D. Peter Siddons

Real-time x-ray studies of Mo-seeded Si nanodot formation during ion bombardment

The formation of self-organized Si nanostructures induced by Mo seeding during normal incidence Ar+ ion bombardment at room temperature is reported. Silicon surfaces without Mo seeding develop only power-law roughness during 1000eV ion bombardment at normal incidence, in agreement with scaling theory expectations of surface roughening. However, supplying Mo atoms to the surface during ion bombardment seeds the development of highly correlated, nanoscale structures (“dots”) that are typically 3nm high with a spatial wavelength of approximately 30nm. With time, these saturate and further surface roughening is dominated by the growth of long-wavelength corrugations.

Revealing hidden paint layers in oil paintings by means of scanning macro-XRF: a mock-up study based on Rembrandt's “An old man in military costume”

Over the past several decades the oeuvre of Rembrandt has been the subject of extensive art historical and scientific investigations. One of the most striking features to emerge is his frequent re-use of canvases and panels. The painting An Old Man in Military Costume (78.PB.246), in the collection of the J. Paul Getty Museum, is an example of such a re-used panel. Conventional imaging techniques revealed the presence of a second portrait under the surface portrait, but the details of this hidden portrait have not yet been revealed. Vermilion (HgS) has been identified to have been used nearly exclusively in the flesh tones of the lower painting, suggesting that element-specific XRF imaging might successfully image the hidden portrait. To test this hypothesis, a full-scale mock-up of the painting was created, including a “free impression” of the hidden portrait, reproducing as closely as possible the pigments and paint stratigraphy of the original painting. XRF imaging of the mock-up painting was conducted using three different XRF imaging systems: a mobile X-ray tube based system and two synchrotron-based setups (one equipped with multiple SDDs and one equipped with a Maia detector). The sensitivity, limits of detection and imaging capabilities of each system under the chosen experimental conditions are evaluated and compared. The results indicate that an investigation of the original painting by this method would have an excellent chance of success.

Diamond crystal X-ray optics for high-power-density synchrotron radiation beams

Man-made perfect single crystal isotopically-enriched diamond is demonstrated to be an excellent X-ray monochromator even when subjected to the highest incident power density expected at third-generation synchrotron source undulator beam lines. Double-crystal rocking curve tests of a diamond (400) wafer exposed to an X-ray power density of 207 W/mm2 (75 W total power) revealed just 1 arc sec of induced thermal distortion integrated across the beam footprint.

Wavelength tunability of ion-bombardment-induced ripples on sapphire

A study of ripple formation on sapphire surfaces by 300-2000 eV Ar{sup +} ion bombardment is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy is performed in order to study the wavelength of ripples formed on sapphire (0001) surfaces. We find that the wavelength can be varied over a remarkably wide range -- nearly two orders of magnitude -- by changing the ion incidence angle. Within the linear theory regime, the ion induced viscous flow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30{sup o}. In this model, relaxation is confined to a few nm thick damaged surface layer. The behavior at high temperature suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation.

ASIC for SDD-Based X-Ray Spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm2 and dissipates with the sensor biased about 15 mW/cm2. As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements at -44°C show a FWHM of 145 eV at the 5.9 keV peak of a 55Fe source, and less than 80 eV on a test-pulse line at 200 eV.

ASIC for SDD-based X-ray spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm2 and dissipates with the sensor biased about 15 mW/cml As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements show a FWHM of 145 eV at the 5.9 keV peak of a 55Fe source, and less than 80 eV on a test-pulse line at 200 eV.

New pressure flow cell to monitor BaSO4 precipitation using synchrotron in situ angle-dispersive X-ray diffraction

A flow cell has been commissioned to monitor in situ precipitation reactions under non-ambient conditions. The majority of high-pressure systems use anvils and presses to obtain high pressures around a small reaction area; however, this prototype is unique in that solutions may be examined as they flow through the cell under pressure. The cell is made of single-crystal silicon, which is capable of withstanding the high pressures created by liquid flow within the cell. With the capability of reaching pressures of up to 4 x 10(7) Pa, the cell is ideal for work on geological and oilfield systems. Here it is used to examine the formation of barium sulfate scale in situ under non-ambient conditions using angle-dispersive XRD on beamline X17b1 at the NSLS.

Comparison of two different methods to produce thin-window silicon drift detectors

We have developed a new method to produce thin-entrance-window Silicon Drift Detectors. To produce the desired thin-entrance-window a double implantation was used. This implantation consists of Boron ions (dose of 1×1014/cm2 at 10 keV) plus a second implant of Phosphorus ions (with a dose of 4×1012/cm2 at 50 keV or dose of 9×1011/cm2 at 80 keV) through 500 A of silicon dioxide. The second Phosphorus implantation compensates for the tail portion of the Boron ion implantation, so that the net Boron ion distribution will result in a thinner “dead” silicon layer and an elevated electric field near the silicon surface. We will compare test results from this newly developed thin-window with those from our previous development, where the thin junction was created using a single implantation of Boron ions (dose of 1×1014/cm2 at 10 keV) through a 500 Å thick silicon dioxide. All testing was done in the U3C beam line at the National Synchrotron Light Source at Brookhaven National Laboratory.

A Guinier Camera for SR Powder Diffraction: High Resolution and High Throughput

The paper describe a new powder diffraction instrument for synchrotron radiation sources which combines the high throughput of a position‐sensitive detector system with the high resolution normally only provided by a crystal analyser. It uses the Guinier geometry which is traditionally used with an x‐ray tube source. This geometry adapts well to the synchrotron source, provided proper beam conditioning is applied. The high brightness of the SR source allows a high resolution to be achieved. When combined with a photon‐counting silicon microstrip detector array, the system becomes a powerful instrument for radiation‐sensitive samples or time‐dependent phase transition studies.

A cryogenically cooled channel-cut crystal monochromator using a helium refrigerator and heat exchanger

Silicon crystals at room temperature employed as x-ray monochromators on synchrotron radiation beamlines, when subjected to high-power-density loading, suffer thermal distortions which compromise their x-ray diffraction efficiency and result in a reduction of the inherent brilliance of the synchrotron beam. At cryogenic temperatures however (below 150 K), silicon crystals suffer little or no thermal distortions under high-power-density loading. The design and implementation of a channel-cut silicon crystal monochromator which is cooled to as low as 50 K, using a commercial helium refrigerator and circulation system and a custom-designed heat exchanger for the monochromator crystal, are described. Test results have been obtained on the National Synchrotron Light Source X13B in-vacuum undulator beamline as well as the higher power X25 wiggler beamline.