Neutron imaging

Contrary to X-rays, neutrons interact predominantly with light, hydrogen and carbon rich materials. The neutron imaging is therefore a complementary method to the X-ray imaging.

Thermal neutrons cannot be detected in silicon sensors directly. However, the sensor can be coupled with a neutron converting material (usually 6Li or 10B based) that captures the neutrons and emits heavy charged particles so that they can easily be detected in the sensor. This way our devices can be adapted for the thermal neutron imaging.

Combined neutron and X-ray computed tomography (CT) of a ball bearing. The X-rays provide high contrast for the steel parts, however, the plastic spacer is not visible. The neutrons on the other hand well show the plastic and the steel is nearly transparent for them.

Combined neutron and X-ray CT of a trimmer. The X-rays again well reveal the metallic parts. The plastic case and internal gear has a better contrast using the neutrons.

The detector in combination with the software delivered with our devices provides tools for very efficient suppression of electron and gamma background that always accompanies neutrons. The detection of neutrons via heavy charged particles (alpha and Tritons) moreover allows measuring the position of the particle and hence the neutron impact with sub-pixel precision.

The following image shows the background suppression and sub-pixel resolution calculation using the pattern recognition:
Gamma and electron background suppression

Individual hits:
a) all detected events – the curly tracks are caused by electrons. The larger circular clusters are events generated by alpha particles and tritons from the 6Li(n,α)T reaction. The energy of alpha particles and tritons is so high that it creates charge in multiple neighboring pixels – a pixel cluster,
b) electron tracks suppressed, only 6Li(n,α)T clusters remain,
c) centers of clusters were calculated.

Images of Al grating filled with 6Li with pitch of 250 μm:
d) no gamma background suppression was applied,
e) background suppressed,
f) effective pixel size reduced by the analysis of individual alpha and triton particle hits to 5.5 μm.

The images were taken using the planar silicon device and cold neutrons at the SANS beamline at FRMII, Garching, Germany. Note: the axes are marked in units of physical pixel number.

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