Spatial Uniformity Of Response Research Articles

Development of a large NaI(Tl) detection system

A large NaI(Tl) detection system was developed to address the increasing demand for high detection efficiency and high energy resolution detectors across various applications. Through Geant4 simulation and experimental research, critical characteristics of the detector prototype were examined, encompassing energy response, energy nonlinearity, and energy resolution. Given the substantial impact of variations in Tl concentration and the complex process of photon propagation on the detection performance of large NaI(Tl) crystals, the detection efficiency and energy resolution with the radioactive source placed at different incident spots were studied to evaluate its spatial response uniformity. The results demonstrate that the detector's energy response range spans from 50 keV to 3 MeV with an energy nonlinearity of 1.5%. The developed large NaI(Tl) detection system achieved an overall energy resolution of 7.93%, along with excellent uniformity in its resolution and detection efficiency, satisfying the application requirements for large dimensions and high detection efficiency.

Characterization and Quantum Efficiency Determination of Monocrystalline Silicon Solar Cells as Sensors for Precise Flux Calibration

As the precision frontier of astrophysics advances toward the one millimagnitude level, flux calibration of photometric instrumentation remains an ongoing challenge. We present the results of a lab-bench assessment of the viability of monocrystalline silicon solar cells to serve as large-aperture (up to 125[Formula: see text]mm diameter), high-precision photodetectors. We measure the electrical properties, spatial response uniformity, quantum efficiency (QE), and frequency response of third-generation C60 solar cells, manufactured by Sunpower. Our new results, combined with our previous study of these cells’ linearity, dark current, and noise characteristics, suggest that these devices hold considerable promise, with QE and linearity that rival those of traditional, small-aperture photodiodes. We argue that any photocalibration project that relies on precise knowledge of the intensity of a large-diameter optical beam should consider using solar cells as calibrating photodetectors.

Electro-Optical Performance Study of 4H-SiC/Pd Schottky UV Photodetector Array for Space Applications

4H-SiC Schottky photodetector array was fabricated by pulsed dc sputter deposition of palladium thin film as UV semitransparent Schottky contact on active pixel. Results of electro-optical characterization of these devices as a function of temperature varying from 33 to 393 K are presented. A peak spectral responsivity of 122 mAW−1 at 280 nm is observed. Further, spatial response uniformity for a 4 mm $\times4$ mm detector, scanned by UV spot of $100~\mu \text{m}$ diameter, is within ±5%, whereas the crosstalk between the pixels in $4\times 2$ detector array with a pixel size of $850\,\,\mu \text{m}\,\,\times 850\,\,\mu \text{m}$ is within 1.3%. Capacitance at near zero bias is around 11 pF and 4.79 pF at a reverse bias of 30 V. From the ${I}$ – ${V}$ characteristics, the calculated Schottky barrier height is 1.37 eV for 70 A thick palladium metal film on 4H-SiC as against the theoretical value of 1.96 eV. Operating temperature studies on detector UV response at 300 nm have shown that the photo-current response at 113 K falls to half of its peak value at ambient. The responsivity of the detectors at 248-nm wavelength is almost constant over a wide temperature range of 33 to 393 K.

Initial assessment of monocrystalline silicon solar cells as large-area sensors for precise flux calibration

As the precision frontier of large-area survey astrophysics advances towards the one millimagnitude level, flux calibration of astronomical instrumentation remains an ongoing challenge. We describe initial testing of silicon solar cells as large-aperture precise calibration photodiodes. We present measurements of dark current, linearity, frequency response, spatial response uniformity, and noise characteristics of the Sunpower C60 solar cells, an interdigitated back-contact 125mm x 125mm monocrystalline solar cell. We find that these devices hold considerable promise as large-area flux calibration sensors and warrant further characterization.

A scintillator geometry suitable for very small PET gantries

In this work we are describing a novel approach to the scintillator crystal configuration as used in nuclear medicine imaging. Our design is related to the coupling in one PET module of the two separate crystal configurations used so far there: monolithic and crystal arrays. The particular design we have studied is based on a two-layer scintillator approach (hybrid) composed of a monolithic LYSO crystal (5–6 mm thickness) and a LYSO crystal array with 4–5 mm height (0.8 and 1 mm pixels). We show here the detector block performance, in terms of spatial, energy and DOI information, to be used as a module in the design of PET scanners. The design we propose allows one to achieve accurate three-dimensional spatial resolution (including DOI information) while assuring high detection efficiency at reasonable cost. Moreover, the proposed design improves the spatial response uniformity across the whole detector module, and especially at the edge region. The crystal arrays are mounted in the front and were well resolved. The monolithic crystal inserted between crystal array and the photosensor, provided measured FWHM resolution as good as 1.5–1.7 mm including the 1 mm source size. The monolithic block achieved a DOI resolution (FWHM) nearing 3 mm. We compared these results with an approach in which we use a single monolithic block with total volume equals to the hybrid approach. In general, comparable performances were obtained.

Examination of the spatial-response uniformity of a microchannel-plate detector using a pulsed high-voltage electron gun

In this paper we describe an alternative method to examine the spatial-response uniformity of a microchannel-plate (MCP) detector to a ∼ 1 ns pulse of soft x-rays. The examination was performed by illuminating the MCP surface with energetic electrons rather than with x-rays. It is shown that the MCP features similar, yet not identical, response to pulses of soft x-ray photons or energetic electrons, making such examinations much simpler and less expensive. The building of the electron-gun system is relatively easy and inexpensive, and in addition to verifying the spatial uniformity of the response of the MCP to incoming particles and radiation, it can be used to detect damaged areas on the detector. A comparison between the results obtained using the electron-gun with those obtained using a laser-produced-plasma x-ray source, demonstrating the reliability of the method, is presented.

Assembly and evaluation of a pyroelectric detector bonded to vertically aligned multiwalled carbon nanotubes over thin silicon

A novel pyroelectric detector consisting of a vertically aligned nanotube array on thin silicon (VANTA/Si) bonded to a 60 μm thick crystal of LiTaO₃ has been fabricated. The performance of the VANTA/Si-coated pyroelectric detector was evaluated using National Physical Laboratory's (NPL's) detector-characterization facilities. The relative spectral responsivity of the detector was found to be spectrally flat in the 0.8-24 μm wavelength range, in agreement with directional-hemispherical reflectance measurements of witness samples of the VANTA. The spatial uniformity of response of the test detector exhibited good uniformity, although the nonuniformity increased with increasing modulation frequency. The nonuniformity may be assigned either to the dimensions of the VANTA or the continuity of the bond between the VANTA/Si coating and the pyroelectric crystal substrate. The test detector exhibited a small superlinear response, which is similar to that of pyroelectric detectors coated with good quality gold-black coatings.

Experimental validation of the six-port design for a highly uniform integrating sphere photometer

We present experimental realization and validation of the six-port design of integrating sphere photometers for total luminous flux measurement, which significantly improves the uniformity of spatial response compared to the conventional single-port design. Construction, measurement procedure, and data acquisition of the realized instrument with a radius of 1m are described. Measurement of the spatial response distribution function confirms the expected effect of improving the uniformity by averaging the signals from the six detection ports. The related spatial mismatch error is determined to be less than 1.4% for all the realistic cases of beam angles and directions of a test lamp mounted in the vicinity of the sphere center. As a result, we confirm that the realized six-port instrument allows us to eliminate the complicated spatial mismatch correction procedure by adding a relative standard uncertainty of only 1.4/3%≈0.81%, which offers a great practical benefit for testing solid-state lighting products of various beam characteristics.

The evaluation of the performance of two pyroelectric detectors with vertically aligned multi-walled carbon nanotube coatings

Two LiTaO3 pyroelectric detectors coated with vertically aligned multi walled carbon nanotube (MWCNT) black coatings were assembled and evaluated using NPL’s detector characterisation facilities. The vertically-aligned nanotube array (VANTA) black coatings were grown on a silicon substrate and subsequently lifted off the silicon and bonded on the pyroelectric crystal substrates. Despite some drawbacks, this method was shown to provide a reliable way of coating delicate substrates such as pyroelectric crystals with VANTA coatings. The performance of the coated and uncoated detectors was evaluated and compared by coating only half of the active area of the test detectors, leaving the other half uncoated. The relative spectral responsivity of the VANTA-coated pyroelectric detectors was shown to be spectrally flat in the 0.8–14μm wavelength range within the uncertainty of the measurements. The spatial uniformity of response of both detectors exhibited fine structure which was assigned either to the thickness of the VANTA coatings or to their bonding to the pyroelectric crystal. Both coated and uncoated detectors exhibited a super-linear response. This observation was expected in the case of the uncoated detectors, but was surprising in the case of the coated detectors and indicates that the thermal conductivity of vertically aligned multi-walled carbon nanotubes is high along their long axis. The spatial variations of the phase delay experienced by the signal propagating through the VANTA coatings indicate that the thermal diffusivity of the coatings is not spatially uniform.

Extension of the NIST spectral power-responsivity calibration service to 2500 nm

The National Institute of Standards and Technology (NIST) is working to extend the upper wavelength limit of the spectral power-responsivity calibration service from 1800 nm to 2500 nm. This extension is based on extended-InGaAs (EIGA) transfer- and working-standard radiometers and low-NEP pyroelectric transfer-standard detectors. The scale is traceable to the electrical-substitution cryogenic radiometer through the transfer-standard EIGA radiometers. The total uncertainty of the extended-responsivity reference scale (realized on four EIGA working-standard radiometers) is 1% (k = 2). The transfer of the reference scale from one of the four working standards to a test EIGA radiometer in direct-current (dc) measurement mode is described. The uncertainty of the transferred scale is 1.6% (k = 2) between 1200 nm and 2300 nm. The transfer was performed on the NIST Spectral Comparator Facility (SCF) without any modifications to the SCF. The EIGA radiometers have been characterized for spatial-response uniformity, temperature-dependent spectral responsivity, linearity, noise, long-term and short-term temporal stability. The preliminary results show that EIGA radiometers can be used over the spectral range between 900 nm and 2500 nm in dc mode and could eventually replace the regular InGaAs detectors in the SCF measurements. The total uncertainty budget for these measurements is discussed.

The evaluation of a pyroelectric detector with and without a sprayed multi-walled carbon nanotube coating

The radiometric properties of a pyroelectric detector based on a LiTaO 3 crystal were studied when the detector had a sprayed multi-walled carbon nanotube (MWCNT) coating and when the coating was removed. The spatial uniformity of response of the bare detector improved as the modulation frequency increased in the 4–70 Hz range whereas the spatial uniformity of response of the coated detector deteriorated as the modulation frequency increased over the same range. The uncoated detector was observed to have a “super-linear” response whereas the linearity of response of the coated detector was shown to be “sub-linear”. The origin and implications of this behaviour are discussed.

Six-port integrating sphere photometer with uniform spatial response

We propose an integrating sphere photometer with six detection ports for total luminous flux measurement, which significantly improves the uniformity of spatial response compared to the conventional single-port detection design. Numerical simulations based on the geometric radiative transfer equation show that a spatial response distribution function of the new design is uniform within 2% with respect to all spatial directions. The related spatial mismatch error is calculated to be less than 0.3% for all the realistic cases of angular intensity distribution of a test lamp. As a result, the new design practically eliminates the spatial mismatch error of an integrating sphere photometer, so that a high-accuracy measurement can be achieved without the complicated spatial mismatch correction procedure.

Characterization of the Linearity of Response and Spatial Uniformity of Response of Two InGaAsP/InP Geiger-Mode Avalanche Photodiodes

The linearity of response and spatial uniformity of response characteristics of two commercially available single-photon avalanche photodiode (SPAD) detection systems were experimentally investigated using a dedicated characterization facility that measures these parameters. Both SPAD detection systems are shown to exhibit a nonlinear response. Moreover, the responsivity of both SPAD systems is shown to be spatially nonuniform and the degree of nonuniformity depends on the single-photon detection probability setting of these systems. The experimentally observed spatial uniformity of response behavior of these detectors is explained in terms of the spatial nonuniformity in the breakdown voltage V_b across the active area of the device. The experimentally observed linearity of response characteristics of these detectors at high count rates can be explained in terms of “pulse collisions.” However, the origin of the experimentally observed linearity of response characteristics of the same detectors at low count rates is currently unknown.

The evaluation of a pyroelectric detector with a sprayed carbon multi-wall nanotube black coating in the infrared

The performance of a pyroelectric detector with a “sprayed” multi-wall carbon nanotube (MWCNT) coating was evaluated in the 0.9 μm–24 μm wavelength range. The relative spectral responsivity of this detector was shown to vary by 8% over this wavelength range. Its responsivity exhibited a super-linear response, while its spatial uniformity of response was strongly dependent on the modulation frequency, indicating that the thermal properties of the “sprayed” MWCNTs play an important role in the spatial uniformity of response profiles. The “sprayed” MWCNT coating is far easier to fabricate than other black coatings and it is relatively durable. This, in combination with the small variation observed in the spectral absorbance of the “sprayed” MWCNT coating over a very wide wavelength range, suggests that these coatings appear extremely promising for thermal detection applications in the infrared.

Electric field distribution and simulation of avalanche formation due to the passage of heavy ions in a parallel grid avalanche counter

Electric field distributions and their role in the formation of avalanche due to the passage of heavy ions in parallel grid avalanche type wire chamber detectors are evaluated using a Monte Carlo simulation. The relative merits and demerits of parallel and crossed wire grid configurations are studied. It is found that the crossed grid geometry has marginally higher gain at larger electric fields close to the avalanche region. The spatial uniformity of response in the two wire grid configurations is also compared.

The use of computed radiography for routine linear accelerator and simulator quality control

Computed radiography (CR) systems were originally developed for the purpose of clinical imaging, and there has been much work published on its effectiveness as a film replacement for this end. However, there has been little published on its use for routine linear accelerator and simulator quality control, and therefore we have evaluated the use of the Kodak 2000RT system with large Agfa CR plates as a replacement for film for this function. A prerequisite for any such use is a detailed understanding of the system behaviour, hence characteristics such as spatial uniformity of response, reproducibility of spatial accuracy, plate signal decay with time and the dose-response of plates were investigated. Finally, a comparison of results obtained using CR for the measurement of radiation field dimensions was made against those from radiographic film, and found to be in agreement within 0.1 mm (mean difference for high-resolution images, 0.3 mm root mean square difference) for megavoltage images and 0.3 mm (maximum difference) for simulator images. In conclusion, the CR system has been shown to be a good alternative to radiographic film for routine quality control of linear accelerators and simulators.

Infrared responsivity of a pyroelectric detector with a single-wall carbon nanotube coating

The performance of a 10 mm diameter pyroelectric detector coated with a single-wall carbon nanotube (SWCNT) was evaluated in the 0.8 to 20 microm wavelength range. The relative spectral responsivity of this detector exhibits significant fluctuations over the wavelength range examined. This is consistent with independent absorbance measurements, which show that SWCNTs exhibit selective absorption bands in the visible and near-infrared. The performance of the detector in terms of noise equivalent power and detectivity in wavelength regions of high coating absorptivity was comparable with gold-black-coated pyroelectric detectors based on 50 microm thick LiTaO(3) crystals. The response of this detector was shown to be nonlinear for DC equivalent photocurrents >10(-9) A, and its spatial uniformity of response was comparable with other pyroelectric detectors utilizing gold-black coatings. The nonuniform spectral responsivity exhibited by the SWCNT-coated detector is expected to severely restrict the use of SWCNTs as black coatings for thermal detectors. However, the deposition of SWCNT coatings on a pyroelectric crystal followed by the study of the prominence of the spectral features in the relative spectral responsivity of the resultant pyroelectric detectors is shown to provide an effective method for quantifying the impurity content in SWCNT samples.

Absolute linearity measurements on LiTaO_3 pyroelectric detectors

The nonlinearity characteristics of a number of LiTaO3 pyroelectric detectors were experimentally investigated using the National Physical Laboratory (NPL) detector linearity characterization facility. All the detectors examined were shown to exhibit a superlinear response, i.e., the responsivity of the detectors increases as the incident radiant power increases. The temperature coefficient of response of some of these LiTaO3 pyroelectric detectors was measured and found to be approximately +0.2% degrees C(-1). The superlinear behavior of the LiTaO3 pyroelectric detectors was attributed to the positive temperature coefficient of response values of these detectors. Moreover, the linearity factor of gold-black-coated LiTaO3 pyroelectric detectors was shown to exhibit a dependency on the area of the spot illuminating the active area of the detector, i.e., on the incident irradiance. Possible reasons for the observed behavior are proposed and discussed. Some variations in the slopes of the plots of the linearity factor versus irradiance for different areas being illuminated have been assigned to the poor spatial uniformity of response of these detectors.

Study and realization of a transfer detector based on porous silicon for radiometric measurements

A photodetector of high accuracy has been developed at the “Laboratoire de Métrologie des Rayonnements (L.M.R)—Tunisia” to serve as a transfer detector for optical power measurements in the visible spectral range, i.e. wavelength from 400 to 800 nm. The originality of the work, comparing with other ones, consists of realizing a pn + junction with a n + porous silicon layer on the front face and a zinc oxide (ZnO) film as a transparent electrode above the porous layer. This detector's structure permits to the porous silicon layer to trap the incident optical radiation and to reduce the reflection coefficient fluctuations of the front face to a value of about 7%. The uncertainties relating to the spatial response uniformity of the detector and especially in the central area with 5 mm diameter are equal to 0.5–0.7% for vertical and horizontal directions, respectively. The realized photodetector can roughly reach 10 mW of measured optical power. This value represents an earning of one decade comparing with commercial photodiodes that cannot measure optical power beyond 1 mW. The photodetector presents a linearity with a cumulated deviation of a few 10 −4 through more than 4 decades with an uncertainty of 1.5×10 −4 at 1 σ level. The photodetector was also indirectly calibrated with the cryogenic radiometer of the “Institut National de Métrologie (INM)–France” and provides a convenient standard with a relative standard uncertainty of a few 10 −3 at the 1 σ level. This uncertainty is equivalent to commercial transfer detectors used in metrology laboratories.

Single Crystal CVD Diamond Detectors: Position and Temporal Response Measurements using a Synchrotron Microbeam Probe

AbstractUltrapure, homoeptaxially grown CVD single crystal diamond is a material with great potential for the fabrication of ionizing radiation detectors for high energy, heavy ion physics, and realtime dosimetry for radiotherapy. Only diamond has suitable transmission properties and can offer the required radiation hardness for synchrotron X-ray beam monitoring applications. We report on experiments made using a synchrotron X-ray microbeam probe to investigate the performance of single crystal diamonds operated as position sensitive, solid state ‘ionization chambers’. We show that for a wide range of electric fields >0.3Vµm−1, suitably prepared devices give excellent spatial response uniformity and time stability. With an applied field of 2Vµm−1 complete charge collection times are ∼1nsec for a diamond plate thickness of 100µm. Position sensitivity was obtained for an X-ray beam incident on the isolation gap between adjacent electrodes of a quadrant device: here, a crossover response region that results from charge carrier diffusion extends over ∼20µm. Using GHz bandwidth signal processing electronics, the signal charge collection process was measured with spatial and temporal resolutions of 1µm and <50ps.