BC-720- Pic du Jour

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DaveC
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BC-720- Pic du Jour

Post by DaveC »

Richard -
Thanks for that "scintillating" photo of your big detector. Your note about the ZnS + Ag screen, reminded me that electron microscopes use the same screen type for the back scattered electron (BSE) pickup. A smaller diameter pmt is there, but the same principle. On the SEMs, the zinc sulfide screen has an overcoating of Aluminum for stray charge drain. So I am guessing the the silver is just a light evaporated coating for the same purpose. And I think you are right about the phosphor being in a light binder adhesive. It probably is just a good grade of thin polyurethane varnish.

Dave Cooper
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Richard Hull
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Re: BC-720- Pic du Jour

Post by Richard Hull »

The silver in the ZnS is a dopant/activator! It is not a coating or overspray. All phosphor/scintillators are doped to enhance their efficiency. They are said to be "activated". As little as 1 part per billion will make a dead chemical a phosphor!!

The manufacture of phosphors is still an art form. A short, but interesting article in an old "Bell Jar" by a hands on guy told how to make your own phosphor, (the easiest) and it was tough!

The normal nomenclature for a phosphor or scintillation material is to give the chemical composition followed by the activator, in parenthesis. i.e., NaI (Tl), ZnS (Ag), Polystyrene (anthracene), Gadolinium oxysulfide (Te), etc.

No source in the literature, that I have ever seen, explains the activation process to my complete satifaction though the poor qauntumists try valiantly with appropriate hand waving, offering theories, feeling that anything photonic, unknown and mysterious falls within the perview of their theory.

How one atom in a pile of tens of millions can tell all the surrounding atoms they are sensitive is spookey. Quantumists won't hear of an aether, but they have stuff so spookey it makes the simple assumption of an aether pale in comparison. Mind you, I am no friend of aetherists either, but have always had the sense of the the pot lecturing the kettle for its blackness. Therefore, I place many of the more bizarre tenets of quantum theory in the same file bin with the aether and attempts at natural philosophy.

Anyway, I have some classic papers on the destruction of the sensitive sites in phosphors by radiation done in the early part of the twentieth century. The radium dials of WW II aircraft no longer scintillate and the term "burned up phosphor" is used to describe this characteristic.

Whether it is the activator that is changed or the host chemical itself, is still debated.

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
DaveC
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Re: BC-720- Pic du Jour

Post by DaveC »

Thanks for the elaboration... I need to read up on the scintillator screens, I guess, because the issue of the dopant didn't cross my mind when I was writing.

But zinc sulfide screens are also used without dopants, too, aren't they? They've been around since the earliest days as energy detectors.

Regarding the ether and such, I am convinced that one neglected aspect of molecular structure is the possibility of strong coupling. With metals, the outer shell electrons are nearly free -(Femto-volts move them!), but with insulating and semiconducting compounds, there seems to me to be no reason at all why outer shell disturbances (in these cases of the valance band) should not couple across a few hunderd molecules. Thus a single atom every so often...( ppm weight doping gives one dopant atom per one hundred surrounding atoms in 3-D) , which is not so far away that the electromagnetic coupling would be insignificant.

I don't know enough about how the metallic dopant works, but it is tempting to try to understand as similar to doped semiconductors.

But thanks for the clues on the role of Ag. I guess since these detectors are used in air, there is no real issue of charge drainage, so a thin conducting overcoating is not needed.

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Richard Hull
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Re: BC-720- Pic du Jour

Post by Richard Hull »

Actually, scintillation screens in the early days (late 1800s) used natural zinc sulfide (sphalerite) this stuff was impure and naturally doped. It turns out they discovered this quickly and sought out and found the best impurity was silver. From here it was "off to th' races" as this and that chemist piddled to find the perfect ratio. By 1910, it was over and scintillators were a fixture, though a total mystery.

ZnS responds to many metal additives manganese is another common additive for a different color scintillation (wavelength).

Rare earth compounds and dopants are king today.

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: BC-720- Pic du Jour

Post by guest »

I strongly suspect that the material used in the BC-720 is something other than polystyrene. The principle upon which this device works requires an optically clear material with high proton content and low energy storage (read rotten fluorescence). Polystyrene is very good at storing energy (lots of conjugated rings ) for transfer to a dopant with visible fluorescence.
I suspect instead that the material used is some sort of acrylic resin, just as used in Hornyak buttons, which work using the same principles. Acrylic resin (lucite, basically) is a good proton source, clear, and easily castable, but a lousy scintillator - just the ticket for this application. BTW, a company in Texas (Enjen, I think) makes a cross to the BC-720.
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Re: BC-720- Pic du Jour

Post by guest »

A correction, the company making the alternate to the Bicron BC-720 is Elgen in Texas. Their product is the EJ-400.
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Richard Hull
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Re: BC-720- Pic du Jour

Post by Richard Hull »

Richard Hester is probably correct on the acrylic resin bit. I had always thought the stuff was polystyrene based on my faulty memory of coversations with folks in the biz.

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: BC-720- Pic du Jour

Post by guest »

Hi Richard Hester, I was interested in using acrylic resin or polyester resin for a homemade hornyak button, would polyester be of any use?.

regards
Mark Harriss
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Re: BC-720- Pic du Jour

Post by guest »

I currently design plastic parts and am intimately familiar with all of the manufacturing and prototyping proceses. I don't know what the demand for these things is, but I would bet that it is not large enough to warrant investing in an expensive injection molding tool.

Richard Hester is most likely correct, the clear material is probably acrylic resin. This resin can be bought in pourable form from a hobby shop. What does the back of the part look like? Are the rings I see male portions of another part, or female indentions into the clear part? As to dopants. If you have the appropriate materials they can be added to the pourable resin.(unless I misunderstand) The phospor coating, if it is typical glo in the dark stuff is available commercially through professional model maker suppliers. You could easily produce a silicone mold, coat it with the phosphor and pour the doped resin into the mold. Phosphor additive comes as a powder, costs about 90 cents per gram.

Not sure we're talking about the same substances here, but it might be worth checking into if you need several of these things at $500 a pop.

I assure you that unless there is some real need for super accuracy in the mixes and tolerances on the part, that anyone capable of building a fusor could do this. Especially if you have some way of calibrating the finished product with a known source.

Perhaps I misunderstand how the dopants are added, or the need for optical clarity here, but it doesn't look like that big of a deal to me. If you have a silicone mold, you can make about 20-30 parts before the mold wears out. It is also possible to make the part oversize, machine to the proper dimensions, and polish. The silicone and the resin are both inexpensive.

Monty
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Richard Hull
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Re: BC-720- Pic du Jour

Post by Richard Hull »

The Silver dopant to the ZnS is a critical component and is usually added in a rather complicated and long running bake it process. The finished property is just a white power to the eye, but a critically doped and processed material to the quantum chemist.

We had, on the old songs fusion list ,an amateur expert in this area. Many of the old timers here will remember Ely Silk. He submitted and had published a very interesting article in the Bell Jar Vol5, #3 1996. This article was entitled "The Production of Phosphors: An introduction.

Steve Hansen, the Editor of the BJ, also has posted in the past in the vacuum forum.

Ely mentions that for repeatability and stability of finished product, the following are necessary.

Familiarity with handling and safety measures involved with highly toxic chemicals.

A high temperature laboratory furnace 1300 degree C and maintainable to within a few degrees, Quartz crucibles with covers, Lab balance, Lab glassware of various sorts, motar and pestle. Ideally you should have a 1600 degree C oven capable of operating under special atmospheres, but you can limp along on the one above.

Ely noted he had a tube furnace as well and could do stuff with flowing gases, such as hydrogen or argon in the 1150 degree range.

The article goes on to talk about how a well equipped amateur could make "Cub". This is a form of ZnS activated by Zn and the last portion of the article describes the process for Cub II which is ZnS (Mn) (orange red phosphor)

To sum it up, it ain't easy and I take my hat off to those late 19th century chemists who toughed out the recipe and processes of phosphors.

You gotta' read the article to realize this phosphor makin' biz is a separate hobby and avocation altogether!!!

Macerating various hazardous chems in a motar and baking at 1150 degrees C for 90 minutes then washing the slug in distilled water pumped with toxic hydrogen sulfide gas is not my idea of fun. Then you have to pulverize the stuff in a ball mill to get a uniform powder for application.

I think I'd attempt to buy correctly done phosphor if I had a love affair with making a BC-720 from scratch. (See my current post on "Got Phosphor?")

By the way, the ZnS (Ag) is needed because it peaks in spectral response where most bi-alkali PMTs also have their spectral peak. If the flash ain't th' right color, you might as well not flash at all.

The real BC-720 has phosphor showing from the front over all the target rings and in between, too. The back is just a solid disk of phosphor backing the target ring system.

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
DaveC
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Re: BC-720- Pic du Jour

Post by DaveC »

Regarding Acrylic resins.... polymethylmethacrylate (PMMA) - I used to mold dielectric samples using the small beads. In this form, it takes a bit of finesse to get clear, bubble free castings. We had to make SS plug molds, and use a hydraulic press with heated platens capable of reaching about 180C at 3000 to 10000 psi. The material is a thermoset resin so once cured, it retains its shape.

I think is is castable from a solution of dissolved, cured resin, but my efforts there were not very good at getting a clear preduct. But the sheet acrylic claims to be "cast" , also the rod material.

The resins I see in the local shops is either polyester, or epoxy resin( amine or amide cured)

Maybe someone here knows the details.

Dave Cooper
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Re: BC-720- Pic du Jour

Post by guest »

I am sure that you can get what ever type of acrylic resin you might need from a professional model making supply house. This is different from the local hobby shop. I know a guy who is a dealer for a couple of companies. I will ask him what the scoop is. As far a getting clear bubble free work, you need to use a pressure pot and low humidity. You can make a pressure pot from an old pressure cooker. use an air compressor to apply pressure
Tom Dressel
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Re: BC-720- Pic du Jour

Post by Tom Dressel »

I worked in the materials lab at Honeywell in the 60's. The plastics guys would mix up the two part plastics then put the mixture in a vacuum chamber, and pull a vacuum until the mixture stopped bubbling. This usually took only a few minuites. They could then pour the mixture into the mold and get bubble free castings.

Tom Dressel
DaveC
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Re: BC-720- Pic du Jour

Post by DaveC »

That would be great if it only takes a little pressure pot. I had to have very high pressures and temps, since the catalyst was activated at about 180 C, Obviously, you need also to keep air away at these temps or rapid oxidation happens.

Dave Cooper
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Re: BC-720- Pic du Jour

Post by guest »

The best way to do this is to first place the plastic mixture in a vacuum chamber to de-gas. Then cast it in a pressure pot. But the vacuum is optional.
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Re: BC-720- Pic du Jour

Post by DaveC »

Right... if it IS a two component, room temperature activated resin... then you do just light pull out the bubbles from each component, the mix them, then quick degas again.. and let it cure at atm pressure.

If the compound is cured at high temp, like PMMA (arylic resin usually is) then you can only vauum first to clear out the air and dry the pellets.

To cure it you first raise the temp to about 110 to 120 C and compress the pellets to fuse them, then rapidly raise the temp to about 180 C and maintain pressure up to about 3000 psi or so for about 5 mins and then turn off the heaters...and maintain pressure all the way to room temp.

Usually that will give a clear void free molded piece.

But the process is a bit of a black art.\\

Dave Cooper
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