## FAQ - BTI bubble detector mathematics

Richard Hull
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### FAQ - BTI bubble detector mathematics

This is a simple exposition dealing with the use of a bubble detector as a crude, but effective standard for recording the isotropic neutron emission from a small, simple, spherical fusor of the type encountered most often in amateur construction.

It involves little more than simple algebra in a solid geometry problem. I will noodle it out tediously in a slowly derived manner rather than give a reduced equation. In this fashion it will ground all in the interesting twists of turning absorbed dose recorded by the BTI detector into flux and vice versa, ending up with isotropic emission rates for the fusor.

This FAQ is based solely on the use of the Bubble Technology Industries (BTI) model BD100R fast neutron bubble detector which has a binned response of 33 bubbles/mrem absorbed dose.

http://www.bubbletech.ca

Fast neutron data:

1. The normal D-D fusion process produces mono-energetic, fast neutrons of 2.54 mev.

2. The normalized fast neutron flux that allows for a dose RATE of 1mrem/hour for neutrons of 2.54 mev is ~8 neutrons/sq cm/second. (You would have to be immersed in this flux for 1 hour to recieve a one mrem dose of fast neutrons in that square centimeter.) It is important to realize that this is a FLUX to DOSE RATE comparison for D-D neutrons only.

3. The BTI BD100R gives a TOTAL ABSORBED DOSE reading ONLY based on 33 bubbles/mrem absorbed!

Relationship BTI to Flux........

Example #1

Q. The BTI detector is seen to collect 15 bubbles over a 60 second exposure in a neutron field. What was the flux at the dosimeter related to this recorded dose?

A. 1/60th of an hour exposure of the BTI detector resulted in a 15/33rem, (0.45mrem), exposure. So it was subjected to a flux of 60 X (0.45) X 8 neuts/sq cm/ sec = 218 n/sq cm/second.

Example #2

Using example #1 if we were 20 cm away from an idealized point source of neutrons what was its isotropic emission rate?

A 20 cm radius (40 cm diameter) sphere has 4X 3.1416 X (20)^2 = 5026 sq cm surface area.

This results in an isotropic emission of 5026 X 218 = ~1,096,000 neutrons per second emitted from that source.

Let us take an example that might exist for a real fusor....

Example #3

We record 28 bubbles in a ten minute run from a fusor that is 6 inches in diameter with the BTI detector placed at 25.4 mm from the fusor wall and at a temperature that allows it to perform at unity rating based on its temperature compensation chart. The unit is thermally shielded and fan cooled below the fusor to maintain the unity temperature state.

1. The 6" fusor has a radius of 3" or 7.62 cm. The BTI detector is, thus, located 7.62 + 2.54 = 10.16cm from the center of the device.

2. This 10.16 cm radius sphere has a surface area of 1297 sq cm.

We record a 1/6 hour exposure dose of 28/33 rem for a flux of

6 X 8 X 28/33 = 40.7 n/sq cm / sec at the range of 10.16 cm from the idealized source. Thus the fusor was producing

1297 X 40.7 = ~53,000 n/sec.

There is much room for skewed error in this as all the neutrons are not produced at the pinpoint center of the device. This would tend to drive the figures computed above, downward.

Likewise, the fusor is tough to keep stable during a ten minute run. There are often times during a run when little production of neutrons occurs as one loses control for a number of seconds to bring the device back to useful output. This result, averaged over time, shows that the fusor must have far exceeded the above emission figure for some periods in order to make it supply the recorded dose in the BTI detector.

Temperature compensation.........

Example #4

Lets us say that the ambient temperature is high and that you have no cooling other than a thermal shield and fan to stabilize the BTI device to ambient. A glance at the compensation chart supplied with the device shows it will respond at a level of 1.26 times normal.

What would be the net output as calculated in example #3 under this elevated temperature conditions.

53,000/1.26 = ~42,000 n/sec.

You can see that the difference is striking. You MUST record and adjust you BTI readings to the actual temperature the device is maintaned at during a recorded run!

Still, we have a crude, average emission rate which was satisfies our original goal.

Due to the very nature of the device and operator skill, a device may operate at the same voltage five times in five weeks or even in a day and give 5 vastly different results based on varied pressures attained over the runs and any number of other operator and fusor operational variables. Thus a fusor is not limited to a specific output at a specific voltage. This is an important point to remember. You can almost always do better, even if voltage limited, up to a point.

Given the above caveats and acceptance of the limitations, one can now co-jointly run a highly sensitive electronic detector like a large, tanked He3 detector along side a BTI detector. Provided the electronic noise can be warranted to be minimal to non-existent in a quiet environment and data taken over several calibration runs, one can arrive at a cross calibration standard for the electronic device that will at least be linked to a noiseless fast neutron detector.

Example #5

A He3 detector during the above #3 example is run co-jointly and records a net count of 7,350 counts over 10 minutes. What data can we attribute to this counter, assuming no noise errors?

We can say that we recorded 7350/600 = ~12 counts per second at its range (whatever that was). In future "quiet" runs, provided we don't move the large, tanked device, we can say that for each count per second recorded on the He3 counter we will record 4417 neutrons isotropically emitted from the device during that same second.

Standardized measurements.

There is a movement about that is driven by the desire to be able to compare one fusor's efficiency or output to another one operated by a different person within this group. Such an effort, while laudable, is tough to achieve in the strictest or absolute sense. This is due to "luck of the draw" operational characteristics, skill of the operator, purity of vacuum, differing voltages, geometries, currents and gas pressures.

With the foregoing in mind, a standardization to a single detector such as the BD100R binned to 33 bubbles per mrem by the bulk of fusor operators will indeed allow some comparison that is real and meaningful. This thankful result is mostly through the fact that the bubble detector is a true fast neutron detector and responds only to D-D fusion and little else. (If you have bubbles you have neutrons and, thus, fusion.) The number of bubbles will give, along with other critical operational data, a fair idea about how well or poorly a fusor is functioning.

More significant is the fact that the bubble detector is absolutely noise free and is the only viable pulsed fusion fast neutron detector.

These results will not tell anything about what was done right to make a fusor function well or what was done wrong to make it perform poorly, but might offer clues based on the entire data sheet offered up with the bubble detector's data.

It is my recommendation that only the bubble detector be used for taking reliable noise free neutron data. However, a very sensitive neutron counter (He3) might well be sought out and purchased as an adjunct, cross check counter to work along side the bubble detector. Such ultra sensitive, large detectors can be used in very low level neutron field where a bubble detector might take a long time to acquire a single bubble.

Standard bubble detector distance from the fusor wall..........

Standardizing this parameter is something we might want to discuss so that I can append it here at some point in the future. However it must be kept in mind that the closer the bubble detector is to the source, the more bubbles we get and the more bubbles we get, the less error is inherent in a reading. The bubble detector placed near a hot, running fusor might drift into an area where its response is continuously changing over a measurement period rendering the device's recorded results rather useless.

The above militates a distance that is close enough to allow for maximum response but distant enough so that simple cooling efforts and protective thermal shielding can keep the device at an even temperature throughout the run.

We actually do not need to do this standization of distance for the detector at all, provided you fully understand the above math and can alter the figures accordingly to bring any distance between the fusor and your detector into line with the other parameters. It is far more important to remember to get as close as possible to the device with the detector while keeping it thermally stable, at a fixed temperature.

Credit for assistance and corrections**************

Thus, far, I would like to thank Carl Willis for catching some errors on my part (now corrected) and for his help in keeping this post accurate.

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.
DaveC
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### Re: FAQ - BTI bubble detector mathematics

Perhaps I missed this in the several discussions, but is there an estimate on the lowest energy the bubble detector will respond to? Seems like if the radiation will penetrate the container, it should produce a bubble.

I may buy a few of these for the lab at work, as backups for our std Xray detectors.

Dave Cooper
Steven Sesselmann
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### Re: FAQ - BTI bubble detector mathematics

Richard,

Maybe one should adopt a metric system.. ie..

Bubbles per second @ 1 meter

No matter at what distance one measures, as you say, it is easy to do the maths and bring it back to b/s, even if this end up being a small number for amateur fusors.

33 bubbles after 10 minutes at a distance of 0.2 m would be around 0.0022 b/s

Hopefully amateur fusors will improve in efficiency over time, so we get some real figures to work with.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Richard Hull
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### Re: FAQ - BTI bubble detector mathematics

To Dave's question........

The BTI fast neutron detector is chemically aligned to not see thermal or epithermal neutrons and has a threshold of about 50kev. After this point, the energy deposited in the medium is of such an energy that bubbles can form. It is fairly linear in its response, I am told.

BTI makes a thermal neutron bubble detector, but it is very expensive and I was told sensitive to gamma rays.

The fast neutron detector is not sensitive to gammas at all.

The whole deal is a chemical nightmare that BTI has a handle on, but limits are there. Development continues, of course. The limits of chemistry and bubble techniology impose a rather permanent limit such that energy and type of radiation is a "slip and slide sort of affair. it is a testament to the genius of the technology that it has matured as far as it has.

In short, for us, the BTI fast neutron detector senses only fast neutrons.

The BTI gamma detector detects mostly gamma, but also very high energy beta that can penetrate the plastic tube and retain enough energy to produce bubbles.

A lot of testing on our end might produce more details, but about the time we get used to a specific effect and range we might chart or discover on a delivered unit, they may change the chemistry to make any discovery on our part rather worthless.

They are only going to warrant what they claim at any given moment, not what we discover.

To Steve................

Bubbles/ meter is fine, but as you note it all has to be reduced to n/sec in the end. Folks might be better served in being forced to develop and noodle out their own math in their own way to arrive at the figure. It forces them to understand the concepts.

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.
DaveC
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### Re: FAQ - BTI bubble detector mathematics

Thanks, Richard -

Sounds like this is really a basic device whose utter simplicity is its strength. Probably more of a validation approach than an instrumentation approach.

With a 50 kev low end cutoff, for Xray/gammas, other low energy detectors are preferable.

Was kind of wondering if the internals (chemicals) were housed in a very thin walled container whether it would give lower minimum detectable energy... unless there is an internal pressure issue.. Does anyone know?

Dave Cooper
Richard Hull
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### Re: FAQ - BTI bubble detector mathematics

The active part is a water clear, fully congealed jello like substance that is immobile for the most part. The shell tubing is most likely a thick, clear, acetal, or polypropylene as it is utterly rigid with no effective give or deformation.

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.
Frank Sanns
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### Re: FAQ - BTI bubble detector mathematics

Richard,

I do think that the bubble detectors are great gems but I have two concerns.

The first concern is the temperature dependence. I can't help but feel some uneasiness with the gamma bubble detector not showing any bubbles in the moderate gamma field in your thermally cold lab this past Friday evening. I saw you put the detector right next to one of those hot insturment dials only to see nothing in the tube. Apparently on Saturday you must have done the same with a warm bubble detector and saw significant response. My concern is that if you only have one detector and it is a bubble detector then no bubbles may not prove that there is no gammas. It just may mean a cold tube. Similarly an old tube made wane in its responsivness and require a higher temperature to operate.

My second reservation is the notion that bubble detectors are impervious to any kind of "noise". The gentleman that was doing lightning research reported measuring neutrons with the bubble detector. The part that struck me as strange (well one of the parts!) was that in one experiment, one of the neutron detectors showed a large number of bubbles after a strike. This alone would seem to say that some or at least one lightning bolt produced neutrons. But there was a second detector on the other side of stroke that had NO bubbles. The gentleman seemed to think that some sharp metal corner in the vicinity reflected the neutrons all into one direction and towards one of the bubble detectors. I have a hard time buying that explaination. I think that some other kind of "noise" caused one of the bubble detectors to fill up with bubbles. Both of the detectors were apparently very close to the lightning strike so a few logical sources of "noise" are E&M (electrical or magnetic), optical, or acoustic noise. The detectors are supposedly tested to be relatively immune to E&M so that leaves optical or acoustical. Both of these are plausible in my mind. Both supply energy into the matrix that could cause a localized boiling and bubble formation and would account for the large numbers of bubbles in one of the detectors. So the detectors seem not to be impervious to all extraneous "noise" from an experiment.

Still, I think these detectors are little gems and are a great price for the money. Short of measuring lightning bolts over the north pole, I think they are nearly fullproof.

Frank S.
Richard Hull
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### Re: FAQ - BTI bubble detector mathematics

There is no issue in my mind with the gamma detectors. they need to be warm to measure lower levels, that is all. Simple really. It must be remembered that BTI doesn't push these!!! They are not in any BTI catalog! They will give no useful data on gamma energy or gamma intensity. That is pretty final!!!

They will detect gamma.....Nuff said.

Anyone expecting useful data from them is deluding themselves. At a given temperature, at any instant in time, crude differential determinations can be observed with two or more sources.

As regards the neutron detectors working with lightning strokes or high energy sparks...... I know the guy using them and I wouldn't be a bit surprised if one of the arcs stroked through or had a streamer or leader attach to the BTI that read bubbles. he did not impress me with his images that I saw or data. No report or paper issued.

Have you ever tried to have lightning or sparking occur just where you want it? Did you see his images? Directing a significant stroke is like putting a leash on a cat and expecting it to walk with you. Good luck.

For any purpose of reasonable men, the fast neutron bubble detectors are noise immune.

Certainly, they are proof against noise in fusor use even with intense localized internal fusor arcs or pulsed systems.

I would tend to question this particular experimenter's procedures long before I would question the BTI device, but then, I am familiar with his work.

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.
Richard Hull
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### Re: FAQ - BTI bubble detector mathematics

I have uploaded to the files forum, a rather large, 8-page paper that I have just completed on the bubble dosimeters for amateurs, should anyone desire a more thourough, if not more tedious epistle than above.

viewtopic.php?f=19&t=7994#p57398

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.