Activation measurement technique

[Richard Hull]

Fast neutron metrology and activation procedures are now rather ancient history, as the advance of nuclear science goes. Bleeding turnips is a laudable past time, I suppose if a 0.1% improvement is world beating.

The same thing tends to get waded through in these forums over and over again. We see the hopeful of future energy and or 100X or more improvement per unit input energy come and go. They will succeed where others have only dabbled. Alas, there are those limits imposed by nature. It seems she just doesn't want the very stuff of the universe, hydrogen, to be allowed to start a nuclear wildfire.

Its mostly a charged world that allows us to detect changes in state. Neutrons are not part of this normal world, interlopers from the guts of atoms that are doomed the instant they are free. A whole three vol;ume set of books was developed way back in the 60's dealing with fast neutrons plus a follow up ancillary volume. Little new has ensued outside of technological and engineering tweeks to the old stuff that created many Phds that built those volumes.

Given a fixed material, a fixed fast neutron flux, it has long been known how to get the most activation from it. What wiggle room is left is for those seeking a third or fourth decimal place and not 10% let alone any order of magnitude advances.

As noted, activation always tells the tale. Few are the number of amateurs here that have achieved significant activations. It is a tough job. Pulsed machines are doomed to settling for activation, for the most part, especially in amateur hands. I understand the desire to claw at this, but limits are set. Experiments here have been done and reported on in the past they jibe with the results long ago set down in print.

Richard Hull

[Carl Willis]

I added a couple more pictures to the end of my last post as promised.

Frank: The best "backing" to put behind the sample is still HDPE among the choices being discussed. Its atom density and the high energy loss per collision in hydrogen makes it desirable here, again to stem leakage of the flux from the vicinity of where the neutrons will be used, which is around a very localized source. Graphite, beryllium, lead, bismuth, uranium, etc. all have their potential applications as reflectors, but a dense hydrogenous moderator will win in this situation. Neutrons effectively diffuse like gas through the moderator once they have slowed down to the energies where capture is probable, rendering their initial direction rather unimportant.

Doug: MCNPX uses energy-dependent data that is detailed, even accounting for the thermal Doppler effect, and for molecular motion in the condensed state in most of these cases (the exception being the Teflon). The quantity calculated in the tallies is INT[F(E) * R(E)]dE where F is the fluence and R is the total absorption cross-section from the carefully-evaluated and very-detailed ENDF/B data.

As I mentioned earlier, the problem shown is set up to NOT consider removal by the activation sample. This problem helps you know where to put the absorber and can accurately tell you what the activation rate will be in thin absorbers, where thin is relative to the mean free path of a low-energy neutron in the absorber. If you want to know what the activation rate accounting for removal would be, you have to specify the thickness or mass of the absorbers and then they have to be explicitly modeled, which, is not hard but the geometry specification becomes more detailed. The problem is set up mostly to show you where to put the absorber, not provide the most accurate measure of induced activity.

I don't know what your third question means exactly, but I guess you are talking about geometric attenuation. For a point source (or sources that sufficiently resemble point sources at a distance), geometric attenuation is proportional to 1/r^2. From a line source, plane source, etc. it isn't. MCNPX does not deterministically approach these problems, but like the natural effect that it models, it "creates" particles at the source location, samples the specified angular and energy distributions, and transports the particles until they either get absorbed in matter or get lost at the problem boundary (and quantities of interest, say, the particle current through a particular surface, are "tallied" as the problem runs). In short, the source geometry is fully accounted for.

I concur with Richard that this discussion is not broadly revelatory; there's been a lot of relevant discussion in the past about moderators and activation and neutron physics and optimum geometries and so forth that I think is pretty comprehensive, and the community's knowledge level is high. The information I am providing from MCNP fleshes out some of the nuances, perhaps, in your own specific project. Within reason I can modify and re-run the problem or write or run new ones.

-Carl

[Richard Hull]

Carl,

I am glad you added the images. Your powerful and definitive posting above with images was of such a caliber that I have added a pointer to it at the end of my two older FAQ's on silver and indium activation techniques, respectively.

I always worry that gems such as yours might be lost in the middle of a posting that might be missed in all but the best of searches, or most avid of readers, so I tagged it onto my relevant activation FAQs.

Again, a sterling piece of work.

Richard Hull

[Doug Coulter]

Yes, very good stuff, kinda deserves its own thread without all this other noise, but I guess that's what Richard has done -- hat's off!

Now to go do something with the new knowledge.

[Joe Gayo]

Any of these images would be also super useful:

ATTACHMENTS
meshtally.jpg
meshtally.jpg (57.25 KiB) Viewed 2413 times
geom1.jpg
geom1.jpg (43.87 KiB) Viewed 2413 times
[The extension txt has been deactivated and can no longer be displayed.]

doug_3.jpg
doug_3.jpg (127.44 KiB) Viewed 2413 times
doug_2.jpg
doug_2.jpg (160.81 KiB) Viewed 2413 times
doug_1.jpg
doug_1.jpg (171.29 KiB) Viewed 2413 times


Hopefully someone has a local copy. I already emailed Carl and he said that he didn't have them readily available.

[Jon Rosenstiel]

I believe I've got everything fixed... except for the txt file.

JonR

[Richard Hull]

Thanks for the effort Jon. So very much in the way of great info., charts and images has been lost forever with the move to Go Daddy last year.

Richard Hull

[Jon Rosenstiel]

Glad to be of help.

Just now fixed the missing txt file, managed to copy its contents into a word document.

JonR