Photon Emission Mechanism Research Articles

3D optically-stimulated-luminescence-based dosimetry using LYSO:Ce scintillators

The search for a reusable 3D dosimeter is ongoing and motivated by the impact it would have on development and verification of complex modalities in radiotherapy. We present a proof-of-concept 3D measurement of a proton-irradiated LYSO:Ce scintillator, using the resettable photon-emission mechanism known as optically stimulated luminescence and a novel optical readout system. Through this demonstration, we show that LYSO:Ce, in addition to being capable of real-time beam imaging, can be employed as a reusable post-irradiation 3D dosimeter with high spatial resolution.

Unravelling the secrets of blazar OT 081: a multiwavelength investigation

ABSTRACTOT 081 is a low-synchrotron-peaked (LSP) frequency blazar target, and has strong emission in the γ-ray band. In July 2016, a significant short-term flare was observed in the optical, X-ray and γ-ray bands. In addition, a long-term orphan flare was observed in the X-ray band from 2009 to 2012. Using the multiwavelength data, we investigate the origin of these two flares and the emission mechanism of γ-ray photons. According to the correlation analysis, we suggest that both flares may have originated from the formation of the new dissipation zones within the jet rather than the change of Doppler factor. The 2016 short-term flare happens on small-scale dissipation zone, while the long-term X-ray flare originates from large scale dissipation zone. Furthermore, we study the spectral energy distribution (SED) to investigate whether the broad-line region (BLR) and the dust torus can provide enough external photons to explain the γ-ray emission of the 2016 flare within the leptonic scenario. We find that the 2016 flare can be explained when the scale of the newly formed dissipation zone is comparable to that of BLR. For the 2009–2012 orphan X-ray flare, we suggest that it may be dominated by the synchrotron self-Compton (SSC) process in a newly formed dissipation zone at pc scale, since both the magnetic field and the external soft photon field energy density are small enough at this region. In summary, the emission mechanism of OT 081 could be explained in the leptonic scenario.

Puzzling time properties of proportional electroluminescence in two-phase argon detectors for dark matter searches

Proportional electroluminescence (EL) in noble gases is a physical process routinely used in two-phase (liquid–gas) detectors for low-energy astroparticle-physics experiments. In this work, the time properties of visible-light EL in two-phase argon detectors have been systematically studied for the first time. In particular, two unusual slow components in the EL signal, with their contributions and time constants increasing with electric field, were observed. This puzzling property is not expected in any of the known mechanisms of photon and electron emission in two-phase media. Time constants of these components is about 4–5 upmu s and 50 upmu s. In addition, a specific threshold behavior of the slow components was revealed: they emerged at a threshold in reduced electric field of 4.8 ± 0.2 Td regardless of the gas phase density, which is about 1 Td above the onset of standard (excimer) EL. There is a conspicuous similarity between this threshold and reduced field threshold of EL in NIR occurring via higher atomic excited states Ar^{*}(3p^{5}4p). An unexpected temperature dependence of slow components was also observed: their contribution decreased with temperature, practically disappearing at room temperature. We show that the puzzling properties of slow components can be explained in the framework of hypothesis that these are produced in the charge signal itself due to trapping of drifting electrons on metastable negative argon ions.

A novel method for investigating optically stimulated luminescence in scintillators

Optically stimulated luminescence (OSL) is a known property in many scintillators, and investigating the branching ratio of thermalized carriers to this photon-emission mechanism could aid both scintillator development and OSL-based dosimetry. We present a novel method for determining the branching ratio of thermalized carriers to OSL in scintillators while monitoring the energy deposition from 511 keV gamma photons on an event-by-event basis. We demonstrate the method using a cerium-doped lutetium oxyorthosilicate (LYSO:Ce) crystal, and validate that the observed signal indeed originates from OSL photons by estimating the lifetime of the OSL traps, which is consistent with values found in the literature. We report a branching ratio of thermalized carriers to the OSL traps in LYSO:Ce of (1.0±0.2)% upon excitation with 511 keV gamma photons, using a one-trap-one-recombination-center model.

Optically stimulated luminescence in state-of-the-art LYSO:Ce scintillators enables high spatial resolution 3D dose imaging

In this contribution, we study the optically stimulated luminescence (OSL) exhibited by commercial hbox {Lu}_{(2-x)}hbox {Y}_xhbox {SiO}_5:Ce crystals. This photon emission mechanism, complementary to scintillation, can trap a fraction of radiation energy deposited in the material and provides sufficient signal to develop a novel post-irradiation 3D dose readout. We characterize the OSL emission through spectrally and temporally resolved measurements and monitor the dose linearity response over a broad range. The measurements show that the hbox {Ce}^{3+} centers responsible for scintillation also function as recombination centers for the OSL mechanism. The capture to OSL-active traps competes with scintillation originating from the direct non-radiative energy transfer to the luminescent centers. An OSL response on the order of 100 ph/MeV is estimated. We demonstrate the imaging capabilities provided by such an OSL photon yield using a proof-of-concept optical readout method. A 0.1 hbox {mm}^3 spatial resolution for doses as low as 0.5 Gy is projected using a cubic crystal to image volumetric dose profiles. While OSL degrades the intrinsic scintillating performance by reducing the number of scintillation photons emitted following the passage of ionizing radiation, it can encode highly resolved spatial information of the interaction point of the particle. This feature combines ionizing radiation spectroscopy and 3D reusable dose imaging in a single material.

Broadband emission from polycrystalline graphite

Visible radiation spectra of polycrystalline graphite under electrical and laser excitation is studied. It is shown that two different mechanisms of photon emission with a broadband spectrum are implemented in this material. The radiation arising as a result of resistive heating is thermal radiation, whose parameters are close to those of blackbody radiation. The laser-induced secondary radiation in the visible range is anti-Stokes luminescence. A red shift of the laser-induced radiation from fine-grained graphite with respect to the similar spectrum of a bulk sample is observed.

Photon emission enhancement studies from the interaction of ultraintense laser pulses with shaped targets.

We study the photon emission by bremsstrahlung and nonlinear Compton scattering from interaction of ultra-intense laser pulses with cone target and flat foil using particle-in-cell simulations. The simulations are performed for laser pulses interacting with Al and Au targets. The strength of the two mechanisms of photon emission from bremsstrahlung and nonlinear Compton scattering are compared. When an ultra-intense (I>10^{22}W/cm^{2}) laser interacts with a cone and a foil target, photon emission by bremsstrahlung is found to be comparable to that from nonlinear Compton scattering. The obtained electron energy as well as the energy and number of photons emitted were found to be higher in case of cone shaped target compared with that of a foil target. The enhanced photon emission from cone shaped target is attributed to the guiding or collimation of hot electrons towards the cone tip from the self-generated magnetic field and electrostatic field along the cone surface which pushes the hot electrons towards the tip.

Switchable photon and phonon emission properties of an atomically precise Ag14core-based two-dimensional silver cluster-assembled material

Changes in structural architectureviainterlayer C–H⋯F interactions triggered by solvent molecules influence the dual (photon and phonon) emission mechanism of a two-dimensional cluster-assembled material, [Ag14(StBu)10(CF3COO)4(4,4′-azopyridine)2].

Use of carrier injection engineering to increase the light intensity of a polycrystalline silicon avalanche mode light-emitting device

This paper demonstrates a polycrystalline silicon avalanche mode light-emitting device. The unique N+PN+PN+ cascade structure is designed to enhance light intensity via carrier injection engineering, in which the minority carriers are injected from the forward-biased junction to the light emission junction. Visible light can be observed at the reverse-biased PN junctions when the device operating voltage exceeds 20 V. In particular, the phonon-assisted indirect interband recombination of carriers with excess energy may be the main mechanism of photon emission. A specific junction model is proposed to explain that the light intensity peaks are generated primarily via carrier injection. Comparing the spectral measurements of a single polysilicon N+P junction device and the proposed cascade device shows that the strategy of improving the luminous intensity via carrier injection engineering is feasible and effective.

Photon radiation in hot nuclear matter by means of chiral anomalies

A new mechanism of photon emission in the quark-gluon plasma is proposed. Photon dispersion relation in the presence of the $CP$-odd topological regions generated by the chiral anomaly acquires an imaginary mass. It allows photon radiation through the decay $q\to q\gamma$ and annihilation $q\bar q\to \gamma$ processes closely related to the chiral Cherenkov radiation. Unlike previous proposals this mechanism does not require an external magnetic field. The differential photon emission rate per unit volume is computed and shown to be comparable to the rate of photon emission in conventional processes.

Preflight performance studies of the PoGOLite hard X-ray polarimeter

Polarimetric studies of astrophysical sources can make important contributions to resolve the geometry of the emitting region and determine the photon emission mechanism. PoGOLite is a balloon-borne polarimeter operating in the hard X-ray band (25–240 keV), with a Pathfinder mission focussing on Crab observations. Within the polarimeter, the distribution of Compton scattering angles is used to determine the polarisation fraction and angle of incident photons. To assure an unbiased measurement of the polarisation during a balloon flight it is crucial to characterise the performance of the instrument before the launch. This paper presents the results of the PoGOLite calibration tests and simulations performed before the 2013 balloon flight. The tests performed confirm that the polarimeter does not have any intrinsic asymmetries and therefore does not induce bias into the measurements. Generally, good agreement is found between results from test data and simulations which allows the polarimeter performance to be estimated for Crab observations.

Investigation of a Ge nanoparticle film by means of electron stimulated photon emission spectroscopy

Electron stimulated photon emission spectroscopy was used for the study of the electronic structure of Ge nanoparticles. A nanoparticle film was prepared by thermal deposition on a quartz substrate. Photon emission was stimulated by electron bombardment at energies of several hundred electron volts. Electron field emission from a W-tip was used at tip voltage Ut=100–600eV. A spectrometer in combination with a liquid nitrogen cooled charge-coupled device (CCD) camera was used for light detection. Light emission spectra were measured in the energy range 1.18–4.2eV. They are characterized by features at ∼1.6 and ∼3.1eV. A comparison with light emission spectra obtained with another excitation, i.e. by applying a voltage of Uf=2V to the Ge nanoparticle film gives further insight into the mechanism of photon emission.

A hybrid superconductor-normal metal electron trap as a photon detector

A single-electron trap built with two superconductor-insulator-normal metal tunnel junctions and coupled to a readout SINIS-type single-electron transistor A (SET A) was studied in a photon detection regime. As a source of photon irradiation, we used an operating second SINIS-type SET B positioned in the vicinity of the trap. In the experiment, the average hold time of the trap was found to be critically dependent on the voltage across SET B. Starting in a certain voltage range, a photon-assisted electron escape was observed at a rate roughly proportional to the emission rate of the photons with energies exceeding the superconducting gap of S-electrodes in the trap. The discussed mechanism of photon emission and detection is of interest for low-temperature noise spectrometry, and it can be of relevance for the ampere standard based on hybrid SINIS turnstiles.

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed.

Photon emission mechanism for the hydrogen atom from a new, non-Copenhagen interpretation of the electron wave functions

Photon emission mechanism for the hydrogen atom from a new, non-Copenhagen interpretation of the electron wave functions

Proton-proton bremsstrahlung: Consequences of different on-shell-point conditions

Background: Proton-proton bremsstrahlung ($pp\ensuremath{\gamma}$) is investigated both theoretically and experimentally. The $pp\ensuremath{\gamma}$ amplitudes can be classified according to the number and location of on-shell points at which they are evaluated. Quantitative understanding of the effect on the $pp\ensuremath{\gamma}$ cross section of using different on-shell-point conditions is lacking, but it is essential to identifying the photon emission mechanism governing the $pp\ensuremath{\gamma}$ process. Method: Four different $pp\ensuremath{\gamma}$ amplitudes, which include four-on-shell-point amplitudes and one-on-shell-point amplitude, are generated from a realistic one-boson-exchange (ROBE) model for $pp$ scattering. These ROBE amplitudes are used to investigate the consequence of using different on-shell-point conditions in calculating the $pp\ensuremath{\gamma}$ cross sections. Purpose: We verify the validity of the ROBE $pp\ensuremath{\gamma}$ amplitudes. We explore similarities between the four-on-shell-point ROBE and two-$u$-two-$t$ special (TuTts) soft-photon $pp\ensuremath{\gamma}$ amplitudes and important differences with the one-on-shell-point ROBE and Low $pp\ensuremath{\gamma}$ amplitudes. We demonstrate that the precision Kernfysisch Versneller Instituut (KVI) data can only be described by the four-on-shell-point (ROBE or TuTts) amplitude. We use the ROBE four-on-shell-point amplitude and one-on-shell-point amplitude to investigate systematically the effect of using different on-shell-point conditions to calculate the $pp\ensuremath{\gamma}$ cross section. Furthermore, we identify a general principle that governs the process. This general principle is also applicable to other bremsstrahlung processes involving the scattering of two identical nucleons. Results: (i) The four-on-shell-point ROBE (or TuTts) amplitude describes the high-precision KVI data much better than does the one-on-shell-point ROBE (or Low) amplitude. Although the contribution from the anomalous magnetic moment of the proton is very significant, it does not completely dominate the KVI $pp\ensuremath{\gamma}$ cross sections. (ii) The four-on-shell-point ROBE (or TuTts) amplitude describes the TRIUMF data better than does the one-on-shell-point ROBE (or Low) amplitude. (iii) The effect on the $pp\ensuremath{\gamma}$ cross section of using different on-shell-point conditions is significant in the hard-photon region, i.e., for small proton scattering angles $\ensuremath{\theta}$ ($={\ensuremath{\theta}}_{3}={\ensuremath{\theta}}_{4}$, symmetric scattering angles) far from the elastic limit ($\ensuremath{\theta}\ensuremath{\rightarrow}{45}^{\ensuremath{\circ}}$); in contrast, the effect becomes insignificant in the vicinity of the elastic limit. Near the limit as $\ensuremath{\theta}$ tends to ${45}^{\ensuremath{\circ}}$ (or as the photon energy $K$ approaches zero), the four-on-shell-point and one-on-shell-point amplitudes approach one another, a general principle applicable to all bremsstrahlung processes because only kinematics is involved. Conclusion: The four-on-shell-point ROBE amplitude provides a quantitative description of $pp\ensuremath{\gamma}$ cross sections. The anomalous magnetic moment is an important component of the photon emission mechanism. The four-on-shell-point property of the full ROBE amplitude is essential to properly describing the complete range of the precision KVI data and the TRIUMF data, although the one-on-shell-point ROBE amplitude is adequate in the region near the elastic limit.

大豆愈伤组织超弱光子辐射的双指数模型

Double-exponential model of ultraweak photon emission of soybean callus was proposed based on the mechanism of ultraweak photon emission in biological system. Treated soybean callus by UV-B radiation (20 μW/cm^2) for 2 h, ultraweak photon emission of soybean callus treated by UV-B for 4 d after was measured. The results show that double-exponential model accurately describes the characteristics of ultraweak photon emission of soybean callus and its changes, delayed luminescence of ultraweak photon emission of soybean callus is composed by both fast and slow phase. Spontaneous luminescence in ultraweak photon emission of soybean callus and some parameters of delayed luminescence such as initial photon number I(0), integrated intensity I(T) and decay parameters of delayed luminescence are obtained through analysis of the double-exponential model, the characteristics of ultraweak photon emission of soybean callus can be quantitative expressed by these parameters. The studies also find that there is an excellent positive correlation between initial photon number I(0) and integrated intensity I(T), and I(0) or I(T) can be used to measure the metabolic intensity of soybean callus. Induced by UV-B radiation (20 μW/cm^2) for 2 h, the decay parameter of fast phase in delayed luminescence decreases first and then increases, the change of spontaneous luminescence is opposite, it suggests that fast phase of delayed luminescence is sensitive to the UV-B radiation and membrane lipid peroxidation could be the reason of the drastic change of fast phase in delayed luminescence of soybean callus.

Bremsstrahlung mechanism of heavy vector meson production

Vector mesons as $\ensuremath{\varphi}$, $\ensuremath{\omega}$, and $J/\ensuremath{\Psi}$ with ${J}^{PC}={1}^{--}$ are produced at colliding electron-positron high energy beams through the annihilation and the scattering channels. We consider here the scattering mechanism, namely, the creation of a vector meson in the fragmentation region of one of the initial leptons. It corresponds to the kinematics where the vector meson is emitted close to the direction of one of the colliding leptons, while the direction of the other lepton remains close to the initial one. The annihilation channel contribution is enhanced by the initial hard photon emission mechanism. It occurs in the kinematical region where the final particles are emitted at large angles and plays the role of background. Differential distributions of the energy fraction and of the transverse component of the vector meson are calculated. The relevant formalism and the numerical estimations are presented.

Photon emission mechanisms of different phosphors

A comparison is made between the different mechanisms involved in the electron and photon excitation processes of four different phosphors. The green luminescence peak of SrAl 2O 4:Eu 2+, Dy 3+ phosphor is normally asymmetrical and tails towards the longer wavelengths. The observed peak in the longer wavelength region is associated with preferential alignment of one of the Eu 2+ d-orbitals. Four Gaussian peaks fitted to the cathodoluminescent (CL) and photoluminescent (PL) spectra of broad band emission spectra of X 1–Y 2SiO 5:Ce phosphor may be attributed to the two different sites (A1 and A2) of the Ce 3+ ion in the host matrix and the difference in orientation of the neighbour ions in the complex crystal structure. Co-activation of Ce 3+ with Eu 3+ quenches the red emission from Eu 3+ and considerably increases the blue emission from Ce 3+ in a SiO 2 matrix. In nanoparticulate PbS the emission data show a blue shift from the normal emission at ∼3200 nm in PbS bulk to ∼560–700 nm. The blue shifting of the emission wavelengths from 3200 to ∼560–700 nm is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles.

Photon emission by nanocavity-enhanced quantum anti-Zeno effect in solid-state cavity quantum-electrodynamics

Solid-state cavity quantum-electrodynamics (QED) has great potential owing to advances such as coupled systems combining a nanocavity and a quantum dot (QD). These systems involve two photon-emission mechanisms: the Purcell effect in the weak coupling regime and vacuum Rabi-splitting in the strong coupling regime. In this paper, we describe a third emission mechanism based on the quantum anti-Zeno effect (AZE) induced by the pure-dephasing in a QD. This is significantly enhanced by the inherent characteristics of the nanocavity. This mechanism explains the origin of strong photon emission at a cavity mode largely detuned from a QD, previously considered a counterintuitive, prima facie non-energy-conserving, light-emission phenomenon. These findings could help in controlling the decay and emission characteristics of solid-state cavity QED, and developing solid-state quantum devices.