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Title: Mitigation of hard x-ray background in backlit pinhole imagers

Abstract

Experiments were performed to mitigate the hard x-ray background commonly observed in backlit pinhole imagers. The material of the scaffold holding the primary backlighter foil was varied to reduce the laser-plasma instabilities responsible for hot electrons and resulting hard x-ray background. Radiographic measurements with image plates showed a factor of >25 decrease in x-rays between 30 and 67 keV when going from a plastic to Al or V scaffold. Here, a potential design using V scaffold offers a signal-to-background ratio of 6:1, a factor of 2 greater than using the bare plastic scaffold.

Authors:
 [1];  [1];  [1];  [1];  [1]; ORCiD logo [1]
  1. Univ. of Michigan, Ann Arbor, MI (United States)
Publication Date:
Research Org.:
Univ. of Michigan, Ann Arbor, MI (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA), Office of Defense Programs (DP) (NA-10)
OSTI Identifier:
1339468
Grant/Contract Number:
NA0002956
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Review of Scientific Instruments
Additional Journal Information:
Journal Volume: 87; Journal Issue: 11; Journal ID: ISSN 0034-6748
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPE AND RADIATION SOURCES; 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; X-ray backlighters; laser-plasmas; laser-generated x-rays; laser-plasma; x-ray radiography; backlighters

Citation Formats

Fein, J. R., Keiter, P. A., Holloway, J. P., Klein, S. R., Davis, J. S., and Drake, R. P. Mitigation of hard x-ray background in backlit pinhole imagers. United States: N. p., 2016. Web. doi:10.1063/1.4962192.
Fein, J. R., Keiter, P. A., Holloway, J. P., Klein, S. R., Davis, J. S., & Drake, R. P. Mitigation of hard x-ray background in backlit pinhole imagers. United States. doi:10.1063/1.4962192.
Fein, J. R., Keiter, P. A., Holloway, J. P., Klein, S. R., Davis, J. S., and Drake, R. P. 2016. "Mitigation of hard x-ray background in backlit pinhole imagers". United States. doi:10.1063/1.4962192. https://www.osti.gov/servlets/purl/1339468.
@article{osti_1339468,
title = {Mitigation of hard x-ray background in backlit pinhole imagers},
author = {Fein, J. R. and Keiter, P. A. and Holloway, J. P. and Klein, S. R. and Davis, J. S. and Drake, R. P.},
abstractNote = {Experiments were performed to mitigate the hard x-ray background commonly observed in backlit pinhole imagers. The material of the scaffold holding the primary backlighter foil was varied to reduce the laser-plasma instabilities responsible for hot electrons and resulting hard x-ray background. Radiographic measurements with image plates showed a factor of >25 decrease in x-rays between 30 and 67 keV when going from a plastic to Al or V scaffold. Here, a potential design using V scaffold offers a signal-to-background ratio of 6:1, a factor of 2 greater than using the bare plastic scaffold.},
doi = {10.1063/1.4962192},
journal = {Review of Scientific Instruments},
number = 11,
volume = 87,
place = {United States},
year = 2016,
month = 9
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Citation Metrics:
Cited by: 1work
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  • Experiments were performed to mitigate the hard x-ray background commonly observed in backlit pinhole imagers. The material of the scaffold holding the primary backlighter foil was varied to reduce the laser-plasma instabilities responsible for hot electrons and resulting hard x-ray background. Radiographic measurements with image plates showed a factor of >25 decrease in x-rays between 30 and 67 keV when going from a plastic to Al or V scaffold. As a result, a potential design using V scaffold offers a signal-to-background ratio of 6:1, a factor of 2 greater than using the bare plastic scaffold
  • Hard x-rays from laser-produced hot electrons (>10 keV) in backlit pinhole imagers can give rise to a background signal that decreases signal dynamic range in radiographs. Consequently, significant uncertainties are introduced to the measured optical depth of imaged plasmas. Past experiments have demonstrated that hard x-rays are produced when hot electrons interact with the high-Z pinhole substrate used to collimate the softer He-╬▒ x-ray source. Results are presented from recent experiments performed on the OMEGA-60 laser to further study the production of hard x-rays in the pinhole substrate and how these x-rays contribute to the background signal in radiographs. Radiographicmore » image plates measured hard x-rays from pinhole imagers with Mo, Sn, and Ta pinhole substrates. The variation in background signal between pinhole substrates provides evidence that much of this background comes from x-rays produced in the pinhole substrate itself. A Monte Carlo electron transport code was used to model x-ray production from hot electrons interacting in the pinhole substrate, as well as to model measurements of x-rays from the irradiated side of the targets, recorded by a bremsstrahlung x-ray spectrometer. Inconsistencies in inferred hot electron distributions between the different pinhole substrate materials demonstrate that additional sources of hot electrons beyond those modeled may produce hard x-rays in the pinhole substrate.« less
  • Pinhole-apertured point-projection x-ray radiography is an important diagnostic technique for obtaining high resolution, high contrast, and large field-of-view images used to diagnose the hydrodynamic evolution of high energy density experiments. In this technique, a pinhole aperture is placed between a laser irradiated foil (x-ray source) and an imaging detector. In the present geometry, the x rays that are not transmitted through the pinhole aperture, ablate the pinhole substrate's surface, and turn it into a flyer plate. The pinhole substrate then breaks apart into shrapnel, and that shrapnel can damage diagnostics inside the target chamber. In this letter, we present amore » technique on mitigating the debris by using a tilted pinhole.« less
  • Backlit pinhole radiography used with ungated film as a detector creates x-ray radiographs with increased resolution and contrast. Current hydrodynamics experiments on the OMEGA Laser use a three-dimensional sinusoidal pattern as a seed perturbation for the study of instabilities. The structure of this perturbation makes it highly desirable to obtain two simultaneous orthogonal backlighting views. We accomplished this using two backlit pinholes each mounted 12 mm from the target. The pinholes, of varying size and shape, were centered on 5 mm square foils of 50 {mu}m thick Ta. The backlighting is by K-alpha emission from a 500 {mu}m square Timore » or Sc foil mounted 500 {mu}m from the Ta on a plastic substrate. Four laser beams overfill the metal foil, so that the expanding plastic provides radial tamping of the expanding metal plasma. The resulting x-rays pass through the target onto (ungated) direct exposure film (DEF). Interference between the two views is reduced by using a nose cone in front of the DEF, typically with a 9 mm Ta aperture and with magnets to deflect electrons. Comparison of varying types of pinholes and film exposures will be presented from recent experiments as well as an analysis of the background noise created using this experimental technique.« less
  • Experiments on the National Ignition Facility (NIF) will require bright, short duration, near-monochromatic x-ray backlighters for radiographic diagnosis of many high-energy density systems. This paper details a vanadium pinhole backlighter producing (1.8{+-}0.5)x10{sup 15} x-ray photons into 4{pi} sr near the vanadium He-like characteristic x-ray energy of 5.18 keV. The x-ray yield was quantified from a set of Ross filters imaged to a calibrated image plate, with the Dante diagnostic used to confirm the quasimonochromatic nature of the spectrum produced. Additionally, an x-ray film image shows a source-limited image resolution of 26 {mu}m from a 20 {mu}m diameter pinhole.