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Activation at Five Paces

Posted: Tue May 27, 2008 10:59 pm
by Jon Rosenstiel
Neutron activation at 15-feet using a Hirsch-Farnsworth fusor? Yes sir! But I did have to push my fusor pretty hard, and I did use an easy to activate material (indium foil).

Activation time (fusor run time) for this experiment was 42 minutes at an output (TIER) of ~3.8E+06 n/s. Due to overheating issues the 42-minute run was broken up into three segments with 20-minute cool-down periods between the first and second and second and third runs. (Carl’s water-cooling scheme is starting to look pretty good)!

To determine if activation had occurred I used a 2-inch pancake detector and a Ludlum 2200 scaler / ratemeter to count the betas given off by the activation product. (54.3-minute In-116m). To minimize the effects of variation in the natural background count 90-minute count times were used.

The In-115 >> In-116m reaction has a thermal neutron cross-section of ~154 barns and a resonance integral of ~3200 barns. The piece of indium used for this experiment was ~two inches in diameter and had a thickness of ~0.025-inches. The indium was placed in my UHMW “neutron oven” which was located 15-feet (5-paces) from my fusor.

(Some pix of the neutron oven):

viewtopic.php?f=13&t=5817#p34261

Data:
90-minute count on indium foil before activation: 41.1 cpm.
90-minute count on indium foil immediately after activation: 44.6 cpm.
90-minute count on indium foil 4-hours after activation period: 41.2 cpm.

I should note that I ran this experiment several times using various fusor run times, and in each and every instance the count immediately after activation was always higher than the count before activation.

Doing a few neutron flux calculations proved quite interesting as well:

Fusor output: 3.8E+06 n/s total isotropic emission rate.
Flux at 15-feet: 1.4 neutrons / sq-cm / second.

To achieve that same flux (1.4 n / sq-cm / s) 15 cm from my fusor’s poissor (my normal activation distance) my fusor would need to be producing ~4000 n/s TIER. Not a lot of output to achieve measurable activation.

Jon Rosenstiel

Re: Activation at Five Paces

Posted: Tue May 27, 2008 11:56 pm
by DaveC
Jon - Once again, a foray into a new area. Very interesting.... Now you need to see if you can activate your mill..!! ( Just kidding,)

Your high grade instrumentation, definitely helps.


Thanks for sharing your data.

Dave Cooper

Re: Activation at Five Paces

Posted: Wed May 28, 2008 4:25 pm
by Richard Hull
Excellent results Jon. That Indium must be absorbing almost every neutron hitting it!
Superb piece of work.

I have often wondered about a neutron counter based on indium activation by wrapping a piece of In around a 1B85 GM tube and dropping it into a thermalizing blanket while shielding the affair with a 0.2 inch thick lead shield at the fusor to stop all gammas.

Seems like it might be doable.

Richard Hull

Re: Activation at Five Paces

Posted: Wed May 28, 2008 8:44 pm
by Q
yes, this seems like a it might work... i had am idea for a similar arrangement some time ago. it seems though that most of us wont have the neutrons to try such an experiment.

Q

Re: Activation at Five Paces

Posted: Wed May 28, 2008 10:06 pm
by Jon Rosenstiel
Great minds think alike.
viewtopic.php?f=13&t=5485#p33929

Jon R

Re: Activation at Five Paces

Posted: Thu May 29, 2008 4:00 am
by Q
heh, well it looks like i may have unknowingly been inspired by your idea, jon.
my first post on the topic was a year or so after yours. i didnt mean to take your idea and run with it.
viewtopic.php?f=13&t=5565#p37288

but yeah, it does seem like a simple and effective way to detect neutrons, though who knows at what effecentcy it would work.

Q

Re: Activation at Five Paces

Posted: Thu May 29, 2008 3:30 pm
by Jon Rosenstiel
Hi Q,

No worries, mate! The activation type of neutron detector has been around for a long time, so it wasn't my idea either.

It could be calibrated (if you could find someone to do it), but at that time I just wanted a simple, EMP-proof back-up detector. Cool thing about this type of detector is that it has a short memory. If it's still clicking away like mad after the fusor power has been killed then you know your fusor kicked out a large burst of neutrons. If the clicking quickly drops off when the power is killed then you know you had a large burst of EMP that produced a bunch of false counts.

Jon R

Re: Activation at Five Paces

Posted: Thu May 29, 2008 4:51 pm
by Richard Hull
If you were absolutely sure you were producing neutrons, the 1B85-indium detector would be tremendously more noise immune over the He3, though less efficient. For high count rate systems, this would be great.

Drawbacks? You bet. No free lunch, ever. You would have to wait for 4 or 5 half lives of the indium isotope to get an equilibrium reading. Even this is at one and only one fixed operating regime. Up your voltage and wait 20 minutes to find out how you are doing. Not very pretty.

Such a scenario would be great to estimate activation levels once a fixed operational level was attained.

Richard

Re: Activation at Five Paces

Posted: Thu May 29, 2008 7:24 pm
by DaveC
Just doing the ratio of cpm of the detector and n/s from Jon's fusor at that distance, it seems like a 0.002 factor..or about 0.2% count efficiency.

But taking into account Richard's notes about the long decay times, then one needs to integrate the counts over the half lives... in order to not underestimate.

Still thinking about whether for an extended run... > several half lives of the longer isotope, 116m2 ...you would reach equilibrium count rates.

Dave Cooper