FAQ - A neutron moderator for activation/detection
Posted: Mon May 09, 2016 2:28 am
Attached are images of a formal neutron moderator. This particular one cost about $40.00 in plastics from a local plastics firm. 100% of everything else came from my stock and junk boxes around the lab. My previous setup left an awful lot to be desired, but worked OK. This works much better.
All D-D fusion produces fast neutrons which can't do much of anything for the experimenter. Valuable neutrons are thermal, (slow), neutrons.
A moderator is a hydrogenous material that slows fast neutrons to thermal velocities. Typical moderator materials are water, paraffin, and polyethylene.
A moderator is a must for neutron detection via various electronic tubes. (boron lined tubes, boron corona tubes, BF3 tube and 3He tubes.)
A modrator is also needed for all activation experiments. Among the most common materials that are easily activated are Silver, Indium and Rhodium.
The moderator in this FAQ is made specifically for activation experiments. Usually such activation is done on metal foils of the metals noted above.
Since the moderator shown thermalizes fast neutrons, I have dubbed it my "Neutron Oven" as it cooks the material to be activated in thermal neutrons.
Out of the oven, after a suitable time, the foil of choice is removed and is radioactive for a very short time, as all the half-lives are in seconds or minutes.
This can be a valuable neutron detection scheme provided you have over 300,000 neutrons per second boiling out of your fusor and is an absolutely perfect, proof positive that fusion has been done. Add to this that a simple geiger counter is all the intrumentation needed to prove fusion. Basically, if you activate a metal and a simple GM counter reads over background radiation immediately upon being pulled from the oven, you have done fusion.
Materials list for this:
A single piece of 1" thick high density polyethylene, (HDPE), 6" wide by 36" long. (local plastic distributor- big cities)
2 - 7" long 1/4 X 20 lengths of "all thread" Hardware stores sell 3 foot lengths that you can cut to length with a hack saw.
2 - 7" long nylon or fiberglass 1/4" diameter rods
4 - Nylon 1/4 X 20 nuts with nylon washers
a few 1/4 X 20 metal nuts and washers (hardware stores)
Mounting materials to suit...Wood, G-10 fiberglass, phenolic, etc.
silver foil
Tools
drill press or hand drill with bits
milling machine if you have one or have a friend or neighbor with one.
set of taps and dies - you really just need a 1/4 X 20 die and handle.
hack saw at least...A chop saw or power hack saw would be better.
method
Saw the polyethylene into 6 equal 6" x 6" squares. I used a power hack saw for this. Note, the plastics place will be more than willing to do this for between $5.00 and $10.00 per cut. (ouch!) I wasn't going to give them my money for that.
Drill 4 accurately placed corner holes of about 5/16" in each slab of the plastic to pass the long bolts through to hold the mass together. I made a master and used it to drill the remaining slabs to match, using a drill press.
Now the hard part.....You need to make a carrier of your own design to hold your foil that can be placed within the moderator. Look at my images as a guide and use your own inventiveness.
My carrier, in the image, was made up of a 2.25" ring of metal from a junk box the ring's rim was only 3/16" wide and thick. (old lens spacer of aluminum). This ring is perfect for mounting the foil since when it is to be measured it is the perfect diameter for the 2" GM pancake detector for the beta particle counting.
Before mounting the foil, I attached a 1.5" long 4-40 hex aluminum standoff for a handle and a knurled top knob as a "stop". In my case I rolled out my foil to a thickness of .002 inches. (2 mils).
I milled a 2.3 " wide slot by 1/4" deep into the face of one of the HDPE slabs with my milling machine. Adjust the length of the slot to allow the foil covered ring to reside in the middle of the slab. The moderator is assembled as shown. using the metal all threads on the bottom and the plastic or fiberglass on the top. (note - you must thread about 3/4" long sections on the two insulating rods with a 1/4 x 20 die.) I did this due to the proximity of the high voltage terminal to the top of the neutorn oven. (see images).
One doesn't need a tempting metallic object close to 45,000 volts. These insulating rods are a safety factor. You could design this to go under the fusor, leaving room for it, where there is no high voltage and use 4 much simpler metal "all thread rods".
The insertion and rapid removal of the foil and holder are an absolute must as the hottest activation of silver lasts only about 80 seconds and,then, it is dead (24 sec half-life). The other radioactive isotope of silver has a nice long 2+ minute half life, but to fully activate it you must do top level fusion in a continuous 10 minute run. Only a few amateur fusors can hold 500,000++ neutrons/sec for this long.
The length of thermalization in HDPE is about 1.8 inches. Thus, I built my oven so that the foil is at this distance. The other slabs are behind this...Why?...once thermalized the neutrons ricochet all through the moderator changing direction and can also activate from behind as well.
Before the questions fly on how I obtained the foil....I have a jewelers rolling mill and rolled my own foil from a 1 troy ounce round 9999 silver Canadian maple leaf I bought years ago for about 6 bucks. It is a tedious process of annealing and rolling and re-annealing and rolling, etc., etc. You burn off lotsa' time and elbow grease. Again, my foil was .002 inches, (2 mils), thick.
Good luck with the project and use your imagination and modify what is shown to suit your skills and tools at hand.
Richard Hull
All D-D fusion produces fast neutrons which can't do much of anything for the experimenter. Valuable neutrons are thermal, (slow), neutrons.
A moderator is a hydrogenous material that slows fast neutrons to thermal velocities. Typical moderator materials are water, paraffin, and polyethylene.
A moderator is a must for neutron detection via various electronic tubes. (boron lined tubes, boron corona tubes, BF3 tube and 3He tubes.)
A modrator is also needed for all activation experiments. Among the most common materials that are easily activated are Silver, Indium and Rhodium.
The moderator in this FAQ is made specifically for activation experiments. Usually such activation is done on metal foils of the metals noted above.
Since the moderator shown thermalizes fast neutrons, I have dubbed it my "Neutron Oven" as it cooks the material to be activated in thermal neutrons.
Out of the oven, after a suitable time, the foil of choice is removed and is radioactive for a very short time, as all the half-lives are in seconds or minutes.
This can be a valuable neutron detection scheme provided you have over 300,000 neutrons per second boiling out of your fusor and is an absolutely perfect, proof positive that fusion has been done. Add to this that a simple geiger counter is all the intrumentation needed to prove fusion. Basically, if you activate a metal and a simple GM counter reads over background radiation immediately upon being pulled from the oven, you have done fusion.
Materials list for this:
A single piece of 1" thick high density polyethylene, (HDPE), 6" wide by 36" long. (local plastic distributor- big cities)
2 - 7" long 1/4 X 20 lengths of "all thread" Hardware stores sell 3 foot lengths that you can cut to length with a hack saw.
2 - 7" long nylon or fiberglass 1/4" diameter rods
4 - Nylon 1/4 X 20 nuts with nylon washers
a few 1/4 X 20 metal nuts and washers (hardware stores)
Mounting materials to suit...Wood, G-10 fiberglass, phenolic, etc.
silver foil
Tools
drill press or hand drill with bits
milling machine if you have one or have a friend or neighbor with one.
set of taps and dies - you really just need a 1/4 X 20 die and handle.
hack saw at least...A chop saw or power hack saw would be better.
method
Saw the polyethylene into 6 equal 6" x 6" squares. I used a power hack saw for this. Note, the plastics place will be more than willing to do this for between $5.00 and $10.00 per cut. (ouch!) I wasn't going to give them my money for that.
Drill 4 accurately placed corner holes of about 5/16" in each slab of the plastic to pass the long bolts through to hold the mass together. I made a master and used it to drill the remaining slabs to match, using a drill press.
Now the hard part.....You need to make a carrier of your own design to hold your foil that can be placed within the moderator. Look at my images as a guide and use your own inventiveness.
My carrier, in the image, was made up of a 2.25" ring of metal from a junk box the ring's rim was only 3/16" wide and thick. (old lens spacer of aluminum). This ring is perfect for mounting the foil since when it is to be measured it is the perfect diameter for the 2" GM pancake detector for the beta particle counting.
Before mounting the foil, I attached a 1.5" long 4-40 hex aluminum standoff for a handle and a knurled top knob as a "stop". In my case I rolled out my foil to a thickness of .002 inches. (2 mils).
I milled a 2.3 " wide slot by 1/4" deep into the face of one of the HDPE slabs with my milling machine. Adjust the length of the slot to allow the foil covered ring to reside in the middle of the slab. The moderator is assembled as shown. using the metal all threads on the bottom and the plastic or fiberglass on the top. (note - you must thread about 3/4" long sections on the two insulating rods with a 1/4 x 20 die.) I did this due to the proximity of the high voltage terminal to the top of the neutorn oven. (see images).
One doesn't need a tempting metallic object close to 45,000 volts. These insulating rods are a safety factor. You could design this to go under the fusor, leaving room for it, where there is no high voltage and use 4 much simpler metal "all thread rods".
The insertion and rapid removal of the foil and holder are an absolute must as the hottest activation of silver lasts only about 80 seconds and,then, it is dead (24 sec half-life). The other radioactive isotope of silver has a nice long 2+ minute half life, but to fully activate it you must do top level fusion in a continuous 10 minute run. Only a few amateur fusors can hold 500,000++ neutrons/sec for this long.
The length of thermalization in HDPE is about 1.8 inches. Thus, I built my oven so that the foil is at this distance. The other slabs are behind this...Why?...once thermalized the neutrons ricochet all through the moderator changing direction and can also activate from behind as well.
Before the questions fly on how I obtained the foil....I have a jewelers rolling mill and rolled my own foil from a 1 troy ounce round 9999 silver Canadian maple leaf I bought years ago for about 6 bucks. It is a tedious process of annealing and rolling and re-annealing and rolling, etc., etc. You burn off lotsa' time and elbow grease. Again, my foil was .002 inches, (2 mils), thick.
Good luck with the project and use your imagination and modify what is shown to suit your skills and tools at hand.
Richard Hull