If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It is bad practice to take the signal from an anode tap, (though very commonly done so that the tube may be hooked up by single core co-ax,) with the centre wire connected to the anode resistor in the remote tube base, and the measurement resistor in the main ratemeter housing. But you can't get a steep cliff face on the leading pulse edge like this, more of a mole-hill after heavy rain, so pulses following in quick succession will blurr and merge, and you'll be helpless in the face of pulse pile-up. However you mean to process the pulses, life will be all that much easier if they are as well defined as possible from the outset.
If you had a data sheet for your tube, it would give us what in GM talk is called the start voltage, the lowest voltage at which any conduction at all occurs, as in a neon bulb for example. If your working voltage is 390V, then the start voltage will probably be about 250V, perhaps 300V.
Your pulse voltage will then be (390V Supply Voltage - 250V Start Voltage)/45 which would be 3.1V across your cathode resistor, an easy sort of voltage to deal with. These figures are merely my guesses, but will give you some idea of the order of pulse voltage to be expected.
By taking the pulses from across the cathode resistor, you will have positive going pulses. Your first amp should be at high impedance so as to present a negligible shunt value across the cathode resistor.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
The GM tubes for my radiacmeter plateau at ~ 700V, and your graph looks like mine would just as I get to the plateau. However Proud Mary is the expert, so I'll defer to her.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Proud Mary wrote ...
It is bad practice to take the signal from an anode tap, (though very commonly done so that the tube may be hooked up by single core co-ax,) with the centre wire connected to the anode resistor in the remote tube base, and the measurement resistor in the main ratemeter housing. But you can't get a steep cliff face on the leading pulse edge like this, more of a mole-hill after heavy rain, so pulses following in quick succession will blurr and merge, and you'll be helpless in the face of pulse pile-up. However you mean to process the pulses, life will be all that much easier if they are as well defined as possible from the outset.
If you had a data sheet for your tube, it would give us what in GM talk is called the start voltage, the lowest voltage at which any conduction at all occurs, as in a neon bulb for example. If your working voltage is 390V, then the start voltage will probably be about 250V, perhaps 300V.
Your pulse voltage will then be (390V Supply Voltage - 250V Start Voltage)/45 which would be 3.1V across your cathode resistor, an easy sort of voltage to deal with. These figures are merely my guesses, but will give you some idea of the order of pulse voltage to be expected.
By taking the pulses from across the cathode resistor, you will have positive going pulses. Your first amp should be at high impedance so as to present a negligible shunt value across the cathode resistor.
I guess if the signal across the 220K cathode resistor is about 3 volts then it won't need much/any amplification and therefore problems caused by interference picked up by the Geiger tube case can be ignored. It still seems a shame not to use its inherent capacity as a shield if earthed, but if it works without it that's all that matters I suppose.
I'm thinking of connecting the top of the 220K cathode resistor directly to the non-inverting input of an op-amp buffer, omitting the ~150pF capacitor, which seems superfluous.
Op-amps I've looked at have extremely low bias current and a couple of pF input capacitance so I'd have though that was a high enough impedance to keep the Geiger pulses sharp? I can't make a judgement about the required GBP though, which makes a comparator more attractive, but then comparators don't come with such nice input characteristics. A comparator set to go high at about 1.5V seems like a good idea, so long as it isn't slowing the Geiger pulses due to poor input characteristics.
This will all get a lot easier when I get a working scope again!
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
MinorityCarrier wrote ...
Have you considered using CMOS logic for your pulse shaping network?
That's an interesting idea. I'd certainly want to include hysteresis in a comparator circuit, which is then basically the same as a Schmitt Trigger.
This pulse stretcher circuit might be handy if I've got lots of spare gates to hand anyway - Fig 12 - I noticed a circuits using a pulse stretcher somewhere online, presumably to make the Geiger clicks more audible, but my test circuit produced perfectly audible pulses without any (intentional) pulse stretching.
But Fig 10 here suggests I might need to add an input capacitor to earth for stability.
Basically I'm stuck until I get a working scope. While I wait I'm going to try to fix the old one...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There's a super circuit using a 4049 Hex Inverting Buffer/Converter in the front end of a particle detector on the cosmicrays.org site here:
As you see, it is a coincidence detector circuit, but you can erase one stream of the circuit, and the circuitry downstream of the 555 for your purpose. The use of the 555 to make a variable output pulse width offers useful flexibility for interfacing with other circuits.
You'll also find other handy and inventive GM circuits on cosmicrays.org. Some of these have no amplification at all, real bare bones GMCs using SCR circuits, which reminds me of the famous, beautifully constructed Meter, Contamination, No. 1 set of 1953, which used cold cathode valves throughout so as to avoid having to provide power for heating filaments:
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Ohh! I had one of those things years ago, complete with the green canvas carrying bag. My high school physics department were throwing it away, because they couldn't get anything out of it.
I tried to get it to work, but never could. The cold cathode tubes glowed and flickered, but it couldn't detect any radiation. (The only radiation source I had to test it was a tritium luminous sign, so maybe it just wasn't radioactive enough? But then again, the GM tube rattled and I suspected it was broken.)
I ended up taking it apart and using the (incredibly high quality!) parts for other projects.
Hope this isn't too off topic, but could you tell me more about it?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Ohh! I had one of those things years ago, complete with the green canvas carrying bag. My high school physics department were throwing it away, because they couldn't get anything out of it.
I tried to get it to work, but never could. The cold cathode tubes glowed and flickered, but it couldn't detect any radiation. (The only radiation source I had to test it was a tritium luminous sign, so maybe it just wasn't radioactive enough? But then again, the GM tube rattled and I suspected it was broken.)
I ended up taking it apart and using the (incredibly high quality!) parts for other projects.
Hope this isn't too off topic, but could you tell me more about it?
The probe head shown was for use with the M2H liquid sampling GM tube which has a capacity of 3.5ml - I say 'has' rather than 'had' because it is still in the currrent Centronics product catalogue. This tube has a little rubber cap on a captive chain, and is meant to be used vertically, the liquid being dropped in from a pipette, I suppose. (I can't imagine how it could be re-used after testing highly active liquids, which would be sure to leave a residue behind them!) I ran a routine test on an M2H (which had a CV number that I've forgotten) a few weeks ago, and noticed that each count was accompanied by a tiny neon-coloured flash in the tube made visible inside because of the tube's opaque black rubber outer shell. At some time, I intend to see how effective it is when I place a photodetector under the rubber cap, so as to have total optical isolation from the tube circuitry.
There was also a very nice beta-gamma probe in the form of an aluminium cylinder with a sliding beta window. I have forgotten what GM tube it used, but remember that it would also detect UV photons with the window opened in bright sunshine, suggesting that the glass tube would have been better painted black.
The superb die-cast construction and the white enamel coating were intended to give the instrument good survivabilitly against the shock and heat flash of an atom bomb. Valve filaments are usually the first things to pack up, so the ingenious cold cathode design, both avoided having to supply filament heating current, and fragile filaments that would give up the ghost when the blast over-pressure gave the instrument and its operator a good seeing to!
It used 150V batteries made by Crompton Parkinson, which had an outer case of heavily waxed cardboard, and I think there was a mains adaptor which slotted into the battery locker space, though I've never seen one. There may also have been a later mod to allow it to use 'D' cells, but I have no details there.
Really nasty bakelite headphones that crushed the ears, and a compartmental canvas haversack completed the basic kit.
This was the main man-portable survey instrument used in the British nuclear tests at Maralinga and Christmas Island. I believe that this instrument stayed in service in Civil Defence inventory until 1982, when it was replaced by the deliberately much less sensitive Plessey PDRM-82, an orange polycarbonate box with an LED readout in cgy.
As for the activity of your tritium luminous sign, H-3 beta emission is completely blocked by the plastic case etc, though I suppose there should in theory be some extremely feeble Bremsstrahlung from the betas impacting on the inside of the case.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Interesting, thanks!
I had a quick look on Ebay, and found a really beat-up looking one, missing the probe head, for 99p.
I also found this:
which might be useful for uranium prospecting. It's the Trainer No. 1, an extra-sensitive counter for use with weakly radioactive practice sources. The readout is in micro-roentgens per hour.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Radioactive Mineral Sample Prospecting Equipment - Advice Sought
