High resolution neutron counting detectors with microchannel plates and their applications in neutron radiography, diffraction and resonance absorption imaging

Neutron News Volume 23 • Number 4 • 2012 35 Microchannel plates The event counting detection technology based on signal amplifi cation in microchannel plates (MCPs) was initially developed for the applications with relatively low fl uxes of photons, electrons and charged particles, such as astrophysical missions, mass spectroscopy, timeof-fl ight (TOF) photoelectron spectroscopy and others. Although the extension of this technology to neutron imaging was suggested in 1990 [1] it was only few years ago when neutron sensitive MCPs became available from Nova Scientifi c [2] for trial experiments. In parallel, fast readout electronics capable of high count rates were developed for the MCP detectors, extending their use to synchrotron and neutron beamline applications where incoming fl uxes may exceed 107 counts/cm2/s. Timepix readout technology [3] developed by the Medipix collaboration at CERN (http://medipix.web.cern.ch/medipix/) installed directly behind the MCP stack provides unique possibilities for neutron counting with spatial resolution of 55 μm (pixel size of Medipix) at high counting rates (>107 counts/cm2/s) and sub-15 μm spatial resolution for the rates of ~5 × 105 counts/cm2/s [4], [5]. One of the main advantages of the MCP/Timepix detectors is their ability to detect both time (~1 μs for thermal and sub-100 ns for epithermal neutrons) and position of many simultaneous events as pixels count neutrons independently. Another attractive feature of these detectors is their ability to operate at a high frame rate of ~1.2 KHz with the readout time of ~290 μs and virtually no readout noise. The active area of these devices is relatively small (28 × 28 mm2 for our present implementation of 2 × 2 Timepix readout) but can, in principle, be extended to 2 × N arrays as Timepix is three-side buttable. The effi ciency of MCP detector reached the predicted levels of 70% for the cold neutron spectrum [6] (ICON beamline, Paul Scherrer Institute, PSI, Switzerland) and ~50% for thermal neutrons. At the same time, the latest experiments at the pulsed neutron beamlines indicate that epithermal neutrons can also be detected with reasonable effi ciencies [7]. Therefore neutron resonance absorption imaging and spectroscopy can be performed simultaneously with Bragg edge imaging at neutron beamlines with short duration initial pulses.