P. Fonte

Perspectives for positron emission tomography with RPCs

Abstract In this study, we address the feasibility and main properties of a positron emission tomograph (PET) based on RPCs. The concept, making use of the converter-plate principle, takes advantage of the intrinsic layered structure of RPCs and its simple and economic construction. The extremely good time and position resolutions of RPCs also allow the TOF-PET imaging technique to be considered. Monte-Carlo simulations, supported by experimental data, are presented and the main advantages and drawbacks for applications of potential interest are discussed.

Dielectron production in Ar+KCl collisions at 1.76A GeV

We present results on dielectron production in 40 Ar+KCl collisions at 1.76A GeV. For the first time ω mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ meson. At intermediate e + e invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for ]

Partial wave analysis of the reaction p(3.5 GeV)+p → pK+Λ to search for the “ppK−” bound state

Employing the Bonn-Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5GeV ) + p !pK + �. This reaction might contain information about the kaonic cluster ”ppK − ” via its decay into p�. Due to interference effects in our coherent description of the data, a hypothetical KNN (or, specifically ”ppK − ”) cluster signal must not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectra like p�. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a KNN cluster. At a confidence level of CLs=95% such a cluster can not contribute more than 2-12% to the total cross section with a pK + � final state, which translates into a production cross-section between 0.7 µb and 4.2 µb, respectively. The

Applications and new developments in resistive plate chambers

Resistive plate chambers are rugged and affordable gas detectors that have found extensive use in high-energy physics (HEP) and astroparticle experiments. The main features of these counters are the very large pulse height, reduced cost per unit area, and good (about 1 ns) time resolution. The field has enjoyed very lively progress in recent years, including the introduction of a new (avalanche) mode of operation, extension of the counting-rate capabilities to levels around 10 MHz/cm/sup 2/, improvement of the time resolution for minimum ionizing particles to 50 ps /spl sigma/, and the achievement of position resolutions of a few tens of micrometers. These new developments have extended the range of REP applications and promise new applications in medical imaging.

Surface streamer breakdown mechanisms in microstrip gas counters

Abstract We have studied breakdown mechanisms in MSGCs. For comparison, “microstrip detectors” without dielectric substrates were also tested. We found that detectors without substrates can always operate at gas gains 5–10 times higher than those with substrates, and that these higher gains are limited by self-quenched streamers. In the case of microstrip detectors with substrates, streamers also occur, but have a very narrow (in voltage) self-quench region and then transit rapidly to a “gliding” discharge. We have tested new geometries of MSGC which allow high gains (>10 5 ) to be reached.

Breakdown limit studies in high-rate gaseous detectors

Abstract We report results from a systematic study of breakdown limits for novel high-rate gaseous detectors: MICROMEGAS, CAT and GEM, together with more conventional devices such as thin-gap parallel-mesh chambers and high-rate wire chambers. It was found that for all these detectors, the maximum achievable gain, before breakdown appears, drops dramatically with incident flux, and is sometimes inversely proportional to it. Further, in the presence of alpha particles, typical of the breakgrounds in high-energy experiments, additional gain drops of 1–2 orders of magnitude were observed for many detectors. It was found that breakdowns at high rates occur through what we have termed an “accumulative” mechanism, which does not seem to have been previously reported in the literature. Results of these studies may help in choosing the optimum detector for given experimental conditions.

Feedback and breakdowns in microstrip gas counters

Abstract The phenomena of feedback and breakdowns in microstrip gas counters have been studied. Tests have been performed on a wide variety of microstrips of different geometries and substrates in various gas mixtures and at several pressures. In addition, for direct comparison, similar microstrip gas counters without substrates were also investigated. In the case of “conventional” microstrip detectors on substrates, it was found that as maximum gains were approached, self-quenched streamers occurred, but that these were present only over a very narrow voltage range after which they rapidly transited to gliding-discharge-type breakdown sparks. For microstrip detectors without substrates, the situation was quite different. At high gains (> ∼ 10 4 ) they transited from proportional to self-quenched streamer-mode and would operate stably in this mode over a very broad voltage range. When breakdown occurred in these detectors it was through a previously unreported mechanism and at gains, without exception, 5–10 times higher than in their counterparts with substrates. The conclusion of these studies is that the substrate represents the weak link in the chain and should ideally be avoided or at least removed from the high-field regions. In cases where this is not possible, then the multiplication region should be concentrated to the smallest volume around the anodes and maximally suppressed elsewhere. Recipes for achieving this are provided, along with new geometries which permit gas gains in excess of 10 5 to be achieved.

A Novel UV Photon Detector with Resistive Electrodes

In this study we present first results from a new detector of UV photons: a thick gaseous electron multiplier (GEM) with resistive electrodes, combined with CsI or CsTe/CsI photocathodes. The hole type structure considerably suppresses the photon and ion feedback, whereas the resistive electrodes protect the detector and the readout electronics from damage by any occasional discharges. This device reaches higher gains than a previously developed photosensitive RPC and could be used not only for the imaging of UV sources, ames or Cherenkov light, for example, but also for the detection of X-rays and charged particles.

Observation of electron multiplication in liquid xenon with a microstrip plate

Abstract We report here on the observation of electron multiplication in liquid xenon in a microstrip chamber with an amplification factor of the order of 10. The measurements were carried out at a temperature between 208 and 215 K (liquid density of about 2.7 g/cm 3 ).

A novel portal imaging device for advanced radiation therapy

At Karolinska Institutet in Stockholm, Sweden a new detector for portal imaging is under development, which could greatly improve the alignment of the radiation beam with respect to the tumor during radiation treatment. The detector is based on gaseous and solid converters combined with GEMs as an amplification structures. The detector will have a large area and will be operated in a very high rate environment. A prototype has been built and extensively tested, both with high rate X-rays as well as in a clinical environment at the Karolinska Hospital. High rates and alpha particles could cause discharges in the GEM and discharge propagation from GEM to GEM and to the readout electronics. Since reliability is one of the main requirements for the portal imaging device, the authors performed systematic studies to find a safe operating range of the device, free from typical high-rate problems, such as discharges.