Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0-nu-2-beta) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q(76Ge) = 2039 keV are 0.93% for events from 60Co, 21% from 226Ra and 40% from 228Th. This background suppression is achieved with 89% acceptance of 228Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0-nu-2-beta-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Q(76Ge). The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.Comment: 22 pages, 16 figures, submitted to JINST: JINST_018P_080

Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0-nu-2-beta) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q(76Ge) = 2039 keV are 0.93% for events from 60Co, 21% from 226Ra and 40% from 228Th. This background suppression is achieved with 89% acceptance of 228Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0-nu-2-beta-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Q(76Ge). The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.Comment: 22 pages, 16 figures, submitted to JINST: JINST_018P_080

Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0-nu-2-beta) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q(76Ge) = 2039 keV are 0.93% for events from 60Co, 21% from 226Ra and 40% from 228Th. This background suppression is achieved with 89% acceptance of 228Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0-nu-2-beta-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Q(76Ge). The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.Comment: 22 pages, 16 figures, submitted to JINST: JINST_018P_080

The background in the neutrinoless double beta decay experiment GERDA

The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta decay of 76Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q-value of the decay, Q_bb. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around Q_bb. The main parameters needed for the neutrinoless double beta decay analysis are described. A background model was developed to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around Q_bb with a background index ranging from 17.6 to 23.8*10^{-3} counts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the background model. The background at Q-bb is dominated by close sources, mainly due to 42K, 214Bi, 228Th, 60Co and alpha emitting isotopes from the 226Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known gamma peaks, the energy spectrum can be fitted in an energy range of 200 kev around Q_bb with a constant background. This gives a background index consistent with the full model and uncertainties of the same size

Bringing bioelectricity to light: all-optical electrophysiology using microbial rhodopsins

My work has focused on the development and application of fluorescent voltage-sensitive proteins based on microbial rhodopsins. These probes led to the discovery of electrical activity in the bacterium Escherichia coli, the first robust optical recordings of action potentials (APs) in mammalian neurons using a genetically encoded voltage reporter, and the development of a genetically targetable all-optical electrophysiology system. I first introduce an engineered fluorescent voltage sensor based on green-absorbing proteorhodopsin. Expression of the proteorhodopsin optical proton sensor (PROPS) in E. coli revealed electrical spiking at up to 1 hertz. Spiking was sensitive to chemical and physical perturbations and coincided with rapid efflux of a small-molecule fluorophore, suggesting that bacterial efflux machinery may be electrically regulated. I then present another microbial rhodopsin, Archaerhodopsin 3 (Arch), whose endogenous fluorescence exhibited a twofold increase in brightness between -150 mV and +150 mV and a sub-millisecond response time. In rat hippocampal neurons, Arch detected single electrically triggered APs with an optical signal-to-noise ratio > 10. A mutant, Arch(D95N), lacked endogenous proton pumping and had 50% greater sensitivity than the wild type but had a slower response (41 ms). Nonetheless, Arch(D95N) also resolved individual APs. Finally, I introduce evolved archaerhodopsin-based voltage indicators and a spectrally orthogonal channelrhodopsin actuator, which together enabled all-optical electrophysiology. A directed evolution screen yielded two mutants, QuasAr1 and QuasAr2, that showed improved brightness and voltage sensitivity relative to previous archaerhodopsin-based sensors, and microsecond response times. An engineered channelrhodopsin actuator, CheRiff, showed high light sensitivity and rapid kinetics. A coexpression vector, Optopatch, enabled cross-talk-free genetically targeted all-optical electrophysiology. In cultured neurons, the Optopatch system probed membrane voltage across temporal and spatial scales, from the sub-cellular and sub-millisecond dynamics of AP propagation, to the simultaneous measurement of firing patterns of many neurons in a circuit. In brain slices, Optopatch induced and reported APs and subthreshold events with high signal-to-noise ratios. In human stem cell-derived neurons, Optopatch measurements revealed homeostatic tuning of intrinsic excitability, a subtle form of plasticity that had yet to be observed in human neurons. The suite of tools and techniques presented here enable high-throughput, genetically targeted, and spatially resolved electrophysiology without the use of conventional electrodes.Engineering and Applied Science

Status of the GERDA experiment

The study of neutrinoless double beta (0nbb) decay is the only one presently known approach to the fundamental question if the neutrino is a Majorana particle, i.e. its own anti-particle. The observation of 0nbb decay would prove that lepton number is not conserved, establish that neutrino has a Majorana component and, assuming that light neutrino is the dominating process, provide a method for the determination of its effective mass. GERDA is a new 0nbb decay experiment which is currently taking data at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. It implements a new shielding concept by operating bare diodes made from Ge with enriched 76Ge in high purity liquid argon supplemented by a water shield. The aim of GERDA is to verify or refute the recent claim of discovery, and, in a second phase, to achieve a two orders of magnitude lower background index than past experiments, to increase the sensitive mass and to collect an exposure of 100 kg yr. The paper will discuss design, physics reach, and status of data taking of GERDA.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Operation and performance of a bare broad-energy germanium detector in liquid argon

The GERmanium Detector Array, GERDA, will search for neutrinoless double beta decay of 76Ge by operating bare high-purity germanium detectors, enriched in 76Ge, in liquid argon. To reduce the background to the required level below 10^(−3) cts/(keV·kg·y), it is necessary to employ active background-suppression techniques. Detectors based on the design of commercially available Broad-Energy Germanium (BEGe) detector are one of the two technologies included in research and development for the second phase of GERDA. BEGe detectors feature an enhanced capability to distinguish between an interaction of an electron from beta-decay and an interaction of a multiple-scattered photon inside the detector, via pulse-shape analysis. A GERDA Phase II prototype BEGe detector mounted in a low-mass holder was operated bare for the first time in liquid argon. The detector showed stable performance over more than one month, with an energy resolution of 1.9 keV (FWHM) at 1.3MeV, and a low leakage current of ≤ 20 pA. Periodic pulseshape analysis checks were performed and the results are equal to those obtained with the same detector in a vacuum cryostat

Test of a fully integrated CMOS preamplifier for HPGe detectors

A CMOS fully integrated charge preamplifier for nuclear physics spectroscopy measurements, designed to work with germanium detectors in the Gerda experiment is tested. The electronics is composed of two distinct circuits: a low noise charge sensitive preamplifier (CSA) and a fully differential low impedance coaxial cable driver. A couple of passive elements (few resistors and the feedback capacitor) are not yet integrated, in order to make the debug process easier and to fit their values to the specific needs of different set-up configurations. The preamplifier operates at +2.5 V / -2.5 V power supplies, with 50 mW power consumption and 8 V differential output bipolar dynamic range at the driver side of the 50 ohm impedance cable. Up to now, we tested it in an experimental set-up consisting of an encapsulated p-type germanium detector operated at liquid nitrogen temperature by means of a custom Dewar container. Reliability and robustness of the CMOS electronics and mounting set-up have been verified, e.g. in terms of immunity to normal operation of the high voltage detector power supply and integrity of IC bonding wires in contact with liquid nitrogen. Long term stability of the preamplifier gain, although operating at almost constant temperature, has also been verified by overnight measurements. Overall noise performance is worse than expected from previous set-ups with simulated detector capacitance, due to relatively larger detector capacitance and leakage current contributions previously not completely taken into account