Hacker News

Locating an unknown source of radiation in a heterogeneous environment (2019)

29 points by toomuchtodo ago | 22 comments

toomuchtodo |next [-]

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

gh02t |root |parent [-]

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.

defrost |root |parent [-]

  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 Unfortunately that's mostly unpublished work at present.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Any comment, or all still a bit unpublished?

gh02t |root |parent [-]

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.

defrost |root |parent [-]

> 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.

gh02t |root |parent [-]

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.

defrost |root |parent [-]

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...

gh02t |root |parent [-]

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.

defrost |root |parent [-]

> Usually you want to avoid anything that blocks gammas as much as possible because you need every one you can get.

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 :/

gh02t |root |parent [-]

Yeah I think I explained it badly. They sorta stream through the detector until they interact, and then potentially undergo multiple compton scatters or photoelectric absorption (or other weird stuff) that produce secondary particles that actually excite a region around the discrete interactions and those excited atoms are what produces the observable signal. Then (at least for a scintillator) the atoms that got excited relax and release a flash of light with a characteristic decay time that is the main limit on how precisely you can resolve individual gammas. Semiconductors are kinda similar but dislodging electron-hole pairs; the higher mobility of these pairs is why the resolution is so much tighter. Imaging detectors have more electronics attached, e.g. two photomultipliers at each end or pixellated photomultipliers, and timing analysis of these pulses is how you get position. Things like a full energy deposition peaks happen when the gamma ray dies in the detector, but it doesn't necessarily dump its energy all in one spot. Since you have to wait for all the excited atoms to decay or the total charge from electron-hole pairs to be collected you have to integrate the total amount of light/charge over some window to get the total energy deposited. Sometimes they escape out the other side and take some of their remaining energy with them, which causes some distinct features in the spectrum too.

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.

3dedb728-3f77 |next |previous [-]

Related to this, is it normal that radioactivity detector are not everywhere?

gh02t |root |parent |next [-]

They are more prevalent than you think, especially in places like ports and airports. But part of the problem is they are typically expensive for anything more sophisticated than a basic Geiger counter, like most commercial offerings start at $5k and can very quickly go into the hundreds of thousands or millions of dollars per unit. And they require a lot of training to use effectively.

At least in the US there is also a pretty substantial network of teams and equipment for providing radiological technical support in an emergency. Examples of this are DoE RAP and the USNG CST. Now part of the motivation for this work was that a lot of the techniques for actually doing a search are fairly primitive, but there is a major support network with most of the tools and skills ready to go 24/7. It tends to all be very low key and fly under the radar, mostly because even a hint of one of these teams being deployed tends to cause a panic.

copperx |root |parent |next [-]

I was born in the city where this happened, right on the US border: https://en.wikipedia.org/wiki/Ciudad_Ju%C3%A1rez_cobalt-60_c...

In fact, the house down the street to my parents' was torn down and rebuilt in the 80s because it had contaminated rebar.

When I was a kid, a neighbor, who is a civil engineer had a Geiger counter and went through the neighborhood and said he found nothing abnormal.

However, there is a cluster of cancer cases of people in the neighborhood of all kinds (breast, testicular, melanoma, liver, and kidney, of which my mother died of), including two friends who still lived there, and were diagnosed in their early 30s. Could be just bad luck, but I still wonder. My father still lives there.

I guess I should at least skim your dissertation, but is there a device that could be employed to sweep the neighborhood?

gh02t |root |parent [-]

The cheapest worthwhile detectors out there are the Radiacode ones. They are excellent for what they are, but they run around $300. Cheaper Geiger counters are unfortunately kinda useless, at least for this. It's also not particularly easy to know what you're looking at and how to interpret it even with a good detector, unfortunately I'm not sure how much luck you'd have so don't waste $300.

copperx |root |parent [-]

I see. Yes, so I just looked and Cobalt 60 has a short half life. If it's still there, it is probably hard to detect.

Still, it's a bit tempting to go check.

wildzzz |root |parent |previous [-]

When doing heavy ion testing for space flight hardware, there's a potential for the hardware to be a little hot after a full day of testing. Not really dangerous but hot enough that you need to let it sit for a day. At one facility I went to, the physicists there said in the past a group didn't let their hardware cool off and it alerted radiation detectors in the Lincoln Tunnel, NYC.

_trampeltier |root |parent |next |previous [-]

At least in Europe, there are a lot.

There is even a public realtime map.[1]

Also at least in Switzerland, all old metal scrap companys, waste incineration plant, and so on, have radiactive detectors on the entrance.

[1] https://remap.jrc.ec.europa.eu/Advanced.aspx

[Edit] It seems since April 2026, Switzerland has also a lot radioactive checks in border/customs control and bigger post/packet centres.

snovv_crash |root |parent |next |previous [-]

I've seen work using smartphone camera sensors to detect radiation, so with the right software maybe they're everywhere.

gh02t |root |parent [-]

Unfortunately smart phone cameras kinda suck performance wise. Radiation detectors fundamentally need some mass to actually absorb and detect radiation, so while cameras are basically similar to semiconductor radiation detectors the performance is so bad as to be useless. I haven't really ever seen the camera detectors be used for anything useful.

Radiacode detectors are about the only affordable detector with actually useful performance out there, and they are around $300. Which is also drastically cheaper than anything else out there, the next cheapest actually useful detector out there that I know of is 10x that price.

defrost |root |parent |next |previous [-]

It's normal that radiation detectors are not everywhere.

It's also normal that radiation can be detected everywhere (subject to terms and conditions with respect to sensitivity and sensing time).

The least radioactive places are just above the middle of large bodies of freshwater.

Still detectable there, just from very stray land sources, drifting airborne sources, and plucky cosmic sources that made it through the atmosphere.

jongjong |root |parent |previous [-]

Yes it is, how else do you propose those in power should get rid of dissidents without alerting people?

atmavatar |previous [-]

But can it find the Tesseract?