Silver and Indium

[Doug Coulter]

I've not done any hard to do or fancy activations at this point, but we use Silver and Indium in a moderated "neutron oven" here as a backstop for our other neutron detectors. These low hanging fruit have nice, short half lives so they are easy to read out, and large cross sections so they are easy to activate in the first place. The big advantage you get using these is they they have no dead time effects, and no sensitivity to EMI, both of which can easily fool real-time counters and detectors. You simply cannot fake it with silver or indium -- you have to have neutrons or they don't get hot, and they catch them all pretty well. So I consider these the gold standard for neutron measurement in the flux ranges we are getting here. Gold itself works too, but is fairly "numb" due to the longer half life it has. And besides, transmuting gold into mercury is kind of going the wrong way, eh?

Our little oven

Above is our little oven. This is made from 4" UHMW HDPE rod stock, a good moderator, and is sized to allow some neutrons in the resonance energy range to get to the samples, where they tend to have better cross sections. The bottom piece was cut to fit over the cylinder sidearm my fusor runs in. and I had to add some carbon rod spacers under there to keep it from melting. The top is just more of the same stuff, wrapped in lead so not so many X rays get out into the room (normally there's lead over the top, too). The samples shown are a silver sheet that just covers the pancake detector we use for counting, and a pea sized bit of indium I got from RotoMetals and just pounded flat with a hammer. The indium doesn't get as "hot" due to its longer half life, but has only one half life, so it's easier to measure if you get things hot enough. Silver has two possible isotopes that get made, with two different half lives, so it's a bit tougher to get any kind of absolute numbers out of it. But due to that real short one, it's also the one that really makes the counter sing after a run.

According to my rather ancient CRC handbook (53rd edition), for silver the main numbers are:

107Ag, stable, 51.82% natural occurence
108 Ag 2.42 min half life (and there is evidently a metastable state with > 5 hr life you can also get)
109Ag, stable 48.18% natural occurence
110Ag, 24.4 second half life, and there's also a long life metastable state.

There are some other states and decay modes of the above listed, but evidently they are rare.

For indium:

113In is stable, 4.28% natural abundance
114In has 3 possible states, half lives, the main one of which is 72 seconds
115In is stable, 95.72% abundance
116In also has several states, the main decay of which is 54 min.

I am leaving out out quite a lot of the data in the book here, each of these has several decay modes, energies, and probabilities, so I'm quoting just the numbers that are near what most experimenters are using.
There is a good chance better numbers are out there now at any rate, and if you know them, please post! Silver and In both take a few book pages of tables each to fully describe. We usually measure the silver first, then the indium if we baked both at the same time, and usually see about 10:1 ratio of counting, silver being the hotter, especially at first.

So I'd class this as a relative measurement as far as metrology goes, but a very reliable one.

[lutzhoffman]

Manganese can also be used for thermal neutron flux monitoring, its half life is = 2.58hr. Yes its somewhat numb at 13.2 barns, but its activated high energy gammas: 2.13, 1.81, and .845 Mev, are easy to isolate from background, this somewhat makes up for the lower thermal neutron absorption figure, plus Mn is low cost and common, so that you can use a lot of it dirt cheap, to compensate for the lower capture cross section. Most commonly it is used in solution, as an MnSO4 in water, or D2O solution. For a fusor, or accelerator, it may be possible to "salt" the cooling solution, with MnSO4, or with an Indium salt to build a semi-passive neutron monitor. Currently on Ebay pure Mn metal powder is $8.00 for 50g.

The government selected MnSO4 at one point for an experimental reactor based food irradiation system. This was done to eliminate the food neutron activation problems, which they had when they first tried to directly use the circulating USO4 enriched fuel. The half life for Mn is short enough to allow for it all to cool down over a couple days. A significant volume of the solution could act as the moderator at the same time. Mn metal should also work, since self shielding should not be a big issue, both ref. its initial neutron absorption, and its gamma emission. Currently on Ebay the metal powder is $8.00 for 50g

For most purposes Indium seems to be the standard, for all of the reasons that you already covered. One neat trick was to suspend multiple In foil pieces on a ruled acrylic rod, one end of which was close to the source in question, the data came by measuring the multiple foils, and then correcting for their distance / decay time. This reduced the error percentage by essentially averaging multiple reading, which were all obtained at the same time.

[Doug Coulter]

Yes, I've seen some interesting data on Mn in some books I recently got. Since I have some, both pure element and in compounds, it's on the list of things to try. One of the things the fusion crowd needs is a good way to compare results that isn't too expensive, and the making of some standard samples for that seems like it would be a good service to the community. Silver and to a not much lesser extent, Indium are kind of pricey compared to Mn. Not to mention gold! Which is basically too numb for good use here anyway. When our output gets up high, we reduce the input power to keep the whole lab from getting hot -- which means that by the time we'd be making gold hot, we have turned it down already.

One neat trick I saw to make things really more sensitive was to use a self moderated bath of MnSO4 in water solution. You don't need that much concentration for this trick. You then make it all precipitate into a tiny, concentrated sample to measure it. This means you can gather neutrons from a large area, but into what amounts to a tiny sample after the precipitation....those dudes from back in the day were pretty clever.

I have an excellent article from a Physics Today about detecting the first nuclear tests in a similar way, in this case by collecting rain water and precipitating/flocking all the impurities out for measurements. NRL ran this program and was able to get enough of a sample set that way to know just what the first Soviet bombs were, isotopic-ally, and how well the reaction in the bomb progressed. They also got good samples from moss and algae growing on/in old swimming pools under the fallout footprint -- they let nature do the concentrating for that. I'll see about posting some fair-use excerpt of it.

[lutzhoffman]

I would be very interested to learn your results ref. MnSO4 in the form of a self moderating solution. The 1 barn water neutron absorbtion figure should be easy to correct for also, plus the D formed is not hot. I priced some MnSO4 on ebay, and it really is dirt cheap: $3.50 a pound ! This if it works then it would be easy to use as a standard,for example: A 10 liter "X"-molar stock solution could be made and sealed into a pile of PE bottles, which could then be distributed as a "standard". This would put to rest any debate on concentration, geometry, etc.

If this works in real life, then it would be better I think, if you presented this idea to Richard, with hard data etc, since then it will be taken more seriously. Doug, I really like your idea of having a standard method here, that everyone can agree on, right now all the fusion N output comparisons made, are simply "apples and oranges" so that you never know if it is real science, or optimism. Maybe Carl could test drive this one at work also?

Now for a bit of isotope humor: I saw a recent post where a guy was wanting to use a fusor to make "cheap heavy water" from regular water! Someone else then posted a reply with the math, and he detemined that his efforts would finally be foiled by our suns "red giant phase"

[Doug Coulter]

Well, I plan to try plain metallic Mn first, maybe MnO2 (also dirt cheap). My first idea was to simply apply them to something like a glass slide for activation, with standard density, this and that.
The cool trick with the solution is that it's self moderated, and that you can then concentrate it for measuring -- but that makes it a one shot deal -- hard to change back again. Kind of
like the old "give them the razor, sell the blades" gig. This might put off people who aren't into paying "rent" -- rather than really being a service to the community that gets adopted as a standard.
Dunno, attractive on one level, not so nice on another. I think first I need to see how effective the plain old, reusable stuff is on a "normal" fusor run here.
So like so many other things, it's a "work in progress" for now. On the other hand, I have the tooling to make thin In sheets too....(it's easy with the right tools). A piece of In the
size of two peas easily rolls out into a 2"+ size sheet.