Bialkali Photocathode Research Articles

A concise quantum efficiency measurement system for gaseous photomultipliers

We have been developing gaseous photomultiplier tubes (PMTs) with a bialkali photocathode combined with micropattern gas detectors (MPGDs). The sensitivity of a PMT is described in terms of its quantum efficiency (QE). The QE is very important in many experiments, particularly when dealing with low photon statistics. A concise QE measurement system has been developed to evaluate the characteristics of a sealed gaseous PMT with a bialkali photocathode. This QE measurement system consists of a Xe arc lamp source, reflective optics and filters, a monochromator, and a reference Si photodiode detector. Using the system, we evaluated the QE, gain, and long-term stability of the gaseous PMT. Here, we report the results of our evaluation.

Performance evaluation of a depth-of-interaction detector by use of position-sensitive PMT with a super-bialkali photocathode

Our purpose in this work was to evaluate the performance of a 4-layer depth-of-interaction (DOI) detector composed of GSO crystals by use of a position-sensitive photomultiplier tube (PMT) with a super-bialkali photocathode (SBA) by comparing it with a standard bialkali photocathode (BA) regarding the ability to identify the scintillating crystals, energy resolution, and timing resolution. The 4-layer DOI detector was composed of a 16 × 16 array of 2.9 × 2.9 × 7.5 mm(3) GSO crystals for each layer and an 8 × 8 multi-anode array type position-sensitive PMT. The DOI was achieved by a reflector control method, and the Anger method was used for calculating interacting points. The energy resolution in full width at half-maximum (FWHM) at 511 keV energy for the top layer (the farthest from the PMT) was improved and was 12.0% for the SBA compared with the energy resolution of 12.7% for the BA. As indicators of crystal identification ability, the peak-to-valley ratio and distance-to-width ratio were calculated; the latter was defined as the average of the distance between peaks per the average of the peak width. For both metrics, improvement of several percent was obtained; for example, the peak-to-valley ratio was increased from 1.78 (BA) to 1.86 (SBA), and the distance-to-width ratio was increased from 1.47 (BA) to 1.57 (SBA). The timing resolution (FWHM) in the bottom layer was improved slightly and was 2.4 ns (SBA) compared with 2.5 ns (BA). Better performance of the DOI detector is expected by use of a super bialkali photocathode.

Spectrometric performances of high quantum efficiency multi and single anode PMTs coupled to LaBr3(Ce) crystal

High quantum efficiency semiconductor photodetectors have recently drawn the attention of the scientific community for their potential in the realization of a new class of scintillation imagers with very high energy and spatial resolution performance. However, this goal does not seem within easy reach, due to various technological issues such as, for example, the difficulty to scale the characteristics of a single detector to an imager with suitable dimensions. Lately a definite technical improvement in increasing quantum efficiency up to 42% for position sensitive photomultipliers was achieved. The aim of this work is thus to test this new technological progress and to study the possible implications in imaging applications. Four Hamamatsu PMTs were tested: two multi anode photomultipliers, one with a bialkali (27% quantum efficiency) and the other one with a super-bialkali photocathode (38% quantum efficiency), and two 1×1 in. PMTs, both equipped with an ultra bialkali photocathode (42% quantum efficiency). In particular one of the ultra bialkali PMT has also an increased efficiency of first dynode charge collection. The results were compared with the ones obtained with a reference PMT (Hamamatsu R6231), mainly used in spectroscopy. The PMTs were coupled to LaBr3(Ce), NaI(Tl) and LSO(Ce) continuous scintillation crystals. The tests were done using two independent electronic chains: one dedicated for spectroscopic application and a second one, using a multi wire 64 channel readout, for imaging applications. The super-bialkali MA-PMTs have shown high energy resolution, both with spectroscopic and imaging setup, highlighting the appropriateness of these devices for the development of imaging devices with high spectroscopic performance.

Large Area Microchannel Plate Imaging Event Counting Detectors With Sub-Nanosecond Timing

Progress towards the development of a 20 cm sealed tube optical detector with imaging and photon event time stamping is presented. Novel microchannel plates employing borosilicate micro-capillary arrays have been tested. These provide many performance characteristics typical of conventional MCPs, but have been made in sizes up to 20 cm, have low intrinsic background (0.08 events cm-2s-1) and very stable gain behavior for at least 7 C cm-2 of charge extraction. Bialkali (Na2KSb) photocathodes with >20% quantum efficiency have also been made on 20 cm borofloat-33 windows compatible with a large sealed tube device.

Terrestrial magnetic field effects on large photomultipliers

The effects of the Earth's magnetic field on the performance of large PMTs for a cubic-kilometer-scale neutrino telescope has been studied. Measurements were performed for three Hamamatsu PMTs: two 8″ R5912 types; one with a standard and the other with a super bialkali photocathode, and a 10″ R7081 type with a standard bialkali photocathode. The main characteristics of the PMTs, such as detection efficiency, transit time, transit time spread, gain, peak-to-valley ratio, charge resolution and fractions of spurious pulses were measured while varying the PMT orientations with respect to the Earth's magnetic field. The measurements were performed both with and without a mu-metal cage magnetic shielding. For the 8″ PMTs the impact of the magnetic field was found to be smaller than for the 10″ PMT. The magnetic shielding strongly reduced the orientation-dependent variations measured for the 10″ PMT and even improved the performance. Although less pronounced, improvements were also measured for the 8″ PMTs.

Influence of the Earth's Magnetic Field on Large Area Photomultipliers

The influence of the Earth's magnetic field on large area photomultipliers proposed for a future deep sea neutrino telescope was studied under the EU-funded KM3NeT design study. The aims were to evaluate variations in PMT performance in the Earth's magnetic field and to decide whether the use of magnetic shielding is necessary. Measurements were performed on three Hamamatsu PMTs: two 8-inch R5912 types, one of these with super bialkali photocathode, and a 10-inch R7081 type with a standard bialkali photocathode. The various characteristics of the PMTs were measured while varying the PMT orientations with respect to the Earth's magnetic field, both with and without a mu-metal cage as magnetic shield. In the 8-inch PMTs the impact of the magnetic field was found to be smaller than that on the 10-inch PMT. The increased quantum efficiency in the 8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\prime\prime}$</tex> </formula> super bialkali PMT almost compensated its smaller detection surface compared to the 10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\prime\prime}$</tex> </formula> PMT. No significant effects were measured upon transit time and the fraction of spurious pulses.

Enhancement of photomultiplier sensitivity with anti-reflective layers

Photomultipliers are fast, sensitive and low noise light detectors which operate across the ultraviolet–optical–near infra red region of the electromagnetic spectrum. Sensitivity is determined by the composition of the photocathode layer in which incident photons excite photoelectrons and the thickness of this layer. Incident light is partially reflected from and partially transmitted through the photocathode layer, which is typically ∼20 nm thick, and this energy is unavailable for photoelectron excitation, limiting sensitivity. Typical reflectance and transmittance values at 500 nm are 21% and 33%, respectively, for KCsSb; 27% and 24% for RbCsSb and 36% and 35% for Na2SbK : Cs. These substantial losses can be reduced by the addition of one or more transparent impedance matching layers between the photomultiplier tube (PMT) window and the photocathode, resulting in enhanced sensitivity without effecting the PMT geometry, photocathode deposition process or altering the acceptance angle of the photomultiplier. The impedance matching serves to reduce reflectance losses, increasing cathode absorptance. By using published measurements of the dispersive properties of bialkali and trialkali (S20) photocathode compositions, accurate modelling of the electromagnetic field distribution within the photocathode layer is possible. This model facilitates the prediction of the enhancement of sensitivity obtainable with an anti-reflective layer of zirconium dioxide (zirconia). The authors present the enhancement factors possible with standard bialkali and trialkali photocathode compositions using the material zirconia for impedance matching. Enhancement factors up to 27% and 38% are predicted for KCsSb and RbCsSb photocathodes, respectively, while an enhancement factor of up to 44% is predicted for the S20 photocathode.

Ion-Feedback Suppression for Gaseous Photomultipliers with Micro Pattern Gas Detectors

Abstract Gaseous photomultiplier tubes (Gas-PMTs) with micro-pattern gas detectors (MPGDs) sensitive to visible light are under development. A micro-pattern gas detector of Kapton GEM is found to be incompatible with bi-alkali photocathode, while a glass capillary plate has no problem. A new production method for a hole type MPGD has been developed. In the development of Gas-PMTs the suppression of ion-feedback is crucial because the ion-feedback limits the gain of the Gas-PMT and cause aging effects of the photocathode. Studies on ion-feedback suppression using a simulation with Garfield and Maxwell 3D programs are presented.

20 cm Sealed Tube Photon Counting Detectors with Novel Microchannel Plates for Imaging and Timing Applications

As part of a collaborative program between University of California, Berkeley, the Argonne National Laboratory, University of Chicago, and several commercial companies, a 20cm square sealed tube microchannel plate detector scheme with a proximity focused bialkali photocathode is being developed. Sealed tube microchannel plate deviceshave good imaging and timing characteristics, but large areas have been previously unavailable. We have madeb considerable progress in fabricating large size microchannel plates. A key feature is the novel implementation of low cost microchannel plates using borosilicate micro-capillary arrays with hollow core tubes. The resistive and secondary electron emissive surfaces are then applied by atomic layer deposition, eliminating the wet etch and thermal reduction processes for normal glass microchannel plates. Initial results with 33 mm format microchannel plates for gain, pulse width, imaging performance and lifetime are comparable to conventional MCPs. Large 20 cm square microchannel plate prototypes with 20 μm and 40 μm pores have been fabricated and initial tests show operational gain. Design and fabrication of a 20 cm sealed tube assembly is well advanced and comprises a borosilicate entrance window, a proximity focused bialkali photocathode, a pair of microchannel plates and a stripline readout anode. The design employs a brazed ceramic walled enclosure and a transfer tube type photocathode with an indium seal. We have adopted a baseline bialkali photocathode to match the anticipated input spectrum, and have made a number of test cathodes with >20% peak quantum efficiency on borofloat-33 window material. Stripline anodes are also being developed which will give less than 1 mm spatial resolution using custom ASIC amplification and timing electronics.

Revealing the Correlations between Growth Recipe and Microscopic Structure of Bi-alkali/Multi-alkali Photocathodes

Abstract For the development of high quantum efficiency photocathodes, the bi-alkali (K-Cs-Sb) photocathode growth process was studied through in-situ X-ray scattering measurements, including X-ray diffractometry (XRD) and X-ray reflectometry (XRR). Characterization during real-time growth revealed correlations between growth parameters and microscopic structural change of the photocathode. This paper presents preliminary results of an ongoing experiment to understand the growth mechanism. X-ray diffractometry enabled the observation of selective growth of specific crystalline orientation during the deposition and evaporation of an Sb layer. A phase transition of the Sb layer was also measured. X-ray reflectometry revealed surface roughness changes during the growth process and permitted layer thickness measurements while the K layer was evaporated on an ex-situ grown Sb layer, in addition to in-situ Sb layer growth.

MCP-PMT development for Belle-II TOP counter

Abstract We have developed a multiple-anode square-shape MCP-PMT for the Belle-II Time-Of-Propagation (TOP) counter. This detector is a hybrid cherenkov ring imaging and timing detector for particle identification in the barrel region of the upgraded detector. The Belle-II experiment will operate at high event rates and needs to withstand the correspondingly high background environment. MCP-PMT's have demonstrated excellent single photon timing resolution. However, the lifetime of photocathode is a known issue. Recently, we successfully improved the lifetime of a square-shape MCP-PMT by a factor of about 10, which is adequate for estimates of the nominal Belle-II background rates in the TOP counter. We have also developed a new MCP-PMT with Hamamatsu photonics, that adopts a super bialkali photocathode. Currently a peak quantum effciency of 28% for 400 nm photons has been achieved.

Atomic layer deposited borosilicate glass microchannel plates for large area event counting detectors

Borosilicate glass micro-capillary array substrates with 20μm and 40μm pores have been deposited with resistive, and secondary electron emissive, layers by atomic layer deposition to produce functional microchannel plates. Device formats of 32.7mm and 20cm square have been fabricated and tested in analog and photon counting modes. The tests show amplification, imaging, background rate, pulse shape and lifetime characteristics that are comparable to standard glass microchannel plates. Large area microchannel plates of this type facilitate the construction of 20cm format sealed tube sensors with strip-line readouts that are being developed for Cherenkov light detection. Complementary work has resulted in Na2KSb bialkali photocathodes with peak quantum efficiency of 25% being made on borosilicate glass. Additionally GaN (Mg) opaque photocathodes have been successfully made on borosilicate microchannel plates.

Development of a gaseous PMT with micro-pattern gas detectors

Gaseous photomultiplier tubes with micro-pattern gas detectors sensitive to visible light are under development. Long term stability of a bi-alkali photocathode is examined for more than 500 days. A micro-pattern gas detector of Kapton GEM is found to be incompatible with bi-alkali photocathode, while a glass capillary plate has no problem. A new production method for a Pyrex glass GEM has been developed. Some studies on ion-feedback suppression are presented.

Novel large format sealed tube microchannel plate detectors for Cherenkov timing and imaging

Large area (20×20 cm 2) sealed tube detectors using novel borosilicate glass microchannel plates, with bialkali photocathodes and strip-line readouts are being developed for Cherenkov light detection. Designs based on conventional sealed tubes with alumina brazed body construction and hot indium seals have been developed. Borosilicate glass substrates with 20 and 40 μm holes have been processed using atomic layer deposition to produce functional microchannel plates. Initial results for these in a 33 mm format show gain, imaging performance, pulse shape and lifetime characteristics that are similar to standard glass microchannel plates. Large area (20×20 cm 2) borosilicate glass substrates with 20 μm pores have also been made.

Effects of the Super Bialkali Photocathode on the Performance Characteristics of a Position-Sensitive Depth-of-Interaction PET Detector Module

Super Bialkali (SBA) photocathode is a new technology that improves the spectral response characteristics of position sensitive PMTs, boosting their quantum efficiency up to 35%. In this experiment, two SBA tubes were introduced into a production line of PET detectors mixed with the regular tubes. The detectors were assembled using the standard factory protocols for detector mounting, calibration and testing. We report an evaluation of the improvement introduced by the SBA photocathode comparing the spatial and energy resolution and the depth-of-interaction (DOI) performance of PET detector modules with DOI capabilities. We conclude that the superior performance of the SBA tube may enable the use of arrays with a larger number of crystals of smaller footprint, thus potentially improving the detector intrinsic spatial resolution without degrading the energy resolution or the phoswich (DOI) discrimination capability.

Performances of four super bialkali large area photomultipliers with respect to the standard R7081

Hamamatsu has recently developed a large area photomultiplier (10 in.) with a new super bialkali photocathode that improves quantum efficiency to 35%. Performances were measured with a gain of 5×10 7 in response to single photoelectrons and were compared to the measurements taken on 72 standard bialkali R7081 PMTs. The results confirmed an increase in quantum efficiency of up to 39% with respect to the standards. In order to define whether the new technology introduces secondary effects, time and charge characteristics, fraction of spurious pulses and dark count rate were measured. The main effects were an increase in the dark count rate and in the type 2 after pulse fraction. It was also found that the fraction of type 2 after pulse decreases linearly with the gain decrease. The local measurements confirmed that the uniformity of the super bialkali photocathode did not deteriorate when compared with standard.

Ion back-flow gating in a micromegas device

As several research and development programs on Gaseous Photomultipliers are running on, the main aging factor on the photocathode, in particular for bi-alkali photocathodes, is the ion back-flow. This effect has also a major impact on the behaviour of Time Projecting Chamber in terms of electron drift perturbations. This paper presents a new device that has been tested with a Micromegas in order to reduce the ion back-flow in this type of detector. Instead of using a single mesh, as it is the case in a classical use of this micropattern detector, a double-mesh is installed. When the good conditions of electric field are applied, the device is transparent to electrons but gates the ions. Several measurements of ion back-flow ratio have been performed using a high power Xenon lamp to create photo-electrons on a Ni photocathode. The currents on the different electrodes are then measured with a pico-ammeter and the ion back-flow ratio is given by the ratio Ic/Ia (where Ic and Ia are respectively the cathode and the anode currents). For a classical mode of working (single mesh mode) the Micromegas ion back-flow is about few percent and is proportional to the field ratio. We have demonstrated that it is possible to reach a back-flow ratio close to few 10−6 in a double mesh mode, for a gain of several thousands, depending on the gas mixture and the electric fields ratio between the different electrodes.

Latest bialkali photocathode with ultra high sensitivity

Among photosensitive devices in use today, the photomultiplier tube (PMT) is a versatile device providing extremely high sensitivity, exceptionally low noise and ultra-fast response. It can detect photons at ultra low light level such as single photon counting level. One of the most important characteristics of the PMT is the quantum efficiency (QE) of the photocathode. Among various kinds of photocathodes, a bialkali photocathode is commonly used for particle physics experiments since it has higher sensitivity and lower dark current than other photocathodes. QE of the conventional bialkali photocathode is 27%.Since 2007, we have made a significant improvement in the QE of the bialkali photocathode, achieving as high as 43% at peak wavelength by precise control of the photocathode activation process. This photocathode was named ultra bialkali (UBA) and was incorporated into PMTs successfully.Additionally, we have developed another new kind of bialkali photocathode for very low-temperature environments. At −175°C, the new bialkali photocathode shows good cathode linearity up to 300nA, and it has 28% QE and lower dark current than the UBA photocathode. A PMT with the new photocathode is expected to be an ideal detector for Dark Matter experiments or double-beta decay.

QUPID, a single photon sensor for extremely low radioactivity

We have successfully developed a new photon sensor, the Quartz Photon Intensifying Detector (QUPID), for experiments such as dark matter searches and observation of double beta decay. The QUPID is a type of hybrid-photo-detector (HPD), consisting of a special bialkali photocathode considering the operation under extremely low temperature such as the temperature of liquid xenon, −108°C, and a special avalanche photodiode for electron bombardment. The QUPID is constructed solely from extremely low radioactive materials, such as quartz and Si-avalanche photodiodes (APDs), which allows it to reach radiation levels of less than 1mBq while being able to detect single photons. In this paper, we report superior characteristics of the QUPID confirmed by the evaluation.

Progress in gaseous photomultipliers for the visible spectral range

Recent progress is described in gaseous photon detectors combining bialkali photocathodes and cascaded patterned gas-avalanche electron multipliers. Efficient avalanche-ion blocking permitted the development and feasibility demonstration of high-gain operation in continuous mode, of gaseous photomultipliers sensitive to single photons in the visible spectral range.