Operation principles and properties of the multi-GEM gaseous photomultiplier with reflective photocathode

Gas Electron Multipliers with a reflective photocathode deposited on the surface of the first multiplying element are very attractive devices for photon detection and imaging over large area at moderate cost. They combine production and operation simplicity, high sensitivity to single photons, fast time response and accurate localization. In this work we present in detail the mechanisms governing the operation of these photon detectors. The results of electron extraction, transfer, multiplication and detection processes in this multi-element structure are presented and analyzed. We discuss the role of important elements and parameters influencing the detector's operation and performance: the GEM geometry, the choice of the different electric fields and the gas mixture.     

Test of a BaF2-TMAE detector for positron-emission tomography

A detector consisting of BaF₂ scintillators and wire chambers has been tested for 511 keV gamma-rays. The wire chamber is filled with photosensitive tetrakis(dimethylamine)ethylene (TMAE) vapour and is operated at a pressure of a few Torr, using different gases at various temperatures. Energy, time, and position resolutions are given for BaF₂ crystals with sections of 10 × 10 mm² and 5 × 5 mm². We discuss the potential of this gamma detector for positron-emission tomography (PET) and compare it with other systems.     

Performance measurements from LYSO scintillators coupled to a CMOS position sensitive SSPM detector

We have designed a 5×5 mm² position sensitive solid-state photomultiplier (PS-SSPM) using a complementary metal-oxide-semiconductor (CMOS) process that provides imaging capability on the micro-pixel level. The PS-SSPM has 11,664 micro-pixels total, with each having an active area and micro-pixel pitch of 30×30 μm² and 44.3 μm, respectively. The PS-SSPM was then examined for its performance characteristics such as its energy and spatial resolution, and LYSO scintillator array imaging capabilities. When coupled to 5×5×3 mm³ LYSO, the energy resolution at 511 keV (²²Na) was measured as a function of bias, and corrected for the PS-SSPM non-linear output. The resolution is 14% (FWHM) at 511 keV with 30 V bias. The LYSO coincidence timing resolution was 9.4 ns (FWHM) at 511 keV. Spatial resolution studies were conducted using a focused (∼30 μm beam spot diameter) pulsed 635 nm diode laser. Scintillator array imaging studies were conducted at 511 keV using a 6×6 LYSO array, having 500 μm pixels (530 μm pitch) and 5 mm tall.     

High accuracy imaging of single photoelectrons by low-pressure Multistep Avalanche Chamber coupled to a solid photocathode

A position sensitive Multistep Avalanche Chamber (MSC), operated with low pressure isobutane (5–20 Torr) was coupled to a solid photocathode. It is shown that gaseous amplification factors above 10⁸ are obtained, thus enabling the efficient detection of single photoelectrons. Imaging of single photoelectrons was performed using the delay-line technique. Position resolution of the order of 200 μm (fwhm) is obtained with a photoelectron detection efficiency close to 100%. Further applications are discussed.     

Solar-driven microbial photoelectrochemical cells with a nanowire photocathode

We report a self-biased, solar-driven microbial photoelectrochemical cell (solar MPC) that can produce sustainable energy through coupling the microbial catalysis of biodegradable organic matter with solar energy conversion. The solar MPC consists of a p-type cuprous oxide nanowire-arrayed photocathode and an electricigen (Shewanella oneidensis MR-1)-colonizing anode, which can harvest solar energy and bioenergy, respectively. The photocathode and bioanode are interfaced by matching the redox potentials of bacterial cells and the electronic bands of semiconductor nanowires. We successfully demonstrated substantial current generation of 200 μA from the MPC device based on the synergistic effect of the bioanode (projected area of 20 cm2) and photocathode (projected area of 4 cm2) at zero bias under white light illumination of 20 mW/cm2. We identified the transition of rate-limiting step from the photocathode to the bioanode with increasing light intensities. The solar MPC showed self-sustained operation for more than 50 h in batch-fed mode under continuous light illumination. The ability to tune the synergistic effect between microbial cells and semiconductor nanowire systems could open up new opportunities for microbial/nanoelectronic hybrid devices with unique applications in energy conversion, environmental protection, and biomedical research.     

Optical absorption of BaF2 crystals irradiated with γ-rays

This paper describes some ways of producing F-centres at room temperature in BaF₂ single crystals, and contains an attempt at understanding the phenomena involved therein.     

Spontaneous ring-like self-assembly of BaF2 nanoparticles

Self-assembly of BaF2 nanoparticles was described. BaF2 nanoparticles were prepared by microemulsion technology. The self-assembly of the particles is spontaneous without coating reagent on the surface or external force being applied during the procedure of sample preparation. XRD examination and ICP data showed the phase purity of the final product; FTIR spectroscopy confirmed that there was no organic species leaved in the product. By depositing one drop of colloid solution containing BaF2 particles on the TEM grid directly, we can get the ring-like self-assembly with larger particles dispersing peripherally to form a ring and smaller particles inside this ring forming circles.     

Properties of BGO and BaF2 detectors

Time and energy response of large BGO and BaF₂ crystals to intermediate energy ions was measured. The timing properties of BaF₂ are close to that of a fast plastic scintillator.     

Microstructure and scintillation characteristics of BaF2 ceramics

This paper examines how the transformation of BaF₂ from a single-crystalline to polycrystalline state as a result of uniaxial hot-forming influences its properties. With increasing strain, the microstructure of the material changes from a nonuniform polycrystalline one, with a block size of about 1 mm, to a classic ceramic structure made up of grains in which dislocations form walls, resulting in a subgrain structure. The best mechanical characteristics are offered by the ceramic produced at a strain on the order of 700%. The intensity of the fast X-ray luminescence component (band at 220 nm) is independent of strain, whereas that of the slow component (band at 310 nm) decreases with increasing strain.     

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.     

Selective etching and dissolution of BaF2 crystals

The selective etching and dissolution of BaF₂ crystals have been studied in aqueous solutions of inorganic salts, inorganic and organic acids. The effect of additives on selective etch rates is demonstrated. It is observed that chemical exchange reactions determine the selective etching and dissolution of BaF₂ crystals. The form of etch pits corresponds to the real dissolution form of BaF₂ crystals (rhombododecahedron). It has been established that the process of dissolution in HNO₃ and HCl is diffusion controlled. The activation energy is independent of acid concentration and has the same value for both HNO₃ and HCl. The pre-exponential factor is concentration dependent. An empirical equation for dissolution kinetics has been obtained.     

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.     

Use of a pulsed magnetic field to increase the position sensitivity of a photomultiplier

It is well known that photomultipliers (PMTs) are sensitive to external magnetic fields. We have used this property to vary the gain of the PMT depending on the position of the light source. Various coil configurations have been evaluated to produce local magnetic fields in the region between the photocathode and first dynode to maximize the amplitude variation in the anode signal with respect to change in the coil current. The aim was to improve the position sensitivity of a PMT used in positron emission tomography (PET). The position sensitivity was tested with an array of collimated light emitting diodes directed towards the photocathodes of a Hamamatsu R1548 PMT. The best coil position was above the photocathodes. A 60% reduction in output for light pulses from two LEDs near the coil was obtained with a current of 43 Ampere-turns while signals from two LEDs far from the coil remained the same. The rise time of the magnetic field was 2 μs.     

Performance of a BGO calorimeter with photodiode readout and with photomultiplier readout at energies up to 10 GeV

Bismuth germanate (BGO) calorimeter arrays, consisting of up to 12 elements of 30 × 30 × 200 mm³ have been tested at the CERN PS with pions and electrons of up to 10 GeV/c momentum, and at SIN with pions, electrons and protons up to 450 MeV/c. Both photomultiplier (PM) and photodiode (PD) readouts were used. Accurate calibration in the 100 MeV energy range was achieved with stopping protons, stopping pions and minimum ionizing pions. With 212 MeV electrons and PM readout, a time resolution of the BGO signal of 640 ps fwhm has been measured. The energy resolution for electrons above 1 GeV (PD readout) was found to be roughly constant at σ/E ～ 1%. This is consistent with a negligible intrinsic resolution for BGO at these energies, after taking into account shower leakage and PD noise. For electrons of 92 and 200 MeV, we obtained (PM readout) energy resolutions close to the theoretical limit given by photon statistics and shower leakage. The electron/hadron separation was better than 1:500 over the energy range of 0.5 to 10 GeV, and improved to better than 1:1000 after a simple pattern cut. The energy deposition of the e.m. showers, both laterally and longitudinally (rear leakage), was found to be in agreement at the 0.1% level with Monte Carlo calculations using the SLAC-EGS program.     

Performance of a photomultiplier tube with transmissive dynodes in a high magnetic field

The performance of a photomultiplier tube with transmissive dynodes was examined. We found that the amplification gain was higher than 10⁴ even in a magnetic field of 7.8 kG. The energy resolution for 662 keV gamma rays from ¹³⁷Cs was 12% fwhm measured using a NaI crystal at 7.8 kG. The time resolution was as good as about 200 ps at 7.8 kG for the light generated by 2 GeV pions when they passed through a 1 cm thick scintillator mounted on the tube.     

On the composition of a multialkali photocathode

During our technological experiments concerning the standard S-20 process, we observed appreciable deviations from the accepted Na₂KSb composition of the multialkali photocathode. First results which were achieved by the method of the double wedge developed earlier by the authors for multialkali composition, which are rather closer to NaK₂Sb, are presented in a short form.     

New results on the scintillation properties of BaF2

Energy resolutions and photoelectron yields are reported for scintillating BaF₂ crystals.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.