The modular neutron array (MoNA) is a detector array specialized to detect neutrons—the neutral particles in the core of atoms. MoNA can measure both the speed and the direction of the neutrons that occur after a rare isotope beam strikes the target. By studying the short-lived neutron-rich isotopes produced at NSCL, scientists can observe how nuclei behave in extreme conditions. The information MoNA generates paints a picture of the interior of these rare nuclei.
The neutrons MoNA detects come from nuclear reactions that occur when the rare isotope beam strikes a target placed in front of the neutron detector. Even though the neutrons are moving at about 30-60 percent of the speed of light, the sensitivity of the detector is so high that it is able to detect 70 percent of all neutrons that hit the detector.
A neutron detector needs to convert the neutrons into charged particles in order to generate a measurable signal. In the case of MoNA, this is done using a plastic scintillator. The incoming neutron strikes the nucleus of one of the atoms of the plastic scintillator. The charged nucleus recoils from the collision and excites other atoms as it moves through the scintillator. The excited atoms give off light as they relax. This process is called scintillation. In addition to plastic, the detector also incorporates thin layers of steel, which have a much higher density and therefore yield more reactions with the neutrons. The charged particles that exit the steel are detected in a scintillator layer which is located behind the steel layer.
The detector array consists of 144 bars of plastic scintillator. Each of these bars measures 10 cm by 10 cm and 2 m wide. The bars are stacked to form a wall that is 2 m wide and 1.6 m high. The ends of each detector bar are equipped with photo-multipliers that are able to detect the faint scintillation light and amplify it by a factor of 30 million. These photo-multipliers also measure when the light arrives very precisely, so the position of the light emission along the bar can be determined within a few centimeters by measuring the time difference of the signals at the left and the right end. This time difference has to be known to within 0.00025 millionth of a second.
With the precise timing information, we also can calculate the velocity of the neutrons. We place a start detector before the reaction target—where the neutron is still part of the rare isotope—and use MoNA as a stopwatch. The neutrons travel a distance of about 10 m in less than 1/10 millionth of a second. The sweeper magnet that is placed between the target and MoNA deflects all charged particles, otherwise they would interfere with the measurement of the neutrons.
When the rare isotope comes close enough to a nucleus of the reaction target, it can break into pieces, and this isotope can be studied through these fragments. For neutron-rich nuclei, some neutrons are usually only very loosely attached, so it does not take much to split them off.
While the tightly bound (and charged) portion of the rare isotope is deflected by the sweeper magnet, the neutrons travel on into MoNA, where they are detected. MoNA tells how many neutrons we detected, their position, and how fast they were going. The properties of the charged fragment are measured in coincidence. All this information taken together can be used to reconstruct the properties of the neutrons within rare isotopes.
Learning about these neutron-rich nuclei will provide a deeper understanding of the structure and interactions of nuclei. It will also provide answers to astrophysical questions. Rare neutron-rich nuclei play a key role in the synthesis of the heavy elements and help drive tremendous stellar explosions such as supernovae and x-ray bursters.
Reconstructing the properties of neutrons can help us understand novel structures that develop when one goes from the stable isotopes out towards neutron-rich isotopes, where the neutron binding becomes weaker and weaker. One example is the phenomenon of halo nuclei. In certain neutron-rich nuclei, some of the neutrons leak out of the main body of the nucleus to form a hazy cloud or "halo" around the core. These nuclei can be many times larger than normal nuclei and exhibit strange properties.
Although none of these neutron-rich nuclei exist on earth because they have extremely short lifetimes—less than a thousandth of a second—they do occur in our universe. The environments inside a supernova or on the crust of neutron stars produce many of these rare isotopes. Supernovae are the source of the heavy elements we find in our universe. If we seek to understand the origin of the world we live in, we need to learn how nuclei behave on the edge of stability. MoNA is an essential tool in that quest.
The modular neutron array (MoNA) is an efficient detector for high-energy neutrons. It is operated by a collaboration between MSU, Florida State University, Central Michigan University, Concordia College, Hope College, Indiana University at South Bend, Marquette University, Wabash College, Western Michigan University, and Westmont College. MoNA consists of 144 horizontal blocks of plastic scintillator arranged in 9 vertical planes of 16 detectors each, covering an area of 2.0 m wide by 1.6 m high. The individual detector blocks are fitted with photo-multiplier tubes on each end. The detector is position-sensitive and has multi-hit capability. The addition of passive iron converters enhances the detection efficiency for neutrons with energies above 100 MeV for an average efficiency of about 70%.
Location: N2 vault
Contact person: Thomas Baumann
Funding acknowledgement: The Modular neutron array was funded by the National Science Foundation through nine separate Major Research Instrumentation (MRI) grants to the participating institutions.
MoNA - The Modular Neutron Array; B. Luther, T. Baumann, M. Thoennessen, J. Brown, P. DeYoung, J. Finck, J. Hinnefeld, R. Howes, K. Kemper, P. Pancella, G. Peaslee, W. Rogers and S. Tabor, Nucl. Instr. and Methods A505 (2003) 33.