FAQ - General Standardized Activation Methodology

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Richard Hull
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FAQ - General Standardized Activation Methodology

Post by Richard Hull »

This is real simple and very straight forward.

Activation is just a shortening of the term "neutron activation"

Neutron activation can be used for a number of purposes. manufacture of useful isotopes, transmutation of elements, creating nuclear particle sources (beta and gamma rays), or as a method of "approximating" the original energy or flux density of a neutron source.

This latter method is the one we are most interested in.

Background...................

Neutron activation occurs whenever neutrons interact with bulk matter.

The most effective activation occurs at lowered neutron energies.

Most often these are termed "thermal neutrons".

A thermal neutron is one that moves at exactly the same velocity as ambient temperature molecules of local matter.

The average thermal energy neutron at STP is figured to be 0.025ev.

A thermal neutron, therefore, is bounced around within matter and is locked into the matter with little chance of direct, straight line escape.

The themal neutron is readily absorbed by a nucleus within the bulk matter.

When this happens, the result is always a radioactive atom of the same atomic number, but a different (higher) atomic weight.

The resultant ISOTOPE usually decays by beta decay, but not always. A beta decay will always move the isotope's nucleus UP one number on the periodic table (transmutation).

Some of the most efficient neutron activation can occur at "resonance energies" in the epithermal range (1ev - 500ev neutron energy). Resonance peaks are often sharp, numerous and jammed up next to one another, whereas thermal captures usually have a steady and progressive (linear) rise as the neutron energy drops below .025ev. (famous 1/v law)


Slowing Neutrons....................................


Most all fusion and neutron producing processes produce fast neutrons. (>1mev).

The best way to slow neutrons is through a process called "moderation". Most moderator materials slow the neutron by "proton recoil". This process involves obtaining a dense, proton rich material and firing the fast neutrons at it. As they enter the material, which is usually a hydrogenous material, (water, oil, parafin, etc.), They impact the hydrogen nuclei within it and knock the protons out of the atoms. In doing so, the collisions transfer some of the momentum of the rocket fast neutron to the proton, thus slowing the neutron with each successive collision. Eventually, if there is enough material thickness, the neutrons exit the material at thermal energies. (.025ev or about 2200 meters/second).

When the thermals exit they can exit in any direction. They can even exit back towards the source of fast neutrons with equal probability!

WHY?

They are truly thermalized, therefore, their paths are totally, statistically, randomized as are all the molecular motions in bulk matter. Thus, a block of parafin where neutrons are totally thermalized will radiate them rather isotropically from the material. Even if the thermalizing medium is not thick enough to totally thermalize the neutrons, neutrons will be scattered over large exit angles.


We need neutrons!! Got any? ........................


The neutron source we have is our fusor. If the fusor is working correctly (producing d-d fusion), about half of all the fusions will produce a neutron at or about 2.45 mev energy, (fast neutrons).

To do neutron activation we must place a water bath or parafin block of some thickness as close to the fusor as possible. This is normally on the order of 2-4 inches thick.

Taped to the other side of the block or tanks, we need to place a foil or solution of some element known to have a high "capture" cross section for epithermal or thermal neutrons.


Activation materials..................


Indium foil is a good choice for thermal neutron activation. Silver is a good choice for epithermal or resonance activation.

It is very important to use only the purest of elemental materials if you don't want to have other tramp materials activated as well. Common old U.S. Silver coins, (1964 or earlier), are all 10% copper. Sterling Silver articles are 8.5% copper. Silver plate is about .05% silver and the rest, God knows what. Copper in these items make the material hard and difficult to roll to sheet and foil.

You should attempt to obtain the thinest foils possible. Indium is so soft and maleable that a simple mallet, a homemade metal rolling pin and some perserverance will reduce a bubble gum sized wad of the metal to almost a square mile of ultrathin, but maddeningly self-adherant, droopy foil.

Likewise, a Canadian silver maple leaf coin, (from a bullion dealer - .9999 pure), can be rolled and worked into usable foil, but with a good deal more effort and several required re-annealings. (work hardens quickly)

*******************************

Shameless advertising - I have small pieces of .9999 silver foil and .9999 indium lump for sale. See trading post.

*********************************

Liquid thermalizer/activator systems are a double whammy! A high cross section, element in the form of a water soluable salt can be disolved in water and this will be a fantastic combo system. Carl Willis did this with a manganese salt. (See Archived Image Dujour work and other reports in this forum.) As the neuts penetrate the water, they are slowed down and scattered. Floating manganese atoms are activated and you are warranted to have activator atoms at every epithermal/thermal energy point in the liquid! (a chemically saturated solution is ideal) It is best to select an activator atom whose isotope or subsequent daughter can be counted in a gamma spectrometer, (gamma emitter), as the liquid target/moderator is not handy for geiger counter data collection/reduction. Of course, this system demands that you have a complete gamma spectrometer system lying about the lab and the knowledge necessary to use it properly.


The epitome of the process..........................


Take a foil and place it hard up against your water tank or block of parafin which is itself very close to the fusor. This is done so that the foil and fusor are separated by the moderator block or tank. In short, the moderator block separates the fusor and the foil.

Now with the fusor turned off, take a digital counter equipped Geiger counter, preferably with a mica windowed detector, make a ten minute long un-activated background check of the foil or activation target. Place the detector hard up against the foil or target.

Record the reading. Divide by ten and record the target-background reading in CPM. Divde the CPM number by 60 and record the activity of the material in Becquerels.

Run your fusor and record the time of the run with all other important data (current, voltage, length of neutron activation run, pulse duration, capacitor used or joules delivered per pulse, etc.)

Immediately upon dropping the high voltage from the fusor, start a stop watch. Quickly place the geiger counter detector in exactly the same place on the foil as you did in the background check prior to you fusor run. Record the time on the still running stop watch just as you start a ONE MINUTE timed count from that point. At the end of that minute record the final time on the stop watch and the count on the geiger counter.

Reduce the count to CPM and record. Reduce the CPM to beq. and record.

If you were really producing neutrons, your one minute count at the end of the fusor run should be noticably higher than the original 10 minute background run reduced CPM data.

The reason for the short timed run after activation is that high cross section materials traditionally have very short half lives, often on the order of one minute to 5 minutes. You must work quickly to get an idea of the activity of the target foil. Remember, you are counting on a decay curve slope.

NOTE*** DO NOT leave the geiger counter detector head in place at the foil during a fusor run! IT WILL BECOME ACTIVATED ITSELF! This will ruin any data taken and make it valueless. Keep the counter and detector head over 20 feet from the fusor while in operation.


Data?............................



Precise data regarding neutron flux cannot be gleaned from activation data in our limited setup here.


Why?

We have no way of knowing how much activation was obtained involving epithermal resonance captures and how much was thermal capture.

You could us a 2 foot cube of parafin and warrant that only thermal activation occurs, at which time you could get a better handle on the input neutron flux, but this would vastly reduce the activation due to inverse square law issues. The secret is to get as close to the fusor as possible with the thinest possible moderator so that maximum activation occurs. Only folks at fission reactors or large accelerators can afford such luxuries as meter long thermalizers because they start with an absolute lethal torrent of fast neuts.

What th' hell did I do all this for if I can't get accurate absolute neutron data?!!

1. You can prove beyond all doubt, and without an expensive neutron counter, that you have done real, honest-to-God fusion.

2. You now have a bench mark recorded in you notebook. As long as you absolutely hold to the same activation setup and materials, you will forever be able to judge the precise degree of improvement or degradation of your fusor or any fusion system over a vast range of voltages and designs against this well understood, fixed system run.

Summary........................

Recordable activation most likely begins around 10e5 d-d fusion neuts per second, isotropic. Someone might do well to check this out. It would demand a real, calibrated neutron counter, inorder to set the fusor to this output level.

If you do anything other than the above tightly controlled process, then you are not doing the process correctly and any results will be suspect or just plain wrong.

Needless to say, part of your lab notebook must contain scrupulously recorded data on the type, thickness and size (area) of the activation material, the type, size and volume of the moderator, etc.

Most of us will be seeking the best activation process from the minimum neutron number produced. To do this a number of runs with a number of moderator types and thicknesses and or activation target type should be tried. Once the most efficient moderator/target combination is found, it should be stuck with doggedly in order to make future data with different systems and fusion devices valuable.

I believe the best results might be obtained with resonance capture as this usually provides phenomenally high cross sections over a moderate energy range at or near 100ev. The subsequent reduced thickness of moderator allows for more neutrons entering the counting system, as well.

Finally, make sure that the target activation material has undergone at least 5-6 half lives before using it again in a data collection run. No one likes putting on soiled underware.

I hope this has clarified the process so that those working without the advantage of a neutron counter, but having a digital readout geiger counter, can get valuable information about their system and record its progess. Needless to say this is just about the only way to get neutron data from pulsed systems. (The classic neutron counter is pretty worthless here.)

I will refer you now to a number of postings in this forum and in the image du jour forum of superb fusor induced activation work done and reported by Carl Willis and John Rosentiel.

I will continue to modify and add to the FAQ as I see fit over time.


Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jon Rosenstiel
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Re: Will recordable activation occur at 10e5 n/s?

Post by Jon Rosenstiel »

Yes!
Count on indium foil before 15 minute activation run, 50.1cpm. Immediately after activation run, 74cpm.

Fusor operating parameters:
23.5kV@10mA at a pressure of 17mTorr gave an indicated 2.6mrem on my Ludlum neutron counter.

Neutron counter:
Ludlum 12-4 BF3 Bonner ball type. Calibrated 10/27/02
Center of fusor to center of detector, 21cm.
21*21*4*pi=5542
5542*7=38794
38794*2.6mrem=1.0e5 n/s

Moderator:
13oz (d=9.9cm, h=13cm, v=1000ml) Folgers coffee can filled with paraffin with the end of can placed hard up against fusor shell. The material to be activated (indium in this case) is placed in a 0.25" (6.35mm) wide slot cut into the side of the coffee can. From the slot to the end of the can up against the fusor is 3.25” (82.6cm). From the slot to the far end of the can is 1.5” (38.1cm).

Activation material:
Indium foil, 2" (5cm) in diameter by 0.009" (0.23mm) thick. Activation time was 15 minutes at 2.6mrem. (10e5 n/s, total isotropic emission)

Geiger Counter:
Ludlum 2200 scaler/ratemeter connected to an Eberline HP-260 probe. ((2” (5cm) mica windowed pancake detector)).

Counts:
A 10-minute background count (pancake detector on indium foil) before activation gave 50.1cpm.
6 seconds after the high voltage was killed a one minute count gave 74cpm.
Two hours after the hv was killed a 10-minute count gave 52.9cpm. (Half-life of In-116 is 54 minutes)

Conclusion:
Measurable activation is a reality at the fairly modest output of10e5 neutrons/second.

Jon Rosenstiel
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Re: FAQ - General Standardized Activation Methodology

Post by Starfire »

Great FAQ Richard ( as usual ) much appreciated - we should have a FAQ Forum -- Now a Question. you said, " The average thermal energy neutron at STP is figured to be 0.025ev. & The themal neutron is readily absorbed by a nucleus within the bulk matter " OK, but, where does the glue come from? - How does the thermal Neutron become absorbed ? it must require some energy to stick? Is 0.025ev sufficient, considering the amount necessary to release it from an atom?
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Richard Hull
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Re: FAQ - General Standardized Activation Methodology

Post by Richard Hull »

No...... No energy is required for it to stick. Neutrons are neutral and see no nuclear potential hill and they just wander or stumble into the nucleus. Once there the theorized strong force takes over and like a nerf ball, it sticks. In almost all cases, this action upsets the stability of the nucleus and, most often, it has to shed an electron. This then down grades one of its neutrons to a proton. This sends the element up the elemental table by increasing the Z by one unit. There can be many routes taken during the decay following activation, but neutron activated material usually follows this path, on average.

Many thanks to Jon for posting on the 10E5 neutron experience and his activation activity. It was very accurate and contained all details needed to act as a base for others to follow.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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