Second pulse from He3 tube

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Lukas Springer
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Second pulse from He3 tube

Post by Lukas Springer » Mon May 07, 2018 8:31 am

Hi!

I finally got myself a Helium-3 corona tube, the SNM-18-1 to be precise.
I don't have a neutron source yet, so I've got to rely on background for testing; the screenshot below shows the signal from my tube for a couple of minutes, without a moderator.
The tube counts about 10 cpm, and some (but not all) of these pulses are followed by a second one with about 650 us delay.
I don't understand where that pulse comes from, my only idea is a recombination event. Maybe this it is from UV ionisation when the proton "catches" an electron?

-Lukas
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He3 tube second peak

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Richard Hull
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Re: Second pulse from He3 tube

Post by Richard Hull » Tue May 08, 2018 4:08 am

Make sure the instrument cables are terminated by their characteristic impedance. This is usually a BNC connector with a built in resistor. The NIM system uses 94 ohms, but both 50 ohms and 75 ohm cables exist in other areas of high frequency electronics.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Andrew Seltzman
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Re: Second pulse from He3 tube

Post by Andrew Seltzman » Tue May 08, 2018 12:36 pm

In this case I don't think termination or ringing of a cable is the issue; a 650us delay would equate to a 400000ft long RG58 cable (5 ns/m (1.54ns/ft) delay).
Andrew Seltzman
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Lukas Springer
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Re: Second pulse from He3 tube

Post by Lukas Springer » Tue May 08, 2018 2:43 pm

I don't think it's from bad / missing termination, since not every pulse is followed by a second one, regardless of pulse height.
There is currently no amplifier installed and I'm measuring with the standard 1M input impedance of my scope, right after the AC coupling capacitor.
All cables are 1m in length and everything is shielded, the passives are in a soldered shut box.

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Richard Hull
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Re: Second pulse from He3 tube

Post by Richard Hull » Tue May 08, 2018 5:58 pm

I would not be too excited about the second pulse if this has no amp or discriminator. This is not a real neutron detector yet. Get some electronics in there and see what you really have.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Lukas Springer
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Joined: Sat Mar 03, 2018 9:37 pm
Real name: Lukas Springer
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Re: Second pulse from He3 tube

Post by Lukas Springer » Tue May 08, 2018 7:05 pm

Of course it isn't a detector yet, it's just testing the tube so I could make use of ebays return policy, if needed.
The scope is placed where the amp would be, the amp will have 1M input impedance as well, so it should amplify that smaller, second pulse.
I know this pulse is not a neutron count, I'm just trying to understand where it comes from.
Once the setup is complete (I've cast the paraffin moderator today) I'm going to ask my university for a californium check source, then I can tell you if it is in any way related to a neutron event.

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