Energy Photon Detector Research Articles

Second2s−2plevel crossing at theFcenter in KCl evidenced by frequency upconversion spectroscopy

Subpicosecond dynamics of resonant secondary emission from the $F$ center in KCl at 5 K is investigated by frequency upconversion method. Detected photon energy range is from 0.65 to 1.55 eV and covers almost all the stationary emission band. The decay time of the luminescence intensity is longer in the low-energy side of emission band than in the high-energy side. A series of transient luminescence spectra shows relatively high intensities in the low-energy side. These two features indicate the existence of the second $2s\text{\ensuremath{-}}2p$ level crossing near the relaxed excited state.

Infrared quantum counting by nondegenerate two photon conductivity in GaAs

We report on infrared quantum counting of photons at optical communication wavelengths based on nondegenerate two-photon absorption in a GaAs photomultiplier tube. The detected photon energy is lower than the GaAs band gap and the energy difference is complemented by a high intensity pump field. This detection setup is simple, compact, has a broad spectral bandwidth, and benefits from the intrinsic low noise and dark counts of large band gap semiconductor junctions.

Excitation dependence of photoluminescence in silicon quantum dots

We have studied the optical properties of silicon quantum dots (QDs) embedded in a silicon oxide matrix using photoluminescence (PL) and time-resolved PL. A broad luminescence band is observed in the red region, in which the time evolution exhibits a stretched exponential decay. With increasing excitation intensity a significant saturation effect is observed. Direct electron–hole recombination is the dominant effect in the red band. A relatively narrow peak appears around 1.5 eV, which is attributed to the interface states overlapping with transition from the ground state of the silicon QDs. The saturation factor increases slowly with detection photon energy between 1.5 and 1.8 eV, which is attributed to the emission from zero-phonon electron–hole recombination. At higher photon energies the significantly increased saturation factor suggests a different emission mechanism, most likely the defect states from silicon, silicon oxide or silicon rich oxide.

The state filling effect in p-doped InGaAs/GaAs quantum dots

The electron dynamics in modulation p-doped InGaAs/GaAs self-assembled quantum dotshave been studied by time-integrated and time-resolved up-conversion photoluminescence.A significant state filling effect is observed with exclusion of the phonon bottleneck effect.The rise time and decay time are found to vary with the excitation intensity forthe ground state and excited states of the quantum dots. In the low intensityregime the rise time decreases with increasing excitation intensity because of theincreased scattering between the photoexcited electrons and excess holes. Bycontrast, in the high excitation regime the rise time exhibits a slight decrease dueto the state filling effect. A simplified rate equation model indicates that themodulation p-doped quantum dots exhibit an increasing saturation factor withincreasing detection photon energy based on the theory of parabolic confinement ofthe quantum dots, which is consistent with the observed excitation dependence.

Linear polarization of the photoluminescence of quantum wells subject to in-plane magnetic fields

The degree and orientation of the magnetic-field induced linear polarization of the photoluminescence from a wide range of heterostructures containing (001) (Cd,Mn)Te quantum wells between (Cd,Mn,Mg)Te barriers has been studied as a function of detection photon energy and of the strength and direction of a magnetic field applied in the plane of the quantum well. Three field-induced contributions to the linear polarization of the photoluminescence are observed which differ in their dependence on the angle, $\ensuremath{\varphi}$, between the magnetic field and the [110] direction, being, respectively, independent of $\ensuremath{\varphi}$, varying as $\mathrm{cos}(2\ensuremath{\varphi})$, and as $\mathrm{cos}(4\ensuremath{\varphi})$. A theoretical description of each of these contributions in terms of an in-plane deformation acting on the valence band states is presented and verified by comparison with the experimental data. In our model, we account for the possibility that the in-plane deformations are distributed in both magnitude and direction. We conclude that it is possible to account for the magnetic-field induced linear polarization of the photoluminescence via in-plane deformations and without invoking terms in the valence band spin Hamiltonian which are cubic in $J$. The models developed in the present paper apply in full measure to nonmagnetic quantum wells as well as ensembles of disklike quantum dots with shape and/or strain anisotropy.

Simulation of X-ray and gamma ray detector response and spectral deconvolution

A detailed Monte-Carlo code has been developed from basic principles that simulates the responses of high energy photon detectors to incident beams of photons. The detector response matrice (DRM)s are calculated using this code. Deconvolution of an artificially generated spectrum is presented.

Fokker‐Planck Modeling of Asymmetric Footpoint Hard X‐Ray Emission in Solar Flares

Chromospheric hard X-ray emission in a solar flare generally occurs in two magnetically connected regions. Recent spatially resolved hard X-ray observations carried out using the RHESSI spacecraft have shown that the ratio of total X-ray fluxes from the two footpoints is time dependent and demonstrates a weak but detectable photon energy dependence. A Fokker-Planck code is used to identify possible scenarios that could reproduce the observed dependence of footpoint asymmetry on time and energy. The code, which is benchmarked against analytical results in the limit of collisionless precipitation from a symmetric flaring loop, includes collisional friction and pitch-angle scattering, asymmetric magnetic mirroring, and a source term that can be prescribed arbitrarily. This model is used to demonstrate that the observed dependence of hard X-ray asymmetry on photon energy can be attributed to an energetic electron source that is isotropic at low energy (presumed to be due to Coulomb collisions) and at high energy (presumed to be due to resonant wave-particle scattering) and strongly anisotropic at intermediate energies.

Electronic states of AgCl nanocrystals embedded in crystalline KCl studied by95−GHzoptically detected magnetic resonance spectroscopy

The properties of shallow electron centers (SEC), self-trapped holes (STH), and self-trapped excitons (STE) are investigated in nanocrystals of AgCl embedded in a crystalline KCl matrix. Time-resolved photoluminescence (PL) measurements reveal a behavior different from that in bulk AgCl: an inhomogeneous distribution of the properties of the recombination centers is revealed by the spectral dependence of the decay rates. In time scales from nano- to microseconds, slower rates are systematically observed at the low-energy side of the emission range. In high-frequency $(95\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ optically detected magnetic resonance (ODMR) measurements, the transitions of SEC and STH centers are only observed in the extreme-low energy tail of the PL emission and they exhibit a broad and asymmetric $g$-value distribution (width $\ensuremath{\delta}g\ensuremath{\approx}0.07$), ranging up from the respective bulk $g$-values. A complex behavior of the ODMR spectrum in the STE region is revealed, showing different spectra at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and at higher temperature, and, different from the bulk AgCl case, exhibiting a strong dependence upon variation of the detection photon energy. Analysis of the measurements at $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ yields a zero field splitting in the STE triplet state in good agreement with the bulk value, while the $g$-values are slightly increased relative to bulk, which is ascribed to the nanocrystal environment.

Field study of the migration of90Sr,237Np,238Pu and241Am in unsaturated Chinese loess under artificial rainfall condition

SummaryThe migration of90Sr,237Np,238Pu and241Am in unsaturated Chinese loess under artificial rainfall condition was investigated at the CIRP´s field test site. Quartz containing90Sr,237Np,238Pu and241Am was introduced into an experimental pit and the pit was backfilled. Artificial rainfall of 10-15 mm/d was applied to the experimental pit for 1078 days. HTO was used for obtaining the migration rate of moisture water in the pit. Vertical core soil samples were taken twice a year and cut into 0.5cm long section soil samples. The section soil samples were weighed and wind dried. Radioactivity of237Np and241Am in the section soil samples was measured with a low energy photon detector (HPGe);90Sr and238Pu were determined with a low backgroundα&βdetector after chemical separation. During the experiment, moisture water at different depths was taken on a daily basis and3H in the moisture water samples was determined with liquid scintillation technique. Experimental result indicates that the downward migration rate of moisture water was 2.27 cm/d and the retardation factors of90Sr,237Np,238Pu and241Am in the loess unsaturated zone were 9.9 × 102, 3.2 × 103, 3.2 × 103and 4.5 × 103, respectively. This result suggests that90Sr,237Np,238Pu and241Am could be effectively retarded by the loess unsaturated zone.

Field study of the migration of237Np,238Pu and241Am in a weak loess aquifer

The migration of 237Np, 238Pu and 241Am in a weak loess aquifer was investigated in-situ in an Underground Research Facility. Quartz containing 237Np, 238Pu, 241Am and 3H was introduced into the aquifer with a stainless steel tube. The local water flow in the aquifer was monitored using a 3-dimensional sampling system. The contaminated area of the aquifer was obtained wholly with the help of a pre-installed stainless steel frame and cut into small pieces (samples) 3-dimensionally and analyzed. 3H in the water samples was determined with a liquid scintillation analyzer (Tri-Camb 2250). Radioactivity of 237Np and 241Am in the soil samples were determined with a low energy photon detector (HPGe), 238Pu was analyzed with a low background α&β detector after chemical separation. The result of the 3-year experiment indicated that 237Np, 238Pu and 241Am were strongly absorbed by the loess aquifer. The retardation factor of 237Np in the loess aquifer was 1.54.104.

Electromagnetic calorimetry with PbWO 4 in the energy regime below 1 GeV

The performance and application of PbWO 4 in electromagnetic calorimetry at energies below 1 GeV has been extensively investigated in parallel with the improvement of the quality of large size crystals. The lineshape, energy and time response to monoenergetic electrons and photons between 50 and 850 MeV energy was measured using detector arrays of up to 5×5 crystals read-out with fast photomultipliers. First results of Tb- and Mo-doped PbWO 4-crystals optimized for very low energy photon detection are presented. As a first application, the proposed concept for a compact photon spectrometer to be implemented into the ANKE magnetic spectrometer at COSY (FZ Jülich) is illustrated.

Localized quantum well excitons in InGaN single-quantum-well amber light-emitting diodes

Optical properties of the InGaN single-quantum-well amber light-emitting-diodes were investigated to verify the importance of localized quantum well (QW) excitons in their spontaneous emission mechanisms. The internal piezoelectric field (FPZ) across the QW of the order of 1.4 MV/cm was confirmed to point from the surface to the substrate. Transmittance and photovoltaic spectra exhibited a broad band tail state, and the emission lifetime increased with decreasing detection photon energy. The electroluminescence spectra did not show remarkable energy shift between 10 and 300 K. The spectra exhibited an exponential tail and the higher energy portion increased more rapidly than that of the lower energy one, reflecting a thermal distribution of the localized carriers within the tail states. Since the well thickness is only 2.5 nm and is atomically flat, the device is considered to exhibit reasonably efficient emission with the external quantum efficiency of 5% at 20 mA in spite of the presence of FPZ and large number of threading dislocations due to radiative decay of the localized QW excitons.

Localized exciton and its stimulated emission in InGaN multiple quantum wells

We have studied exciton localization and its stimulated emission in InGaN multiple quantum wells at 2K. Mobility edge was determined by the use of site-selectively excited photoluminescence (PL) measurement. A time-resolved PL spectroscopy was performed to study exciton relaxation processes. The PL decay time increased with the decrease of the detection photon energy, indicating the dynamical features of exciton localization. Under the high-density excitation, we observed the localized exciton luminescence turned into the stimulated emission just below the mobility edge. In addition, a nonlinear luminescence measurement was performed to study the detailed optical property of the stimulated emission.

Encapsulation of germanium detectors for space flight

A hermetic enclosure for a high-purity germanium detector was developed as part of the detector assembly for the Transient Gamma-Ray Spectrometer flown on the WIND satellite. High G forces during launch (/spl sim/100 G), long term stability (lifetime of at least 5 years), passive cooling and low energy photon detection were critical parameters addressed. These design parameters dictated rigid containment of the detector and the selection of enclosure materials, hermetic high voltage and signal feedthroughs, electrical contacts to the detector and enclosure seals. Sealing of the germanium detector within a valveless hermetic enclosure was achieved with a custom vacuum bell-jar press equipped with a vacuum isolated hydraulic ram. Both vacuum and nitrogen gas filled encapsulations (pressurized to 2 atmospheres) were examined; nitrogen gas filled encapsulations were more rugged.

Elemental concentrations in the spinach reference material determined by k0-based INAA

The IAEA-331 spinach material NIST SRM 1570a Spinach Leaves, submitted to an intercomparison run by the IAEA, has been analysed by k0-based INAA with counting using both low and high energy photon detectors. The results have been compared with (i) the certified values of the NIST SRM 1570a; the agreement is good, taking into account the uncertainties; and (ii) the certified and “consensus” values of the former NIST SRM 1570 Spinach Leaves, their composition in minor and trace-elements are quite similar; however a lower content has been observed for Co, Fe and Sc in the IAEA-331 and for Zn in the SRM 1570. For quality control, the NIST 1573 Tomato Leaves and the NIST 1575 Pine Needles have been analysed using the same conditions as for the IAEA-331. The results agree quite well with the certified and “consensus” values given in the literature.

Wave accessibility effects on lower hybrid current drive in JT-60U

The effects of the wave accessibility condition on lower hybrid current drive (LHCD) are investigated in JT-60U. The density dependence of the detectable maximum photon energy of hard X rays in LHCD plasmas is consistent with that of the maximum electron energy which is limited by the accessibility condition. The current drive efficiency and the driven current profile are affected by the accessibility condition. Inaccessible wave power enhances impurity content, particle recycling and main plasma radiation loss, and in extreme cases triggers a MARFE. The wave intensity on the plasma periphery, which is detected by a Langmuir probe located on the divertor plate, is also consistent with an inaccessible power fraction going into the plasma interior. A ray trace analysis taking account of toroidal effect indicates that the inaccessible wave power is localized on the plasma periphery and finally propagates into the divertor plasma region

K fluorescence yield measurements in rare earth and heavy elements

K fluorescence yields were measured for 16 elements Cu, Mo, Ag, Cd, Sn, Pr, Sm, Gd, Dy, Ho, Yb, Ta, W, Pb, Th and U using a HPGe low energy photon detector employing the reflection geometry set-up. The target atoms were excited using 59.5 keV and 279.2 keV gamma rays from Am-241 and Hg-203 radioactive sources of strengths 20 mCi and 40 mCi respectively. An effective shielding arrangement was used to minimize the background and multiple scattering. Elemental foils of uniform aerial density and purity better than 99.9% were used as targets. The data were recorded in a 4K multichannel analyser. Among the K fluorescence yields measured in this work nine cases constitute the first measurement. The results are compared with the literature experimental values for available cases and also with the semiempirical fits and theoretical predictions.

Angle-Resolved Inverse Photoemission Spectra of Layered1T-VSe2, 1T-TiS2, 1T-TaS2, 2H-NbSe2and 2H-TaSe2

Angle-resolved inverse photoemission spectra have been measured at room temperature on the layered transition metal dichalcogenides, 1T-VSe 2 , 1T-TiS 2 , 1T-TaS 2 , 2H-NbSe 2 and 2H-TaSe 2 , in the isochromat mode with a detection photon energy of 9.4 eV. Their energy dispersion curves are obtained along symmetry directions in the Brillouin zone and compared with theoretical band calculations.

Time-resolved measurements of the radiative recombination in GaAs/AlxGa1-xAs heterostructures.

The radiative recombination of two-dimensional (2D) carriers in an n-type-channel modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction has been studied with time-resolved photoluminescence (PL). Two emission bands related to the recombination of 2D electrons, the so-called H band 1 (HB1) and H band 2 (HB2), are observed in PL. Their spectral shape and position are strongly dependent on the sample and the experimental conditions, and are, e.g., found to shift within a large photon-energy range with the excitation intensity. We have in this study measured the decay time of these PL bands as a function of recombination photon energy, under different experimental conditions. We find that the measured decay times of both the HB1 and HB2 emissions are strongly dependent on the detection photon energy. The decay times are found to increase with decreasing photon energy, in the range 1--100 ns for HB1, and 100 ns to 10 \ensuremath{\mu}s for HB2. This increase is explained as due to a spatial separation between the recombining electron and hole. The results are consistent with a recombination process involving 2D electrons, confined in the interface notch, and holes either from the valence band (HB1) or from neutral acceptors (HB2) in the active GaAs layer. We further show that the band bending in the active GaAs region influences the observed decay times of the HB2 emission, while the HB1 emission is almost unaffected.

Proportional counters as low energy photon detectors

We have investigated the response of an argon methane proportional counter to monochromatic X-rays in the range 99–277 eV. The apparent nonlinearities in mean pulse height as a function of photon energy and the detailed shape of the pulse height distribution for each energy can be predicted quite precisely using the extensive atomic data available for argon. Based on this understanding, we propose a semiempirical system for using a limited amount of calibration data to predict the full response of counters employing less well characterized gasses. Such accurate models of proportional counter response are required to maximize the spectral information that can be derived from observed pulse height distributions.