Neutron diffusion time in a moderator with 3He tube

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Richard Hull
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Neutron diffusion time in a moderator with 3He tube

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The URL below is a well done experiment by the Maryland NPL group.

Pulsed fusion systems would tend to have a very short high current pulse, (~10us), produce a burst of neutrons during the peak pulse duration. Could a good 3He detection tube even detect a single neutron during this period? The answer is yes, It could detect many neutrons due to thermal neutron diffusion over time within the moderator. Thermal neutrons move at an average velocity of 2200m/s. To travel .3 meter in a "long counter" moderator would take ~120us. It is seen that ultra slow neutrons will take over 500us to finally either exit the moderator or be counted by the 3He tube.

In any pulsed power neutron event, the RFI environment around the neutron detection system must be superlatively shielded to be of any value. In such systems, peak powers in the megajoule range are common while the average power consumed might only be 300 watts per pulse.

The experiment in question gave an average diffusion time of 93us, over which neutron counts were still being detected from the short power pulse induced burst of fusion. In this experiment, counts extended out to over 400us after the pulse event as the last of the extremely pokey neutrons wandered in to be detected. For those looking to do pulsed fusion, the repetition rate of the pulses is important and only a bubble detector might be used to correlate the neutron count over time to a suitable TIER. In short, you can count many neutrons per pulse in a "long counter" 3He setup using a pulsed neutron source. Check out this fine experiment.


http://www.nuclearphysicslab.com/npl/np ... asurement/

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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