Single High-energy Electron Research Articles

Photon Statistics of Incoherent Cathodoluminescence with Continuous and Pulsed Electron Beams

Photon bunching in incoherent cathodoluminescence (CL) spectroscopy originates from the fact that a single high-energy electron can generate multiple photons when interacting with a material, thus, revealing key properties of electron–matter excitation. Contrary to previous works based on Monte Carlo modeling, here we present a fully analytical model describing the amplitude and shape of the second order autocorrelation function (g(2)(τ)) for continuous and pulsed electron beams. Moreover, we extend the analysis of photon bunching to ultrashort electron pulses, in which up to 500 electrons per pulse excite the sample within a few picoseconds. We obtain a simple equation relating the bunching strength (g(2)(0)) to the electron beam current, emitter decay lifetime, pulse duration, in the case of pulsed electron beams, and electron excitation efficiency (γ), defined as the probability that an electron creates at least one interaction with the emitter. The analytical model shows good agreement with the experimental data obtained on InGaN/GaN quantum wells using continuous, ns-pulsed (using beam blanker) and ultrashort ps-pulsed (using photoemission) electron beams. We extract excitation efficiencies of 0.13 and 0.05 for 10 and 8 keV electron beams, respectively, and we observe that nonlinear effects play no compelling role, even after excitation with ultrashort and dense electron cascades in the quantum wells.

A precision luminometer for future linear collider experiments

The FCAL collaboration develops the technologies of compact and fast calorimeters to measure the luminosity at Linear Collider experiments both with high precision using small angle Bhabha scattering, and bunch-by-bunch using beamstrahlung pairs. Beside the luminosity measurement, the capability of detecting high energy electrons at low angles is important for many search experiments. A small Molière radius facilitates the measurement of Bhabha events in the presence of background and allows the detection of single high energy electrons on top of the widely spread background of beamstrahlung. A multi-plane prototype of a compact precision luminometer was studied in an electron beam with energies between 1 and 5 GeV at DESY. The results for the longitudinal and the transverse shower profiles are compared with Geant4 simulations of the setup. Very good agreement was obtained. The effective Molière radius of the prototype was determined. Again, very good agreement between data and Monte Carlo simulations was found. The value of the Molière radius approached the technological limit. A dedicated multi-channel ultra-low power readout ASIC is under development in 130 nm CMOS, comprising an analogue front-end and fast 10-bit ADC in each channel, followed by fast serialization and data transmission. In addition, an ASIC with a dual readout scheme allowing for a fast feedback to the accelerator and simultaneous data taking and calibration is under development. The paper summarizes the results on design optimization, beam-tests and the status of the readout ASICs.

Multiple-Cell Upsets Induced by Single High-Energy Electrons

Multiple-cell upsets (MCUs) in static random access memory-based field-programmable gate arrays from three different technology nodes are recorded from single-particle interactions with 20–52 MeV electrons. Indirect ionization events associated with an electronuclear reaction are shown to be the cause of the MCUs. As has been observed with other ionizing particles such as heavy ions and neutrons, the electron upset cross section per bit is shown to decrease with smaller technology nodes, but the prevalence of electron-induced MCUs increases with scaling.

Protein structure determination by electron diffraction using a single three-dimensional nanocrystal.

Three-dimensional nanometre-sized crystals of macromolecules currently resist structure elucidation by single-crystal X-ray crystallography. Here, a single nanocrystal with a diffracting volume of only 0.14 µm3, i.e. no more than 6 × 105 unit cells, provided sufficient information to determine the structure of a rare dimeric polymorph of hen egg-white lysozyme by electron crystallography. This is at least an order of magnitude smaller than was previously possible. The molecular-replacement solution, based on a monomeric polyalanine model, provided sufficient phasing power to show side-chain density, and automated model building was used to reconstruct the side chains. Diffraction data were acquired using the rotation method with parallel beam diffraction on a Titan Krios transmission electron microscope equipped with a novel in-house-designed 1024 × 1024 pixel Timepix hybrid pixel detector for low-dose diffraction data collection. Favourable detector characteristics include the ability to accurately discriminate single high-energy electrons from X-rays and count them, fast readout to finely sample reciprocal space and a high dynamic range. This work, together with other recent milestones, suggests that electron crystallography can provide an attractive alternative in determining biological structures.

Hot carrier multiplication on graphene/TiO2 Schottky nanodiodes.

Carrier multiplication (i.e. generation of multiple electron–hole pairs from a single high-energy electron, CM) in graphene has been extensively studied both theoretically and experimentally, but direct application of hot carrier multiplication in graphene has not been reported. Here, taking advantage of efficient CM in graphene, we fabricated graphene/TiO2 Schottky nanodiodes and found CM-driven enhancement of quantum efficiency. The unusual photocurrent behavior was observed and directly compared with Fowler’s law for photoemission on metals. The Fowler’s law exponent for the graphene-based nanodiode is almost twice that of a thin gold film based diode; the graphene-based nanodiode also has a weak dependence on light intensity—both are significant evidence for CM in graphene. Furthermore, doping in graphene significantly modifies the quantum efficiency by changing the Schottky barrier. The CM phenomenon observed on the graphene/TiO2 nanodiodes can lead to intriguing applications of viable graphene-based light harvesting.

Search for MeV dark photons in a light-shining-through-walls experiment at CERN

In addition to gravity, there might be another very weak interaction between the ordinary and dark matter transmitted by U'(1) gauge bosons A' (dark photons) mixing with our photons. If such A's exist, they could be searched for in a light-shining-through-a-wall experiment with a high energy electron beam. The electron energy absorption in a calorimeter (CAL1) is accompanied by the emission of bremsstrahlung A's in the reaction eZ -> eZA' of electrons scattering on nuclei due to the \gamma - A' mixing. A part of the primary beam energy is deposited in the CAL1, while the rest of the energy is transmitted by the A' through the "CAL1 wall" and deposited in another downstream calorimeter CAL2 by the e+e- pair from the A'->e+e- decay in flight. Thus, the A's could be observed by looking for an excess of events with the two-shower signature generated by a single high energy electron in the CAL1 and CAL2. A proposal to perform such an experiment to probe the still unexplored area of the mixing strength 10^{-5} < \epsilon < 10^{-3} and masses M_{A'} < 100 MeV by using 10-300 GeV electron beams from the CERN SPS is presented. The experiment can provide complementary coverage of the parameter space, which is intended to be probed by other searches. It has also a capability for a sensitive search for A's decaying invisibly to dark-sector particles, such as dark matter, which could cover a significant part of the still allowed parameter space.

Structural Basis for Reductive Radical Formation and Electron Recycling in (R)-2-Hydroxyisocaproyl-CoA Dehydratase

The radical enzyme (R)-2-hydroxyisocaproyl-CoA dehydratase catalyzes the dehydration of (R)-2-hydroxyisocaproyl-CoA in the fermentation of l-leucine by the human pathogenic bacterium Clostridium difficile. In contrast to other radical enzymes, such as bacterial class II ribonucleotide reductase or biotin synthase, the Fe/S cluster containing (R)-2-hydroxyisocaproyl-CoA dehydratase requires no special cofactors such as coenzyme B(12) or S-adenosylmethionine for radical generation. Instead it uses a single high-energy electron that is recycled after each turnover. The catalyzed reaction, an atypical α/β-dehydration, depends on the reductive formation of ketyl radicals on the substrate generated by injection of a single electron from the ATP-dependent activator protein. So far, it is unknown how the active electron is recycled and how unwanted side reactions are prevented, allowing for up to 10,000 turnovers. The crystal structure reveals that the heterodimeric protein contains two [4Fe-4S] clusters at a distance of 12 Å, each coordinated by three cysteines and one terminal ligand. The cluster in the α-subunit is part of the active site. In the absence of substrate, a water/hydroxide ion acts as the fourth ligand. The substrate replaces this ligand and coordinates the cluster via the carbonyl-oxygen of the thioester group. The cluster in the β-subunit has a terminal sulfhydryl/sulfido ligand and can act as a reservoir to protect the electron from unwanted side reactions via a recycling mechanism. The crystal structure of (R)-2-hydroxyisocaproyl-CoA dehydratase serves as a model for the reductively radical-generating metalloenzymes of the (R)-2-hydroxyacyl-CoA dehydratase and benzoyl-CoA reductase families.

Ultrasound transmission gel as a bolus device for skin irradiation of irregular surfaces: technical note

This paper describes an uncommon radiation treatment of the external ear, with ultrasound (US) transmission gel used as bolus device to compensate for the irregularity of the target surface. Postoperative radiotherapy for cutaneous carcinoma was performed with a single high-energy electron beam directed over the ear auricle. Due to the irregular surface of the target, a "missing tissue" compensator was employed. Daily, after patient positioning, the concha was filled and flattened with US gel, and a dose of 54 Gy in 27 fractions was delivered. Water equivalence of the gel was verified by comparing the gel's computed tomography (CT) number [Hounsfield units (HU)] and density with the corresponding values for water and another commercial bolus device. Whereas ultrasound gel and water had comparable values (HU: 0; density 1 g/cm(3) for both), the corresponding values for the commercial device were slightly higher (HU: 80; density 1.02 g/cm(3)). Ultrasound gel proved to be an easy, fast and cheap compensating tool. Its water equivalence allows it to be used as an alternative to water, though easier to position and with lower risk of displacement. Thus, it is recommendable as a practical tool for most irregular sites. Further investigations are warranted to validate this solution in more complex irradiation techniques.

Observation of Single Compton-Electron Tracks in Superfluid Helium-4 and Trace Detection of Metastable Helium Molecules by Laser-Induced-Fluorescence Imaging

We use laser-induced fluorescence enhanced by a cycling transition to image metastable helium molecules in liquid helium-4. We demonstrate that the method achieves sufficient sensitivity to image the trail of helium molecules that forms along the track of a single high-energy electron created by Compton scattering of a 511 keV gamma ray in liquid helium. The experiments show that a liquid helium detector with optical readout can function as an electron-tracking Compton camera when combined with conventional gamma-ray detectors. The demonstrated sensitivity for imaging helium molecules could also find application in the spectroscopy and imaging of neutrons, detection of electronic and nuclear recoil events, and investigation of superfluid or normal-fluid flow in liquid helium using the molecules as passive tracer particles.

Long-Time Components in the Emission of Luminescent Materials

The decay of light emission in the time range from 150 microseconds to 20 milliseconds, caused by excitation by single high-energy electrons and alpha particles at room temperature, was studied for several different types of phosphors. The zinc cadmium sulfides investigated were found to exhibit emission under alpha particle excitation longer than can be accounted for by the bimolecular decay theory, indicating that traps are of importance even with this kind of excitation. Several alkali halides also emit a considerable part of their total light in this interval when excited by alpha particles. Of the organic phosphors studied, only diphenylacetelene showed measurable emission in this time range, and only when excited by alpha particles. A phosphor with a very large number of traps (Standard 7) had practically no emission in the time range investigated.

The Transition Effect for Cosmic-Ray Bursts at Small Thicknesses of Lead

The transition curve for cosmic-ray bursts under lead at sea level is analysed to find the contributions from stars, extensive showers, electromagnetic interactions of μ-mesons, and single high-energy electrons and photons. When this is done, a substantial fraction of the bursts under lead thicknesses less than about 2.5 cm remains unaccounted for. It is suggested that they are produced by `narrow air showers' consisting predominantly of photons of about 2.5×108 eV and having a sideways spread of less than 0.3 m.

The Origin of Large Cosmic-Ray Bursts

Integral size-frequency distribution curves for cosmic-ray bursts of more than 100 particles were obtained by using an ionization chamber shielded by 1.25, 12, and 35 cm of iron. These distribution curves can all be represented by an inverse power law with exponent = 2.0\ifmmode\pm\else\textpm\fi{}0.2. The transition curve plotted from these data shows a pronounced maximum. Five G-M counter coincidence sets were arranged to register on the ionization chamber trace so that coincidences between bursts and air showers could be measured. For a thickness of 1.25 cm of iron, about 85 percent of the bursts were coincident with air showers, while for 12 cm of iron, this fraction was about 20 percent and for 35 cm of iron, only 5 percent of the bursts were observed to be coincident with extensive showers. A discussion is given to account for the origin of the bursts which were not coincident with air showers. For the analysis of the transition curve three different types of bursts are considered: bursts due to extensive atmospheric showers, bursts produced by narrow air showers or single high energy electrons, and bursts produced by mesotrons (knock-on and bremsstrahlung processes). Data for bursts of more than 500 particles under 12 cm of lead are compared with the corresponding data obtained under 35 cm of iron in order to determine the dependence of burst frequency on atomic number; the results are in substantial agreement with production of these bursts by bremsstrahlung of the mesotron. Integral size-frequency distribution curves are also plotted for data obtained during a five-year period at Huancayo (3350-meters elevation) and at Cheltenham (72 meters) for bursts under 12 cm of lead. The Cheltenham data are compared with the theoretical calculations of Christy and Kusaka for burst production by mesotrons of spin 0, \textonehalf{}, and 1 and it is concluded that either the spin 0 or the spin \textonehalf{} curve fits the experimental data but the evidence definitely excludes spin 1. The altitude dependence for burst under 12 cm of lead shows that the ratio of the burst frequencies at Huancayo and Cheltenham is constant for bursts up to 2400 particles, but increases sharply for still larger bursts. This increase is discussed on the basis that bursts under thick shields at higher altitudes may be caused either by extensive showers or by possible spin 1 mesotrons having so short a mean life that most of them fail to reach sea level.