In the framework of the ATTRACT-uRANIA project, funded by the European Community, we are developing an innovative neutron imaging detector based on micro-Resistive WELL (μ-RWELL) technology. The μ-RWELL, based on the resistive detector concept, ensuring an efficient spark quenching mechanism, is a highly reliable device. The detector is composed of only two elements, i.e. a readout-PCB which comprises the amplification stage and a cathode. The amplification stage of the detector, realized by photolithography as a matrix of wells (with a pitch of 140 μm and a diameter of 60-70 μm) on a 50 μm thick polyimide substrate, is embedded in the readout board through a resistive layer realized by means of an industrial process with DLC (Diamond Like Carbon). The required surface resistivity, typically ranging from few tens to hundreds of MOhm/square, is clearly a parameter that must be optimized as a function of the detector performance, such as rate capability, spark amplitude quenching and maximum achievable gain.
The cathode electrode defining the gas conversion-drift gap completes the detector mechanics: depositing few μm of Boron-10 on the copper surface of the cathode, will allow thermal-neutron detection through the release of an alpha particle, inside the active volume of the device, and a 7Li atom (10B + 1 n → 7Li + α). Boron-10 is one of the technologies being developed as an efficient and convenient alternative to the He-3 shortage.
The goal of the project is to prove the feasibility of such a novel neutron detector by developing and testing small planar prototypes with readout boards suitably segmented with strip/mini-pad readout, equipped with existing electronics or readout in current mode.
The standard cathode PCB (18 micron of Cu layer on a 1,6 mm FR4 glass epoxy plate) of the μ-RWELL has been coated with different thickness (1,5 – 2,0 – 2,5 – 3,0 – 4,3 microns) of 10B4C deposition by the ESS Coating Workshop in Linköping, Sweden. The thickness of the Boron layer is crucial for the conversion efficiency, a Geant4 simulation has been used to optimize the geometry of the detector.
A preliminary characterization of the prototypes has been done at the ENEA-HOTNES a calibrated 241Am-Be thermal neutron source placed in a cylindrical cavity delimited by polyethylene walls at the ENEA – Frascati Laboratory. HOTNES exploits a polyethylene shadow bar that prevents fast neutrons to directly reach the samples. The effect of the shadow bar and of the cavity walls, combine in such a way that the thermal neutron fluence is nearly uniform. The nearly uniform thermal neutron fluence rate at HOTNES reference irradiation plane is 758±16 cm-2s-1.
For this test, the detectors, operated with the gas mixture Ar/CO2/CF4 (45/15/40), have been read-out in current mode through a CAEN HV module A1561HDM. To extract from the current measurement the neutron detection efficiency, a detailed simulation of the thermal neutron detection process as well as a fine calibration of the gas gain of each detector used during the test has been performed.
The thermal neutron efficiency epsilon is extracted from the measurement of the average current drawn by the irradiated detector through the formula: I = e N epsilon G R, where e is electron charge, G is the gain of the detector and R is the thermal neutron fluence irradiating the detector, while N, estimated with the simulation, is the average ionization generated in the gas gap by the alpha or Lithium ions coming from the interaction of the thermal neutron with 10B atoms. An overall neutron detection efficiency ranging between 1.5-2.0 (±0.2) % has been measured with 10B deposition in the range 1.5-4.3 micron thick. Systematic effects (measured and simulated) due to the absorption and back-scattering of the thermal neutron on the FR4 glass epoxy structure of the cathode PCB have been taken into account.
Further studies will be focused on the increasing of the detection efficiency and on the spatial resolution.
To increase the detection efficiency, a stack of Boron-coated aluminum mesh will be placed between the cathode and the readout PCB. Geometry optimization of the mesh and the spacing will be object of future studies with prototypes and simulation.
To achieve a spatial resolution of the order of 100 microns a combined charge and time readout will be used. A reconstruction software is being developed to allow exploiting the full potential of the readout electronics by means of processing charge and time information to estimate the position of the neutron interaction. Charge centroid and μTPC clusterization algorithms, developed for the BESIII Cylindrical GEM detector, will be adapted to the new detector configuration.
Preliminary results from the test with different prototypes, showing a good agreement with the simulation, will be presented together with construction details of the prototypes and the future steps of the project.