Home Made Scintillator

[John_Poleto]

Hi,
If anyone has any details on a homemade scintillator that they have made, i would very much like ideas...I would especially like more information on John Hendron's daughter's home made scintillator. I mainly would like to know what kind of scintillation fluid/plastic works the best for neutron detection, and what kind of Photomultiplier tubes are used. One thing that i am especially interested in is where one can purchase a survey meter that one could hook up to a home made scintillator. I have read about people using a ludlums survey meter, but these seem to be very expensive, howeverr not nearly as expensive as a complete unit from ludlums. I am fairly constrained on money, so the cheapest possible solution would be the most usefull to me.
Thankyou,
John

[Starfire]

The Scintillator use’s a BC-720 look-a-like ( the white ringed plastic button ), made by Tom Dressel. It is of the highest quality due to Tom's meticulous attention to detail and care during manufacture. May I suggest you contact Tom – you won’t be disappointed. You can also make your own, but there are many techniques required. You can also buy clear Scintillating plastic from http://www.oetech.com The plastic Scint’s have a better response to Neutrons and higher rejection of Gamma. Check out the many postings and Richard Hull’s ‘Nice URL on Scintillation materials 2004-01-29 16:06 ‘ in this forum. To check out the many past postings on this subject in this forum – hit the black right arrow at the bottom and surf the past postings. You will find masses of great information of the features and performance of BC-720 clones and Hornyak buttons, both commercial and amateur built. The moderator to slow down the fast Neutrons to thermal Neutron energies can be any hydrocarbon, such as paraffin wax. The Plastic Scintillator must be optically coupled to the PMT with an optical gel. The PMT should be a ‘ Bialkali ‘ tube and must be screened against Electrostatic and Magnetic interference, as it is a very sensitive instrument. Read up on their use and don’t power up when it is exposed to even room light levels. There are many postings by Richard Hester on the great work he has done with simple HV power supplies, pre-amps and SCA’s to make a powerful detector system. Experience with an instrument such as this, is a great basis for understanding the properties of Neutrons, Gamma etc. and the more serious measurements obtainable from sophisticated commercial instruments and MCA’s. It is also fun to make and gives great satisfaction ( I have built several others than Tania’s ) – I heartily recommend building at least one.

[John_Poleto]

On the ortech site, i cannot seem to find just the scintillation plastic....am i missing something? Also, with the mentioned PMT, can this be used with any PMT base what includes a preamp? I noticed that ortech sell PMT bases, that have a direct BNC plug out, which is nice, so that one does not have to fabricate one.
Thanks,
John

[Starfire]

Opps sorry john - try;- http://www.oetech.com I always mix them up but ortech has lots of useful info which relates.
Some of the bases have pre-amps or amps builtin also different PMTS may have different pin outs - you need to check. --- A number of bases with pre-amps were on ebay reciently

[John_Poleto]

This brings me to another question...tow, actually,
I read in the past forums about home made scintillation, and the person who made the paint can scintillator, which was my first idea, said that his design worked only for the detection of gamma rays and muons. his set up used a PMT and the scintillation plastics available from oetech. will the same plastics work for the detection of neutrons? also, i have been un abel to locate a "bialkali" PMT. is it absolutly neccisary to have this kind, or can i use another? I found a site that specializes in vacuum tubes, nixie tubes, crt's, PMT's and the like, and they have a PMT for $85 that has a 3" base, that operates at about 1,250v, and the website says that it is often used for scintillation...will this tube work?

[Richard Hester]

Check the past posts - it's all explained. Plastic scintillators are good detectors of fast neutrons due to recoil protons generated by elastic collisions between the neutrons and hydrogen nuclei in the scintillator plastic.

[Tom Dressel]

The BC-720 clone is made with clear casting resin doped with ZnS phosphor (purchased from Richard Hull) poured into the teflon "negative" mold shown on the upper left of the attached photo. This produces the doped core.

The doped core is then back filled with clear resin in the mold shown in the upper right. The resin and catalyst mix must be degassed in a vacuum and mixed to provide a SLOW (24 hr) cure to prevent cracks.

The walls of the molds are tapered at 3 degrees and the aluminum rods in the mold center allow for extraction of the casting. The finished scintillator is 2" diameter 5/8" thick with 1/8" thick solid and doped faces. The clear face is coupled to the PMT with OCM pocket-pond silicone gel http://shop.store.yahoo.com/pocketscope/pocketpond.html

For some reason I cant remember the ratio of phosphor to resin but I think it was a 5% mix. Maybe Richard Hull could coment.

Tom Dressel

[Starfire]

John check out http://www.hpk.co.jp/eng/products/ETD/p ... uction.pdf it will give you a detailed explaination of PMTs. The bialkali tubes ( page 3 of the PDF ) have an better responce to the blue and UV end of the spectrum and are fast. Can't comment on the tube you refer to -- need its number etc.

[John_Poleto]

How do you make the molds for the scintillators?

[Tom Dressel]

I would nave preferred to use a lathe but my brother has the family machine shop and he lives 25 miles away. I actually used an end mill which is a flat bottomed cutting tool that will cut an its side (like a router bit), in a drill press. My "rotery table" consists of a dowel pin on a board clamped to the drill press table. The dowel pin runs through the center hole in the mold. The drill press table was tilted at 3 degrees to get the taper. Each channel in the mold for the core required two passes one to cut the outer wall and one to cut the inner wall. I used a paint program to make a series of concentric circles that I printed out to use as a template. Fortunately the teflon block is easily machinable and it only took a few min. to make.

I tried to use high density polyetheline for the mold but I couldn't get the any of the castings out of the mold. Even with the teflon I still used a silicone mold release spray.

Tom Dressel

[John_Poleto]

Where can i get a block of teflon, in order to make the mold? Also, is the 3 degree taper abolutely neccisary?

[Tom Dressel]

Look for a plastics supply house in your area. The 2-1/2" diameter rod was a special order $40.00/foot. In a pinch, I could sell you a foot, but it would be expensive to ship because it is suprisingly heavy. Where do you live?

I think the taper is necessary. It is called "draft" in molding jargon, it allows you to get the casting out of the mold .

Tom Dressel

[Richard Hull]

The bi-alkali is the tube that responds the best to all plastic scintillators and to theZnS flashes. You should use only a bi-alkali tube.

Making a loaded Scintillator is definitely a mission that is full of extra effort and some hassle.

Richard Hull

[Tom Dressel]

Richard:

What is the weight percentage of phosphor in the doped core of the BC-720?

Tom Dressel

[ningauble]

The taper is necessary, my 5" mould does not have it and
requires grease pumped in from below to hydraulic the moulding
out of the mould. I'll have to try some slow setting resin
sometime though, I made mine fast and hot and had a cracked
base which i sanded off once it was backfilled.

regards
Mark Harriss

[Richard Hull]

I never knew this and have no way of figuring it out. I don't think the weight enters into the picture at all, as one is just trying to get maximum surface area of the phosphor staring up into the PMT. I would think 1000:1 might be about right. 1000 parts resin to 1 part phosphor.

If I were making these puppies, I would make part A with all the grooves and ridges, then shoot the works with a spray adhesive and sprinkle the phosphor on in bulk and then dump it all out. This would leave all surfaces coated. I would then just fill the remainder with clear resin.

Richard Hull

[Richard Hester]

I've been going by the Hornyak button proportions listed in Marion and Fowler. They cite a 5.7% by weight doping for Hornyak buttons to detect Mev-energy neutrons. Since the BC-720 and its clones work on the same principle as the Hornyak button, I've opted for the same doping level.

[Richard Hull]

For a Hornyak button, yes, the ratio is somewhat critical as they have to have a transparency of sorts making a milky thin mass.

The advice in Marion and Fowler's "Fast Neutron Physics", vol I is good, I am sure. The Hornyak button has the disadvantage that the pulses are never all that uniform with the background gamma levels miming real neutron hits on the far side of the button. (attenuation of the light signal) This makes discrimination a bit difficult to near impossible in a mixed gamma and neutron field, which is often the case in the real world, but not so much for we fusioneers.

In the real BC 720 the phosphor is absolutely opaque!!!!! This is merely a buried surface of phosphor bound up in a proton rich clear soup.

Richard Hull

[Richard Hester]

I mentioned the Hornyak button specs as the only guide I have seen for reasonable phosphor-plastic ratios. The original Hornyak formula had a much higher percentage of phosphor. The altered formula was based on empirical observations. With the thin sections of doped plastic used in the BC-720 scintillator and its clones, self absorbtion and scattering is less of a problem. However, as thin as the sectors are in my clone detectors, you still can't see through them. ZnS has a radically different refractive index from that of the plastic matrix material, so light scattering in the mixture is pretty intense.

[Tom Dressel]

The doped core was made by using a 5.7% by weight of the phosphor uniformly distributed throught the resin, not just sprinkled on the surface. The fact that the clone performed essentially the same as the original makes me think that the original BC-720 has a uniformly distributed phosphor also.

Tom Dressel

[John_Poleto]

I think i may have found a sloution for my neutron detection issue. on ebay, i found an eberline model PNC-4 neutron counter. as i am unable to locate any information on this counter, becase the eberline site does not have information on it, nor do any other sites, i was wondering if this would be a suitable neutron detection device. Attached are pictures of the unti selling on ebay.

[Richard Hull]

The thing appears to be functional and the seller talks like he knows what is going on. I have never seen a PNR-4 attached to a detector base, ala PNC-1. Still, this may be a new one on me. It could indeed have a moderator base with BF3 tube in it.
I wish the guy had given a 3/4 or side view.

I see it is already up to $255.00 If it is truly functional and complete it would be worth up to $300.00, I would say.

The PNR-4 usually demanded a rem ball for detection. Not shown for sale here.

I would be wary here. You are on your own.

Richard Hull

[John_Poleto]

I asked the seller if it was a complete unit or not, and he said that he bought it from eberline this way, and that it is a complete unit. He also said that it works, because he has a pulsing neutron source that he tested it on, so i know now that it is a fully functional complete unit.
-John

[Richard Hull]

Good luck to all bidders on this one.

Richard Hull