Fusor Forums

I'm trying to figure out the correct operating voltage for this tube (Ludlum 42-31).

The website for Model 42-31H Neutron Detector says 1200V but the LND referenced by the same page says 750V (https://www.lndinc.com/products/neutron ... ors/25185/).

It's also very possible that the "H" means He3 and the non-H variant is a BF3 tube. The BF3 tube referenced 01-5018 (https://ludlums.com/images/pdf_files/Mo ... f_tube.pdf) is said to be a Nancy Wood G-5-1 (https://www.qsl.net/k/k0ff/0%2018Manual ... 0jun98.pdf) that operates at 1600-1900V.

Through a window on the tube it says 201 (could be partial) and this lines up to a couple of LND BF3 tubes but nothing exactly.

I would work my way up in voltage starting at 750v. You have a great working fusor and you will see neutrons aplenty as you raise the voltage, slowly.
This assumes you never get a firm piece of data on the tube. BF3 tubes tend to take a lot more bias than 3He tubes, Boron lined detectors are more in the 600-800 volt range and are very critical of voltage adjustment.

Richard Hull

BF3 tubes are usually operated between 1800 and 2000 volts. The "sweet spot" for mine is at 1900 volts.

I see some reasonable response at 1100 - 1200 V but I have the secondary amplifier gain set significantly higher (128x versus 4x) than my other detection system. I suspect that I need to raise the voltage but I'm reluctant since I don't want to damage my tube.

If indeed, you have the 42-31 3He tube, that should be operated no higher than 1100 volts. But if your model number is just "42-31" that would be the BF3 tube which is rated at 2000 volts max.

Bob,

Just so I'm clear you have a 42-31 and bias it at 1900V, or you are talking about BF3 tubes in general.

Thanks, Joe

BF3 tubes in general operate between 1800 and 2000

One way to determine if you have a BF3 or He3 tube is to look at the output pulse height spectrum. The two spectrums are significantly different. Knoll’s book covers this in detail, but there are many practical examples in past fusor posts. One He3 pulse height spectrum I captured is described in this thread:

viewtopic.php?t=10569

Here are some other useful ones:

viewtopic.php?t=5613
viewtopic.php?t=6308

With your fusor's outstanding neutron production the features of these different spectrums should be easily identifiable.

Bruce

I definitely have a BF3 tube and the sweet spot is 1850V. Thank you, everyone, for your help.

P.S. I now have 2 NIM electronic detection systems, 1 portable electronic system, and a bubble dosimeter.

Hopefully you are ready to make neutrons soon, as the bubble detectors have a limited shelf life, used or not. Use the bubble detector to calibrate your electronic detectors.

Bob is correct. Assuming you have a good, new, bubble detector, use its calibrated dosimeter calculation (bubs/mrem) to calibrate your electronics.
I created a FAQ to explain how I did it years ago.

Richard Hull