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2012-11-07 xBSM meeting, Dan Peterson, Coded Aperture 1 1 A new Coded Aperture design In this talk, I will discuss a means to evaluated different coded.

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Presentation on theme: "2012-11-07 xBSM meeting, Dan Peterson, Coded Aperture 1 1 A new Coded Aperture design In this talk, I will discuss a means to evaluated different coded."— Presentation transcript:

1 xBSM meeting, Dan Peterson, Coded Aperture 1 1 A new Coded Aperture design In this talk, I will discuss a means to evaluated different coded aperture designs in different conditions, To this end, I have revised the image calculation program to output images with a common normalization for all conditions: energy and filters. The images, and image differences, can be compared. The normalization to number-of-photons is unknown, but the same for all images. investigate the current Coded Aperture at and 1.8 GeV, with varying gold thickness, compare the Coded Aperture to the Pinhole, investigate a preliminary design for a new Coded Aperture. We are getting ready to purchase a new Low Energy Coded Aperture chip. We do not do this often; the cost is about 15K$ each. We have decided that the Fresnel Zone Plate will not be useful and can be replaced with an alternate Coded Aperture. Possibly, an alternative design, including gold thickness and pattern, will provide improved resolution (especially) at low beam energy.

2 xBSM meeting, Dan Peterson, Coded Aperture 2 2 The current Coded Aperture with 7 μm beam size, GeV, 4.0μm diamond, 2.5μm silicon, 0.7μm gold. (red) straight-through: no gold, no silicon. x-ray energy distribution incident on gold (blue) = 3.50 KeV The CA works because of the high average transparency. The standard Coded Aperture design has a total vertical opening of 150 μm, distributed over a total distance of 280 μm. This total distance projects to 3.34 * 280 μm = 935 μm on the detector. With GeV beam energy, with Diamond filter, the average transmitted light on the detector, relative to straight-through, is calculated from the images from the images to be 270/1246 = ( A simpler calculation based on the pattern predicts a ratio 150 * 3.34 /1600 = 0.31 ) The other loss, 0.22/0.31, is due to the silicon substrate.

3 xBSM meeting, Dan Peterson, Coded Aperture 3 3 The Pinhole, 45μm slit width, with 7 μm beam size, GeV, 4.0μm diamond, no silicon, 10μm gold. (red) straight-through: no gold, no silicon. The Pinhole (without silicon filtering), has average transmission, (ref: CA with 0.7μm gold was 0.22 ). From a simple calculation, using the Pinhole opening, expect 45 * 3.34 /1600 = Without the silicon filtering, the average x-ray energy, incident on gold (blue) is reduced to = 3.19 KeV The Pinhole works with less transmission because the light is concentrated in one peak. The feature width limits the resolution at small beam size.

4 xBSM meeting, Dan Peterson, Coded Aperture 4 4 Evaluate the resolving power of the Pinhole. In the plot below, compare the image for 9μm beam size (blue) relative to 7μm beam size (red). Define the relative χ 2 /dof as… χ 2 /dof = 1/(#bins) bin ( PH i (9μm) - PH i (7μm) ) 2 / ( PH i (7μm) ). This is the sum over bins of differences (squared) due to the beam spread, relative to statistical accuracy (squared). In this case, comparing 9μm vs.7μm, with the Pinhole, χ 2 /dof = The pulse height is not calibrated to number-of-photons. I do not evaluate the beam smeared image above 10 μm because I integrate over a limited number of bins.

5 xBSM meeting, Dan Peterson, Coded Aperture 5 5 Evaluate the resolving power of the Coded Aperture; with 0.7 μm gold thickness, χ 2 /dof = As expected, this is better than the Pinhole, ref: χ 2 /dof = We have seen agreement between Coded Aperture and the Pinhole measurements from December data. Fluctuations in the single turn measurements indicated that the Coded Aperture measurement is more accurate. Shown at right are the images for 0.7 μm, 0.3 μm, and 10 μm gold. With the 0.3 μm gold, the image dips below the baseline, the minimum-to-maximum swing is larger when compared to maximum gold. the total transmission, and statistical weight, is higher. There is a large background, but it is not in phase. 0.7 μm gold 0.3 μm gold 10 μm gold Coded Aperture, GeV, with Diamond

6 xBSM meeting, Dan Peterson, Coded Aperture 6 6 χ2 /dof is shown as a function of beam size. for both Coded Aperture and Pinhole. This is for a constant δσ/σ (not constant δσ). Note: the Pinhole increases to a maximum at a beam size slightly higher than the subtractor (16μm), while the Coded Aperture is best at low beam size. Note: calculations for beam sizes below the detector pixel size are not useful GeV, with Diamond GAP is optimized at 45 μm subtractor ~18 μm

7 xBSM meeting, Dan Peterson, Coded Aperture 7 7 Resolving power as a function of gold thickness. χ2 /dof is maximum with 0.50 μm gold. I do not advocate 0.5 microns; 0.7 is OK and may be more robust. The transmission is shown relative to GeV, diamond, straight through. Increased gold thickness reduces the total transmission. Coded Aperture, GeV, with Diamond

8 xBSM meeting, Dan Peterson, Coded Aperture 8 8 Evaluate the resolving power of the pinhole In this case, comparing 9μm vs.7μm, with the pinhole, χ 2 /dof =0.089 Recall, for 2.085, with Diamond, χ 2 /dof = The χ 2 /dof is significantly reduced compared to GeV; the Pinhole will not work at 1.8 GeV. Pinhole, 1.8 GeV, No Diamond (45 μm, not re-optimized). GAP is optimized at 73 μm subtractor ~29 μm

9 xBSM meeting, Dan Peterson, Coded Aperture 9 9 With 1.8 GeV beam energy, and no diamond filter, the average x-ray energy, incident on gold (blue) is reduced to = 1.96 KeV. ref: GeV, with Diamond, = 3.50 KeV The current Coded Aperture with 7 μm beam size, GeV, NO diamond, 2.5μm silicon, 0.7μm gold. (red) straight-through: no gold, no silicon. Coded Aperture, GeV, NO diamond The CA collected light is 270/1246=0.063 relative to GeV straight-through. ref: GeV, with Diamond, transmission =0.22. The reduced light collection will affect the resolving power.

10 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture 10 Just for illustration, this shows the two images: GeV, Diamond, 7 μm beam size, (blue) GeV, No Diamond, 7μm beam size, scaled to equal area (red). χ 2 /dof = 40.9

11 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture μm gold 10 μm gold 0.3 μm gold Evaluate the resolving power of the Coded Aperture. With 0.7 μm gold, χ 2 /dof = As expected, this is worse than Gev, with Diamond, χ 2 /dof =0.614 Shown are the images for 0.7 μm, 0.3 μm, and 10 μm gold. With the 0.3 μm gold, AGAIN the image dips below the baseline, the minimum-to-maximum swing is larger when compared to maximum gold. the total transmission, and statistical weight, is higher. AGAIN, there is a large background, but it is not in phase. Coded Aperture, GeV, NO diamond

12 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture 12 χ2 /dof is shown as a function of beam size. for both Coded Aperture and Pinhole. This is for a constant δσ/σ (not constant δσ). Note: the Pinhole increases to a maximum at a beam size slightly higher than in the case of 2.085, with Diamond, while the Coded Aperture is best at low beam size. Note: calculations for beam sizes below the detector pixel size are not useful GeV, No Diamond GAP is optimized at 73 μm subtractor ~29 μm

13 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture 13 Resolving power as a function of gold thickness. χ2 /dof is maximum with ~0.32 μm gold, but is always less than for GeV with Diamond. The transmission is shown relative to GeV, diamond, straight through. Coded Aperture, GeV, NO diamond

14 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture μm gold Alternate CA This an early attempt; it is not optimized. DPP Standard BKH Nov 07 Oct 31 Total transmitting μm Total range μm χ 2 /dof μm beam 0.7 μm gold χ 2 /dof μm beam 0.7 μm gold The BKH Oct 31 pattern has low total transmission. A variation of my starting point, from the Oct 23 , does not work at 1.8 GeV. 0.7 μm gold GeV, No Diamond 0.7 μm gold

15 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture μm gold Alternate CA This an early attempt; it is not optimized. DPP Standard BKH Nov 07 Oct 31 χ 2 /dof μm gold The alternate CA provides improved resolving power at both 1.8 and GeV. Hopefully, further refinement is possible. 0.7 μm gold GeV, with Diamond

16 xBSM meeting, Dan Peterson, Coded Aperture xBSM meeting, Dan Peterson, Coded Aperture 16 The analysis indicates at GeV, with Diamond, at 7 μm beam size, the CA (0.7 μm gold) performs better than the Pinhole, χ 2 /dof =0.614 (CA) vs (PH), at GeV, with Diamond, the optimum gold thickness is 0.5 μm, at 1.8 GeV, No Diamond, at 7 μm beam size, the pinhole is not usable, χ 2 /dof =0.028, at 1.8 GeV, No Diamond, at 7 μm beam size, the CA (0.7 μm gold) performs at about the resolution of the pinhole at 2.085GeV, χ 2 /dof =0.225, at 1.8 GeV, No Diamond, the optimum gold thickness is 0.32 μm, it is possible to design a new CA with improved resolution, the example shows improved χ 2 /dof for both 1.8 GeV and GeV. The peak-to-peak separation for the example CA is 10 pixels. (Is this small enough for beam motion?) I do not advocate gold thickness less than 0.6 μm because I think it is fragile.


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