Standard counters

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

Re: Standard counters

Postby Doug Coulter » Sun Feb 06, 2011 2:46 pm

Now we just have to get you a scanner, doggone it! The one I use is from one of those all in one cheapo inkjets where they give you the thing, but charge more than cocaine for the ink. I don't use the printer anymore (have a color laser), but the scanner is nice...and works with all my software as well. I'd bet some stuff in that book belongs in our library. I'll ask Bill if he can find a copy when he gets back in town -- he really gets around.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Standard counters

Postby chrismb » Sun Feb 06, 2011 2:52 pm

Don't panic. I have a good scanner at work.
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Standard counters

Postby Doug Coulter » Sun Feb 06, 2011 3:17 pm

Ah, Hitchhikers guide to the galaxy, eh? I have the original BBC tapes of that one.

In other news, I noticed something cool last night on the prototype for the standard counter. I've been running 320v on it, and just because it was easy, used a .47uf polypropylene capacitor as the supply filter for the HV (have a ton of them). The charge on that cap will run the counter for minutes before the pulses go below 50v! So maybe it's not such a dumb idea to think "battery operated portable" if the uP in there can simply turn on the power supply only at low duty cycle, and itself go to sleep (leaving the hardware counter running) when computing isn't required.
Cuts the power drain way way down.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Standard counters

Postby johnf » Sun Feb 06, 2011 5:14 pm

Here the aricle from the book

scan1.pdf
(1.81 MiB) Downloaded 353 times
johnf
 
Posts: 433
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand

Re: Standard counters

Postby chrismb » Sun Feb 06, 2011 6:12 pm

{Thanks, John. Quick work - quicker than Amazon!!}

This was, essentially, much the same scintillator-sandwich construction I was planning to try but with indium for near-real-time response, and a smaller setup due to probable higher sensitivity [a quicker decay of the In116 than silver's shortest, therefore higher beta flux] and the fact that I'll only have a small PMT/crystal.

In my calcs, and due to the desire to cost-save on precious metals, my back-of-envelope suggested scintillator thicknesses of 5 to 7 mm would be a more optimum balance of beta capture versus beta's crashing into the next foil along. There again, I was looking at a graph for indium's 3.3MeV betas when I did that. All the same, I'd liked to have seen a calc to show why they went for 3.2mm thick scintillator wafers.

So this setup is 1 cps for a 5 half-life fluence of 3 neut/cm^2 [during 5 half-lives of the fastest decay rate, I think they did].

[cf. Jon Rosentiel's indium activation detector at 2.35 cps for total fluence of 490 neut/cm2 for 5 half-lives of In116 14sec, or 1 cps for 200 neut/cm^2. Not sure if you can quite compare like-with-like]

Given the base trade price of that much silver is $3,500, I'll be sticking with my few oz. of indium for now!
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Standard counters

Postby Doug Coulter » Sun Feb 06, 2011 6:41 pm

Thanks John!


Right, the expense of that vs one small silver sample and one cheap piece of HDPE, plus a cheap, standard geiger counter (more reliable than phototubes) is what motivated the design I've been working on. Something that big and fancy and expensive can't be a standard for us -- no one will have it due to $$$. Interesting that they mention "setting the threshold is critical" as I've mentioned occurs with phototubes, and also that they used a "long counter" presumably a BF3 or 3He, to calibrate this -- shows which one they consider more sensitive and trustworthy, though as far as I know, no one has really well-calibrated either of those types either. All based on assumptions all the way down until you get to the point of thermocouples and scales to get real numbers, then work backwards.

I'm not sure why they say the side blanket shields from epithermal and thermal neutrons from the sides, when the same stuff doesn't do it from in front -- that's just weird, doesn't make sense.

BTW, here a piece of UHMW HDPE 6" diameter by 3' costs $300 all by itself. I know as I recently bought one for my 3He tube to replace the wax I'm using now. Ne-110 isn't cheap either by the way, probably costs similar to the silver in this design.

This also makes my point that if you want more sensitivity, it's on the end of collection, not later detector sensitivity. I'm not sure it's needed anyway. Actually, I'm real sure it's not. If you're not doing better than a fusor that still needs many, many orders of magnitude to get to breakeven....what's the point? Chris, go ahead and run your thing! Get over the performance anxiety! You might be wasting time and money on something you don't need and can't use -- if the device works, and I think it can be made to work, myself. We were able to see activation on our silver with impure D and only 16kv in our first fusor just fine, and that's real small potatoes compared to now. An amount that would barely put 3 bubbles in the most sensitive BTI, or less than 1/10 mrem.

I'm not being convinced here. Looks to me like any extra sensitivity they get is due to sheer money thrown at it (to the point where a 3He tube is much cheaper, they're only about $1500 for a decently big one, new, I checked and they are available again -- and Tech just got one), and the ability to count that first couple seconds after the source is cut off to see the short stuff off silver.

This was meant to be put in a beam of neutrons -- obvious from the sheer size, which wouldn't intercept any much extra from our apparatus, which are nearly point sources.

So, I'd say it is a cool thing (and especially nice for Lerner's toy which is inherently pulsed) but too rich for my blood entirely. I'd bet the cosmic background is huge -- sheer area there, and that variations in scintillation from the betas that scatter out of the plastic without depositing full energy mean if you get above the point where you can count each count (rather than looking at total current) you're hosed with this design.

In 100% of our tests, silver is much better (sensitivity) than indium unless you count the indium for a very long time. I am highly suspicious of counting things from metastable states, as things other than neutrons can create those.

Now, there's this football game on that everyone in the US has to watch or be shunned from all public discourse, so I'm outa here for awhile.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Standard counters

Postby chrismb » Sun Feb 06, 2011 7:03 pm

I agree with the adage that it is better to be able to discriminate from background than to have higher sensitivity.

One thing to note/ask - why do folks seem to prefer UHMWPE for moderators? HDPE is denser, so you get more H in there.

According to my data sheets;

UHMWPE is 0.93 g/cc == 8.3E22 hydrogen atoms/cc

HDPE is 0.95-0.96 g/cc == 8.5E22 hydrogen atoms/cc
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Standard counters

Postby Doug Coulter » Sun Feb 06, 2011 8:09 pm

I think there is such a thing as UHMW (ultra high molecular weight) High Density PE, which is "both". At least that's the catalog listing...what do I know? That's about what I know.

At any rate, according to Doctor C. Willis, it's the total density of both hydrogen and carbon, which does some of the moderation too. And this stuff is heavy stuff (goes to see if it even floats),
and it does, barely, a 5" long 4" diameter piece in a bucket of ice cold water sticks about 1/4" out of the water, so your density numbers appear to check. Interesting that it was stable long way down in the water, which was quite cold (the bucket was brought indoors to thaw out from solid ice).

The point being that the smaller it is, the shorter the "moderation length", the less scattering out of it there will be as well -- important here since we only have neutrons coming from one direction. At any rate, the ~1.5" thickness was calculated by Carl to be about the best you could do to put the most neutrons into the silver/indium resonances as possible at a few ev, and I check that with actual measurements here (it's not critical at all, though). There is so much plastic in that paper design that I'd guess most of the neutrons are down to thermal, well below the max sensitivity point for either silver or indium for most of it -- the Ne110 is also a moderator plastic.

I doubt there's a serious difference between them other than the value of the scraps around the shop. The UHMW stuff is very like super strong teflon and has a lot of uses for things that slide, or need HV insulation, as well. I can tell you it is a son of a gun to work with in sizes over 4" diameter. An extremely sharp Japanese wood saw simply skips over this stuff, doesn't even leave a mark. My metal bandsaw cuts it, kind of, but tends to melt it and get stuck in the cut, which is bad for both. I finally cut a piece off that long rod I got (for a B10 tube) with a chainsaw, and nearly ruined that when the molten plastic jammed up in the clutch housing and re-hardened. And basically, if you can't melt it, you can't cut it. Drilling that same piece ruined my arm running the drill press table up and down to clear the molten chips before they seized the drill and to put on ever longer drill extensions -- took about 4 hours solid hard work to make an 18" long hole through it -- about half an inch at a time.

I have some offhand near-evidence of how good this is as a moderator compared to wax. I have my 22" long 3He tube in wax, 6" diameter, and almost touching the tank where the action is.
On the floor, about 4 feet from the action on the opposite side is a 16" B10 tube in this HDPE stuff, also 6" OD. It counts 1/2 or 1/3 as fast during a run. We know the B10 tube isn't supposedly as good as the 3He....I have them on stereo channels in a little amp with speakers I built into my equipment rack, which is how I got that -- pure "feels about like 1/2 or 1/3", no real numbers yet. (In fact, I've been building tube to ttl preamps for both this weekend so I can put them into my data logger stuff for the next run -- as they are, they won't work a logic level counter and I've just been using them on audio for realtime feedback as I adjust things). Very interesting the things one's ears can tell you about time correlations in bursting.

I was scratching my head about their claimed 50cps background in that article until I remembered they had set threshold above the positron annihilation energy. Else I was wondering how they got it so low - here a tube with a 4" sq scint set to "normal energies" counts thousands/second sometimes, and about 100 always -- with a lot less square area involved. They are incorrect about being able to count 10 meg cps reliably with that scintillator, it's not fast enough, the tube's not fast enough and so on for there to not be very significant pile ups at that rate. Merely having tubes and logic able to go 10x the count rate isn't good enough by far; you're already missing some counts due to pile ups. The linearity curve starts to look like log(x) instead of x.

Doesn't matter to us. We can always run shorter times, wait longer to start counting (and get the more reliable longer half life stuff alone), move it farther away....
I've been running 5 minutes mostly as a convenient runtime, and taking about 20 sec to get to start of count (which time is logged so I can know it exactly). I should be able to put up some plots soon, but I'm going to finish the neutron tube counter electronics before I run again. When the tech guys were here, I had to do a short run due to equipment issues, but got to the 700 cpm region with the system as is in about 4 minutes of running. No fancy counting for 10 minutes to curve fit a scatter plot needed at all for that, it was way above the background.

Note data at this link, which shows what happens when an H atom captures a neutron (it happens enough to require enriched U to make a reactor with regular water). That's one hot gamma, and might itself be used as a neutron detector just fine, indirectly! I'll soon be reporting on what I see with one of my NaI heads during a run -- I'll set it the same way so the 511 kev gammas from Cs137 don't fire it, and see what we see. I couldn't tell much last run (the tech guys were here) but it looked fairly promising.
Pretty much everything that captures neutrons does that, so borated wax and a high threshold gamma counter might be ideal -- if the fusor itself didn't make so many, and enough EMI to make using phototubes troublesome.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Standard counters

Postby johnf » Sun Feb 06, 2011 8:42 pm

As far as I know they are almost identical except on how the molecular chains are crosslinked ie UHMWPE has much longer chains of PE atoms that are crosslinked to other long chains .This is probably what causes it to be so troublesome to machine /cut
johnf
 
Posts: 433
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand

Previous

Return to Metrology

Who is online

Users browsing this forum: No registered users and 5 guests