(a) Pixel histogram, event histogram, and -pixel event histograms of an source measurement. The base line is shown in each case. (b) Analogous histograms from a measurement of an source. The inset compares the CCD histogram with data obtained using a Ge point detector, showing the rapidly deteriorating high energy response of the CCD. (c) Comparison of the base line and 5.9 keV widths in the same data set. (d) High energy events tend to span more pixels. The inset to panel (a) is an aerial sketch of a pixel, showing regions where an event of radius will span 1, 2, 3, and 4 pixels, applicable in the case of fully registered events.
A simplified sketch of a pixel volume in the CCD. Photons approach from above and are attenuated exponentially. The surface layer is associated with weak field gradients, allowing diffusion of electron clouds both vertically and laterally, and resulting in partial registry of electron clouds forming in this layer. The depletion layer is associated with strong field gradients and a potential minimum (buried channel) in the vicinity of the gate layer. Thermalized electron clouds contained within this layer cannot diffuse out, and are fully registered.
Parallel measurement of an X-ray spectrum of the broadband laser plasma source using the He using the Ge point detector and the CCD, after normalizations for filters, pileup, energy binning interval, solid angle, and number of laser shots. Slope differences are due to larger electron clouds from high energy events being less able to fit in the CCD’s depletion layer thickness. The CCD’s depletion depth is adjusted to give a common zero energy extrapolation for both detectors, where the cloud size is expected to vanish. This interpretation allows quantitative estimation of the cloud size variation with energy, shown in the inset (see text).
Selection of laser plasma source experiments motivating CCD characterization. Requirement for single or multiple shots is indicated in the panels. Data measured using the CCD are shown in each case, except as follows; (a) X-ray photons impinge on the CCD in discrete pixels enabling histogram spectra. (b) Aggressive hardware binning allows real time optimization of x-ray flux (impression only, vertical binning sketched). (c) Blurred shadow edges of high contrast objects reveal source dimensions. (d) Biological specimens can be imaged by x-ray absorption. (e) Crystal dispersion of x-ray energies (von Hamos, absorption edge of Ti foil is illustrated, vertically binned). (f) Energetic electron beams are produced when using double or impaired temporal contrast laser pulses, traversing filters (with scatter and secondary radiation) to impinge directly on the CCD. (g) Magnetic deflection confirms electron beam polarity and energy. (h) Indirect observation of electron beams by x-ray fluorescence from element arrays. (i) Single shot distinction of x rays vs electron beams (CCD half obscured by thick absorber, other half by thin element foil; x rays produce sharp edge shadow with absorption edge in foil transmission, electrons give a diffuse shadow (scatter) with x-ray emission lines from the foil in the shadow; impression in figure applies to electron beam).
Example of an x-ray spectrum from the laser plasma source when operated under aspirator vacuum, here using 38 laser shots at 4.0 mJ/pulse with estimated pulse duration (see text).
Event counts obtained by thresholding data obtained using the CCD.
Fits to (photons/ per eV bandwidth per sr per laser shot) under aspirator vacuum, with fixed laser pulse duration (estimated ). The apparent anomaly at 6.0 mJ is noted in the text.
Fits to (photons/eV bandwidth per sr per laser shot) under aspirator vacuum, with fixed laser energy per pulse (4.0 mJ).
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