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Although event counting detectors with Microchannel Plates (MCPs) were originally developed primarily for applications with low input fluxes, recent progress in readout electronics substantially extended their counting rate capabilities, while still maintaining very low dark count rates and no readout noise. Detection of many simultaneous events and operation at rates exceeding 100 MHz has been demonstrated with combination of MCPs and a Timepix readout. The spatial resolution in such devices can be an order of magnitude better than the 55 μm pixel size of Timepix chips when event centroiding is implemented. However, a compromise between spatial resolution and the number of accepted events has to be found for such detectors operating at high rates. Here we present an MCP/Timepix detector capable of simultaneous detection of an image with the highest spatial resolution of ~6.5 μm, but reduced number of counts and an image with all registered photons but reduced 55 μm spatial resolution. A compromise between detector resolution and the acquisition duty cycle is also discussed. It was confirmed that high spatial resolution can still be maintained through event centroiding even with 60% of pixels hit by more than one photon per acquisition frame.

IMAT is a new cold-neutron imaging facility at the neutron spallation source ISIS at the Rutherford Appleton Laboratory, U.K.. The ISIS pulsed source enables energy-selective and energy-resolved neutron imaging via time-of-flight (TOF) techniques, which are available in addition to the white-beam neutron radiography and tomography options. A spatial resolution of about 50 μm for white-beam neutron radiography was achieved early in the IMAT commissioning phase. In this work we have made the first steps towards achieving higher spatial resolution. A white-beam radiography with 18 μm spatial resolution was achieved in this experiment. This result was possible by using the event counting neutron pixel detector based on micro-channel plates (MCP) coupled with a Timepix readout chip with 55 μm sized pixels, and by employing an event centroiding technique. The prospects for energy-selective neutron radiography for this centroiding mode are discussed.

Part of 15th International Workshop on Radiation Imaging Detectors (IWORID2013) A photon counting, microchannel plate (MCP) optical imaging tube has been fabricated using a 2 × 2 array of Timepix application specific integrated circuits (ASICs) as the readout anode. A Timepix ASIC is a 256 × 256 pixellated CMOS readout chip with each pixel containing an amplifier, discriminator and counter. The counter values, representing either time of arrival, total count or time over threshold, can record the position and time of arrival of the electron pulses from the MCP if the charge collected on its input pads exceed the adjustable lower threshold value. Below we present initial results of the tube's performance, the quantum efficiency of the bi-alkali photocathode, uniformity of response, spatial and temporal resolution, and dynamic range. Planned improvement to the design based on the new Timepix3 chip will be discussed.

The capability of a Timepix readout to detect both position (with 55 ?m accuracy) and time (with up to 10 ns resolution) of multiple simultaneous events enables novel non-destructive testing methods. The first generation Timepix ASIC provides timing only for one event per pixel per frame. With fast parallel readouts capable of > 1 KHz frame rates up to ~ 3 ? 106 counts cm?2s?1 can be detected with 10% overlap probability. For some applications, where incoming fluxes exceed this rate the event overlaps distort the measured timing signal. In this case many particles arriving late in the frame are not registered as many pixels are already occupied by preceding counts. In this paper we demonstrate how the count rate capability of Timepix readout can be increased in the experiments where incoming flux has a periodic structure. It is shown how the timing characteristics of input flux can be accurately reconstructed even for ~ 80% pixel occupancy, where the probability of detection is reduced for the later events of the acquisition frame. The implementation of the correction algorithm is demonstrated for a UV flux varying with a 60 Hz frequency. The post-measurement reconstruction method is implemented for each individual pixel of the acquired images and thus can be applied even for the experiments where the incoming flux has a strong variation across the active area.

Novel large area microchannel plates (MCPs) constructed using micro-capillary arrays functionalized by atomic layer deposition (ALD) have been successfully demonstrated in large format detectors (10 cm and 20 cm) with cross delay line and cross strip readouts. Borosilicate micro-capillary substrates allow robust MCPs to be made in sizes to 20 cm, the intrinsic background rates are low ( 7 C cm−2 extracted charge. We have constructed a number of detectors with these novel MCPs, including a 10 × 10 cm cross strip readout device and 20 × 20 cm delay line readout sensors. The cross strip detector has very high spatial resolution (the 20 μm MCP pores can be resolved, thus obtaining ~ 5k × 5k resolution elements), good time resolution ( 5 million counts/s at 20% dead time), while operating at relatively low gain ( ~ 106). The 20 × 20 cm delay line detectors have achieved spatial resolutions of ~ 50 μm and event rates of several MHz, with good gain and background uniformity and 20% quantum efficiency and good uniformity for large area (20 cm) bialkali photocathodes.

This paper reports on the edge enhancement and refraction/scattering effects in neutron radiography measured at thermal and cold neutron beams with a high resolution ( ~ 55 μm) microchannel plate neutron counting detector. These effects in some cases can enhance the contrast of certain features in the neutron radiographic images. At the same time, the edge effects introduce image distortions, as in case of tomographic reconstructions. The edge effects in radiographies of several steel and aluminum objects are shown for different beam divergences and sample orientations relative to the beam. It is also demonstrated how novel microcapillary neutron collimators can enable refraction and scattering contrast imaging in some cases, where the refraction and scattering angles are relatively large. These collimators can also be used to reduce some refraction artifacts, namely remove bright edges in the transmission images.

The existing Berkeley neutron sensitive MCP/Timepix hybrid detector has been very successful at demonstrating energy resolved spatial imaging with a single Timepix ASIC read out at a ~ 30 Hz frame rate where each neutron's position and time (energy) is determined (X,Y,E). By increasing the detector format using a quad arrangement of Timepix readouts and increasing the frame rate to 1 kHz, we can increase our total event throughput by a factor of 120, thereby taking full advantage of the high fluxes of modern pulsed neutron sources (106 n cm?2 s?1). The key to this conversion is a new design for the ASIC readout, called the Berkeley Quad Timepix detector, consisting of 3 major subsystems. The first is a quad (2 ? 2) bare Timepix ASIC board mounted directly behind the neutron sensitive MCPs in a hermetic vacuum enclosure with a sapphire window. The data from the Timepix ASICs flow to the second subsystem called the Interface board whose field programmable gate array (FPGA) rearranges and converts the digital bit stream to LVDS logic levels before sending downstream to the third subsystem, the Roach board. The Roach board is also FPGA based, and takes the data from all the ASICs and analyses the frames to extract information on the input events to pass on to the host PC. This paper describes in detail the hardware and firmware designs to accomplish this task.

The spatial resolution in neutron imaging is often limited by the neutron beam divergence, especially in experiments where the distance between the sample and the detector cannot be very small. Imaging of relatively large or extended objects or tomographic experiments, where samples have to be rotated, typically require several to tens of centimeters distance between the sample and the detector. Moreover, some neutron imaging techniques require placement of components between the sample and the detector, as in the case of magnetic field imaging with polarized neutrons. Compact and efficient policapillary collimators can be an attractive alternative to the conventionally used beam limiting aperture approach. The possible improvement of spatial resolution by a policapillary collimator is studied in experiments where beam divergence limits the quality of neutron transmission imaging. A ~ 7 mm thick collimator with ~ 6 ?m capillaries demonstrates the possibility to decrease the neutron beam divergence and to diminish the image blurring. The improvement of out-of-angle neutron rejection and peak transmission of our future collimators will be crucial for their effective application in neutron imaging experiments.

Neutron imaging is a non-invasive method for material research on the macroscopic level. It is carried out at laboratories equipped with powerful neutron sources, suitable neutron beam lines and neutron detection systems. Decades ago neutron radiography began capturing images with film techniques. These techniques yielded excellent spatial resolution even over large fields of view. In the recent years, improvements in the detection techniques and their digitization have been the main forces driving successes in neutron imaging. Several detector options have been developed, implemented and used in practical applications in order to achieve digital information from the neutron transmission process which is needed for a quantitative evaluation of image data by sophisticated methods like neutron tomography, phase contrast imaging, neutron interferometry and time dependent studies. The most common approach in digital neutron imaging is a conversion of the neutron field information into visible light by a scintillation process, where a neutron converter is needed because neutrons do not excite directly due to their neutral charge. Low level light signals can be observed either with sensitive camera systems or by using amorphous silicon based semiconductor plate devices. However, these now established detection techniques are still limited in respect to spatial and time resolution. The best possible spatial resolution which can be achieved today is available by a system built at PSI with about 10 μm pixel size. Recently, it was upgraded with a tilted option for an increased resolution by a factor of 4 in one direction. Scintillator based techniques are limited by the dissipation of the secondary particles. This limitation has motivated the search for new detector options. One approach is a pixilated system where the readout per incoming neutron can be used to calculate precisely the position of its impact. Such devices are realized as the TIMEPIX system already. The system was tested successfully at PSI and other neutron imaging facilities. Other options might be to use MEDIPIX devices with neutron absorbing/converting materials. A similar pixel detector, EIGER, has been developed recently at PSI and its performance in the field of neutron imaging is under investigation. For future applications at the upcoming pulsed spallation sources the time = energy resolving aspect becomes even more important. This will push the future use of pixilated systems. This article intends to describe the present state-of-the-art even if most of the presented results are obtained at PSI's leading facilities, sometimes in collaboration with our partners.