primary cosmic radiation consists of no neutrons (too short of a half life.) All neutrons in ground level cosmic radiation are from impact and decay stars in the upper atmosphere. The neutron intensity is heaviest about 10-12 miles up.
Primary cosmic radiation striking earth's atmosphere consists of ~ 90 percent protons, 9 % alpha particles, and about 1% heavy nuclei. Gamma and x-radiation is virtually non-existant in space. The very weakest of these matter particles are about 1 billion electron volts, (10e9ev) and the very strongest are on the order of 100 quintillion ev (10e20 ev).
Neutrons that filter down from the 10-12 mile max neutron zone, reach ground level with an average energy of 5mev. At the 50,000 foot level you receive 10 mrem per day dosage, but at sea level that falls to a fast neutron dose of only .05 mrem/day. Cosmic ray generated, secondary, neutrons account for about 13% of your total daily rad dose whether eating, sleeping or playing football. The bulk of your secondary cosmic ray dosage is composed of various muons. The bulk of your total background, sea level, dose is made up of natural sources in the ground and air.
LET (linear energy transfer) is usually stated in kev/micron of tissue. This value is related to the type of radiation absorbed and how it interacts with tissue. The most dangerous LET values lie between 40-60kev/micron.
Alpha particles have a huge LET value (~200-300kev/micron), but their RBE (relative biological effectiveness) is lower than neutrons due to the LET value being so high. This is because although they do a lot of damage in the very short range of penetration, they do not have the ability to damage major organs. Their range in air is also very short (~3-4 inches). (all this is based on external exposure to the particles)
Neutrons in the 1-2mev class have ranges of hundreds of kilometers in air! In tissue, this narrow range of neutron energies dumps about 30-40kev/micron in tissue. The neutron induced proton recoils inside the tissue themselves account for a LET of about 28kev/micron. This works out to about 400 ionizations per micron of tissue with a range of 23 microns. These ionizations release an electron. Many of these inturn have more than enough energy to create ion pairs of their own. 100ev electrons have ranges of a few millimicrons in tissue. Some electrons with 1kev energies are call delta rays (in tissue). They have ranges about 25% of the proton. Thus, the bulk of the damage occurs deep with the tissue and near the end of recoil proton tracks.
Electrons and x-rays are given an RBE of 1. Fast neutrons are given a 10 or more based on energy and absorbing medium. Protons within tissue are given an RBE of 5.
Fast neuts are very dangerous to humans.
The current flux accepted for workers in a continouos work environment is on the order of 20n/sqcm/sec This is down from the 60n/sqcm/sec quoted in the earlier posting which related more to the continuous flux, daily, occular damage limit.
Actual studies done around acclerators using the d-d & t-d reactions where beam port intensities on the order of 10e9 neutrons/sqcm/sec show that even fairly close to the machine where scattered levels of 200-300n/sqcm/sec are seen, it would take well over two hours at this nearby point to built up an appreciable dosage. Workers around these machines and cyclotrons are limited to 25n/sqcm/sec as a working flux level.
We use the total isotropic neutron emission when talking about our fusors. This is our method. Flux is what all the professionals use. Our flux levels are far below the worry point and our time of exposure at these low flux levels is negligable. Nonetheless, we increase the neutron background around our fusor by about 1000 fold.
Richard Hull
Created on Monday, March 12, 2001 11:05 AM EDT by Richard Hull