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The Typical Backscatter Factors of Nickel and Copper

Backscatter is a technique used to determine the depth of a metal coating by bouncing particles in the form of waves or rays off a second layer of metal beneath the surface. Backscatter started out as a way to measure cast-steel rolls, but advances in technology have allowed the measurement of thinner materials. Two metals that may be the subject of backscatter measurements, either as coatings or substrates, are copper and nickel.

Elemental Metals
- Copper, No. 29 on the Periodic Table of Elements, is a common metallic element known for its reddish-brown color. Nickel, next to copper on the Periodic Table at No. 28, is also a common metal, with a silvery appearance. The name "nickel" derives from the German word "kupfernickel," meaning "Devil's copper." The name came about when copper miners found the different, less-valuable ore -- to their disappointment.
Uses of Copper and Nickel
- Both copper and nickel are commonly used in industrial alloys, either as the hardened surface or the unaltered substrate. Copper can be used as a coating, even over plastic, or as the substrate under gold, for instance. Like copper, nickel can be both a coating and a substrate, and the two metals may even be combined in "cupro-nickel" alloys. Common uses include tubes and fasteners.
Backscatter Methods
- Backscatter can be measured using transverse ultrasonic waves or a beam of beta particles. The wave method involves beaming the waves at the alloy on an angle, then measuring the time between the waves hitting the alloy and the beginning of an increased backscatter, indicating the waves have reached the substrate. The beta backscatter method measures the coating thickness in similar fashion, but uses a beam of particles emitted from an isotope. Backscatter is not the only way of measuring coating thickness, but it allows users to measure a wide variety of surface/substrate combinations in a nondestructive fashion.
Backscatter Factors
- The major backscatter factor is the thickness of the coating, which is what backscatter is designed to measure. However, the measurements used in the thickness calculation are affected by the metals used, such as copper or nickel, and whether they are used as the surface coating or substrate. Another factor is the Periodic Table number of the elements used. Backscatter might not be an effective way to measure a cupro-nickel alloy, for instance, as it works best if the elements' atomic numbers differ by a minimum of 20 percent. However, backscatter can be used on alloys that include either copper or nickel in conjunction with a more diverse element or organic compound.

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