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Proud Mary

Mon Jun 25 2012, 09:14PM

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

IntraWinding wrote ...

Could you direct backscattering electrons away from your Beryllium window by applying to it a suitably large negative bias voltage, reducing the heating effect and thus enabling a thinner window to be used?

That is indeed the modus operandi of most end-window tubes, where backscatter is steered back to the anode, and so is removed from the window's thermal budget.

In practice, this means that Be windows of as thin as 75 Âµm can be used, so much longer wavelengths can exit the tube.

As you see in this graph, the 75 Âµm window is able to transmit useful amounts of photons down to about 1.5 keV, whereas the 300 Âµm side windows were found to be effectively opaque below 2.3 keV in my first experiments above.

Needless to say, I have several end-window tubes whose emissions below 2.3 keV I will be investigating in this thread once I've sorted out the ionisation chamber to detect them.

Proud Mary

Tue Jun 26 2012, 10:29AM

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

From Russia With Love

Arrived in the post this morning.

1340705217 543 FT0 10 Gigaohm Glass Upload

10 GÎ© resistors - body length 45mm

I measured these with the Takeda Riken, and they both came in at 10.7 GÎ© - so they're throwing in a free 700 MÎ© with each resistor!

Now I have the dimensions, I can get on with the circuit layout.

Proud Mary

Wed Jun 27 2012, 12:03AM

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

TIA Circuit No. 3

This circuit is about getting the femtoamps to queue in an orderly line and not drift off while they are being counted. It's not much different to
the circuit with which I started, and changes - anti-drift measures - have been about lay-out, mechanical stability, cleanliness, and the dielectric properties of materials.

Proud Mary

Mon Jul 02 2012, 11:32AM

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

I am on my summer holiday and will put up the photos of the prototype front end board when I get back.

In the meantime:

radhoo

Mon Jul 09 2012, 02:02PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

what's the tolerance for those high ohmic resistors?

when I last checked ebay, I could only find 5% tolerance resistors.

ps: make that from russia with love for money :)

Proud Mary

Sun Jul 15 2012, 11:26PM

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

radhoo wrote ...

what's the tolerance for those high ohmic resistors?

when I last checked ebay, I could only find 5% tolerance resistors.

ps: make that from russia with love for money :)

Just a very quick reply as I am still away on holiday. The tolerance on the resistors was determined empirically by me to be +7.0 % using a Tadeka Riken high ohm meter with a guard electrode wound round the outside of the glass tubes after cleaning with methanol and baking. Sample was n = 2. That's all I know, Radhu!

Conundrum

Sat Aug 11 2012, 08:47AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Just a thought, what about immersing the resistor in dry vegetable oil?
That ought to work, at least for a while.
-A

radhoo

Wed Nov 07 2012, 09:23AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Hi Stella,

It seems we now share a common goal, as I also plan to build a ion chamber - at first just for testing and learning, as I know almost nothing about this field. Judging from my Geiger Counters experience, I expect this to be challenging as well.

I got myself nice 100G and 1000G glass resistors, with a few more on the way (had some luck finding them, as someone scrapped those from working equipment). I'll be starting a new thread, to keep this one clean, but I would surely love to see this thread revitalized.

Proud Mary

Wed Nov 07 2012, 07:19PM

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

Hi Radhu,

I haven't made much progress with this because I have had to put a lot of work into other areas of my life, as we all have to do sometimes.

I have had especial difficulty with reliable dosimetry below 4.99 keV using the standard ionisation chamber configurations, and was wondering what to do until I re-read the details of this Hamamatsu ultra-soft source.

Notice that the measuring device is nothing more than a metal plate 150mm*150mm of capacitance 20 pF charged to 1 kV. Hamamatsu is a world leader in radiation sensor technology, yet they are using what is basically a gold leaf electroscope for measurements below 4.9 keV!

I guessed that the distance between Hamamatsu's plate and a second plate must be 10mm, and that is what it turns out to be where

C= K*Eo*A/D, where Eo= 8.854x10-12

where:

K is 1.00054
A is 22500 mmÂ²
d is 10mm distance to a second plate of equal area.
C is 19.9 pF - the capacitance between the two plates.

If it's good enough for Hamamatsu, it's good enough for me, so I've started to look at the problem from this new direction, and hope to have something to show you and Plazmatron by Christmas.

All the best, Stella.

radhoo

Sun Jun 09 2013, 04:07PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Hi Stella!

I was wondering if you managed to invest more time into this , for some additional progress?

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