they may affect considerably the decay rate. These nuclear effects are summarized as the nuclear matrix elements (NMEs), containing information about the initial and final states of the involved atomic nuclei and the decay mechanism. Under the assumption of light Majorana neutrino exchange, the inverse half-life is proportional to the effective Majorana neutrino mass, a kinematic phase-space factor, the involved NMEs and the fourth power of the weak axial-vector coupling gA. The search for the 0νββ-decay is driven by experiments and an accurate description of the nuclear structure effects is essential to estimate the required sensitivity to cover a certain mass range. In order to match theoretical calculations and the results of β-decay and ββ-decay studies, there is a scientific discussion regarding quenching effects of gA in nuclear media. Different methods are being investigated to determine an effective gA at the energy scale of nuclear transitions. One of those recently proposed methods exploits the dependency of the spectrum-shape of highly forbidden β-decays on gA. An ideal candidate for such an investigation is the fourfold forbidden non-unique β-decay of Cd-113, which is the most prominent signal in the current stage of the COBRA experiment searching for 0νββ-decays with cadmium zinc telluride (CZT) solid state detectors. The detector material CZT acts as a semiconductor at room temperature and contains intrinsically several candidates for rare nuclear transitions. The experiment is located at the LNGS underground facility in Italy, which is shielded against cosmic rays by a mean rock coverage of about 1400 m. In the present demonstrator phase, it consists of 64 coplanar grid (CPG) detectors that are arranged in four layers of 4 x 4 crystals. In the scope of this thesis, conventional and novel prototype CPG-CZT detectors, which are the basis for an anticipated large-scale experiment, are characterized by evaluating homogeneous and localized γ-ray irradiation measurements. Moreover, a novel pulse-shape discrimination (PSD) technique is established, optimized and applied in the analysis of the physics data obtained with the demonstrator array. The PSD optimization is complemented by dedicated laboratory measurements with the aim to create a pulseshape library of signal-like single-site events and high-energy cosmic muon interactions for which an analytic reconstruction model has been developed. For the first time, the COBRA demonstrator’s full exposure from Sept.’11 to Nov.’19 is analyzed, including a detailed background characterization as well as an automatized data partitioning to identify periods with increased backgrounds. The main subject is the study of the Cd-113 β-decay’s spectrum-shape to address the quenching of gA in lowmomentum exchange nuclear processes. The analysis of the experimental data is carried out in the context of three nuclear frameworks and confirms the idea of a significantly quenched gA. Furthermore, the data are analyzed with respect to the 2νββ-decay of Cd-116 and the long-lived α-decay of Pt-190 as a localized contaminant in the CZT detectors’ electrode metalization. Finally, the prospects of a search for excited state transitions of the ββ-nuclides Cd-116 and Te-130 are studied with elaborate Monte-Carlo simulations. The analysis section is concluded with an estimate of the achievable 0νββ-decay half-life sensitivity for multiple ββ-nuclides given the full exposure of the COBRA demonstrator and the recently achieved upgrade to the COBRA eXtended DEMonstrator (XDEM)

Confirmation of gA quenching using the revised spectrum-shape method for the analysis of the 113Cd β-decay as measured with the COBRA demonstrator

In this article we present an updated spectrum-shape analysis of the113Cd fourfold forbidden non-uniqueβ-decay transition in order to address the quenching of the weak axial-vector coupling gA in low-momentum exchange nuclear processes. The experimental data were collected in a dedicated low-threshold run with the COBRA demonstrator at the LNGS and resulted in 44 individual113Cdspectra. These data are evaluated in the context of three nuclear model frameworks based on a revised version of the spectrum-shape method and the conserved vector current hypothesis. The novel idea devised in the present work is to fit the value of the small relativistic nuclear matrix element (s-NME) driving the nuclear model calculations, which remained essentially as a free parameter in previous studies. This is done by tuning the nuclear structure calculations and making use of the interplay of gA and the s-NME such that the experimentally known113Cd half-life gets reproducible by the different frameworks. In this way, a best fit s-NME value can be derived for each of the considered nuclear models, which finally enters the template calculations used to perform the spectrum-shape analysis for each of the obtained113Cd spectra. The primary analysis strategy results in significantly quenched values of the axial-vector coupling for all three nuclear models: gA(ISM)=0.907±0.064,gA(MQPM)=0.993±0.063andgA(IBFM-2)=0.828±0.140. Moreover, with our data-driven approach one of the main shortcomings of the spectrum-shape method has been resolved. This achievement is a milestone in the description of strongly forbiddenβ-decays and adds to the indications for the existence of a quenching of gA in low-momentum exchange nuclear processes.peerReviewe