Neutron detector build

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Luke Weston
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Neutron detector build

Post by Luke Weston » Wed Mar 20, 2019 3:10 am

This is a neutron detector experiment, a build using a USSR-made SNM-11 boron-10 corona tube.

Here the tube is slid into the moderator and the PCB sits just outside.
The tube can easily be slid off for the all-important "moderator on vs. moderator off" test.
D2AbCWkUYAALKkN.jpg
Here is the exposed assembly before sliding into the moderator.
3M copper foil tape with conductive adhesive is used to make the contact onto the tube.
D2AbCWnU0AAoXtu.jpg
Here is detail of the PCB.
D18oroQU0AAXjxq.jpg
This is the starting HDPE stock.
The HDPE is 90mm OD, which is a little thin for complete thermalisation of neutrons, but this is the material that I had available easily at a reasonable price. It should be good for producing epithermalisation of neutrons, maybe for activation experiments one day.

The HDPE rod is cut to 350mm length (slightly longer than the SNM-11) and drilled to 20mm down the centre.
D0jNPPBU4AEydmC.jpg
This is the amplifier schematic, using a single BJT (MMBT3904 in this case.) R2 is changed to 68k, the output bias R5/R6 are not populated,
R1 is 330k, R3 is 5.6k. (I changed these slightly from the drawing to suit the components on hand.)
DzZ6QyNXQAEyeCb.png
The bias tee section consists of a 100pF HV ceramic, a 47M HV resistor and an SHV connector fed from a 0-5kV adjustable (low current) power supply module. I haven't installed the 16k resistor yet.
DzZ6QyMXgAEV_9O.png

John Futter
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Re: Neutron detector build

Post by John Futter » Wed Mar 20, 2019 6:19 am

Luke
your amplifier is not matched to your detector circuit in fact from resistor values presented there is not much gain after the amp
Corona tubes do not have much output so I would not expect R2 to be 16K --more like 100's of K or megs
I would have expected a jFET amp as the first stage to keep the impedance high to match your tube circuit.
BC547 is a general purpose transistor not a low noise fast one as I would expect
edit i see you have at least used the mmbt3904

where did you find this amplifier circuit??
edit I see you got it from Lucas
I see lucas used exactly what i'm saying straight after his tube conditioning circuit that you reference here.
and others question his use of the value for R2 at least you have raised the value
Steven Sesselman replied with his circuit (it works)
A Cremat amp module or above are good options --some ramdom transistor circuit will not provide you with what you want unless you have a heap of pulse related test gear, a very good scope, and you are a very good fast analog design engineer.

look carefully in this site as to what works!!!!
do not cross pollinate one solution with another unless you are an analog EE
Last edited by John Futter on Wed Mar 20, 2019 6:15 pm, edited 1 time in total.

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Richard Hull
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Re: Neutron detector build

Post by Richard Hull » Wed Mar 20, 2019 2:50 pm

Remember.....Once you think you are detecting neutrons to put a very hot piece of U ore near the naked tube. If it counts....You are not actually counting only neutrons. Boron lined tubes are tricky to bias.

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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Re: Neutron detector build

Post by Lukas Springer » Wed Mar 20, 2019 9:12 pm

I don't see why you seem to dislike the russian SNM tubes and "my" circuit that much.
The circuit with a 100 pF / 16 kOhm high pass filter is recommended by the manufacturer, what else can you wish for.
The low impedance filters out pretty much everything of the high impedance corona noise, leaving only clean pulses behind.
Filling gas in those B10 lined tube is at a somewhat low pressure, so gamma interaction is very weak and it needs something like a neutron induced fission of Boron 10 to actually ionize something in there. I wonder why they designed it that way...

The biasing of those tubes is childs play, as the corona is stabilising the gas multiplication extremly well, as is described in the attached paper.

>>Proportional counting in the corona zone is attractive since operation at maximum gas gain, independent of variation in counter e.h.t. voltage, is available. In such a counter, however, the minimum amount of particle ionization that can be detected in practice is limited by the amplitude of fluctuations in corona current (background corona noise).

This means that rejecting everything below ~1.5 times the corona noise (value taken from experimental experience, see: viewtopic.php?f=13&t=12474 ) results in a very clean output with very high neutron certainty.

My amp design is far from spectroscopy grade or even stable, but it's sufficient, since the tube and filter itself reject most of the unwanted signals.

I've tested the SNM11 and SNM-18-1 tubes with proper equipment, a known neutron source and gamma / beta sources up to several mSv/h (yes, MILLISIEVERT) and presented my results here, yet you still don't seem to accept them.

Keep in mind "you" refers to no specific person, but rather the mindset of this forum.

And also my name is Lukas, with a "k".
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Richard Hull
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Re: Neutron detector build

Post by Richard Hull » Thu Mar 21, 2019 4:37 am

I was merely warning all who use the Russian tubes to take all precautions with biasing to eliminate all detections other than neutron detections. Not everyone is a careful or knowledgeable electronics person, nor do all people have materials at hand to warrant only neutrons are detected. No neutron detection tube is, by nature, purely a neutron detector. Many skills are needed to avoid falling into the trap of false neutron detection.

On fusor.net, far more people have failed with the Russian tubes than have passed muster. For the newbie or the uninformed in electronics, but particularly those with zero nuclear measurement experience tend to hear that a tube is a neutron detector and assume it will count only neutrons.

The rush to get into the neutron club tends to leave many falling short on neutron detection in a quick dabble with the inexpensive Russian "neutron" detector tubes. The classic fusor running 20kv @ 6ma with a Russian tube giving counts of 3,000 per minute. (far too high) and background counts of 100/min. We know that such numbers are not neutrons.

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.

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