Detecting radiation in a standoff geometry with mid-IR laser breakdown

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