Neutron Transmutation Doping

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Samuel Low
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Neutron Transmutation Doping

Post by Samuel Low »

So, I'm intending to finish the construction of a working fusor and a polywell with a funded student initiative I've had in school. I'm interested to place samples of raw pure silicon near the source of neutron radiation (of course assuming that is first achieved, which is the hard part!) and study the effects of doping (exposure time versus dopant concentration, assuming uniform distribution). Has anyone had any experience with this? Would be good to hear any opinions or shared advice by some of the seasoned fusioneers out in the field :)

EDIT: For the ones not too familiar with NTD, basically, bombarding Silicon with neutrons causes a transmutation effect where some of the Si atoms transmute into P atoms, creating a uniform n-type distribution across the silicon sample. IAEA has fully documented this phenomena here: http://www-pub.iaea.org/MTCD/Publicatio ... 81_web.pdf
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Richard Hull
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Re: Neutron Transmutation Doping

Post by Richard Hull »

A laudable experimental goal to be sure, but you might want a good deal more flux than a fusor can supply. What flux levels are commonly used in this endeavor. Do you know?

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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Samuel Low
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Re: Neutron Transmutation Doping

Post by Samuel Low »

Hi Richard. I'm not too sure! I'm hoping I can vary the flux level to a maximum (hoping to achieve at least 10^6 neutrons a second), and plot out the graphs of conductivities versus the neutron flux! I don't really have quantitative measure now as to what would be the maximal neutron flux, but the neutron capture rate can be estimated with the equation attached in the post. And given that Doping concentration for silicon semiconductors may range anywhere from 10^13 cm−3 to 10^18 cm−3, I suppose its just a matter of time before the cumulative captured number of neutrons equal to that of the desired concentration you need in the silicon crystal.

Another interesting application of neutron irradiation is to apply the fusor for gemstone irradiation - and doing a study on how neutrons flux levels can vary with colour intensity or the wavelength of colour observed... This is another interesting application of the fusor. There's a wiki article on it here.

https://en.wikipedia.org/wiki/Gemstone_irradiation

These dopant techniques seem bent on transmuting different objects into various isotopes for various purposes. Perhaps it could be applied to making medical isotopes too? Just some ideas to share. Would like to know the opinions or experiences if anyone has any to share! :)
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neutron flux calculation.png
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Rich Feldman
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Re: Neutron Transmutation Doping

Post by Rich Feldman »

Short answer to Richard's well-pointed question: a boat-load of neutrons. More detail below.

Neutron transmutation doping isn't mainstream in microcircuit fabrication. Never was. But since the 1970's, it's been a critical enabler for electric-utility-scale power semiconductors. For example, thyristors in high voltage DC power transmission. http://tdworld.com/sponsored-articles/conversion
valve-suspended-from-ceiling-of-valve-hall_20150202.jpg
Nowadays the individual devices, each using a whole 6 inch wafer, are rated to block at least 8000 volts and to conduct at least 4500 amps. It still takes dozens in series to make a single "valve" for, say, a 400 kV converter station, designed and budgeted to operate for decades. Those mainstream 12 inch fab lines, cranking out billions of IC's for smartphones, tolerate process parameter gradients that would be show-stoppers in the power switching world.

It's that market where neutron transmutation works routinely to produce wafers with exceptionally uniform and well-controlled n-type doping. Thermal neutrons transmute 30Si to 31P in place. But you need a real nuclear reactor to get enough neutrons in a reasonable amount of time. Here's one set up for the job: http://nrl.mit.edu/facilities/ntds

>> its just a matter of time before the cumulative captured number of neutrons equal to that of the desired concentration
Yup. There's a handy chart in this note: http://www.topsil.com/media/56052/ntd_a ... er2013.pdf
doping.PNG
Exercises for Samuel:
1. Say there's a fusor releasing 10^6 neutrons per second. You put some silicon 10 cm away. How long will it take to deliver 10^16 n/cm^2 ?
2. Does the capture cross-section formula you posted agree with the dose values in the topsil table?

[edit] Remember to factor in the natural abundance (3.1%) of the target isotope in silicon. http://www.tracesciences.com/si.htm
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Re: Neutron Transmutation Doping

Post by Dan Knapp »

I recall that one of the Japanese groups doing IEC work published on transmutation doping using an IEC fusion neutron source. I don't have the reference at hand, but one could probably turn it up with a google search.
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Re: Neutron Transmutation Doping

Post by Richard Hull »

I am at a loss for the "neutron dose" part of the table. What does it mean??? Perhaps a flux on the first line of 10e13n/cm2/sec for 86x10e3 seconds? If so, total neutron bombardment might be a better term. Doses are usually giving in rem or grays with RBE adjustment.

If it is total bombardment then we know that only a tiny fraction actually interacted with the very low cross section sillycon and dopants. Still, I guess you have to quantify the work in some fashion that is easily replicated and controlled.

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Re: Neutron Transmutation Doping

Post by Rich Feldman »

Perhaps Topsil was a bit loose with their terms. Could have said fluence instead of dose.

Looks like the practical NTD business (for Si) needs thermal neutron fluences that start around 10^16 cm^-2 and often go to higher exponents.
Dan, please let us know if you find that paper connecting NTD with IEC fusion.

The word dose is pretty broad, and generally depends on qualifiers (often implicit).
Tables in safety standards give, for neutrons at 2.5e-8 Mev, a Q factor of 2 and a neutron to rem factor of 980 million.
So the biological injury from 1e16 n/cm^2 would be an Equivalent Dose of 10.2 million rem = 102,000 sieverts.
The unweighted Absorbed Dose in tissue (not silicon) would be 51,000 grays.

51,000 grays means 51 kJ/kg are absorbed, enough to warm tissue by up to 12 °C if done quickly. I don't think that's why the neutrons are called thermal. Therapeutic diathermy is better left for non-ionizing forms of radiation. 1e16 n/cm^2 to whole body would probably leave your corpse pretty hot in the sense of being a radioactive hazard.
Last edited by Rich Feldman on Tue Oct 18, 2016 8:01 pm, edited 2 times in total.
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Re: Neutron Transmutation Doping

Post by Dan Knapp »

The reference I mentioned on NTD was a presentation at one of the US-Japan IEC workshops:
http://www.iae.kyoto-u.ac.jp/beam/iec20 ... ts/P-2.pdf
It looks like the publication of this work was in Japanese.
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Re: Neutron Transmutation Doping

Post by Dan Knapp »

I found that Hotta's group published a paper in English on this in 2010:
IEEJ Transactions on Fundamentals and Materials 130(9): 787-792, 2010.
But I don't have online access to this journal. I've requested it from my library and will post it here if they can turn up a copy. Unfortunately, this project is likely dead because Hotta retired recently.
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Re: Neutron Transmutation Doping

Post by Samuel Low »

Hmm... at 10cm away, there'll be about 796 neutrons per centimetre squared at that intensity... that will take orders of magnitude of 10^13s just to dope a concentration of 10^16 P / cm^2 on a silicon wafer... which is about 400,000 years of running the fusor just to dope a silly lump of silicon... and that is assuming a 100% one-to-one interaction... Seems the performance of an amateur fusor will be far from capable of making its name heard in the semiconductor field. Thats a good question to provoke thought, thank you Rich :)
Looks like the practical NTD business (for Si) needs thermal neutron fluences that start around 10^16 cm^-2 and often go to higher exponents.
Dan, please let us know if you find that paper connecting NTD with IEC fusion.
Yes... and it seems fission reactors are more capable of producing in those amounts...
I found that Hotta's group published a paper in English on this in 2010:
IEEJ Transactions on Fundamentals and Materials 130(9): 787-792, 2010.
But I don't have online access to this journal. I've requested it from my library and will post it here if they can turn up a copy. Unfortunately, this project is likely dead because Hotta retired recently.
Thank you for the share Dan! I am looking for literature on this right now. Seeing if there is a way I can play with the system to make an improvement somewhere. Seems they use Fission Reactors for this process...

Is there ever a record set for most number of neutrons produced in amateur fusion so far?
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Re: Neutron Transmutation Doping

Post by Richard Hull »

Fission reactors are the only solution to such continuous flux levels on earth.

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Fusion is the energy of the future....and it always will be
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Re: Neutron Transmutation Doping

Post by Dan Knapp »

Turns out the 2010 Hotta group paper was in Japanese also. There was an English abstract:

Neutron Transmutation Doping (NTD), which is one of the semiconductor manufacturing methods, can produce impurity semiconductor with high quality. The neutron source currently used in the NTD is a nuclear reactor, which has some problems such as to become too old for use. Hence development of a new neutron source, which enables uniform irradiation, is desired. A new Inertial Electrostatic Confinement (IEC) device with a coaxial double cylindrical structure, which is especially designed to be capable of uniform neutron irradiation, was developed. As the feature of the device, it has triple electrode structure which consists of a cylindrical grid cathode between inner and outer anodes. Neutrons are centralized in the center of the device, and uniform neutron flux is obtained there. The device achieved a neutron production rate of 1.5×106 n/s, and uniform neutron flux distribution was provided in the central hole of device. The largest size of the uniform neutron flux area was 35.2% (25 cm) of sample irradiation area in the axial direction, and 54.3% (10 cm) in the radial direction.

As you can see, their neutron production rate was nowhere near what would be needed for NTD.
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Re: Neutron Transmutation Doping

Post by Samuel Low »

Interesting... I found the article online too, the one you just shared, Dan. Thank you very much, it sheds light on how far we are from using IEC devices for neutron production!

Richard, do you happen to know the highest record for neutron production in an amateur fusion device? Would be interesting to know how far out a fusor can go even with all the engineering limitations :)
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Re: Neutron Transmutation Doping

Post by Richard Hull »

Steve Sessleman keeps the records on this, but I think that on this site, 2-3 million neutrons/sec emission (isotropic) is the maximum range, but this is typically only limited by the ultimate HV the fusioneer has to apply to his fusor. (Thus far, this maximum is in the 40-60kv range for amateurs here)

We are limited to D-D fusion, by law in the case of what fusion fuel we can easily aquire and by economics keeping us from using D-3He fuel. I imagine, with a 100kv power supply, an amateur might be limited to under 10 million neutrons/sec (isotropic).

A good, honest evaluation would say that 1 million neutrons/sec is easy to achieve by a determined fusioneer with decent, but not perfect, apparatus provided his power supply could readily reach 40kv and supply a stable 20ma of current at that voltage. Using this same setup, and operational experience, over time, close to 2 million neutrons per second might be possible. Again, the limit here is set by the high voltage supply and limitations of the original insulation within the fusor chamber and wiring.

As I have noted many times here.....At any given degree of perfection of equipment and any given limitations in the applied voltage and current, the maximum performance at this point is solely a matter of operational experience by the amateur fusioneer.

There is some artifice involved in operating a fusor, beyond the physics and the science of it.

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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Samuel Low
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Re: Neutron Transmutation Doping

Post by Samuel Low »

Thank you Richard, for once again sharing the insight and wisdom. Its truly appreciated.

Artifice is a good word indeed... :)
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Re: Neutron Transmutation Doping

Post by Richard Hull »

Thank you for recognizing "artifice" for what it means in relation to a fusor. I often wonder if it is understood at the core level by many here.

The fusor is an item that works due to the physics and scientific principals only!! No art involved at the theoretical level.
However, the operation of a fusor by the human hand is an art. The fusor operates on a thin sliver of gas discharge physics that is a moving target as operation progresses. The human operator is the active hysteresis loop in this operational regime. Learning how to be a human bungee cord within the operational loop is an art. The active and successful operation of a fusor demands artifice.

This artifice can be hinted at in the demo fusor as the operator spends time trying to achieve star mode and observing gas discharge characterisitcs at different levels of evaucation. Moving on to the fusor, ever more critical control is demanded, but the basics should be in place to succeed having already run and studied the demo fusor.

It took me two years of demo use to, finally, do fusion and 2 more to do it well and another 2 to perfect technique. Over this course of time I was out in front and had no forebearers or extensive knowledge base beyond my own reading and hands-on-imperative. Fusor net's informational volume grew as I did and ultimately I learned even more from my betters and more extensively learned that showed up here in the 2004 time frame and to date.

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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