Uniform large-area x-ray imaging at 9 keV using abacklit pinhole

Jonathan Workman, James R. Fincke, George A. Kyrala, and Tim Pierce

The development and application of point backlighting at high x-ray energies is an essential step indiagnosing radiation-driven experiments. The point-backlighting technique provides uniform backlighterirradiance over a large field of view. This technique circumvents the large laser energy required for areabacklighters at energies of 9 keV and above. We present the results of a Zn 9 keV point-backlighter sourceusing the technique of pinhole aperturing to define the source size and hence the resolution. Details of thedesign and application of this technique to an undriven gold-walled hohlraum are described.

OCIS codes: 340.7740, 340.7480, 110.7440.

1. Introduction

X-ray imaging in laser-driven high-energy densityphysics and inertial fusion experiments comprise anessential, well-established tool for observing dynamicphenomena such as shock trajectories, interface mo-tion, and instability growth.1–3 The standard config-uration uses an x-ray source from a laser-irradiatedfoil of the order of the size of the object to be observedand a pinhole as the image-forming element betweenthe object and detector. The location and size of thepinhole determine the magnification and resolution.

For nanosecond laser pulses, area backlighting re-quires sufficient laser energy over a large area togenerate thermal plasmas capable of exciting He-liketransitions in the laser-irradiated foil. For targets ofsignificant �l (density � length), such as a gold-walled hohlraum, a high-energy source such as Zn isrequired in order to obtain sufficient signal and con-trast in the image. For 9-keV quasi-monochromaticemission from a Zn backlighter, a laser irradiance of5 � 1015 W�cm2 is necessary. Moreover, to provide asufficient field of view, an illumination diameter of1 mm requires 40 kJ of laser energy for a nanosecondpulse duration. In addition to the laser energy bud-get, one must consider nonuniformities in the spatialprofile of the laser beam that imprint in the x-rayemission. The spatially nonuniform emission must besubtracted out of the image or removed by appropri-

ate overlapping of the laser beams. Another success-ful approach makes use of spherically and toroidallybent crystals for two-dimensional imaging.4–7 Theseimagers present a trade-off between high spectralresolution and reflectivity. Matching crystals tostrong-line emitters may limit the extension to higherenergies.8 An alternative approach is described be-low.

The pinhole-apertured point-backlighter (PAPBL)configuration mitigates the issues associated withproducing high-energy x rays, uniform illuminationover a large area, and significant laser energy. Thetechnique using a backlit pinhole has been describedand applied at lower energies elsewhere.9–12 In fact,the use of x-ray point sources has been applied forimaging13,14 and spectroscopy15–19 on several laserfacilities. We describe the development and sourcecharacterization of a specific application, at 9 keVused to image an indirect-drive ignition or high-energy density physics configuration.

2. Experimental Results

Experiments were carried out in two phases utilizingthe Omega laser facility at the University of Roches-ter.20 The first phase characterized the generation of9-keV He� x-ray photons from Zn foils under varyinglaser irradiance conditions.21 These results were thenfolded into the PAPBL imaging design of staticplastic-coated thin gold-walled hohlraums.

A. X-Ray Yield and Conversion-Efficiency Scaling

The first experiments measured the scaling of ZnHe-like x-ray yield as a function of the laser irradi-ance. Data taken with 3-mm-diameter, 12-�m-thicksolid flat Zn disks measured x-ray emission from the

The authors are with Los Alamos National Laboratory, Box1663, Mail Stop E-526, Los Alamos, New Mexico 87545. J. Work-man’s e-mail address is workman@lanl.gov.

Received 29 January 2004; revised manuscript received 4 June2004; accepted 1 November 2004.

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He-like n � 2–1 transition �1s2–1s2p� at 9 keV. Aconvex crystal spectrometer22 recording on DEF (Di-rect Exposure Film, Kodak) x-ray film23 was used toobserve the emission from the irradiated side of thefoils. Film data were digitized to determine exposureand converted to source emission using known dis-tances and theoretical crystal reflectivities.24–26 Spec-tra were integrated to include the He� and Lyman�

transitions as well as their satellites. The scaling ofconversion efficiency with laser irradiance was ac-complished by varying only the focal-spot diameterand laser energy. Laser energy and focal-spot diam-eter were varied from 1.5–6.0 kJ and 100–300 �m,respectively; using a 1-ns-square temporal profile. Anattempt was made to look at the effects of spot diam-eter for the same laser irradiance by varying only theenergy.

The conversion efficiency of incident laser energyinto Zn x rays is shown in Fig. 1. Data taken withthree beams at tight focus (� 100-�m focal-spotdiameter) had similar irradiance to that takenwith 12 beams with a 200-�m focal-spot diameter��2 � 1016 W/cm2�. Data taken with 6 beams with a200-�m focal-spot diameter were similar in irradi-ance to that taken with 12 beams with a 300-�mfocal-spot diameter �9 � 1015 W�cm2�. The conversionefficiency was higher for the larger focal-spot diame-ter in one case and lower in the other, leading toinconclusive evidence whether the spot size (two-dimensional effects) was important. Unfortunately,experimental constraints limited our ability to makedrastic changes in spot diameter while retaining thesame laser irradiance. Two of the irradiance condi-tions, 1 � 1016 and 4 � 1016 W�cm2, with the samefocal-spot diameter and energy were repeated exhib-iting good shot-to-shot consistency. Results were re-producible to the level of 20% or better throughout

the data set. The peak in the conversion efficiency hasnot been determined since the lowest irradiance of4.5 � 1015 W�cm2 shows the conversion efficiencycontinuing to increase. The maximum conversion ef-ficiency was �0.8%, while the lowest measured was� 0.25%, consistent with measurements on the Novalaser.13

Very little cold K� emission is seen in the time-integrated Zn x-ray emission spectra, Fig. 2. Specifi-cally, the spectrum in Fig. 2 was taken at anirradiance of 8 � 1015 W�cm2 using �6 kJ in a300-�m focal-spot diameter. Little cold K� emissionindicates low hot-electron-produced x-ray emission27;however, a step-filtered diagnostic indicated signifi-cant hard x-ray emission peaked near 15–20 keV.Typical filtration in an imaging system would use aZn filter to eliminate some of the background below15 keV while allowing the He-like 2–1 transition topass at 9 keV.

Ultimately, the data show that a conservativelylarge spot diameter (much larger than the pinholediameter) may be used for the point-backlighter con-figuration. This allows for unavoidable error in place-ment of the pinhole and pointing of the laser beams.As discussed below, it was found that a 300-�m focal-spot diameter is a good choice both from a standpointof conversion efficiency and geometric throughput ofthe pinhole.

B. Static Radiograph

For the second phase of experiments, the object to beradiographed is placed near target chamber center ofthe Omega laser facility with a total magnification ofthe system �18�. The pinhole is placed at a distanceof 9.5 mm from the object with the laser-producedx-ray source at a distance of 10.5 mm. Between thepinhole and Zn foil is a 1-mm region containing 0.5-mm-thick Be with a diameter of 5 mm and 0.5-mm-thick plastic with a diameter of 3 mm on which the Znfoil is placed. The low-Z material in between the Znfoil and pinhole substrate is used to avoid pinhole

Fig. 1. Zn conversion efficiency and yield as a function of laserirradiance obtained from a time-integrated spectrometer. Conver-sion efficiency is in x-ray joules into 4�/laser joules incident, shownas the triangles. The open triangles indicate the conversion effi-ciency obtained from a pinhole camera in the radiography exper-iment. Total x-ray yield is plotted as joules into 4�, shown as thestars. It is interesting to note that the conversion efficiency ap-pears to peak at 2–5 � 1015 W�cm2.

Fig. 2. Time-integrated spectrum of Zn K-shell emission at alaser irradiance of 8 � 1015 W�cm2 using �6 kJ in a 3-�m focaldiameter. He-like n � 2–1 transitions dominate the x-ray emissionnear 9 keV, while a small peak from H-like emission is evidentnear 9.3 keV. There is also a peak from the cold K� emissionproduced by nonthermal electrons.

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closure from the soft x-ray emission.10 The pinholesubstrate is coated with �75 �m of CH to avoid anynoise sources from hot-electron-produced high-energy x-ray background. In an attempt to avoidlaunching the backlighter assembly directly into thediagnostic, possibly destroying it, a 10° wedge is ma-chined into the plastic to allow the backlighter as-sembly or pinhole to “take off,” through shock or therocket effect, at an angle away from the direct line ofsight of the detector.28 The backlighter configurationis shown in Fig. 3. The detector, x-ray film in thiscase, is placed at a distance of 160 mm from the ob-ject. Unlike true point sources and fiber sources, thefinite-thickness pinhole aperture looks more like atube. High-energy x-ray sources require that the pin-hole substrate be fabricated from a high Z material sothat x rays do not pass through the substrate, in thiscase a 125-�m-thick Ta substrate. For high spatialresolution with sufficient throughput, a single 20-�m-diameter pinhole is machined into the Ta sub-strate. The pinhole aspect ratio gives a fullacceptance angle of �17°. Projected 1 mm back to-ward the x-ray source, the largest usable source sizeis 330 �m in diameter [see the “pinhole (expandedview)” schematic inset in Fig. 3]. Larger sources sub-tend a larger angle than that accommodated by thepinhole, while a smaller source generates a smallerillumination cone, not necessarily subtending the en-tire object to be radiographed.

For this experiment, we initially configured sixoverlapping laser beams, without any beam smooth-ing applied, with an overall focal-spot diameter of�300 �m. This configuration was consistent with thehighest conversion efficiency obtained in the firstphase of experiments as well as the geometric con-straint of the pinhole. In practice, we found that thisconfiguration gave too much signal, and therefore thenumber of beams was reduced to 2 (950 J). The finalconfiguration gave an estimated irradiance of justover 1 � 1015 W�cm2, a surprisingly low irradiance.

Estimates from pinhole-camera images recorded onx-ray film indicate that the conversion efficiency usedfor the radiographs was �2–3 � 10�3 (below the high-est conversion efficiency recorded in the earlierphase). Figure 4 shows the results of radiographing a1.2 mm � 2.0 mm hohlraum. Three pieces of DEFx-ray film were stacked in the film holder with a20-mil Be filter directly in front of the film. Figure4(a) shows the frontmost piece of DEF x-ray film thatrecorded the image. The image is of a gold hohlraumof wall thickness 2.7 �m with a 42-�m period grid(10-�m bars) glued to the side opposite of the diag-nostic. The grid is used to determine spatial resolu-tion.29 Because of the grid’s distance from the centralaxis of the hohlraum, the magnification was �20 atthe plane of the grid, as expected.

The hohlraum diameter is 1200 �m with a 90-�mlayer of epoxy coated on the outside for rigidity. Theepoxy layer �C32H36O9� is clearly seen in the secondlayer of film in Fig. 4(b), which is a magnified view ofthe lower right-hand quadrant of the hohlraum. Thecontrast in the epoxy at 9 keV is excellent, aided bythe increased dynamic range possible with multiplelayers of film (the transmission of DEF film at 9 keVis �40%).24 It is of interest to note the faint white linesurrounding the epoxy in Fig. 4(b). The peak in signal(white curve) is not consistent with bulk absorption,

Fig. 3. Backlit pinhole imaging model with a hohlraum as theobject. At the left is the side view showing the wedged backlightersubstrate and projection of the illumination at the object. The insetat the bottom of the figure shows the tubelike structure of thepinhole defining the x-ray geometry. The right-hand side shows anend-on (tilted to show the hohlraum) view of the configuration fromthe backlighter substrate side.

Fig. 4. Radiographs of a 2.7-�m gold-walled hohlraum recordedon DEF x-ray film: (a) the full image on the front-most piece of film;(b) magnified view of the rear end of the hohlraum on the secondlayer of film showing the 90-�m epoxy layer.

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but rather refractive effects producing phase-contrast imaging.30 Future experiments might possi-bly enhance this effect by increasing the source-to-object distance.31 The white circle in Fig. 4(a) is theopening of the nosetip of the diagnostic that definesthe field of view. From this corresponding region onthe second layer of film, the uniformity of the unat-tenuated x-ray source is determined. It could also beused for absolute density determination. Scans of theunattenuated x-ray region to the right of the hohl-raum in Fig. 4(b) shows a variation of a factor of 2 inthe signal level over a distance of 1 mm with a peaktoward the center. This variation is in fact consistentwith the distribution predicted by ray-trace calcula-tions from the pinhole aspect ratio used for the ex-periment. Some part of this variation comes frombackground as it is observed in scans of the regionobscured by the filter holder frame. It should be notedthat a factor-of-2 variation in backlighter uniformityis very good when compared with a typical area back-lighter profile.32 Outside the white circle, the expo-sure may be due to fluorescence from the 1-mil Znfilter placed at the front of the nosetip or scattering inthe Be filter near the film plane that is stopped in a“square” pattern from the outer dimensions of therear 2-mm-thick Al filter frame. It is not likely thathigh-energy x rays that might penetrate the outsideof the nosetip would produce this background, as theywould also pass through the rear aluminum filterframe.

In order to give an idea of the proximity of thediagnostic nosetip to the target, Fig. 5 shows thenosetip and backlighter positions with respect to thehohlraum. The nosetip had an open diameter of 3 mmand was located at a distance of 5 mm from the centerof the target. The backlighter is shown in the top-right corner driven from the opposite side by severallaser beams. The hohlraum would normally be drivenby 12 laser beams, which must be avoided by thediagnostic nosetip.

Figure 6(a) shows an expanded view of the grid inFig. 4(a). The grid consisted of square profile goldbars with 16-�m and 10-�m widths and a period of42 �m. The configuration of the two bar sizes isshown in Fig. 6(a) with white dashed curves. Figure6(b) shows a horizontal lineout of the grid after rota-tion and vertical averaging over �20 �m. Because thegrid sits on top of the gold hohlraum, measurementshave been taken in a region of low transmission andthus low signal-to-noise ratio. The lineout shows anaverage full width at half-maximum (FWHM) of15.5 �m for the 10-�m bar and 21.7 �m for the16-�m bar. The 10%–90% rise on the bars is10–12 �m. These values are consistent with the res-olution expected from a 20-�m pinhole giving apinhole-limited image resolution.

Spectral purity of the x-ray source is a concernwhen trying to determine relative or absolute mate-rial densities from a radiograph. In laser-producedplasmas, the image can be degraded by the presenceof hot electrons that can couple to high-energy�10–100 keV� x-ray bremsstrahlung and fluorescenceemission. High-energy x rays can complicate theanalysis of relative exposure information as well asreduce resolution and dynamic range from the back-ground. Figure 7 represents a measure of the spectralpurity of the x-ray source through a comparison ofactual and predicted transmission across the cylin-drical section of the hohlraum. The transmission in

Fig. 5. Diagnostic nosetip opening was 3 mm in diameter at adistance of 5 mm from the hohlraum center. The backlighter as-sembly, driven from the far side, was at a distance of 10.5 mm fromthe hohlraum. The hohlraum is shown in the center, and wouldtypically be driven by 12 beams.

Fig. 6. (a) Expanded view of the grid in Fig. 4(a) showing thepattern of 10- and 16-�m bars. (b) An averaged lineout of the 10-and 16-�m-bar, 42-�m-period grid. The 10-�m bars show a widthof 15.5 �m with a rise from 10–90% of 10–12 �m.

862 APPLIED OPTICS � Vol. 44, No. 6 � 20 February 2005

Fig. 7 is normalized to the transmission through thecenter of the cylinder after subtraction of the back-ground, which drops rapidly toward the edges owingto limb darkening (increased apparent materialthickness). Included in the calculations is the 90 �mof epoxy, used as a support structure, coated aroundthe 2.7-�m gold-walled hohlraum as well as the

20-�m diagnostic resolution. Comparisons are madeat the expected energy of 9 keV and at a higher en-ergy of 20 keV. One can see that the predicted trans-mission widens significantly and is more uniformtoward the center at 20 keV when compared with thedata. The broadened profile becomes more pro-nounced at even higher energies. The predictedtransmission at 9 keV compares well with the data onthe right side of the cylinder. Deviations in the trans-mission on the left side of the cylinder fall within thethickness uncertainty of the gold 2.7 � 0.5 �m.Transmission in the data does not fall to zero owingto saturation in the digitized film, some slight overallnonuniformity in the backlighter, and some hardx-ray background.

3. Direct Comparison to Pinhole-Camera Imaging

The image obtained using the PAPBL technique canbe directly compared with an image taken of the Znx-ray source from the irradiated side of the disk usinga standard pinhole camera. Figure 8(a) shows anx-ray pinhole image of the Zn source recorded on DEFx-ray film using a 10-�m-diameter pinhole located157.5 mm from the source at a 4� magnification,filtered though 0.5-mil Zn. This image was recordedon the same shot as the PAPBL image looking di-rectly opposite the PAPBL detector. The x-ray emis-sion is produced from two “overlapped” beams. The

Fig. 7. Comparison of observed transmission through the cylin-drical hohlraum compared with calculations for 9 and 20 keV. Thesolid dark curve is the experimental data, the long-dashed curve isthe prediction at 9 keV, and the dotted curve is the prediction at20 keV. The light dashed curves on the left-hand side representtransmission predictions for 2.2- and 3.2-�m gold wall thicknessesat 9 keV. The match in shape to the 9-keV calculation shows arelatively pure backlighter source.

Fig. 8. (a) Static x-ray pinhole image of the irradiated side of the Zn source at 4� magnification using a 10-�m pinhole. (b) and (c)Averaged lineouts of the pinhole-camera image in the vertical and horizontal directions. The dashed curves are the Gaussian fits. (d)Lineout from the hohlraum Fig. 4(a). The curve shows the raw cylindrical profile peaked at 0.5 photons��m2, while the top flat profile hasbeen corrected for the known cylindrical profile and transmission. The flat profile shows a standard deviation of 3.5% over 1 mm. Alllineouts are averaged over 100 �m at the respective object planes.

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emission is very peaked and well described by twoGaussian distributions with an area of approximately200 �m � 300 �m at the 1�e intensity level. The un-corrected peak exposure on the film is approximately0.4 Zn x-ray photon��m2. Gaussian fits to the x and yprofiles are shown as dashed curves in Figs. 8(b) and8(c). The profiles are averaged over 100 �m in theobject plane with small-scale variations most likelydominated by film noise.

In contrast, the x-ray source detected from thePAPBL arrangement has a very flat profile over a1-mm region, as shown in the top trace of Fig. 8(d)with a standard deviation of �3.5%. The top trace isthe raw lineout of the cylinder corrected for the ex-pected transmission through a cylinder, shown inFig. 7. The curved trace of Fig. 8(d) is the lineoutthrough the cylinder, taken from Fig. 7, averagedover 100 �m at the object plane. The peak exposureat the center of the cylindrical region in Fig. 8(d) is 0.5Zn x-ray photon��m2. A direct comparison of signalefficiency between the PAPBL image and the pinhole-camera image requires incorporation of all the knownfilters, magnification effects, the pinhole diameters,and distances from the source. The relative film-density correction for the pinhole camera can be ex-pressed as

fPAPBL

fPHC� drel

Irel � �MPAPBL

MPHC�2 �

�PHC

�PAPBL, (1)

where f is the throughput of a given aspect pinhole ata given distance from an extended source, drel is therelative contribution between two pinhole sizes, Trel isthe relative transmission difference between two sys-tems, M is the magnification of the source, and � isthe signal density recorded on the film. Differences infiltration give a factor of 13, the square of the mag-nification ratio gives a factor of 1.8 � 103, and the10-�m pinhole (pinhole camera) versus the 20-�mpinhole (PAPBL arrangement) gives us a factor of 4.The pinhole distance gives an additional factor of6.9 � 103.33 The direct comparison of the PAPBL topinhole camera, all factors included, gives��6.9 � 103 � 4���13 � 1.8 � 103�� � �0.4�0.5� � 0.95.This is very consistent, showing that the PAPBLtechnique extrapolates to the pinhole-camera config-uration as expected.

4. Conclusions

A high-contrast high-resolution uniform, large-areax-ray image has been demonstrated using a pinhole-apertured point projection zinc backlighter at anx-ray energy of 9 keV. Conversion efficiency scalingshows that the laser irradiance for peak conversionmay be below 4 � 1015 W/cm2 with bright images ob-tainable with two beams �950 J� on the Omega laser.Equivalent area backlighting would require muchmore laser energy. The spatial resolution obtained

from the grid shows that the pinhole diameter limitsthe spatial resolution of the configuration. Analysis ofthe x-ray transmission through the cylindrical part ofthe hohlraum demonstrates good spectral purity ofthe x-ray source. Comparison on the same shot witha pinhole image of the source empirically demon-strates extension of the pinhole camera to the PAPBLtechnique.

The authors would like to thank P. Walsh, S.Evans, T. Sedillo, and the MST 7 target fabricationteam of Los Alamos National Laboratory, V. Rekowand R. Costa of Lawrence Livermore Laboratory, andthe Omega operations crew. This work was per-formed at Los Alamos National Laboratory under theauspices of the U.S. Department of Energy undercontract W-7405-ENG-36.

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28. The backlighter assembly did strike the detector off-center;however, this is not sufficient to show that the wedge made asignificant contribution.

29. The location of the grid was chosen to reduce debris launchedat the detector by the radial expansion of the hohlraum walls.

30. A. Snigirev, I. Snigireva, V. Kohn, S. Kuznetsov, and I. Sche-lokov, “On the possibilities of x-ray phase contrast microimag-ing by coherent high-energy synchrotron radiation,” Rev. Sci.Instrum. 66, 5486–5492 (1995).

31. D. Montgomery, Los Alamos National Laboratory, Los Alamos,N.M. 87545 (personal communication, 2004).

32. S. H. Batha, C. W. Barnes, and C. R. Christensen, “Backlighterpredictive capability,” Rev. Sci. Instrum. 74, 2174–2177(2003).

33. This ratio is from a calculation that includes the extendedsource and finite pinhole thickness.

20 February 2005 � Vol. 44, No. 6 � APPLIED OPTICS 865