Matthias Kronberger

Searching for Faint Planetary Nebulae Using the Digital Sky Survey

Recent Hα surveys such as SHS and IPHAS have improved the completeness of the Galactic planetary nebula (PN) census. We now know of ∼3000 PNe in the Galaxy, but this is far short of most estimates, typically ∼25 000 or more for the total population. The size of the Galactic PN population is required to derive an accurate estimate of the chemical enrichment rates of nitrogen, carbon, and helium. In addition, a high PN count (>20 000) is strong evidence that most main-sequence stars of mass 1-8 M⊙ will go through a PN phase, while a low count (<10 000) argues that special conditions (e.g. close binary interactions) are required to form a PN. We describe a technique for finding hundreds more PNe using the existing data collections of the digital sky surveys, thereby improving the census of Galactic PNe.

Probing the Concepts of Photonic Crystals on Scintillating Materials

The high refractive index of current scintillating materials puts severe restrictions on their effective light yield. In this paper, we describe an approach that uses a photonic crystal pattern machined into the coupling face of the scintillator to partly overcome the problem of total internal reflection. Simulations are performed for 2 mm times 2 mm times 8 mm LuAP and LSO pixels with and without photonic crystal and different types of wrapping. It is shown that by tuning the structure of the photonic crystal and the size of its elements, the extraction efficiency of the surface can be significantly improved compared to a plain exit surface.

Identifying close binary central stars of PN with Kepler

Six planetary nebulae (PN) are known in the Kepler space telescope field of view, three newly identified. Of the 5 central stars of PN with useful Kepler data, one, J193110888+4324577, is a short-period, post common envelope binary exhibiting relativistic beaming effects. A second, the central star of the newly identified PN Pa5, has a rare O(He) spectral type and a periodic variability consistent with an evolved companion, where the orbital axis is almost aligned with the line of sight. The third PN, NGC~6826 has a fast rotating central star, something that can only be achieved in a merger. Fourth, the central star of the newly identified PN Kn61, has a PG1159 spectral type and a mysterious semi-periodic light variability which we conjecture to be related to the interplay of binarity with a stellar wind. Finally, the central star of the circular PN A61 does not appear to have a photometric variability above 2 mmag. With the possible exception of the variability of Kn61, all other variability behaviour, whether due to binarity or not, would not easily have been detected from the ground. We conclude, based on very low numbers, that there may be many more close binary or close binary products to be discovered with ultra-high precision photometry. With a larger number of high precision photometric observations we will be able to determine how much higher than the currently known 15 per cent, the short period binary fraction for central stars of PN is likely to be.

A Time-Based Front End Readout System for PET & CT

In the framework of the European FP6s BioCare project, we develop a novel, time-based, photo-detector readout technique to increase sensitivity and timing precision for molecular imaging in PET and CT. The project aims to employ Avalanche Photo Diode (APD) arrays with state of the art, high speed, front end amplifiers and discrimination circuits developed for the Large Hadron Collider (LHC) physics program at CERN, suitable to detect and process photons in a combined one-unit PET/CT detection head. In the so-called time-based approach our efforts focus on the systems timing performance with sub-nanosecond time-jitter and -walk, and yet also provide information on photon energy without resorting to analog to digital conversion. The bandwidth of the electronic circuitry is compatible with the scintillators intrinsic light response (e.g. les40ns in LSO) and hence allows high rate CT operation in single-photon counting mode. Based on commercial LSO crystals and Hamamatsu S8550 APD arrays, we show the system performance in terms of timing- and energy resolution as well as its rate behavior (SPICE, simulating a high intensity X-ray beam). If proven viable, this technique may lead to the construction of a compact, radiation tolerant, and cost effective PET/CT detection head in one unit.

Improving light extraction from heavy inorganic scintillators by photonic crystals

The impact of photonic crystal (PhC) slabs on the extraction of light from the heavy inorganic scintillators LuYAP and LYSO is evaluated by combining numerical transmission calculations of a scintillator with PhC coupling face with simulations of the light propagation inside the scintillator. The transmission of the scintillator-PhC coupling face is determined by means of a scattering-matrix algorithm. The PhC slab is assumed to consist of a bulk material with a triangular pattern of air holes that is sandwiched between the scintillator substrate and a layer of optical grease. By folding these data with the angular distribution of the scintillation photons arriving at the coupling face, the light collection efficiency η<i>L</i> of the system is estimated. The results indicate that a scintillator coupling face equipped with a PhC slab can exhibit a significant gain in η<i>L</i>. This gain is due to the extraction of photons that are lost in a scintillator with plain exit surface due to total internal reflection. The largest simulated gains of up to a factor of two are observed for small scintillators and for PhC coupling faces with <i>n</i>_bulk ≪ <i>n</i>_sub, <i>n</i>_avg ≈ √(<i>n</i>_sub·<i>n</i>_grease), and <i>d</i> ≤ <i>a</i>, where <i>n</i>_bulk, <i>n</i>_sub and <i>n</i>_grease are the respective refractive indexes of the PhC bulk, the scintillator substrate, and the optical grease, <i>a</i> the lattice constant of the PhC pattern, <i>d</i> the thickness of the PhC slab, and <i>n</i>_avg the average refractive index of the PhC slab determined by <i>n</i>_bulk and the filling factor <i>f</i>. Due to the approximations and idealizations of the model, these gains in light collection efficiency may be lower in practical applications and are expected to be achieved only with specular reflectors with reflectivities above 90%, and PhC bulk materials with absorption coefficients α_abs ≤ 10³-10⁴ cm^-1 over the whole wavelength range of emission of the scintillator.

Commissioning of the new H- source for Linac4

As part of the CERN accelerator complex upgrade, a new linear accelerator for H(-) (Linac4) will start its operation in 2014. The source for this linac will be a 2 MHz rf driven H(-) source which is a copy of the very successful source from DESY. In this paper the design and the first results of the commissioning are reported. The commissioning has progressed successfully, and no major obstacles have been identified which will prevent reaching the goal of 80 mA H(-) beam current, 45 keV beam energy, 0.4 ms pulse length, and 2 Hz repetition rate. The source is producing up until now a stable beam of 23 mA, 35 keV, and with a repetition rate of 0.83 Hz.

Determination of the absolute light yields of LuYAP and LYSO

The results of light yield (LY) measurements of 2×2×8 mm³ and 2×2×10 mm³ LuYAP and LYSO pixels are compared with Monte-Carlo simulations of equivalent systems performed with the light ray tracing program LITRANI. It is shown that the differences in LY observed for different setup configurations are well explained by the different light collection efficiencies η<inf>L</inf> of the setups derived from the simulations. Combining the results on LY and η<inf>L</inf> for a reference system makes it possible to determine the absolute light yield LY<inf>abs</inf> for LuYAP and LYSO.

High duty factor plasma generator for CERN's Superconducting Proton Linac

CERNs Linac4 is a 160 MeV linear accelerator currently under construction. It will inject negatively charged hydrogen ions into CERNs PS-Booster. Its ion source is a noncesiated rf driven H(-) volume source directly inspired from the one of DESY and is aimed to deliver pulses of 80 mA of H(-) during 0.4 ms at a 2 Hz repetition rate. The Superconducting Proton Linac (SPL) project is part of the luminosity upgrade of the Large Hadron Collider. It consists of an extension of Linac4 up to 5 GeV and is foreseen to deliver protons to a future 50 GeV synchrotron (PS2). For the SPL high power option (HP-SPL), the ion source would deliver pulses of 80 mA of H(-) during 1.2 ms and operate at a 50 Hz repetition rate. This significant upgrade motivates the design of the new water cooled plasma generator presented in this paper. Its engineering is based on the results of a finite element thermal study of the Linac4 H(-) plasma generator that identified critical components and thermal barriers. A cooling system is proposed which achieves the required heat dissipation and maintains the original functionality. Materials with higher thermal conductivity are selected and, wherever possible, thermal barriers resulting from low pressure contacts are removed by brazing metals on insulators. The AlN plasma chamber cooling circuit is inspired from the approach chosen for the cesiated high duty factor rf H(-) source operating at SNS.

A new extraction system for the Linac4 H− ion sourcea)

As part of the CERN accelerator complex upgrade, a new linear accelerator for H(-) (Linac4) is under construction. The ion source design is based on the non-caesiated DESY RF-driven ion source, with the goal of producing an H(-) beam of 80 mA beam current, 45 keV beam energy, 0.4 ms pulse length, and 2 Hz repetition rate. The source has been successfully commissioned for an extraction voltage of 35 kV, corresponding to the one used at DESY. Increasing the extraction voltage to 45 kV has resulted in frequent high voltage breakdowns in the extraction region caused by evaporating material from the electron dump, triggering a new design of the extraction and electron dumping system. Results of the ion source commissioning at 35 kV are presented as well as simulations of a new pulsed extraction system for beam extraction at 45 kV.

Magnetic Cusp Configuration of the SPL Plasma Generator

The Superconducting Proton Linac (SPL) is a novel linear accelerator concept currently studied at CERN. As part of this study, a new Cs‐free, RF‐driven external antenna H− plasma generator has been developed to withstand an average thermal load of 6 kW. The magnetic configuration of the new plasma generator includes a dodecapole cusp field and a filter field separating the plasma heating and H− production regions. Ferrites surrounding the RF antenna serve in enhancing the coupling of the RF to the plasma. Due to the space requirements of the plasma chamber cooling circuit, the cusp magnets are pushed outwards compared to Linac4 and the cusp field strength in the plasma region is reduced by 40% when N‐S magnetized magnets are used. The cusp field strength and plasma confinement can be improved by replacing the N‐S magnets with offset Halbach elements of which each consists of three magnetic sub‐elements with different magnetization direction. A design challenge is the dissipation of RF power induced by edd...