A compact and gridless channel plate start detector

A small gridless time detector consisting of an electron emitting foil and two channel plates with central holes and without any grid or wire, i.e. with 100% transmission, is described. For α-particles a time resolution of 90 ps and an efficiency up to a maximum of about 50% have been obtained. The efficiency for heavily ionizing particles like evaporation residues or fission fragments is 100% and with an elastically scattered 26 MeV/amu ³²S beam an overall time resolution of 60 ps was achieved.     

Thermal neutron imaging detectors combining novel composite foil convertors and gaseous electron multipiers

The potential of a thermal neutron imaging system based on a composite neutron convertor foil combined with a low-pressure, multistep avalanche chamber is demonstrated. Neutron-induced charged particles from a primary convertor element induce multiple low-energy electrons escaping from a second thin film of high electron-emissive material. We investigated the performance of detectors with Gd- and Li-based primary convertors coated with CsI as a secondary electron emitter. It is shown, that the detector can be operated with high stability at a sufficiently high gain to detect all escaping particles. A localisation resolution of 0.4 mm (fwhm) was obtained. With Li-based convertors a very low γ-ray sensitivity was established. The good imaging performance, free of parallax errors in divergent neutron beams, fast time resolution, low occupation time and high count rate capability, make this instrument an excellent tool for time-resolved neutron scattering experiments and for neutron radiography and tomography.     

Accurate Measurement of the Velocity History of a Laser-Driven Foil Plate with a Push-Pull-Type VISAR

We describe a technique that provides high time resolution and high accuracy in the velocity-history measurement by coupling an electronic streak camera with a push-pull-type velocity interferometer system for any reflector. This technique shows strong potential for the study of the dynamic process associated with a rapid velocity change, such as the acceleration of a foil plate driven by a pulsed laser beam. Also, by using a micrometer-size spot optical probe, we demonstrate the acceleration histories of Al 10-mum-thick foil plates at laser intensities ranging from 30 to ~400 GW/cm(2) with a subnanosecond time resolution and within a 1-2% error for peak velocity.     

Development of cryogenic alpha spectrometers using metallic magnetic calorimeters

Cryogenic particle detectors have recently been adopted in radiation detection and measurement because of their high energy resolution. Many of these detectors have demonstrated energy resolutions better than the theoretical limit of semiconductor detectors. We report the development of a micro-fabricated magnetic calorimeter coupled to a large-area particle absorber. It is based on a planar, 1 mm² large paramagnetic temperature sensor made of sputtered Au:Er, which covers a superconducting meander-shaped pickup coil coupled to a low-noise dc-SQUID to monitor the magnetization of the sensor. A piece of gold foil of 2.5×2.5×0.07 mm³ was glued to the Au:Er film to serve as an absorber for incident alpha particles. The detector performance was investigated with an ²⁴¹Am source. The signal size comparison for alpha and gamma peaks with a large difference in energy demonstrated that the detector had good linear behavior. An energy resolution of 2.83±0.05 keV in FWHM was obtained for 5.5 MeV alpha particles.     

Time resolution of liquid argon detectors: Part I. Optimum filter

A simple dependence of the ultimate time resolution of a LAr detector on the transconductance of the FET, the capacitance of the detector, the ionization and purity of the argon is presented in this paper. For small capacitance of the detector (～ 100 pF) and for minimum ionization particles, the accuracy of time measurement is about ～ 10 ns.     

New 2-D dosimetric technique for radiotherapy based on planar thermoluminescent detectors

At the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ) in Kraków, a two-dimensional (2-D) thermoluminescence (TL) dosimetry system was developed within the MAESTRO (Methods and Advanced Equipment for Simulation and Treatment in Radio-Oncology) 6 Framework Programme and tested by evaluating 2-D dose distributions around radioactive sources. A thermoluminescent detector (TLD) foil was developed, of thickness 0.3 mm and diameter 60 mm, containing a mixture of highly sensitive LiF:Mg,Cu,P powder and Ethylene TetraFluoroEthylene (ETFE) polymer. Foil detectors were irradiated with (226)Ra brachytherapy sources and a (90)Sr/(90)Y source. 2-D dose distributions were evaluated using a prototype planar (diameter 60 mm) reader, equipped with a 12 bit Charge Coupled Devices (CCD) PCO AG camera, with a resolution of 640 x 480 pixels. The new detectors, showing a spatial resolution better than 0.5 mm and a measurable dose range typical for radiotherapy, can find many applications in clinical dosimetry. Another technology applicable to clinical dosimetry, also developed at IFJ, is the Si microstrip detector of size 95 x 95 mm(2), which may be used to evaluate the dose distribution with a spatial resolution of 120 microm along one direction, in real-time mode. The microstrip and TLD technology will be further improved, especially to develop detectors of larger area, and to make them applicable to some advanced radiotherapy modalities, such as intensity modulated radiotherapy (IMRT) or proton radiotherapy.     

Mass resolution of recoil fragment detector telescopes for 0.05–0.5 A MeV heavy recoiling fragments

The factors governing the mass resolution for 0.05–0.5 A MeV recoil nuclei have been investigated for detector telescopes in which carbon-foil time zero detectors and ion-implanted silicon detectors are used to determine the time of flight and energy respectively. Experimentally determined second moments of the mass distribution have been compared with theoretical estimates based on literature data. The experimental mass resolution is in reasonably good absolute agreement with theoretical estimates. For low energy (< 0.3 A MeV) particles the mass resolution is dominated by the contribution from the silicon detector and thus largely independent of timed flight length. In fact for detection of very low energy (0.1 A MeV) recoil nuclei timed flight lengths of less than 0.22 m are sufficient.     

Time-resolved diffraction and interference: Young's interference with photons of different energy as revealed by time resolution

We present time-resolved diffraction and two-slit interference experiments using a streak camera as a detector for femtosecond pulses of photons. These experiments show how the diffraction pattern is built by adding frames of a few photons to each frame. It is estimated that after 300 photons the diffraction pattern emerges. With time resolution we can check the speed of light and put an upper limit of 2 ps at our resolution to the time for wave function collapse in the quantum measurement process. We then produce interference experiments with photons of different energies impinging on the slits, i.e. we know which photon impinges on each slit. We show that for poor time resolution, no interference is observed, but for high time resolution, we have interference that is revealed as beats of 100 GHz frequency. The condition for interference is that the two pulses should overlap spatially at the detector, even if the pulses have different energies but are generated from the same pulse of the laser. The interference seems to be in agreement with classical theory at first sight. However, closer study and analysis of the data show deviations in the visibility of the interference fringes and of their phase. These experiments are discussed in connection with quantum mechanics and it may be concluded that the time resolution provides new data for understanding the longstanding and continuing arguments on wave-particle duality initiated by Newton, Young, Fresnel, Planck and others. A thought experiment is presented in the appendix to try to distinguish the photons at the detector by making it sensitive to colour.     

Improvement of time resolution in a TOF PET system with the use of BaF2 crystals

In the course of the construction of a new type of time-of-flight positron emission tomograph (TOF PET), the time resolution of a system with BaF₂ crystals has been tested. It was found that low-viscosity silicon oil is one of the best optical coupling media between the BaF₂ crystal and the photomultiplier tube, and that the time resolution is less sensitive to the geometry between the detector and the source position. The dependence of the time resolution on the size of the BaF₂ crystal has been determined as well. Absorption and attenuation of photons in the detector system are expected to be of particular importance for the better performance of the TOF PET.     

A modular NaI(Tl) detector for intermediate energy photons

We describe the properties of a detector array made up of 64 NaI(Tl) 406 × 63 × 63 mm³ modules, used as an intermediate energy photon spectrometer. We obtain an energy resolution of 6% FWHM at 129 MeV, a time resolution of 1 ns FWHM and a resolution of 48 mm FWHM for the location of the impact point on the front face of the detector. The modularity allows to some extent a discrimination between photons and neutrons. We also present the response of the detector to 69 MeV neutrons.     

Time-of-flight neutron imaging at a continuous source: Proof of principle using a scintillator CCD imaging detector

Energy resolved imaging has recently gained attention for the potential of spatially resolved texture, crystallographic phase and strain investigations. Especially a time-of-flight (TOF) approach that takes maximum advantage of the new generation of pulsed spallation neutron sources is currently in the focus of investigations. Here, we present results of corresponding TOF measurements recorded at the continuous source of Helmholtz Center Berlin. The critical component for TOF imaging is however the availability of a high resolution imaging detector capable of the required time resolution. Here, a gated time-integrating detector without the corresponding continuous time resolution has been used and the measurements therefore have to be interpreted as proof-of-principle experiments as will be discussed. Measurements of different series of samples revealing structural differences related to their crystalline structure will be presented as well as a strain measurement on a dieless drawn wire.     

A driftless gas proportional scintillation counter for muonic hydrogen X-ray spectroscopy under strong magnetic fields

An experiment involving muonic hydrogen requires an X-ray detector having large area and working under strong magnetic fields (5 T) with good energy and timing resolution. A compact, driftless gas proportional scintillation counter (GPSC) capable of operating under such magnetic fields is investigated. This GPSC uses a CsI photocathode deposited onto a microstrip plate as the UV scintillation readout photosensor. This photocathode has the advantage of operating in direct contact with the scintillation gas. The detector is filled with pure xenon and is designed to have a high detection efficiency for 2 keV X-rays. Energy resolutions of 23% and 22% were obtained for 1.74 and 2.3 keV X-rays, respectively. The low-energy detector limit due to the electronic noise is 300 eV. Its performance in the presence of strong magnetic fields was tested. At magnetic field of 5 T the detector pulse amplitudes are reduced by less than 25%, while the detector energy resolution and pulse rise time present a relative increase of less than 10%.     

A detector for filtering γ-ray spectra from weak fusion–evaporation reactions out of strong background and for Doppler correction: The recoil filter detector, RFD

A detector has been designed and built to assist in-beam γ-ray spectroscopy with fusion–evaporation reactions. It measures with high efficiency the evaporation residues that recoil out of a thin target into the angular interval from 1.8° to 9.0° at an adjustable distance of 1000–1350 mm from a target, in coincidence with γ-rays detected in a Ge-detector array. This permits filtering of such γ-rays out of a much stronger background of other reaction products and scattered beam. Evaporation residues are identified by their time-of-flight and the pulse height using a pulsed beam. The velocity vector of the γ-emitting recoil is also measured in the event-by-event mode, facilitating to correct the registered γ-ray energy for the Doppler shift, with the resulting significant improvement of the energy resolution. The heavy-ion detection scheme uses emission of secondary electrons caused by the recoiling ions when hitting a thin foil. These electrons are then electrostatically accelerated and focused onto a small scintillator that measures the summed electron energy, which is proportional to the number of electrons. The detector is able to operate at high frequency of the order of 1 MHz and detect very heavy nuclei with as low kinetic energy as 5 MeV. The paper describes the properties of the detector and gives examples of measurements with the OSIRIS, GAREL+ and EUROBALL IV γ-ray spectrometers. The usefulness of the technique for spectroscopic investigations of nuclei with a continuous beam is also discussed.     

Method of calculating the image resolution of a near-infrared time-of-flight tissue-imaging system

A new method for calculating the image resolution for a near-infrared time-of-flight tissue-imaging system is presented. The image resolution is calculated from the full width at half-maximum of the photon-path function along the midplane of the medium, integrated over all times of flight, and weighted by the time-resolved detector response. Detailed treatment of the optical gating mechanism shows that for some types of gating mechanism, there exists an optimal gating time beyond which the image resolution is not improved by arbitrarily decreasing the gating time. This theory predicts a limiting image resolution of~20% of the medium thickness, which is consistent with the research of others.     

High-efficiency fast scintillators for "optical" soft x-ray arrays for laboratory plasma diagnostics

Scintillator-based "optical" soft x-ray (OSXR) arrays have been investigated as a replacement for the conventional silicon (Si)-based diode arrays used for imaging, tomographic reconstruction, magnetohydrodynamics, transport, and turbulence studies in magnetically confined fusion plasma research. An experimental survey among several scintillator candidates was performed, measuring the relative and absolute conversion efficiencies of soft x rays to visible light. Further investigations took into account glass and fiber-optic face-plates (FOPs) as substrates, and a thin aluminum foil (150 nm) to reflect the visible light emitted by the scintillator back to the optical detector. Columnar (crystal growth) thallium-doped cesium iodide (CsI:Tl) deposited on an FOP, was found to be the best candidate for the previously mentioned plasma diagnostics. Its luminescence decay time of the order of approximately 1-10 micros is thus suitable for the 10 micros time resolution required for the development of scintillator-based SXR plasma diagnostics. A prototype eight channel OSXR array using CsI:Tl was designed, built, and compared to an absolute extreme ultraviolet diode counterpart: its operation on the National Spherical Torus Experiment showed a lower level of induced noise relative to the Si-based diode arrays, especially during neutral beam injection heated plasma discharges. The OSXR concept can also be implemented in less harsh environments for basic spectroscopic laboratory plasma diagnostics.     

A BaF2-MWPC gamma camera for positron emission tomography

The construction and performance of a position sensitive detector for 511 keV annihilation radiation are described. The detector consists of a barium fluoride crystal sandwiched between two wire chambers which are operated with 45 and 50°C TMAE vapour. Energy, time and position resolution are presented. The advantages and limitations of the detector are discussed. We also present some preliminary data on other UV scintillators.     

Time resolution study of a CPPM 320 UV microchannel-plate photomultiplier with a BaF2 scintillation crystal

The time resolution has been measured of a BaF₂ scintillator mounted onto an ITL CPPM 320 UV channelplate photomultiplier. We obtained a intrinsic time resolution of 235 ps (fwhm) at 1.3 MeV. Compared to 120 ps for a Pilot U-XP2020 reference detector this result is rather poor. The explanation is found in the relatively poor quantum efficiency of the channelplate photomultiplier.     

Time-multiplexed three-dimensional displays based on directional backlights with fast-switching liquid-crystal displays

An autostereoscopic display using a directional backlight with a fast-switching liquid-crystal (LC) display was designed and fabricated to obtain a better perception of 3D images by enhanced resolution and brightness. A grooved light guide in combination with an asymmetric focusing foil was utilized to redirect the emitting cones of light to the left and right eyes, respectively. By designing the groove structures of the focusing foil with rotation from -1.5 degrees to 1.5 degrees in the gradient and having the pitch ratio of the grooved light guide to the focusing foil of less than 3, the boundary angle then shifts from normal viewing and the moiré phenomenon can be suppressed. Cross talk of less than 6% and a LC response time of faster than 7.1 ms further improve the stereoscopic image perception. Additionally, 2D-3D compatibility is provided.     

Dose distribution around a needle-like anode X-ray tube: dye-film vs. planar thermoluminescent detectors

The dosimetry around the X-ray tube with a needle-like anode (NAXT), developed at the Institute of Nuclear Studies, for interstitial brachytherapy has been performed using (1) dye films (Gafchromic XR-T), (2) large-area thermoluminescent (TL) detectors--prepared either by gluing TL powder onto thin Al foil (so-called planar detectors with spatial resolution of 0.1 mm) and (3) miniature (2 mm diameter and 0.5 mm thick) TL detectors. The measurements were performed in following geometries. (1) Needle inside a PMMA cylinder--the planar TL detector mounted on the surface of the cylinder. (2) Needle inside a thick block of PMMA and TL detector mounted vertically 7 mm from needle axis. TL detectors were read with the planar (2D) thermoluminescence reader, developed at IFJ, with a sensitive CCD (charge couple device) camera. Gafchromic films were evaluated with a system based on Agfa Arcus 1200 scanner and calibrated with X rays (35 kV) filtered with 0.03 mm Mo and with Co-60 photons. The intensity distribution of TL light on the planar detector was calibrated in terms of absorbed dose to water, using (137)Cs gamma-rays. TL planar detectors seem to be a promising tool for 2D dosimetry of miniature X-ray sources. Obtained results for TLDs and Gafchromic films seem to be comparable but differences have been found. Both methods are useful for measurements of dose distribution around the NAXT X-rays source.