Optimizations of Pt/SiC and W/Si multilayers for the Nuclear Spectroscopic Telescope Array

The Nuclear Spectroscopic Telescope Array, NuSTAR, is a NASA funded Small Explorer Mission, SMEX, scheduled for launch in mid 2011. The spacecraft will fly two co-aligned conical approximation Wolter-I optics with a focal length of 10 meters. The mirrors will be deposited with Pt/SiC and W/Si multilayers to provide a broad band reflectivity from 6 keV up to 78.4 keV. To optimize the mirror coating we use a Figure of Merit procedure developed for gazing incidence optics, which averages the effective area over the energy range, and combines an energy weighting function with an angular weighting function to control the shape of the desired effective area. The NuSTAR multilayers are depth graded with a power-law, d_i = a/(b + i)^c, and we optimize over the total number of bi-layers, N, c, and the maximum bi-layer thickness, d_(max). The result is a 10 mirror group design optimized for a flat even energy response both on and off-axis

W/SiC x-ray multilayers optimized for use above 100 keV

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E ~150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail, compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope

X-ray study of W/Si multilayers for the HEFT hard x-ray telescope

This paper outlines an in-depth study of the W/Si coated mirrors for the High Energy Focusing Telescope (HEFT). We present data taken at 8, 40 and 60 keV obtained at the Danish Space Research Institute and the European Synchrotron Radiation Facility in Grenoble. The set of samples were chosen to cover the parameter space of sample type, sample size and coating type. The investigation includes a study of the interfacial roughness across the sample surface, as substrates and later as coated, and an analysis of the roughness correlation in the W/Si coatings for N = 10 deposited bilayers. The powerlaw graded flight coating for the HEFT mirrors is studied for uniformity and scatter, as well as its performance at high energies

Performance of ePix10K, a high dynamic range, gain auto-ranging pixel detector for FELs

ePix10K is a hybrid pixel detector developed at SLAC for demanding free-electron laser (FEL) applications, providing an ultrahigh dynamic range (245 eV to 88 MeV) through gain auto-ranging. It has three gain modes (high, medium and low) and two auto-ranging modes (high-to-low and medium-to-low). The first ePix10K cameras are built around modules consisting of a sensor flip-chip bonded to 4 ASICs, resulting in 352x384 pixels of 100 $\mu$m x 100 $\mu$m each. We present results from extensive testing of three ePix10K cameras with FEL beams at LCLS, resulting in a measured noise floor of 245 eV rms, or 67 e$^-$ equivalent noise charge (ENC), and a range of 11000 photons at 8 keV. We demonstrate the linearity of the response in various gain combinations: fixed high, fixed medium, fixed low, auto-ranging high to low, and auto-ranging medium-to-low, while maintaining a low noise (well within the counting statistics), a very low cross-talk, perfect saturation response at fluxes up to 900 times the maximum range, and acquisition rates of up to 480 Hz. Finally, we present examples of high dynamic range x-ray imaging spanning more than 4 orders of magnitude dynamic range (from a single photon to 11000 photons/pixel/pulse at 8 keV). Achieving this high performance with only one auto-ranging switch leads to relatively simple calibration and reconstruction procedures. The low noise levels allow usage with long integration times at non-FEL sources. ePix10K cameras leverage the advantages of hybrid pixel detectors with high production yield and good availability, minimize development complexity through sharing the hardware, software and DAQ development with all other versions of ePix cameras, while providing an upgrade path to 5 kHz, 25 kHz and 100 kHz in three steps over the next few years, matching the LCLS-II requirements.Comment: 9 pages, 5 figure

Overview of segmented glass optics development for the Constellation-X hard X-ray telescope

We report recent work on segmented glass optics for the Constellation-H hard x-ray telescope. This effort seeks to both improve the figure of the free-standing glass substrates, and to refine a newly-developed mounting technology for the substrates. We discuss metrology on recently characterized glass shells both unmounted and mounted. We also present plans for several prototype optics to be constructed in the upcoming year

Report on 240Am(n,x) surrogate cross section test measurement

The main goal of the test measurement was to determine the feasibility of the {sup 243}Am(p,t) reaction as a surrogate for {sup 240}Am(n,f). No data cross section data exists for neutron induced reactions on {sup 240}Am; the half-life of this isotope is only 2.1 days making direct measurements difficult, if not impossible. The 48-hour experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in August 2011. A description of the experiment and results is given. The beam energy was initially chosen to be 39 MeV in order to measure an equivalent neutron energy range from 0 to 20 MeV. However, the proton beam was not stopped in the farady cup and the beam was deposited in the surrounding shielding material. The shielding material was not conductive, and a beam current, needed for proper tuning of the beam as well as experimental monitoring, could not be read. If the {sup 240}Am(n,f) surrogate experiment is to be run at LBNL, simple modifications to the beam collection site will need to be made. The beam energy was reduced to 29 MeV, which was within an energy regime of prior experiments and tuning conditions at STARS/LIBERACE. At this energy, the beam current was successfully tuned and measured. At 29 MeV, data was collected with both the {sup 243}Am and {sup 238}U targets. An example particle identification plot is shown in Fig. 1. The triton-fission coincidence rate for the {sup 243}Am target and {sup 238}U target were measured. Coincidence rates of 0.0233(1) cps and 0.150(6) cps were observed for the {sup 243}Am and {sup 238}U targets, respectively. The difference in count rate is largely attributed to the available target material - the {sup 238}U target contains approximately 7 times more atoms than the {sup 243}Am. A proton beam current of {approx}0.7 nA was used for measurements on both targets. Assuming a full experimental run under similar conditions, an estimate for the run time needed was made. Figure 2 shows the number of days needed as a function of acceptable uncertainty for a measurement of 1-20 MeV equivalent neutron energy, binned into 200 keV increments. A 5% measurement will take 3 days for U, but 20 days for Am. It may be difficult to be the sole user of the LBNL cyclotron, or another facility, for such an extended period. However, a 10% measurement will take 19 hours for U, and 5 days for Am. Such a run period is more reasonable and will allow for the first ever measurement of the {sup 240}Am(n,f) cross section. We also anticipate 40% more beam time being available at Texas A&M Cyclotron Institute compared to LBNL in FY2012. The increased amount of beam time will allow us to accumulate better statistics then what would have been available at LBNL