A laser hits a target of thin wires, and within tens of femtoseconds (billionths of a billionth of a second) knocks out a stream of protons. These protons fly into a converter made of lithium and deuterium, where neutrons are born. In an experiment with a laser of one petawatt (a million billion watts), they got up to eight million neutrons per joule of laser energy.
The key trick is the spacing between the wires. If they are too close, the laser light cannot penetrate deep. If too far apart, the protons scatter sideways. The chosen pitch allowed the protons to be gathered into a dense beam and directed into the converter. A computer model reproduced the experiment and showed that replacing the converter with a beryllium one would increase neutron yield by another factor of four.
A compact laser-driven neutron source can operate at high repetition rates — thousands of shots per second. This paves the way for portable neutron devices instead of giant accelerators.
One joule is the energy your heart uses for a single beat. The laser in this experiment delivers billions of times more energy per pulse, but the neutrons are born from just one joule of that energy.
Compact neutron sources resemble the handheld analyzers of science fiction, capable of scanning objects and revealing their composition on the spot.