Current-Biased Transition Edge Detector of $ \hbox{MgB}_{2}$ Nanowires for Neutrons: Imaging by Scanning Laser

Abstract

The superconducting MgB2 nanowire detector is biased by a current by a battery-class low voltage, and uses the nuclear reactions with a single neutron and the 10B contained in MgB2. The MgB2 detector consists of a 200-nm-thick MgB2 thin film meander structure of 3 μm line width protected by 300-nm-thick SiO layer. We use enriched boron 10B to enhance the sensitivity. The thermal energy of 2.3 MeV released by the reaction 10 B(n, α)7Li gives 1.83 MeV to 4He and 0.47 MeV to 7Li. The energy dissipation in a restricted space causes a local resistive state in the MgB2 wire, which yields a subtle change in bias current. Through a mutual inductance connected in series with current-biased detector, we are able to measure the event as a voltage pulse. The position-dependent response of the MgB2 detector is investigated by scanning a focused laser spot with the aid of an XYZ piezo-driven stage and an optical fiber with a focusing lens. A clear two-dimensional imaging pattern of the sensor signal well corresponds to the detector geometry. According to our results, the inhomogeneity and position-sensitive nature of the detector are supposed to be critical in governing the detection efficiency of the current-biased transition edge detector, which works only in the narrow temperature regime near Tc.