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: Re: p + B11 reaction
Date: Feb 23, 1:23 am
Poster: Richard Hester

On Feb 23, 1:23 am, Richard Hester wrote:

On the P-B11 front, the thing that will frustrate most researchers is that the reaction cross-section is virtually non-existant below about 50 kev. Around 60-70 kev, the cross section rises rapidly due to a resonance and is actually equal to or greater than the cross-section for the D-D reaction. Of course, the problem is getting to the required voltage, which will require careful pumping and pressure monitoring, not to mention meticulous high voltage technique. Most folks are trying to pump past the pressure of interest... If the pressure is too high, you'll probably arc and destroy your grid (and maybe your power supply). We are talking here about submicron pressures with a third grid to provide ionization.

Another challenge is getting the boron into the system. I was thinking initially of using diborane as a source of both boron and hydrogen, but the more I read about this stuff, the less willing I am to have it as a house guest. Diborane is poisonous, pyrophoric, and apparently unstable as well. In the pure form, diborane is shipped in cylinders in a barrel of dry ice. Even the diluted mixtures have a shelf life. As an interesting aside, some researchers use diborane to plate the inside of their reaction chambers with boron, to reduce the amount of high-Z impurities that get introduced into the plasma by wall reactions.

Other possibilities for introducing boron are lithium borohydride (keep it away from air and moisture - extremely reactive solid), or using a hydrogen fill and placing a boron target in the fusion chamber. The boron would be vaporized with a pulsed laser (another use for that e-bay toy...).

Detecting the alphas from the P-B11 reaction would be fairly straightforward, using a drift tube and magnetic fields to strain out the electrons. The alphas, after all, are several thousand times heavier than electrons and will scarcely notice a magnetic field that would slam electrons into the drift tube wall. The alpha pulses in the detector could be readily differentiated from the X-ray background using a pulse height analyzer. Detection efficiencies should be much higher than for neutrons, since they can be detected directly rather than through an intermediate reaction like proton recoil or reaction with boron (with another intervening step here being the required moderation).

Richard Hester