Abstract: Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.

Abstract: Double beta decay (ββ) is a nuclear decay mode expected to appear in at least two varieties, the double-neutrino (2ν) and the zero-neutrino (0ν) mode. The 0νββ-decay is of particular interest as it requires the neutrino to be a Majorana particle. The search for such a decay is presently being carried out or planned in a number of experiments, such as EXO, MAJORANA, GERDA, CUORE, COBRA, NEMO-III and SNO+. The 0ν-decay rate depends on the neutrino mass but, unfortunately, also on a rather complex nuclear matrix element, making the extraction of the mass heavily dependent on the underlying theoretical nuclear model. However, all theoretical models can readily be tested against the 2ν mode, which, unlike its 0ν counterpart, only involves simple Gamow–Teller nuclear matrix elements. These elements can be determined experimentally either through charge-exchange reactions or, for the ground-state transition, through the electron capture (EC) or single β-decay of the intermediate odd–odd nucleus. The present program is geared towards the measurement of the EC branching ratios (BR). In most cases, these ratios are poorly known or not known at all, because EC is usually suppressed by several orders of magnitude compared to the β-decay counterpart due to energy considerations. Traditional methods for measuring these ratios have so far suffered from overwhelming background generated by these high-energy electrons. Recently, a unique background-free method for measuring EC branching ratios was proposed using the TITAN ion trap at the TRIUMF ISAC (Isotope Separator and ACcelerator) radioactive beam facility. The measurements will make use of the EBIT (Electron Beam Ion Trap) operating in Penning mode where electrons from the β−-decay will be confined by the magnetic field. K-shell X-rays from EC will be detected by seven X-ray detectors located around the trap, thus providing orders of magnitude background suppression and thus ideal low-BR measurement environment.

Abstract: In this Letter, we report a new mass for 11Li using the trapping experiment TITAN at TRIUMF's ISAC facility. This is by far the shortest-lived nuclide, t_{1/2}=8.8 ms, for which a mass measurement has ever been performed with a Penning trap. Combined with our mass measurements of ;{8,9}Li we derive a new two-neutron separation energy of 369.15(65) keV: a factor of 7 more precise than the best previous value. This new value is a critical ingredient for the determination of the halo charge radius from isotope-shift measurements. We also report results from state-of-the-art atomic-physics calculations using the new mass and extract a new charge radius for 11Li. This result is a remarkable confluence of nuclear and atomic physics.

Abstract: A high-precision Penning trap mass measurement of the exotic 8He nuclide (T1/2=119 ms) has been carried out resulting in a reduction of the uncertainty of the halo binding energy by over an order of magnitude. The new mass, determined with a relative uncertainty of 9.2×10-8 (deltam=690 eV) is 13 keV less bound than the previously accepted value. The mass measurement is of great relevance for the recent charge-radius measurement of 8He [P. Mueller et al., Phys. Rev. Lett. 99, 252501 (2007).]. The 8He mass is the first result from the newly-commissioned Penning trap: TITAN (TRIUMF's Ion Trap for Atomic and Nuclear science) at the ISAC (Isotope Separator and Accelerator) radioactive beam facility at TRIUMF.

Abstract: The in-pile positron source NEPOMUC of the Munich research reactor FRM II delivers a low-energy positron beam of highest intensity. Different source potentials can be applied in order to vary the primary kinetic energy of the positrons in the range between 15 eV and 1 keV. The maximum yield of slow positrons is up to 4 × 107 and 5 × 108 moderated positrons per second, respectively. New instruments for beam diagnostics have been implemented for the determination of the positron intensity and for positron beam profile measurements. The long-term stability and the degradation of the platinum moderator as well as the recovery procedure were investigated. In the present arrangement of NEPOMUC's instrumentation the monoenergetic positron beam is magnetically guided to different experiments: a coincident Doppler broadening spectrometer (CDBS) and an analysis chamber for positron induced Auger electron spectroscopy (PAES). In addition, an apparatus for the production of the negatively charged positronium ion was connected to the beam line in order to investigate this bound leptonic system in collaboration with the Max-Planck Institute for nuclear physics. An overview of the beam performance, the current status of the positron beam facility and the experiments is presented.

Abstract: The moderation efficiency of the commonly used positron moderators, such as tungsten or solid rare gases, is very low and amounts to less than 10-2. For this reason a novel spectrometer was developed that enables the determination of the positron workfunction + and the temperature dependent moderation efficiency of various sample materials. An additional feature of the experimental setup is the ability to investigate the temperature dependence of positronium (Ps) formation. Either with three BGO detectors the emitted gamma-rays of the ortho-Ps decay can be detected in coincidence or the decay spectrum can be measured with a high resolution Ge detector. The layout of this novel spectrometer and simulations are presented.