Electron Drift Length Research Articles

SiPM array of Xenoscope, a full-scale DARWIN vertical demonstrator

The DARWIN project aims to build and operate a next-generation observatory for dark matter and neutrino physics, featuring a time projection chamber with a proposed active target of 40 t of liquid xenon. As an R&D facility to test fundamental components of the future detector, Xenoscope, a full-scale vertical demonstrator with ∼400 kg of liquid xenon and up to 2.6 m electron drift length, was built at the University of Zurich. Its main objective is to demonstrate electron drift over unprecedented distances in liquid xenon—first in a purity monitor setup with charge readout, followed by a dual-phase time projection chamber. In this second phase, an array of 48 VUV4 MMPCs from Hamamatsu (model S13371-6050CQ-02) with a 12-channel readout will be placed above the liquid xenon column and operated as a light readout for the secondary proportional scintillation signals coming from extracted electrons in the time projection chamber. This work presents the design and development of the silicon photomultiplier array of Xenoscope, covering the structural and electronic design, sensor characterisation at cryogenic temperature and signal simulation.

Changes in detection characteristics of CdTe X-ray sensors by proton irradiation

This paper reports on the performance changes in CdTe:Cl semiconductor X-ray detectors seen after irradiation with 50 MeV protons up to a fluence of 1.6⋅1011cm−2. The rationale for the irradiation parameters is described. Performance was quantified before and after irradiations by obtaining X-ray spectra with a barium-133 radioactive source and detectors cooled to −20°C. Energy resolutions, electron and hole drift lengths, and energy calibration parameters were extracted by fitting model spectra to the data. Ten crystals of dimension 10×10×1 mm3 were used, with two each being irradiated to 20%, 40%, 60%, 80%, and 100% of the maximum fluence. Each crystal is pixelized and the eight large pixels of area 9.6mm2 are analysed separately to allow estimation of uncertainties.The main observations are a strong decrease of the electron drift length and an increase of the electronic noise with fluence. The hole drift length shows a tendency to increase with fluence. A basic interpretation of some observations in terms of an increasing density of acceptor trap levels with fluence is given.

Effects of transverse electron beam motion in a relativistic backward wave oscillator operating at low guiding magnetic field

We found that the electron drift length, L1, between a diode area and a resonant reflector in a relativistic backward wave oscillator operating at a low guiding magnetic field has a periodical influence on device power, which is a special characteristic due to periodical transverse motions of electron beams. In this paper, electron motion characteristics under both TM02 and TM01 fields are investigated using single-particle simulation, and the result shows that the condition of beam expansion is dependent on the beam state that spatially meets the TM02 standing-wave field. A larger beam expansion will lead to a larger axial energy spread in the modulation area, which is unfavorable for efficient beam-wave interaction. Furthermore, rapid transverse expansion of electron beams can be observed under a TM01 travelling-wave field, which is the main cause for the obvious decline in the current collected by the collector as the output power reaches saturation.

A Conical Ion Diode with Self-Magnetic Insulation of Electrons

The results of studying the generation of a gigawatt-power pulsed ion beam formed by a diode in the mode of self-magnetic insulation of electrons are presented. Studies were carried out at the TEMP-4M accelerator in the mode of generating two pulses: the first is negative (500 ns, 150–200 kV) and the second is positive (150 ns, 250–300 kV). The formation of anode plasma occurs during the explosive emission of electrons during the first pulse. To improve the efficiency of generating an ion current, a conical geometry of the diode was proposed in which the electron-drift length exceeds that in the previous diode structures by factor of 2. It was found that the energy efficiency in the conical diode increased to 15–17%, while the ion-beam energy density at the focus increased to 2–3 J/cm2; the beam consisted of protons and carbon ions. The efficiency of suppression of the electron component of the total current in the diode was analyzed and the calculations of the electron-drift duration and ion acceleration were performed. It is shown that in the diode of the new design, efficient plasma formation occurs on the entire working surface of the graphite anode, while the plasma concentration may limit the ion current.

Zero-Dimensional Cs2TeI6 Perovskite: Solution-Processed Thick Films with High X-ray Sensitivity

We demonstrate a potential candidate, the 0D “all-inorganic” perovskite material Cs2TeI6, as a sensitive all-inorganic X-ray photoconductor for the development of the new generation of direct photon-to-current conversion flat-panel X-ray imagers. Cs2TeI6 consists of high atomic number elements, has high electrical resistance, and exhibits high air and moisture stability, making it suitable as a sensitive X-ray photoconductor. In addition, we identify that Cs2TeI6 film can be prepared under a low-temperature process using electrostatic-assisted spray technique under atmospheric conditions and achieved resistivity of 4.2 × 1010 Ω·cm. The resulting air- and water-stable Cs2TeI6 device exhibits a strong photoresponse to X-ray radiation. An electron drift length on the order of 200 μm is estimated under an applied electrical field strength of 400 V·cm–1. A high sensitivity for Cs2TeI6 thick film device is realized, with the value of 192 nC·R–1cm–2 under 40 kVp X-rays at an electrical field of 250 V·cm–1, which...

Performance of cryogenic charge readout electronics with the ARGONTUBE LAr TPC

ARGONTUBE is a liquid argon time projection chamber (TPC) with an electron drift length of up to 5 m equipped with cryogenic charge-sensitive preamplifiers. In this work, we present results on its performance, including a comparison of the new cryogenic charge-sensitive preamplifiers with the previously used room-temperature-operated charge preamplifiers.

Comparative research for GaAs photocathodes with graded compositional AlxGa1−xAs buffer layer

The graded Al compositional structure design in AlxGa1−xAs buffer layer can well increase the quantum efficiency than the stationary Al compositional structure design, especially at short-wave band. For deep study, the samples of two structures were prepared by metal organic chemical vapor deposition for carrying out the comparative research between surface photovoltage before Cs–O activation and spectral response current after Cs–O activation. Through fitting calculations and analyzes, the mixed structure of graded Al composition in AlxGa1−xAs buffer layer and exponential doping concentration in GaAs active layer cannot only form continuous internal electric field throughout window layer and active layer, but also decrease the misfit dislocations and stacking faults arising from lattice mismatch at the AlxGa1−xAs/GaAs interface. The better interface properties can further decrease the back interface recombination velocity than the standard structure design. Furthermore the continuous internal electric field can well increase the electron diffusion and drift length for active layer and surface escape probability for surface barriers and can also drive more photo-excited electrons at short-wave band in buffer layer to cathode surface.

The LUX prototype detector: Heat exchanger development

The LUX (large underground xenon) detector is a two-phase xenon time projection chamber (TPC) designed to search for WIMP–nucleon dark matter interactions. As with all noble element detectors, continuous purification of the detector medium is essential to produce a large (>1ms) electron lifetime; this is necessary for efficient measurement of the electron signal which in turn is essential for achieving robust discrimination of signal from background events. In this paper, we describe the development of a novel purification system deployed in a prototype detector. The results from the operation of this prototype indicated heat exchange with an efficiency above 94% up to a flow rate of 42 slpm, allowing for an electron drift length greater than 1m to be achieved in approximately 2 days and sustained for the duration of the testing period.

Comparison between exponential-doping reflection-mode GaAlAs and GaAs photocathodes

A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.

Modelling charge transport lengths in heterojunction solar cells

A drift-diffusion model is used to estimate the minority carrier transport length and depletion width in heterojunction solar cells from measured external quantum efficiency (EQE) data. The model is applied to Cu2O-ZnO heterojunctions synthesized by electrodeposition and thermal oxidation, and the electron drift and diffusion lengths are estimated: Ldrift ≈ 110 nm for electrodeposited Cu2O and Ldrift ≈ 2790 nm and Ldiff ≈ 310 nm for thermally oxidized Cu2O. Better fitting of EQE data is obtained than with traditional models that neglect recombination in the depletion region.

Comparison between gradient-doping and uniform-doping GaN photocathodes

In order to enhance the quantum efficiencies of negative electron affinity (NEA) GaN photocathodes, gradient-doping reflection-mode GaN photocathodes are grown by metal organic chemical vapor deposition (MOCVD)at doping concentrations of 1×1018cm-3, 4×1017cm-3, 2×1017cm-3 and 6×1016cm-3 from the body to the surface, with the thickness of each doping region being about 45nm and the total thickness of GaN 180 nm. The gradient-doping GaN photocathodes are activated in an ultra-high vacuum system and are compared with two kinds of uniform-doping GaN photocathodes whose thicknesses are both 150 nm and doping concentrations are 1.6×1017cm-3 and 3×1018cm-3 separately. The results show that both the photocurrent growth rate and the maximum photocurrent of the gradient-doping GaN photocathodes are greater than those of the uniform-doping GaN in the Cs/O activation process, and the multi-test system measured maximum quantum efficiency of the gradient-doping NEA GaN photocathode is about 56% which is as high as the double of the uniform-doping. Calculations show that the energy band bendings of the gradient-doping GaN photocathodes are 0.024eV, 0.018eV and 0.031eV from the body to the surface, a larger electron drift and diffusion length are gained due to the built-in electric field formed by the energy band bending, because of the 0.073eV total energy band bending the photoelectrons reaching the surface have higher energies and pass through the surface barrier more easily. Therefore the gradient-doping NEA GaN photocathodes have greater quantum efficiencies.

Determination of Physical Parameters in Organic Bulk Heterojunction Solar Cells Using a Genetic Algorithm

A method is presented to extract the physical parameters of illuminated organic bulk-heterojunction solar cells based on genetic algorithms and Levenberg-Marquardt method. The solar cell model is the well known single diode equivalent circuit containing four components. The voltage dependence of the photocurrent in the organic solar cells due to electron and hole drift lengths is taken into account. The applicability of the present method is demonstrated by fitting current - voltage characteristics of poly-3-hexylthiophene (P3HT) and [6,6]- phenyl-C61-butyric acid methyl ester (PCBM) bulk-heretojunction solar cells at different light intensities and annealing temperatures.

Influence of exponential doping structure on the performance of GaAs photocathodes

Obtaining higher quantum efficiency and more stable GaAs photocathodes has been an important developmental direction in the investigation of GaAs photocathodes. One significant approach to this problem is to improve the electron diffusion length. We put forward and investigate an exponential doping mode GaAs photocathode. It was proved by theoretical and experimental results that, because the exponential doping structure is in favor of forming a directional constant built-in electric field, the electron diffusion and drift length of the cathode material can accordingly be enhanced. The mathematical expression of the electron diffusion and drift length L(DE) of an exponential doping photocathode was deduced, and the relationship between the doping coefficient and the electron diffusion and drift length is made certain. This investigation contributes to the understanding of varied doping GaAs photocathodes and provides guidance to optimize the doping structure of GaAs photocathodes for higher quantum efficiency.

Equivalent Method of Solving Quantum Efficiency of Reflection-Mode Exponential Doping GaAs Photocathode

The mathematical expression of the electron diffusion and drift length LDE of exponential doping photocathode is deduced. In the quantum efficiency equation of the reffection-mode uniform doping cathode, substituting LDE for LD, the equivalent quantum efficiency equation of the reffection-mode exponential doping cathode is obtained. By using the equivalent equation, theoretical simulation and experimental analysis shows that the equivalent index formula and formula-doped cathode quantum efficiency results in line. The equivalent equation avoids complicated calculation, thereby simplifies the process of solving the quantum efficiency of exponential doping photocathode.

Using a Wavelength Shifter to Enhance the Sensitivity of Liquid Xenon Dark Matter Detectors

Liquid xenon dark matter detectors have been successfully employed to search for WIMPs, a proposed dark matter candidate. Increasing the scintillation wavelength from 175 nm to 300-400 nm can increase light collection and consequently improve sensitivity to dark matter. Such a shift increases the reflectivity of common reflector materials, decreases the amount of Rayleigh scattering and boosts the quantum efficiency of photomultiplier tubes. In this paper, we show that depositing p-terphenyl (p-TP) solid wavelength shifter on PTFE reflectors and the entrance window of a photomultiplier tube enhanced light-collection efficiency by 21%. We also observed an anti-correlation between the liquid xenon temperature and electron drift length, indicating that the p-TP dissolves into liquid xenon with a temperature dependent solubility. Possible ways to solve the drift length problem are discussed in the paper.

Fano factor and nonuniformities affecting charge transport in semiconductors

The Fano factor is a measure of the variance in the number of charge carriers produced in materials by ionizing radiation interactions, which determines the ultimate energy resolution achievable by a semiconductor spectrometer. Similar to charge production, charge transport in semiconductors suffers variation due to material nonuniformities. A reanalysis of published data illustrates that the variance in electron drift length, which is typically neglected in the estimation of the Fano factor, is significant for CdZnTe. In fact, at low electric fields, signal variance due to inhomogeneous charge transport can dominate. Our analysis shows that the standard deviation in the electron drift length is on the order of hundreds of microns for the published data.

Optimum conditions of the runaway electron beam generation during a uniform gas breakdown

It was shown that a uniform breakdown of a gas gap must be accompanied by the Stoletov effect, i.e., a current maximum in pressure at a given voltage pulse. Analogues of the Stoletov constant were calculated for the pulsed non-self-sustained discharge in various gases. These constants also determine the minimum breakdown delay time. It was shown that the maximum current of the electron beam generated in the gas-filled diode is reached at a pressure corresponding to the maximum current and for an electrode spacing corresponding to the electron drift length during the pulse.

Acceleration element for femtosecond electron pulse compression.

An acceleration element is proposed for compressing the electron pulse duration in a femtosecond photoelectron gun. The element is a compact metal cavity with curved-shaped walls. An external voltage is applied to the cavity where a special electric field forms in such a way that the slow electrons in the electron pulse front are accelerated more than the fast electrons, and consequently the electron pulse duration will be compressed. The distribution of the electric field inside the acceleration cavity is analyzed for the geometry of the cavity. The electron dynamics in this acceleration cavity is also investigated numerically. Numerical results show that the electron pulse front and pulse duration can be improved by compensating for the effects of space charge and the initial energy spread of photoelectrons with a Lambertian angular distribution. Depending on the design parameters and the shape of the electron pulse, for a femtosecond electron gun with an electron energy of 30 keV, 10(3) electrons per pulse, and an electron drift length of 40 cm, the electron pulse duration can be reduced from 550 to 200 fs when using a compensating cavity with an average radius of 1.7 and 5.6 cm in length. Electron pulses shorter than 200 fs can be achieved if the length of the drift region is reduced.

Photoelectronic properties of HgI2 crystals for nuclear radiation detection

Photoelectronic properties of red mercuric iodide single crystals, grown from its saturated solution in tetrahydrofuran, have been studied for the wavelength range 450–700 nm at temperatures 80,110, 175, 235 and 300 K. Various aspects of the optical generation of charge carriers have been discussed. The computer simulation of the room temperature photoconductivity has generated the optimized values of the mobility-lifetime products μeτe = 5.67 × 10−5 cm2/V, μhτh = 0.18 × 10−5 cm2/V), and surface recombination velocities (Se = 3.2 × 105 cm/s, Sh = 4.5 × 105 cm/s) of the charge carriers in these crystals. The estimated values of the electron and hole drift lengths for typical electric fields suggest that, under the negative electrode illumination, THF α-HgI2 crystals have high potential as regards to their use as photodetectors in most of the scintillation spectrometers.

Evidence of Enlarged Drift Length in Nanocrystalline Porous Silicon Layers by Time-of-Flight Measurements

The electron transport mechanism of nanocrystalline porous silicon (PS) has been studied for a self-supporting PS layer by a time-of-flight (TOF) measurement. The observed TOF transient photocurrent curves are different from those of both single crystalline silicon (c-Si) and amorphous silicon (a-Si). On the basis of the mobility-lifetime product (µτ) deduced from TOF measurements, it is shown that the electron drift length of PS is enhanced up to 3.8 µm at 100 K and 2.6 µm at 300 K.