FAQ - Neutron flux vs. emission

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Richard Hull
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FAQ - Neutron flux vs. emission

Post by Richard Hull »

Neutrons are measured in many ways for purposes of health physics, total fluence and power reactor output classification. This FAQ will explain some real differences so that you can know the differences and not allow large numbers to intimidate or befuddle you.

Pulses are different from continous operation and a neutron flux is different from an emission rate. Health physics is concerned with the time averaged absorbed neutron dose.

Total Isotropic Emission Rate (TIER)

This is the rate, in neutrons per second emitted by a continuously functioning reactor and is of no value to anyone in the field other than we amateur fusioneers. We actually use it as a simple way of judging the relative performance of our systems. It is a difficult figure to arrive at without having a formal neutron counter on hand.

Most continous duty amateur D-D fusors produce between 100,000 (10e5) and 10,000,000 (10e7) neutrons every second. The ISOTROPIC term in the TIER acronym means that these per second rates are spread out over the entire sphere of emission or a 4pi area.

Pulsed Isotropic Emission (PIE)

This is the total isotropic emission of neutrons in a single burst or pulse from a pulsed fusor or reactor. The numbers here can be very high but may pose no real hazard due to the very slow pulse rate.

To convert from PIE to TIER you will need to know the Pulse Repetitiion Rate (PRR) of the pulsed device. To obtain the TIER, multiply the PRR X PIE. Often, a pulsed fusor can't do more than one pulse per minute and one pulse per second is a very fast rate.

No neutron counter can effectively count pulsed neutrons. The most often used method of determining the PIE is to neutron activate materials and figure backwards to the Flux then to the TEIR and finally the PIE. This is always laden with lots of room for errors due to a large number of assumptions. There is no way to genuinely estimate the PIE based on cross sections from the front end due to the vast number of variables in the pulse's bulk ionization and fusion process that can't be controlled.

Neutron Flux

This is a term used by the nuclear energy and health physics people and conveys real data about just how much power is contained in the neutron source or absorbed by the human body.

Flux is what all the big boys talk in and will laugh at TIER and PIE related reports of fusion.

It is flux that boils water and kills people.

FLUX is the number of neutrons impacting or passing through a square centimeter area of space or material per second. Knowing the FLUX and some distances, TIER and PIE can be calculated and vice-versa. More importantly, knowing FLUX will allow energy delivery rates to be calculated.

Flux is the number of neutrons present at a particular point in space per unit area and is somewhat uncoupled from the source.

The bottom line is to make sure that the number your quote for your fusion effort are qualified as TIER, PIE or FLUX. For our purposes, TIER is by far the most useful as it relates to the total neutron output of the device per second. In our small sizes there will never be any power production so flux isn't needed until we talk about neutron exposure for health physics purposes. Once TIER is known, you can figure flux easily for any range.

PIE is often a large and impressive figure until it is time ordered to a TIER by multiplying by the PRR.

Some example calculations to put things in perspective will follow below:

A flux of 30n/sq.cm/sec is considered the most that one should endure in a normal work exposure.

Q. At what range would this criterion be met in a fusor with a TIER of 10e6 neutrons per second.

A.
1. Surface area of a sphere is 4pi X r^2 (r= centimeters radius)
2. Our equation is flux = 10e6 / 4pi x r^2
3. Solving for r = sqrt (10e6 /(flux x 4pi))
4. r= ~51 cm or about 20 inches from the inner grid region (assuming point source.)

So, with a fusor producing 1 million neutrons per second (TIER), to be relatively safe for short term exposures, one would only need to stand about 2 feet away from it. (less than 30n/sq.cm/sec FLUX)

Q. What kind of flux am I getting with a 10 billion neutron per pulse blast, (10e10) PIE, if it takes 50 seconds to recharge my capacitor ( PPR =0.02) and I am standing at a range of 2 meters from my pulsed fusor?

A.
1. For each pulse your are only absorbing in each second averaged over the PRR about 1/50th the burst of neutrons as time ordered flux so we are down to 2 x10e8 neutrons/per second average.
2. FLUX = 2x10e8 / (4pi x r^2)
3. FLUX = 20x10e7 / 5x10e5 = 400 n/s/sq.cm

This is a respectable average flux but nothing to run from and certainly not dangerous for isolated, short term exposures.

Q. What is a power producing FLUX?

A. Reactor cores have fluxes on the order of 10e12 to10e15 n/sq.cm/sec so you can see that the 1 million Tier contiuous fusor with a flux of 30 or the 10 Billion PIE pulse fusor with a flux of 400 are both ~12 orders of magnitude or one trillion times shy of the mark for bulk power production and none of this even speaks to efficiency which is abysmal for both systems.

So huge PIE pulse numbers at slow PRRs and large TIER numbers are just not in the power arena, even with scale ups in size.

Q. What is a very dangerous neutron flux level that would cause major radiation injury in a short exposure.

A. Medically the human body is a crap shoot. What injures one man might only make another man weak, however, it is generally assumed that a flux of 10e6 n/sq.cm/sec will leave you ill and injured physically and may even shorten your life span if exposure lasts for some minutes. No fusor, pulsed or otherwise, can hit any where near these levels and are about 100,000 times weaker than this harmful level.

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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Re: FAQ - Neutron flux vs. emission

Post by 3l »

Hi Richard:

Thank you for posting this .
I have answered till I'm blue in the face on this very topic.
Maybe you can get the flux connection established in a coherent
fashion.

Btw my state of the art is now pushing on 50 cps for 37 KV voltages.
I now use 100 nf at 40 kv ceramic cap banks with a 1 ns discharge rate. Those values tend to make easier charging with established wall power and produces a reasonable amperage for making neuts. It does not stress out my SS thyratron that charges the cap bank thru a 2uf at 50kv capacitor and the grid lasts many times longer. I still use a overvoltage spark gap in a microwave oven to trigger the fusor tho.

Only 50kv-100 kv pulse is in the one pulse per second realm today.

But that will all change when the 100kv pseudospark becomes standard practice at 30 cps.

I just bought a Hippotronics megaohmmeter that will allow pulse design at even higher voltages with homemade chargers.

Store bought resistors can't really be found over 150 kv.
Capacitor peak out at 100 kv also.
All will have to be hand made.
A tip of the hat to Jim Lux's hv voltage site.

More to come!

Happy Fusoring!
Larry Leins
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Adam Szendrey
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Re: FAQ - Neutron flux vs. emission

Post by Adam Szendrey »

Thanks Richard! Another great FAQ! :)

This one shows that its not the neutrons that are dangerous in case of a fusor, but the x-rays.
**
Though if you can pulse 1/s and each pulse produces 1e10 neuts than the TIER value is 1e10 n/sec and at a distance of 20 cm from the center of the device that should give an average flux of about 2e6 n/s/sqcm, which is twice of what is concidered harmful. That is much more admirable. Though i'm not sure it is enough to ignite a fission reaction in boron (sorry, im a bit weak on this side of the issue i have to educate myself on nuclear physics much more).
I think a next. gen. pulsed system will be capable to produce a lot more neutrons than that (i would estimate a TIER value in the 1e12-14 n/sec range). That means a flux of 2e8-10 n/s/sqcm @ 20 cm. That sounds quite unpleasant,especially the latter, reaching nuke reactor levels.

Adam
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Re: FAQ - Neutron flux vs. emission

Post by Richard Hester »

If you take 10^10 neutrons delivered, say, in a 10ns pulse, then average this over one second, the neutron flux becomes a lot more sane. It's really the total number of neutron impacts that gets you. Think of the pulsed effort as taking the one second output of a fairly weak fusor and cramming it into a much shorter interval. If the pulse rate goes up, that's another matter . Still, it would take a high pulse rate to equal the total neutron output of a good continuous fusor.
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Re: FAQ - Neutron flux vs. emission

Post by Adam Szendrey »

Hi Richard,

Well that's what i'm talking about. Crancking up the repetition rate.
I'm not sure i know why the high repetition rate is needed to equal a cont. fusor.
In Richards example the pulsed fusor has a far higher flux rate than the continious fusor, although the repetition rate is quite low (0.02/sec). Sure, if there is a cont. fusor with TIER values in the 1e10-12 range, that is far better than the pulsed fusor in the example.
But i think the flux rate we are interested in (for hybrid use) is in the 10-20 cm range. That gives much higher flux values for both continious and pulsed systems.
BTW i think a pulse rate of 0.5/sec (one pulse per two seconds) can be achieved without too much problem.

Adam
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Re: FAQ - Neutron flux vs. emission

Post by Richard Hull »

Richard Hester is correct. The continuous fusor is a lot better neutron producer per unit time-watt second by orders of magnitude.

It is a matter of watt-second or joule delivery rates for a high voltage supply in the pulsed fusor game in order to get the PRR up there. Forget the attrition on capacitors.

Such supplies are no trivial matter!

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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Re: FAQ - Neutron flux vs. emission

Post by 3l »

Hi Richard:

I reduced my caps from 100lb humongous caps to small cap banks in order to meet the charging requirement. Thirty cps is easy to meet when you are using an xray trani to charge with.
I precharge a 2uf cap with the supply then when it is full the the ss thyratron moves voltage to the ceramic caps through a small inductor choke at the required cycle rate. I have used multigap trigatron and overvoltage spark gaps to run the fusor. The joules in the cap bank are 8 joules with a 1 ns switching time that becomes 8 x 10^9 Watts.....with a slow switch . But if you use a 100 picosecond switch the gain becomes 8 x 10^12 Watts.
Richard you were right tho these supplies are not trivial to design..but I had time and a willingness to blow up expensive parts.

Enough to play with.

Happy Fusoring!
Larry Leins
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Carl Willis
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Re: FAQ - Neutron flux vs. emission

Post by Carl Willis »

Richard,

Thanks for the FAQ. A valuable resource as always, and it should be a first stopping point for anyone getting ready to measure neutrons. It is right on the money on everything.

I'll add a few fine points.

First, concerning the relationship between neutron FLUX, neutron CURRENT, and Richard's TIER. Flux is commonly given in units of neutrons/cm^2/sec, as is neutron current. The big difference is that flux is defined as the product of neutron density (number per volume) and velocity, whereas current is a vector quantity defined as the number of neutrons per unit time passing through a unit area. When most of us mention "flux", we are really thinking "current." It is a more intuitive quantity.

The problem is, neutron detectors detect flux (they don't care which way the neutrons are going) but when we use geometry and the detector output to calculate the TIER for our fusor (distance from isotropic point source, continuity principle, all that), we are thinking in terms of current. We are justified in equating the magnitude of the current with that of the flux ONLY if our point source is surrounded by materials that reflect very few neutrons, such as vacuum and air. Put absorber / reflectors like water around a fusor, or between the fusor and the detector, and suddenly the relationship between the measured flux, the current, and the TIER is not so simple anymore! The moral of the story is that when doing a flux measurement to find a fusor's TIER, keep the immediate surroundings clear of moderators and reflectors.

Finally, it should be noted that there are really two systems of nomenclature for particle rates and densities. One system, advocated by the Particle Detector BriefBook at CERN, says:

"Flux" is the time rate of change of the number of particles (dN/dt), analogous to the TIER that Richard defined.
"Fluence" F is the total number of particles impinging on an area element (dN/da); and
"Fluence rate" (dF/dt) is equivalent to neutron current as mentioned earlier.

All this last part just goes to show that one really has to look at the units to know what quantity is being expressed. The words by themselves may mean different things to different folks. What a nuisance!

-Carl
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Re: FAQ - Neutron flux vs. emission

Post by Richard Hull »

Thanks carl for the fine points. This should be read by all who read the FAQ above for as our measurements become more sophisticated, so must out ability to talk intelligently regarding those measurements. Neutrons are the first real tough nuclear particle to measure with accuracy, but just easy enough to measure indirectly that one can fall into complacency very easily.

I have mentioned before that absolute neutron numbers are among the most difficult of all measurements to make. Fortunately, we can use relative counts to determine improved performance provide our technique is fixed and rigidly replicated, measurement-to-measurement.

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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