the Strontium 90 is detectable, that you just don't see that. Of course, if we are talking about concentrated fission products and no other solid material in the upper atmospher around 150, 000 feet, then with a large collection of many small particles, or a collection of just a few middle-sized particles around 0,2 micron in radius, there would be a possible chance of analysis and classification. Col. Russell: I was thinking of this in terms of a possible application to study of materials produced by burnup of a reentry vehicle. Dr. Friend: Right. I think that that is one of the important problems faced by this meeting, in that the mesosphere is essentiaily the place where re-entering vehicles will burn up, that is, between 50 and 80 kilometers. Floor: One might comment that, if this is possible, the place to look is well above the layer that you have been studying. The background should be less. All this is assuming that-we do have a preferred region of holdup or layer of holdup at higher levels. Dr. Friend: According to Chris's (Junge) balloon data, the number concentration drops off very rapidly above 75, 000 feet. Of course, we haven't made too many measurements at 90,000 feet to determine exactly what is going on there and above it, but the problem with re-entering vehicles is: fraction so that you know what you are studying? can you get them to burn up in a significant This is also the problem of injecting a tracer at these altitudes. If you could be guaranteed that it is all going to be nonsedimenting material, it would be glorious; but, if you have the sedimenting material mixed in with it, the resulting problem is difficult. Floor: There is the problem of what is a micrometeor material background that you are contending with even here. You have a large enough mass that is unique enough at high atmosphere to discriminate this particular source, Dr. Friend: This is essentially right. I might put in a plea from the people who are studying micrometeorites: Please, let's not put the elements in the higher atmosphere which will confuse the micrometeorite problem. Dr. Kalkstein: To get back to the last question, in a more general sense of this panel - the use of this method as an alternative technique of assay - let me add a hopeful comment. When we talk about 1-dpm beta as a limit of what one can do, appreciate that this figure was gotten by lots of experience in low-level counting. Part of electron microprobe work is the count, and the people, so far, in electron microprobe work are not low-level people as 93