The NEPOMUCupgradeand advanced positron beam experiments

Abstract

The neutron-induced positron source NEPOMUC at the FRM II provides a mono-energetic positron beam of high intensity of the order of 10 9 moderated positrons per second. The new layout of NEPOMUC upgrade is presented and the constraints for operating an in-pile positron source at a research reactor are discussed. Inside the tip of the new beam tube, 80% 113 Cdenriched Cd is used as a neutron-γ -converter that has a projected lifetime of 25 years of reactor operation and thus ensures positron beam experiments in the long term. The source consists of Pt foils that both generate positrons, by pair production, and moderate them. The layout of these foils, the electric lenses and the magnetic fields for positron extraction and beam formation have been improved. In addition to a higher beam intensity, it is expected that the beam brightness will improve by at least one order of magnitude. The present and planned experiments range from fundamental studies in nuclear, atomic and plasma physics to high-sensitivity and element-selective investigations in surface and solid state physics to applications in materials science. The upgrade of several positron spectrometers as well as new positron beam experiments are presented. In addition, a new switching and remoderation unit will allow us to toggle from the high-intensity primary beam to a brightness enhanced remoderated positron beam.

Characterization of the NEPOMUC primary and remoderated positron beams at different energies

“…Geometrically this remoderator set-up is located between the primary Pt source and the position of the user experiments. The kinetic energy of the positrons within NEPOMUC is set, for the primary beam, as the potential difference between the extracting Pt structure and the beam line [9]. For the remoderated beam, the energy is set by biasing the W remoderator crystal accordingly.…”

“…The results for the remoderated beam at the three different energies are summarized in Table 3. For completeness we include also the beam brightness which can be defined [9] as…”

“…Another novel topic is the creation of an electronpositron plasma in the laboratory, as suggested by some of the current authors [7,8]. Many of these projects benefit from the use of high-flux, high-brightness positron beams where the beam brightness is defined as flux normalized to the product of the beam diameter squared and the perpendicular energy [9]. One of the most intense sources of low-energy positrons worldwide is the NEutroninduced POsitron source MUniCh (NEPOMUC) [9,10], which is part of the FRM II research reactor in Garching, Germany.…”

“…Many of these projects benefit from the use of high-flux, high-brightness positron beams where the beam brightness is defined as flux normalized to the product of the beam diameter squared and the perpendicular energy [9]. One of the most intense sources of low-energy positrons worldwide is the NEutroninduced POsitron source MUniCh (NEPOMUC) [9,10], which is part of the FRM II research reactor in Garching, Germany. Here, high-energy γ-rays produced from neutron-capture in 113 Cd generate positrons by pair production in Pt foils.…”

Characterization of point defects in CdTe by positron annihilation spectroscopy

“…Processing protocols are commonly designed, in part, to suppress inferred relevant point defects, progress in unambiguous experimental identification in device, or near device, material is of clear importance. Facilities are in place that enable both PALS and CDBS to be performed on thin films, 38,39 in addition to the more widely available conventional studies of bulk samples.…”