Hacker News Reader

Comment by defrost | Hacker News Reader

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...

View on HN

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.

> 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 :/

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.

Cheers for that, we're a lot more on the same page now - I got a little time with fancy detectors but principally spent years with straight up crystal packs and scintillation energy bucket counting, calibrations against doped concrete pads, stacked atmosphere flights, open water and known ground truth flights, geometry configurations, temperature control, and a banquet of derived corrections for radiometric mapping - we also ran passive magnetics, LIDAR, gravity, microwave et al in parallel.

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.

Reply on news.ycombinator.com