Electron imaging with Medipix2 hybrid pixel detector

The electron imaging performance of Medipix2 is described. Medipix2 is a hybrid pixel detector composed of two layers. It has a sensor layer and a layer of readout electronics, in which each 55 microm x 55 microm pixel has upper and lower energy discrimination and MHz rate counting. The sensor layer consists of a 300 microm slab of pixellated monolithic silicon and this is bonded to the readout chip. Experimental measurement of the detective quantum efficiency, DQE(0) at 120 keV shows that it can reach approximately 85% independent of electron exposure, since the detector has zero noise, and the DQE(Nyquist) can reach approximately 35% of that expected for a perfect detector (4/pi(2)). Experimental measurement of the modulation transfer function (MTF) at Nyquist resolution for 120 keV electrons using a 60 keV lower energy threshold, yields a value that is 50% of that expected for a perfect detector (2/pi). Finally, Monte Carlo simulations of electron tracks and energy deposited in adjacent pixels have been performed and used to calculate expected values for the MTF and DQE as a function of the threshold energy. The good agreement between theory and experiment allows suggestions for further improvements to be made with confidence. The present detector is already very useful for experiments that require a high DQE at very low doses.     

Measurements of the fast electron bremsstrahlung emission during electron cyclotron resonance heating in the HL-2A tokamak

A fast electron bremsstrahlung (FEB) diagnostic technique based on cadmium telluride (CdTe) detector has been developed recently in the HL-2A tokamak for measurements of the temporal evolution of FEB emission in the energy range of 10-200 keV. With a perpendicular viewing into the plasma on the equatorial plane, the hard x-ray spectra with eight different energy channels are measured. The discrimination of the spectra is implemented by an accurate spectrometry. The system also makes use of fast digitization and software signal processing technology. An ambient environment of neutrons, gammas, and magnetic disturbance requires careful shielding. During electron cyclotron resonance heating, the generation of fast electrons and the oscillations of electron fishbone (e-fishbone) have been found. Using the FEB measurement system, it has been experimentally identified that the mode strongly correlates with the electron cyclotron resonance heating produced fast electrons with 30-70 keV.     

Determination of radial location of rotating magnetic islands by use of poloidal soft x-ray detector arrays in the STOR-M tokamak

A technique is presented for determining the radial location of the rotating magnetic islands in the STOR-M tokamak by use of soft x-ray (SXR) detector arrays. The location is determined by examining the difference in the integrated SXR emission intensities through two adjacent lines of sight. A model for calculating dependence of the line integrated SXR emission intensity on the radius, the mode numbers and the magnetic island geometry, has been developed. The SXR difference signal shows phase inversion when the impact parameter of the line of sight sweeps across the magnetic islands. Experimentally, the difference SXR signals significantly reduce noise and suppress the influence of background plasma fluctuations through common mode rejection when a dominant mode exists in the STOR-M tokamak. The radial locations of the m = 2 magnetic islands have been determined under several experimental conditions in the STOR-M discharges. With the decrease in the tokamak discharge current and thus the increase of the safety factor at the edge, the radial location of the m = 2 magnetic islands has been found to move radially inward.     

Saving the photons: mapping X-rays by position-tagged spectrometry

Since the early years of X-ray spectrometry in electron microscopes, mapping the locations of chemical elements has been important. The X-rays needed in large numbers for this are rare, owing to poor production efficiency compared with electron signals, and at risk of loss by many mechanisms such as missing the limited solid angle of the detector, absorption before reaching the detector and pulse pile-up conventional digital mapping hardware reduces the information contained in the X-ray spectrum at each pixel to the itegrated counts from a few regions of interest.
The acquisition technique of position-tagged spectrometry eliminates the conflict between the desire to see full frame X-ray images quickly versus the analytical advantages of having complete spectra for each pixel. As the beam is scanned rapidly relative to traditional X-ray mapping, photons are counted in a virtual 3-D multichannel analyser on disk, preserving both spatial and spectral information. Along with the sophisticated post-processing allowed by storing an entire spectrum per pixel, a unique degree of dynamic interaction with the developing data is made possible by integrating many short scans instead of using a single long dwell time at each pixel.     

Optimizing a low-energy electron diffraction spin-polarization analyzer for imaging of magnetic surface structures

A newly designed scanning electron microscope with polarization analysis (SEMPA or spin-SEM) for the acquisition of magnetic images is presented. Core component is the spin detector, based on the scattering of low-energy electrons at a W(100) surface in ultrahigh vacuum. The instrument has been optimized with respect to ease of handling and efficiency. The operation and performance of a general low-energy electron diffraction (LEED) detector for SEMPA have been modeled in order to find the optimum operating parameters and to predict the obtainable image asymmetry. Based on the energy dependence of the secondary electron polarization and intensity, the detector output is simulated. For our instrument with optimized performance we demonstrate experimentally 8.6% polarization asymmetry in the domain structure of an iron whisker. This corresponds to 17.2% image contrast, in excellent agreement with the predicted simulated value. A contrast to noise ratio of 27 is achieved at 5 ms acquisition time per pixel.     

Detective quantum efficiency of electron area detectors in electron microscopy

Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.     

Discrimination of particles by the scintillation pulse shape in the range of low energies (6–100 keV for electrons) using the linear filters

The use of the linear filters for particle discrimination by the scintillation pulse shape is considered. The particle separation process has been simulated by the Monte Carlo method, taking into account that the slow scintillation component decays according to the hyperbolic law and that its relative contribution is energy dependent. The best figures of merit of particle discrimination attainable with this technique have been obtained assuming that the PMT and electronic noises are zero. It is shown that, by contrast to the zero crossing method, pulse shape discrimination of particles using the linear filters can ensure rejection of the γ-ray background to a level of ∼10⁻⁴ at particle energies of up to 12 keV of the electron equivalent. For energies of <24 keV, it is expedient that the signal acquisition time be increased to a few microseconds.     

Detection of nanosecond-scale, high power THz pulses with a field effect transistor

We demonstrate detection and resolution of high power, 34 ns free electron laser pulses using a rectifying field effect transistor. The detector remains linear up to an input power of 11 ± 0.5 W at a pulse energy of 20 ± 1 μJ at 240 GHz. We compare its performance to a protected Schottky diode, finding a shorter intrinsic time constant. The damage threshold is estimated to be a few 100 W. The detector is, therefore, well-suited for characterizing high power THz pulses. We further demonstrate that the same detector can be used to detect low power continuous-wave THz signals with a post detection limited noise floor of 3.1 μW/√Hz. Such ultrafast, high power detectors are important tools for high power and high energy THz facilities such as free electron lasers.     

High energy resolution bandpass photon detector for inverse photoemission spectroscopy

We report a bandpass ultraviolet photon detector for inverse photoemission spectroscopy with energy resolution of 82 ± 2 meV. The detector (Sr(0.7)Ca(0.3)F(2)/acetone) consists of Sr(0.7)Ca(0.3)F(2) entrance window with energy transmission cutoff of 9.85 eV and acetone as detection gas with 9.7 eV photoionization threshold. The response function of the detector, measured using synchrotron radiation, has a nearly Gaussian shape. The n = 1 image potential state of Cu(100) and the Fermi edge of silver have been measured to demonstrate the improvement in resolution compared to the CaF(2)/acetone detector. To show the advantage of improved resolution of the Sr(0.7)Ca(0.3)F(2)/acetone detector, the metal to semiconductor transition in Sn has been studied. The pseudogap in the semiconducting phase of Sn could be identified, which is not possible with the CaF(2)/acetone detector because of its worse resolution.     

Note: A portable pulsed neutron source based on the smallest sealed-type plasma focus device

Development and operation of a portable and compact pulsed neutron source based on sealed-type plasma focus (PF) device are reported. The unit is the smallest sealed-type neutron producing PF device. The effective volume of the PF unit is 33 cm(3) only. A compact size single capacitor (4 μF) is used as the energy driver. A battery based power supply unit is used for charging the capacitor and triggering the spark gap. The PF unit is operated at 10 kV (200 J) and at a deuterium gas filling pressure of 8 mb. The device is operated over a time span of 200 days and the neutron emissions have been observed for 200 shots without changing the gas in between the shots. The maximum yield of this device is 7.8 × 10(4) neutrons/pulse. Beyond 200 shots the yield is below the threshold (1050 neutrons/pulse) of our (3)He detector. The neutron energy is evaluated using time of flight technique and the value is (2.49 ± 0.27) MeV. The measured neutron pulse width is (24 ± 5) ns. Multishot and long duration operations envisage the potentiality of such portable device for repetitive mode of operation.     

A sectioned spectrometer of fast neutrons

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched ⁶Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ～10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10^?4 to 10² cm^?2 s^?1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 10² s^?1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (～1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ～600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.     

A Gaseous Radiochemical Neutron Monitor

Real-time neutron detection (monitoring) is based on the ability of atoms of inert radioactive gases generated by nuclear reactions to easily escape from the crystalline lattice of certain solid substances. An inert radioactive gas produced in a detector ampule is transferred by a carrier gas to a proportional gas-flow counter. The decay rate of the inert gas, which is uniquely related to the neutron-flux density in the ampule, is measured with this flow counter. This method is applied to monitor the neutron flux in the RADEX pulse neutron target driven by the linear proton accelerator of the Moscow Meson Facility at the Russian Academy of Sciences' Institute for Nuclear Research. By simultaneously using several nuclear reactions with different threshold energies of inert-gas production, it is possible to measure neutron spectra.     

Imaging and spectrometry of a microscopic gaseous target by SEM and TEM

Microscopic gas jets are experimentally investigated using a scanning electron microscope/electron microprobe (JXA 50A, JEOL, Tokyo) equipped with a transmitted electron detector, and the CEM 902 (Zeiss, Oberkochen) which allows electron spectroscopic imaging. Both capillary microjet devices and circular orifices have been used. Imaging of the microgas jet (argon, air, artificial gas mixtures) is done by transmitted electrons (dark field), electrons as collected by the conventional Everhart-Thomley secondary electron detector, and by electron spectroscopic imaging, both at the argon M- and L edges. X-ray spectra and electron energy loss spectra taken at an axis point just a few micrometers downstream from the orifice are discussed.     

Quantitative water mapping of cryosectioned cells by electron energy-loss spectroscopy

A direct technique based on electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has been developed to map subcellular distributions of water in frozen-hydrated biological cryosections. Previously, methods for water determination have been indirect in that they have required the cryosections to be dehydrated first. The new approach makes use of spectrum-imaging, where EELS data are collected in parallel at each pixel. Several operations are required to process the spectra including: subtraction of the detector dark current, deconvolution by the detector point-spread function, removal of plural inelastic scattering and correction for the support film. The resulting single scattering distributions are fitted to standard reference spectra at each pixel, and water content can be determined from the fitting coefficients. Although the darkfield or brightfield image from a hydrated cryosection shows minimal structure, the processed EELS image reveals strong contrast due to variations in water content. Reference spectra have been recorded from the major biomolecules (protein, lipid, carbohydrate, nucleic acid) as well as from vitrified water and crystalline ice. It has been found that quantitative results can be obtained for the majority of subcellular compartments by fitting only water and protein reference spectra, and the accuracy of the method for these compartments has been estimated as ± 3·5%. With the present instrumentation the maximum allowed dose of 2 × 10³ e/nm² limits the useful spatial resolution to around 80 nm for ± 5% precision at a single pixel. By averaging pixel intensities a value of 56·8% with a precision of ± 2·0% has been determined for the water content of liver mitochondria. The water mapping technique may prove useful for applications to cell physiology and pathophysiology.     

Medipix 2 detector applied to low energy electron microscopy

Low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) traditionally use microchannel plates (MCPs), a phosphor screen and a CCD-camera to record images and diffraction patterns. In recent years, however, MCPs have become a limiting factor for these types of microscopy. Here, we report on a successful test series using a solid state hybrid pixel detector, Medipix 2, in LEEM and PEEM. Medipix 2 is a background-free detector with an infinite dynamic range, making it very promising for both real-space imaging and spectroscopy. We demonstrate a significant enhancement of both image contrast and resolution, as compared to MCPs. Since aging of the Medipix 2 detector is negligible for the electron energies used in LEEM/PEEM, we expect Medipix to become the detector of choice for a new generation of systems.     

Electron multiplier-scintillator detector for pulse counting positive or negative ions

A general-purpose ion detector has been developed for fast pulse counting in mass spectrometry. It consists of a six-stage electron multiplier joined to part of a scintillator detector. This novel arrangement combines the virtually noiseless pulse-amplification response of an electron multiplier with the bipolar voltage flexibility of a scintillator detector. The resulting hybrid detector is superior in performance to either an electron multiplier or a conventional scintillator detector. It is designed to detect a beam of positive ions or negative ions focused to a line image or to a spot at the focal plane of a mass analyzer. Background counting noise is less than 1 count/min when the detector is used in either the positive ion or negative ion detecting mode. Mass-resolved ions are post-accelerated through an electric field to give them an additional kinetic energy of 10 keV before they strike the ion-to-electron converter in the electron multiplier. This allows low-energy ions to be detected with an efficiency close to unity, over a range of ion masses from 1 amu up to several hundred amu. Randomly spaced pulses can be counted at rates from 0 Hz to 10 MHz. The design of the detector and its operating characteristics are discussed.     

Discrimination of particles by the scintillation pulse shape (in the electron energy range of 0.5–4.0 MeV) using the zero-crossing method

The ratios of the fast to slow components of scintillation pulses produced by neutrons and γ rays have been calculated on the basis of experimental data for several energies in the range of 0.5-4.0 MeV of the electron equivalent. The procedure for discriminating between neutrons and γ rays by measuring the zero-crossing time of a bipolar pulse formed by RC circuits has been simulated for organic scintillators using the Monte Carlo method in the range of 0.012-4.000 MeV of the electron equivalent. It is shown that pulse shape discrimination of particles based on the zero-crossing technique allows rejection of γ-ray background down to a level of 10-4 at particle energies of >100 keV of the electron equivalent (for energies of <50 keV, the γ-ray background is suppressed to a level of 10^-1- 10^-2 and this technique becomes ineffective in principle).     

自动超声探伤的抗干扰研究

在自动超声探伤领域中，传统的多通道超声波探伤仪的抗干扰性能不甚理想的主要原因是其相关报警电路的抑制干扰性能较差。本文介绍了设计新颖的干扰抑制电路——符合式缺陷脉冲鉴别电路的构成与工作原理，并进一步叙述了采用该电路的ＣＴＳ－４６型六通道超声波探伤仪的抗干扰性能实验及现场测试实验。结论表明符合式缺陷脉冲鉴别电路的抗干扰性能很强，对自动探伤用多通道超声探伤仪而言，是一个很好的相关报警电路。     

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung

The x-ray bremsstrahlung spectrum emitted by the electron population in a 14.5 GHz ECR plasma source has been measured using a NaI(Tl) detector, and hence the electron temperature of the higher energy electron population in the plasma has been determined. The x-ray spectra for Ne and Ar gases have been systematically studied as a function of inlet gas pressure from 7 × 10(-7) mbar to 7 × 10(-5) mbar and for input microwave power ～1 W to ～300 W. At the highest input power and optimum pressure conditions, the end point bremsstrahlung energies are seen to reach ～700 keV. The estimated electron temperatures (T(e)) were found to be in the range 20 keV-80 keV. The T(e) is found to be peaking at a pressure of 1 × 10(-5) mbar for both gases. The T(e) is seen to increase with increasing input power in the intermediate power region, i.e., between 100 and 200 W, but shows different behaviour for different gases in the low and high power regions. Both gases show very weak dependence of electron temperature on inlet gas pressure, but the trends in each gas are different.     

Note: a high performance pulse energy detector for Q-switched laser based on photoacoustic effect

A high performance pulse energy detector is developed based on photoacoustic effect. Different from the detectors reported before which also utilized photoacoustic effect, our detector can measure the energy of each pulse output from a Q-switched laser and monitor the pulse energy fluctuation in real time owing to the signal processing circuit designed. By comparing with a commercial laser energy meter, our detector is proved to be of high sensitivity and accuracy. We test the detector under illumination of different pulse energy at varied wavelengths, and the results demonstrate that the detector has a broad spectral response and a dynamical energy range. Besides, the measurements of this detector will not be affected by the background light according to the principle of photoacoustic effect.