Induced Positron Analysis

Five years ago, AMT published an article on photon induced positron annihilation (PIPA). In the article, we discussed a new technology that allows an inspector to detect damage to material at an atomic level before any visible damage is apparent. In this article, we will discuss some advances made by Positron Systems in this inspection technology that allows inspectors greater flexibility to inspect airframe and engine components.

No longer PIPA
The technology formerly known as PIPA is now known as induced positron analysis (IPA) There are two types of IPA tests; IPA - volumetric (IPA-V), which is the old PIPA test, and a newer test developed called IPA - surface (IPA-S).

IPA
IPA technology is based on a nuclear physics concept called positron annihilation. It was developed by the U.S. Department of Energy at the Idaho National Laboratory, and Positron Systems now owns the worldwide license to the patents on the technology.

The process starts by inducing positrons into a component. The energetic positrons slow down until they “annihilate” with electrons, emitting low-energy gamma rays. During this process, the positrons seek defects in the material such as dislocations in the lattice sites. When the positrons become trapped in a lattice structure defect, they annihilate with free electrons, which have a very low momentum level.

The test is highly sensitive at low levels of damage induced either during fabrication, or from fatigue, embrittlement, high-temperature creep, corrosion, or other failure mechanisms. IPA offers the ability to detect these defects before visual indications of the defect are present.

IPA-V
As mentioned earlier, there are two types of IPA test methods, IPA-V and IPA-S. The IPA-V test has a large advantage. It is able to measure defects up to 7 inches (184 mm) deep. But there’s a catch – the test requires a linear accelerator. The test is not portable – it has to be performed at Positron System’s Idaho facility. As you could imagine, the TSA, airport security, and other security agencies might have a small problem with a linear accelerator located within the secure area of an airport.

The linear accelerator generates electron beams that pass through a tungsten target to generate high energy photons. The photons then enter the material being tested and interact with its atoms, dislodging neutrons from the atoms. This results in an unstable neutron-deficient isotope. As the isotope decays into a more stable element, a positron is ejected. Then the generic process of reading the gamma response takes place.

IPA-S
Positron System’s new IPA-S test now offers a solution for end-users. The test can detect surface damage between 2 and 3 mm deep, depending on the probe and material being tested. There is no linear accelerator involved, so all the safety and security issues around that are not an issue. The test can be performed using a mobile work bench. This test technique is being developed for widespread commercial use.

In order to test using IPA-S, a test probe is positioned in a jig in close proximity to the part being tested. In order to determine the proper placement of the probe, and whether or not a test can be performed successfully, sample tests are performed on the part at various stages. Martin Hedley, president of Positron Systems, shares, “When a customer approaches us about using IPA-S, we first prove the feasibility and business case for the test. We take a material that is brand new, that is partially used, that is halfway used, that is used up, and one that is broken. Upon development of a business case, we take lots of samples so that we have a statistically valid curve. And we show what exactly the response from the test would be as the part goes through its service life.”