Prompt Neutron Activation Spectrum

[Carl Willis]

What happens when your fusor neutrons land in borax shielding? For one thing, a 478 keV gamma is released in about 75% of absorbtions by B-10 (and alpha particles are as well.) Tonight I put an NaI(Tl) spectroscopy detector up against a small plastic bottle of borax solution WHILE THE FUSOR WAS RUNNING...and, as can be seen in the spectrum, there is indeed a 478 keV gamma. Also the hydrogen absorption peak at 2.23 MeV is evident. Since no background was subtracted, various natural peaks also accumulated, including K-40 (labelled 1444). My big question, still unresolved, concerns that 857 keV peak. It is not an artifact and is not a natural peak, but my brief look through some prompt gamma emission libraries did not shed any light on the issue.

Now a note about the process: this might be bad for an NaI(Tl) crystal, at least in the short term, because of activation of the crystal's constituents. Try running a background right after the experiment, and it will be noticed that there is a good bit of noise in the scintillator spectrum. Probably betas from activation? Anyway, that stuff goes away very quickly so no permanent harm is done.

Also noticed that iodine crystals and a cobalt sulfate solution seem to activate (get above-background counts in a Ludlum 44-9 pancake tube). So I will be trying some activation experiments on these materials later with the NaI(Tl). This weekend should see some activation experiments with gold and uranium! I'd like to get a prompt gamma spectrum of cadmium as well. Gadolinium is also good for this, and I have some monazite that should be relatively full of it. However...it is also full of thorium daughters, and the natural spectrum is just too intense.

This neutron stuff is just really cool. Perhaps we should open a new forum just for neutron experiments?

-Carl

[Richard Hull]

Incredible Carl! Good going. I am really super pissed that I currently have no fusor up and running! Gotta get back in th' game. Your and Jon's work is intriguing. This 800kev gamma is a mystery but as with all such gamma spec mysteries it can be solved.

Odd, and coincidental, I just received a new 4" NaI all-in-one last night by Harshaw using an RCA 6025 5"PMT. I spent the entire evening humming it in to my NIM spectrometer setup and taking a lot of spectrums with it of various materials on hand.

I did get a bizarre spectrum at the low end of a Co60 sourced Varian T-R tube I have. I repeated it three times and got about 8 peaks in the 100-300kev range. I freaked out! I ate dinner around 9:30 and went back to the lab and the next time I ran the spectrum, got zippo in that area! I can't explain that one either. Probably don't need to either.

I think that for now, at least, the neutron and radiation forum will contain the neutron experiments postings.

Thanks again for the spectrum and postings of your work.

Richard Hull

[pfostini]

How does a Nim Spectrometer setup work? I have a nim bin/crate with different modules and I am interested in setting up a spectrometer.

[Carl Willis]

Phil,

The pulse-height spectrometer essentially sorts pulses collected from a detector according to amplitude. It does this by (1) digitizing incoming pulses; (2)"binning" the pulses by amplitude, e.g. by using a chain of comparators or some more sophisticated method; and (3) storing in memory a record of the number of times a pulse gets sorted into a particular "bin."

How is this used for gamma spectroscopy? When a gamma ray is fully absorbed in a detector with proportional response, like an NaI(Tl) crystal, the output pulse from the detector is linearly related to the amount of ionization caused by the radiation, and higher energy gamma rays cause more ionization. Thus, the data accumulated [bin, number of counts] in a PHA attached to such a detector can be used as a gamma spectrum. The quality of the spectrum depends mainly on the detector used.

For NIM crates, you can get single PHA units (usually three or four slots wide). In addition, you want a preamplifier (like an Ortec 113) and a main shaping amplifier to go ahead of the MCA. My personal NIM MCA is a Tracor Northern 1706, and I have the HV bias supply, preamp, and a CRT display unit in the crate as well. I took it home when Guilford College majorly one-upped me and got a UCS-10 from Spectrum Techniques--that is what I have been using for the measurements posted on activation stuff. It is not a NIM unit.

The modern trend is to put the whole MCA on a standard PC card or in a computer-controlled external box. In my opinion these are real sweet for data collection.

If you have an SCA and counter, you can do what I think Jon Rosenstiel has been doing--that is, do the binning of pulse heights by hand and manually record the data. Not a small job though.

Good luck! Hope this helps some!
-Carl

[Jon Rosenstiel]

Carl, you're havin' way too much fun!

Got a question.....I don't understand where the B-10 478keV gamma value came from. I've looked at just about every table I know of and can't find that number. What am I doing wrong?

Jon Rosenstiel

[Carl Willis]

Hi Jon,

You're doing what I did, and getting the same search results. I believe the data was omitted or simply not present in the major tables. However, if you search on Google for stuff on boron capture gamma radiation, you get plenty of stuff that cites the 478 keV number...in fact, I just found that you can search for "boron" + "478 keV" and get a whole lot. Here is one link:

web.mit.edu/22.54/ProblemSets/PS1solutions.pdf

I do wonder why this very important peak never makes it into the prompt neutron activation tables.

-Carl

[pfostini]

Thats great help. No I can read the posts and understand how the units work.
Thanks

[Jon Rosenstiel]

Thanks for the clarification Carl, now I don’t feel quite so lame!

I had done several Goggle searches using "B-10" + "neutron gamma" and "neutron prompt gamma" and "prompt gamma" and came up empty-handed. (I imagine if I would have taken the time to wade through the tens of pages I might have eventually found it).

You’re right, when I plug in "boron" + "487keV" I hit the jackpot! For some reason the link to MIT wouldn't work this morning. Maybe they're down for maintenance.

Yeah, it is kind of odd that the 478keV B-10 peak is missing.

Jon

[Jon Rosenstiel]

Carl,

Isn't this what's happening?

10B + n --> 7Li + a
Plus some 478keV gammas from the recoiling 7Li nucleus. (Same reaction as the BF3 detector).

Jon

[Carl Willis]

Yep, that's the one. You mentioned the gamma comes "from the recoiling Li-7." Does the lithium nucleus form in an excited state? If for some reason the (a, n) reaction is classed differently from standard neutron capture where the product mass goes up by one, that may explain the omissions of the peak from tables. I'd like to track down the 870 keV peak in my spectrum and I wonder if it, too doesn't come from the B-10 (n, a) reaction products.

-Carl

[Jon Rosenstiel]

Carl Willis wrote:
> Yep, that's the one. You mentioned the gamma comes "from the recoiling Li-7." Does the lithium nucleus form in an excited state?

I'm learnin' this stuff as we "speak", so I'm certainly no expert here, but my understanding so far is that, yes, the Li nucleus does form in an excited state. (Maybe I should have picked an easier hobby)!

You know, Richard Hull is to blame for getting me into this stuff. (He would be the “pusher”, so to speak). It started off with Tesla coils, then headed towards high-energy capacitor discharge stuff, then on to radioactivity, fusors, nuclear physics, etc, etc. Where’s it all gonna’ end?

Jon

BTW.....Thanks, Richard! Fusion and fission is much more fun than fishin'!

[Richard Hull]

Jon,

It'll all end for me when I die! I enjoy following the physics of things. It has been one of the most enriching adventures of my life. The joy of physics is in personal discovery and making things happen.

Richard Hull