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tobias

Wed Jun 09 2010, 01:30AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

I think a good plan would be to take a few more 24h measurements to be able to evaluate the uncertainty in measurement.
I would certainly do it !
ISO GUM (Guide to the expression
of uncertainty in measurement):

Proud Mary

Wed Jun 09 2010, 05:41AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

tobias wrote ...

I think a good plan would be to take a few more 24h measurements to be able to evaluate the uncertainty in measurement.
I would certainly do it !
ISO GUM (Guide to the expression
of uncertainty in measurement):

As the count rate is randomly distributed around a mean, the minimum excess count rate that is detectable depends on the counting interval and the background rate.

In this experiment, as you will have read above, I decided to use 1,440 consecutive one minute counting intervals totaling 24hr in order to overcome the problem of semi-diurnal cosmic ray background variation.

Given the other experimental uncertainties - the 99.8% purity of the KOH sample, the scales weighing only to the nearest 10mg, and so on - I do not believe that repeating the initial 1,440 one minute counting intervals would alter the outcome in any way that I could measure, and that would have practical significance for measurements made by me in the real world. If I were publishing these results in the British Journal of Peer Review, I'm sure I'd repeat the measuring process till everyone was bored to death, or died of old age. Of course, there may have been some apocalyptic space weather event during the period of the experiment - but this is amateur science, and as no extraordinary claims are being made, no extraordinary levels of proof are necessary.

Proud Mary

Wed Jun 09 2010, 06:14AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

Excellent! I'm very impressed by the sensitivity of that tube. Is that the final instalment of Geigermania, or do you have other plans?

Yes, I'm very pleased with SI-22 - it has the right combination of sensitivity and robustness for a practical radiation tool.

I'll be constructing a rugged, waterproof, instrument for outdoor use next.

Fabio

Thu Jun 17 2010, 09:33AM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

EXCELLENT work stella, all informations will be useful for my experiments!

Proud Mary

Sun Jul 18 2010, 07:26PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

While watching paint dry on my voltage divider project, I've found time to restore and rebuild another one of the seven trashed Mini Instruments Mini Monitors I bought for Â£17 in an ebay sale of Harwell cast offs. (i.e. the main UK gov't nuclear research facility)

1279479350 543 FT89183 Gm Counters Restoration Of 3 400

The instrument on the right - a "Mini Instruments Mini Monitor GM Meter type 5.10" - I've treated as a conservation project, by which I mean that I have restored it to its original condition without modifications. I did no more than re-paint it with the same original Hammerite, replace all the electrolytics, and one or two other components that had wandered in value over the years. I fitted this one up with a Centronics ZP1430 mica end-window alpha-beta-gamma GM tube, and re-set the HT for it to 450V. ZP1430 is a rather sluggish tube - the dead time is 230 microsec - but as 500 cps is top of scale for the 5.10 Mini Monitor, speed isn't an issue here.

The instrument at centre is heavily modified. The probe is now connected by a BNC socket with dustcap, and a second BNC socket outputs 1 millisec TTL level GM event pulses which I can take to a counter-timer, as in the K-40 experiments above.

The instrument on the left has the same modifications, but with the addition of a 2kV PIV diode and a 10nF/3kV disc ceramic capacitor to double the HT voltage. Cranking the HT-set trimpot on the board now covers the voltage range 600V - 1.2kV to suit the nutritional needs of some of my more baroque GM tubes, like the giant G26 (1.15kV), and the mad blue Rumanian glass beta-gamma tubes (1.125kV) that somehow survived the Cold War unharmed. (The glass is so fragile you barely have to breathe on it to break it)

Now, I still have enough Mini Monitor spare parts left over to re-build a fourth instrument, as time permits, and have a board left over, which I am going to fit into a slim diecast metal box and use as a prospecting instrument.

And now it's time to be getting back to my Voltage Divider!

Conundrum

Mon Jul 19 2010, 06:04PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

ooh, nice..
reckon some of those 8 digit IN-30 tubes would make a good counter display?

i also have two micro tubes left (the very small Russian ones) with a centre pin, any use?

regards, -A

Proud Mary

Mon Jul 19 2010, 08:09PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

ooh, nice..
reckon some of those 8 digit IN-30 tubes would make a good counter display?

i also have two micro tubes left (the very small Russian ones) with a centre pin, any use?

regards, -A

I've got an old linear ratemeter with 6 sig figs of flying dot neon tubes in it knocking around somewhere like a ghost of the Thermionic Age.

And I do have a box of mini dosimeter GM tubes - mostly ZP1313s, last time I looked - and there's not a lot you can do with them because of their built-in insensitivity, and tiny capture area.

The challenge for the amateur lies in developing very sensitive equipment so that one can do interesting experiments with low level natural sources of radioactivity like my K-40 experiment above, with minerals, and with cosmic rays, none of which is accessible with insensitive sensors designed for other purposes entirely.

Proud Mary

Fri Jul 23 2010, 02:06PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GM Prospector
Re-homing a Mini Monitor 5.10 board

While waiting for more parts for my Voltage Divider project to arrive, I thought I'd get on with re-homing a spare Mini-Monitor type 5.10 GM board.
This six transistor board is dated July 1975.

At top right are the pads for the 18V supply, originally provided by two PP6 9V zinc carbon batteries in series. Nowadays these batteries are hard to get at a reasonable price, so I'll be using two PP3 9V 280mA/hr NiMH batteries, which will power the board for 36 hours before re-charging.

The two holes at right were for a 2P3W switch providing front panel options of OFF, BATTERY TEST and ON.

At top centre are two take off pins providing audio clicks to a 100R earphone insert fitted over a hole in the bottom of the case.

The two holes a little below the centre line and to the left took the screw terminals of a Sifam 100uA panel meter, calibrated to 500 cps at FSD.

Lastly, at lower left are the take-off pins for the GM tube.

Immediately to the left of the step-up transformer, I have added an extra diode and ceramic capacitor, doubling the voltage which is now 600V-1.2kV, adjustable by the black trimpot on the centre line. You can see how many times this has been adjusted in its 35 year life by the ablation of the screwdriver slot. I will replace this.

1279892330 543 FT89183 Gmprospector Parts Overview 400

Here's my vision of how the parts will come together in an Eddystone diecast metal box, but I haven't finalised the order of the front panel parts yet.
The two U-bolts top and bottom are the front panel protector, and will also take velcro straps from the carry case.
The other parts from top to bottom are:

Grayhill 3P4W switch.
Locking two pin audio output socket and plug for single Racal 100R earphone.
Crompton 100uA meter.
Racal right-angle BNC socket.

Rear: battery charger input socket.

1279892851 543 FT89183 Gmprospector Panel Meter Detail

Finally, we have the 100-0-100uA panel meter. I'm going to modify the circuit slightly, so that the meter will deflect to the left in BATTERY TEST mode, and to the right in RUN mode. I'll calibrate it with a known probe so that 100uA FSD = 100uGy/hr - the typical gamma activity of best Cornish pitchblende at 3cm.

Proud Mary

Sat Jul 31 2010, 01:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GM Prospector

Marking Out The Front Panel

The ebay seller from whom I ordered some nickel-plated washers for my 75kV Voltage Divider 3GZ Meter project is away on his summer holidays for another week, a fact I hadn't noticed when I placed my order, so I'll have to get on with this project in the meantime.

1280582963 543 FT0 Prospector Marking Out Front Panel Smaller 001

I needed the protractor in this marking out because the sides of the metal box are not at rightangles to the top and bottom, and the face side angle is not the same as the side elevation angle. A section through the box would take the form of a severely truncated triangle, a bit of a rum do, but the only diecast box I have to hand in this size.

1280582992 543 FT0 Prospector Marking Out Front Panel Smaller 004

I cover the work with paper masking tape so I can draw upon it with a pencil. The slight tackiness of the upper surface also stops the instruments sliding about during marking out.

Quite a bit of room is lost at the interior corners, where the lid screw channel runs down them. I mark out not just the holes to be cut, but the area of the surface to be taken up by knobs, bezels, and so on, to make sure everything will fit with enough room to turn the switch, plug and unplug the GM BNC connector, the headphone connector and so on.

Proud Mary

Wed Aug 18 2010, 06:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GM Prospector
Fitting Out

1282152466 543 FT0 1gmprospectorwetanddry

Sanding down the drill burrs with wet & dry in the kitchen sink

1282152499 543 FT0 2gmprospectorcountersunkclearance

After painting with Red Oxide primer and two coats of black Hammerite, I clean out the countersink holes with a drill bit, and deepen them just enough for a straight edge to pass over the bolt heads without catching. In the final tarting up, I'll paint over the bolt heads, thread-sealing and waterproofing them in one go.

1282152540 543 FT0 3gmprospectorhardwaretools

These are the tools I needed to tighten all the fastenings.

1282152571 543 FT0 4gmprospectorswitchwiringandtools

The wiring of tiny switch lugs is no joke for bovine ungulates, but I managed to get the job done with these tools and the help of a magnifying ring lamp and a 12W 12V soldering iron which I run off a bench PSU. (The cigarette lighter is for cooking the heatshrink on the terminations)

1282152600 543 FT0 5gmprospectorswitchwiringinplace

I colour coded all the wires and kept a note of what was supposed to go where, so I wouldn't get the switch wiring all in a muddle.

1282152641 543 FT0 6gmprospectorpcbinstalled

I added a new waterproof Racal 60R single earpiece - bottom right - as an afterthought, for use on the bench and elsewhere where the relatively low volume would not be a problem. It is wired in parallel with the locking circular audio output connector on the front panel. This late modification meant that I had to secure the PCB out-of-rightangle, but this cosmetic blemish has no effect on functionality, and is invisible from the outside.

1282152690 543 FT0 7gmprospectorfrontpanel

And finally the front panel, pictured here at about twice life size.. From left to right we have:

Circular locking connector - audio O/P to single earphone.

Switch: Off-On-Battery Test

Meter: (+ve) 100uV = 100uSv/hr gamma*, (-ve) battery test, FSD = Full Charge

BNC Rightangle Connector with dust cap for GM probe.

*The scale is non-linear and bunches up towards FSD, a matter of no importance in prospecting, where one simply collects interesting samples for later examination in detail on the bench.

Still to do: (1)make a constant current battery charger module using an L200 I & V regulator IC, and fit it into the rear of the case close to the power input socket (2) Make a clamp to hold two PP3 9V NiMH batteries against the back wall.

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