S Das, P R Vaidya and B K Shah 624 Insight Vol 48 No 10 October 2006 1. Introduction Most of the radiation testing methods are based on the transmission method where the X-ray or gamma-ray passes through the material and an attenuated beam is recorded by a detector on the other side of the object. In practice, there are some objects where the transmission method cannot be used because of the inaccessibility of the other side of the object. The Compton back-scattering method is an alternative option under such circumstances. When a collimated gamma-ray beam impinges on a point in a solid, it may interact with matter in three ways; it can be transmitted without attenuation, completely absorbed by the photoelectric effect or it can be Compton scattered. Attenuation by pair production generally does not occur as it is a high energy (>1.02 MeV) interaction phenomenon. Some of the scattered photon will travel back and re-emerge at the surface. The energy of the scattered photon can be described by the well known Compton single scatter relationship: .............................(1) where E is the energy of the scattered photon, Eo is the energy of incident photons, θ is angle of scatter and moc 2 is the rest-mass energy of the electron (511 keV). The scattered photon can be detected by a radiation detector and the signal is available for further processing. 2. The Compton effect In Compton scattering, a gamma photon makes an elastic collision with an electron in the target material. Such electrons may be considered free as the binding energy is much less than the photon energy. In the collision, both momentum and energy are conserved and part of the energy of the incident photon is transmitted to the electron. In the process, a scattered photon of lower energy comes out and moves in a new direction, as shown in Figure 1. The energy of the scattered photon can be related to scattering angle as described in equation (1). As the Compton interaction is between the photon and the orbital electrons, the probability of occurrence is directly proportional to the number of orbital electrons, ie the atomic number. The energy dependence of Compton effect is too complicated to reproduce here, but it can be concluded that the probability of a Compton interaction is less for high-energy photons. Therefore, as a rough approximation, it can be stated: Probability of Compton interaction ≈ constant ............(2) where Z is the atomic number of the scattering material and E is the energy of the incident photon. 3. Examination method A well-collimated beam of gamma-rays from a lead shielded container irradiates the target. Since the target material is aluminium, the isotope chosen is Co-57 of energy 122 keV with estimated activity of 8 mCi. The back scattered photon is detected by a NaI(T1) detector. A high density material, ie lead, is used to shield the detector against background radiation. The distance between the target object and detector is 70 mm. The aperture size in front of the detector housing is a rectangular slot of 3 mm x 8 mm to simulate a narrow beam condition of examination. The schematic arrangement set-up is shown in Figure 2. A scintillation detector, ie NaI(T1), is preferred over a semiconductor high-purity germanium detector as NaI(T1) detector has a high detection effciency and NDT FUNDAMENTALS Use of the Compton scatter spectrometry technique in non-destructive evaluation of engineering components Non-destructive testing of engineering components is a prime requirement of a quality control programme before putting them into service. Several techniques are available for non- destructive inspection, such as radiography, ultrasonic testing, eddy current testing and magnetic faw detection. In all the techniques, scanning energy, in different forms such as electromagnetic waves, ultrasound waves, electrical energy, polarised light, is applied to the component under test and their interaction with a faw is recorded and later this signal is analysed to extract information. It is known that each technique has its limitations. For example, magnetic particle inspection is limited to ferromagnetic material, eddy current testing is applicable to electrically conducting materials, radiography is less sensitive in detecting planar faws. Therefore, newer methods are emerging in the feld of non- destructive inspection technology for specifc requirements. Infrared thermography, holography, neutron radiography, Compton scatter spectrometry are all such techniques. In the Compton scatter technique, material is scanned with a photon beam. It is known when an X-ray or gamma-ray impinges on a target material that some of the photons are scattered and deviate from their initial path. This Compton scattered radiation provides useful information. In this paper, the use of Compton scatter radiation for the non-destructive evaluation of materials has been explored. Sanjoy Das, P R Vaidya and B K Shah are with the Quality Assurance Division, South Site, Bhabha Atomic Research Centre, Mumbai – 400 085, India. E-mail: sanjoy_95@rediffmail.com Figure 1. Compton scattering process