Session B7 - Mini-Symposium on Double-Beta Decay and Dark-Matter Search.
FOCUS session, Saturday morning, April 05
Regency Ballroom C1, Loews Philadelphia Hotel

[B7.001] Double beta decay - a key to the neutrino mass scale

If neutrinos are Majorana particles, neutrinoless double beta decay will occur, with a rate proportional to the square of the effective neutrino mass, which in turn depends on the absolute neutrino masses, mixing angles, and unknown Majorana CP phases. Oscillation experiments have established the approximate values of differences of mass squares,and of mixing angles. They show that at least one neutrino has a mass above about 50 meV, and another one above about 10 meV. To reach effective mass sensitivity to these scales, ton-size searches of neutrinoless double beta decay must be performed. I will review the existing proposals and ideas for such experiments. At present, the uncertainties in nuclear matrix elements are the stumbling blocks in relating the effective neutrino mass to the decay rate. I will also discuss issues related to this problem. Determination of the charge conjugation properties and of the absolute scale and/or pattern of the neutrino masses (normal or inverted hierarchy, degenerate masses) is one of the main challenges in particle physics today. Double beta decay clearly will play a major role in meeting this challenge.

[B7.002] FIRST EVIDENCE FOR NEUTRINOLESS DOUBLE BETA DECAY from the HEIDELBERG-MOSCOW EXPERIMENT

Double beta decay is one of the rarest nuclear decay modes and is under investigation already for more than sixty years. The neutrinoless mode is of particular interest since it would violate lepton number and its occurrence would determine the neutrino to be a Majorana particle.

Double beta decay experiments are indispensable to solve the structure of the neutrino mass matrix. They further probe, complementary to high energy colliders, other fields of beyond standard model physics.

The present experimental status is reviewed including the recent evidence for the neutrinoless decay mode from the Heidelberg-Moscow experiment, which yields a half-life of

T_1/2= 1.5 (+16.8, -0.7) x 10^25 y (95% c.l.). This corresponds to an effective neutrino mass of 0.39 (+0.45, -0.34) eV (95% c.l.), assuming that contributions to neutrinoless double beta decay from processes other than exchange of Majorana neutrinos are negligible. Consequences and future perspectives are discussed.

[B7.003] Cuoricino and CUORE:calorimetric search on Double Beta Decay of 130Te

Cuoricino is an array of 62 TeO2 bolometers running at LNGS (Italy) and searching for the Double Beta Decay of 130Te. CUORE is a proposed second generation experiment consisting of a tightly packed array of 1000 TeO2 bolometers with a design optimized for ultralow background searches.

[B7.004] Production of NTD Ge Thermistors for CUORE

The Cryogenic Underground Observatory for Rare Events (CUORE) will be a cryogenic bolometer experiment designed to search for the neutrinoless double beta decay of ^130Te. It will be located in the Gran Sasso underground laboratory in Italy. The experimental apparatus will consist of 1000 750-gram TeO_2 crystals cooled to less than 20 mK. Neutron-transmutation-doped (NTD) germanium thermistors will be attached to each crystal to measure the small temperature changes produced by the double beta decay events. We have begun the production of the thermistors by irradiating ultra-pure germanium wafers at the Univ. of Missouri Research Reactor. Monitor foils were placed in the reactor to determine the thermal, epithermal, and fast neutron fluxes seen by the germanium. Following the irradiations, these foils and germanium wafers were counted at LBNL's Low Background Counting Facility. After suitable decay periods, the resulting Ga and As doping levels and the the resistivity versus temperature characteristics of the thermistors were measured. Results of these measurements and their implications for CUORE will be presented.

[B7.005] The First Several Months of Operation of the Double Beta Decay Experiment NEMO 3

The NEMO 3 experiment began its official operation in July of 2002. The detector has 10 kg of double beta decay isotopes in the form of thin foils. The majority of the foils are composed of 100-Mo and 82-Se. These foils are focused on the search for zero neutrino double beta decay, while the remaining isotopes are focused on refining two neutrino measurements. The performance characteristics of the detector will be discussed along with background measurements and preliminary results for two and zero neutrino double beta decay.

[B7.006] Majorana and MEGA: A Proposed Germanium-Based Search for 0\nu\beta\beta Decay*

The Majorana experiment is a proposed search for neutrinoless double-beta decay, utilizing high-purity Ge detectors isotopically enriched to 85% ^76Ge. With 9 close-pack arrays of 57 crystals each, the current lower-limit half-life of 1.9 \times 10^25 years will be matched within six months of the start of the experiment and improved by an order of magnitude within 2.5 years. Within five years our half-life lower-limit of 10^27 years will be obtained, with an expected asymptotic limit of 4.0 \times 10^27 years. MEGA (Multiple Element Germanium Array) is a supporting experiment currently under construction that uses 18 unsegmented HPGe detectors in an electroformed copper annular cryostat. MEGA will be used in the development of construction and analysis techniques for Majorana, and also will be used for independent studies of materials.

* This work was supported by the US DOE Office of Nuclear Physics, the NSF Division of Physics, and the US DOE under contract DE-AC06-76RLO 1830.

[B7.007] SEGA: a prototype detector element for Ge double beta-decay experiments

We present here a description of the geometry and performance of SEGA (Segmented Enriched Germanium Assembly), a 1.37 kg, n-type, segmented Ge detector enriched to 85% 76Ge. SEGA was developed as a possible prototype detetctor element for the Majorana double beta-decay experiment, and is a cylindrical Ge detector, divided into six azimuthal and two axial segments. In this talk we present the achieved energy resolution of SEGA for standard gamma-ray lines; the efficacy of SEGA segmentation in the identification and reconstruction of multiple-Compton-scatter events; and the use of pulse-shape discrimination to separate single-site events from multiple-site events. Single-site events include, for example, those produced from photoelectric absorption of a gamma-ray or double-beta decay to the ground state of 76Se. Multi-site events include those produced from the absorption of the full energy of a gamma ray via multiple-Compton scattering. This work was partially supported by DOE and NSF.

[B7.008] Status of the MOON ^100Mo Double Beta Decay and Solar Neutrino Project.

MOON is three experiments based on ^100Mo: searches for neutrinoless double beta decay to the ground and 0_1^+ states of ^100Ru, and a tagged charged-current solar neutrino experiment with a threshold of 168 keV. The use of the tags provided by the \beta decay of ^100Tc and by the cascade gamma decay of the 0_1^+ state of ^100Ru gives strong defenses against backgrounds. The status of research and development will be described.

[B7.009] Status of the EXO project

The EXO (Enriched Xenon Observatory) Collaboration is building an advanced double-beta decay detector. We are intending to use a novel approach to the study of neutrino masses that combines quantum optics techniques with radiation detectors. In that way the search for the Majorana neutrino masses down to 10 meV might become accessible. The experimental technique aims to detect individual Ba^+ ions in the final state of ^136Xe double-beta decay. This is to be achieved via resonant excitation with a set of lasers and a large Time Projection Chamber (TPC). Our collaboration is preparing a 200 kg prototype experiment using isotopically enriched Xe. Contributions of our group at University of Alabama to the project include the selection of radio-pure materials to be used as TPC components. We employ low-background gamma spectroscopy and Neutron activation Analysis. The WIPP (New Mexico) underground site has been characterized for the experiment. The status of the project will be discussed.

[B7.010] Simulation of the DRIFT Experiment

The DRIFT Experiment [1] is an underground search for WIMP Dark Matter using a novel detector invented for this purpose: the Negative Ion TPC (NITPC). To aid in interpreting the results, a simulation code system has been developed. The system uses the CERNLIB program GEANT [2] and the NRC package EGS4 [3] to simulate particle interactions in the detector. These are linked directly to the CERNLIB program GARFIELD, which simulates signal production in the NITPC. Finally the GARFIELD output is converted into the format of the DRIFT DAQ for presentation to the analysis code.

The physics and software issues dealt with in this development will be discussed.

[1] Low Pressure Negative Ion TPC for Dark Matter Search. D. P. Snowden-Ifft, C. J. Martoff, J. M. Burwell, Phys Rev. D. Rapid Comm. 61, 101301 (2000) [2] GEANT Manual, CERN Program Library Long Writeup W5013, Copyright CERN, Geneva, 1993 . [3] EGS4, National Research Council, Canada. Note PIRS-701. http://www.irs.inms.nrc.ca/inms/irs/EGS4/get_egs4.html . [4] GARFIELD Manual, version 7.04, CERN Program Library Long Writeup W5050, Copyright CERN, Geneva, 2001 .

[B7.011] Analysis Methods for the DRIFT Dark Matter Search

The DRIFT Experiment [1] is an underground search for WIMP Dark Matter using a novel detector invented for this purpose: the Negative Ion TPC (NITPC). Data is collected in the form of digitized time-records of signals received on each active anode wire of the NITPC endcap. Analysis procedures developed to characterize this data and discriminate backgrounds (x-rays, gamma rays, alpha particles) from potential Dark Matter signals (simulated with neutron elastic scattering) will be discussed.

[1] Low Pressure Negative Ion TPC for Dark Matter Search. D. P. Snowden-Ifft, C. J. Martoff, J. M. Burwell, Phys Rev. D. Rapid Comm. 61, 101301 (2000)

[B7.012] Neutrino-Proton Elastic Scattering Detection in Scintillators

We propose that neutrino-proton elastic scattering, \nu + p \rightarrow \nu + p, can be used for the detection of supernova neutrinos. Though the proton recoil kinetic energy spectrum is soft, with T_p \simeq 2 E_\nu^2/M_p, and the scintillation light output from slow, heavily ionizing protons is quenched, the yield above a realistic threshold is nearly as large as that from \bar\nu_e + p \rightarrow e^+ + n. In addition, the measured proton spectrum is related to the incident neutrino spectrum, which solves a long-standing problem of how to separately measure the total energy release and temperature of \nu_\mu, \nu_\tau, \bar\nu_\mu, and \bar\nu_\tau. The ability to detect this signal would give detectors like KamLAND and Borexino a crucial and unique role in the quest to detect supernova neutrinos

[B7.013] Linux-PC based 1024-Channel Transient Digitizer System for the DRIFT Experiment Acquisition System

The DRIFT Experiment [1] is an underground search for WIMP Dark Matter using a novel detector invented for this purpose: the Negative Ion TPC (NITPC). The data acquisition system for DRIFT had to allow acquisition of long duration time digitized data from the 1024 analog channels at an affordable price. This was accomplished with a system based on a Linux PC, the Comedi [2] open-source device driver software, the inexpensive PCI-DIO-32HS National Instruments high speed digital I/O board, and custom 32-channel preamp+digitizer boards built at SLAC.

System architecture, testing, and performance will be discussed, as well as further upgrade plans.

[1] Low Pressure Negative Ion TPC for Dark Matter Search. D. P. Snowden-Ifft, C. J. Martoff, J. M. Burwell, Phys Rev. D. Rapid Comm. 61, 101301 (2000) [2] Comedi: linux Control and MEasurement Device Interface : http://stm.lbl.gov/comedi/

Part B of program listing