Several recently proposed methods for detecting radioactivity at range involve driving laser induced avalanchebreakdown seeded by electrons or negative ions whose density are elevated in the vicinity of a radioactive source. Usinga chirped, mid-IR laser, we drive breakdowns at 1 meter standoff distances and monitor the breakdown timing using thebackscattered spectrum. In addition to the on/off radiation detection based on the increased probability of finding a seedelectron in the focal volume, we also can determine the spatial distribution of these seed electrons in the focal volumethrough temporal information encoded in this backscatter spectrum. We demonstrate that the backscatter spectrum is asuperior detection method relative to visible plasma fluorescence, total pump backscatter, or absolute backscatter timingin its ability to determine the relative radiation level. We discuss scaling to longer focal geometries inherent in remotesensing and possible limitations to the technique, supported by modeling.
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