New Developments in Neutron Detectors

[Dan Knapp]

I just returned from a summer school on neutron detectors (NDRA2016) where I heard four days of lectures on the current state of development of neutron detectors. For those interested, the presentation slides and poster abstracts are available at:
https://agenda.infn.it/conferenceDispla ... nfid=11646
I had hoped I might hear about some new simple, inexpensive, off the shelf detector that would be useful to the fusor community and not have the short lifetime of the bubble detector. Unfortunately, I didn't; but I did learn about some new developments that would be within the reach of the amateur scientist who is willing to do some construction and experimentation. Indeed, work on detectors is one area where a fusor could be applied in ongoing experiments once the builder has an operating neutron source.

The work on neutron detectors continues to be driven primarily by the shortage of helium-3. The price of helium-3 has actually come down some in Europe. Having shot up from about 8€ to over 3000€ per liter, the price has recently leveled off at about 2000€/liter. In addition to being an obstacle for large volume homeland security applications, this still high price is also an obstacle in large array detectors; so there is much work being done on new detectors. Most of the development work is centered on detectors using (n, alpha) reactions on the high cross section isotopes boron-10 and lithium-6 with the resulting alphas (and other energetic nucleus products) being detected by gas, scintillation, or semiconductor detectors.

Boron-10 is being used in all three detector types with sputtered boron carbide thin films being used in gas detectors and on semiconductors. The amateur could use natural abundance boron with 20% boron-10 and just get a less sensitive detector. While sputter coating is within the reach of the amateur with a high vacuum system, and one can buy small boron carbide grinding discs to use as sputter sources on eBay, sputter coating does present additional technical challenges. Very fine boron carbide powders are readily available for polishing and lapping. It would be interesting to see if the amateur could produce functional thin films from such powders.

Lithium-6 (usually as lithium fluoride) is also being used in all three types of detectors. The most interesting report in terms of what the amateur might do was a report of putting Li-6 on bare silicon photodetectors (these can be purchased bare). Solutions of LiF were painted on the silicon surface and dried to make thin films. The speaker said he actually did the preparations in his office with no sophisticated facilities. The Li-6 was purchased as LiF from Sigma-Aldrich for $7 per gram (which is quite cheap, and a gram would be enough for lots of thin films) but he bought a kilogram to get this price. It is probably sold in smaller quantities, but another problem for the amateur is that Sigma-Aldrich usually won't sell to individuals.

The amateur could use natural abundance lithium and get a detector with about 7% of the sensitivity of an enriched Li-6 detector, if one could obtain material with actual natural abundance. But there is a problem in doing so. Since purified lithium isotopes (both 6 and 7) have been produced in large scale, and the depleted material is still chemically valuable lithium, many of the lithium chemicals being sold are actually enriched (or depleted depending upon your viewpoint) in one or the other isotope. The buyer has no way to know unless one is purchasing an enriched isotope with an abundance specification. Very large amounts of lithium-7 fluoride have been produced as potential coolant for molten salt reactors, so there should be lots of lithium-6 enriched by-product out there somewhere. If the amateur could obtain some of this, he could get enriched Li-6 without paying pure isotope prices. Does anyone out there in the fusor community have connections in this area? Alternatively, someone with the appropriate mass spectrometer could check the isotope ratio of some commercial lithium salts and might get lucky.

A detector type not yet mentioned is the proton recoil detector. People are putting thin plastic (CH2) films on silicon photodiodes and detecting recoil protons from fast neutron collisions with hydrogen nuclei. The amateur could conceivably take plastic encapsulated photodiode wafers and remove all but a thin film of the plastic with solvent. The plastic encapsulated photodiodes would probably even give some response with full thickness plastic from recoil events close to the silicon interface.

The darling of the new semiconductor detectors is single crystal diamond, but cost is a major obstacle. Given that (polycrystalline) diamond film coated machine tool cutters can be had at affordable price, it would be interesting to see if an amateur might be able to make a crude neutron detector by adding a second contact (using the metal tool substrate as one contact). The diamond detector is the ultimate in simplicity with just a thin crystal (undoped) and two contacts. Fast neutron collisions with C produce charged particles for detection. These detectors are not vulnerable to the radiation damage silicon detectors experience in high neutron flux conditions, so they are of considerable interest in the fusion community. Some people are making semiconductor detectors by just painting contacts onto the wafer. I've seen small diamond crystal wafers on eBay in the past; an amateur might just add contacts to such wafers to produce a detector.

Gas proportional counter detectors are available on eBay occasionally at affordable price, and they can also be made relatively easily. Using a boron carbide film in such a detector produces a neutron detector. Counting gas and plumbing is an added cost, but one can buy small argon tanks and regulators on sale at Harbor Freight (and using those 20% coupons!), and some of the welding shield gases can be used for counting (see further below).

Finally, one of the most interesting reports was a poster describing the use of "thick" gas election multiplier (GEM) devices made from standard two sided PC board material by drilling an array of holes. GEM's are atmospheric pressure charge amplification devices that exploit the electrical fields produced by small holes in a dielectric with conductor layers on each side. Originally they were expensive devices produced by microfabrication methods. They can now be made by just drilling PC board material (using micro carbide drills from Harbor Freight - keep clipping those coupons!).These devices are described in a review available on arXiv (A. Breskin, et al. A concise review on THGEM detectors). In the reported work, a layer of boron carbide was used for neutron to charged particle conversion. They used 10% CO2 in argon as the detector gas, which is easily obtainable at reasonable cost as a welding shield gas.

In any of these detectors, one can still face the problem of discriminating signals from neutrons and gammas; but in many cases, the speakers implied that the detector was sufficiently specific for neutrons as to not require peak shape discrimination, or that simple single channel analyzer thresholds suffice. Gammas not produced within the detector can also be blocked by metal shielding that will still pass the neutrons. While peak shape discrimination can be a complex subject, and most modern detectors now do it digitally, it can be done in analog mode with readily available NIM modules. Retired time of flight mass spectrometers have produced a ready supply of constant fraction discriminator modules on eBay. It would be good if someone knowledgeable in the subject (I'm not) could write a FAQ on how to configure a PSD device with NIM modules.

In summary, while there is still no other neutron detector comparable in cost and simplicity to the bubble detector, there are a variety of types that would be within the reach of the amateur scientist to build; and development work on detectors could be a subject of ongoing investigation once one achieves the goal of building a working fusor.

[Richard Hull]

Great report and update Dan. Thanks for taking the time and effort to explain, in simple terms, where the art in neutron detection is making some headway. 3He will be expensive virtually forever. We used up the world's supply after 9-11 making giant 3He tubes for portal monitors, until someone decided to reorder 3He and found out there was none to be had. Marketing and government mandates on the stuff took over from there pushing the stuff into the stratosphere in Europe and on specific allocation here.

Richard Hull

[Jerry Biehler]

Boron carbide will be a little difficult for most people to coat. It is an insulator so you will need a RF source to do the excitation at probably around 1-200 watts for a 2" magnetron gun. You also need to mix methane with the argon. The problem there is methane with catalyze vacuum pump oil which will turn the oil into your vacuum pump into a giant block of plastic. Fun!

So oil free or fomblin pump and a turbo. We typically sputter at around 8mtorr for most materials so while a rouging pump might be able to get down there I would recommend having a turbo and a throttle valve.

[Dan Knapp]

Jerry
What is the purpose of the added methane in sputtering boron carbide?

[Steven Sesselmann]

FWIW I don't really understand why fusioneers want high efficiency He3 neutron detectors when even the cheapest russian corona tubes are at least 125 times more sensitive than a bubble detector.

It may be satisfying to hear a lot of clicks, but any reasonable fusion effort can be measured with a low efficiency counter like the SNM-12. If anyone is interested I can supply a complete detector or for the DIY'ers here I can supply the pre-amp only or just the board for the tinkerers, just send me a PM.

Steven

[Jerry Biehler]

Jerry
What is the purpose of the added methane in sputtering boron carbide?

Some materials will try to revert to simpler forms when sputtered or evaporated. Silicon dioxide and titanium dioxide like to become SiO or TiO which for optics is a bad thing, so a small partial pressure of 02 is added to the process to keep it in the desired forms, or you can use an ion gun like I am putting in my system.

In the case of boron carbide the methane provides extra carbon to keep the boron combined, I believe.

Yikes, Lesker wants $660 for a boron carbide target, 2" x 1/8". Oh well, cheaper than the Pt/In target we have at work, $6k!