HPGe Repair And Fet Swap

Scintillation crystals, PMTs, voltage dividers etc...
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GigaBecquerel
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HPGe Repair And Fet Swap

Post by GigaBecquerel » 11 Jan 2022, 02:08

Hi Guys,

It’s time for a HPGe repair! This one was broken and needed a new JFET. The detector itself is not mine, but a friends Canberra GC1520 on a portable Big MAC Dewar. It came from eBay, that’s all we know about it. It looks to be in good condition, but the endcap was bent inwards, like it got a strong blow from the front. Electrically it was dead, with a constant +12 V output, zero microphonics and no reaction to any kind of test signal.

This made it clear to me that the front end JFET was dead, so I went ahead to swap it. I set up the detector on my pump and opened the plug, which made the pressure immediately jump to a few 10s of mBar, showing significant pressure inside the cryostat. Most datecodes on the Preamplifier hint to the detector being made in the early ’90s, and judging from the pressure inside my guess is that it never had any maintenance done before, or it has a leak somewhere.

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I evacuated the cryostat until my pirani bottomed out at 1E-3 mBar, backfilled it with Argon to flush residual air and repeated that two more times. After the last flush I opened up the detector endcap and had a small, but constant Argon stream into the dewar in order to keep it at positive pressure and displace some air where possible. The inside of the cryostat looked pretty ok, showing an older design of crystal holder and thermal isolation pad, but nothing to worry about. I quickly located the JFET and snipped off its legs to avoid introducing dirt and thermal issues due to unnecesary soldering. Sadly the name and all other text on the JFET was missing, so I had to gamble on the pinout. As a replacement I used a 2N4391, which is not ideal in terms of noise, but entirely adequate. As Carl Willis has shown the JFET isn’t as critical as one might think, and even with a “cheapo” audio JFET and still get a very respectable resolution. Since the gate matched on both pinouts I decided to replace it as is, and as it turned out that was the right call.

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Out with the old, in with the new!

After swapping the JFET I cleaned up the solder joints with a toothbrush and some brake cleaner, isopropanol and lastly some pure ethanol without any added bittering agents. It is very important to keep the cryostat very clean, as any outgassing will cause a local pressure rise and might result in a high voltage breakdown, which will definitely kill the fet again.

The crystal itself is protected by a thin metal plate, held by some PTFE tape. The plate showed some scorch marks, and the endcap had the fitting counterparts on inside. I think it’s fair to assume, that the bad vacuum combined with reduced distance between the metal plate and endcap resulted in some arcs, which killed the fet. I applied some more PTFE tape to prevent further issues, as well as bending the endcap out again to regain some isolation distance.

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Once I was done with these repairs I evacuated the dewar again, flushed a few times with argon and then closed it up again. With the preamplifier reinstalled it was time to test the detector, and it showed a beautiful output to the applied test signal, as well as the expected microphonics when tapping it!
After finally acquiring some liquid nitrogen to test the detector I am happy to report that it is fully operational again! It cooled down nice and quick and reported operational temperature after around 2 hours. I then applied the bias voltage and got nice, clean pulses. It’s been ~2 weeks since I evacuated the dewar and it didn’t show any signs of condensation during the cooldown, so it’s fair to assume that it doesn’t have a major leak.

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It appears that the crystal has suffered a bit, as even after a long cooling time it becomes very leaky and noisy above 3.5 kV, compared to the rated 4.0 kV on the specsheet. This is a small loss, as detectors are usually spec’d for far above the depletion voltage, so 500 V less won’t change much. All in all it still gives a very respectable FWHM of 4 keV at 662 keV, and I’m calling this repair a success.

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After working with scintillators for the least year or so it was really nice to go back to semiconductor detectors, and I enjoyed this work a lot.

This is an abridged version of the original article on my website:
https://gigabecquerel.wordpress.com/202 ... -ever-was/

Feel free to ask any questions you have, and I hope you enjoyed reading this!
Lukas

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Sesselmann
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Re: HPGe Repair And Fet Swap

Post by Sesselmann » 14 Jan 2022, 10:09

Lukas,

Yes I enjoyed reading your post, even though I don't have any experience with HPGE detectors.

I don't quite understand which parts are under vacuum. The rubber O'ring and cap with screw holes in the picture doesn't look as if it could provide for a descent vacuum seal.

What kind of vacuum pressure is required here?

Steven

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GigaBecquerel
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Re: HPGe Repair And Fet Swap

Post by GigaBecquerel » 14 Jan 2022, 19:12

Hello Steven,

Thanks, I'm glad you like it!
The O-Ring and Cap are exactly what's holding the vacuum. The crystal, fet and all are inside the vacuum, and the FR4 structure keeps it centered, as well as keeping the temperature gradient between the outside at room temperature, and the cold finger down the middle.
The vacuum doesn't have to be ultra high, a good rotary vane pump will do the job. It only needs to insulate the temperature and prevent arcovers / glow discharges, as well as keeping oxygen and water away from everything. The dewar has some molecular sieves that act as a cryopump when you fill it with LN2, so the whole system is fairly forgiving. First sign of a bad vacuum is condensation on the outside of the cryostat when it's warming up to room temperature again, as then the sieves have released their gas again, and it can conduct heat between the still cold crystal and the outside.
Generally I'm surprised how much of the internals goes against "good vacuum design", but then again I guess it just doesn't have to be that good of a vacuum.

Some HPGes have metal to metal seals everywhere and never need to be pumped again, give or take diffusion of hydrogen / helium, but most are o-ring sealed and need to be pumped every 5 to 20 years, depending on the unit, temperature cycles and many more factors.

Lukas

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Re: HPGe Repair And Fet Swap

Post by Sesselmann » 15 Jan 2022, 11:19

Lukas,

Yes I agree, strange design if I wanted that to seal properly I would have made it like a KF flange with a centring ring, such that the screws clamped the seal flat rather than just a push on fitting.

Steven

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Re: HPGe Repair And Fet Swap

Post by GigaBecquerel » 15 Jan 2022, 20:15

The O-Ring needs to be compressed, and if that's done right it seals well.
It obviously works here, and as far as I know canberra has not changed their cryostat design in over 20 years.
While the proprietary valves are very annoying to work with I am glad that they don't use common flanges and easily accessible valves, that would be a very quick way to kill a lot of perfectly good detectors.
Physicists like to fumble around with everything, especially if they don't know what it does, and something like a KF is very inviting to do something with it.
Many also check if a valve is open or closed by trying to open it...

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Re: HPGe Repair And Fet Swap

Post by luuk » 19 Jan 2022, 07:55

Hello Lukas,
Great work you did on the HpGe detector, with a very nice result!
When I was building Ge detector a long time ago we used most times amplifier with the Fet outside the cryostat, the cooled Fet was just beginning to get more popular then.
The Fet’s were very expensive 300$ (in 1982) we bought them from Canberra or Philips they were special selected on low noise and low leakage they were mostly unstamped.
If the amplifier needed to be repaired I used 2n4419 (N-Channel J-FET) just for testing because it was cheap and if something went wrong while repairing the amplifier I simply replaced it.
We found out the if we want to replace the real Fet we could also use a 2n4857(N-Channel J-FET) we did some selecting and they work almost as good as the original but cost just a few dollars and not the crazy price for an original selected Fet from Canberra or Philips.
We build later also some HpGe with cooled FET and then the 2n4857 work really very good.
Maybe it is interesting for you to test one, the next time.
Kind regards,
Luuk

Kind regards,
Luuk

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GigaBecquerel
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Re: HPGe Repair And Fet Swap

Post by GigaBecquerel » 19 Jan 2022, 08:12

Hi Luuk,

Thank you for the FET Names!
I am never sure what exactly to use, but those are a good number of canidates.
I have heard of some fets working well at room temperature, but not working when cooled, have you made that experience at all?

Best Regards
Lukas

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Re: HPGe Repair And Fet Swap

Post by luuk » 21 Jan 2022, 06:41

Hi Lukas,
As far as I remember the 2n4857 worked also good as cooled FET.
Btw I looked at the vacuum seal that is used in your detector, interesting to see it work with an O ring, in the "old" days we used always Indium sealing, but then the hood of the detector was mounted with a flange to flange construction.
Kind regards,
Luuk

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Re: HPGe Repair And Fet Swap

Post by GigaBecquerel » 21 Jan 2022, 19:24

Hi Luuk,

All the new detectors use O-Rings, I guess both due to advances in the materials, as well as cost.
The O-Ring sealed ones also have to be pumped more often, which is another source for money...
One could also argue to keep the detectors neutron cross section as small as possible.

Lukas

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