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.     

Carbon nanotube based photocathodes

This paper describes a novel photocathode which is an array of vertically aligned multi-walled carbon nanotubes (MWCNTs), each MWCNT being associated with one p-i-n photodiode. Unlike conventional photocathodes, the functions of photon-electron conversion and subsequent electron emission are physically separated. Photon-electron conversion is achieved with p-i-n photodiodes and the electron emission occurs from the MWCNTs. The current modulation is highly efficient as it uses an optically controlled reconfiguration of the electric field at the MWCNT locations. Such devices are compatible with high frequency and very large bandwidth operation and could lead to their application in compact, light and efficient microwave amplifiers for satellite telecommunication. To demonstrate this new photocathode concept, we have fabricated the first carbon nanotube based photocathode using silicon p-i-n photodiodes and MWCNT bunches. Using a green laser, this photocathode delivers 0.5?mA with an internal quantum efficiency of 10% and an I(ON)/I(OFF) ratio of 30.     

Investigation of timing properties of a thin multiwire proportional chamber with Csl photocathode

A measurement of the time distribution of signals due to single electrons created at the CsI photocathode in a thin multiwire proportional chamber is described. The full width at half maximum (FWHM) has been measured to be 8.3 ns. Calculations, which include different drift lengths, diffusion and electronic noise, agree well with the measured distribution. A timing resolution of about 1 ns FWHM is expected for Cherenkov radiation in 1 cm of NaF crystal, giving rise to approximately 30 photoelectrons from the CsI photocathode.     

Evaluation of the photoelectron contribution to the efficiency of open discharge

Estimates of the energy necessary to provide for the emission of a single photoelectron from a cathode in open discharge show that (i) the discharge cannot be maintained at the expense of photoemission and (ii) the contribution of photoelectrons to the energy efficiency of the electron beam formation is negligibly small.     

Measured conversion efficiencies of P45, paraterphenyl, tetraphenyl butadiene, and sodium salicylate phosphors in the soft-x-ray wavelength range

The measured conversion efficiencies at 9.89, 23.6, 44.7, and 160 ? of P45 phosphor screens, as well as those of paraterphenyl, tetraphenyl butadiene, and sodium salicylate at 9.89, 44.7, and 67.6 ?, are presented. The conversion efficiency is defined as the ratio of photoelectrons ejected from the photocathode of a visible detector, which are excited by the scintillated photons that are emitted from the phosphor in a solid angle of 2π, to the number of soft-x-ray photons incident on the phosphor. The effect of the phosphor's thickness on the conversion efficiency was studied. The P45 phosphor converts the soft-x-ray photon (10-200 ?) into an order of magnitude more visible photons than the low-Z phosphors. The P45 phosphor screen used in conjunction with a photomultiplier tube offers a soft-x-ray photodetector with a conversion efficiency that ranges from 0.5 at 160 ? to 12 at 9.89 ? and a high electronic gain.     

Proton-recoil detectors for time-of-flight measurements of neutrons with kinetic energies from some tens of keV to a few MeV

For experiments at the superconducting electron accelerator ELBE, where neutrons in the kinetic energy region from some tens of keV to a few MeV will be produced by bremsstrahlung, neutron-time-of-flight detectors have been developed. These detectors are made from the plastic scintillator material EJ-200. Efficiency calibration showed more than 10% efficiency for kinetic energies down to 30 keV. The calibration was done at the “accelerator facility for fast neutron research” at Physikalisch-Technische Bundesanstalt in Braunschweig, using pulsed quasi-monoenergetic neutron fields with a well-determined fluence. The low detection threshold was obtained by coincident readout of two Hamamatsu R2059-01 photomultiplier tubes per scintillator and by triggering just below the single-photo-electron peak of these photomultiplier tubes, which additionally gives a well-reproduceable detection threshold.     

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%.     

Development of a 24-anode linear-array fine-mesh PMT with 85% photoelectron detection efficiency and 100 ps TTS at B≤1 T

By shortening the distance between the photocathode and the first dynode to 1 mm and using a finer mesh size of 2500 lines/in. with 4 μm^φ, a 24 anode linear-array of fine-mesh photomultiplier tube has produced a clear single-photon peak in its pulse-height distribution under a strong magnetic field of B≤1 T, and provided σ_TTS=100 ps of transit time spread (TTS) and 85% of photoelectron detection efficiency.     

Statistical treatment of photon/electron counting: extending the linear dynamic range from the dark count rate to saturation

An experimentally simple photon counting method is demonstrated providing 7 orders of magnitude in linear dynamic range (LDR) for a single photomultiplier tube (PMT) detector. In conventional photon/electron counting methods, the linear range is dictated by the agreement between the binomially distributed measurement of counted events and the underlying Poisson distribution of photons/electrons. By explicitly considering the log-normal probability distribution in voltage transients as a function of the number of photons present and the Poisson distribution of photons, observed counts for a given threshold can be related to the mean number of photons well beyond the conventional limit. Analytical expressions are derived relating counts and photons that extend the linear range to an average of ～11 photons arriving simultaneously with a single threshold. These expressions can be evaluated numerically for multiple thresholds extending the linear range to the saturation point of the PMT. The peak voltage distributions are experimentally shown to follow a Poisson weighted sum of log-normal distributions that can all be derived from the single photoelectron voltage peak-height distribution. The LDR that results from this method is compared to conventional single photon counting (SPC) and to signal averaging by analog to digital conversion (ADC).     

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.     

A comment on the measurement of efficiency of the electron beam formation in an open discharge

Methods used for the measurement of the efficiency of the electron beam formation in an open discharge are critically analyzed. It is shown that a high efficiency determined as the ratio of the beam current to the total current, η=j _e/j, can serve neither as evidence for one of the open discharge mechanisms nor as a true criterion of the energy efficiency of the beam formation. Under typical open discharge conditions, when electrons in the entire discharge gap occur in the runaway regime, the η value is close to the geometric transparency of the anode grid and is independent of the coefficient of electron emission (γ) from the cathode. It is also demonstrated that recent experiments reported by P.A. Bokhan and D.E. Zakrevsky [Tech. Phys. Lett. 28, 74 (2002) do not prove the photoelectron nature of the discharge studied but, on the contrary, confirm once again that the open discharge is not induced by photoelectrons.     

Gated wire photodetector for high flux operation

A method of gating the wire gas discharge photodetector with electrical and fibre-optical readout is investigated. No restrictions due to positive space-charge effects have been revealed in the flux range up to 5 × 10⁵ ph.e./mms per anode wire. Hodoscope photomultipliers are used for light detection at the fibre exits.     

A concept for a hadron calorimeter with photodiode readout

A concept for a hadron calorimeter will be described. The calorimeter is a scintillator sandwich type with WLS-bars and photodiode readout. Emphasis is put on compactness, high stability, easy fabrication, and safety.     

Pulse-shape analysis of Cs2LiYCl6:Ce scintillator for neutron and gamma-ray discrimination

Cs₂LiYCl₆:Ce (CLYC) is one of the most promising new scintillators for detecting both neutrons and gamma-rays. Its neutron and gamma-ray discrimination capability using pulse-shape analysis has drawn much attention, and there is significant interest in its use in field applications. For such applications, compact and low-power readout electronics capable of exploiting the pulse-shape discrimination (PSD) capabilities of CLYC will be essential. A readout system centered around a PSD-capable application specific integrated circuit (ASIC) that is well-suited for use with CLYC has been characterized, tested, and validated. As part of this study, automated analysis of CLYC data collected with a fast waveform digitizer extracted optimized charge integration windows for PSD. Additionally, several different CLYC samples were studied in order to gain understanding of the dependance of pulse shapes on parameters such as crystal size, ⁶Li enrichment level, crystal packaging, and choice of PMT. Extremely good PSD performance was obtained from CLYC scintillator and the ASIC-based readout system.     

Single Stage Photoelectronic Image Intensifiers

The limit of image detectobility determined by the shot noise of the incident photons is discussed and the sensitivity of the ideal detector is evaluated. The advantages of a phatoelectric. as compared with a photographic, primary detector are enumerated and some of the practical methods ond problems involved in taking advantage of the greater quontum efficiency of the photoelectric effect ore then described. The methods described are limited to single-stage image intensifiers. The first of these is Lallemand’s electronic camera in which the electrons from a photocathode are focused at high energy on a photographic plate in the same vacuum chamber. The second is the modified type of the some device in which the vacuum chamber is divided into two compartments by a thin membrane; the photoelectrons being projected through this membrane from the photocathode on the high vacuum side onto the photographic plate in the relatively low vacuum side. Thirdly the method is described in which the electron image is formed on a phosphor screen and the light from this image is recorded on a photographic emulsion. The development of a ~hin window to enable contact records to be made and the reduction of spurious background are the main points of this development. Lastly. as a result of this work. it has become possible to project the electron image through a very thin mica window and record it directly on a photographic plate in contact with the window.     

A high-speed gateable image pipeline

We present the performance of a high-speed gateable vacuum image pipeline, which permits individual images to be delayed and selected from continuous non-repetitive image stream. This device is composed of a vacuum tube equipped with a photocathode at one end, a phosphor screen at the other end, and a system of metal grids in between. Photoelectrons produced by the images focused on the photocathode, are guided by a uniform magnetic field, parallel to the tube axis. By changing the grid potentials, the drift time of the photoelectrons inside the tube can be varied from 0.35 to 1.5 μs. An image can then be selected by an external trigger with a time resolution in the range of 4–30 ns, depending on the delay time. The selected photoelectrons are finally accelerated onto the phosphor screen, set at 10 kV, where they reproduce the desired image. With a magnetic field of 0.1 T, a spatial resolution of 33 lp/mm was obtained. The high spatial and time resolution make this device an interesting tool for high-energy physics and astrophysics experiments, and for high-speed photography.     

A water Cherenkov counter sensitive to nonwaveshifted ultraviolet Cherenkov photons

We describe a prototype water Cherenkov counter which has been built and tested with relativistic cosmic ray muons. An analysis of the expected photoelectron yield is described. The predicted result of 315 ± 31 photoelectrons is compared with the experimental result of 272 ± 30 photoelectrons. We find that over 70% of the Cherenkov photons detected have wavelengths less than 400 nm.     

CdWO4 scintillator as a compact gamma ray spectrometer for planetary lander missions

The objective of this work is to develop a gamma ray spectrometer (GRS) suitable for use on planetary rover missions. The main characteristics of this detector are low weight, small volume low power and resistance to cosmic ray radiation over a long period of time. We describe a 3 cm diameter by 3 cm thick CdWO₄ cylindrical scintillator coupled to a PMT as a GRS for the energy region 0.662–7.64 MeV. Its spectral performance and efficiency are compared to that of a CsI(Tl) scintillator 2.5 cm diameter by 6 cm thick coupled to a 28 mm×28 mm PIN photodiode. The comparison is made experimentally using ¹³⁷Cs, ⁶⁰Co, 6.13 MeV gamma rays from a ¹³C(,γn)O¹⁶* source, 7.64 MeV thermal neutron capture gamma rays emitted from iron bars using a ²⁵²Cf neutron source, and natural radioactivity 1.46 MeV ⁴⁰K and 2.61 MeV ²³²Th gamma rays. We use a Monte Carlo method to calculate the total peak efficiency of these detectors and the full energy, first and second escape peak efficiencies. The experimental and calculated results agree well. We investigated the usefulness of these detectors for a GRS on a Mars lander mission. Although both detectors meet desired specifications, it was found that CdWO₄ has advantages over CsI(Tl) being a more compact detector of higher efficiency. Using a shaping amplifier of 24 ms, CdWO₄ spectrometer exhibited a 6.8% FWHM at 662 keV. At 6.13 MeV, CdWO₄ detector possesses an intrinsic total and full energy peak efficiencies of 16.7% and 6.3%, respectively. These efficiencies are nearly a factor of 1.6 and 4 greater than the corresponding efficiencies of the CsI(Tl) detector.

A proposed gamma ray spectroscopy system to be placed on a rover, consists of a central detector surrounded by a Compton suppressor shield. The central detector is a cylindrical CdWO₄ detector and the Compton suppressor shield is made of segmented CdWO₄, coupled to PIN photodiodes. The shield also prevents thermal neutron activation of the central detector.     

High precision axial coordinate readout for an axial 3-D PET detector module using a wave length shifter strip matrix

We describe a novel method to extract the axial coordinate from a matrix of long axially oriented crystals, which is based on wavelength shifting (WLS) plastic strips. The method allows building compact 3-D axial gamma detector modules for PET scanners with excellent 3-D spatial, timing and energy resolution while keeping the number of readout channels reasonably low. A voxel resolution of about 10 mm³ is expected. We assess the performance of the method in two independent ways, using classical PMTs and G-APDs to read out the LYSO (LSO) scintillation crystals and the WLS strips. We observe yields in excess of 35 photoelectrons from the strips for a 511 keV gamma and reconstruct the axial coordinate with a precision of about 2.5 mm (FWHM).