FAQ - NEUTRON SAFETY

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FAQ - NEUTRON SAFETY

Post by Richard Hull » Thu Jan 16, 2003 5:30 pm

An occasional e-mail drifts through, often anonymous, scolding us for supporting the amateur fusor effort and the attendant neutron production. Most of the bad vibes are due to the casual, "nervous nellie" observer to this site failing to really read back into this very forum to view the numerous discussions on neutron and x-ray safety issues.

This FAQ is an attempt to mollify the "chicken littles" and, at the same time, place in one single location basic information on neutron safety.

First of all, very few on this list will ever make a working fusor. Most who do build here will produce only a demo fusor where no radiation of any form is encountered, being happy with just seeing star mode plasmas. Secondly, all amateur fusing fusor efforts produce pitiably few neutrons compared to even the most timid of baby neutron generators sold on the open market. Third, The average amateur might operate his or her fusor for microscopic blocks of time compared to any other viable user of neutron sources.

Thus, the exposure to neutrons of any actively fusing amateur here is so minimal that it is almost, (but not quite), a non-issue. Far more problematic are the X-rays from glass or large viewported fusors. While the x-rays are weak compared to a real x-ray machine, it is this very weakness that can cause bad skin surface burns. Fortunately, the simplest of thin lead shielding precautions can totally elimenate this problem.

The above being said, it must be clearly stated that in the mind of modern radio-medicine all radiation exposure, no matter how minimal is undesirable. This in spite of recent studies showing that a continuous increase in background radiation levels in control groups tend to indicate a reduction in some forms of cancer!!

Medicine must always err on the side of caution.

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THE NEUTRON
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The neutron is no longer considered a primary particle of matter. Instead, it is looked at as a separate piece of condensed matter that is unstable when alone and flying free. It has a life span in this state of only about 10 minutes.

The neutron appears to be stable only when residing within the nucleus of an atom. All atoms except protium, (hydrogen), have neutrons within their nucleus.

Natural radioactive elements all decay through the emission of electrons, gamma rays, or helium nuclei (alpha particles). On rare occassions and in some non-earthly, man made isotopes, positrons can be emitted.

The one particle never emitted from natural elements or long lived nuclides of those element, is a neutron. This is very strange indeed as it appears that the even the most unstable of atoms up to the last nautral element, Uranium, jealously guard their neutrons. Such atoms would rather throw off an entire helium nucleus rather than forfeit a single lone neutron. Only a self-fissioning heavy natural element will release neutrons.

* note* in normal minus beta decay, the nucleus decays a neutron internally rather than emit it. So a nucleus can rid itself of a neutron, but only does it internally and never externally in any natural element or isotope of same that is a natural decay product. Spontaneous fission in U238 and Th232 can have neutrons fly off as the entire nucleus fails.

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THE FUSOR AND NEUTRONS
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The fusor is designed to do nuclear fusion. In the amateur effort this involves only D-D fusion where deterium nuclei (deuterons) are collided in velocity space after being electrostatically accelerated.

The only particle from this reaction that can escape the fusor vessel is a neutron. This neutron is normally considered rather mono-energetic and is said to have a velocity which gives it a kinetic energy of 2.45 mega-electron volts. (2.45 mev).

This is called a "fast neutron" in the parlance of neutron physics.

Normal, simple amateur fusors, operating at lower voltages, rarely emit a total isotropic neutron count exceeding 100,000 neutrons per second.

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RBE and fast neutrons
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All forms of radiation have an RBE or "relative biological effectiveness" associated with each type. The rating system used to run from 1-10, 1 being low risk or small effectiveness in doing damage and 10 being considered very high risk and very damaging to human tissue, organs and genetics. Of late, the scale has been extended in some of the literature to 1-20.

The fast neutron is rated at 10 on the lower scale and 20 on the higher one, making it a real super dangerous particle.

WHY?

It has to do with the fact that we are really just big bags of water with skin covers. Neutrons are not absorbed easily in any form of matter due to their neutral charge. They effortlessly glide through matter with out being deflected by intermolecular or nuclear magnetic or electric fields. They move in a straight line like a bullet. However, they do occassionally hit a nuclei that is in their way. Hydrogen nuclei (protons) look rather large to the approaching FAST neutron. The apparent relative size of a nucleus as seen by a neutron of a specific energy is a measure of the target nuclei's relative "cross section".
Without going into neutron physics here in depth, hydrogen looms large before an approaching fast neutron. As such, in hydrogenous targets, (water, plastics, wax, human bodies, etc.), the fast neutron impacts a lot of protons in its path. These highly charged protons are sent flying in such collisions. It is these high speed protons which do tremendous damage over their extremely short path in the target material by ionizing tens of thousands of normal atoms they encounter. These can be skin cells, genes, heart or eye tissue, or whatever. All radiation damages, ultimately, by this latter process.

The high speed or "fast neutron" is so ugly due to the fact that it can fly through the entire body unlike other radiations. Fast neutrons create millions of proton "recoils" in their wake actually leaving the body still a neutron although much reduced in energy and speed. So, one can readily see that the neutron can, through secondary reactions, cause a lot of ionizations along its entire path through the body. This is why it is rated amoung the most biologically dangerous of all radiations.

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NEUTRON CAPTURE
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If a neutron is slowed down enough to near "thermal velocity", (2200 meters/sec or a mere .02 electron volt energy), it can be captured rather readily by atoms of high cross section. If and when this occurs it almost always creates a radioactive isotope of the atom it is captured by. This is refered to as "neutron activation". As such, this atom is now, itself, radioactive and most normally decays via beta radiation (high speed electron).

Now this means that not only can you get zapped by recoil protons as a fast neutron zips through you, but you can also be made internally radioactive at atom sites where neutrons slowed to thermal energy in your body are captured! A sort of double whammy, if you will.

Likewise, stuff around you of high cross section will be made radioactive, but only if the fast neutrons have been slowed to thermal energies by penetrating your body, thick wood masses, parafin blocks, etc. Most likely though the fast neutrons will move off many miles in air, into the ground or space before this happens.

Are you now sufficiently frightened? You are certainly now rather fully informed about fast neutrons and their nasty habits in passing through your blubber and vital organs.

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THE GOOD NEWS
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If you undertake to advance beyond the relatively safe, non-fusing, science fair demo fusor and make a real functional fusor, you must be prepared to deal with minimal levels of neutrons during the short runs of your rather whimpy neutron machine.

There are three big ways to limit reduce or elimenate your exposure. We will list them and then dicuss them in some detail.

1. Inverse square law
2. Shielding
3. Exposure time

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Inverse Square Law - solid angle
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"Nothing beats getting the hell away from a source of radiation"........Old proverb - unattributed

Distance between you and the source of any radiation is a real life saver.

Most sources can be viewed as a pinpoint source of radiation radiating over the classic 4pi radians in 3D space. A sphere!

The fusor has its pinpoint source of radiation ideally located at the geometric center of the inner grid which is also often the center of the fusor vessel, if spherical.

Real he-man neutron sources don't even fiddle with isotropic emission rates the way we do. The big boys and health physicists deal in "neutron flux" This is the number of neutrons passing through a square centimeter of surface area per second. As a flux producer the fusor is a real ultra whimp not even garnering the title of "also ran". Flux levels at one meter from a 100,000 n/s source is a rather laughable 0.80 neutrons/cm sq/sec. This demostrates that an apparent fearsome source of radiation can be reduced significantly in rather short distances. I have covered the simple calcs needed to figure out flux with known isotropic emission rates in the neutron measurment FAQ posted earlier.

Again.............nothing tames radiation blasts like physical separation. Use it! It will help keep you safe even in non-shielded, long exposure scenarios.

From this, it can be seen that a 1 meter distance between the operator and a small working fusor might be considered a MINIMUM safe working distance.

Most of us are remote operators anyway, using video cameras in the view port with remote power and operating controls. You should definitely consider this as a major design criterion in any planned fusor operation.

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SHIELDING - bullet proofing the fusor
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Neutrons aren't easily stopped. They can be attenuated to a great degree.

The best one might hope for is to thermalize and capture a huge fraction of the fast neutrons from any fast neutron source.

The best shield to construct "on the cheap" is a combination of parafin and borax. A small wall of parafin about 3-4" thick should thermalize most of the fast neutron flux impacting it. Backing this up with a wall of boxes of borax ~2" thick (Boraxo) will stop the a large fraction of the thermals exiting the parafin. You can rest assured that virtually zero fast neuts will exit this shield. If your fusor operation is in an out building, you are lucky. The shield need only provide an "operator's shadow cone". In this manner a very small and inexpensive shielding arrangement can be placed hard up against the fusor in between the operators position and the fusor. a 10" X 10" shield placed 2" from the fusor body of a 6" fusor should provide a 64 degree shadow cone pyramid which translates into a total shielded area of 8 feet X 8 feet only 2 meters away. With this combined scenario of distance and a very small shield, the operator of even a 500,000 n/s fusor would be exposed to virtually zero neutrons.

If the fusor is assembled and operated in a home, (not a good idea), it would absolutely require a full shielding castle built around it.

Common sense would tell you that you would not even need this if you operated it when no one but you were home. This leads us to our final and third safety tip.

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TIME OF EXPOSURE - there is such a thing as too much of a bad thing
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Total radiation dosage, as relates to medical damage, is as much a function of exposure time as it is of neutron flux levels.

No amateur to date has really operated a fusor for more than a few minutes at a time. They are just too finicky. Most of the time the fusor is operated just long enough to see if any improvement has occurred following some whiz-bang idea being implimented. Often, the total weekly on-time at fusion levels is well under 20 minutes. I doubt if I got 30 minutes per month total fusing time at the peak of my activity.

The secret is...operate for as short of an interval as is possible to collect data and finish any experiments. If leaving a fusor in "idle mode", drop the acceleration voltage to zero and, if possible, shut off the D2 flow. The latter saves you money by conserving the precious D2 gas.

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WELL ARMED IS FOREWARNED
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If you must make a real fusor, you MUST obtain a neutron counter of some sort. You should already own a geiger counter, needless to say.

It is only through neutron metrology that you can truly experiment viably. It is also the only way to determine neutron levels, the effectivness of shielding, distance separtion, or get an idea of total absorbed dose over time.
The latter should be so close to zero as to, again, be a non-issue.

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FOR SALE- IRRATIONAL FEAR 25 CENTS PER TON.....REASON AND LOGIC $2,000,000 PER OUNCE
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For many the word radiation means evil, bad things. Their concept of the word it is limited to what the media and noisey crybabies feed to them in regular drumbeat doses. They can't bother to study the issue. Instead they let others who have the public's ear do their thinking and analysis for them.

Most knee-jerk reaction to radiation is found in the un-educated masses. If one finds a lettered person blindly attacking this or that radiation issue one will usually note that their considerable education is often in the humanities or social studies.

The human being is like any other biological organism. It tends to respond to stimulus in a number of predictable ways. In the human and higher animal species that often live in organized groups or have developed a social behavior of sorts, some of these stimulated reactions are truly irrational when coldly observed by the logic and reason of a scientist in light of the facts. Nonetheless, social stigmas, prejudices, poor education, etc. often show up in knee-jerk reactions and irrational behavior. All are very human and often very predictable.

You can't change the nature of human beings!

The release of radiation is a simple, well understood physical process occurring, for the most part, within a nucleus. The radiation then moves out into space and can interact with other matter. Biological systems can be damaged or interact with the radiation at the atomic, molecular and cellular level. A good deal is now known about the results of exposure to varying levels of most all forms of radiation. Scientifically, there is no truly agreed upon threshold level or minimal safe level for any form of radiation. There is no such thing as a level of radiation that does no damage. There are, however, certain higher levels where definite and permenant damage can occur. These levels are rather well documented. Wisdom tells us not to deliberately or needlessly expose ourselves or others to radiation levels even approaching these upper levels and to try and remain far below those levels once thought to be rather safe. Thus, if the descision is made to work or experiment around radiation, some level MUST BE ACCEPTED by the worker/experimenter. The current government guidelines for workers in the nuclear industry are a good point to establish your personal limits.

Once you establish your personal, minimal level, you should remain alert, but should also be content with the decision.

Likewise, upon being confronted with a knee-jerk radiation reactionary, you should smile and allow them to spiral out of control for it is what they do well. They are not reasonable entities and you are under no obligation to teach them anything. Doing battle with them is a fool's errand and you might just freak them out more creating a danger zone that did not exist prior to the encounter.

A wise researcher lets all sleeping dogs lie and out of control knee-jerkers spin on out of control toward other "reactive targets", (which is what they seek).

Wisdom and informed reason and logic always wins the day in science.

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WRAPPING IT ALL UP
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Armed with the above truthes you can see that even the simplest and easiest radiation protection (separation by distance) can make neutrons produced by the amateur fusor a virtual non-issue.

If you use all three of the above methods. Neutron problems melt away to be never dealt with again.

Worry about the normally attendant neutron scattering is a non-issue. Worry about neutron activation of laboratory materials or fusor components is also another non-issue. All of this is due to the whimpy levels of neutron production in a simple amateur fusor and the near zero flux levels at just 1 meter out from the fusor. Don't let anyone tell you different.

Still, it is ultimately your decision as to whether you move onto neutron production or not. It is also your responsibility to protect yourself and others around the fusor.

I will not operate my fusor to produce neutrons for visitors or friends. I run it in demo glow mode which is satisfactory to give them the look and feel of the system without the issues of radiation.

The fusor is a research tool in the hands of an amateur and should be treated as such. Operate it only for as long as needed to conduct an experiment and either shield or remove your person from its vacinity or both. In this manner neutron hazards approach zero.

The reader is advised to comb this forum in great detail. You can also find more information on X-ray hazard protection. You should also intently read the entire NEUTRON COUNTING FAQ which explains neutron metrology in some moderate detail.

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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Re: FAQ - NEUTRON SAFETY

Post by Captain_Proton » Fri Jan 17, 2003 9:18 pm

Well said, Richard...

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Re: FAQ - NEUTRON SAFETY

Post by Brett » Sat Feb 22, 2003 11:12 pm

I'd only take exception to,

"Natural radioactive elements all decay through the emission of electrons, gamma rays, or helium nuclei (alpha particles). On rare occassions and in some non-earthly, man made isotopes, positrons can be emitted.

The one particle never emitted is a neutron...."

I'd point out that spontaneous nuclear fission, which certainly IS a decay process for many natural isotopes, is usually accompanied by the emission of one or more neutrons. This is, as I understand it, because the decay products are in a highly excited nuclear state.

Granted, even a pathetic Fusor would completely swamp the neutron emission of any naturally occuring isotope. But the point is, any random chunk of pitchblende IS going to be naturally emitting neutrons as a result of nuclear decay.

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Re: FAQ - NEUTRON SAFETY

Post by Carl Willis » Sun Feb 23, 2003 12:29 am

Brett,
Indeed there will be a very few SF neutrons from U-238. The branching percentage for this decay mode is some 10^-5 percent, with the usual alpha decay comprising the remaining ~100%. Yields are many orders of magnitude lower for all other naturally occurring isotopes. Interestingly enough, some heavy nuclei are reported to spit out light nuclei as a rare decay mode, mainly isotopes of carbon thru magnesium (just noticed that.) Natural neutrons are more likely to be produced as a result of bombardment of aluminum, beryllium, carbon, etc. by nearby alpha emitters. There are indeed natural neutrons but they are too rare to be of use...with the exception of the Oklo natural fission reactors!! Now those were killer neutron sources back in the day (millions of years ago.)
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Re: FAQ - NEUTRON SAFETY

Post by Richard Hull » Mon Feb 24, 2003 4:41 pm

Fission is not a decay process. It is a destruction process. The atom is truly fractured into, mostly, nearly equal sized masses. The neutrons are uncoupled from the mass and not primarily emitted, (at least there is no hard evidence that the atom splits because a neutron needs out). Decay simply steps and atom down a notch or two, It never disrupts the entire nucleus. Decay has a predicatble parent and daughters. Fission can literally wind up as two of anything in the periodic table.

This is why I specifically stated neutrons are not emitted in the DECAY process of any natural element. There are no natural neutron sources in the sense of alpha, beta or gamma emitters. Atoms demand a catastrophic energy event to give off a neutron. They are never seen unless matter is being assembled or destroyed through collision or fracturing.

They are most jealously guarded by nature. She went to too much trouble to make them. I have for years held and still hold the belief that neutrons are the nuclear glue and that there is no strong fource. I do not swallow, in wholeform, all that modern physics preaches.

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
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Re: FAQ - NEUTRON SAFETY

Post by longstreet » Tue Sep 06, 2005 2:23 am

I was just wondering how a boron shields anything. I mean, if the boron absorbed a neutron doesn't it then become radioactive and emmit even more radiation? Maybe it blocks neutrons, but then you have gammas and other fast things. I guess those products are easier to stop then? Idealy, should you then line the thermal-boron shield with lead to stop the resulting radiation?

Carter

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Re: FAQ - NEUTRON SAFETY

Post by Richard Hull » Tue Sep 06, 2005 3:50 pm

Boron neutron capture is common and yes it can make an isotope which decays in milliseconds. Check out

http://ie.lbl.gov/education/parent/B_iso.htm

Boron has two stable isotopes B10 and B11...

1. With neutron capture, normal stable B10 pumped up to B11 which is the other stable isotope of boron. no radiation here.

2 When B11 captures a neutron it is bumped up to B12 which beta decays in milliseconds to stable carbon leaving no effective long term activation products from neutron absorbing boron.

It must be noted that it is B10 with the huge slow neutron cross section so this is the real absorber in boron. B12 is normally produced with faster neutrons, not thermals. So Boron is a total stopper, but is far more suitable as a thermal neut. stopper due to the monsterous B10 cross section for same.

I am fairly sure I got this right. I am no nuclear expert here, but have tried to learn my nuclide ABC's over the years. Carl will undoubtedly have comments. Regardless, Boron poses no activation issues and is why it is used extensively in Neutron shielding.

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
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Re: FAQ - NEUTRON SAFETY

Post by Alex Aitken » Tue Sep 06, 2005 5:12 pm


B-10 is indeed the active isotope, and on taking in the neutron it immediately fissions, or as neer as, spitting out an alpha particle and a Li-7 nucleus in oppasite directions. Most of the time it also produces a gamma ray as well. Its the same reaction that the BF3 tubes use to detect neutrons. The alpha and nucleus fragments are highly ionisiing as they lose the energy which gets swept by the field in the tube and amplified by cascade and electronics.

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Re: FAQ - NEUTRON SAFETY

Post by Carl Willis » Tue Sep 06, 2005 11:43 pm

Carter,

Neutron shielding is always a coupled neutron-photon problem at least. There is pretty much nothing that does not kick out gamma rays upon capture or inelastic scattering of a neutron, and that holds for boron as well.

Neutron capture in B-10 leads to frequent production of prompt 480 keV gammas from the excited Li-7 product nucleus. This radiation can be seen in some of the prompt gamma spectra that Jon Rosenstiel and I have posted, such as

view.php bn=fusor_neutrons&key=1085179675

The only thermal neutron shielding material commonly in use that does NOT produce prompt gamma rays is lithium. However, the physical / chemical properties of its compounds make it unattractive. It also produces tritium upon neutron capture, which in high fields can be a problem in its own right.

Inelastic scattering is an abundant source of very penetrating (up to 20 MeV) gamma rays in fast neutron apparatus. Lead, which is a favorite photon shield, is unfortunately good at making hard photons from inelastic scattering. It is usually replaced by bismuth in critical neutron-photon shielding.

For all of the secondary radiation produced by neutron shielding, it must be remembered that nothing does biological damage like a neutron. A fast neutron incident on human flesh has all the problems of high penetration, effectively high LET, and induction of further radioactivity. Since they are going to cause secondary radiation anyway, one might as well stop them right close to the source with boron and cut down the gamma rays in a separate exterior shield.

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Re: FAQ - NEUTRON SAFETY

Post by longstreet » Wed Sep 07, 2005 2:32 am

If I start to get into higher neutron counts, what I was thinking was to melt paraffin with borax and cast them into big 2'x2'x4" blocks, maybe sandwiched between steel/lead plates. But what you're saying is maybe the steel and lead aren't necessary, or even make things worse?

Carter

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