B. Bachmann, R. Chow, N. Palmer, M. Hoover, E. Huffman, J. Lee, E. Romano, C. Kumar, R. Hulbert, F. Albert, E. Dewald, L. Divol, M. Hohenberger, O. Landen, A. Warrick, T. Döppner
KEYWORDS: National Ignition Facility, Hard x-rays, Diagnostics, Electrons, Optical filters, Signal to noise ratio, X-rays, Copper, Sensors, Image filtering
We recently designed, built and commissioned a new pinhole / filter assembly for the equatorial hard x-ray imager
(eHXI) at the National Ignition Facility (NIF). In this paper we describe the design and metrology of the new diagnostic
as well as the spectral and spatial response of the hard x-ray detector. The new eHXI assembly has improved the photon
collection efficiency along with spectral and spatial resolution by making use of 1D imaging channels and various hard
x-ray filters. In addition we added a Ross pair filter set for Au K-alpha emission (67-69 keV). The new eHXI design will
improve our understanding of the sourcing of hot electrons, generated in laser-plasma-instabilities, along the vertical
hohlraum axis. This information is an important input for simulating and eventually limiting the DT fuel preheat in ICF
implosions.
This paper describes a method to calibrate image plate sensitivity for use in the low energy spectral range. Image plates, also known as photostimulable luminescence (PSL) detectors, have often proved to be a valuable tool as a detector for plasma physics studies. Their advantages of large dynamic range, high stopping power, and resistance to neutron damage sometimes outweigh the problems of limited resolution and the remote processing required. The neutron damage resistance is required when the X-ray source is producing a high neutron flux. The Static X-ray Imager (SXI) is a key diagnostic on the National Ignition Facility (NIF) target chamber at LLNL for use in determining the symmetry of the laser beams. The SXI is essential to proper interpretation of the data from the Dante diagnostic to determine the X-ray radiation temperature. It is comprised of two diagnostics located at the top and the bottom of the target chamber. The usual detector is a large array CCD camera. For shots giving high yields of neutrons, the camera would not only be blinded by the neutrons, it would be damaged. To get around this problem, an image plate (IP) is used as the detector. The NIF application covers the energy range from 700 to 5000 eV. The type of image plates typically used for plasma physics are the Fuji BAS-MS, BAS-SR, and BAS-TR models. All models consist of an X-ray sensitive material made of BaF(Br,I):Eu2+ embedded in a plastic binder. X-rays incident on the phosphor ionize the Eu 2+ producing Eu3+ and free electrons that are trapped in lattice defects (F-centers) produced by the absence of halogen ions in the BaF2 crystal. An image plate readout scanner irradiates the IP with a red laser causing reduction of the Eu3+ and emission of a blue photon. The photon is collected using a photomultiplier and digitized to make an electronic image. Image plates are cleared of all F-centers by putting them under a bright light for about 10 minutes. They are then ready for producing a new X-ray image. The MS IP model has the higher sensitivity and the SR IP and TR IP models are designed for higher resolution. The MS and SR IPs have a thin Mylar coating that protects the sensitive layer. The TR model has no protective layer and is more sensitive at the lower X-ray energies but must be handled more carefully. The raw image data from the Fuji scanner can be converted to units of PSL that are proportional to the photon count. The equation relating PSL to the raw greyscale value is: PSL = (R/100)2(4000/S)exp10{L(G/(2B-1)-1/2)} where R is the resolution in μm S is the sensitivity setting L is the latitude B is the dynamic range (8 or 16 bits) G is the raw image greyscale value. The IP photon sensitivity is defined as the PSL output per photon input and is a function of the photon energy. Meadowcroft et al in 2008 published the sensitivity for the three types of image plates in the spectral range from 1 to 100 keV. Maddox et al measured the sensitivity for type MS and SR image plates from 8 to 80 keV using the NSTec High Energy X-ray (HEX) source, a fluorescer type X-ray source. The Meadowcroft and Maddox measurements used similar X-ray sources for the higher spectral and the same type of IP scanner, the FLA 7000. There is reasonable agreement between the Maddox and Meadowcroft sensitivity measurements of MS and SR type IP for the at spectral energies above 20 keV, but the Maddox sensitivities are much lower than those of Meadowcroft in the energy range below 20 keV. Recently Bonnet et al published a model for the photon sensitivity based upon the amount of energy deposited and Monte Carlo calculations to incorporate the specifics of the X-ray absorption and the readout process. The model was calibrated for sensitivity using radioactive sources. The model was compared to the previous publications cited. The Bonnet model tends to agree with the Meadocroft measurements at the low spectral energies. The present paper describes the measurement of IP sensitivity in the spectral range from 700 to 8000 eV. The sensitivity in this spectral range had not previously been measured and was needed for the NIF application. A calibration at the low energy range was done using a diode source and a band pass filter. X-ray beam is filtered and limited by the applied voltage to provide a spectral band that is about 1/10 of the average spectral energy. The X-ray flux is measured using a photodiode that is traceable to National Institute for Standards and Technology (NIST). The spectrum for each X-ray band is measured using a silicon drifted detector. The photodiode calibration method is described. Measurements were made on SR, TR, and specially coated TR image plates. The measurement results will be presented and the uncertainties in the measurement will be discussed. The results will be compared to other measurements and estimation methods.
KEYWORDS: X-rays, Scanners, Electrons, X-ray imaging, Luminescence, Data modeling, National Ignition Facility, Calibration, Monte Carlo methods, Chemical species
Periodic sensitivity calibration of imaging plates (IP) is crucial for quantitative understanding of x-ray data obtained at the National Ignition Facility (NIF). To test the x-ray sensitivity of the IPs and the scanners, we developed an x-ray exposure station based on radioactive isotopes (55Fe, 109Cd, and 241Am). This apparatus provides a convenient setup for a periodical test of the IP’s and the scanners. On NIF implosion experiments with deuterium-tritium mixture fuel, the neutrons produced in the capsule hit the imaging plates and impose background signal. Therefore it is also important to know the neutron sensitivity of the IPs. The sensitivity for 14 MeV neutrons was measured on high neutron yield shots at the OMEGA laser facility. The measured sensitivities were compared with the results of Monte Carlo simulations.
Charge-injection devices (CIDs) are solid-state 2D imaging sensors similar to CCDs, but their distinct architecture
makes CIDs more resistant to ionizing radiation. CID cameras have been used extensively for X-ray imaging at the
OMEGA Laser Facility with neutron fluences at the sensor approaching 109 n/cm2 (DT, 14 MeV). A CID Camera X-ray
Imager (CCXI) system has been designed and implemented at NIF that can be used as a rad-hard electronic-readout
alternative for time-integrated X-ray imaging. This paper describes the design and implementation of the system,
calibration of the sensor for X-rays in the 3 - 14 keV energy range, and preliminary data acquired on NIF shots over a
range of neutron yields. The upper limit of neutron fluence at which CCXI can acquire useable images is ~ 108 n/cm2
and there are noise problems that need further improvement, but the sensor has proven to be very robust in surviving
high yield shots (~ 1014 DT neutrons) with minimal damage.
The Static X-Ray Imager (SXI) is a National Ignition Facility (NIF) diagnostic that uses a CCDcamera to record timeintegrated
X-ray images of target features such as the laser entrance hole of hohlraums. SXI has two dedicated
positioners on the NIF target chamber for viewing thetarget from above and below, and the X-ray energies of interest are
870 eV for the “soft” channel and 3 – 5 keV for the “hard” channels. The original cameras utilize a large formatbackilluminated
2048 x 2048 CCD sensor with 24 micron pixels. Since the original sensor isno longer available, an effort
was recently undertaken to build replacement cameras withsuitable new sensors. Three of the new cameras use a
commercially available front-illuminatedCCD of similar size to the original, which has adequate sensitivity for the hard
X-ray channelsbut not for the soft. For sensitivity below 1 keV, Lawrence Livermore National Laboratory (LLNL) had
additional CCDs back-thinned andconverted to back-illumination for use in the other two new cameras. In this paper we
describethe characteristics of the new cameras and present performance data (quantum efficiency, flat field, and dynamic
range) for the front- and back-illuminated cameras, with comparisons to the originalcameras.
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