Oak Ridge neutrons have a worldwide impact
Free neutrons in Oak Ridge. They are drawing hundreds of scientists from all over the world to the High Flux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.
“We are overbooked,” said Hans Christen, director of ORNL’s Neutron Scattering Division, in this month’s lecture to Friends of ORNL. One reason is that a few other neutron sources in the world are temporarily shut down.
Another reason is that the SNS and HFIR offer scientists some of the brightest of neutron beams (a million billion neutrons striking a tiny area every second), providing high resolution and sensitivity. It’s like observing dust floating in room air only when a beam of sunlight shines through a window.
The free neutrons are free in several ways. They are free of charge in the sense that they have no electrical charge like protons, their positively charged cousins with which they are confined in atomic nuclei. Because they have no charge, they can penetrate deeper into material than other probes such as X-rays and electrons.
Christen said they are free of charge for scientific users of the instruments costing millions of dollars that receive neutrons flowing from the beamlines of HFIR and SNS, provided the researchers publish the results of their experiments in the open literature.
He explained that the free neutrons had been liberated from nuclei in uranium-238 atoms bombarded by neutrons in HFIR or from the nuclei of mercury atoms in a steel vessel at the SNS target station, which are subjected to accelerated pulses of protons.
Neutrons:Learn about how neutrons help scientists make discoveries on Tuesday
For every proton striking a mercury nucleus, at least 20 neutrons are freed and then channeled into a beamline. They are aimed at a sample – a solid or liquid material in an instrument, ranging from plastics to proteins. Neutrons may bounce off a sample’s atomic nuclei at angles that can be measured. Or they may pass through the sample or get absorbed by it.
Detectors of the scattered neutrons that have different velocities and arrive at different times help researchers learn the positions of various atoms or molecules, indicating a material’s structure….
