Item — HN Reader

Original title "Bayesian Parameter Estimation for the Localization of a Radioactive Source in a Heterogeneous Urban Environment" compressed to fit within title limits.

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

1 comments

Ha, that's me. Thanks for posting this. It's late where I am but I can answer questions. I took a brief break on this topic after I graduated and worked on other stuff, but the company I'm at now has done a lot of subsequent work on this, including making it practical in the field. Though as I mentioned in the prior thread, bridging the valley of death and selling it as a product is still a challenge.

  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 Unfortunately that's mostly unpublished work at present.

~ https://news.ycombinator.com/item?id=49214068

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Any comment, or all still a bit unpublished?

Not us, but probably an imaging radiation detector with LIDAR. Its another technology for search, with its own tradeoffs and advantages. We are friends with a couple companies that make that tech, but it's a different thread of R&D. Arguably a bit more mature; doing localization like we do with "normal" detectors on the ground is pretty challenging. We've done pretty realistic joint exercises with the military with our tech (called QkRad) but nobody has actually fielded it in a real emergency yet. We are around TRL 7 if you know that terminology.

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.

> probably an imaging radiation detector with LIDAR.

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.

It was probably an imaging radiation detector regardless, the spinny bit would likely be some sort of camera to add context to the radiation image.

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.

Ha - my mental model was to spin a partial shield about a crystal and watch the incoming counts and timings - dipping when shield passes between source and crystal, increasing on the gap transit.

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...

Spinning shield is a thought people have had but it doesn't work very well in practice (for gamma imaging). Usually you want to avoid anything that blocks gammas as much as possible because you need every one you can get. I wanted to try spinning one of the scintillator logs we used, which is more or less the same idea, but it isn't practical either for a bunch of reasons.

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.