MoNA (the Modular Neutron Array) is a detector array specialized in detecting high-energy neutrons. The neutrons that MoNA detects come from nuclear breakup reactions that occur when the rare isotope beam strikes a target placed upstream from the neutron detector. MoNA measures both the velocity and direction of the neutrons leaving the target. Even though the neutrons are moving at about 30–60% of the speed of light, the detector is designed so that it is able to detect 70% of all neutrons that reach the detector. A powerful dipole magnet, the Sweeper, which is placed between the target and MoNA, deflects all charged particles, otherwise they would interfere with the measurement of the neutrons.
Since neutrons are not charged, they can not be directly detected. Instead, they need to react with a nucleus, for instance scatter off it. This is what happens inside the plastic scintillator, the active detector material that MoNA utilizes. Because of the relatively long distance the neutron will travel in matter before it scatters, an efficient neutron detector requires a large volume of detector material.
The recoiling charged nucleus from the collision excites atoms as it moves through the scintillator. The excited atoms give off light as they relax. This process is called scintillation.
The detector array consists of 144 individual bars of clear plastic scintillator. The bars are stacked 16 high and 9 deep to form a large array. Each of these bars measures 10×10 cm² and is 2 m wide. The ends of each detector bar are equipped with photo-multipliers that are able to detect the faint scintillation light and amplify its initial signal by a factor of 30 million. The MoNA electronics enables us to determine the position of the light emission along the bar 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 250 picoseconds. With the precise timing information, we also can calculate the velocity of the neutrons from their flight time. The neutrons travel a distance of about 10 m in less than 100 nanoseconds.
The information we get from MoNA will enable us to build a picture of the interior of rare neutron-rich nuclei. Learning about these nuclei will give us a deeper understanding of their structure. 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.