x-ray proportional counter issues

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ChristofferBraestrup
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x-ray proportional counter issues

Post by ChristofferBraestrup »

I've recently purchased a cute little x-ray proportional counter, Philips/Norelco 62031 - 20 mm dia, 8 cm long!

Made a quick fixture for it, with the bias circuit attached below. HV filter, 40 mohm bias, and a coupling cap. I wanted to see if it was at all alive on the scope before making a more permanent box for it.

It requires roughly +1700V for normal operation, I gather from an old Norelco catalog. It's a (methane?) quenched xenon filled tube.
And it was, briefly giving pulses. Then it didn't.

it seems to go into some strange oscillating mode where the output swings 200 mv at 3ish kHz, undoubtedly the HV PSU inverter ( Ortec 456)
This PSU is working fine with all the scintillation probes I have, so I'd find it strange if it can handle high-ish currents for hours at 1200-1300V but not even microamps at 1700!

To test if it was my scope that was too low impedance to see the pulses (no preamp yet!) I tried my MCA with built in CSP - only 3 or 4 pulses over 20 mins.

Does anyone have experience in this area? What are the likely causes? Can a prop. tube go so 'bad' that it sees no pulses at all? even if it leaked, it would still act as an 'air proportional counter' surely.

Or do I just need a proper preamplifier? I've bought a couple of Cremat modules, 110 and 113, they're for sale on ebay at the moment. I will definitely build a prop. counter preamplifier once they arrive to definitively answer that question.
prop cnt bias.jpg
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Richard Hull
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Re: x-ray proportional counter issues

Post by Richard Hull »

This so obviously out of an old XRF system. I have taken these systems apart and many have Xenon filled tubes for x-ray detection. you circuit looks typeical, although a charge sensitive preamp is normally in the box with the tube before passing the signal to a full blown amp or o'scope. The 50 ohm resistor is not needed the scope at 1 megohm would develop all the voltage across it. 50 ohm impedance is normally reserved for large signal amp outputs, though NIM is typically a very oddball 94 ohm signal impedance.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
ChristofferBraestrup
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Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

Either XRF or an old school diffractometer/goniometer, I've seen these tubes in norelco diffraction systems.

I'm not sure what the tube body is, it's a very hard dark metal, not brass or bronze. Could be tungsten to absorb stray bremsstrahlung.

The bias circuit is the input copied from the schematic of the Ortec 109pc preamp - with 40Mohm instead of 100.

I've come to the conclusion that it must be the lack of preamp. Even with oscillating bias (still at 1700v) one should see pulses even if the tube was more or less just filled with air now.

If a quenched prop. Tube uses up all it's quench gas, it would still act as a proportional counter, right?
ChristofferBraestrup
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Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

I decided to splurge on this project. I got a NOS NIB proportional detector LND 45114 with Be window, and a Cremat CR-110 fpr preamplifier.

After a lot of no pulses I figured my back to back diodes in the preamplifier input for protection ruined it. Maybe too slow.

Either way with the diodes removed, I get beautiful counts with various low energy gamma sources!

I also found a male MHV bulkhead connector, so I could couple the detector directly to the preamp box! Preamp is almost self explanatory from the picture, I have about 25 Mohm as bias R, a bit on the low side maybe. I wanted to remove aging hi-ohm resistors as a variable in the debugging stage. 100-200 M would be ideal.

Will post spectra once I manage to tame the system.

What is a good metal for x-ray fluorescence spectrum test/calibration?

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Richard Hull
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Re: x-ray proportional counter issues

Post by Richard Hull »

Very nice setup and your noise should be very low due to the direct input, cable-less, connection to the detector. The CR110 is not cheap, but it is great as many folks here that are electronics capable to a limited degree have turned out fine proportional pre-amps using that simple component. Most any metal from aluminum on up should be fine. The heavier metals might be an issue depending on your x-ray source. Your spectroscope electronics must be very good with a good ADC in it.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
ChristofferBraestrup
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Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

Thanks! My source is at first just going to be an Am241 source and a foil of whatever metal to generate the XRF - so basically anything that can overcome the work function while exited by 59keV gamma's. Maybe iron would be good.

The rest of the setup follows my gamma spec rack: Ortec 572 amp, and then either a NIM Lecroy 3001 MCA or PC ND/Canberra Accuspec MCA.

My bias PSU has some issues though, upon heating it gets this oscillation. not many volts but enough to modulate the baseline of the preamp out. I will try filtering it away before trying to refurbish it.
ChristofferBraestrup
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Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

First meaningful xray spectrum with the LND45114!

Source is Am241, 2cm from the beryllium window.

Cremat CR113 preamp
Ortec 572 spec. Amp; 0.5us shaping time, 50x gain
MCA 256 conv. Gain, LLD 10 ULD 255, 0=0,
15min live time at around 40 cps.

I am not completely sure what I am looking at, the uppermost peak could be the 59.5 keV peak, reduced n intensity because some pases through the detector unattenuated.

Spectrum looks a bit like this random AM241 xray spec I found, if one can accept the tallest peak is the 13.8 or 17.7 keV line. Hm. I'll play around some more. The fact that the Am241 is deposited on the metal button complicates things because some lines may be XRF from this.
20210825_223133.jpg
reference (from researchgate):
Image
ChristofferBraestrup
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Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

Ortec actually made an application note for x-ray spectroscopy with proportional counters.

https://www3.nd.edu/~wzech/Application- ... ent-11.pdf

An Am241 spectrum is included, this looks more like it! The smaller peaks could require a longer live-time to resolve.
George Dowell
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Re: x-ray proportional counter issues

Post by George Dowell »

Nice work ChristofferBraestrup

Welcome to my end of the spectrum

Am-241 changes to Np-237 via alpha particle radioactive decay.

The lowest peaks you are seeing are therefore from the 100% Np-237 atoms, which are a dead giveaway that there are Am-241 atoms present.

Starting at is the 13.7597keV Np La2 transition X-Ray
Next higher 13.944keV is your Np La1 transition X-Ray
Then up a notch at 16.840keV is the Lb2 transition X-Ray
Above that is the17.7502keV Lb1 transition X-Ray
And finally topping the important L X-Ray cluster is 20.7848keV Ly1(gamma) Transition X-Ray.

In this context X-Rays refer to the photons generated within the atomic shell area of an atom, due to the quantum shifting of electrons from one shell or sub shell to another one, not the energy position (wavelength) on the spectrum chart. Other mechanisms can cause photons in the same area but are qualified by specific names other than X-Ray. Some sub discipline's group them all together as X-Rays but in XRF we don't.

Further on up are 26.34keV and 59.4 Gamma Rays, which also come from the Np-237, but the nucleus.

Interspersed among those may be other, usually not as tall peaks, from the safety coating required on Am-241 generally licensed sources. This can be Au, sometimes Pd, maybe others I'm not aware of.

Hope that helps.

You have the makings of a good XRF set.

Now a quick question for you, what is the clear window material shown in your first proportional tube project? Mica?

When you get good at this, you can use your new tube to look inside the window of the old tube to see if there is noble gas still inside. And if you get just a little better you can analyze the alloy of the old tube's central wire.

George Dowell
(K0FF)
ChristofferBraestrup
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Re: x-ray proportional counter issues

Post by ChristofferBraestrup »

Thank you for the info!

The details on setting up a prop. counter is a bit more exotic than a scintillation system. All info is greatly appreciated.

The original detector, i believe, has mica windows.
I've recently gotten a new detector, an Outokumpu 0490 Be-window Xe prop. counter, along with a Canberra 2006E prop. preamp, NIM amps and bin, my greatest dumpster dive achievement.
So far it looks like the new detector is the best of the bunch but I could be wrong. I will post more spectra soon.
George Dowell
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Re: x-ray proportional counter issues

Post by George Dowell »

If links are OK here's a re a few words onthe subject:

https://www.qsl.net/k0ff/0_Nuclear_Phys ... 0Probe.pdf
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