title: Investigation of the hard x-ray background in backlit pinhole imagers.
abstract: 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-α 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. Radiographic 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.
authors.affiliation: Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Fein
authors.affiliation: Center for Energy Research, University of California, San Diego, La Jolla, California 92093, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Peebles
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Keiter
authors.affiliation: Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: J P
authors.initials: JP
authors.lastname: Holloway
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Klein
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: C C
authors.initials: CC
authors.lastname: Kuranz
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: M J-E
authors.initials: MJ
authors.lastname: Manuel
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, USA.
authors.forename: R P
authors.initials: RP
authors.lastname: Drake