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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Progress is very slow as I have lots of other things distracting me. Could I possibly take you up on your offer of the tiny HT supply to keep delays to a minimum? I'm sure my tube will turn out similar to yours.
If you'd like to send me your postal address in a private message, I'll put one in the post for you with a couple of spare GM tubes for you to play with.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Calibration Standard Preliminary Practical
Object: To better understand the size of the effects and uncertainties to be anticipated in the main calibration experiment.
Problem sticking its snout in: Semi-diurnal variation in cosmic ray strikes which is likely to impose a goofy sine wave on background radiation counts.
Method: To neutralise the cosmic ray semi-diurnal variation effect, I'll make the counting period 24hr, for both shielded and un-shielded background counts, beginning at the same time on consecutive days. I don't know the magntiude of the semi-diurnal variation effect to be expected, but this method should cancel it out whatever it is. (Measuring the semi-diurnal variation itself can be a project for another day)
Apparatus: An SI-22 GM probe is set up on a stand. I have modified a Mini Instruments Mini Monitor GM Meter type 5. 10 to output a 5V 1000us pulse into a 75R line with each Geiger event - see BNC socket with dust cap added on lower right-hand side of case.* 75R coax conducts the pulses to a Black Star Nova 2400 counter timer, set up as a totaliser.
First Counting Period Start time: 1803A 4 June 2010 End Time: 1803A 5 June 2010
And we're off: Hurrah!
Definitely the best part of the experiment, because now I can go and sit in the garden and enjoy the early evening sun with a tall glass of Pimm's clinking with ice.
* I have also added a BNC socket for the GM tube input on the left-hand side, so the tube can readily be changed, - though tube changing also involves adjusting a pot on the PCB to re-set the HT voltage to that of the new tube. The original type 5.10 design was intended to be foolproof, and my mod means that it would now be possible to get an electric shock from the BNC socket when the instrument is on, but no tube is plugged in, a very small hazard in my scheme of things.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Calibration Standard Preliminary Practical
Part Two
Time is up!
Total counts after 24hr 00' = 137, 089 Geiger events (conjectured to be made up of terrestrial gammas, and muons from above*, together with a light sprinkling of alphas from Po-210 and Pb-210 found naturally in the solder used in the internal construction of the GM tube) which is 95,2 counts per minute.
Geiger events per minute, being a result of random nuclear decay, are generally deep fried in batter - Poissonian - - but the long counting period gives our 95.2 cpm figure a juicy confidence interval.
Second Counting Period With 3mm Al surrounded by 20mm Pb shielding
Start Time: 1805A 5 June 2010 End Time: 1805A 6 June 2010
*The tubes are mounted vertically in this experiment to give them the smallest cross-sectional capture area for cosmic ray muons coming from above.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Calibration Standard Preliminary Practical
Part Three
After 24hr 00' inside the Al/Pb shield the total count was exactly:
98,800 Geiger events.
Now, in the previous 24 hour trial without shielding,
137,089 Geiger events were logged.
Thus the Al,3mm/Pb,20mm shielding stopped 137,089 - 98,800 = 38289 Geiger events from happening
And 98,800/137,089 = 72.07%. count reduction, the same as saying that
27.93% of inbound radiation was rendered undetectable.
(I'd have liked to say it was stopped, but all I know for sure is that it wasn't detected.)
This isn't exactly the amazing % discount that would make you rush off and buy that must-have summer outfit, but it is what I have to work with.
To add an additional 20mm Pb would mean adding a cylinder of lead measuring
10cm ID x 14cm OD x 36.5 cm.
I work out the volume by subtracting the volume of a cylinder of radius 5cm X 36.5cm from a larger cylinder of radius 7cm x 36.5cm
Thus I would need to buy 5621cc - 2868cc = 2753cc of lead sheet
As w = dv and d for Pb is 11.34 gm/cc
the weight of an additional 20mm of shielding would be
2753 x 11.34 = 31.22kg.
Added to the first 20mm of Pb weighing 22kg, this would take the weight of lead alone up to 53.33kg, which isn't my idea of a table top device. Not to mention the cost of a further 4,5m of 365mm x 2mm lead sheet, which seems only to come in 3 and 6 metre rolls.
So I'll carry on with the 20mm shield for as long as it's useful.
Part Four
100gm KOH was weighed into a container that made a good fit with the well in the calibration shield.
I decided to increase the sample size by a factor of 10 over my original plan for two reasons:
1. My simple Ohaus 310 balance can only weigh to the nearest 10mg, so by increasing the sample size by one order of magnitude, I decrease the significance of these 10mg increments by the same amount.
2. The difference between the shielded and the unshielded count rate doesn't give me a big margin, so I thought it best to multiply by ten the effects to be measured.
The plastic drum containing the KOH sample was sealed and placed inside the shield with the CI-22 GM tube on top of it.
(The container first came to Haus Stella full of black peppercorns from Tesco)
Count start time = 1808A 6 June 2010 Stop time = 1808A 7 June 2010
So that's that until tomorrow evening, when, hopefully, more will be revealed...
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
I'm torn between whether it makes sense to have your counter mounted horizontally or vertically to minimise cosmic interference.
Vertical minimises cross section as viewed from space, but then you don't get any lead/aluminium shielding in that direction. An equal or thicker end cap of lead/aluminium on the top with wiring exiting at the bottom would fix that worry. I have no idea how much difference it would make in practice.
I like your 24 hour compensation for the diurnal effect, but here's another effect which might scupper your results if your weather's been anything like as changeable as mine in the last few days. Worth reading the whole page in fact as this chap finds quite a lot of unexplained variation in the background count.
" ...eventually it became obvious that the background radiation can increase quite dramatically when it rains. This is likely the precipitation washing the natural Radon daughters out of the air and concentrating them nearer the detector"
Alan Yates' Laboratory - Background Radiation Monitoring
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
I'm torn between whether it makes sense to have your counter mounted horizontally or vertically to minimise cosmic interference.
Vertical minimises cross section as viewed from space, but then you don't get any lead/aluminium shielding in that direction. An equal or thicker end cap of lead/aluminium on the top with wiring exiting at the bottom would fix that worry. I have no idea how much difference it would make in practice.
I like your 24 hour compensation for the diurnal effect, but here's another effect which might scupper your results if your weather's been anything like as changeable as mine in the last few days. Worth reading the whole page in fact as this chap finds quite a lot of unexplained variation in the background count.
" ...eventually it became obvious that the background radiation can increase quite dramatically when it rains. This is likely the precipitation washing the natural Radon daughters out of the air and concentrating them nearer the detector"
Alan Yates' Laboratory - Background Radiation Monitoring
That's just the sort of reason that this section's stated objective (see above) is -
Object: To better understand the size of the effects and uncertainties to be anticipated in the main calibration experiment. - which can hardly be 'scuppered,' as you put it, by discovering what these known unknowns and unknown unknowns may be. If I knew the outcome, it wouldn't be an experiment, would it
The calculation of the the axial to radial detection ratio of SI-22 features next in the experimental series to begin this evening after I've had some fish and chips.
As for your 'rain stops play' pessimism:
"Long as I remember The rain been coming down Clouds of mystery falling Confusion on the ground Good men through the ages Trying to find a sun And I wonder, still I wonder Who'll stop the rain?"
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Calibration Standard Preliminary Practical
Part Four Continued
Girl Interrupted:"At this moment he was unfortunately called out by a person on business from Porlock, and detained by him above an hour, and on his return to his room, found, to his no small surprise and mortification, that though he still retained some vague and dim recollection of the general purport of the vision, yet, with the exception of some eight or ten scattered lines and images, all the rest had passed away like the images on the surface of a stream into which a stone has been cast..." - Coleridge
So to return to my experiment:
Start Time: 1808A 6 June 2010 Stop Time: 1808A 7 June 2010.
Total counts = 102,628
As the shielded count was 98,800 Geiger events in 24 hours, we now have a surplus of
102,628 - 98,800 = 3828 Geiger events due to K-40 1.461 MeV gamma rays
which is 2,66 gamma detections above background per minute over 1,440 one minute counting periods. (significant)
I will work up these figures, and explain their relationship to GM tube efficiency and ultimate Bq sensitivity after the next experimental section.
The thing to remember is that this niggardly excess over background was with an axial presentation of the GM tube to the source - its least sensitive direction.
Calibration Standard Preliminary Practical
Part Five
In order to measure the efficiency and ultimate Bq sensitivity of the SI-22 tube radially, I have to surround the tube with the source.
100gm of 99.8% KOH prills were used as before.
The 100gm KOH prills were put into a Tesco medium resealable food bag and reduced and distributed with a rolling pin. The KOH prills are harder than they look, and would probably puncture the bag with too vigorous an application of the rolling pin.
An old vacuum cleaner tube found on a rubbish tip was cut down to be slightly longer than the active length of the SI-22 GM tube when in its housing I had made earlier (see above). A second Tesco resealable food bag was covered on both sides with self-adhesive aluminium tape, and the bag containing the rolled KOH prills was slid into it. It was then carefully sealed.
The sealed bag was then wrapped round the plastic pipe, and self-adhesive heat-shrink threaded onto either end to secure it.
The partly assembled tube was then placed on a stand and the heat-shrink sweated down with a hot air gun until the glue oozed from the ends, looking rather like a futuristic chocolate eclair, I thought. A larger diameter piece of heat-shrink was then cut to size, and sweated onto the centre section in the same way. The finish was slightly uneven, because I durst not over-heat the assembly to shrink the centre section down evenly all round, lest steam and fumes evolve, and eject hot KOH at high speed through a vent. I'll be honest and admit that I didn't think of this risk until I'd started, so I would urge readers not to copy this particular process. There will be many less hazardous ways of making a coaxial KOH-containing cylinder, but I didn't think of them.
My effort (as in "What's that effort s'posed to be then, Stella?") was complete.
The radial coaxial K-40 1.461 MeV gamma source is ready to roll! Hurrah!
Counting period with radial coaxial source:
Start Time: 1810A 7 June 2010 Stop Time: 1810A 8 June 2010
Will the radial coaxial source produce a much higher count than the axial one for the same amount of disintegrations? How will the efficiencies of axial and radial GM tube sensitivity compare? What is the ultimate sensitivity of the tube in Bq?
So until tomorrow evening, when all this and more will be revealed exclusively on 4HV:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Calibration Standard Preliminary Practical
Part Five Continued
Start Time: 1810A 7 June 2010 Stop Time: 1810 8 June 2010
Total counts after 24hr 00' = 157,384
COWABUNGA!
The 100gm radial coaxial K-40 1.461 MeV gamma source has produced a Geiger event count over mean background of
157,384 - 98,800 = 58,584 per 24hr or
40.68 counts over mean background per minute
As we saw in my Preliminary Calculations above, my 100gm KOH sample is home to 125,900 disintegrations per minute.
The efficiency of my SI-22 GM tube when detecting 1.461 MeV gamma rays in this configuration is therefore
(40.68/125,900) x 100 = 0.0323% - very much a typical gamma figure for GM tubes.
From this efficiency figure, we can now say that 1 gamma count of the SI-22 is equivalent an activity of
1/0.000323 = 3096 Bq.
As one event is the minimum count possible, we can say that the gamma sensitivity of the tube is 3096 Bq - which is equivalent to the activity of 0.000000084 gm of radium.
And so, at last, with the aid of a little ordinary potassium hydroxide, we have turned a device that seems to do little but click at random into a useful scientific measuring instrument.
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Geigermania
- but the long counting period gives our 95.2 cpm figure a juicy confidence interval.
