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: Dose, Dose rate, Flux, etc.
Date: Jan 24, 7:19 pm
Poster: Richard Hull

On Jan 24, 7:19 pm, Richard Hull wrote:

Well, I have more trivia which is of great importance to those stuck with a neutron rate meter or survey dosemeter and contemplating a real working fusor.

In the two volume book, "Neutron Dosimetry", International Atomic Energy Agency, Vienna, 1963, I have uncovered some good data and info.

I was given the volumes by Kim Goins of our local physics group. She is a young physicist with the Naval Research Lab in DC. She got them for free in the stacks of old books being purged regularly from the lab's huge, multi acre, library facility.

Dose is given in rem (roentgen equivalent man)this is merely a number of neutrons falling on a given area (usually sq cm.). This is tempered by the RBE curve at a given energy. Dose rate is the number falling on this area per unit time. (usually in rem or mrem/hr. Flux is the number of neutrons per unit area per unit time also, but can't be rem until it is adjusted by the RBE curve for the specific neutron energy. Thermal neutrons are about 10 times less dangerous than fast neuts!!! The most dangerous radiation known to mankind is a fast neutron of 1mev energy (RBE of 10) with the fusion 2.45mev neutron very close to it (RBE 9.90).

So, it would be prudent of us to study fusion neutron blasts from our fusors with interest and respect.

Based on some handy charts in a paper by S. Hagsgard and C.O. Widell, "Personal Neutron Monitoring at AB Atomenrgi" (Sweden), the number of 2.45 mev neutrons in a rem equals ~2.5X10e7 neuts slamming into a square cm. That mean 2.5X10e4 neuts of this energy in a square cm is a millirem. This is what most neutron survey meters are cal'd for. They follow the inverse rpg curve relatively faithfully at the high end of neutron energies for the most part. Actually they are a rate meter and read in mrem/hr. So dividing by 3600 we get a 2.45mev neutron rate of ~6 or 7 neut/sec/sq. cm. per mrem The key info you must now supply is your meter's relative efficiency to this radiation. Most are on the order of 50% efficient so a millirem at a given range is about equal to (10)- 2.45 mev neutrons/sq. cm./s (again, check your efficiency to be sure).

A fusor boiling away at .5mrem/hr at a range of 30cm from a survey meter detecting every other 2.45 mev neutron in its volume would represent about 113,000 neutrons/ second isotropically emitted from the central grid region (+or- 20%). Don't kid yourself into thinking the meter is more accurate than this.

I hope this helped those who are looking at actually makin' neuts!

Richard Hull