11.1 RT Physics

Radiographic Testing: The Physics of Penetrating Radiation

Radiographic testing (RT) uses short-wavelength penetrating radiation - X-rays or gamma rays - to pass through a part and form a shadow image on a detector. Where the part has less material (a void, porosity, or missing weld metal), less radiation is absorbed, so more reaches the detector and the image reads darker on film. A Level III must understand where the radiation comes from, how it interacts with matter, and how its intensity changes with distance and thickness, because every technique decision - energy, source, distance, exposure time, and shielding - flows directly from this physics.

X-ray Generation

An X-ray tube produces radiation by accelerating electrons from a heated cathode filament toward a positively charged target (anode), almost always tungsten (atomic number Z = 74, melting point 3,422 degrees C). When the fast electrons strike the target, most of their energy becomes heat, but a small fraction converts to X-rays through bremsstrahlung ('braking radiation'), which forms a continuous spectrum, plus discrete characteristic X-rays. Two controls define the beam:

• Peak kilovoltage (kVp) sets the accelerating voltage and therefore the maximum photon energy and penetrating power. Higher kVp penetrates thicker or denser sections.
• Tube current (milliamperes, mA) sets the number of electrons per second, controlling beam intensity (photon quantity), not maximum energy.

So kVp governs penetration and contrast, while mA and time govern total exposure. The effective (apparent) focal spot appears smaller than the physical spot because the target is angled - the line-focus principle - which sharpens the image. X-ray machines have one decisive safety advantage: switch them off and the hazard disappears.

Gamma-Ray Sources

Gamma rays are emitted from the nucleus of a decaying radioactive isotope, not from a tube; the source is 'always on' and cannot be turned off, only shielded. Three isotopes dominate industrial radiography, each matched to a thickness range.

Higher photon energy means greater penetration but lower contrast. Co-60 reaches thick steel but yields lower contrast; Se-75's low energy gives excellent contrast on thin-wall pipe; Ir-192 is the versatile everyday workhorse. Because isotopes decay, source activity - measured in curies (Ci) or becquerels (Bq), where 1 Ci = 3.7 x 10^10 Bq - drops continuously, so exposure times must lengthen as a source ages.

The Electromagnetic Spectrum

X-rays and gamma rays occupy the high-frequency, short-wavelength, high-energy end of the electromagnetic (EM) spectrum, above ultraviolet light. They are physically identical forms of energy - the only difference is origin: X-rays are generated in a tube by electron interactions, while gamma rays come from an atomic nucleus during radioactive decay. Their very short wavelength lets them penetrate solids that visible light cannot, which is exactly what makes volumetric inspection possible.

Attenuation and Absorption

As radiation passes through material, its intensity decreases by attenuation - a combination of absorption (photoelectric effect) and scatter (Compton scattering). The loss is exponential:

I = I0 x e^(-mu*x)

where I0 is incident intensity, x is thickness, and mu is the linear attenuation coefficient. mu increases with material density, atomic number, and thickness, and decreases as photon energy rises. This is why lead (high Z, high density) shields well, and why thicker or denser parts demand higher energy or longer exposure. The thickness differences across a part create the intensity differences that become the radiographic image.

The Inverse-Square Law

Radiation from a small source spreads over an expanding sphere, so intensity falls with the square of the distance:

I1 / I2 = (d2 / d1)^2

Doubling the distance cuts intensity to one-quarter; tripling it cuts intensity to one-ninth. This single relationship drives both exposure (moving film farther from the source demands longer exposure) and safety (stepping back is the cheapest way to reduce dose).

Half-Value Layer - Worked Example

The half-value layer (HVL) is the material thickness that reduces radiation intensity by 50 percent. From I = I0 x e^(-mu*x), setting I/I0 = 0.5 gives:

HVL = ln 2 / mu = 0.693 / mu

A larger mu means a thinner HVL. HVLs stack multiplicatively: after n half-value layers, intensity = I0 x (1/2)^n.

Worked example: the steel HVL for Ir-192 is roughly 0.5 in. A beam enters a plate at 100 units of intensity - how much emerges through 2.0 in of steel? That is 2.0 / 0.5 = 4 half-value layers, so I = 100 x (1/2)^4 = 100 / 16 = 6.25 units; about 94 percent is attenuated. This is why thick sections need high-activity sources or long exposures, and why shielding calculations count HVLs (or the related tenth-value layer, TVL = about 3.32 HVL) to size barriers around a shot.

Radiation Quality and Beam Hardening

Radiation quality describes the penetrating character (hardness) of a beam, not its quantity. A low-kVp beam is 'soft' - easily absorbed and high-contrast - while a high-kVp or Co-60 beam is 'hard' - penetrating but lower-contrast. As a polyenergetic X-ray beam passes through material, low-energy photons are preferentially absorbed and the average energy rises, an effect called beam hardening, which flattens contrast in thick sections. Added filtration (a thin copper or aluminum sheet at the tube port) removes soft photons up front so the exposure is more uniform through varying thickness.

Common Level III Physics Traps

• Confusing kVp with mA: kVp = energy/penetration, mA = intensity/quantity. Raising mA never lets a beam reach thicker steel; only higher kVp or a more energetic source does.
• Assuming a gamma source can be 'switched off' - it cannot; only shielding and distance reduce its field.
• Forgetting that source activity decays, so an aging Ir-192 source needs progressively longer exposures to hold the same film density.