Indium activation.

Isotope making and measuring no matter where the neutrons came from

Indium activation.

Postby chrismb » Wed Dec 29, 2010 4:55 pm

Doug,

Have you posted much about indium activation yet? I was debating whether this would ever be something I might want to do, over and above silver (and if so do I need to buy indium while it is still around 30 bucks an oz?). I'm not that interested in the finer details of activation work, only to be able to have the best coverage of neutronometry.

[PS, the reason I am asking rather than saying I have done the search and found nothing is because the search never seems to work for me. I just got "The following words in your search query were ignored because they are too common words: indium.", which is typically what I get.]
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Re: Indium activation.

Postby Doug Coulter » Wed Dec 29, 2010 5:35 pm

Nope, haven't said much about it yet, but it's useful. I bought a quarter pound from rotometals (100 bucks), chopped off a pea sized piece to smash out (easy) into a disk about 2" diameter.
I use it to backstop the silver -- I put both in the oven at the same time; you do get more activation if there's less in there, but I'm going for very conservative numbers so as to be sure I'm not claiming more than I really can deliver.

It's useful stuff to have for a lot of reasons, so if you can grab a chunk, do it. Molten, it wets glass, for example, and it doesn't take much in a solder alloy to bestow that property.
Imagine you're having trouble making glass-metal seals for vacuum feedthroughs that won't get too hot. This is the magic glue for the final seal, and it will appease the tempco monster by yielding, while keeping the seal -- the stuff is about as malleable as chewing gum, but a better seal than viton, and it wets practically everything. So if you leave a bit of gap between your glass and metal, and fill with indium alloy, you're good to go, and actually a lot more handy than epoxy in many cases.

I am getting on the very rough order of 10% the cpm on indium after a run of that I get on silver. A few reasons for that, even though the cross sections are nearly the same. One is indium has a longer half life, so when I do the counts, I do it last after getting a good number of points on the silver decay curve. Second is, it only has one major decay route (I think!), where silver (according to my CRC book) has many-many. Turns out silver has some (a page in the table) metastable states with decays of everything from seconds to the oft quoted ~12 minutes...which complicates things and makes plotting an actual curve a lot smarter than just taking a point, especially if you are getting to the measurement fairly fast -- say under 30 seconds after a run. In the case you should ever be so successful as to "saturate" the silver, indium would give you a much more accurate number, and you'd be using indium and gold, rather than silver and indium, most likely. To be honest, I myself need to do a bit more study of the CRC numbers to say anything that could be considered authoritative, but what I think I know seems to lay pretty well with what I actually measure, so I've been a bit lazy. For me, relative measurements still suffice, and I'm calibrating my neuts/second numbers off Richard's 2008 HEAS run and his and Carl's calculations then (along with our own measurements we snuck in during the visit). Since then, with claimed zero changes in setup, he's claiming double that (but refuses to re-run with silver and the same measuring for activation on a timed run) - so I ought to be safe basing it on the older numbers indeed. Because I've never seen a double without having to improve something to get it.

Truth be told, I'm probably doing several times better than I claim if the relative incidence of those hot gammas matches what I see in cross section tables, but I'm not going to claim that yet.
I'll let some doubter visit with their own measuring gear and find it out themselves -- more fun that way. But I do see a lot of something that makes a counter count when it's inside a solid block of lead with ~2" thick walls.... I kind of doubt it's all neutron capture gammas in the lead (which is surely some, but this is a good distance from the action too).

We once just barely got gold for sure out of the background here -- and that was one expensive piece of kit, costing us about $350 for 2" sq piece of very pure foil (which worked out to about $3000/oz). Not sure why I didn't just use jewelers grade 24k, but I suppose you could make a case that in that grade, there's some silver in there as a contaminant. I got *pure* gold to 5 nines claimed so it's going to be a lot more numb than say the odd class ring or gold coin.

Part of that is as we get things better, we shorten our timed runs to keep the numbers about the same, then extrapolate. That's to reduce the exposure of, well, us, and things in the lab.
So we'll probably stay with silver and indium for quite some time, at least until we put a fusor in the backyard cave so we can just run it all day with 30 feet of rock between it and us. With our standard 5 min timed run, we get no where near saturating the silver (secular equilibrium) or in other words, it's still going up pretty linearly in activation all the way to 20-30 minute runs (which make it scary-hot).
Fwiw, when Jon Rosenstiel reported that fantastic count on silver from his fusor (15k cpm), he was using a scintillation counter that was roughly 10x more sensitive than a geiger counter for the radiations involved, and I get even bigger numbers on our "henny penny" scintillator, myself. But the good old pancake geiger seems a good (and inexpensive) way to make measurements that track across all our labs. I tend to somewhat discount high claims for someone who also boasts the most sensitive measuring gear on earth, for some reason. Did they really make more fusion, or do they just have a ten times more sensitive measurement device? Hmmm.

I also plan to try manganese, which is one the historical guys used a lot -- cheap, readily available as MnO2 in batteries and elsewhere, and chemically interesting. A cool trick they did was to use a soluble manganese compound (either sulfate or potassium permanganate) in a water moderator (speaking of perfect dispersion) and then precipitate it out to concentrate it for activation measurements. That might turn out to be a pretty nifty trick, we'll have to test it someday.

But the nicest thing about silver and indium -- they cool right back down, so reuse is easy. You probably won't want to reuse indium on the same day unless runs are pretty far apart, but on the other hand, if you're going to run a long time, or integrate a couple of runs, it's a better choice. Nice of the universe to give us so many ways to do this!

For what it's worth, yes, the search function stinks. I will look into that one further. I've been getting help from Jerry and Jon on the board software around various of these issues, so I guess this is a good place to say "thanks guys" for that.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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