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Stroboscopic Imaging with Thermal-Neutrons

31.01.2011

Thermal-neutron radiography and tomography became a powerful tool to investigate distributions of hydrogenous material embedded in high-Z matrices, not visible with X-ray radiographic techniques. An overview on the techniques, imaging methods and applications can be found in [1,2].

At PTB a 9-frame gated optical imaging system (TRION [3,4]) for time resolved fast-neutron and gamma radiography was developed. It allows exposure time control from several seconds down to 5 nanoseconds in pulsed fast- and thermal neutron beams. The imaging system is able to acquire 9 frames, up to eight of them with independent and freely selectable exposure timing (time-framing). Each of these 8 frames can be triggered repetitively, so for periodic processes multiple exposures may be integrated in a single CCD image, applying a phase locked repetitive exposure scheme. Thus excellent quantum statistics can be achieved even for very short exposure time windows within a fast periodic process.

In thermal neutron radiography time resolved imaging becomes of increasing interest. Mainly the advent of powerful pulsed spallation sources opens new possibilities in radiography, e.g. energy selective imaging can be used to utilize Bragg edges for element sensitive imaging. But also with unpulsed reactor beams the possibility of investigating the dynamics of fast (µs) processes opens new and challenging applications.

While originally developed for energy resolved fast-neutron imaging, PTB’s TRION system was modified for thermal neutron radiography - mainly by replacing the neutron converter. With the new converter the system is capable for performing stroboscopic imaging with a time resolution of down to 1 µs.

Figure 1 : Image of a PC hard disk

Figure 2 : CTF of TRION with thermal neutrons

In late 2009 an experiment with the modified (thermal-neutron sensitive) TRION was performed at the neutron radiography facility "ANTARES" at the research reactor (FRM 2) of the Technical-University Munich, to evaluate the performance of the device in a thermal neutron beam. Fig. 1 shows as an example the static image of a computer harddisk. Here all 9 frames were exposed simultaneously for the same duration, Fig. 2 the CTF (Contrast Transfer Function) of the imaging system in the ANTARES neutron beam.

In a second experiment we have tested the ability of stroboscopic imaging of dynamic processes by imaging a PC processor fan in a phase locked repetitively triggered exposure scheme. A sequence of half a rotation of the fan can be seen in Fig. 3. A small piece of Cd was glued to one fan blade and occurs as a darker spot in the radiographs. It is visible in Fig 3 inside the red circle. About 600 exposures cycles were summed up in the images, the exposure window per cycle was 25 µs.

Based on this promising first results we have now proposed to apply time resolved thermal neutron radiography with TRION for the investigation of the dynamics of hydrogen containing liquid and gaseous media ( fuel and oil) in internal combustion engines. Linking the exposure timing of the TRION camera to the phase angle of the crank shaft we aim to image the process of fuel injection into the cylinder and oil lubrication of the piston in stroboscopic imaging sequences by neutron-radiographic methods. This proposal was granted a 5 days beam time at FRM 2 in early September 2010 in collaboration with the ANTARES group and a bavarian car manufacturer.

Figure 3 : Stroboscopic imaging of a PC-processor fan at ANTARES using the TRION detector in a phase locked repetitive exposure mode. The small black spot inside the red circle is a piece of Cd glued to one fan blade.

References:

  1. E. Lehmann et al.:
    Neutron Imaging at the spallation source SINQ,
    Paul-Scherrer Institut, Villigen/CH, Jul2006, neutra.web.psi.ch/publication/Neutron_Imaging_e_Nutzer.pdf
  2. E. H. Lehmann and V. Dangendorf:
    Neutronen-Imaging - Eine Alternative zum normalen Röntgen,
    Bulletin der electrosuisse SEV/AES 11/2006, p. 9 http://www.electrosuisse.ch/cms.cfm/s_page/63880
  3. I. Mor et al.:
    High spatial resolution fast-neutron imaging detectors for Pulsed Fast-Neutron Transmission Spectroscopy,
    Journal of Instrumentation (2009) JINST 4 P05016
  4. V. Dangendorf et al.:
    Multi-Frame Energy-Selective Imaging System for Fast-Neutron Radiography,
    IEEE TNS 56 (2009) 1135