Soft X-ray Quantum Research Articles

Measuring the soft x-ray quantum efficiency of a hybrid CMOS detector

Next-generation X-ray observatories, such as the Lynx X-ray Observatory Mission Concept or other similar concepts in the coming decade, will require detectors with high quantum efficiency (QE) across the soft X-ray band to observe the faint objects that drive their mission science objectives. Hybrid CMOS Detectors (HCDs), a form of active-pixel sensor, are promising candidates for use on these missions because of their fast read-out, low power consumption, and intrinsic radiation hardness. In this work, we present QE measurements of a Teledyne H2RG HCD, performed using a gas-flow proportional counter as a reference detector. We find that this detector achieves high QE across the soft X-ray band, with an effective QE of $94.6 \pm 1.1 \%$ at the Mn K$\alpha$/K$\beta$ energies (5.90/6.49 keV), $98.3 \pm 1.9 \%$ at the Al K$\alpha$ energy (1.49 keV), $85.6 \pm 2.8 \%$ at the O K$\alpha$ energy (0.52 keV), and $61.3 \pm 1.1 \%$ at the C K$\alpha$ energy (0.28 keV). These values are in good agreement with our model, based on the absorption of detector layers. We find similar results in a more restrictive analysis considering only high-quality events, with only somewhat reduced QE at lower energies.

Comparison of Back-Thinned Detector Ultraviolet Quantum Efficiency for Two Commercially Available Passivation Treatments

Back-thinned silicon detectors offer a high response over a very broad spectrum for direct detection by providing an efficient optical path into the sensing silicon avoiding front face structures manufactured from metal, polysilicon, nitrides, and oxides that may absorb the incident light before reaching the sensing silicon. We have tested two CCDs with different back-surface shallow p+ implant thicknesses (basic and enhanced) at the M4 line (wavelength between 40 and 400 nm) at Physikalisch-Technische Bundesanstalt (PTB)'s Metrology Light Source in Berlin. This characterization in the ultraviolet spectral range extends the soft X-ray quantum efficiency (QE) data set previously acquired with the exact same devices. Due to the short absorption depth and the scope for many types of interactions of the device materials with ultraviolet photons, QE measurement and stability of the device against extended exposure in the UV are of ongoing interest. Therefore, QE measurements have been carried out before and after exposures to quantify any change in behavior. To allow characterization of the passivation processes only, the devices have no antireflection coating. The measured QE of the standard back-thinned CCD is below 10% between 70 and 370 nm. An average additional 5% efficiency is achieved in the enhanced device within the same range. At the limits of the measured spectrum, toward soft X-rays or toward the visible range, the QE increases and the difference between the standard and the enhanced process is reduced as the photon absorption length increases beyond the immediate back-surface. The measured QE after long high-flux exposures at 200 nm shows remarkable improvement.

EUV and Soft X-Ray Quantum Efficiency Measurements of a Thinned Back-Illuminated CMOS Active Pixel Sensor

We report the first absolute effective quantum efficiency (e-h pairs collected/predicted) measurements of a monolithic thinned back-illuminated CMOS active pixel sensor (APS) in the extreme ultraviolet and soft X-ray region (13-600 Å). The sensor was designed and fabricated under a joint Rutherford Appleton Laboratory/e2v research program and characterized in the Lockheed Martin Solar and Astrophysics calibration facility. We compare our QE results to the data and models developed for thinned CCDs with similar back surface passivation. Our results demonstrate that CMOS APS arrays show significant promise for use in space-based solar physics and astrophysics missions.

Soft X-ray spectroscopy and quantum chemistry characterization of defects in onion-like carbon produced by nanodiamond annealing

The electronic structure of onion-like carbon (OLC) consisted of quasi-spherical and polyhedral nanoparticles and produced as a result of nanodiamond annealing at 1800 K and 2140 K has been probed by soft X-ray emission and X-ray absorption spectroscopy. The enhanced density of states in the vicinity of the Fermi level was revealed for OLC prepared at moderate temperature. Ab initio calculation on carbon models showed the only unpaired electrons at the zigzag edges of graphitic fragments can provide localized states observed in the X-ray spectra. With the increase of the annealing temperature the defects, which likely constitute the boundaries of incompletely closed onion shells, are healing.

Soft X-ray quantum efficiency of the CCD camera for the Astro-E

Soft X-ray quantum efficiency of the CCD camera for the Astro-E

Soft X-ray quantum efficiency of the CCD camera for the Astro-E

Soft X-ray quantum efficiency of the CCD camera for the Astro-E

Measuring the soft X-ray quantum efficiency of charge-coupled devices using continuum synchrotron radiation

We have measured the soft X-ray (250–4500 eV) quantum efficiency of a charge-coupled device using continuum synchrotron radiation. The spectrum of the synchrotron radiation can be calculated accurately, so the observed pulse height spectrum is a direct measure of the detector efficiency over the synchrotron bandpass. A key function in modeling the efficiency is the energy dependence of the fraction of detected single pixel events. We find that a model including the effects of charge drift and diffusion plus two different depletion depths (corresponding to the collection and barrier phases under the gates) is required. With this model the absolute QE can be determined to within ∼5% everywhere over the 500 to 4500 eV bandpass.

Soft X-ray spectroscopy with sub-electron readnoise charge-coupled devices

We demonstrate use of a charge-coupled device (CCD) with sub-electron readnoise performance as a non-dispersive X-ray spectrometer. The exceptionally low readnoise (0.9 electrons rms) was achieved by applying a floating gate output amplifier with 16 readouts per pixel. The soft X-ray quantum efficiency was enhanced over other front-side illuminated devices by using a novel thin-poly gate structure. The combination of sub-electron noise and good soft X-ray quantum efficiency have enabled us to detect photons in the EUV energy range ( E < 100 eV) in photon counting mode with this detector. These low energy events are well separated from the readnoise floor. The measured energy resolution is 16.3 eV (FWHM) at Al L ( E = 72 eV), 33.8 eV at C K α ( E = 277 eV) and 120 eV at Mn K α ( E = 5.9 keV).

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium chloride photocathode layers on microchannel plates

We present measurements of the quantum detectio efficiency (QDE) of potassium chloride, applied directly to the surface of a microchannel plate (MCP), over the 44-1460-A wavelength range. The contributions of the photocathode material in the channels, and on the interchannel web, to the QDE have been determined. Two broad peaks in the QDE centered at lambda congruent with 500 A and lambda congruent with 900 A are apparent, the former with ~40% peak QDE and the latter with ~30% peak QDE. The photoelectric threshold is observed at lambda ~1400 A, and there is a narrow QDE minimum at lambda~ 670 A, which correlates with 2xthe band gap energy for KC1. The angular variation of the QDE from 0 to 35 degrees to the channel axis has also been examined. We describe a simple QDE model and show that its predictions are in accord with our QDE measurements. Assessment of the stability of KC1 shows that there was no significant degradation of the QDE at wavelengths of </=1066 A during a 4-month storage period in dry nitrogen with a limited exposure to air.

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium bromide photocathode layers on microchannel plates

The quantum detection efficiency (QDE) of potassium bromide, applied directly to the surface of a microchannel plate (MCP), has been measured over the wavelength range from 44 to 1216 A. We present the first measurements for the QDE of KBr between 44 and 256 A. These show that there is a high QDE peak (~70%) centered at ~70 A. The results at wavelengths above 256 A agree with our previous study. Investigation of the angular dependence of the QDE indicates that maximum efficiencies are achieved for graze angles </=10 degrees at wavelengths below 171 A. Examination of the photocathode surface with a scanning electron microscope shows that the KBr layer is rough, with granularity at the 0.5-microm level.

The soft X-ray detection efficiency of coated microchannel plates

An improved model of the soft X-ray quantum detection efficiency of microchannel plate electron multipliers is presented. The model takes account of the low-energy secondary component of the photoelectric yield and therefore can be used to study microchannel plates bearing high-yield deposition photocathodes such as CsI. Experimental data and theoretical calculations are presented for both front surface and open area contributions to the detection efficiency. The effects of vacuum deposition geometry on the uniformity of response of coated plates are discussed.

The stability of CsI-coated microchannel plate X-ray detectors

Coating with caesium-iodide has been shown to increase the soft X-ray quantum detection efficiency of microchannel plate (MCP) electron multipliers. Measurements are now presented of the stability and reproducibility of such CsI deposition photocathodes. The effects of limited exposure to humid air and of storage in poor vacuum, high vacuum and dessicated air are described. The stability of CsI coatings under prolonged X-ray bombardment is discussed and the mechanical strength of thick (up to 14000 Å) CsI layers briefly assessed.

The soft X-ray quantum detection efficiency of microchannel plates

An account is given of the quantum detection efficiency of microchannel plates in the X-ray energy range 0.02–20 keV. The predictions of a detailed theoretical model are compared with measurement.