Characterization of Agfa Structurix series D4 and D3sc x-ray films in the 0.7–4.6 keV energy range

In recent years, opacity experiments at Sandia’s Z facility^1,2 and the National Ignition Facility (NIF)^3,4 have studied x-ray propagation through high-energy-density (HED) plasmas at temperature, density, and ionization conditions relevant to the radiative-convective boundary of the sun. Radiation propagation produced in the center of the sun is impeded from escaping by trace elements’ opacity. This is due to the sun’s outer cooler layers having more bound electrons, increasing the opacity. The opacity becomes large enough to stop radiative transport, and convective transport takes over. The lower-temperature experiments (>160 eV) were in good agreement with the theory. However, experiments on Fe and other elements have shown disagreement with the theory as the temperature increases above 160 eV.^3,4

The soft x-ray Opacity Spectrometer (OpSpec) used on the NIF has recently incorporated an elliptically shaped crystal to address these concerns. The original OpSpec used two convex cylindrical crystals for time-integrated point-projection absorption spectra measurements from 540 to 2100 eV.⁵ However, the convex geometry suffered from high backgrounds due to scattered x rays and reflections from alternate crystal planes (ACPs) in the spectrum’s low-energy portion.⁶ The new elliptically shaped crystal allows an acceptance aperture at the crossover focus between the crystal and the detector, reducing the background and eliminating nearly all ACP reflections. The current elliptical design improves the convex cylindrical design but has a usable energy range of 900–2100 eV.

Discriminating between Z and NIF data, the opacity models require opacity accuracy within 10%, which requires transmission measurements accurate to 0.02% and individual OpSpec spectra accurate to 1%.³ The push to increase the spectral resolution of OpSpec has led to an effort to replace the original OpSpec design using image plates (IPs) as the detection media and switch to x-ray film. Figure 1 shows a comparison of the IP and x-ray films from opacity experiments at the NIF, demonstrating a clear improvement in the spectral resolution of the OpSpec when the film is used. Wallace et al. discussed using the film as a detector and predicted the total resolving power E/δE to be greater than 950 at all energies of interest. The effective spatial resolution for OpSpec shows an improved resolution ranging from 2 to 5 times.⁷ In these calculations, the films’ effective pixel size is calculated with 20 μm spatial size, and the image plate is 100 μm, based on previous calibration efforts.⁸ 

The high sensitivity and reasonable resolution of Agfa D4 and D3sc x-ray films in the 0.7–4.6 keV energy range makes it the primary x-ray film used for imaging HED experiments on the NIF. The NIF darkroom developed all the films using the standard NIF film development protocol. This differs from the Lanier et al.⁹ protocol for Agfa D4 and the D3sc x-ray films. Both protocols are shown in Table I. The main difference is the NIF temperature develops the film 7 °F above the Lanier et al. protocol. The time in the presoak, GBX developer, and wash differs as well. The past work focused on photon energies above 4 keV.⁹ The characterization work in Sec. II expands the films’ characterization down into lower energy ranges using the new film development protocol required by NIF.

This paper presents absolute calibration techniques obtained for the Agfa D4 from 705 to 4620 eV measured using the Nevada National Security Site (NNSS) Manson x-ray source. The calibration technique will be expanded to the D3sc films. Section II describes the x-ray source, the experimental setup, and the characterization technique. Section III presents the result of the characterization, and Sec. IV presents future goals.

The measurements were conducted at NNSS Livermore Operations (NNSS-LO) using the medium resolution x-ray source (MRXS),¹⁰ also known as a Manson source, which is a diode type x-ray source used to test and calibrate various devices. The Manson source holds multiple anodes for ease in selecting various spectral lines in the spectral energy range from 400 to 9000 eV.¹⁰ The Manson source comprises three independent vacuum compartments: the source chamber and two test chambers (diagnostic arms). The source chamber contains the anode array, the filament, and the filters. The anode array rotates to adjust between the six anode materials in front of the filament. The electron beam impact forms a small spot, ∼1 mm diameter, on the anode, providing a flat spot at the target. The two test chambers are connected to the main chamber by stainless steel vacuum components that include an isolation gate valve and a mechanical shutter. The film is mounted to the end of the chamber with enough distance to ensure the film is covered uniformly by the source. Figure 2 details the setup for exposure of the film cassette on one of the diagnostic arms, with the step-wedge filter placed directly in front of the film cassette on arm No. 1.

Test chambers have a photodiode and an energy dispersive detector for measuring x-ray flux and the x-ray spectrum. They are mounted on pneumatic actuators so that they can be moved into or out of the beam. The typical spectrum of the selected anode will emit a broadband spectrum, including bremsstrahlung and the anode material’s spectral lines. The filters in the source chamber are used to isolate a narrow wavelength band of x rays. Figure 3 shows the spectrum resulting from each of the six anodes/filter pairs used for these calibration efforts. The spread of photon energies incident on the film was >1 keV. The dominant line feature FWHM is typically in the range of less than a few hundred eV and makes up the majority of the photons. This is using a similar technique described by Rosenberg et al.¹¹ for IP characterization using a Manson source. The x-ray beam overfills the area of the photodiode detector. The photodiode detector produces a current proportional to the x-ray beam intensity, which is then converted to a fluence using the exposure time.

The Manson source’s flux is collected on both diagnostics arms (arm Nos. 1 and 2) using a NIST traceable silicon detector. Arm No. 2 of the Manson source continuously measures the flux during film exposures. The Manson source’s two arms are cross calibrated, ensuring a representative flux at the film in arm No. 1. The distance, solid angle, and filters are used to determine the exposure (photons/μm²) in the step wedge region of interest (ROI).^9,12–14

The film cassette is light-tight and vacuum compatible. Once exposed, the film is then taken to the NIF darkroom for development. The NIF darkroom develops the films using the NIF protocol described in Sec. I. A PerkinElmer PDS microdensitometer, 5 × 20 aperture scan, 12-bit digitizer, calibrated from 0 to 5 Optical Density (O.D.) is used to scan the developed x-ray film.^9,15,16 A filter schema was fielded to increase the amount of data collected per exposure run. We 3D printed a gridded step wedge and then used various filter thicknesses to control the photon flux on different film regions. Figure 2 shows the step wedge.

Two types of step wedges were constructed using filters of either aluminum or Mylar. To maximize the filters’ functionality, each column (a, b, c) and row (e, f) has a distinct filter thickness. The combined filter thicknesses, along with the empty slot, gives 12 different transmission coefficients. Tables II and III show the thicknesses of the filters used in the aluminum and Mylar step wedges, respectively.

The Mylar step wedge filter is paired with the lower photon energy anodes, Fe (Lα 705 eV) and Cu (Lα 930 eV). The aluminum step wedge filter was selected for the higher energy photons for the Mg (Kα 1245 eV), Al (Kα 1487 eV), Mo (Lα 2345 eV), and Ti (Kα 4620 eV). Figure 4 has the modeled CXRO¹⁶ transmission for given photon energy plotted alongside the calculated O.D. for each of the 12 regions of the step wedge normalized to 1 of the region with no filter. All six anodes are shown with their respective step wedge transmission, all normalized to the unfiltered region. The plot was used to verify if the given photon energy transmission and a given film thickness were accurate. However, there is a slight deviation from the predicted transmission. The deviation could mean that the filters’ thickness is not accurate, which would increase our error on the exposure measurement. We are currently developing a technique that would measure and verify each step wedge region’s transmission coefficient. The filter thicknesses were chosen to maximize the difference in energy deposited into the film over a given exposure.

This section presents the initial finding for each of the six anodes (Fe, Cu, Mg, Al, Mo, and Ti) for the Agfa D4 film. More data are to be collected in the future, and a more thorough analysis will compare against theoretical models. Each film was exposed for a set amount of time to achieve the required O.D. distribution needed for the analysis. The average O.D. and the average standard deviation are calculated along with the flux for each ROI. The background or fog of the film is measured from the unexposed region of the film. The fog is then subtracted out from the total O.D. measurement. A second identical exposure is done without the filters to create a flat field for each photon energy exposure. The flat field is used to correct the non-uniform x-ray beam using the standard correction given by^17,18$nj=(pj−b)/(fj−b),$ where n_j is the O.D. of the corrected exposure, p_j is the O.D. of the original exposed image, f_j is the O.D. of the flat field image, and b is the O.D. of background or fog of the film.

The exposure in each region is calculated from the photodiode current, the time interval the film was exposed, and the solid angle made for each region of the step wedge and then corrected for the transmission of the given step-wedge filter or filter combinations. The photodiode is calibrated, allowing a current to flux at the selected photon energies to be calculated. Currently, the transmission coefficient for the filters is given by the database from CXRO¹⁶ (see Fig. 4) and is not measured at this time. Diode response, geometric, and electron current meter errors are approximated and folded into the flux measurement error. The photodiode current errors used for the exposure are a percentage error estimated from past diode characterizations. This analysis does lack the transmission coefficient errors from, the filters and will be discussed in more detail later in the section.

The O.D. is plotted as a function of exposure (photons/μm²), which is then repeated for each for the selected photon energies of a given anode. The error associated with the O.D. is calculated from the standard deviation of the average O.D. The error analysis lacks the PerkinElmer PDS microdensitometer error, which will be added in the future. Table IV beaks down the fog, exposure time, flux, film sensitivity, and calculated slopes with fit errors for the six anodes. The standard error of the estimated slope is calculated under the assumption of residual normality. Figure 5 shows O.D. vs exposure calculated from the data collected from exposing the film using photon energies of 705 eV (Fe anode), 930 eV (Cu anode), 1245 eV (Mg anode), 1487 eV (Al anode), 2345 eV (Mo anode), and 4620 eV (Ti anode) with the lines used to fit the data. The film response to the low energies of 705 and 930 eV have produced O.D. well below the typical NIF data signal, 1–4 O.D, with O.D. > 0.10 showing to be unreliable and producing trends for the film sensitivity that may or may not agree with the higher O.D. sensitivities. However, at these lower photon energies and flux, it would require exposure five times longer, or more, than those used in the initial experiments (see Table IV). To increase the O.D. for these exposures above an O.D. of 1 would require days of continuous exposure or a different brighter source. In Fig. 5, the film sensitivity for 1245 eV (Mg anode), 1487 eV (Al anode), 2345 eV (Mo anode), and 4620 eV (Ti anode) seems to be trending linearly in the given O.D. ranges. Lower and higher O.D. is required for a proper trend.

The data do show some deviation from trend lines, and some do not go through the origin. This systematic error could be attributed to photodiode drift or a difference in the photodiode’s old calibration. The current gold standard for the photodiodes used in this experiment has been sent back for a new calibration. If the diode response drifted for some photon energies, we could see an overall increase or decrease in the photon/μm².

Another systematic error could be from the transmission coefficients for the filters. The current filter transmission correction is given by the CXRO database¹⁶ for the filter material and thickness. If the filter thicknesses were different, this would have an impact on the transmission coefficients. Since the filters are stacked in six regions of the step wedge, the filter pairs could significantly deviate from the model predicted by CXRO¹⁷ and have the most considerable impact on the six lowest O.D. exposure measurements, effectively changing the film sensitivity slope..

A comparison between previously published work by Lanier and Cowan¹² for similar photon energies and film suggest a slight deviation from their film sensitivity trends (Fig. 5), Ti anode (4620 eV). Note that two film sensitivity trends by Lanier et al.^9,12 for the Agfa D4sc film at Ti-4738 eV differed from each other. While the energies, film, and development protocol were the same, the x-ray sources are different. In this work, the NIF development protocol is different, but the x-ray source is different, which could be responsible for the differences in film sensitivity trends.

Preliminary results of Agfa D4 film response for photon energies 705, 930, 1245, 1487, 2345, and 4620 eV are presented. Photon energies <930 eV show a linear trend in the 1–3 O.D. range and with comparable film sensitivities shown in Table IV. O.D. data >0.2 do suggest a deviation from this linear trend. No data above 3 O.D. have been collected, and we cannot compare how the film will react as it approached saturation. The data for photon energies of 4620 eV shows trends similar to the film response based on previously published data.¹² We have identified systematic errors that could lead to changes in trends that we are exploring. Film response models for the Agfa D4 and D3sc film will be conducted. We will continue the characterization of the film Agfa D4 and will be expanding to the D3sc film. We are developing a characterization method for the NNSS operated beamline at the Stanford Synchrotron Radiation Lightsource (SSRL).¹⁹ This is a low-energy beamline from 50 to 2400 eV perfectly suitable for these x-ray film characterizations. The new characterization method will be used to fill in the energy gaps for the x-ray films, to reduce exposure time, and to characterize the x-ray film’s energy deposition dependence on the photon angle of incidence.

This work was performed under the auspices of the U. S. Department of Energy (DOE) by the Los Alamos National Laboratory (Contract No. 89233218CNA000001), the Lawrence Livermore National Laboratory (Contract No. DE-AC52-07NA27344), the Sandia National Laboratory (Contract No. DE-NA0003525), and the Nevada National Security Site, Mission Support and Test Services (Contract No. DE-NA0003624) supported by National Nuclear Security Administration, Office of Defense Programs. The United States Government (USG) retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for USG purposes. The U.S. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan) (No. DOE/NV/03624-0972).

The data that support the findings of this study are available from the corresponding author upon reasonable request.