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NIM/He3 Frustrations

Posted: Wed May 22, 2019 10:31 pm
by Luke Harrill
Right now I pretty much have everything complete. I have made runs with H2 and more than adequate current/voltage. However, it isn't worth anyone's time to report on that without neutron measurements. Some problems have left me in the dark.

My setup currently consists of the following ran out of a Tennelec Tennebin3:
(Tennelec TC940 HV Power Supply)>(Tennelec TC175B Charge-Sensitive Preamp)>(Texlium "Special" He3 tube 2ft w/ paraffin mod.)>(Tennelec TC211 Linear Amp)>(Tennelec TC441 SCA)>(60's pulse counter)

After hooking everything up, I power the bin and counter. Next, I turn on the power supply and work up from 0, 500, 1000, and finally 1500 volts. I then place a hot gamma source next to the tube. I start to bring up the main amp gain, and see counts starting. With each stage, the counts increase.

Here's the catch- When I remove the source, nothing happens. I've tried to change settings on the amp, but to no avail. The SCA seems to do a good job of removing the noise(?) from the chain. If I increase the threshold a little, the counts stop.

I am wondering if this indicates a bad preamp, or am I doing something wrong? On hand I have another TC175B, but I would need to swap the SHV's out to BNC.

-Luke Harrill

(Note: I may have uploaded one photo sideways)

Re: NIM/He3 Frustrations

Posted: Wed May 22, 2019 11:40 pm
by Mark Rowley
I’m not too versed on the modules but your counter is so old it probably hit official antique status in the late 70’s. I love the vintage equipment and actually run a few restoration pages on FB. But for this application, you may want to upgrade to something digital. Lots of relatively modern counters out there you can get for under $100. I can’t help but think the caps in your unit are dried out and leaky (unless you changed them).

Regardless, if your concerns are rectified, using the old equipment to quantify and prove fusion is nothing short of awesome. Best of luck!

Mark Rowley

Re: NIM/He3 Frustrations

Posted: Thu May 23, 2019 3:20 am
by Luke Harrill
Under normal circumstances, I would agree with you about that being an issue. However, it was given a restoration by Richard just a few years back, and seems to still be running well. (The thing has more 12ax7's than you can shake a stick at anyways...)

I do wonder if you can somehow run the output of an SCA into a regular ratemeter such as a Ludlum 3. It would be nice to have an acutal ratemeter to run alongside of the counter.

-Luke Harrill

Re: NIM/He3 Frustrations

Posted: Thu May 23, 2019 9:49 pm
by Richard Hull
A decent O scope would be nice to look at the various outputs. It sounds as if you have reduced the noise correctly, assuming you have a really good neutron source. As you note the counter is fully functional and is not the issue as long as your NIM is sending signals of over a volt. (easy to get out of the NIM amplifier using the gain control). When you say you have good counting with a neutron source and when it is removed you get no counts at all.

This is a great sign, but you should actually get 2-8 counts per minute due to cosmics with that 3He tube properly biased. The bias I usually use is on the order of 1400-1600 volts. I always strongly recommend using a scope to view all waveforms. I look first, from the pre-amp and then the amplifiers so that you are just getting good 1 volt or more pulses at the end of the chain with a neutron source and zero pulses with a hot gamma source like a 10uCi Cs137 source touching the tube body. I like to observe happiness from each NIM system component on a scope.... Then either a rate meter or a counter is reading the "right stuff"

An analog rate meter can be hand fashioned by a suitable electronic guru, but the standard NIM module in working condition often brings $200 or more. Nim readout modules are always expensive and near $200 or more if warranted to work, be they digital or analog.

Try and contact a ham or someone with a decent o'scope (preferably digital storage type) and look at you outputs one by one.

Richard Hull

Re: NIM/He3 Frustrations

Posted: Wed May 29, 2019 7:08 pm
by Luke Harrill
I have not tested the setup with a neutron source yet, but only a decent amount of radium, to which it did not respond at all.

This morning I went and bought an o-scope, as no one near me had one. It is a Leader LBO-524 40 MHz. (I also repaired my LBO-508 20 MHz today.) Although it does not have a storage, I hope it will suffice...

What kind of pulses should I be seeing from a good, working preamp?

-Luke H

Re: NIM/He3 Frustrations

Posted: Thu May 30, 2019 5:07 am
by John Futter
there are plenty of posts on this forum of good scope pulse captures but a unipolar pulse of around a few microseconds.
you realy need to add a test pulse input to your preamp so that you can get a steady stream of pulses to view especially important when you do not have a storage scope beit analog or digital

Re: NIM/He3 Frustrations

Posted: Thu May 30, 2019 8:26 am
by Richard Hull
John offers another solution. A pre-amp can be tested with a NIM pulser plugin, provide you fully understand the multiple attenuation switches on the front panel. Most such modules let you select a base level 1 or 10 volt pulse with a toggle switch. I like to set mine to 1 volt... Now it is up to you to attenuate this using the attenuators to reduce this to some much lower voltage to apply to the D.U.T, (device under test).

For a preamp test you might reduce the pulse input to the pre-amp to a few millivolts and feed the output to your amp set to a gain of 10 or 50 and try and drive your digital ratemeter. if it won't count, try increasing the gain on the amp. If nothing, then move the pulser output to the input of the amplifier. Change the attenuation to give a .5 volt output from the pulser and set the amp gain to 10 or 20 and see if the counter counts. If it counts, the amp is good. You may have issues with the pre-amp.

If you doubt the counter, take out all of the attenuation and flip the 1 or 10 volt pulse switch to 10 volt pulse and feed it to the counter. It should count. I think some pulsers work at 60hz others work as high as 120hz (pulses). I realized you noted you did have the counter working. Remember there is a variable input voltage control on the front panel. That Counter works great with a positive 5 volt pulse.

As with all tube gear give it time to warm up. It has a crystal oven in it and they want a 5- 10 minute warm up to let it stabilize.