Alan D. Conder

Radiation hardening of gated x-ray imagers for the National Ignition Facility (invited)

The National Ignition Facility will soon be producing x-ray flux and neutron yields higher than any produced in laser driven implosion experiments in the past. Even a non-igniting capsule will require x-ray imaging of near burning plasmas at 10(17) neutrons, requiring x-ray recording systems to work in more hostile conditions than we have encountered in past laser facilities. We will present modeling, experimental data and design concepts for x-ray imaging with electronic recording systems for this environment (ARIANE). A novel instrument, active readout in a nuclear environment, is described which uses the time-of-flight difference between the gated x-ray signal and the neutron which induces a background signal to increase the yield at which gated cameras can be used.

Experimental study of neutron induced background noise on gated x-ray framing cameras

A temporally gated x-ray framing camera based on a proximity focus microchannel plate is one of the most important diagnostic tools of inertial confinement fusion experiments. However, fusion neutrons produced in imploded capsules interact with structures surrounding the camera and produce background to x-ray signals. To understand the mechanisms of this neutron induced background, we tested several gated x-ray cameras in the presence of 14 MeV neutrons produced at the Omega laser facility. Differences between background levels observed with photographic film readout and charge-coupled-device readout have been studied.

Miniature, vacuum compatible 1024×1024 charge‐coupled device camera for x‐ray, ultraviolet, or optical imaging

We have developed a very compact (60×60×75 mm3), vacuum compatible, large format (25×25 mm2, 1024×1024 pixels) charge‐coupled device (CCD) camera for digital imaging of visible and ultraviolet radiation, soft to penetrating x rays (≤20 keV), and charged particles. This camera provides a suitable replacement for film with a linear response, dynamic range, and intrinsic signal‐to‐noise response superior than current x‐ray film, and provides real‐time access to the data. The spatial resolution of the camera (<25 μm) is similar to typical digitization slit or step sizes used in processing film data. This new large format CCD camera has immediate applications as the recording device for streak cameras or gated microchannel plate diagnostics, or when used directly as the detector for x‐ray, x‐ray ultraviolet, or optical signals. This is especially important in studying high‐energy plasmas produced in pulse‐power, inertial confinement fusion, and high powered laser‐plasma experiments, as well as other medical and industrial applications.

Characterization of an x-ray framing camera utilizing a charge coupled device or film as recording media

A compact charge coupled device (CCD) camera system has been designed and characterized for use in the six inch manipulator (SIM) at the Nova laser facility. The camera system has been designed to directly replace the 35 mm film packages currently used in SIM-based x-ray imaging diagnostics. The unit’s electronic package has been constructed for small size and high thermal conductivity which reduces the overall camera size and improves its performance when operated within the vacuum environment of the Nova target chamber. Measurements of the x-ray imager’s contrast transfer function (CTF) were made under a variety of operating conditions on a static x-ray Manson source using both the CCD and Kodak T-Max 3200 film as recording media. The CTF data were converted to an equivalent modulation transfer function (MTF). The MTF plots show that the microchannel plate has a uniform response within our measurement accuracy along its strips. In a direction normal to the strip, however, the MTF is reduced due to the slant angle of the pores in the MCP. The measurements show that the CCD camera has a lower MTF response than T-MAX film for all spatial frequencies and configurations measured. However, data obtained from the film exhibited reciprocity failure and border effects that are not observed in the CCD data. Measurements indicate that the signal-to-noise ratio for the CCD data is four to six times larger than that obtained with film and higher photon flux levels were recorded. The CCD-based diagnostic offers immediate access to the data, improved dynamic range, and reduced turnaround time, while eliminating the need for film development, digitization, equipment, and personnel.

Detection of 1-100 keV x rays from high-intensity 500-fs laser-produced plasmas using charge-coupled devices

We describe a compact, vacuum compatible, large format, charge-coupled device (CCD) camera for scientific imaging and detection of 1 - 100 keV x-rays in experiments at the Lawrence Livermore National Laboratory JANUS-1 ps laser. A standard, front-illuminated, multi-pin phase device with 250 k electron full well capacity, low dark current (10 pA/cm2 at 20 degree(s)C) and low read noise (5 electrons rms) is cooled to -35 degree(s)C to give the camera excellent 15-bit dynamic range and signal-to-noise response. The intensity and x-ray energy linear response have been determined for optical and x-ray (< 65 keV) photons and are found to be in excellent agreement. Departure from linearity has been measured to be less than 0.7%. The inherent linearity and energy dispersive characteristics of CCD cameras are well suited for hard x-ray photon counting techniques in scientific applications. X-rays absorbed within the depletion and field-free regions can be distinguished by studying the pulse height spectrum. Results are presented for the detection of 1 - 100 keV Bremsstrahlung continuum, K-shell and L-shell fluorescence spectra emitted from high intensity (1018 W cm-2), 500 fs laser-produced plasmas.

Modeling of neutron induced backgrounds in x-ray framing cameras.

Fast neutrons from inertial confinement fusion implosions pose a severe background to conventional multichannel plate (MCP)-based x-ray framing cameras for deuterium-tritium yields >10(13). Nuclear reactions of neutrons in photosensitive elements (charge coupled device or film) cause some of the image noise. In addition, inelastic neutron collisions in the detector and nearby components create a large gamma pulse. The background from the resulting secondary charged particles is twofold: (1) production of light through the Cherenkov effect in optical components and by excitation of the MCP phosphor and (2) direct excitation of the photosensitive elements. We give theoretical estimates of the various contributions to the overall noise and present mitigation strategies for operating in high yield environments.

Absolute calibration of charge-coupled devices to hard 8- to 98-keV x rays

We describe experimental techniques for characterizing the absolute response of charge-coupled devices (CCD) to incident hard x-rays using the high energy x-ray source at the Lawrence Livermore National Laboratory. We present responsivity and quantum detection efficiency measurements for a standard, front-illuminated, scientific CCD to monoenergetic 8-98 keV K-(alpha) x-rays. This systematic study out to high energies reveals the contribution of different absorption processes to the CCD detection efficiency. For lower energies below 20 keV the CCD behaves like an ideal photoelectric detector as expected. Increasingly above 40 keV the photoelectric effect in the CCD epitaxial region is augmented by incoherent or Compton scattering where a fraction of the energy from the photon scattering event is transferred to the electrons and subsequently detected. The Compton scattering mechanism dominates the photoelectric effect above 100 keV giving the CCD a predicted detection efficiency which remains constant from 150 keV to 1 MeV assuming that the scattered electrons finally come to rest within the active region. These physics issues will be briefly discussed and are particularly relevant to deep active region solid-state detectors with application for hard x-ray detection above 40 keV.

On-sky demonstration of the GMT dispersed fringe phasing sensor prototype on the Magellan Telescope

The GMT is an aplanatic Gregorian telescope consisting of 7 primary and secondary mirror segments that must be phased to within a fraction of an imaging wavelength to allow the 25.4 meter telescope to reach its diffraction limit. When operating in Laser Tomographic Adaptive Optics (LTAO) mode, on-axis guide stars will not be available for segment phasing. In this mode, the GMT’s Acquisition, Guiding, and Wavefront Sensing system (AGWS) will deploy four pickoff probes to acquire natural guide stars in a 6-10 arcmin annular FOV for guiding, active optics, and segment phasing. The phasing sensor will be able to measure piston phase differences between the seven primary/secondary pairs of up to 50 microns with an accuracy of 50 nm using a J-band dispersed fringe sensor. To test the dispersed fringe sensor design and validate the performance models, SAO has built and commissioned a prototype phasing sensor on the Magellan Clay 6.5 meter telescope. This prototype uses an aperture mask to overlay 6 GMT-sized segment gap patterns on the Magellan 6.5 meter primary mirror reimaged pupil. The six diffraction patterns created by these subaperture pairs are then imaged with a lenslet array and dispersed with a grism. An on-board phase shifter has the ability to simulate an arbitrary phase shift within subaperture pairs. The prototype operates both on-axis and 6 arcmin off-axis either with AO correction from the Magellan adaptive secondary MagAO system on or off in order to replicate as closely as possible the conditions expected at the GMT.

Prototyping the GMT phasing camera with the Magellan AO system

The future diffraction-limited performance of the 25.4 meter Giant Magellan Telescope (GMT) will rely on the activeand adaptive wavefront sensing measurements made by the Acquisition, Guiding, and Wavefront Sensor (AGWS)currently being designed by SAO. One subsystem of the AGWS, the phasing camera, will be responsible for measuringthe piston phase difference between the seven GMT primary/secondary segment pairs to 50 nm accuracy with full skycoverage using natural guide stars that are 6-10 arcmin off-axis while the on-axis light is used for science operations.The phasing camera will use a dispersed fringe sensor to measure the phase difference in rectangular subaperturesspanning the gaps between adjacent mirror segments. The large gap between segments (>295 mm, compared to 3 mmfor the Keck telescope) reduces the coherence of light across the subapertures, making this problem particularlychallenging. In support of the AGWS phasing camera technical goals, SAO has undertaken a series of prototypingefforts at the Magellan 6.5 meter Clay telescope to demonstrate the dispersed fringe sensor technology and validateatmospheric models. Our latest on-sky test, completed in December 2015, employs a dual-band (I and J) dispersedfringe sensor. This prototype uses an adaptive optics corrected beam from the Magellan AO adaptive secondary system.The system operates both on-axis and 6 arcmin off-axis from the natural guide star feeding the MagAO wavefrontsensor. This on-sky data will inform the development of the AGWS phasing camera design towards the GMT first light.

Fast-frame-rate 512 x 512 CCD digital camera system

This paper details the performance and shows images obtained from a 512 X 512 CCD camera capable of recording 400 digitized frames per second. A brief description of the data acquisition hardware and image analysis software is also included.