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

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 T_c.