11D607-2
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 2. MCP response from 6 to 20 keV. The MCP response is present as output electrons divided by input photon energy in keV. The inset shows the L-edges of lead in 13–16 keV range. FIG. 1. Test apparatus set inside the NSLS X15A beamline experimental hutch as shown in the picture. The schematic of the experimental setup is shown as (1) input x-ray beam, 6–25 keV, (2) x-ray shutter, (3) attenuation filters, (4) calibrated Si photodiode, (5) vacuum chamber with a beryllium window, (6) vacuum bellows, and (7) MCP detector and recording system.
source spectrum with useable flux from 6 to 25 keV. The energy resolution (E/E) is about 2 × 10−4 . The monochromator was calibrated using the known K absorption edge of Zr at E = 17.998 keV. The desired x-ray energy was selected by setting the corresponding Bragg angle for the crystal. To suppress higher orders of the fundamental x-ray energy, the monochromator was slightly detuned by “rocking” the dual crystals until 50% of the peak transmission was achieved. Typical x-ray flux was about 1 × 108-9 photons/sec/mm2 over the tested energy range. The nominal synchrotron ring current was ∼200 mA with electron beam energy of 2.8 GeV. The output x-ray beam spot size was collimated to about 15 H × 2 V mm2 . Calibrated sets of aluminum and titanium foils were used to attenuate the incident x-ray flux, and a Uniblitz x-ray shutter was used to set 10 ms time exposures. With a fixed exposure time, selected foil material and thicknesses were driven into the beam path by remote control to vary the incident x-ray flux to prevent the MCP from saturation. Between measurements, the incident flux on the MCP was monitored by inserting a calibrated Si photodiode detector (IRD AXUV100G, 09-7-#53) in front of the framing camera. The MCP detector was mounted behind a bellows on a vacuum chamber with P < 10−6 Torr. The bellows provided ±25◦ bend angles from normal incidence for studies on angular dependence. Due to space limitations in the experimental hutch, only ±15◦ was allowed. A fiber plug was used to couple the MCP detector and a Spectral Instruments SI-800 CCD camera with 9 μm pixels. III. RESULTS AND DISCUSSION A. MCP response
A set of Aluminum and Titanium filters was precharacterized at NSLSX15A to attenuate the incident x-ray
flux to maintain the MCP detector under test in a linear response regime. The MCP bias was set at 650 VDC, still above the uniform gain region and well below saturation. In general, a 100 eV/step increment was used to study MCP response. In the range of 13–16 keV, some judicious smaller step sizes were used to resolve Pb edges. The experimental results are shown in Figure 2. As expected due to a decrease in the interaction cross section, MCP sensitivity continued to decrease for increasing energy between 6 and 20 keV. Small jumps between 13 and 16 keV were observed, corresponding to the L-edge energies of Pb. The variation in MCP sensitivity per incident photon energy dropped about a factor of seven between 6 to 20 keV, which was smaller than results obtained by Hirata et al.3–6 These differences could be attributed to a different geometry and compositions of lead glass for our MCP. B. Spatial resolution
As x-ray energy increases, penetration and scattering of energetic photons into multiple pores also reduce and smear the spatial resolution. For TiGHER, the incident angles of the x-ray spectrum onto the detector are expected to vary significantly. Our experiments at NSLS were conducted to determine if the MCP detector is sufficiently sensitive over this energy range and able to meet the specifications of the spectrometer. A 508 μm thick copper/tungsten (Cu/W, 30/70% weight) target, shown in Figure 3(b) was specially designed and fabricated to measure the spatial resolution of the MCP detector (framing camera) for incident x-rays up to 25 keV. The knife edge features have a slightly flat area at the lip of slit (50-100 μm) and are EDM (electric discharge machining) to create a 60◦ edge. The x-ray transmission in this region is shown in Figure 3(a). The resolution target was carefully inspected using an optical microscope and x-ray CCD camera, and was shown to be well suited for resolution measurements to better than 50 μm for 1.49 keV (Al anode). For an image contrast ratio that is greater than three orders of magnitude, this
11D607-3
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 3. (a) Estimated transmission of x-ray in the range of 5–20 keV for a 508-μm thick Cu/W target. (b) Picture of resolution target for Gen-II MCP camera. There are nine slots with 2 mm widths and 37.6 mm height. The spacing between the slots is 4 mm.
resolution target yielded a LSF (line spread function) with a FWHM ∼20 μm at 15 keV. The resolution target was mounted on top of the MCP with a gap spacing of 0.5 mm for resolution tests. Typical images acquired from tests using the Manson and synchrotron sources are shown in Figure 4. With eight 4-mm wide horizontal stripes on the Gen-II MCP framing camera, eighteen knife edges can be seen in each horizontal extent (as masked by the nine vertical bars of the resolution target). In contrast, at NSLS X15A, only four knife edges are available for calculating the LSF due to the small x-ray beam spot. An IDL based image processing software developed at NSTec-LAO has been customized for SI-800 captured data. Background subtraction and star removal are performed be-
FIG. 4. (a) Typical image from Manson source, eight metal strips were coated on the MCP and nine slot opening in the resolution target. 72 blocks of images can be recorded in single exposure. 144 knife edges can be obtained for the resolution measurements. (b) Typical image from NSLS X15A beamline, due to limits of beam size of synchrotron light source, image can be recorded in only one MCP strip. Only 4 knife edges can be obtained in each exposures under unfocused beam.
fore MTF (modulation transfer function) and LSF analysis. For a defined area of interest (AOI), a typical intensity distribution, LSF and a Gaussian fit near the knife edge are obtained as shown in Figure 5. High frequency noise is clearly evident in the raw LSF data due to the spatial convolutions of fiber plug coupling (INCOM BLI58-4: 4.5 μm diameter fibers), imaging (Kodak CCD chip: 9 μm pixels), and MCP pore size (10 μm). A direct determination of the LSF FWHM for each AOI can result in a large variation (or error bar) for a composite set of image data. A better approach is to apply a best-fit Gaussian for the LSF peaks (left and right) of an AOI as shown in Figures 5(c) and 5(d) and then average over the peaks to determine the LSF FWHM. For these spatial resolution studies, x-ray energy was varied from 7 to 19 keV at 2 keV steps. The phosphor bias voltage was varied from +1000 to +3000 VDC at 500 V steps. The experimental results listed in Table I show the Gen-II MCP detector to have a spatial resolution about 45 ± 10 μm when the phosphor bias voltage is set at +3000 VDC and the x-ray energy is less than 20 keV. More importantly, no change in spatial resolution was observed in this energy range. The variation of spatial resolution followed theoretical prediction, being inversely proportional to the square root of the phosphor bias voltage, or linear to V−1/2 . An interesting note is that the spatial resolution did not change even when the MCP response was saturated. For example, at 15 keV, the MCP was exposed to full beam intensity about a factor of 700 above the linear operating regime. The observed spatial resolutions were almost identical. The spatial resolution variation with x-ray incident angles was measured at 15 keV. Due to hutch space limitations, this experimental study was only conducted between +15◦ and −7◦ . A comparison between the measurement and simulation is shown in Figure 6. Agreement between them is generally
11D607-4
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 5. Typical results from image processed 15 keV data taken at NSLS X15A. (a) Intensity distribution in the area of interest; (b) LSF of the area of interest; (c) a Gaussian fit of left peak of LSF; and (d) Gaussian fit of right peak of LSF.
within their respective uncertainties. It is important to know that according the simulations, a better than 100 μm spatial resolution can be achieved in our MCP detector even with a 40◦ incident angle.
TABLE I. Summary of spatial resolution measurements (FWHM of LSF, μm). Phosphor bias voltage (V) X-ray energy (keV) 7.0 9.0 11.0 13.0 15.0 17.0 19.0
1000
1500
2000
2500
3000
63 63 66 66 64 69 67
56 56 57 57 60 58 59
52 49 50 52 53 52 52
49 45 48 48 49 47 47
45 43 44 45 45 45 45
FIG. 6. Spatial resolution variation with x-ray incident angles at 15 keV. The MCP and phosphor were biased at −650 and +3000 VDC, respectively.
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 2. MCP response from 6 to 20 keV. The MCP response is present as output electrons divided by input photon energy in keV. The inset shows the L-edges of lead in 13–16 keV range. FIG. 1. Test apparatus set inside the NSLS X15A beamline experimental hutch as shown in the picture. The schematic of the experimental setup is shown as (1) input x-ray beam, 6–25 keV, (2) x-ray shutter, (3) attenuation filters, (4) calibrated Si photodiode, (5) vacuum chamber with a beryllium window, (6) vacuum bellows, and (7) MCP detector and recording system.
source spectrum with useable flux from 6 to 25 keV. The energy resolution (E/E) is about 2 × 10−4 . The monochromator was calibrated using the known K absorption edge of Zr at E = 17.998 keV. The desired x-ray energy was selected by setting the corresponding Bragg angle for the crystal. To suppress higher orders of the fundamental x-ray energy, the monochromator was slightly detuned by “rocking” the dual crystals until 50% of the peak transmission was achieved. Typical x-ray flux was about 1 × 108-9 photons/sec/mm2 over the tested energy range. The nominal synchrotron ring current was ∼200 mA with electron beam energy of 2.8 GeV. The output x-ray beam spot size was collimated to about 15 H × 2 V mm2 . Calibrated sets of aluminum and titanium foils were used to attenuate the incident x-ray flux, and a Uniblitz x-ray shutter was used to set 10 ms time exposures. With a fixed exposure time, selected foil material and thicknesses were driven into the beam path by remote control to vary the incident x-ray flux to prevent the MCP from saturation. Between measurements, the incident flux on the MCP was monitored by inserting a calibrated Si photodiode detector (IRD AXUV100G, 09-7-#53) in front of the framing camera. The MCP detector was mounted behind a bellows on a vacuum chamber with P < 10−6 Torr. The bellows provided ±25◦ bend angles from normal incidence for studies on angular dependence. Due to space limitations in the experimental hutch, only ±15◦ was allowed. A fiber plug was used to couple the MCP detector and a Spectral Instruments SI-800 CCD camera with 9 μm pixels. III. RESULTS AND DISCUSSION A. MCP response
A set of Aluminum and Titanium filters was precharacterized at NSLSX15A to attenuate the incident x-ray
flux to maintain the MCP detector under test in a linear response regime. The MCP bias was set at 650 VDC, still above the uniform gain region and well below saturation. In general, a 100 eV/step increment was used to study MCP response. In the range of 13–16 keV, some judicious smaller step sizes were used to resolve Pb edges. The experimental results are shown in Figure 2. As expected due to a decrease in the interaction cross section, MCP sensitivity continued to decrease for increasing energy between 6 and 20 keV. Small jumps between 13 and 16 keV were observed, corresponding to the L-edge energies of Pb. The variation in MCP sensitivity per incident photon energy dropped about a factor of seven between 6 to 20 keV, which was smaller than results obtained by Hirata et al.3–6 These differences could be attributed to a different geometry and compositions of lead glass for our MCP. B. Spatial resolution
As x-ray energy increases, penetration and scattering of energetic photons into multiple pores also reduce and smear the spatial resolution. For TiGHER, the incident angles of the x-ray spectrum onto the detector are expected to vary significantly. Our experiments at NSLS were conducted to determine if the MCP detector is sufficiently sensitive over this energy range and able to meet the specifications of the spectrometer. A 508 μm thick copper/tungsten (Cu/W, 30/70% weight) target, shown in Figure 3(b) was specially designed and fabricated to measure the spatial resolution of the MCP detector (framing camera) for incident x-rays up to 25 keV. The knife edge features have a slightly flat area at the lip of slit (50-100 μm) and are EDM (electric discharge machining) to create a 60◦ edge. The x-ray transmission in this region is shown in Figure 3(a). The resolution target was carefully inspected using an optical microscope and x-ray CCD camera, and was shown to be well suited for resolution measurements to better than 50 μm for 1.49 keV (Al anode). For an image contrast ratio that is greater than three orders of magnitude, this
11D607-3
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 3. (a) Estimated transmission of x-ray in the range of 5–20 keV for a 508-μm thick Cu/W target. (b) Picture of resolution target for Gen-II MCP camera. There are nine slots with 2 mm widths and 37.6 mm height. The spacing between the slots is 4 mm.
resolution target yielded a LSF (line spread function) with a FWHM ∼20 μm at 15 keV. The resolution target was mounted on top of the MCP with a gap spacing of 0.5 mm for resolution tests. Typical images acquired from tests using the Manson and synchrotron sources are shown in Figure 4. With eight 4-mm wide horizontal stripes on the Gen-II MCP framing camera, eighteen knife edges can be seen in each horizontal extent (as masked by the nine vertical bars of the resolution target). In contrast, at NSLS X15A, only four knife edges are available for calculating the LSF due to the small x-ray beam spot. An IDL based image processing software developed at NSTec-LAO has been customized for SI-800 captured data. Background subtraction and star removal are performed be-
FIG. 4. (a) Typical image from Manson source, eight metal strips were coated on the MCP and nine slot opening in the resolution target. 72 blocks of images can be recorded in single exposure. 144 knife edges can be obtained for the resolution measurements. (b) Typical image from NSLS X15A beamline, due to limits of beam size of synchrotron light source, image can be recorded in only one MCP strip. Only 4 knife edges can be obtained in each exposures under unfocused beam.
fore MTF (modulation transfer function) and LSF analysis. For a defined area of interest (AOI), a typical intensity distribution, LSF and a Gaussian fit near the knife edge are obtained as shown in Figure 5. High frequency noise is clearly evident in the raw LSF data due to the spatial convolutions of fiber plug coupling (INCOM BLI58-4: 4.5 μm diameter fibers), imaging (Kodak CCD chip: 9 μm pixels), and MCP pore size (10 μm). A direct determination of the LSF FWHM for each AOI can result in a large variation (or error bar) for a composite set of image data. A better approach is to apply a best-fit Gaussian for the LSF peaks (left and right) of an AOI as shown in Figures 5(c) and 5(d) and then average over the peaks to determine the LSF FWHM. For these spatial resolution studies, x-ray energy was varied from 7 to 19 keV at 2 keV steps. The phosphor bias voltage was varied from +1000 to +3000 VDC at 500 V steps. The experimental results listed in Table I show the Gen-II MCP detector to have a spatial resolution about 45 ± 10 μm when the phosphor bias voltage is set at +3000 VDC and the x-ray energy is less than 20 keV. More importantly, no change in spatial resolution was observed in this energy range. The variation of spatial resolution followed theoretical prediction, being inversely proportional to the square root of the phosphor bias voltage, or linear to V−1/2 . An interesting note is that the spatial resolution did not change even when the MCP response was saturated. For example, at 15 keV, the MCP was exposed to full beam intensity about a factor of 700 above the linear operating regime. The observed spatial resolutions were almost identical. The spatial resolution variation with x-ray incident angles was measured at 15 keV. Due to hutch space limitations, this experimental study was only conducted between +15◦ and −7◦ . A comparison between the measurement and simulation is shown in Figure 6. Agreement between them is generally
11D607-4
Wu et al.
Rev. Sci. Instrum. 85, 11D607 (2014)
FIG. 5. Typical results from image processed 15 keV data taken at NSLS X15A. (a) Intensity distribution in the area of interest; (b) LSF of the area of interest; (c) a Gaussian fit of left peak of LSF; and (d) Gaussian fit of right peak of LSF.
within their respective uncertainties. It is important to know that according the simulations, a better than 100 μm spatial resolution can be achieved in our MCP detector even with a 40◦ incident angle.
TABLE I. Summary of spatial resolution measurements (FWHM of LSF, μm). Phosphor bias voltage (V) X-ray energy (keV) 7.0 9.0 11.0 13.0 15.0 17.0 19.0
1000
1500
2000
2500
3000
63 63 66 66 64 69 67
56 56 57 57 60 58 59
52 49 50 52 53 52 52
49 45 48 48 49 47 47
45 43 44 45 45 45 45
FIG. 6. Spatial resolution variation with x-ray incident angles at 15 keV. The MCP and phosphor were biased at −650 and +3000 VDC, respectively.
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
