Accounting for neutron absorption that shifts the gauge volume’s neutron-weighted centre of gravity

What is it about?

The representative position in the gauge volume is the neutron-weighted centre of gravity. This takes into account neutron attenuation and variations in intensity across the gauge volume. When the gauge volume is not completely filled at a surface or sharp interface between different materials, the so-called surface effect can occur. The resultant different geometry of gauge volume and different average neutron wavelength can be seen as a shift on the detector which is not due to real strain and is referred to as ‘pseudo-strain’ . This work however concentrates on the shift of the neutron-weighted centre of gravity due solely to neutron attenuation and neutron divergence when the gauge volume is fully immersed in the specimen (i.e. not at a surface or sharp interface). In this case a correct value of strain is determined but the position of the neutron-weighted centre of gravity relative to the geometrical centre of the gauge volume needs to be determined. This is referred to as an ‘absorption shift’ in this work.

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Why is it important?

Often (unlike X-rays) the effect of absorption using neutrons is not taken into account as their ability to penetrate materials deeply is taken for granted. In aluminum for example (which has a low neutron absorption coefficient) one can indeed assume the neutron-weighted centre of gravity of the gauge volume is very closely coincident with its geometrical centre. However highly absorbing materials such as Nickel and Nickel alloys, one can not make this assumption. Making this assumption leads to potential errors when one measures in reflection geometry of a flat plate, especially when there is a steep strain gradient and if there is a chemical compositional gradient.

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