Refurbishing an Ortec HPGe detector

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Carl Willis
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Refurbishing an Ortec HPGe detector

Post by Carl Willis »

An Ortec HPGe detector from the "GEM" line (cylindrical, pi-doped, positive high voltage) came to me by way of Taylor Wilson quite a while ago, and I have had the project of restoring it to service on the back burner. But I've seen the value of such a detector first-hand at Jon R.'s lab, and that has spurred me to make recent progress.

Here are a bunch of photos and data from my efforts so far. I have many questions too, mostly aimed right at Jon Rosenstiel, but anyone else with experience please chime in.

When I first examined the detector, I determined that the input FET was blown. The original part was a 2N4393 in a metal package. I decided to replace it with two 2SK152 jfets in parallel (I have a big sack of these on hand, and the parallel option allows low-noise matching to a wide variety of detector capacitances). To make the replacement, it was necessary to open the vacuum envelope and, later, re-pump the Dewar. I opted to add a permanent pumping nipple to the Dewar rather than deal with the hassle of the manufacturer's proprietary and supremely inconveniently-located pump port. That way, future transistor replacements won't be so painful. Another component I focused on was the potted HV filter, which in Jon's experience should not be overlooked as a source of trouble. It is not as simple electrically as meets the eye. An x-ray image sheds some further detail, but still leaves this part a bit mysterious.

Image 1: The business end of the detector, showing the outer crystal can, the external preamp electronics, and the black HV filter. There are three pots on the preamp: one controls quiescent current through the FET (but I'm forgetting which one! Gosh, I thought I got some documentation from Jon about that!), one probably calibrates the bias-cutoff thermocouple circuit, and one is probably an inter-stage pole-zero or gain pot. The preamp has a number of proprietary ICs on it.

Image 2: The detector innards with the can off (vacuum seal broken). You can see the white ceramic insulator that separates the grounded cold finger attachment from the crystal can (at several kV positive during operation). I hope I did not damage this detector beyond repair by having it up to ambient air for several days.

Image 3: The crystal cover can has a layer of transparent plastic insulation and some kind of a getter or trap taped inside.

Image 4: Detail of the cold finger attachment, with leads visible for the bias-cutoff thermocouple, the FET source and drain, the FET RC feedback, the HV bias, and the test pulse to the FET gate.

Image 5: Macro shot of the transistor mounting area (original transistor removed) and the feedback RC. I don't know the value of these components. Scribed on the cold finger plate in this area are the following: "FET H1295 res. 385". Help with interpretation, anyone?

Image 6: Replacement transistors, 2SK152 x2, are installed. It's a sloppy job, done with regular rosin-core solder, and without any attention paid to heatsinking the FETs to the cold finger. They were tricky enough to get in even so. After this installation I powered up the preamp and applied pulses to the test input from a pulser. The preamp reported out a nice tail pulse, so I think we're in business again.

Image 7: The black potted HV filter. Power enters through the coaxial cable; the HV socket to the detector is the indented circular impression on the top surface. The through resistance of the filter is 200 MOhm as measured with an electrometer. The braid of the HV cable DOES NOT have continuity to the flying pigtail that is tied down to the chassis of the HPGe detector when installed. In fact, the resistance between pigtail and braid appears to be infinite on the electrometer. The capacitance of the input (between center conductor and braid) is too low to measure on a Fluke 87-V. The output capacitance between female socket and braid is about 7 nF; between female socket and pigtail, about 4 nF.

Image 8: X-ray of the HV filter shows two larger capacitors and one probable ceramic disc cap (probably all output caps, according to the measurements). The filter resistor is invisible to the x-rays but its position is clearly defined by its leads. Two smaller, mystery components seem to be disposed between the flying pigtail and the HV input lead at the bottom of the image.

Image 9: KF25 pumping port and Hysol 1C epoxy seal into the back of the Dewar to evacuate the volume. I'm glad I drilled this hole extremely carefully, because the inner can of the Dewar is surrounded by a fluffy aluminized material a few millimeters thick.

Image 10: On the suck. I've been pumping for three hours as I write this. The turbo-drag pump is at full speed; the pressure with the gate valve open to the pump is in the low single-digit millitorr range, and when the valve closes, the pressure swings up to a steady value of about 40 mtorr. This value is slowly marching downwards. I hope to get to 10 mtorr or less before testing the detector with LN2.

I need HELP identifying the FET bias pot on the preamp! Also, if anyone can comment on the choice of a replacement FET, on whether or not the measurements of the HV filter's resistance, capacitance, and continuity make sense, or on anything at all, I would be very appreciative.

-Carl
Attachments
HPGe_filter.JPG
HPGe_can_1.JPG
HPGe_crystal.JPG
HPGe_can_inside.JPG
HPGe_coldfinger.JPG
HPGe_xistor_mount.JPG
HPGe_transistors.JPG
xray_filter.JPG
HPGe_vacuum_port.JPG
HPGe_teh_suck.JPG
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Re: Refurbishing an Ortec HPGe detector

Post by Richard Hester »

Looking at the x-rays of the HV filter, I would have guessed it contained maybe a couple of film and foil caps and an MOV. Regarding JFET replacement - the PN4393 would be a suitable replacement for the 2N part, and still available from catalog distributors, if you're willing to settle for Fairchild parts. I've used them successfully for both nuclear and audio preamps. They have low input and reverse transfer capacitance, and low noise. I'm curious about the specs on the 2SK152s, and will nose around for some data.
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Re: Refurbishing an Ortec HPGe detector

Post by hjerald1 »

I understand there should be some molecular sieves somewhere to help scavenge
undesireable volatile stuff (e.g. H2O) in the vacuum, especially when the detector is warmed and then cooled again. Image 3 is a packet of such??? Not what I expected it to look like though.
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Re: Refurbishing an Ortec HPGe detector

Post by Richard Hull »

I foolishly took apart 2 perfect Ge-Li detectors and heads about 18 years ago to get at the crystals. The FET's on both very very much a part of the cold finger and were metal canned types and bolted into a thick solid copper holder on the finger. ( noise reduction, I assume). I'll check my FET collection and see what I have.

It looks like a great project Carl. All the best in getting it up and running again.

Richard Hull
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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
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Re: Refurbishing an Ortec HPGe detector

Post by Richard Hester »

For those needing metal cans, you can still find 2N4393 as "new old stock" here and there, especially from people who sell surplus. Linear Systems in Fremont is manufacturing new replacement parts, but I would expect those to be pricey (still withinn reason, compared to the cost of a Ge detector rig).
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Re: Refurbishing an Ortec HPGe detector

Post by Jon Rosenstiel »

Hi Carl,

Schematics and installation instructions for the Ortec 137N (your preamp) and the 237N (a more modern version) located here:

https://gamma.triumf.ca/8pi/Manuals%20A ... GG%20ORTEC

On the 137N preamp the drain current pot is located (referencing your image 1) on the board nearest the cryostat flange. The two pots on the other board (the one nearest the Dewar) are the pole-zero adjustment and the temperature monitor adjustment. (The blue pot next to the yellow cap is the pole-zero pot).

Re: Image 2. The first time I opened up one of these detectors I was appalled to see ugly channel-lock marks all over the flange the FET is mounted on. Gave me the impression that a plumber had attempted repairs at one time. Judging by similar marks on your detector I guess these channel-lock marks are just an undocumented feature.

Re: Image 3. I’ve seen the clear plastic insulation in other detectors, but have never seen the getter or trap or whatever it is that’s inside of your detector.

Re: Image 5. The 137N schematic gives values of 2000 Megohms and 0.25 pF for the feedback components. I don’t know what the numbers scribed on the cold finger plate mean.

Re: Image 6. These detectors are normally somewhat microphonic; something to keep in mind is that the two unsupported FET’s may increase microphonics. Sounds like you’re back in business if you’re getting a nice pulse out of the preamp.

From what I’ve found specially selected 2N4393’s were used in older Ortec detectors while newer detectors (and all Ortec supplied replacement parts) use an SNJ132L14. (The “S” in SNJ132L14 means “Select”). From checking good, removed-from-detector FET’s I have been able to determine that at least one part of the selection process involves selecting FET’s with the lowest possible gate-source cutoff voltage. For example, an SNJ132L14 removed from a LO-AX detector had a Vgs of -0.7 V at room temp and -0.3 V at LN temps, considerably lower than the Vgs of off-the-shelf parts. BTW, a new FET from Ortec runs around $150.00

Re: Image 7 & 8. On a known good filter I also got 200 meg thru resistance, so yours sounds ok in that respect. Using a BK 875A LCR meter I measure ~50 pF between the filter’s pigtail and the coax cable’s center conductor, 49 pF between the coax cable’s center conductor and ground (coax shield), and about 4.7 nF between the filter’s output socket and ground. (Coax shield). I would guess that as long as none of the caps are shorted all is ok with your filter.

(As Carl mentioned, I have run across a couple of bad filters. One had ~4000 megs thru resistance and the other had a shorted output cap. BTW, a new hv filter from Ortec runs around $450.00).

You’ll find the bias pot adjustment procedure in “Preamp Installation Instructions at the above linked site… and may your pump-down progress with nary a hitch.

Jon Rosenstiel
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Re: Refurbishing an Ortec HPGe detector

Post by John Futter »

Carl
The funny metalized plastic in the vacuum space is "super insulation"
basically it is metalised mylar it works the same as underfloor reflective foil ie it reflects heat from the outside jacket back out and reflects cold from the inner surface back to the cold.Usually several layers of loosely fitting turns are used
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Re: Refurbishing an Ortec HPGe detector

Post by Carl Willis »

Thanks all for your input.

Jon, the info at that TRIUMF page is awesome. Thanks particularly for that. Interesting insight into the selection process for FETs. I will probably try to avoid shelling out $150 to Ortec if it can be helped.

Tomorrow I'll take my 10-liter Dewar out to Matheson for a chilly drink right off their distillation tower. I've been quoted $12 to fill it. The right guy has to be there, or I'll be admonished for transporting the juiced nitrogen in my passenger sedan, which is technically a no-no. But I think I've got a good deal lined up.

-Carl
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Re: Refurbishing an Ortec HPGe detector

Post by Jon Rosenstiel »

$12 for 10 L is dirt cheap, the lowest I've ever paid around here (LA area) is $2 / liter.

I've been admonished for not wearing gloves while handling the LN Dewar.

Jon R
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Re: Refurbishing an Ortec HPGe detector

Post by Richard Hull »

Here in Richmond, the LN2 is about $2.20/liter, as of last October. It remains about the same price and tracks beer pretty closely, (volumetrically) and disappears at about the same rate.

The only no-no here is they will not fill up a dog bowl, a milk bottle or a coffee pot with LN2. You must have a formal dewar and be an adult. Once it is in th' dewar, they figure you are on your own and they have done their bit. No gloves needed by the purchaser and you can lock yourself in a bank vault with the dewar.....they don't care, once you are out the door.

I have a friend to always go with me and place the 10 liter dewar between his legs to avoid tip over on the trip home and we keep all windows fully rolled down on the drive back.

I did not have any metal canned (TO18) FETs of the number needed in my first pass through my FET boxes.

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
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Re: Refurbishing an Ortec HPGe detector

Post by Carl Willis »

I just went to Matheson and filled my Dewar...10 liters for $10.42 including tax, no fuss.

The rules at this place are that (1) you must have an account; (2) they can't "see" you (wink, wink) drive off with LN2 in a passenger car; and (3) no bringing Dewars into the office (they are all mindful of this policy and although a small Dewar was tolerated as an exception, toting it through the office was clearly a faux pas...next time, better carry the Dewar directly to the loading dock where they fill it). I was not hassled over bare hands, but had a pair of work gloves at the ready just in case.

Tonight I will fill the detector and adjust the charge loop per the instructions linked by Jon. Maybe I'll even get some spectra.

Thanks Richard for checking your FET inventory. If you don't have it, then it's a rare breed indeed.

-Carl
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Re: Refurbishing an Ortec HPGe detector

Post by Steven Sesselmann »

Carl,

I would be interested in knowing what the peak to peak voltage and length of the output pulse is.

Steven
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Re: Refurbishing an Ortec HPGe detector: SUCCESS

Post by Carl Willis »

The HPGe detector is working tonight, making me a member of the rarified and elite High Purity Germanium Club (which, as Jon joked, has an unfortunate National Socialist ring to it).

Anyway, I balanced the preamp charge loop once the detector had cooled on LN2 for four hours. This task went straightforward, by-the-book.

At this point, still with zero bias voltage applied, I brought a bottle of thorium nitrate near the detector (image 1 below) and noticed pretty obvious signal spikes on the background noise (image 2 is signal without thorium, image 3 WITH thorium), a solid indication that the detector crystal itself was intact and capable of operating in photovoltaic mode.

I began raising the bias slowly and measuring resolution of a Cs-137 signal at 662 keV. Even at 0 volts, a photopeak and some structure are evident (image 4). At 200V, the resolution exceeds the capabilities of scintillation detectors. The crystal is essentially fully depleted by 2500V (image 5). To improve the resolution further, the Ortec 571 amplifier's pole-zero was set properly, baseline restorer turned on, and shaping time constant boosted to 10 microseconds. Under those conditions, a spectrum exhibiting 0.22% FWHM at 662 keV was obtained (image 6). I should point out that the preamp on this detector is still missing a shield shroud and some 60 Hz ripple is still in the circuit. I will probably fashion a shield out of aluminum flashing. My last suggested improvement to the resolution number will come from the use of a triangular shaping amplifier in the near future. I think under ideal conditions, I might be able to get under 0.2%. Maybe revisiting the charge loop tuning is also in order. That number is pretty much the best I figured I could get out of this geometry under ideal conditions, and to be here riding on my sloppy FET replacement job is pretty surprising.

Image 7 shows a typical preamp output pulse (for Steven).

The last photo documents the filth in my workspace that is typically characteristic of good work getting done. Thank God this is not my mother's house. During operation of the detector, the pumping station was shut down and the detector Dewar valved off. It has no detectable leaks.

-Carl
Attachments
thorium_chargeloop_adj.JPG
nothorium_noise.JPG
thorium_noise_0v.JPG
HPGe_lowV_spectra.jpg
HPGe_resvbias.jpg
HPGe_suckonthis_jon.jpg
preamp_output.JPG
mess.JPG
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Re: Refurbishing an Ortec HPGe detector: SUCCESS

Post by Richard Hull »

Fabulous work Carl! I am very envious. The work and descriptions you give are perfect for any person, in the future, to slip into the HPGe work should they be fortunate enough to pick up the system.

This will change the entire complexion of your future Gamma spec work and I look forward to the reports.

Thanks to both you and Jon for the insights.

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
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Re: Refurbishing an Ortec HPGe detector: SUCCESS

Post by Jon Rosenstiel »

Carl,

Congrats on your successful repair job and welcome to the HPGC!

The fact that your replacement FET(s) are working well is very encouraging, but I don't really understand what paralleling them does... if you would please enlighten me. (My only “formal” electronics education took place near the end of the vacuum tube era; some of this silly-con stuff is still somewhat mysterious to me).

Also encouraging is the fact that your clear and concise descriptions of the cool-down, power-up, and tuning processes mirror to a “T” what I’ve experienced.

Jon R
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Re: Refurbishing an Ortec HPGe detector: SUCCESS

Post by Steven Sesselmann »

Carl,

Like Richard, I would just like to register my envy as well...

Looks fantastic...

Steven
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Re: Refurbishing an Ortec HPGe detector

Post by billslugg »

I just got three old Princeton Gamma Tech GeLi detectors with preamps. They appear in good shape and all three give consistent resistances to ground from all pins. One is a 20%, one a 22% and one a 60% well detector. I plan to cool them one at a time and look for frost on the housing denoting a bad vacuum. If OK, then I plan to power up the electronics and listen for clicks with no HV bias at all. If OK then I plan to raise the bias until the noise takes off to see if they can tolerate the least bit of bias, given that they have been at room temperature for 15 years. Then I plan to take them up to 200°F for three days in an effort to redissolve the Lithium into the Germanium lattice and repair some of the dislocation damage done to the crystal by the precipitation of the Lithium. Test it out again to see if I did any good. I'll be happy just to be able to do counting at a few per second, I don't need a high count rate. In any event, I will not break the seals on them. If they are junk, then I will try and sell them in the scrap market still sealed.
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Re: Refurbishing an Ortec HPGe detector

Post by corrado_tine »

Hi Jon,

reading your answer, I notice that you posted a link with the installation instruction manual for 237N ortec preamplifier. Unfortunately I am not able to get this manual througth this link because a password is required. Could you send me it?
I need for this manual because I have an old GMX ortec detector with 237N preamplifier and I need to setup it.
My supplier is not able to provide me this manual either because it's very old.

Thank you in advance.

Corrado

Jon Rosenstiel wrote: Tue Mar 29, 2011 4:36 pm Hi Carl,

Schematics and installation instructions for the Ortec 137N (your preamp) and the 237N (a more modern version) located here:

https://gamma.triumf.ca/8pi/Manuals%20A ... GG%20ORTEC

On the 137N preamp the drain current pot is located (referencing your image 1) on the board nearest the cryostat flange. The two pots on the other board (the one nearest the Dewar) are the pole-zero adjustment and the temperature monitor adjustment. (The blue pot next to the yellow cap is the pole-zero pot).

Re: Image 2. The first time I opened up one of these detectors I was appalled to see ugly channel-lock marks all over the flange the FET is mounted on. Gave me the impression that a plumber had attempted repairs at one time. Judging by similar marks on your detector I guess these channel-lock marks are just an undocumented feature.

Re: Image 3. I’ve seen the clear plastic insulation in other detectors, but have never seen the getter or trap or whatever it is that’s inside of your detector.

Re: Image 5. The 137N schematic gives values of 2000 Megohms and 0.25 pF for the feedback components. I don’t know what the numbers scribed on the cold finger plate mean.

Re: Image 6. These detectors are normally somewhat microphonic; something to keep in mind is that the two unsupported FET’s may increase microphonics. Sounds like you’re back in business if you’re getting a nice pulse out of the preamp.

From what I’ve found specially selected 2N4393’s were used in older Ortec detectors while newer detectors (and all Ortec supplied replacement parts) use an SNJ132L14. (The “S” in SNJ132L14 means “Select”). From checking good, removed-from-detector FET’s I have been able to determine that at least one part of the selection process involves selecting FET’s with the lowest possible gate-source cutoff voltage. For example, an SNJ132L14 removed from a LO-AX detector had a Vgs of -0.7 V at room temp and -0.3 V at LN temps, considerably lower than the Vgs of off-the-shelf parts. BTW, a new FET from Ortec runs around $150.00

Re: Image 7 & 8. On a known good filter I also got 200 meg thru resistance, so yours sounds ok in that respect. Using a BK 875A LCR meter I measure ~50 pF between the filter’s pigtail and the coax cable’s center conductor, 49 pF between the coax cable’s center conductor and ground (coax shield), and about 4.7 nF between the filter’s output socket and ground. (Coax shield). I would guess that as long as none of the caps are shorted all is ok with your filter.

(As Carl mentioned, I have run across a couple of bad filters. One had ~4000 megs thru resistance and the other had a shorted output cap. BTW, a new hv filter from Ortec runs around $450.00).

You’ll find the bias pot adjustment procedure in “Preamp Installation Instructions at the above linked site… and may your pump-down progress with nary a hitch.

Jon Rosenstiel
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Re: Refurbishing an Ortec HPGe detector

Post by Richard Hull »

It is to be remembered that this entire thread was created and ended in 2011. In internet time, the 12 years since this post thread is an ice age! Many things disappear forever. If the information or manual is still there, there may be a paywall in place for your inconvenience. Mostly, stuff just disappears forever, companies change hands, they go under (out of business), entire websites go down, etc. 12 years on the internet is a great epoch of change and loss.

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
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Re: Refurbishing an Ortec HPGe detector

Post by Jon Rosenstiel »

Corrado,

I no longer have anything on the 237N preamp. Below is a Dropbox link to what could find, hope this helps.

https://www.dropbox.com/sh/ul3tmyk7ypkw ... IfvWa?dl=0

Jon
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Re: Refurbishing an Ortec HPGe detector

Post by corrado_tine »

Thank you very much Jon.
Even if you don't have the manual of 237N, I solved anyway my problem because the schematic of the 137N model which you sent me is very similar to the 237N one.

Corrado
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Re: Refurbishing an Ortec HPGe detector

Post by ccipp »

Jon Rosenstiel wrote: Mon Jan 09, 2023 4:47 pm Corrado,
I no longer have anything on the 237N preamp. Below is a Dropbox link to what could find, hope this helps.
https://www.dropbox.com/sh/ul3tmyk7ypkw ... IfvWa?dl=0
Jon
Hi Jon,
Do you happen to have a similar set of instructions for a GEM type preamplifier?
Sorry to prolong the discussion for this old topic.
Ciprian
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