X射线散射光子对剂量当量电离室影响

为了测量X射线辐射散射光子对剂量当量电离室的影响,根据ISO-4037建立X射线防护水平参考辐射场.采用PTW-32002 1L和PTW-32003 10L两个不同体积的球形空腔电离室在不同辐射质条件下通过影锥屏蔽法测量散射光子对不同电离室的影响程度.实验结果表明:散射光子对不同体积电离室影响程度不同,对于10 L球形电离室散射份额在2％左右,而对于1L球形电离室散射份额在4％左右.能量越高,散射光子的散射份额也会相对增大.     

An approach to Monte Carlo simulation of low-energy electron and photon interactions in air

In this paper we present an approach to an energy deposition model for low energy (10 eV–10 keV) photons and electrons. The model is based on Monte Carlo simulations of the tracks and energy loss resulting from single electron scattering events with the constituent molecules of air, namely N₂ and O₂. The simulation code is based on the Geant4 toolkit. The original physics in Geant4 low-energy processes for photon interactions was used for the incoming photons and secondary X- and gamma-rays, from energies below 1–10 keV. Two new electron interaction processes were developed,and both of them were considered as discrete processes. The new implemented physics is based on experimental cross-sections for elastic and inelastic scattering, complemented with theoretical calculations. Ionization is treated as a subprocess of the inelastic process. The implementation of the new processes in the standard Geant4 frame is considered. Validation tests and comparison with other Monte Carlo results are discussed. The current limitations of this version of the code are also outlined. The code can be of interest in areas where energy deposition at microscopic level is crucial, such as microdosimetry.     

Penetration depth for diffusing-wave spectroscopy

The depth at which diffusing photons are assumed to be deposited in a random scattering medium has traditionally been treated as a phenomenological parameter comparable to the photon transport mean free path. We show how to average properly over an exponential distribution of depositions weighted additionally by the transmission probability, and compare our prediction for the autocorrelation of intensity fluctuations in the transmitted light with experimental data on an ideal system. The improved correlation function, where distinguishable from the prior form, provides slightly better agreement with data as long as the sample is thicker than approximately 10 transport mean free paths. However, in contrast with static transmission, proper averaging over a range of penetration depths does not extend the validity of diffusing-wave spectroscopy to significantly smaller slab thicknesses. The most significant errors in the theory must therefore arise from approximations other than the treatment of the source of diffusing photons.     

Measurement of the local optical properties of turbid media by differential path-length spectroscopy

We report on the development of an optical-fiber-based diagnostic tool with which to determine the local optical properties of a turbid medium. By using a single fiber in contact with the medium to deliver and detect white light, we have optimized the probability of detection of photons scattered from small depths. The contribution of scattered light from greater depths to the signal is measured and subtracted with an additional fiber, i.e., a collection fiber, to yield a differential backscatter signal. Phantoms demonstrate that, when photons have large mean free paths compared with the fiber diameter, single scattering dominates the differential backscatter signal. When photons have small mean free paths compared with the fiber diameter, the apparent path length of the photons that contribute to the differential backscatter signal becomes approximately equal to 4/5 of the fiber diameter. This effect is nearly independent of the optical properties of the sample under investigation.     

Forward-adjoint fluorescence model: Monte Carlo integration and experimental validation

The adjoint form of the photon transport equation is applied to a generalized fluorescence detection problem, and its accuracy is empirically tested. This approach can be interpreted as mathematically reversing the temporal flow of fluorescent photons; that is, they are tracked from the detector back to potential sites of origin in the scattering medium. The result is a distribution of potential fluorescing sites that, when properly normalized, gives a probability field of the relative importance of the photon starting position and direction to the resulting signal. This adjoint solution can be combined with the temporally forward-derived distribution of absorbed excitation photons to evaluate the fluorescence excitation detection scheme. This bypasses the normal, temporal derivation wherein the fluorescence transport solution is dependent on the result of the excitation transport solution.     

Alanine blends for ESR measurements of thermal neutron fluence in a mixed radiation field

In this paper, the results of a study on the electron spin resonance (ESR) dosimetry to measure thermal neutron fluence in a mixed radiation field (neutron and photons) are presented. The ESR responses of alanine dosemeters with different additives are compared. In particular, the (10)B-acid boric and the Gd-oxide were chosen to enhance the sensitivity of alanine dosemeters to thermal neutrons. Irradiations were carried out inside the thermal column of the TAPIRO reactor of the ENEA center, Casaccia Rome. The main results are a greater neutron sensitivity and a smaller lowest detectable fluence for the dosemeters with gadolinium than for dosemeters of alanine with (10)B, which is well known to be much more sensitive to thermal neutrons than simple alanine.     

Cold electron source with an electron multiplier illuminated by ultraviolet photons

A cold electron source has been developed using a microchannel plate (MCP) electron multiplier. Primary electron emission is initiated by illumination with ultraviolet photons from a diode. Final electron emission currents of up to 10 μA in continuous wave (CW) mode and 10 mA in pulsed mode have been achieved from a MCP with an active diameter of 5.5 mm. Electron emission generated by UV photons with a wavelength of 260 nm has been shown to be about 30 times more efficient than emission generated by photons with a wavelength of 300 nm. Application of this cold electron source in mass spectrometry is discussed.     

Kapitza resistance between silicon and helium-4

By simultaneously measuring the phonon scattering at or near a silicon surface and the phonon transmission from it into liquid helium, we have correlated quantitatively these two processes. For a clean polished silicon surface, the scattering of thermal phonons below 0.3 K becomes very small, and the transmission probability in this case approaches the prediction of the acoustic mismatch theory, which is 0.27%. The transmission probability increases, i.e. the thermal boundary resistance decreases, as the diffuse scattering increases either as a result of increasing the phonon frequency or the disorder at the surface. In the limit of completely diffuse scattering, the phonon transmission probability exceeds 30%, and the thermal boundary resistance approaches the value predicted by the diffuse mismatch model to within a factor of three.     

Krypton fluoride laser pulse shortening by stimulated thermal scattering on a liquid surface

Pulse shortening of the backscatter of a KrF excimer laser beam focused on the surface of n-hexane is reported. Simultaneous measurements of the backscattered beam, a Fresnel reflected beam, incoherent scattered photons from a liquid surface, and a transmitted beam are performed. The results show that the surface reflected beam disappears followed by beam expansion and the transmitted beam is reduced to form a filamentlike structure with the onset of backscatter. The pulse width of the backscattered beam shows a clear dependence on the focal position in the liquid. Incoherent scattering is sharply enhanced when the laser beam is focused at the liquid surface where the backscattered beam is especially short. The observed phenomena indicate that stimulated thermal scattering is the pulse shortening mechanism.     

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.     

Multiple scattering of a photon flux: implications for the integral average cosine of the underwater light field

It is shown that where mu (s) is the average cosine of scattering, then for any set of photons that undergoes exactly n scatterings per photon, the average cosine after scattering is mu (0)mu (s)(n), where mu (0) is the average cosine of the photon flux before scattering. For a set of photons that has traversed distance d through a medium with scattering coefficient b, the average cosine is mu (0) exp[-bd(1 - mu (s))]. For water bodies in which loss of upward-scattered photons through the surface is small enough to be disregarded, the value of mu (c) (the average cosine of all the photons instantaneously present in the water column) for any given incoming flux of photons with average cosine mu (0) is determined entirely by the inherent optical properties of the water in accordance with mu (c)= mu (0)/[1 + (b/a)(1 - mu (s))], where a and b are the absorption and scattering coefficients.     

X-Ray Efficiency and Modulation Transfer Function of Fluorescent Rare Earth Screens,Determined by the Monte Carlo Method

The overall, X-Rays to light conversion efliciency and the modulation transfer function of radiographic single front screens have been computed using the Monte Carlo method to simulate the generation and the multiple-scattering diffusion and absorption of light photons in the screen. The computation is based on a set of probability functions related to the absorption of X-Ray photons, free-path lengths and absorption of light photons and reflection conditions at the boundaries. A constant scattering angle has been used for light photons colliding with phosphor grains. For a given phosphor volume fraction, results are shown as a function of phosphor grain size, screen thickness and light wavelength. Some computed results have been checked experimentally.

A comparison is made between screens based on mainly green emitting terbium activated gadolinium oxysulphide, and conventional screens based on blue emitting lead activated calcium tungstate. Screens having equal packing volume, thickness and phosphor grain size show a much higher conversion efficiency and improvement in modulation transfer functions when made of Gd₂O₂S:Tb.     

Single-photon generation by electron beams

We propose a drastically new method for generating single photons in a deterministic way by interaction of electron beams with optical waveguides. We find a single swift electron to produce a guided photon with large probability. The change in energy and propagation direction of the electron reveals the creation of a photon, with the photon energy directly read from the energy-loss spectrum or the beam displacement. Our study demonstrates the viability of deterministically creating single guided photons using electron beams with better than picosecond time uncertainty, thus opening a new avenue for making room temperature, heralded frequency-tunable sources affordable for scientific and commercial developments.     

Vertex/propagator model for least-scattered photons traversing a turbid medium

The least-scattered photons that arrive at a detector through highly scattering tissues have the potential to image internal structures, functions, and status with high imaging resolution. In contrast, optical diffusing tomography is based on the use of the late-arriving photons, which have been diffusely scattered, leading to very low imaging resolution. A good model of the early-arriving photons, i.e., the least-scattered photons, may have a significant effect on the development of imaging algorithms and a further understanding of imaging mechanisms within current high-resolution optical-imaging techniques. We describe a vertex/propagator approach that attempts to find the probabilities for least-scattered photons traversing a scattering medium, based on analytical expressions for photon histories. The basic mathematical derivations for the model are outlined, and the results are discussed and found to be in very good agreement with those from the Monte Carlo simulations.     

Monte Carlo study of coherent diffuse photon transport in a homogeneous turbid medium: a degree-of-coherence based approach

We employ a Monte Carlo (MC) algorithm to investigate the decoherence of diffuse photons in turbid media. For the MC simulation of coherent photons, the degree of coherence, defined as a random variable for a photon packet, is associated with a decoherence function that depends on the scattering angle and is updated as a photon interacts with a medium via scattering. Using a slab model, the effects of medium scattering properties were studied, which reveals that a linear random variable model for the degree of coherence is in better agreement with experimental results than a sinusoidal model and that decoherence is quick for the initial few scattering events followed by a slow and gradual decrease of coherence.     

Surface plasmon-driven hot electron flow probed with metal-semiconductor nanodiodes

A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO(2)) Schottky diodes. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowler's law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flow generated by internal photoemission and localized surface plasmon resonance.     

Effects of electron scatterings on thermal conductivity of thin metal films

In the framework of the statistical conduction models for polycrystalline and monocrystalline metal films, the thermal conductivity due to electron transport is calculated under the further assumption that the electron relaxation time in the bulk material depends on electron energy. It is shown that the same function describes the size effect in thermal and electrical conductivities, and that the Wiedemann-Franz law holds for any electron scattering in thin metal films.     

Statistical treatment of photon/electron counting: extending the linear dynamic range from the dark count rate to saturation

An experimentally simple photon counting method is demonstrated providing 7 orders of magnitude in linear dynamic range (LDR) for a single photomultiplier tube (PMT) detector. In conventional photon/electron counting methods, the linear range is dictated by the agreement between the binomially distributed measurement of counted events and the underlying Poisson distribution of photons/electrons. By explicitly considering the log-normal probability distribution in voltage transients as a function of the number of photons present and the Poisson distribution of photons, observed counts for a given threshold can be related to the mean number of photons well beyond the conventional limit. Analytical expressions are derived relating counts and photons that extend the linear range to an average of ～11 photons arriving simultaneously with a single threshold. These expressions can be evaluated numerically for multiple thresholds extending the linear range to the saturation point of the PMT. The peak voltage distributions are experimentally shown to follow a Poisson weighted sum of log-normal distributions that can all be derived from the single photoelectron voltage peak-height distribution. The LDR that results from this method is compared to conventional single photon counting (SPC) and to signal averaging by analog to digital conversion (ADC).     

Theory of the double-edge technique for Doppler lidar wind measurement

The theory of the double-edge technique is described by a generalized formulation that substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared with the single-edge technique. Use of two high-resolution edge filters reduces the effects of Rayleigh scattering on the measurement by as much as an order of magnitude and allows the signal-to-noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined. The effects of Rayleigh scattering are then subtracted from the measurement, and we show that the correction process does not significantly increase the measurement noise for Rayleigh-to-aerosol ratios as high as 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photons, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh-to-aerosol ratio of 5. Methods for increasing the dynamic range to more than +/-100 m/s are given.     

Near-field microwave microscope and electron-spin-resonance detection: ruby crystal surface

Microwave photons can image a surface by using near-field geometry with spatial resolution close to the nanometer-length scale. We detected electron-spin resonance (ESR) on ruby surfaces by using microwave photons at the S-band frequency (3.73 GHz). The spatial locations of the electron-spin centers were pinpointed with localized incident microwave photons generated by using evanescent microwave microscopy (EMM). We show that the EMM probe is capable of resolving 20,000 spin transitions, compared with the approximately 10(10) minimum detectable spins of the conventional continuous-wave ESR spectrometer. This represents roughly a 6-order-of-magnitude enhancement in sensitivity. Our ultimate goal is to achieve the minimum detectable spin transition of a single electron and nanometer-level spatial resolution by using microfabricated atomic force microscopy-EMM probes.