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: Not very active
Date: Mar 31, 08:57 am
Poster: Richard Hull

On Mar 31, 08:57 am, Richard Hull wrote:

Stephan Coley was correct. The floor is the source for a lot of radiation. Actually, radon rises from the ground especially following heavy rains as it is displaced such that following a heavy rain in many areas the background count out of doors near the earth can double!!!

Radon can permeate concrete readily as well. In offices with a lot of traffic, this mixes uniformly with the air, but in infrequently traveled areas, hugs the floor as it is more dense than air. Radon, is rarely a problem to most folks inspite of the huge Radon scare in the 70's and 80's. We gets our doses and we takes our lumps. Jet setters (frequent flyers) and air crews on commercial flights get massive doses of radiation compared to we stay at homes, even with radon trapping basements.

Now to Scott's readings........ I had a deep feeling that we were not or should not really be neutron activating our chambers. The flux is far too low, the neuts are all fast, and the cross section for stainless is very low, making the chamber look like it isn't even there to the neuts. This is great news as the radiation in geiger mode is probably mostly tritium. I will ask one thing.... Scott... how long after you dropped the nuclear fires did you test the chamber?? If we are activiating the chamber, most of the activation should be of very short lived isotopes. (Some on the order of minutes to hours.)

Why? Because activation occurs en mass for short lived stuff first based on run time versus half life. If you run a chamber for 20 minutes, that would highly activate only materials with a 3-5 minute half lives, and then only in a flux of 100% slow neuts! Hour or more half lived items whould only partially activate and long stuff would barely be detectable.

Tritium, however, is brought into existence in 50% of our fusions. This is not an activation process! Tritium lasts over 12 years!!! So virtually every tritium atom made (in a totally sealed fusor chamber) will remain hot for years!!

If Scott opened his chamber more than 24 hours after his last run, even if it was real hot from activation, the stuff should have mostly decayed off. The tritium he made probably got pumped out into his lab. Not to worry, the tritium level would have be no more than ~1million tritium atoms for each minute of operation. In his continuously pumped system, that load would easily be handled by normal company ventilation systems.

However a twenty minute run in a valved off chamber might have over 20 million trapped atoms of tritium in it. At the internal pressures of 3u, the mean free paths of the betas would be almost chamber wide! Thus, virtually every 3-15KV beta particle could be counted.

Before the LANL labs went super secure, I pulled off and printed the on-line offical government Tritium researchers handbook, with tables charts, danger levels and the like. Now the security firewall is up and the site just can no longer be accessed by the general public! This is a one year ago thing!

Nice to know we are not really activating long term isotopes to create a throw away hazard. That would have been a real bummer, but I had little fear of that.

Richard Hull

>I have completed a quick survey of my now-dormant fusor chamber with a 2" NaI detector looking for signs of neutron activation of elements in the stainless steel housing.
>To my surprise, there is no configuration of counter+chamber that does not count a little LESS than the counter by itself!! Obviously, what's happening is that the chamber is serving as a shield for the counter against background radiation. The greatest reduction (~18%) occurs when the chamber is place between the counter and the concrete floor. The smallest reduction (~7%) occurs when the chamber is placed directly above of the counter.
>Obviously, my chamber is not very active...