10.1 Radiation Physics: X-ray vs Gamma, Energy, and Attenuation

The ASNT NDT Level II radiographic general exam opens with official RT topic 1, Review of Basic Radiographic Principles. Those items are not asking you to recite a medical physics textbook. They ask whether you know what kind of radiation makes the image, how an X-ray tube differs from a radioisotope projector, how energy trades penetration against contrast, and why steel, lead, and a gas pore do not transmit the same intensity.

Radiographic testing (RT) in this program is photon radiography. An industrial X-ray tube or a sealed gamma source sends a beam through the part. Thicker, denser, or higher-atomic-number material removes more photons. The film, imaging plate, or digital detector records the transmitted intensity as a pattern of optical density or gray level. A gas-filled pore transmits more radiation than the surrounding steel and records as a darker spot on conventional film. A tungsten inclusion does the opposite. Everything in this chapter is in service of that differential transmission.

Industry practice is written around the same physics. ASTM E94 (standard guide for radiographic examination), ASTM E1032 (radiographic examination of weldments), and ASME Boiler and Pressure Vessel Code, Section V, Article 2 assume a photon source, a known energy or isotope, and a technique that produces required density and image quality indicator (IQI) sensitivity. They do not publish a secret ASNT energy number. If a stem gives an energy, a half-value layer, or a thickness, use the number in the stem.

Photons versus particles

Exam stems still mix radiation types. Sort them before you talk about kV or curies.

X-rays and gamma rays are the same kind of thing — photons traveling at the speed of light — once they have left the source. A 300 keV X-ray photon and a 300 keV gamma photon interact with steel the same way. The difference that the written exam cares about is origin and control:

• X-rays are made in a tube (or a linear accelerator) when electrons strike a target. You choose kV and mA. You switch the beam off.
• Gamma rays are made when a radioisotope decays. Energy is a nuclear property of that isotope. You cannot switch the nucleus off; you only shield, collimate, and lock the projector.

Do not answer that gamma rays are "stronger particles" or that X-rays are "weaker electricity." Photons are not alpha particles. Particle radiation is a safety and shielding topic; it is not the imaging beam on a film weld shot.

How an X-ray tube makes the beam

A conventional industrial tube is a vacuum diode.

1. A filament (cathode) is heated. Thermionic emission boils off electrons. Tube current, measured in milliamperes (mA), is the number of electrons per second crossing the tube. More mA means more photons per second — intensity — not a higher maximum energy.
2. A high voltage, stated as kilovolts (kV) or kVp when the supply is pulsed, accelerates those electrons toward the target (anode). Kinetic energy per electron in keV equals the applied kV if the electron falls through the full voltage.
3. Electrons strike a high-melting, high-Z target, almost always tungsten in portable industrial tubes (copper-backed, often angled). More than 99% of the electron energy becomes heat. A few percent becomes X-rays.
4. Two photon families leave the target: a continuous bremsstrahlung spectrum and, if kV is high enough, characteristic lines of the target.

Controls the Level II must keep separate:

The tube produces no useful beam when high voltage is off. That is the operational difference from a projector: an X-ray bay can be entered after a survey once the generator is secured. A gamma camera still holds a live source.

Characteristic radiation versus bremsstrahlung

Bremsstrahlung ("braking radiation") is the continuous spectrum. An electron misses a tungsten nucleus, decelerates in the Coulomb field, and radiates a photon. The photon can have any energy from nearly zero up to the electron's full kinetic energy. Therefore:

• E_max (keV) = applied kV. A 250 kV tube cannot emit a 300 keV photon.
• Most photons are well below E_max. The useful, or effective, energy of a filtered industrial beam is often roughly one-third to one-half of kV, not the peak. The exam will not give you a secret fraction; it will ask whether raising kV raises the ceiling of the spectrum.
• Filtration (the tube window, added copper or brass, and the part itself) removes the softest photons and hardens the beam.

Characteristic X-rays are discrete lines. An incident electron (or a photon) ejects a K-shell electron of the target. An outer electron drops in and emits a photon whose energy is the difference of those electron binding energies. For tungsten the strong K lines sit near 59 keV and 67 keV. They appear only when tube voltage exceeds the K-absorption edge (about 70 kV for tungsten). Characteristic radiation does not form a continuum, and its energies are a property of the target element, not of the mA setting.

A gamma source does not have a bremsstrahlung knob. Iridium-192 and cobalt-60 emit line spectra from nuclear transitions (plus some weaker lines and X-rays from internal conversion). You pick the isotope; you do not "turn Co-60 down to 200 kV."

Radioisotopes the general exam expects you to name

Field radiography in general industry and pressure equipment is still dominated by two sealed sources. A third, selenium-75, appears in some shops for thinner pipe; treat it as industry practice, not as a substitute for knowing Ir-192 and Co-60.

Half-life is the time for activity (decays per second, stated in curies or becquerels) to fall to one-half. After one half-life you have half the output; after two, one-quarter; after three, one-eighth. Ir-192's ~74-day half-life is why a Level II decay-corrects the exposure chart every day or uses a dated calculator. A source that was 100 Ci on the assay date is not 100 Ci six weeks later. Co-60's ~5.27-year half-life changes slowly; you still correct it, but not on the same frantic calendar.

Do not invert the pair. A stem that says "the isotope with the 74-day half-life" is Ir-192. A stem that says "the 5.27-year isotope with 1.17 and 1.33 MeV lines" is Co-60. Cesium-137 (~30 years, 0.662 MeV) and thulium-170 appear in older handbooks; they are not the two sources this section drills.

Activity sets output, not energy. A 20 Ci Ir-192 source and a 100 Ci Ir-192 source emit the same photon energies. The 100 Ci source simply emits more of them per second, so exposure time drops. You cannot turn a weak Ir-192 source into Co-60 by waiting, and you cannot soften Co-60 by using fewer curies.

Energy, penetration, and subject contrast

Penetration is the beam's ability to put enough photons through the thickest section to reach the required film density in an allowed time. Subject contrast is the difference in transmitted intensity between two adjacent thicknesses or materials — the physical contrast that exists before film or a monitor touches it.

Those two quantities move in opposite directions with energy:

• Raise energy (higher kV, or switch Ir-192 → Co-60). More photons survive the steel. Thick sections become radiographable. Differential absorption shrinks, so a 2 mm cavity in 40 mm of steel is a smaller fractional change in transmission. Subject contrast falls.
• Lower energy. Photoelectric absorption becomes more important, especially in higher-Z details (lead IQI numbers, copper-nickel, tungsten inclusions). Small thickness changes produce larger transmission changes. Subject contrast rises, but the thick section may be blank white because nothing got through.

The Level II rule is not "always use the hardest beam." It is: use the lowest energy that still penetrates to the specified density, IQI, and time. The technique sheet, not a habit, names that energy.

Worked comparison (qualitative numbers a stem might hand you):

• The same 25 mm steel plate is shot with a filtered 200 kV X-ray beam and with Co-60. The 200 kV shot, if density can be reached, will show more subject contrast at a small pore. The Co-60 shot will be flatter — easier to penetrate, harder to see a small thickness change.
• A 75 mm steel section that will not reach density 2.0 at 300 kV in a practical mA·min product is a candidate for Ir-192 or Co-60, not for "just hold the button longer" at a soft kV that the part is stopping.

Photoelectric absorption, Compton scatter, and pair production

A photon that enters the part does one of three industrially important things (coherent scatter exists but is not the exam workhorse).

Photoelectric effect

The photon is completely absorbed. Its energy ejects a bound electron, usually from an inner shell. The atom may then emit characteristic X-rays or Auger electrons. Photoelectric probability:

• Falls rapidly as photon energy rises (very roughly as 1/E³ in the handbook picture).
• Rises strongly with atomic number (often taught as about Z³ to Z⁵, depending on the shell and energy).
• Dominates at low energy and in high-Z materials — lead foil, gold-platinum, the lead numbers on an IQI, the reason a thin lead screen eats scatter.

Photoelectric absorption is why subject contrast is high on a 120 kV aluminum shot and why a lead letter shows sooty-black on a light steel radiograph.

Compton scatter

The photon collides with a loosely bound electron, loses some energy, and changes direction. The scattered photon may still reach the film — from the wrong angle — and lay down a fog that reduces contrast. Compton probability:

• Depends mainly on the number of electrons in the path, so it tracks density and electron density more than high powers of Z.
• Dominates through much of the industrial RT range: a few hundred kV X-rays and Ir-192, and it is still major at Co-60.
• Is the physical reason for collimators, masks, lead screens, and not radiographing with a huge uncollimated field onto a small coupon.

If the stem asks which interaction produces the scatter that fogs a weld radiograph, the answer is Compton (plus some secondary radiation), not pair production in a 150 kV bay.

Pair production

If the photon energy is at least 1.022 MeV (twice the rest energy of an electron), the photon can convert in a nuclear field into an electron-positron pair. The positron later annihilates into two 511 keV photons. Pair production:

• Cannot occur below 1.022 MeV. A 400 kV tube (0.40 MeV max) does not pair-produce.
• Becomes more likely as energy rises above the threshold and as Z rises.
• Matters more for Co-60 (1.17 and 1.33 MeV) and for high-energy accelerators than for Ir-192 or ordinary kV X-ray.

You do not need Klein-Nishina integrals. You need the ranking: photoelectric at low E / high Z, Compton across the middle, pair production only above 1.022 MeV.

Half-value layer and tenth-value layer

Attenuation in a simple narrow-beam picture is exponential:

I = I₀ e^(−μx)

where μ is the linear attenuation coefficient and x is thickness. The half-value layer (HVL) is the thickness of a stated material that reduces intensity to one-half. The tenth-value layer (TVL) reduces it to one-tenth.

Useful exam arithmetic:

• After n HVLs, I = I₀ / 2ⁿ.
• 1 TVL ≈ 3.32 HVL, because 2^3.32 ≈ 10.
• Two TVLs leave 1/100 of the incident narrow-beam intensity.
• Higher energy → larger HVL of the same material. Co-60's HVL in steel or lead is thicker than Ir-192's. That is penetration, restated as a shop number.

Worked HVL example. A stem says the HVL of a given beam in steel is 12 mm, and the incident intensity at the plate is I₀.

• 12 mm of steel → I = I₀ / 2.
• 24 mm (2 HVL) → I = I₀ / 4.
• 36 mm (3 HVL) → I = I₀ / 8.
• 48 mm (4 HVL) → I = I₀ / 16.

If a second stem says a TVL of lead for that same beam is 15 mm, then 30 mm of lead is two TVLs and the narrow-beam intensity is I₀ / 100. Real broad-beam shielding is worse than this because of buildup (scatter that still reaches the survey meter). On a principles item, use the HVL/TVL arithmetic the stem gives; do not invent a buildup factor ASNT did not print.

HVL is not only a vault-wall number. It is also why a 3 mm difference in steel is a large contrast change at low energy (a sizable fraction of one HVL) and a small contrast change at Co-60 (a tiny fraction of a much thicker HVL).

Attenuation versus atomic number, density, and thickness

Three part properties remove photons:

1. Thickness along the beam. More path, more interactions. A 20 mm doubler next to 10 mm plate is a classic density step on the film.
2. Density. More atoms per centimetre. Steel stops more of a given beam than the same thickness of aluminum; a gas pore (essentially density zero) stops almost none.
3. Atomic number, especially where photoelectric absorption matters. Lead (Z = 82) is a far better absorber per millimetre than iron (Z = 26) at X-ray energies. That is why IQI lead numbers, lead screens, and lead letters work.

On negative film, more transmitted radiation → more blackening → higher radiographic density. Therefore:

• A pore, cavity, or thinner ligament → darker indication.
• A high-Z inclusion or extra thickness → lighter indication.
• Scatter arriving from the side → added density that is not an image of the weld, so contrast falls.

Mass attenuation (μ/ρ) is the handbook way to compare materials on a per-gram basis. For the written exam it is enough to say: at a given energy, raising Z or density or thickness reduces transmitted intensity and, if the change is local, creates subject contrast.

Realistic exam scenarios

A candidate is asked why a 250 kV tube radiograph of a 12 mm aluminum casting shows more contrast at a small gas hole than a Co-60 shot of the same casting. Answer from this section: the lower-energy X-ray beam has more photoelectric absorption and a shorter HVL, so a small missing thickness is a larger fraction of the beam's attenuation. Co-60 penetrates easily and flattens contrast.

A projector is loaded with Ir-192. The assay was 80 Ci thirty-seven days ago. One half-life is about 74 days, so the source is not yet at 40 Ci, but it is well below 80 Ci. Using the original 80 Ci exposure time will under-expose the film. Decay-correct the activity, then compute Ci·min.

A stem says pair production is the main reason 200 kV radiographs of carbon steel look foggy. That statement is false. 200 keV is below 1.022 MeV. The fog is Compton scatter (and technique: collimation, screens, field size).

A tungsten inclusion in a steel weld is lighter than the surrounding deposit on film. Tungsten's high Z and density attenuate more photons than steel. That is photoelectric-plus-density, not "the inclusion emitted gamma rays."

A Level II claims characteristic radiation is the continuous spectrum and bremsstrahlung is the 74-day Ir-192 line. Reverse of the truth. Characteristic lines are discrete and target-specific; bremsstrahlung is the tube continuum; Ir-192's half-life is a nuclear property, not an X-ray-tube setting.

What topic 1 physics items are really testing

If the stem names the radiation, ask: photon or particle, and which origin? If it names kV versus mA, ask: energy or quantity? If it names Ir-192 or Co-60, ask: half-life and whether energy is adjustable. If it names contrast versus penetration, ask which way energy moved. If it names an interaction, rank photoelectric / Compton / pair by energy and Z. If it names HVL, do the 2ⁿ arithmetic. Those six questions are the physics half of Review of Basic Radiographic Principles.