Geophysicists moderate neutrons

There was a brilliant article in last November’s EOS (The American Geophysical Union newsletter) that described an experiment that measured the moderation of cosmogenic neutrons by the attendees at the 2006 annual conference.

Neutron moderation is the decrease in kinetic energy of neutrons caused by the elastic collisions between the neutron and atomic nuclei. Because the collisions are elastic, the neutrons loose energy to light atoms much more easily than to heavy ones, so hydrogen is by for the most efficient moderator that naturally occurs in the geologic environment. The stated goal of the research project is to use cosmogenic neutron moderation to calculate soil moisture, snowpack, and other hydrologic purposes; the measurement of conference attendance was meant as a clever technology demonstration.

I honestly have no idea how cosmic rays generate neutrons in the first place. But the process generates fast neutrons, which are moderated until their kinetic energy is equal to that of the particles with which it is colliding. Since that energy is related to temperature, such neutrons are called ‘thermal neutrons’. These thermal neutrons typically travel much shorter distances before getting absorbed in a nuclear reaction (such as n + ¹⁴N -> p + ¹⁴C, the cosmogenic isotope of dating fame), compared to fast neutrons. Thus, increased moderation reduces the area of cosmogenic neutron production that is sampled by their neutron detector, lowering the count rate.

Desilets et al. placed their instrument in “a ground floor exhibit booth in the low-rent (far southwest) corner of Moscone Center West, which was optimally located between the afternoon beer cart and some restrooms.”

The main sources of hydrogen to use as a moderator in this location were the bodies fo the geoscientists attending the meeting, who the authors modeled as “

The corrected for changes in cosmic ray flux, magnetic field location, and elevation. They addressed the issues of neutron attenuation by the upper floors of the building and the potential moderation of neutrons produced by people on those floors, by modeling those distinguished researchers as, “an equivalent depth of water and bone spread across the upper floors.”

There results show a decrease in neutron flux while the conference was in session. This is “consistent with an increased presence of hydrogen in Moscone Center West, which we interpret to be mostly from conference attendees.” The flux recovered during the lunch break, before declining for the afternoon session as “high-fluid-content AGU conference attendees returned to the conference floor.”

Their estimate for the number of people on the ground floor during the sessions? About 1700.

Full article: Desilets D, Zreda M, Ferre T, 2007. Scientist Water Equivalent Measured with Cosmic Rays at 2006 AGU Fall Meeting. EOS 88, 48, 521-522.