The Neutron Emission Ratio Observer

The neutron emission ratio observer (NERO) is a device that detects neutrons—tiny electrically neutral particles that together with protons are the building blocks of atomic nuclei. The formation of elements in the explosions of stars involves extremely rare nuclei that decay within fractions of seconds. One goal of NERO is to find out whether these nuclei eject neutrons when they decay.

Expanded Description

Neutrons play a critical role in the formation of elements in the universe. Basically all elements heavier than iron—including important elements like gold or uranium—have been, and still are, created in stars or star explosions by processes that involve neutrons. The goal is to find out how stars can create all these elements. NERO can simulate the nuclear processes in the laboratory and “see” the neutrons involved.

The formation of elements in the explosions of stars involves extremely rare nuclei that decay within fractions of seconds. However, these nuclei which exist in exploding stars are very hard to produce on earth. Even though NSCL is the leading laboratory in the world for this type of experiment, only a few of these nuclei can be made within the few days of an experiment. NERO has therefore been designed to be extremely efficient in observing neutrons.

Because neutrons are electrically neutral, it is very difficult to detect them. NERO uses about 400 pounds of plastic to slow the neutrons down. Once they have low velocities they enter tubes filled with gas that contains helium or boron. When a neutron strikes one of these gas nuclei, a charged particle is created—either a proton or an alpha particle. The charged particles knock electrons off the gas atoms, and these electrons are collected by high voltage electrodes that generate an electrical signal. This electrical signal is processed by a computer and tells us that there was a neutron around.

The Importance of NERO

The origin of the chemical elements that make up the world is one of the most fundamental scientific questions. After the big bang, the universe consisted only of hydrogen and helium with traces of lithium. All the other elements—including the carbon in our bodies, and the iron, silicon, and oxygen that makes up most of earth—have been created later by nuclear reactions in stars. When stars explode, they eject their freshly made nuclei into space. This stardust can then form new stars that continue the process of the formation of elements, and also planets, like the earth. However, the origin of many elements heavier than iron—including gold and uranium—is still a mystery. With NERO, experiments can be performed that simulate some of the nuclear processes that might possibly be responsible for the origin of these elements.

One possible place for the formation of heavy elements in the cosmos is the explosions of stars (supernovae). NSCL can create the extremely rare nuclei that are part of the reactions that form new elements in these explosions. Those nuclei decay within fractions of seconds—exactly as they do during a stellar explosion. NERO can help determine whether they eject a neutron during their decay. If that is the case, a different nucleus is created in the process than if a neutron had not been ejected. Results from NERO can show whether star explosions are able to explain the proportions of heavy elements in the world.

Another application of NERO is to measure the efficiency of the nuclear reactions that create neutrons in stars needed for building new elements. These nuclear reactions can be simulated in the laboratory at stellar temperatures (about 100 million Kelvin), and NERO will tell how many neutrons are made. The results are used to test whether a given astrophysical model of a star makes enough neutrons to synthesize new elements.

Technical Information

The neutron emission ratio observer (NERO) is a low-energy neutron detector consisting of three concentric rings of 3He and BF3 proportional counters embedded in a 60 x 60 x 80 cm³ polyethylene matrix and centered around a 22.4 cm diameter beam line opening. NERO detects neutrons ranging in energy from 1 keV to 5 MeV with an efficiency of approximately 30%–40%. A rough estimate of the neutron energy distribution can be obtained from ratios of counts within the three rings. Layers of boron carbide and water can be placed around the detector to minimize neutron background.

Status: Operational

Location: N2 vault

Contact person: Hendrik Schatz

Funding acknowledgement: The neutron emission ratio observer (NERO) is funded by the National Science Foundation and the Alfred P. Sloan Foundation.

Illustrations:

	Schematic drawing of NERO indicating the sizes of the various detector rings and the beam line hole. (gif 18.2 kByte eps 872 kByte)
	Photograph of the full setup of NERO including electronics and shielding as seen from the back of NERO. (jpg 800 2400 px wide)