The Central Neutron Detector installed in Experimental Hall B. Silvia Niccolai and her team at the Laboratory of the Physics of the two Infinities Irène Joliot-Curie (IJCLab), a joint research unit of CNRS in Orsay, France, Paris-Saclay University, and Paris-City University, began constructing the detector in 2011 with funding from the French National Institute of Nuclear and Particle Physics. (Photo courtesy Silvia Niccolai)
An inaugural measurement of the neutron will help physicists learn about nucleon structure and spin
Protons and neutrons–known collectively as nucleons–are the building blocks of matter, but one of these particles has received a bit more attention in certain types of nuclear physics experiments.
Until now. New results published in Physical Review Letters describe a first-time glimpse of the internal structure of the neutron thanks to the development of a special, 10-years-in-the-making detector installed in Experimental Hall B at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
“We detected the neutron for the first time in this type of reaction, and it’s a quite important result for the study of nucleons,” said Silvia Niccolai, a research director at the French National Centre for Scientific Research (CNRS).
Niccolai proposed the experiment that enabled this measurement, which will help physicists better understand the structure and spin of both neutrons and protons.
A new way to detect neutrons
Nucleons are made up of smaller particles called quarks and gluons. Physicists don’t yet fully understand how these constituent particles are distributed inside nucleons, or how they contribute to overall nucleon spin. Experimenters use the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility, to probe these particles, scattering electrons off nucleon targets and detecting the final products of these reactions.
One reaction is called deeply virtual Compton scattering (DVCS). In DVCS, an electron interacts with a nucleon target. The nucleon absorbs some of the electron’s energy and emits a photon, but doesn’t break. In the end, three particles can be detected: the impinged nucleon, the photon it emitted, and the electron that interacted with the nucleon.
Researchers have studied DVCS extensively using the CLAS12 detector, which stands for the CEBAF Large Acceptance Spectrometer at 12 GeV beam energy, as well as its predecessor, CLAS. However, the CLAS and CLAS12 detectors in Hall B have mostly been used to explore DVCS on the proton, which is easier to measure than DVCS on the neutron.
Neutrons involved in DVCS are more difficult to detect because they tend to scatter 40 degrees up from the beamline, an area CLAS12 cannot access.
“In the standard configuration, there was no detection for neutrons possible in these angles,” Niccolai said. In 2007, she started thinking about how the CLAS collaboration of nuclear physicists could measure these neutrons. Her solution? The Central Neutron Detector.
Niccolai and her team at the Laboratory of the Physics of the two Infinities Irène Joliot-Curie (IJCLab), a joint research unit of CNRS in Orsay, France, Paris-Saclay University, and Paris-City University, began constructing the detector in 2011 with funding from the French National Institute of Nuclear and Particle Physics.
The team completed the detector in 2015. Two years later, it was installed in CLAS12. Pierre Chatagnon, a Ph.D. student at Paris-Saclay University at the time, joined the IJCLab team at Jefferson Lab to install the detector. He also wrote software to calibrate it. Today, he has returned to Jefferson Lab as a postdoc in Hall B.