THE MODULAR NEUTRON ARRAY
MoNA is composed of 144 individual detector bars that can be assembled in a variety of configurations. The standard 16 by 9 configuration is shown in the photo below.
The MoNA bars measure 200 x 10 x 10 cm3. The attached light guides measure 6.25" in length tapering to a 2" diameter cylinder after 5 in. An aluminum flange is mounted at the base of the cylinder with the end extending 1" beyond the flange. The scintillator material is BC-408 with a bulk attenuation length greater than 3.8 m. The light guide material is a UVA acrylic plastic known as BC-802.
Photonis X2262B 12 stage 51 mm (2") photomultiplier tubes are attached to two ends of the detector bar. These tubes provide a nominal gain of 6.0 x 107 and an anode pulse rise time of 2 ns.
By measuring the time difference between the phototubes at each end the position of the neutron strike along the bar can be determined within 7.0 cm while the bar size (10 cm) determines the precision of the position in the other two directions. Momentum and energy of the neutron is determined by time of flight between a thin scintillator in front of the target and the position of the strike in MoNA.
MoNA can achieve a 70% detection efficiency for neutrons between 50 and 250 MeV. With an 8 meter flight path MoNA has a better than 1% energy resolution for 100 MeV neutrons and an angular resolution of 12.5 mrad.