FAQ - Neutrons, neutrons, neutrons

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Richard Hull
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FAQ - Neutrons, neutrons, neutrons

Post by Richard Hull » Mon Dec 19, 2016 3:06 pm

The only reason I ever got into this effort was to get my hands on neutrons. To hell with fusion. It is a means to an end for me. The fusor is the best and easiest neutron source for the amateur. Let's amplify that to a very, very serious and commited amateur with a goal and a number of skills and some cash in hand.

Fusion, as a process, is abysmally easy to do at, both, the professional and amateur level. At the amateur level, it can only be done one way. This path is one of deuterium-deuterium fusion, (D-D fusion). This is the only possible fusion fuel that the amateur can get his or her hands on and hope to be able to fuse. Six specific radiations are produced in the D-D fusion process in an amateur fusor. Two of the radiations are electromagnetic and four are particulate.

Eample: Let us assume an amateur fusor doing one million D-D fusions per second....How many of these radiations and particles are produced each second?

Electromagnetic
1. X-rays are produced in dangerous and copious amounts (many millions/sec) Most of the worst and bulk of this radiation will never penetrate the fusor shell or body!)
2. A fantastically energetic gamma ray of many meV energy (about 100 gammas/sec) This is so rare in the D-D reaction as to be a non-radiation and is just not worth even considering as existing at all)

Particulate
1. A 4He helium nucleus (about 100/sec) This is party balloon helium and is so rare in D-D fusion that it is produced at the same non-rate as the high energy gamma noted above. IT NEVER EXITS THE FUSOR CHAMBER or SHELL!)
2. A 3He helium nucleus (about 500,000/sec) A rare and non-earthly isotope of helium. (IT NEVER EXITS THE FUSOR CHAMBER or SHELL!)
3. A Proton (about 500,000/sec) A common hydrogen nucleus. (IT NEVER EXITS THE FUSOR CHAMBER or SHELL!)
4. A Triton (about 500,000/sec). This is a radioactive form of hydrogen called tritium made up of a proton and two neutrons. (IT NEVER EXITS THE FUSOR CHAMBER or SHELL!)
5. A Neutron(about 500,000/sec). This is a highly energetic, (2.5meV), "fast neutron". This will be the only thing ever exiting the fusor chamber or shell other than the x-rays and rare gamma
This neutron is the darling of our desirings! The pesky x-radiation can easily be shielded and knocked out.

The above is a quick rinse for those too lazy to locate and read the highly detailed D-D fusion nuclear process discussed elsewhere in these FAQs.

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Now, to the neutron physics. There are three or four general forms or types of neutrons based solely on their energy. As the neutron has a fixed mass, a neutron's energy is related solely to its velocity. Nuclear scientists and amateurs refer to any particulate energy in terms of electron volts. (Don't be lazy! Look up and understand the "electron volt" as a form of energy)

Fast neutrons - What the fusor puts out .. Fast neutrons are neutrons with energies over 300ev, but normally, near or over 1 meV This is the most dangerous form of neutron radiation

Epithermal neutrons - A neutron that is in the energy range from about 1 to 300 ev. (a real fuzzy and arguable definition)

Thermal neutrons - These are the neutrons that get things done! Thermal means that the neutrons are moving at the same velocity as the molecules of free air at Standard Temperature and Pressure, (STP). This is about the same speed as a high velocity rifle bullet and the energy is .025 ev. Neutrons well below .025ev are still considered thermal by most parties.

Cold neutrons - These are of no concern to us, but they are very torpid and are in the .001 ev range or well below and could be considered deeply sub-thermal neutrons.


Neutron emission and other cool stuff you need to know about fusor neutrons.............................................


In D-D fusion the 2.5 meV neutrons are emittied randomly in all directions for the purpose of amateur efforts. There are no neutron beams, preferred beaming directions, or tricks to create intense neutron beams, etc. The neutrons come out randomly in all directions, simultaneously. This is called, (isotropic neutron emission), and is what the fusor does. In general, in this amateur effort, neutron numbers are expressed in Isotropic emission numbers.

FLUX..............................

Imagine a giant pumpkin with a black powder bomb in its center, going off. In this explosion, the closer you are to the pumpkin, the more juice, seeds and chunks of pumpkin will hit you. Same..same neutrons from a fusor. Move back 50 feet and a few seeds, some juice and only a few minor bits of pumpkin will hit you. What follows is important to grasp! The number of neutrons or pumpkin debris bits that hit you at a given range, over you body's surface area, is call the "flux" The closer you stand, the higher the flux. In neutron radiation physics, the flux is referred to as the number of neutrons passing through an area of one square centimeter each second and is always related to the distance from the source of neutrons, (more distance, less flux). At most any range, the flux of neutrons from a fusor doing 1 million fusions per second is very low compared to professional neutron sources. In general, we do not use flux statements from our fusors, but choose to state it as (neutrons emitted per second, isotropic). From the above example, the fusor would be producing 500,000 neutrons per second, isotropic. Yes, the flux at any point is easily calculable and once calculated would mean little to the amateur save for the fact that the flux of dangerous fast neutrons 8 feet away from such a fusor would present no danger to a naked person over the short operational period of the average amateur fusor!! Still, they are there, so, Nervous Nellies and ALARA freaks take note and do what makes you happy in the way of neutron shielding to keep you in your little rad-free womb.

Fast neutrons - Nasty RBE = 20+ (Relative Biological Effectiveness).....................

All neutrons tend to scatter in most hydrogenous materials, after striking the contained hydrogen nuclei. (wood, plastics, paper, cloth, waxes, petroluem, human beings and pets.) When they scatter, they are slowed, (lose energy), and alter DNA and the matter's atomic nuclei they finally stop within. This is why the fast neutrons are so deleterious to humans. We are big bags of water which slow fast neutrons splendedly. With each slowing collision, damage is done in the form of fast, free ions in our bodies. Bodies do not like free ions in DNA and other stuff ricocheting around in it. The new, slower velocity means the neutron will travel far less distance before scattering again. Thus, the average human will absorb virtually all of the 2.5 mev energy throughout their body. Shielding for fast neutrons is dealt with in great detail in other FAQs within this forum..... Enough on the danger of fast neutrons to humans.......

Happy neutrons, (thermal), the experimenters joy............................

If you want to experiment with neutrons from a fusor, you will need to slow them from fast to thermal velocities. This is normally accomplihed with parrafin wax or polyethylene plastic.
We tend to build "neutron ovens" around and adjacent to our fusors. These ovens are said to "moderate" the neutron energies and the materials are called "moderators". The oven should have a minimum thickness of 3-inches of high density polyethelene all around the "activation chamber". This will warrant a continuous thermal neutron "bath" in the activation chamber. Thermalization continues unabated within a moderator as long as the neutron is within it. Once at thermal velocities, an inverse energy relationship, called the "1/v law" occurs as the neutron continues to slow and moderate. Thus, the thermal cross section for all materials gets bigger and bigger as neutrons slow to sub-thermal energies due to the 1/v law. Larger cross sections mean easier and more complete activation in any given neutron flux!

Activation???

A number of materials have their atoms altered in a thermal neutron field. The rate at which this alteration occurs is related to the neutron flux and its energy and the material's "absorption cross section", at that neutron energy. In general, materials of low cross section are not altered as much or as fast at high neutron energies. Thus, the neutron oven, effectively, makes a bath of slow, thermal neutrons in its core chamber from outside impinging fast neutrons from our fusors. Many materials of high cross section have their atoms altered in the neutron oven in a way that they are made radioactive following a suitable exposure to the slow or thermal neuton bath. This process of taking a suitable, non-radioactive, high cross section material and bathing it in slow neutrons which results in the item so irradiated becoming radioactive is called "Activation". Activation is a great way to prove you have done fusion! Unfortunately, few amateur fusors ever get so far up the neutron production number curve, (250,000 n/s istropic or more), to ever activate anything. Thus, the need for a much more sensitive neutron detector for weak fusors.

The best materials to activate are those with very large cross sections and very short half lives of the desired finished or "baked in" activation product. The good part is activated materials are easily detected with common, low cost geiger counters. Likewise, they "die" fast due to their short half lives and thus, no radioactive material remains after a few minutes and your activation sample is now radiologically "dead" again. The absolute best materials are, in order of ease of activation, Silver, Rhodium, Indium, Manganse and few others. Silver foil is the cheapest and most easily obtained and the number 1 choice. Indium foil is next and Rhodium foil, while expensive, is realy nice. Manganese requires a mega-neutron per second fusor.

Concentration of neutrons from a fusor..............................

Again, there is no way to beam neutrons, but a properly designed "oven" surrounding the fusor and scattering the neutrons over large useful areas of fusor emission can make more use of the fusor's neutron output and do some mild concentration in your oven. However, this will always be expensive as large slabs and amounts of high density polyethylene will be needed. For fusors in the 2 million fusions per second range, ( 1million neutrons per second isotropic), such an expense will produce a great amateur activation system.

Neutron detection issues.......

Fast, epithermal or slow neutrons are all electrically neutral and, as such, no direct electronic detection method is possible as in the case of charged particles. Thus secondary, two step processes are needed for electronic detection of neutrons. Two step detection automatically means rather low detection efficiency of no where near even 30% in the finest methods and as poor as 1% in the worst methods. The best methods involve slow neutron reactions in high pressure gas tubes where the neutrons react in a nuclear fashion with the gas atoms creating high energy charged particle debris in the gas which are detected in a proportional manner. This means weak signals and very special preamplifiers. In addition, some of these detection methods will also detect a lot of other radiations slamming into the gas environment. More electronics, (pulse height discrimination), circuitry is needed to filter out of the signal all but neutron detection pulses.
As noted above, just about all of neutron detection demands only thermal neutrons enter the detection system. This places more distance between a weak neutron source and the detector, further reducing net efficiency of detection.

Much of this is covered in great detail in other FAQs in this forum. Neutron detection is no walk in the park and a great deal of effort and perhaps even a chunk of the money substances must be devoted to it.

This is a quick rinse in neutron physics

The fusor makes only fast neutrons and, in general, for an operator or observers over 8 feet away, presents no real radiation hazard.
These neutrons are isotropically emitted during D-D fusion.
Activation requires thermal neutrons and only a few materials lend themselves to amateur activation.
The slower the neutron the higher the absorption cross section for any mateiral due to the 1/V law.

Don't go believing you can beat the laws of physics. Instead, learn to use them profitably and wisely. DO a lot of follow up reading on neutron physics and be smarter, still.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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