Singlet exciton fission-sensitized infrared quantum dot solar cells

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.     

A specific voxel phantom for in vivo measurements of 241Am in the knee

Calibration phantoms for in vivo measurements of low-energy photons should be anatomically realistic to minimise the uncertainties in the activity assessment. The calibration of the detection system can be performed using computational techniques based on numerical phantoms. The purpose of this work is to approach a numerical calibration by Monte Carlo (MC) technique of a Germanium detection system for the determination of 241Am in the knee. A specific voxel phantom was built from a computerised tomography of the calibration Spitz knee phantom. The phantom and the procedure to generate the associated input file for the MC code, namely MCNPX, have been described, as well as the characterisation of the detectors according to the manufacturer data and the energy calibration curves of the spectrometer. The detection efficiency and the pulse-height distribution have been determined for a homogeneous contamination of 241Am in bone.     

A method to register hidden photons with the aid of a multi-cathode counter

We propose to detect hidden photons (HPs) by using single electrons emitted from the surface of a metal cathode under the action of HPs of cold dark matter. A multiwire gas-discharge proportional counter of special design with four cathodes has been developed. Calibration measurements were performed for determining the efficiency of counting single electrons. Single-electron pulse counting rate in various detector configurations was measured. The upper limits of mixing parameter χ, which characterizes the kinetic mixing of HPs and ordinary photons, are estimated.

     

Estimation of a radiation weighting factor for 99mTc

Decaying (99m)Tc does not only emit a gamma ray (140.5 keV), but also low-energy Auger and conversion electrons. These electrons cause a serious problem in the determination of a radiation weighting factor for (99m)Tc due to their extremely short range in tissue. Therefore, for comparison ultrasoft X rays are used here, which deposit their energy mainly via the photoeffect thus also initiating low-energy photoelectrons. Monte Carlo computer codes provided electron emission spectra of (99m)Tc and subsequent track structure calculations simulated the induction of DNA damage of different degrees of complexity. For the modelling of ultrasoft X rays carbon K photons with an energy of 270 eV were selected, for which experimental results are available from the literature. On average, four electrons were found to be emitted per (99m)Tc decay. Simulation of DNA damage revealed a nearly identical spectrum of primary strand breaks for (99m)Tc and C-K radiation. On this basis, a total radiation weighting factor of 1.2 was evaluated for (99m)Tc.     

Simulation study for cloud detection with space lidars by use of analog detection photomultiplier tubes

Output signal electrons from photomultiplier tubes (PMTs) have neither a Gaussian nor a Poisson distribution because of changes induced by multiplication when the number of input signal photons and dark electrons is fewer than approximately 100. Therefore the assumption of a Gaussian distribution of signal electrons cannot be used in simulations for space lidar observations with PMTs, for which the number of return signal photons is normally small. A theory is introduced for analog detection with PMTs that have Poisson-distributed secondary-electron emission at each dynode stage. The theory is validated by straightforward numerical simulations. It is shown that the multiplication in PMTs is a multiply stochastic Poisson process and that the distribution of output signal electrons can be interpreted basically as Neyman type A. Analysis by the threshold method of cloud detection with a space lidar shows considerable difference between a Gaussian approximation and the exact distribution. The result indicates that the threshold level must be optimized for the exact distribution. Return signals were simulated for a proposed space lidar, and cloud detection with the threshold method was demonstrated.     

Gamma-ray rejection, or detection, with gadolinium as a converter

Gadolinium is a competent neutron conversion material for neutron detection due to its extremely high neutron capture cross section. It differs from the other neutron reactive materials by emitting large amounts of low-energy electrons for the consequent signal generation in a detector. Such low-energy electrons, though abundant, are prone to be contaminated by internal and/or external gamma rays, such as the activated 43.0 keV K-X rays, given the high atomic number of gadolinium. While the 43.0 keV K-X ray ought to be rejected as it originates in part from the external gamma rays when neutron detection is concerned, the ability to separate this energy line from other signals points to a practical mode of gamma-ray detection by a thin-film semiconductor with gadolinium as a converter. In this paper, a gamma-ray discrimination scheme for neutron detection is studied, which also provides insight into gamma-ray detection with a small semiconductor device with gadolinium as a converter, in line with the same principle of isolating the K-X rays activated by high- or medium-energy gamma rays.     

Scanning tunneling microscope excited cathodoluminescence from ZnS nanowires

The electronic excitation spectrum of ZnS nanowires (NWs) is characterized by cathodoluminescence (CL) excited using the tip of a scanning tunneling microscope as a highly localized and bright source of low-energy (150-350 eV) electrons with impinging power densities in the range of up to 60 kW/cm(2) or more. The CL spectra reveal significant differences when compared with the photoluminescence spectra of ZnS NWs. The differences can be associated with the properties of the surface region of the NWs, which are preferentially emphasized in the CL spectra owing to the small probing depth of low-energy electrons.     

Correlation of beam electron and LED signal losses under irradiation and long-term recovery of lead tungstate crystals

Radiation damage in lead tungstate crystals reduces their transparency. The calibration that relates the amount of light detected in such crystals to incident energy of photons or electrons is of paramount importance to maintaining the energy resolution the detection system. We report on tests of lead tungstate crystals, read out by photomultiplier tubes, exposed to irradiation by monoenergetic electron or pion beams. The beam electrons themselves were used to measure the scintillation light output, and a blue light emitting diode (LED) was used to track variations of crystals transparency. We report on the correlation of the LED measurement with radiation damage by the beams and also show that it can accurately monitor the crystal recovery from such damage.     

Single photon response of photomultiplier tubes

Beta or gamma rays, when directly incident on the window of an optically shielded photomultiplier tube, yield a typical single photon spectrum. The single photons are possibly generated in the glass window of the photomultiplier tube through excitation of atoms in glass by electrons. The coincidence resolving time has also been measured with a ⁶⁰Co gamma source and a pair of optically shielded photomultiplier tubes detecting single photons.     

表面电晕预电离与火花预电离之比较

本文报道了表面电晕预电离与火花预电离的比较结果。针式火花预电离源利用热电弧发出强烈的紫外光 ,其发射光谱范围从可见光连续延伸到紫外。表面电晕预电离源 ,通过介质极化发射电子 ,在预电离电路中产生电晕电流 ,其强度决定了发射光子的数量 ,而发射电子的能量分布确定辐射的光谱分布 ,其发射光谱分布不连续。对两种预电离源的放电电压和电流进行测量 ,得到瞬态信息。计算得到火花预电离源中消耗的能量约是表面电晕预电离源的 4 0 0倍。比较分析可知 ,表面电晕预电离的效率远高于火花预电离     

Two-dimensional infrared and mid-infrared imaging by single-photon frequency upconversion

Single-photon frequency upconversion is an effective method of infrared single-photon detection and imaging by converting the long-wavelength photons to shorter wavelengths to match the detector’s spectral response. We realized few-photon level 2D infrared imaging with a coincidence frequency upconversion system in a bulk periodically poled lithium niobate crystal. Moreover, the infrared photons carrying orbital angular momentum were converted to the visible regime with high efficiency, while the orbital angular momentum of the photons was well conserved during the frequency upconversion process. The single-photon frequency upconversion method was also used for mid-infrared imaging at 3.39 µm with high efficiency and low noise.     

Quantum communications and beyond

We introduce the concept of encoding and manipulation of information on single photons. This is exploited in the technique of quantum cryptography to distribute random bit strings in a secure way. More general quantum information processing requires a conditional interaction between separate photons. This can be achieved by exploiting the interference between two photons at a beam-splitter and the non-linearity inherent in detection. As yet the efficiency of such gates is low.     

Pulsed X-ray luminescence of composites consisting of inorganic particles and organic phosphors

A fast luminescence component with a duration of ??2 ns has been observed upon pulsed X-ray excitation of composites composed of microparticles of a heavy constituent (heavy-metal oxides and fluorides) and optically active polymer adhesive. The intensity and temporal parameters of this component depend on neither the structural state of the heavy constituent nor the presence of optically active impurity. A mechanism of the formation of the fast luminescence component of composites upon pulsed X-ray excitation is proposed; according to it, when high-energy X rays interact with the heavy constituent of the composite, electrons and low-energy X-ray photons, which are intensely absorbed by the polymer adhesive and thus cause its luminescence, are generated due to the photoelectric effect and X-ray scattering.     

Optimizing frequency-domain fluorescence lifetime sensing for high-throughput applications: photon economy and acquisition speed

The signal-to-noise ratio of a measurement is determined by the photon economy of the detection technique and the available photons that are emitted by the sample. We investigate the efficiency of various frequency-domain lifetime detection techniques also in relation to time-domain detection. Nonlinear effects are discussed that are introduced by the use of image intensifiers and by fluorophore saturation. The efficiency of fluorescence lifetime imaging microscopy setups is connected to the speed of acquisition and thus to the imaging throughput. We report on the optimal conditions for balancing signal-to-noise ratio and acquisition speed in fluorescence lifetime sensing.     

A Monte Carlo method for calculating the energy response of plastic scintillators to polarized photons below 100 keV

The energy response of plastic scintillators (Eljen Technology EJ-204) to polarized soft gamma-ray photons below 100 keV has been studied, primarily for the balloon-borne polarimeter, PoGOLite. The response calculation includes quenching effects due to low-energy recoil electrons and the position dependence of the light collection efficiency in a 20 cm long scintillator rod. The broadening of the pulse-height spectrum, presumably caused by light transportation processes inside the scintillator, as well as the generation and multiplication of photoelectrons in the photomultiplier tube, were studied experimentally and have also been taken into account. A Monte Carlo simulation based on the Geant4 toolkit was used to model photon interactions in the scintillators. When using the polarized Compton/Rayleigh scattering processes previously corrected by the authors, scintillator spectra and angular distributions of scattered polarized photons could clearly be reproduced, in agreement with the results obtained at a synchrotron beam test conducted at the KEK Photon Factory. Our simulation successfully reproduces the modulation factor, defined as the ratio of the amplitude to the mean of the distribution of the azimuthal scattering angles, within 5% (relative). Although primarily developed for the PoGOLite mission, the method presented here is also relevant for other missions aiming to measure polarization from astronomical objects using plastic scintillator scatterers.     

Graphene Microbolometers with Superconducting Contacts for Terahertz Photon Detection

We report on noise and thermal conductance measurements taken in order to determine an upper bound on the performance of graphene as a terahertz photon detector. The main mechanism for sensitive terahertz detection in graphene is bolometric heating of the electron system. To study the properties of a device using this mechanism to detect terahertz photons, we perform Johnson noise thermometry measurements on graphene samples. These measurements probe the electron–phonon behavior of graphene on silicon dioxide at low temperatures. Because the electron–phonon coupling is weak in graphene, superconducting contacts with large gap are used to confine the hot electrons and prevent their out-diffusion. We use niobium nitride leads with a \(T_\mathrm {c}\approx 10\)  K to contact the graphene. We find these leads make good ohmic contact with very low contact resistance. Our measurements find an electron–phonon thermal conductance that depends quadratically on temperature above 4 K and is compatible with single terahertz photon detection.     

On the possibility of eliminating the energy threshold for electron detection in semiconductor detectors

A method of eliminating the energy threshold for electron detection in semiconductor devices is described. The class of devices used for the detection and measurement of electron emission is sufficiently large, including electrostatic analyzers, gas-filled devices, microchannel plates, etc. An alternative to these types of devices for electron detection is offered by semiconductor radiation detectors based on the p-n junctions, which are widely used in the spectrometry of nuclear particles including medium-and high-energy electrons (10² keV and above). These detectors are obviously advantageous in comparison to the devices of other types, but there are several factors hindering the use of semiconductor detectors for the detection and analysis of low-energy electrons (in the kiloelectronvolt range). We propose an approach that allows the energy threshold for electron detection in semiconductor detectors to be eliminated by means of preliminary acceleration of the detected particles in an electrostatic field created between the emitter and the detector. This approach removes the basic factor limiting the use of semiconductor detectors in a number of diagnostic methods based on the analysis of electron emission, such as the extended X-ray absorption fine structure (EXAFS), surface EXAFS, and X-ray absorption near-edge structure (XANES) techniques.     

A technique for the absolute measurement of the W-value for X-rays in counting gases

A technique was developed for the absolute measurement of the W-value (the mean energy for the production of an electron-ion pair) for low-energy X-rays in a wide range of gases at atmospheric pressures, with a standard uncertainty better than 1%. This technique is based on the absolute measurement of the primary ionization charge produced by X-ray photons from a constant intensity monoenergetic X-ray source, e.g. a long lifetime radioactive source. The ionization charge is calibrated by the number of X-ray photons absorbed in the gas, counted with a photon detector. For this purpose, a hybrid detector system was tested and its use in W-value measurements was investigated. The technique was applied to pure xenon at 825 Torr with 5.9 keV X-rays and a W-value of 21.61_−0.10^+0.14 eV was obtained for a 68% confidence level. The required corrections and the different factors contributing to the accuracy of the results are discussed. The advantages and limitations of this technique are explored and future developments are discussed.     

Radiograph X-ray source based on interaction of femtosecond laser pulse with solid-state targets in an air medium

We study the charcteristics of X-ray radiation of a laser plasma generated in the interaction of a femtosecond pulse with solid targets in an air medium and conside the possibility of using it for radiography. We show that the mechanism for generating X-ray radiation in the interaction of short, powerful laser pulses with solid targets in a gas atmosphere is related to the generation of fast electrons in the gas breakdown region. We prove experimentally that under such conditions, even under relatively low laser radiation densities <10¹⁵ W/cm², a copper target quite effectively emits photons with energies of up to 10 keV. We measure the spectrum of such an X-ray source containing the expressed Cu K_α and Cu K_β lines and moderate bremsstrahlung. We demonstrate that the source can be used to obtain micron-resolution absorption images of low-contrast objects in an air medium, and in the same way, open possibilities for studies of medical-biological objects in vivo.     

A Novel Type of Absorber Presenting a Constant Detection Efficiency for up to 25 keV X-Ray Photons

A novel type of absorber, dedicated to cryogenic detectors, was conceived to reach a high and constant intrinsic detection efficiency (>98%) for up to 25?keV X-ray photons. The absorber consists of two layers having a different atomic number?Z. The role of the first layer (large?Z) is to make negligible the transmission through the absorber; while the second layer (medium?Z) has to reabsorb the escape photons from the first layer. A?metallic magnetic calorimeter was realized with an Au-Ag absorber. The required thicknesses of both layers were determined using Monte Carlo simulation of the efficiency. To show the advantages of such a detector, its efficiency and its energy resolution are compared to the efficiency and energy resolution of a gold absorber using a ²⁴¹Am source.