Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: There be neutrons!
Date: Feb 22, 00:09 am
Poster: Richard Hull

On Feb 22, 00:09 am, Richard Hull wrote:


I have worked three long straight days and nights from Friday until midnight Sunday. The result of about 36 hours of work is a working and fusin' Fusor III.

The key was the neutron detector, its setup, and calibration. A large number of controls were in place and specific hoops had to be jumped through in order that I not deceive myself with the instrument.

Neutron measurement is no trivial matter and at the lower levels of fusion, the need for extreme care is in order as the measurements can get tangled up in noise and false sources.

I used the Bicron BC-720 fast neutron detector coupled to a Hamamatsu R-1306 bi-alkali extended blue range PMT with preamp. This was sealed in a light tight shell of aluminum lined with 1/2" of lead for radiation shielding. Electrostatic shielding was taken care of by the outer aluminim shell and an inner wrapping of copper around the PMT tube itself. Also wrapped around the PMT was a mu-metal shield to fend off any magnetic fields.

I used a Stanford Research photon counter system,(hamfest buy), as the power unit,and discriminator. Both a digital Tektronix scope and Hewlett Packard digital counter were used to set the discriminator levels and count the neutron pulses.

Extensive long term checks were used to establish a base level for background radiation. Large gamma and X-ray sources were placed near the system and the discriminator set to ignore them.

A full power run with air, (no D2), was used to check the counter against the no power background and the two tests indicated that the fully powered fusor with air only, was idenetical to background levels so far as the neutron counter was concerned.

The first test with deuterium was performed at a continuously pumped D2 pressure of 0 microns on my TC gauge. The fusor was run and held to 18kv @ 5ma for 10 minutes. Using the equations developed for this counter, the results gave 2.1 X 10E3 nuetrons per second for this run. I was elated.

After supper, I was emboldened to make two more separate runs. This time at 3 microns and 20kv @10ma (200 watts). The average of the two runs was 5.3X10E3 neutrons/second. Current is the key to neutron production. The secret is to put as much D2 into the chamber as possible and keep the voltage and current up as well.

I was like a spider monkey twirling dials, setting gas flow, tweeking the supply, carefully checking the current meter, working the stop watch and counter system as well as keeping a close eye on the tantalum grid on the video monitor to insure it stayed only orange hot. I can tell that this is a two man operation for data acquistion and control.

It might only be 5000 neuts/second, but it is an honest count and 10E4 is just around the bend. Not bad for a 1 year total effort and two interim fusors.

I am close to the limit for the design of this unit (25KV) and have to play with current and D2 pressure from here on to improve the system. I really hope to hit 10E4 with this thing before starting on fusor IV!

In the fusion sweet spot the star spokes become sharply focused needles and the central glow focuses down as well to a tight hot little knot.

There it is and I am exhausted, it is now about 1 am here and tomorrow is a work day. A very well spent weekend.

Richard Hull