Author was inspired to write their PhD dissertation on radiation source search due to the Goiânia incident. Thank you to them for sharing this with us.
Goiânia Accident - https://news.ycombinator.com/item?id=49202635 - August 2026
There's been a lot of development since this dissertation to let it run in real time, and to integrate with tactical radiation detectors that the military/police/civil defense teams tend to use. Unfortunately that's mostly unpublished work at present.
Australia had some fun with a lost mining source - rather than run a geophysics plane 80m above ground from mine to city they held off to test a gadget with some whirly stuff about a crystal pack ... as I understand it.
Any comment, or all still a bit unpublished?
These sort of loss events happen much more often than is reported widely. Not like something that happens every day, but there are a non negligible number of events every year. The Nuclear Threat Initiative catalogs publicly known events if you want to browse.
Sorry - had to laugh at "LIDAR" - the source bounced off a truck in W.Australian Pilbara and the Eastern States specialists rolled along the road in a van picking up the lost source by direction.
The environment would be best described as flat. Very flat. Road, flat ground. No buildings, no reflections or refraction, not a lot for a LIDAR to do.
I'm still impressed with the direction to source capabilities over what we started with many years ago; crystals stacked with lead plate separating top crystal from lower crystals .. to get a sense of what was cosmic and what was not.
Imaging detectors can get a lot fancier than just two planes. One design is basically segmented crystals and you look at the timing between the separate segments light up as a gamma ray passes through each quadrant. You can then work backwards from those sub nanosecond time difference to get the direction, and stack up a bunch of events to make a sort of picture (more like a blobby heat map).
For neutrons there is an even crazier imaging technique called a coded aperture that I won't even try to describe here. Suffice to say it's very clever and unintuitive.
With a few tweaks.
> the separate segments light up as a gamma ray passes through each quadrant.
Err, one single ray (we doing wave fronts or particles in our slit experiment here) surely only impacts and flashes once in one segment .. or passes through with no impact?
A whiteboard might be easier for this convo, or perhaps we let it rest. Coded aperture looks to be more or less as I imagined: https://www.sciencedirect.com/science/article/pii/S135044871...
Gammas tend to act more like particles in the detector. They don't stop all at once, they tend interact multiple times and deposit their energy in multiple places along a line via a ton of different physical interactions. My description was simplifying and conflating things a bit, you can look up optically segmented detectors, as well as Compton scatter cameras if you want a better answer. There are other imaging approaches, too.
Agreed- my first order assumption run with that one was that any shielding would be light weight.
> they tend interact multiple times and deposit their energy in multiple places along a line via a ton of different physical interactions.
Huh. Okay, although all of the form gamma energy packet interacts with doped crystal and scintillates I'd assume. I can see that you've got a bit more going on here in the detection instrumentation.
Fast enough to catch everything or throwing in calibrations for deadtime?
Hmm, anyhow, seems like much fun was had in the lab working on the dissertation, good job :-)
My time was almost always pressed to get more fieldwork done and never quite got the amount of playing about time I wanted :/
The timing and electronics for doing position sensing of the gamma path through the detector (i.e., what you need to reconstruct an image) are... complex. Dead time and false correlates (e.g. another gamma in the detector at the same time, aka false coincidences) are a thing and they are factors that contribute to uncertainties. There's a bunch of statistical analysis you have to do to try and correct for these effects and pull the signal out of the noise. In a Compton camera, you can correlate the first scatter's trajectory back into a cone of possible incident directions, which you then stack up to form a picture. Hence why I said the image you get out is very blobby and more like a heat map than an image. But you pay a price in efficiency, as you can only count some fraction of the actual interactions in the detector now. I'm a bit rusty on the really nuts and bolts details, I don't work with this type of detector anymore. We mostly try to avoid using this type of detector for our localization, because they are crazy expensive and complex.
Notable sites included most of the hotter mine sites about the globe, Finnish / Russian sea borders, and the odd live underground test here and there (Pokhran-II exchange) .. but enough about that.