Light Output Data Research Articles

Anisotropic neutron response of trans-stilbene and impact on a handheld dual particle imager

This manuscript details measurements of the anisotropic light-output response of trans-stilbene, henceforth referred to as stilbene, along the three primary crystal planes and analyzes how this anisotropic response impacts neutron image reconstruction in a handheld dual particle imager (H2DPI). A prototype H2DPI composed of stilbene pillars (6×6×50 mm3) coupled to silicon photomultipliers was built and is capable of imaging both fast neutrons and gamma rays from kilogram quantities of special nuclear material. Stilbene was chosen as the scintillating medium for the imager because of its pulse shape discrimination capability, time resolution and relatively high light output. A drawback to using stilbene, however, is the anisotropic response. A recoiling proton from a neutron elastic-scattering event in stilbene will yield different amounts of scintillation light depending on the direction of the recoiling proton with respect to the crystal lattice. This manuscript analyzes how this anisotropic response impacts neutron image reconstruction in 4π. The light output in the three primary crystal planes of stilbene were independently measured 6–7 times for proton recoil energies ranging from 0.5–5.0 MeV using a quasi-monoenergetic time-of-flight neutron source. The measured light-output data in the three crystal planes were fit with a semi-empirical function based on Birks’ formula. These fits were used to reconstruct the location of a 252Cf source in front of the imager (0°, −0.8°), to the left of the imager (−90°, −0.8°) and directly above the imager (0°, 90°) to determine if it is necessary to apply the directionally-dependent response of stilbene to accurately reconstruct source locations. A bootstrapping technique was applied to the measured data sets to produce 1000 images composed of 2000 cone projections for each source location and each measured light-output curve. List-mode maximum likelihood expectation maximization was applied to each image; the highest pixel location and the full width at half maximum (FWHM) were then recorded. The average and standard deviations of these parameters were taken for each set of images. The maximum difference in the average azimuthal pixel location when varying between the appropriate and incorrect light-output curves for the source locations at (0°, −0.8°) and (−90°, −0.8°) were respectively found to be 1.85±1.39° and 2.03±1.78°. All other pixel locations and FWHMs were within a single standard deviation of uncertainty. Taken together, the anisotropic response of stilbene has negligible impact on the neutron image reconstruction capability of the H2DPI.

Feasibility study of a proton fluence monitor for LEO-Nanosatellite missions based on displacement damage induced in GaAs-LED

High-energy protons trigger irreparable displacement damage (DD) in unbiased Gallium Arsenide (GaAs) Light-Emitting Diode GaAs-LED (GAL) . The DD is caused by non-ionizing-energy-loss (NIEL) process and results in reduction of light emission of the GAL when connected to a power source. Based on this finding a lightweight, low current consuming proton detector for space applications was developed. A commercial off the shelf (COTS) yellow GAL optically coupled to a Light-to-Frequency converter chip (LFC) made the basic assembly of the proposed proton detector. The device was calibrated using selected GAL samples irradiated with a 180 MeV proton beam from a proton-therapy medical cyclotron to 2, 10, 50, 100, 200 and 300 Gy dose levels. The light output of the GAL was measured with the detector assembly and presented as frequency (kHz). Protons of energy distribution between 100 keV to ∼500 MeV are trapped in the inner-shell of the Van Allen belt (VAB) surrounding the earth, whereas the intensity of trapped electrons (Emax ∼10 MeV) remains insignificant. The GAL light output data (frequency) was fitted with a second-degree polynomial function of imparted proton dose (∼ number of protons). The results were parameterized for widely used NASA radiation belt models AP8-Max and AP8-Min using the proton NIEL distribution in Gallium Arsenide (GaAs). Spacecrafts, in particular Nanosatellites operating in low earth orbit (LEO) environment are primarily exposed to those trapped protons in the VAB . Nanosatellites endure severe radiation exposure while passing through the South Atlantic Anomaly (SAA) region. The footprint, mass and average current consumption of the proton detector was 0.027 cm2, 0.28 g and 2.5 mA respectively. The highest detectable level proton fluence (protons⋅cm−2) was evaluated to be 1.17× 109 and 1.01× 109 for AP8-Max and AP8-Min models respectively. Hence the proposed GAL based radiation detector is only suited for proton fluence monitoring on board LEO-Nanosatellites (NanoSat) of short lifetime, usually 6–18 months.

Measured neutron light-output response for trans-stilbene and small-molecule organic glass scintillators

Abstract The neutron light-output response from quasi-monoenergetic neutrons was measured for a O5.08 × 5.08 cm trans-stilbene and a O5.08 × 5.08 cm small-molecule organic glass scintillator . Quasi-monoenergetic neutrons were isolated from a time-of-flight measurement of a Cf-252 spontaneous fission source with a flight distance of 200 cm. Two different methods of waveform analysis were implemented, where the neutron light output response proportional to the detected pulse height distribution (PHD) and pulse integral distribution (PID) were extracted for both types of scintillators. The light output response proportional to the pulse integral was determined by integrating the digitized waveform with an integration window length of 150 ns, which contained > 90% and > 95% of the scintillation light for an averaged neutron and photon waveform above 0.5 MeVee. The extracted light-output data were fitted with a semi-empirical function based on the Birks’ formula. The results show that the small-molecule organic glass scintillator produced more light than the trans-stilbene scintillator for a range of neutron energies of 0.79 ± 0.04 MeV to 3.65 ± 0.38 MeV. The fitted semi-empirical function was used in MCNPX-PoliMi with MPPost to simulate the detector response from an independent measurement of a Cf-252 spontaneous fission source to test the fidelity of the extracted light output response functions. The simulated and measured total neutron count rate agreed to within ± 3 % and ± 1% for the trans-stilbene and small-molecule organic glass scintillators. Due to the different light-output response of the two scintillators, the intrinsic neutron detection efficiency was calculated for equal observable ranges in light output and neutron-equivalent energy units. The intrinsic neutron detection efficiency in the observable light-output range of 0.06 MeVee to 2.4 MeVee was calculated to be 28.66 ± 1.43 % and 34.66 ± 1.73 % for pulse height and pulse integral analysis, respectively. For the same observable light-output range, the intrinsic efficiency of small-molecule organic glass was calculated to be 32.54 ± 1.63 % and 37.39 ± 1.87 % for pulse height and pulse integral analysis, respectively. When observing equal light-output ranges, the small-molecule organic glass was 11.92 ± 6.23% and 7.88 ± 7.35% more efficient than the trans-stilbene using pulse height and pulse integral analysis, respectively. When observing equal neutron-equivalent energy ranges, the trans”–stilbene scintillator was 8.4 ± 6.46% and 11.95 ± 6.23% more efficient than the small-molecule organic glass for pulse height and pulse integral analysis, respectively.

Evaluation of neutron light output response functions in EJ-309 organic scintillators

An accurate model of the nonlinear detector response of organic scintillators to neutrons is required to correctly simulate fast neutron detection, as well as interpret measured pulse height data. Several empirical and semi-empirical models are available to fit measured scintillator light output data. In this work, EJ-309 light output data from neutrons depositing 1.15–5.15MeV on hydrogen were analyzed using empirical models as well as semi-empirical models based on the work of Birks and Voltz. Although all tested models fit the experimental light output data well in the measured range, the models were observed to diverge in low-energy extrapolation. The models were then tested by comparing a measurement and MCNPX-PoliMi simulation of an EJ-309 detector response to fast neutrons from a 252Cf spontaneous fission source. The agreement between the measured and simulated pulse height distributions varied significantly depending on the light output model used. The best agreement between simulated and measured neutron pulse height distributions was achieved by using the Birks model. The bin-by-bin agreement was better than 5% over the range 0.08–2.18MeVee, and better than 10% from 2.18 to 3.13MeVee. The integral count rate over the range 0.08–3.14MeVee differed by less than 1% in absolute units.

Re-Evaluating the Economic Costs of Conflicts

This paper studies the economic costs of conflicts at the country and ethnic group settlement level with light output data as measured by orbital satellites and conflict data spatially mapped to latitude and longitude coordinates. Using a worldwide data set of 7,704 individual ethnic group settlements of 862 ethnic groups in 177 countries, we find that conflicts strongly reduce light output in settlements directly affected by fighting over the period 1992-2008. In addition, conflicts have large negative spillovers both across and within countries: light output in settlements not directly exposed to fighting declines significantly once a conflict begins; neighboring countries also experience large negative effects. The negative effects of conflicts are particularly pronounced in Eastern Europe, but also observable in sub-Saharan Africa, Latin America, and South and South-East Asia. In contrast, conflicts have, on average, no negative effects on light output in the Middle East and in the West. We contrast these results with cross-country regressions with GDP data, which suggest much smaller negative effects of conflicts.

Prognostics of lumen maintenance for High power white light emitting diodes using a nonlinear filter-based approach

High power white light emitting diodes (HPWLEDs), with advantages in terms of luminous efficacy, energy saving, and reliability, have become a popular alternative to conventional luminaires as white light sources. Like other new electronic products, HPWLEDs must also undergo qualification testing before being released to the market. However, most traditional qualification tests, which require all devices under testing to fail, are time-consuming and expensive. Nowadays, as recommended by the Illuminating Engineering Society (IES, IES-TM-21-11), many LED manufacturers use a projecting approach based on short-term collected light output data to predict the future lumen maintenance (or lumen lifetime) of LEDs. However, this projecting approach, which depends on the least-square regression method, generates large prediction errors and uncertainties in real applications. To improve the prediction accuracy, we present in this paper a nonlinear filter-based prognostic approach (the recursive Unscented Kalman Filter) to predict the lumen maintenance of HPWLEDs based on the short-term observed data. The prognostic performance of the proposed approach and the IES-TM-21-11 projecting approach are compared and evaluated with both accuracy- and precision-based metrics.

Measurement of the proton light response of various LAB based scintillators and its implication for supernova neutrino detection via neutrino–proton scattering

The proton light output function in electron-equivalent energy of various scintillators based on linear alkylbenzene (LAB) has been measured in the energy range from 1 MeV to 17.15 MeV for the first time. The measurement was performed at the Physikalisch-Technische Bundesanstalt (PTB) using a neutron beam with continuous energy distribution. The proton light output data is extracted from proton recoil spectra originating from neutron-proton scattering in the scintillator. The functional behavior of the proton light output is described succesfully by Birks' law with a Birks constant kB between (0.0094 +/- 0.0002) cm/MeV and (0.0098 +/- 0.0003) cm/MeV for the different LAB solutions. The constant C, parameterizing the quadratic term in the generalized Birks law, is consistent with zero for all investigated scintillators with an upper limit (95% CL) of about 10^{-7} cm^2/MeV^2. The resulting quenching factors are especially important for future planned supernova neutrino detection based on the elastic scattering of neutrinos on protons. The impact of proton quenching on the supernova event yield from neutrino-proton scattering is discussed.

VUV spectroscopy of luminescent materials for plasma display panels and Xe discharge lamps

In plasma display panels and Xe discharge lamps, phosphors with a high quantum efficiency under vacuum ultraviolet (VUV) excitation are required. Absolute quantum efficiencies of such phosphors are usually be determined by using synchrotron radiation since the excitation wavelength can be freely tuned over a wide energy range. A serious drawback of the use of synchrotron radiation for this purpose is the accessibility towards research facilities and the time-consuming measurement procedure which is not in line with the fast pace of industrial phosphor development. We have built up a VUV spectrophotometer which allows the measurement of absolute excitation and reflection spectra in the wavelength range of 115–350 nm. Additionally, a procedure was developed to derive spectrally resolved quantum efficiencies from these excitation spectra. In this work, the set-up of the VUV spectrometer is described and its performance is documented by comparing the obtained light output data of several phosphors with results recorded by using synchrotron radiation. It is shown that also a relatively simple VUV spectrometer can be used to determine absolute light output and quantum efficiency data in the excitation range between 115 and 350 nm with an experimental error lower than 10%.

Light output measurements of C 6H 6 and C 6H 12 scintillators for protons

Light output of C 6H 6 and C 6H 12 proton scintillators has been measured for neutrons using a 241Am-Be source. In these measurements the energy of the neutrons (recoil protons) was measured using time-of-flight technique. The light output of the C 6H 6 scintillator was measured for seven proton energies ranging from 2.185 to 4.236 MeV while that of the C 6H 12 scintillator was measured for eight neutron energies ranging from 2.201 to 5.147 MeV. The measured light output data of the C 6H 6 scintillator is in good agreement with the published data. Additionally the proton and deuteron response ratio of a C 6D 6 scintillator identical in shape and size was derived from these measurements. The response ratio of the C 6D 6 scintillator agrees with the published data.

Correlation of time-resolved electroluminescence and cathodoluminescence measurements on quantum well light emitters with varying barrier widths

We present results of a comparative study of the time-dependent luminescence properties of multiple quantum well structures with varying barrier widths which are embedded in the active area of a light-emitting device. The carrier kinetics is investigated by different experimental approaches: Cathodoluminescence and electroluminescence experiments where excitation is on/off-modulated for the purpose of time-resolved measurements and time-resolved electroluminescence experiments in the small signal regime which allow for observation of the carrier kinetics under flatband conditions. Due to the exact determination of the excess carrier density the latter technique provides a sensitive tool for a precise estimation of the mono- and bimolecular recombination coefficients. Comparison with light output data yields radiative and nonradiative parts. We find that coupling of quantum wells dramatically favors nonradiative interface recombination as expected from a theoretical model accounting for the superlattice wavefunctions. On the other hand, the bimolecular recombination rate remains unaltered even when the barrier width is lowered from 18 to 0.9 nm. In contrast, on/off modulated experiments reveal that luminescence decay is strongly influenced by carrier drift out of the active area. A barrier width dependent carrier mobility in growth direction accounts for these results if phonon assisted hopping rather than Bloch transport is presumed. Thus, an estimation of device quality of quantum well light emitters by conventional time-resolved cathodo- (or photo-) luminescence experiments is found to be possible if internal field induced carrier drift processes are taken into account.

Observation of gain compression in a GaAlAs diode laser through a picosecond transmission measurement

The carrier density and light output of a pulse modulated GaAlAs diode laser were measured with picosecond time resolution. A new technique for measuring carrier density by transmission of picosecond light pulses through a diode laser is described. The measured carrier density was compared with a density calculated from the light output data using a rate equation analysis. Gain compression of a magnitude expected from relaxation oscillation damping rates was observed. Because the experiment measures the density at the center of the waveguide, and not the average across the active layer, the compression mechanism is not spatial hole burning.

Light output and response function of an NE-213 scintillator to neutrons up to 100 MeV

Light output of the NE-213 organic scintillator to 22.6, 27.6 and 48.7 MeV protons were obtained by the analysis of recoil proton spectra by monoenergetic neutron injection into the scintillator. The neutrons were generated by the proton bombardment on a thin lithium target backed with a thick carbon beam stopper. The measured light outputs were slightly smaller than the extrapolation of other works. Birks' semiempirical formula was fitted to these measured data and our previous experimental data of 11.03, 9.01 and 8.05 MeV protons and the formula failed to fit them completely over this wide energy region. A Monte Carlo code to calculate response functions of an NE-213 scintillator was improved by, 1) using these measured light output data, 2) using new cross sections of ENDF/B-IV, O5S and Guerra's compiled data, and 3) replacing the evaporation model of the C(n, p), C(n, pn) and C(n, 2n) reactions with the two-body reaction model. Results of this revised Monte Carlo calculation generally agreed well with the measured response functions of other works. Response functions of a 7.62-cm-diameter by 7.62-cm-long NE-213 scintillator to neutrons of energies up to 100 MeV were calculated by this Monte Carlo code.

Minority carrier lifetime and luminescence in MOVPE-grown (Al,Ga)As epilayers and DH lasers

Minority carrier lifetime measurements on MOVPE-grown GaAs and Al x Ga 1− x As ( x ≲ 0.17) are reported and compared with similar data on LPE material. Two types of measurements are used. (i) Decay of the band edge luminescence, as optically excited by the 647.1 nm radiation of a mode-locked Kr + laser. This decay yields the effective minority carrier lifetime τ eff at low excitation conditions (δ n, δ n < n 0, p 0); that is, both the radiative and non-radiative part contribute to τ eff. (ii) A novel way of analysing efficiency and light-output data of double-heterostructure lasers operating in the spontaneous high-injection regime (δ n ⋍ δ p > n 0, p 0). With this method, the non-radiative part of the carrier lifetime can be obtained, even when the radiative recombination rate largely exceeds the non-radiative rate. No essential differences in carrier lifetime were found between MOVPE and LPE-grown material. In GaAs, the value of τ eff measured at low excitation densities apparently is limited by the radiative lifetime. The value of τ eff strongly decreases, however, with increasing Al content, showing that effective killer centres are incorporated both during LPE and MOVPE. The recombination through these centres shows saturation effects with increasing excitation densities. It is shown, that ∼10 ppm of water vapour in the reactor during MOVPE has strong adverse effects on the luminescence quality of the (Al, Ga)As as well as on the steepness of the Al gradient in DH structures. High laser threshold currents observed in DH lasers grown under such conditions are related to losses due to superlinear carrier leakage from the active layer to the (Al, Ga)As confinement layers. Finally it is shown that with ample care taken to reduce water and oxygen concentrations during MOVPE, DH lasers operating at 775 nm can be made having nearly the same laser threshold current density as state-of-the-art LPE grown lasers.

Transfluxor Controlled Electroluminescent Display Panels

The feasibility of displaying images according to electrical signals using an array of electroluminescent cells magnetically controlled has been demonstrated by a working model. A transfluxor is associated with every electroluminescent cell. Each transfluxor is set by the coincidence of row and column current pulses to a level determined by the amplitude of the video signal, stores this level, and transmits energy continuously to the cell in proportion to the stored level. Both halftone and on-off control of brightness is possible. Scanning is accomplished by magnetically switching row and column conductors. The array has 1200 elements arranged in 30 rows of 40 each. The picture is uniform, exhibits good halftones, and has a maximum brightness of four foot lamberts. To obtain sufficient voltage from the transfluxor to excite electroluminescence, drive by short, fast-rising pulses is used. Light output data for such pulses were obtained and it was found that a steeper dependence of light output upon voltage, a different color spectrum, and higher efficiency resulted than is obtained with sine-wave drive. A modification using a linear transformer to couple the transfluxor and electroluminescent cell and driven by sine waves to operate as a transfluxor-tuned resonant circuit was demonstrated to be economical in driving and setting power. For some applications, the full storage afforded by a transfluxor at each picture element is not necessary. A system was investigated which uses fewer transfluxors storing only a row of picture information at a time.