Needing more than a spark test?

WobblyHand said:
My Pocket Geiger is to arrive on Saturday. Is C16 (4700pF) really hanging off the end of U3B (LM393) voltage comparator?

Edit: What is the part U4? It claims it is an LT1651, but that doesn't seem to be an active part, anywhere. Ah, an LT1615 seems to have the same outline and function. Jeesh. I think according to the numbers, the switcher would supply about 13.5V to the NJM regulator. Not positive if that is a doubler, or tripler. Tripling the voltage would make it about 39V, give or take. That's a nice high bias voltage. What's interesting, or rather sad, is there's no information, at least on the Sparkfun site on how this works, or even how it is wired. What is the function of the JP2 block? What is the baseline configuration?

It would seem the top comparator threshold is 3.089V, this is "pulse". The bottom comparator threshold is 2.686V, this is noise. I understand the comparator outputs won't be used in our application, but it would seem they are using these "signals" to validate the pulse.
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The comparator is an open collector job so C16 is part of a pulse stretcher. That's my take on it, anyway.

I believe that 4-diode network is a doubler. IIRC, I measured the bias voltage to be something over 26V, also not too bad -- but the switcher couples a lot of noise into the amplifier chain. Not too surprising, considering the high gain.

I don't know if JP2 is there for testing the board once it's been assembled, or if it is an option that isn't used. The footprint is on the board but it's not populated with anything. It looks like the board could be operated with outboard power supplies via JP2 so the detector, amplifiers and comparators could be tested before mounting all of the on-board power supply stuff. I haven't checked prices on those components so I don't know if it would make sense to do that or not. Test time can be expensive so doing it twice may not pencil out.

I think the "LT1651" is a typo. I didn't find a DS for it, either; and the LT1615 looks right.
 
Very interesting discussion. Out of my league. 100 years ago +-. in my college chemistry it was a whole lot easier to do qualitative analysis than quantitative. Some pretty crude systems were used in the good ol days. When I was in the Navy we X-rayed welds on high pressure steam lines, while at sea, with some hot radioactive slug that was kept in a very heavy container that 2 men carried with long handle out each side. The lid was removed at a distance and a long fishing rod & real used to pick the slug out of the container. It was held in front of the pipe with photo film on the back side. They would count seconds of exposure then put the slug back in the container. Processing the film would show how good the weld was. High pressure steam was about 1200°F & 1400 psi. Memory might not be dead on here but close, I think. I was an engineering officer on the carrier during Vietnam. After months at sea things would start to fail.
 
Larry$ said:
Very interesting discussion. Out of my league. 100 years ago +-. in my college chemistry it was a whole lot easier to do qualitative analysis than quantitative. Some pretty crude systems were used in the good ol days. When I was in the Navy we X-rayed welds on high pressure steam lines, while at sea, with some hot radioactive slug that was kept in a very heavy container that 2 men carried with long handle out each side. The lid was removed at a distance and a long fishing rod & real used to pick the slug out of the container. It was held in front of the pipe with photo film on the back side. They would count seconds of exposure then put the slug back in the container. Processing the film would show how good the weld was. High pressure steam was about 1200°F & 1400 psi. Memory might not be dead on here but close, I think. I was an engineering officer on the carrier during Vietnam. After months at sea things would start to fail.
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Hi Larry
At least they knew it was harmful. Lots of things were done in the early days that were pretty risky. Giving sailors brooms and buckets to "wash down" deck contamination at a test site was just a good way to spread it about everywhere. I think, by now, most educated and aware people know there is a difference between "radiation", and having some of the stuff that is actually doing it, either on you, or in you. Even so, if folk around me ask, I say "household smoke detector" instead of "radioactive Americium 241 nuclear isotope emitting gamma rays".

During this thread, it has been an amazing spell of self-education and experiment. Nobody is expected to read through the whole thing, though if you skim through, you can catch the gems. I am not, and was never an "expert" on nuclear physics, but there is enough here that has rubbed off on us to have us designing mechanical shielded mounts, and calculating about gamma ray photon energy for transimpedance amplifiers.

We are at the point where we are now reasonably sure of what we are doing, though there is still lots that can happen on the way. Good Lord - I am going to try and machine a lump of lead! Oh boy! The thought of being able to put something up to a piece of metal, and have it deliver an alloy analysis is just delightful!

Unfortunately, I can't devote as much time to it as I would like, and I am sure that is the case with most of us, but we are close. The hope is that at the end, anyone wanting to get one together, regardless their expertise in electronics, or physics, has a reasonable chance with help from here.
 
WobblyHand said:
Attempting to install LTSpice on my linux laptop. Used to have it before upgrading the OS. Installed via wine. It started the install and it offered to do an upgrade. This is as slow as Windows upgrades of yore... Downloading countless library files at 10KB/sec from the website... Feel like I have an old modem.
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Hmm - when mine updates, it's about 30 seconds. Are you sure that old Sportster is not still in there? :)
 
graham-xrf said:
Hmm - when mine updates, it's about 30 seconds. Are you sure that old Sportster is not still in there? :)
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It was a very old version that I started from. It finally updated. Strange they downloaded individual models, rather than a bigger zip file and extracting. It's up and running and I can use the tool on my old files.
 
I have attached a document I generated awhile back to help me make and assemble the source stuff and properly orient the pocket geiger board to my source assembly. It predates my real-world discovery that I needed to up my game, lead-shielding wise; but it does provide some dimensions for the aperture hole and so-called "focus ring" configuration I'm using at the moment. I'm not saying this is the way it "has" to be done, just putting it out there so folks have a better idea about my current setup.

For just a little background, I was (am) concerned regarding the relatively low count rate I was seeing with my oscilloscope so I worked pretty hard to get everything as close as possible, while still preventing the primary 60Kev xrays from hitting the detector. It looked to me like the best way to do that was to tilt the sources toward a "focus point" on the sample. Keep in mind that the sources MUST be arranged around a hole in the lead shield, and that hole is about .55" in diameter. So if the sources are mounted flat on that plate and the sample is moved to within a cm or so of the hole, that part of the sample "seen" by the detector is at a relatively shallow angle relative to the sources. Since the xray emission pattern is approximately Lambertian that means the xray fluence is much lower in the region that our detector can see. Tilting the samples toward that same view-point should increase the xray fluence there, and thus the count rate.

Clear as mud? I can post a couple of sketches if need be, just let me know.
 

Attachments

homebrewed said:
I have attached a document I generated awhile back to help me make and assemble the source stuff and properly orient the pocket geiger board to my source assembly. It predates my real-world discovery that I needed to up my game, lead-shielding wise; but it does provide some dimensions for the aperture hole and so-called "focus ring" configuration I'm using at the moment. I'm not saying this is the way it "has" to be done, just putting it out there so folks have a better idea about my current setup.

For just a little background, I was (am) concerned regarding the relatively low count rate I was seeing with my oscilloscope so I worked pretty hard to get everything as close as possible, while still preventing the primary 60Kev xrays from hitting the detector. It looked to me like the best way to do that was to tilt the sources toward a "focus point" on the sample. Keep in mind that the sources MUST be arranged around a hole in the lead shield, and that hole is about .55" in diameter. So if the sources are mounted flat on that plate and the sample is moved to within a cm or so of the hole, that part of the sample "seen" by the detector is at a relatively shallow angle relative to the sources. Since the xray emission pattern is approximately Lambertian that means the xray fluence is much lower in the region that our detector can see. Tilting the samples toward that same view-point should increase the xray fluence there, and thus the count rate.

Clear as mud? I can post a couple of sketches if need be, just let me know.
Click to expand...
As I recall, you designed this structure in SCAD?
 
homebrewed said:
For just a little background, I was (am) concerned regarding the relatively low count rate I was seeing with my oscilloscope so I worked pretty hard to get everything as close as possible, while still preventing the primary 60Kev xrays from hitting the detector. It looked to me like the best way to do that was to tilt the sources toward a "focus point" on the sample.
Click to expand...
Yes - I came to the same conclusion about tilting. The 52° I came up with was built around the constraint of having the lead shield cylinder around the PIN photodiode, the sources arranged around, also shielded from radiating backwards or sideways, and how close one could get consistent with physically fitting the sources around the detector.

Re: the low count. Is the advice to squeeze in 8 instead of 6 sources?
Are we saying we should max out with more rows of sources?
I can imagine a circular support about 75mm diameter, a shielded PIN diode in the middle, and all the remaining area packed with sources!
 
Hi Mark
Just to get me clear on the LMC662. Is it always operated with negative power input grounded, and positive power input at 9V?
Also, is the LMC662 on the signal conditioner board also like that?
 
Ordered some sources from AliExpress. Couldn't find the old smoke detectors, must have recycled them.
Should arrive in a month! While I was on AliExpress, I also ordered some glass scales for my lathe - but that's off topic of this thread.
 
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