Dr. Friend: I can at least say where I saw it. people from Brooks. It's in the Physics and Medicine of Outer Space, the book edited by the There is an article on re-entry in there and, from what I gathered, the plan was to try and make maximum burnup occur somewhere in the mesophere. Floor: Getting back to adequacy of radioassay techniques, I'd like to request comments on this: Dr. Machta has shown us what possible levels we might be able to attack, and Dr. Martell pointed out that if we had some very low- yield nuclear bursts put a few kilotons of fission product debris into the higher atmosphere (assuming the atmosphere is perfectly clean, that is, without any background material) we should be able to detect the fresh debris. The question is: what kind of background presently exists and how long do you feel it will exist to ob- scure some of these low levels that Dr. Machta suggested as measurable? Dr. Friend: Have you considered neutron activation? Col, Russell: Neutron activation is so sensitive that it poses background problems. Dr. Lockhart: What kind of neutron activator do you have on the bomb itself that these things were exposed in, unless you wait long enough for that original activity to decay? Col, Russell: There are many things, about a kilogram of which, if irradiated to 30 days or to top of saturation (whichever comes first) with about 5 x 10'4 neutrons per square centimeter per second, are detectable in 19° !4 microgram quantities. In other words, if you put up a kilogram of material and you get yourself a very high rocket fracture back, or whatever you want to call it, you can detect it; you get something like 10 counts a minute on activation. The problem, though, is: I challenge anybody to select something that isn't already up there, radio- active in many, many times these quantities. Dr. Martell: This is a problem, Irv (Russell), but we have looked at it in a couple of more sensitive cases. or europium, for example, which yield activation products on a very high cross section. Take gold You can, in principle, see 10! atoms, which means that only a 10 1° sample of a uniform layer is necessary, and 200 square centimeters is adequate in principle. this. The main point is: what is background? There are ways of getting around Let's assume that we have to go to 100 or 1000 kgm - this is still feasible to carry - to match backgrounds, and we are looking at high levels where these backgrounds are quite low. could carry on such experiments. Europium is a very interesting case. It is not out of the question that one In the background you can distinguish