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20th May 2010 18:55

Alan Yates wrote...


While I didn't use a packaged darlington like the MPSW45 I did make a Darlington from the MPSA18 and the 2N3904, the combined beta should be at least 50000, perhaps more.

It is certainly insensitive compared to an avalanche device like a Geiger Muller tube, but still "relatively" sensitive. If my Dad's Tc-99m tracer injection would set if off the next morning then it may be a bit too sensitive for the original "joke" purpose. An x-ray machine or a smoke detector element is enough to make it alarm.



19th May 2010 22:18

tarsonis wrote...

Hi !

I'm french so there are some mistakes in my comment ;)

You said that your project is quite insensitive. But I read that you used MPSA18 (Hfe 500) as detector, and 2N3904 (Hfe 100) to amplify the signal.

But the original kit used MPWS45A transistor; it is a darlington wich Hfe is pretty high (30000 min), amplified by MPSA64 (Hfe 20000 min).

With appropriate transistor (MPWS45, BC517, BC516, 2N6426etc...), you must have more sensitive ion chamber ;)

7th June 2009 22:05

vesa wrote...

For charge amplifier ideas I suggest taking a look at US pat 4053847. is your friend :^)

PS. There is still some Ru available if you'll ever need it.

11th May 2009 22:25

Alan Yates wrote...

G'day Arv,

Nah mate, not holding out on secret news of an impending nuclear exchange. :)

I guess I am just fundamentally curious about everything. I've studied nuclear physics on paper for some time, but I've never done any practical work on it. Seemed like time to try.

Many are absolutely terrified of this completely natural feature of our universe. It has always interested me, and the technology required to do fundamental investigations is pretty basic really. Particle and high-energy quanta radiation are another unseen window on the universe, it isn't all that different than my other hobbies with longer wavelengths.

Medical wise, well that's yet another geeky obsession. Biological machines fascinate me. Apart from self interest - my biology supports my mind - biological machines are just so different to engineered ones. They are the most amazing "nanotechnology" - nothing like what our current Silicon and electromagnetic-centric technology would attempt. Exquisitely complex, yet highly ordered. Messy, yet practical and creative in ways that would impress even the most jaded human programmer. Champion of the good-enough but ever improving solution, locked in eternal battle with Thermodynamic law.

Probably most of all, biological machines are a true frontier of human understanding. Physics is in a bit of a rut at the moment, although the glimmers of new physics are there... Software and Electrical engineering are more of a craft now days. Not that I am complaining, Software pays my way in this world yet is creative enough to keep me sane. Mathematics - well I'm just not talented enough at that wonderful art. I dearly wish I had been blessed with a better mathematical ability - that said it is so beautiful, if I had been so gifted I would probably have been caught up by the sheer pleasure of "doing math" and never experienced the similarly intoxicating joys of building things within this wonderful universe.



PS: Charles has lots more experience with nuclear instrumentation. His devices use ultra-low Idss JFETs (aka electrometer JFET - low leakage). Even your garden variety JFET has pretty good leakage, for a large chamber with a low JFET bias current they would probably be OK. I sourced some 80 uA Idss devices, but I haven't tried them yet. I am currently trying LMC622 Op-Amps, which have exceedingly good input bias current characteristics, femto-Amperes. Amazing stuff.

First step is to build something that can measure large resistances with some precision. With that I can use these beautiful op-amps in a transimpedance configuration and actually know what kind of chamber current I have. That should make more scientific use practical. I can calibrate the set up with natural Potassium-40 if that isn't practical, I've already observed the effect of a bag of Potassium Chloride on my Geiger tube, it is quite measurable. I plan to press some dried KCl into a puck, which from its mass and geometry I can compute its absolute activity and by controlling the geometry, calibrate an ion chamber with it.

I'll post the Geiger tube experiments shortly, they have been quite interesting. Especially fluctuations of the background with local rainfall and ventilation changes in the shack.

I suspect my next obsession might be scintillators. Apart from a desire to do spectroscopy I've long wanted to prepare my own phosphors for other purposes. It is quite an art, at least to make good ones. It may also be fairly hard to source ZnS:Ag material in large enough quantities for some of my more outlandish ideas. My previous experiments in pyrotechnic chemistry have furnished the basic chemicals and a lot of the equipment required... I also have a rather strong desire to build my own large Geiger/avalanche tubes for a correlation counting idea I have. I need an Argon tank and regulator for that, and to actually set up a basic vacuum system. At the completely nuts end of the scale I wouldn't mind trying to build a spark chamber either.

11th May 2009 04:52

Arv - K7HKL wrote...


Why the sudden interest in biometrics and radiation detectors? Do you know something about the near future that the rest of do not?

FET devices have quite high gate impedances, but admittedly not a lot of gain. Is their noise figure low enough for you to use one of these as your giga-ohm input impedance device?

I enjoy your projects. Even if I don't build all of them myself, they are still educational and quite interesting.

Arv - K7HKL