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The resistive plate chamber (RPC) concept for time-of-flight positron emission tomography (TOF-PET) is based on the converter-plate gamma detection principle and takes advantage of the naturally layered structure of RPCs, of its simple and economic construction, excellent time resolution and very good intrinsic position accuracy. These characteristics may be of interest for the detailed imaging of small animals and for high-sensitivity whole-body human TOF-PET. In this communication we will present detailed simulations concerning the efficiency and imaging accuracy of the RPC-PET concept as a function of the main structural parameters, supplemented by measurements in a large-area chamber targeted at human RPC-PET.

We report the first results on the operation of an optical double GEM TPC, read out by four photomultipliers in a square array. The detector, operated with Ar + 5 % CF 4 at atmospheric pressure, was tested with 241Am alpha particles emitted in various orientations. We obtained a typical track angle resolution of 2 ∘ ( σ ) at 45 ∘ and achieved a point position resolution of better than 1 mm (FWHM) for the X- and Y-directions in the GEM plane.

The status of the resistive plate chamber (RPC)–PET technology for small animals is briefly reviewed and its sensitivity performance for human PET studied through Monte-Carlo simulations. The cost-effectiveness of these detectors and their very good timing characteristics open the possibility to build affordable Time of Flight (TOF)–PET systems with very large fields of view. Simulations suggest that the sensitivity of such systems for human whole-body screening, under reasonable assumptions, may exceed the present crystal-based PET technology by a factor up to 20.

A prototype composed of four resistive plate chamber layers has been exposed to quasi-monoenergetic neutrons produced from a deuteron beam of varying energy (300 to 1500 AMeV) in experiment S406 at GSI, Darmstad, Germany. Each layer, with an active area of about 2000 × 500 mm2, is made of modules containing the active gaps, all in multigap construction. Each gap is defined by 0.3 mm nylon mono-filaments positioned between 2.85 mm thick float glass electrodes. The modules are operated in avalanche mode with a non-flammable gas mixture composed of 90% C2H2F4 and 10% SF6. The signals are readout by a pick-up electrode formed by 15 copper strips (per layer), spaced at a pitch of 30 mm, connected at both sides to timing front end electronics. Measurements of the time of flight jitter of neutrons, in the mentioned energy range, point to a contribution of the resistive plate chamber in the order of 150 ps, independent of the neutron energy.

The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomultiplier tubes, with the purpose of studying the scintillation response of xenon to gamma-rays and neutrons, is described. Measurements with gamma-rays indicate a high VUV light collection efficiency resulting in ~5.5 photoelectrons per 1 keV of deposited energy. The energy resolution (FWHM) is 18% and 22%, for 122 keV and 511 keV gamma-rays, respectively. An algorithm for the reconstruction of the scintillation coordinates in (x,y) plane was developed and tested. The position resolution is estimated to be 6.9 mm (sigma) for 122 keV gamma-rays.

The accuracy of the pulse height weighting technique for the determination of neutron capture cross-sections is investigated. The technique is applied to measurements performed with C6D6 liquid scintillation detectors of two different types using capture samples of various dimensions. The data for well-known (n, gamma) resonances are analyzed using weighting functions obtained from Monte Carlo simulations of the experimental set-up. Several causes of systematic deviation are identified and their effect is quantified. In all the cases measured the reaction yield agrees with the standard value within 2%. (C) 2003 Elsevier B.V. All rights reserved. Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment

In this work, we report the development and performance of single-gap timing RPCs equipped with a 3 mm FWHM resolution bidimensional position-sensitive readout and reaching a timing resolution of 55 ps [sigma] at an efficiency of 75% for MIPs. These chambers are aimed to be applied in a multilayer geometry, forming small and very accurate TOF systems. The avalanche localization capability would provide information for accurate flight-path determination and allow the offline correction of any position dependent effects in the chamber, like gap inhomogeneities or edge effects. The multilayer geometry would provide redundant timing information, which, besides improving the timing resolution, can be used for reducing the timing tails and for the self-calibration of the device. From the experimental single-chamber data it can be calculated that a four-layer configuration of such chambers would yield a time resolution of 33 ps [sigma] at a 95% efficiency, essentially free of timing tails. http://www.sciencedirect.com/science/article/B6TJM-4891NNX-8/1/697dcc04cf9483865a813afea8aca954

In this work, the spatial resolution of a human wide axial field of view RPC-PET system was studied. It was verified that physical processes involving the detection of 511 keV photons do not degrade the spatial resolution, compared with that attainable with systems based on inorganic scintillation crystals coupled to photomultiplier tubes. Considering the photon acollinearity effect, a detector depth-of-interaction resolution of 3.44 mm, and detector readout binnings of 1.0 and 2.0 mm, the spatial resolutions computed as the mean of full width at half maximum of point spread functions in three orthogonal directions (X, Y, and Z) were 1.4±0.0 and 2.1±0.1 mm, respectively. The corresponding mean spatial resolutions computed as the full width at tenth maximum were 3.9±0.4 and 4.8±0.3 mm, respectively.

The successful operation of a liquid-xenon ionisation chamber with imaging capability is reported. A bi-dimensional readout based on a mini-strip plate—a glass slab with metal strips deposited along two perpendicular directions—was developed and studied both experimentally and by computation. An image of a 122 keV γ-source of 5 mm diameter was obtained with a liquid-xenon ionisation chamber equipped with such readout. A position resolution better than 2 mm was achieved.

In a previous work it has been pointed out that scintillation light, extending up to the infrared, emitted in microstructure based gaseous detectors (microstrips, microgaps, GEMs, etc.) can be used for non destructive testing of these detectors when they are associated to a CCD readout system. The choice of the gas mixture is an important issue, in so far as its emission spectrum should overlap efficiently the sensitivity region of the CCD (400-1100 nm). In the present work we report on a systematic study for several gas mixtures which includes measurements of the total light yields as a function of the electric field and of the spectrometric distribution of the light emitted, in the wavelength region between 250 and 930 nm. Results are presented for pure argon and argon and xenon based gas mixtures. A comparison is made between the results obtained with the CCD coupled to a GEM detector and with a gaseous scintillation proportional counter.

We developed a very low resistivity RPC-type detector, the anode of which was a plate made from materials with resistivity up to 5]107 ) cm, the cathode being a metallic mesh preceded by a drift region. In such a detector it was actually possible to combine the versatility and high counting-rate capability of metallic PPACs with the extreme robustness and ‘protectivenessa of Resistive Plate Chambers. Occasional discharges triggered by large deposits of primary ionisation or by extreme counting rates are quenched by the resistive anode and are constrained to the streamer phase of the sparking process. The study shows that this discharge a!ects the detector only locally and that the charge released is limited to a few tens of nC. Proportional counting rates up to 105 Hz/mm2 were achieved at gains above 104. The energy resolution at 6 keV was 20% FWHM. The observed gain-rate trade-o! is well described by an analytic model and further improvements may be expected by lowering the resistivity of the anode material. The properties of several custom-made, controllable resistivity, anode materials are described and prospects of improvement in the performance of the detector are discussed. ( 1999 Elsevier Science B.V. All rights reserved.

A multiwire ionisation chamber filled with liquid xenon as a position sensitive detector for Positron Emission Tomography is studied. Both xenon scintillation, which provides a fast trigger, and ionisation charge for position and energy measurements are read. The detection efficiency of the detector is assessed in separate measurements of the probability of detection of the scintillation light and the charge signals due to 511 keV gamma-rays. These were measured to be about 1 and 0.82, respectively.

Detectors with highly resistive electrodes, particularly RPCs, have been studied extensively in recent years. Several authors considered the effect of rate on the detector response (efficiency) and a few also associated with the rate, in a rather qualitative manner, the charge per event, i.e., the current. Here, without loss of generality of the results obtained, rate effects are studied with a classical cylindrical counter geometry and different gas mixtures. To a high accuracy, the observed behavior, both for the proportional mode and for the transition to streamers, is quantitatively and accurately interpreted as loss of voltage on the dielectric due to the current generated in the detector, with direct consequences for the local electric field in the avalanche region. Results obtained in the self quenching streamer (SQS) mode fail the previous interpretation and clearly need further research.

The coincidence time resolution and the intrinsic position resolution along the electron drift direction of a liquid xenon multiwire ionization chamber for PET were measured, being, respectively, 1.5 ns (fwhm) and 0.5 mm (fwhm). Data on the low temperature performance of a photomultiplier tube operating on the VUV are presented.

Some aspects of the performance of a liquid xenon PET detector prototype were studied by means of Monte Carlo simulation.

The liquid xenon multiwire chamber has been proposed as a detector of 511 keV gamma-photons for positron emission tomography (PET). Both the scintillation and the ionization signal, read from the anode wires, are used. A test chamber of about 300 cm3 and containing six multiwire ionization cells has been built. The cell is formed by two plane cathodes, 10 mm apart, and 20 anode wires spaced by 2.5 mm tied to pairs resulting in 10 channels, read out independently. The first experiments were performed for a single cell. The number of the anode wire on which the signal is induced gives the depth of interaction in the detector with a precision of 5 mm. This is important information allowing one to reduce the parallax error and, therefore, to decrease the tomograph size and improve the solid angle. The position sensitivity across the cell (or along the tomograph ring) is achieved by measurement of the drift time of the electrons from the point of primary ionization to the anode, triggered by the scintillation detected with a photomultiplier tube. The scintillation also supplies a fast trigger for the coincidence analysis.

The authors describe the time evolution of VUV photons emitted in Ar/CH4 and Ar/CO2 mixtures, during the development of electron avalanches occurring in cylindrical coaxial radiation detectors. Particular attention is paid to emissions considered as relevant to the photon feedback mechanisms, namely the C I atomic lines at 156.1, 165.7 and 193.1 nm. A correlation is obtained between the time spectra and the corresponding mean anode current signals, both for the proportional and the self-quenching streamer modes of operation. A comparison is also made between the data obtained with two detectors having different cathode diameters (16 and 30 mm).

The counting rate capability of RPCs is strongly conditioned by the availability of suitable resistive materials for the electrodes. For time-of-flight measurements in heavy-ion high-energy physics, the extension of the counting rate capabilities achievable with glass electrodes, around 2 kHz/cm2, to much higher values is of fundamental importance. To address this issue we developed single-gap tRPCs with electrodes made from metal and from a commercially available ceramic material free of charge-depletion effects. Tests performed with 511 keV photons yielded a time resolution around 90 ps [sigma]. Neither the time resolution nor the efficiency have been affected by background rates of up to 20 kHz/cm2 produced with photons from an X-ray generator. The present result establishes the practical feasibility of accurate timing measurements with RPCs at rates up to 20 kHz/cm2, while keeping a time resolution below 100 ps [sigma]. http://www.sciencedirect.com/science/article/B6TJM-4MCW883-5/1/fcc92c6a2416f5c510774013efa35d6f

A prototype composed of four resistive plate chamber layers has been exposed to quasi-monoenergetic protons produced from a deuteron beam of varying energy (200 to 800 AMeV) in experiment S406 at GSI, Darmstadt, Germany. The aim of the experiment is to characterize the response of the prototype to protons in this energy range, which deposit from 1.75 to 6 times more energy than minimum ionizing particles. Each layer, with an active area of about 2000 × 500 mm2, is made of modules containing the active gaps, all in multigap construction. Each gap is defined by 0.3 mm nylon mono-filaments positioned between 2.85 mm thick float glass electrodes. The modules are operated in avalanche mode with a non-flammable gas mixture composed of 90% C2H2F4 and 10% SF6. The signals are readout by a pick-up electrode formed by 15 copper strips (per layer), spaced at a pitch of 30 mm, connected at both sides to timing front end electronics. Results show an uniform efficiency close to 100% along with a timing resolution of around 60 ps on the entire 2000 × 500 mm2 area.

A microgap plate was tested in liquid xenon. The maximum anode voltage before discharges, 450 V, was not high enough for producing charge multiplication. Simulations of microgap and leak microstructure were done.

Following some previous work, we report here considerable improvements on the counting rate capability of timing RPCs by the use of ceramic electrodes with a resistivity of 109 [Omega][dot operator]cm. The X-ray sensitivity of the detector depends linearly on the counting rate with a slope of 9% per 100 kHz/cm2, free from charge depletion effects, while keeping a timing accuracy, measured with 511 keV synchronous photon pairs, around 90 ps [sigma] up to 75 kHz/cm2. http://www.sciencedirect.com/science/article/B6TVD-4KV4MR8-K/1/ea4432be2a0e736afc8220e837ad51bb

In this study we present experimental results from a first prototype of a positron emission tomography system based on the resistive plate chamber (RPC) technology. The system is composed of two counting heads, each one containing 16 single-gap RPC detectors capable of detecting the photon interaction point in the transaxial plane. Uniformity studies were performed for image resolution and sensitivity, yielding a rather uniform image resolution close to 0.3 mm FWHM across the field of view. The contribution of the photons noncolinearity effect to the intrinsic spatial resolution was also studied, causing a variation from 0.52 to 0.63 mm when the system diameter ranges from 60 to 120 mm, in agreement with calculations. http://www.sciencedirect.com/science/article/B6TJM-4K5HV7F-9/1/7b655e08199d2b28acc8050106c2271c

It has been demonstrated in previous work that the RPC-PET technology is able to deliver radioisotope image resolutions approaching the physical limits of the PET principle. Here we study, by simulation, the effect of the magnetic field on the positron range to evaluate whether the spatial resolution of the RPC-PET could be improved by using an intense magnetic field. Six positron emitters of interest to small animal PET imaging (18F, 11C, 15O, 68G, 62Cu and 86Y) are considered. Results suggest that a three-fold improvement on the spatial resolution may be obtained under a magnetic field of 10 T for the higher energy radioisotopes like 86Y or 62Cu, and by about 20% for the lower energy ones, like 18F or 11C. http://www.sciencedirect.com/science/article/B6TVD-4KV4MR8-15/1/efa4356ba16a85fd7740937bd0e672a9

We present imaging results of needle-like and planar 22Na sources obtained with a prototype of a high-acceptance small-animal positron emission tomograph based on resistive plate chambers (RPC-PET). The maximum-likelihood expectation-maximization (MLEM) reconstruction of the acquired data yielded an excellent and stable resolution of 0.4 mm FWHM.

In this study, we demonstrate a strong influence of the working temperature on the counting rate capability of large-area timing resistive plate chambers (RPCs) incorporating industrial flat glass electrodes. The effect relies on the sharp dependence of glass resistivity on temperature, approximately one order of magnitude for each 25 °C, allowing a considerable extension of the counting rate capability merely by a moderate warming of the detector. The results confirm operation at 1.5 kHz/cm 2 for irradiation with gamma photons, and may be extrapolated to 6 kHz/cm 2 for the detection of minimum ionizing particles (MIPs) with a timing resolution below 100 ps σ.

For many applications of RPCs to time-of-flight counters in heavy ion experiments the expansion to much higher values of the counting rate capability, so far limited to around 2 kHz/cm2 is of fundamental importance. To address this issue we developed single-gap timing RPCs with resistive electrodes made from a commercially available plastic material. Tests performed in photon beams yielded a time resolution around 90 ps [sigma], essentially unchanged from 2 kHz/cm2 to 27 kHz/cm2. This result establishes the basic feasibility of timing measurements with RPCs at rates of tens of kHz/cm2, keeping a time resolution below 100 ps [sigma] and using plastic electrode materials. http://www.sciencedirect.com/science/article/B6TJM-4CXHH16-1/1/294a04bd430f914a4663436357d765de

A small positron emission tomography system, based on the timing RPC technology has been built and tested. This first prototype is aimed at validating the expectations, derived from simulations, of a very high spatial resolution, which could be of value for the imaging of small animals. The system is composed of two counting heads, able to measure the photon interaction point in two dimensions, the transaxial dimension and the Depth of Interaction. Each head is composed of 16 independent stacked RPCs made from copper and glass (anode) electrodes. Point-like 22Na sources were so far reconstructed, using the standard filtered back-projection algorithm, with a spatial accuracy of 0.6 mm FWHM, free of parallax error. http://www.sciencedirect.com/science/article/B6TJM-4CXB0P8-4/1/5e71356440cf220c1ce79dbe820b564d

We show the feasibility of large area timing RPC detectors for experiments dealing with fragmentation-like primary interactions and/or high detector multiplicities, without significative degradation of the detector capabilities. The tested prototype is constituted by three independent cells placed into grounded aluminium boxes, which reduce the cross-talk to the level of 0.4%. Each 2 × 2 × 60 cm 3 box consists of a 4-gap glass–aluminium RPC. At 200 Hz / cm 2 the counters have shown a time resolution of 60 ps σ t 80 ps with 3 σ -tails below 8%. High mechanical robustness and homogeneity of the physical properties along the longitudinal direction of the detector were also observed.

The optical readout of GEMs, presented a few years ago, is currently being considered for several detector projects. Although using suitable gaseous mixtures the GEM avalanches emit a large number of photons in the visible and/or NIR regions, many gaseous mixtures of great interest for detectors scintillate mainly in the UV, and the readout of these detectors is usually inefficient or very expensive. In this work we report on the operation of GEMs coated with a wavelength converter. We show that using a GEM with an optimal geometry the wavelength conversion process can be quite efficient and does not degrade the intrinsic position resolution of the detector, an usual drawback of the utilisation of wavelength shifters. http://www.sciencedirect.com/science/article/B6TJM-4C7B66V-1/1/ea672f73675fd42cd4f864c479c5bfbe

Here, we report on the present state of development of a liquid xenon detector composed by multiwire ionisation chambers, which has been developed for positron emission tomography (PET). It uses the fast scintillation signal for coincidence and subsequent trigger of the event processing. The charge signals induced on the wires are used for position determination along the radial and transaxial directions of the tomograph, as well as for energy information. The following detector characteristics have been achieved: (i) coincidence time resolution better than 1.5 ns (fwhm); (ii) position precision along the radial direction equal to 5 mm; (iii) position resolution in the transaxial direction of 0.8 mm (fwhm); (iv) energy resolution of 17% (fwhm) for 511 keV /spl gamma/-rays; (v) efficiency of trigger or about 100%; (vi) overall efficiency of detection of 60%. The prospects for the development of this detector in view of its application in PET are discussed.

The time structure of light pulses from GEM detectors operated in scintillation mode with He/CF4 mixtures, previously used with integrating readout devices for neutron detection, has been investigated. A system using photomultipliers (PMs) and charge preamplifiers coupled to a fast digitizer has been used to measure simultaneously both the charge and light signals of the avalanches and the dependence of these signals on the operating conditions of the GEM, such as gain, transfer yield and collection field. The present results show that detectors relying on photon counting techniques, using of position sensitive PMs or arrays of avalanche photodiodes, are most promising as fast position sensitive neutron detectors. http://www.sciencedirect.com/science/article/B6TJM-48408MT-T/1/d28fe3c77462b34770621afe4d9da89d

Although the GEM has been mainly used as a gaseous preamplifier device, it has been shown that using suitable gaseous mixtures the avalanches emit a large number of photons, in the UV, visible and/or NIR bands. This scintillation, readout by CCDs, has been used recently for the development of imaging detectors to be used with X-rays, alpha particles and neutrons. The luminescence process in the GEM and a summary of its applications will be presented. The latest developments, including the operation of the GEMs in photon counting mode, will also be referred. http://www.sciencedirect.com/science/article/B6TJM-49FGX8X-B/1/95c4ce32cbe642552741e1461297c1de

The prototype of a recently proposed liquid xenon drift chamber for PET with simultaneous detection of scintillation and ionization signal has been built. First tests of the chamber under irradiation by 511 keV /spl gamma/-photons are reported. >

The applications of Resistive Plate Chambers (RPCs) have recently been extended by the development of counters with time resolution below 100 ps sigma for minimum ionising particles. Applications to HEP experiments have already taken place and many further applications are under study. In this work we address the operating principles of such counters along with some present challenges, with emphasis on counter aging., Presented at "PSD6 - 6th International Conference on Position Sensitive Detectors", 9-13 September 2002, Leicester, UK. Submitted to Nuclear Instruments and Methods A

We report on the optical readout of the gas electron multiplier (GEM) operated with a gaseous mixture suitable for the detection of thermal neutrons: 3He-CF4. A CCD system operating in the 400-1000 nm band was used to collect the light. Spectroscopic data on the visible and NIR scintillation of He-CF4 are presented. Images of the tracks of the proton and triton recorded with a triple GEM detector are also shown. http://www.sciencedirect.com/science/article/B6TJM-45HFC9M-2D/1/c77549a88a84ef08c2d169c0b4e97ed8

Resistive Plate Chambers (RPC) made with glass and metal electrodes forming accurately spaced gas gaps of a few hundred micrometers have reached timing accuracies below 50 ps σ with efficiencies above 99% for MIPs. This type of detector, operating at atmospheric pressure with non-flammable gases, seems well suited for large TOF systems, providing performances comparable to the scintillator-based TOF technology with significantly lower price per channel and being compatible with magnetic fields. In this work, we report recent developments of the timing RPC technology, including a large area counter (0.16 m 2 ) and smaller, position-sensitive, single-gap counters. The latter devices are aimed to be applied in small and accurate TOF systems using a multilayer structure. The large counter has shown a timing resolution between 50 and 75 ps σ over the whole active area, with 96% efficiency and very small timing tails, along with a longitudinal position-resolution of 1.2 cm σ . The single-gap counters had a timing resolution of 55 ps σ and a bi-dimensional position-resolution better than 3 mm FWHM. It was calculated that a 4-layer configuration of such counters would yield a time resolution of 33 ps σ essentially free from timing tails.

In the present paper, we report on our developments of liquid xenon detectors for medical imaging, positron emission tomography and single photon imaging, in particular. The results of the studies of several photon detectors (photomultiplier tubes and large area avalanche photodiode) suitable for detection of xenon scintillation are also briefly described., Comment: 13 pages, 5 figures, presented on the International Workshop on Techniques and Applications of Xenon Detectors (Xenon01), ICRR, Univ. of Tokyo, Kashiwa, Japan, December 3-4, 2001 (submitted to proceedings)

A large area Resistive Plate Chamber (RPC) with a total active surface of 160x10 cm2 was built and tested. The surface was segmented in two 5 cm wide strips readout on both ends with custom, very high frequency, front end electronics. A timing resolution between 50 and 75 ps sigma with an efficiency for Minimum Ionizing Particles (MIPs) larger than 95% was attained over the whole active area, in addition with a position resolution along the strips of 1.2 cm. Despite the large active area per electronic channel, the observed timing resolution is remarkably close to the one previously obtained (50 ps sigma) with much smaller chambers of about 10 cm2 area. These results open perspectives of extending the application of timing RPCs to large area arrays exposed to moderate particle multiplicities, where the low cost, good time resolution, insensitivity to the magnetic field and compact mechanics may be attractive when compared with the standard scintillator-based Time-of-Flight (TOF) technology., 22 pages, 10 figures (submitted to Nucl. Instr. and Meth.)

We present an overview of results from our recent studies on the use of the visible and NIR scintillation emitted by the gas electron multiplier (GEM) and on the possibility of using detectors operated with cascaded GEMs to build tracking chambers. http://www.sciencedirect.com/science/article/B6TJM-44DHYY7-10/1/0076b77e06804d2654d892c3d6126c90

Non destructive quality control of microstructures at the manufacturing stage is an important issue in the foreseen use of huge numbers of such gaseous detectors in the future high luminosity colliders. In this work we report on the use of the scintillation light emitted by the avalanches in GEM channels for checking defects in the foils. The test system is described and data on the relative efficiency of several gaseous mixtures are presented. The foil images obtained with a low-noise CCD system are analysed and compared with the optical images obtained with an industrial inspection system of high magnification. The validity of this test method is established and possible extensions of its use are discussed., 11 pages, 9 figures

Here we report on a particular type of RPC that presents up to 99% efficiency for minimum ionizing particles and a very sharp time resolution, below 50 ps sigma in the most optimized conditions. Our 9 cm2 cells, made with glass and metal electrodes that form accurately spaced gaps of a few hundred micrometers, are operated at atmospheric pressure in non-flammable gases and can be economically produced in large quantities, opening perspectives for the construction of large area time of flight systems., Comment: 10 pages, 8 figures

The left-right ambiguity in drift chambers can be solved, to a large extent, in a chamber operated in the self-quenched streamer mode, by using the charges induced in two cathode strips. An induced charge ratio of about 3 was obtained in the two cathode strips placed on each side of the anode wire, using the first 30 ns of the signal. The tests were carried out with a prototype of the forward muon drift chambers of DELPHI in which the only modification was the splitting of its central cathode plate in two.

One of the main drawbacks of the use of microgap structures when used at high pressure (6 bar) at high count rates (~106 s-1 mm-2) is the limited maximum safe gain at which they can be operated. The use of a preamplification device such as the GEM (gas electron multiplier) can overcome this limitation; however, secondary space charge effects due to charge accumulation at the non-metallic surfaces cannot be neglected and should be minimised. These positive charges are due to the intrinsic multiplication of the GEM device and also to the drift of ions from the anode of the main detector. In this study we present data on the variation of the positive ion collection by the drift electrode versus drift field at several pressures up to 6 bar using a Kr-CO2 mixture. Data were collected with microgaps having several anode and insulator widths. The influence of the drift field in the pulse rise time is also considered. http://www.sciencedirect.com/science/article/B6TJM-3VHW9D0-1H/1/7e6cb7ab07e3df379a279e38a943af61

The number of cyclotrons capable of accelerating protons to about 20 MeV is increasing throughout the world. Originally aiming at the production of positron emission tomography (PET) radionuclides, some of these facilities are equipped with several beam lines suitable for scientific research. Radiobiology, radiophysiology, and other dosimetric studies can be performed using these beam lines. In this work, we measured the Bragg peak of the protons from a PET cyclotron using a stacked target consisting of several aluminum foils interleaved with polyethylene sheets, readout by in-house made transimpedance electronics. The measured Bragg peak is consistent with simulations performed using the SRIM/TRIM simulation toolkit. Furthermore, we report on experimental results aiming at measuring proton beam currents down to 10 pA using a thin aluminum foil (20-μm-thick). The aluminum was chosen for this task because it is radiation hard, it has low density and low radiation activity, and finally because it is easily available at negligible cost. This method allows for calculating the dose delivered to a target during an irradiation with high efficiency, and with minimal proton energy loss and scattering.

he CERN Neutron Time-Of-Flight (n_TOF) facility is well suited to measure low cross sections as those of neutron-induced fission in subactinides. The cross section ratios of (nat)Pb and (209)Bi relative to (235)U and (238)U were measured using PPAC detectors and a fragment coincidence method that allows us to identify the fission events. The present experiment provides first results for neutron-induced fission up to 1 GeV. Good agreement is found with previous experimental data below 200 MeV. The comparison with proton-induced fission indicates that the limiting regime where neutron-induced and proton-induced fission reach equal cross sections is close to 1 GeV.

The liquid rare gases have excellent scintillation properties and there are increasingly good prospects for their use in radiation detectors having a wide range of applications in different fields, such as medical radionuclide imaging (Chepel, 1993; Chepel et al., 1994), high and medium energy particle physics (Akimov et al.,1993), astrophysics (Aprile et al.,1989). Despite the new photosensitive devices that have been developed in recent years, photomultipliers are still the photodetectors most widely used for the detection of the liquid rare gases scintillation light. To increase the efficiency of light detection it is very often mandatory to place the photomultiplier very near, or even inside, the detector, which means operating the PM at low temperature (the boiling point of xenon and krypton is -108"C and -153"C respectively). The manufacturers usually give -30°C as the lowest working temperature and very little information on the behaviour of photomultipliers at lower temperatures is available in literature (Ichige et al.,1993). Thus, it is necessary to study the performance of photomultipliers in the range of 7r: temperatures of interest in view of the liquid rare gases detectors.

A method is presented to solve the left-right ambiguity in single wire drift chambers operated in the Self-Quenching Streamer (SQS) mode. Experimental results obtained with a prototype of the DELPHI forward muon drift chambers are presented.

Following the development of a signal in a detector operated in self-quenching streamer mode, a small part of the anode wire becomes insensitive for some time due to the presence of localized space charge. Experimental evidence of azimuthal dependence of dead time is presented.