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Modular Neutron Array


The Modular Neutron Array (MoNA) is a large-area, high efficiency neutron detector that is used in basic research of rare isotopes at Michigan State University's National Superconducting Cyclotron Laboratory (NSCL), a nuclear physics research facility. It is specifically designed for detecting neutrons stemming from breakup reactions of fast fragmentation beams.

The Modular Neutron Array consists of 144 individual detector modules. Each module is based on a plastic scintillator measuring 10 cm by 10 cm by 200 cm. This scintillator bar is fitted with light guides on each end that direct the light into one photo-multiplier tube on each end. Each detector module is wrapped in a light-tight material, allowing the detector array to be arranged in different configurations.

In its original configuration, MoNA consisted of 9 vertical layers of 16 detectors stacked closely, having an active area of 2.0 m wide by 1.6 m tall. In its current arrangement (depicted in the adjacent image), it is stacked in four separate sections of 2, 2, 2, and 3 layers each, respectively, separated by spaces ranging from 0.5 to 0.8 meters. It measures both the position and time of neutron events with multiple-hit capability. The energy of a neutron is based on a time-of-flight measurement. This information together with the detected position of the neutron is used to construct the momentum vector of the neutrons.

The detection efficiency of MoNA is maximized for the high-beam velocities that are available at the NSCL's Coupled Cyclotron Facility (CCF). For neutrons ranging from 50 to 250 MeV in energy, it is designed to have an efficiency of up to 70% and expands the possible coincidence experiments with neutrons to measurements which were previously not feasible. The detector is used in combination with the Sweeper magnet and its focal plane detectors for charged particles. In addition, MoNA’s modular design allows it to be transported between experimental vaults and thus to be used in combination with the Sweeper magnet installed at the S800 magnet spectrograph. Due to its high-energy detection efficiency, this detector will be well suited for experiments with fast fragmentation beams at the proposed ISF.

When the NSCL upgraded their capabilities to the Coupled Cyclotron facility, a Florida State University/Michigan State University consortium built the Sweeper magnet to be used with two existing neutron walls to perform neutron–fragment coincidence experiments. The neutron walls were originally built for lower beam energies and had only an efficiency of about 12% for the neutron energies expected from the CCF. During the 2000 NSCL users meeting, a working group realized the opportunity to significantly enhance the efficiency with an array of more layers using plastic scintillator detectors.


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