[ RadSafe ] Time for calibration: Detector? <Check> Source? <Check> Train? <Check> -- huh?

Cary Renquist cary.renquist at ezag.com
Thu Jan 7 18:14:03 CST 2010

Interesting approach to performing a measurement.

Cary Renquist
cary.renquist at ezag.com

Choo-Chooing Along to Aid in Measure of Neutrons

Published: December 28, 2009

During the holiday season, many people place toy trains on circular
tracks beneath their Christmas trees.

TRAINED Sly Vinson in a fusion reactor at a Princeton laboratory with
the toy engine that was modified and run in circles carrying a
radioactive element.

This month, at the Princeton Plasma Physics Laboratory, physicists and
engineers built tracks inside one of its fusion reactors and ran a toy
train on them for three days.

It was not an exercise in silliness, but in calibration.

The modified model of a diesel train engine was carrying a small chunk
of californium-252, a radioactive element that spews neutrons as it
falls apart.

"We needed to refine the calibration technique to make sure we are
measuring our neutrons as accurately as possible," said Masa Ono, the
project head of the National Spherical Torus Experiment.

The spherical torus experiment is a small reactor designed to test new
approaches to fusion, in which hydrogen atoms are fused together at
ultrahigh temperatures to produce energy - as the Sun does. Fusion
generates copious numbers of neutrons, which tell how well the reaction
is proceeding.

The reactor has been shut down for improvements, and the downtime
provided an opportunity for recalibrating the neutron sensors.

A stationary neutron source was previously used for the calibration, but
that did not fully capture how the neutrons bounced around. Putting the
californium on the moving train improved the accuracy by about a factor
of 10, Dr. Ono said. (The same technique had been used two decades
earlier at one of Princeton laboratory's older reactors.)

Experiments at the reactor are set to restart in March.

Californium is only slightly radioactive, so the toy train did not glow
green after its ride in the fusion reactor. And now that it has
completed its physics duties, the train, back in its original state, is
running around the laboratory's Christmas tree in the lobby.

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