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Proud Mary

Mon Jun 11 2012, 01:18AM

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

Ash Small wrote ...

@ Les and P.M.: I'm sure I'm not the only one here who'd be interested to know how to convert a 'ceiling mounted burnt toast' alpha particle detector into a weak gamma ray/X-ray detector.

It may well be possible to modify a smoke detector to detect radiation other than its own built-in americium source, but I feel my chances of making reasonably accurate measurements are better if I stick with simple techniques and building blocks.

Ash Small wrote ...

Will this still require the photo-multiplier, etc. that conventional detectors use?

The detector I am constructing uses a free air ionisation chamber, which belongs to the general class of gaseous detectors, and a PMT is not required.

Despite its apparent simplicity, the ionisation chamber remains the gold standard for dosimetry because the number of ion pairs created by ionising radiation - and hence the ion current - in a given mass of a gas is proportional to the dose received. As 1 Gray is defined as the absorption of 1 joule by 1 kg of matter, you'll see that the ionisation chamber works directly with first principles - if we know the mass of the air in the chamber, and the charge collected from it, we can calculate the dose accordingly - though real world corrections usually have to be applied to account for such things as the chamber geometry, temperature, pressure, and energy dependent effects.

Proud Mary

Mon Jun 11 2012, 01:43PM

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

I've decided - for constructional reasons, and because I have all the needed parts to hand - to go with the parallel plate ionisation chamber.

1339422055 543 FT0 Parallel Plate Ionisation Chamber Concept Circuit

Proud Mary

Mon Jun 11 2012, 06:28PM

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

Ionisation Chamber Raw Materials

An Affair of Little Boxes

Proud Mary

Mon Jun 11 2012, 09:04PM

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

Triaxial Feedthrough Construction

The triaxial feedthrough provides a fully guarded connection from the collector plate through the ionisation chamber wall to the trans impedance amplifier in the box next door.

I drilled down through the centre pin of an SO239 co-axial socket, leaving the walls of the centre pin intact.

1339447541 543 FT0 Triax Feedthru Drill Press Shot Upload

I then threaded a length of semi-rigid micro co-ax through the hollow centre of the inner conductor, to create the triaxial feedthrough.

1339447567 543 FT0 Triaxial Feedthrough Upload

Note: I looked at three different sorts of SO239 socket, and selected one where I could already see a chink of light down the centre of the inner conductor, which made drilling it out the work of seconds. I chose a drill bit the same diameter as the copper sheathing on the semi-rigid co-ax, to make for a snug fit.

I first devised this technique for turning an SO239 socket into a triaxial feedthrough using thin glass capillary tubes (melting point determination tubes) with a fine wire threaded through them, but haven't used the method here because glass isn't such a good dielectric in the fA world, and is easily cracked.

Proud Mary

Tue Jun 12 2012, 11:46PM

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

I have just learned something I should have known at the outset:

For physical reasons, a proportion of the electrons are always scattered on the surface of the anode. The extent to which these backscattering electrons arise depends, among other factors, on the anode material and can be as much as 40%. In the side-window tube, these backscattering electrons contribute to the heating up of the surrounding material, especially the exit window. As a consequence, the exit window must withstand high levels of thermal stress and cannot be selected with just any thickness. The minimum usable thickness of a beryllium window for side-window tubes is 300 Âµm. This causes an excessively high absorption of the low-energy characteristic L radiation of the anode material in the exit window and thus a restriction of the excitation of lighter elements in a sample. (In Bruker's Introduction to X-ray Fluorescence)

If we assume that the Be window thickness of the two tubes tested so far is 300 Âµm, as per Bruker, then the transmission of photons below 4.999 keV looks like this:

1339543318 543 FT139308 Transmission 5 Kev 300 Microns Be

Looking in more detail at the transmission at 2.3 kV - the voltage level at which I was first able to detect photons with the ZP1430 GM tube -we see that only ~6.5% of the photons generated at the anode were actually able to exit the tube.

And of this venturesome 6.5%, only about 65% succeeded in crossing the 1 cm air gap to the detector window, as we see here:

1339543775 543 FT139308 Transmission Air 1 Cm 2

This means that of all the photons generated only about 6.5%*0.65 = 4.2% made it as far as the detector window.

Given that a 300 Î¼m window is practically opaque to photons below 2.0 keV, we see that all the activity detected
can be accounted for by photons in the energy range 2.0 keV - 2.3 keV.

Given that Bruker calls 300 Î¼m "the minimum usable thickness for side-window tubes", and I have no information on the window thickness of the two tubes
tested so far, it may be that this 2.3 kV anode potential represents their absolute rock bottom for getting photons through the window,
and is a limitation of the tube rather than the mica end-window GM detector, which might yet be able to detect photons of lower energy.

More research is needed, as papers never fail to conclude!

Proud Mary

Wed Jun 13 2012, 01:19PM

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

Marking Out the Ionisation Chamber

The interior angle of base to sides is not 90Â°! The outside angle is a rightangle, but inside the wall thickens slightly towards the base, a common occurrence in diecast metal boxes.

HB pencils can draw directly on the bare metal, but the lines are easily smudged away.

Ash Small

Thu Jun 14 2012, 12:31AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

More research is needed, as papers never fail to conclude!

P.M., just a quick question here, given your knowledge of VT technology, is it possible that a detector built around a PMT 'could' be more sensitive to emissions @ these levels?

Proud Mary

Thu Jun 14 2012, 09:55AM

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

Ash Small wrote ...

Proud Mary wrote ...

More research is needed, as papers never fail to conclude!

P.M., just a quick question here, given your knowledge of VT technology, is it possible that a detector built around a PMT 'could' be more sensitive to emissions @ these levels?

Ah, now I think I understand what you meant in an earlier question about 'weak' x-rays.

The dosimetry problem here isn't the same as, for example, trying to detect radiation from typical 'weak' amateur sources such as thoriated gas mantles and uranium glass marbles, where quite energetic rays and particles are emitted - but in the small or 'weak' quantities appropriate for consumer goods. These can be detected and measured with an ionisation chamber, but it wouldn't normally be first choice.

Now, in our case, we have no shortage of low energy photons <4.999 keV. Our sources are very bright, and at the short ranges used in these experiments have a high fluence - a high number of photons per square centimeter. To put this in perspective, the dose rate 1 cm from a thoriated gas mantle will be just a few micrograys an hour (Î¼Gy/hr) while the dose rate below 4.999 keV of a hefty electron-impact source like BSV-25 at the same distance will be in the kilogray per hour regime (kGy/hr) so that care must be taken if adverse events are to be avoided.

Now back to your PMT question. Measuring pulse height at energies below about 10 keV with a thallium-doped sodium iodide scintillator (NaI/Tl) is difficult - but not impossible - because of the short attenuation length of Grenz ray absorption, with all the impinging photons being absorbed extremely close to the scintillator surface. If this problem can be overcome, then there is the problem of detector saturation - the zillion zillion scintillations per second produced by kilogray absorption would require specially built equipment - which is why ionisation chambers are almost always first choice for measuring x-ray and gamma ray beams. There are no 'pulse pile-up' or 'dead time' issues with an ionisation chamber.

Because the ionisation chamber deals with primary physical concepts - one Gray being the the absorption of one joule of ionising radiation energy by one kilogram of matter - it is the standard against which all other dosimetry is calibrated, so-called 'reference ionisation chambers' being kept in the vaults of the Bureau of Standards in Paris.

And finally, I don't have an NaI/Tl crystal!

Proud Mary

Thu Jun 14 2012, 03:19PM

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

Ionisation Chamber First Fixings

1339686903 543 FT0 Ion Chamber Ground Floor

The triaxial feedthrough, the ceramic supports for the collector plate, and the copper mesh faradic screen over the photon port are now in place.

plazmatron

Thu Jun 14 2012, 03:20PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

@Ash,
I suppose one could coat a microscopically thin layer of a suitable Scintillator onto a PMT, and while technically possible, I would imagine it would be very expensive.

I think the main reason this has not been done, is that very little research has been carried out at sub 5KeV levels.

Ion chambers of one sort or another have been around since the dawn of x-ray technology, and as Proud Mary says, much is known about them. Using standard 'knowns' is an important practice when investigating virgin territory like this.

@Proud Mary,
very nice work on the chamber

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