10.3 Radiographic Technique: kV, mA-min, Screens, and Source Selection

Physics and geometry do not yet make a radiograph. Official RT topic 1 still expects a Level II to turn those ideas into a technique: a source or kV, an exposure product, screens, filters, and a source choice that matches thickness. The specific exam will then ask whether you followed the procedure and the technique sheet. This section is where those two worlds meet.

ASTM E94 describes how to build an exposure chart. ASME Section V, Article 2 and ASTM E1032 (welds) tell you that the written procedure must name the essential variables — source or voltage range, screens, SFD or Ug, IQI, density. ASNT does not publish a hidden kV-versus-thickness table for the general paper. If the stem quotes a sheet, the sheet wins.

Raising kV: more penetration, less subject contrast

Kilovoltage is the quality knob on a tube. Increase kV and three things happen at once:

1. E_max rises. Photons that could not exist at 180 kV exist at 250 kV.
2. The beam hardens. A larger fraction of the spectrum sits where μ is smaller, so a given steel thickness transmits more. Penetration increases. Thick sections that were blank can reach density.
3. Differential absorption shrinks. A 1 mm cavity is a smaller fraction of a longer HVL. Subject contrast decreases. The radiograph looks grayer, flatter, easier to penetrate, harder to read for fine thickness change.

If mA·min is held constant while kV rises, density also rises, because more photons reach the film. Shop practice is therefore: raise kV only as far as penetration requires, then cut mA or time to land in the specified density band. Raising kV "to save time" on a thin, high-contrast job is how a lack-of-fusion edge disappears into a flat gray.

The same contrast story applies when you change isotopes. Ir-192 is already harder than a 200–300 kV tube on thin steel. Co-60 is harder still. Switching a 15 mm pipe weld from a 250 kV crawler to Co-60 because the projector was closer to the spool is a contrast error, not a clever shortcut.

Film contrast (the slope of the D-log E curve, gamma of the film) is a different knob from subject contrast. A high-contrast film cannot restore subject contrast the beam never created. Technique-factor items that say "the image is flat" want you to think energy too high or scatter too high, not "buy a different brand of developer first."

Exposure product controls density

Radiographic density on film is D = log₁₀ (incident light / transmitted light) in a densitometer. Codes and procedures name an acceptable band — ASME Section V practice commonly requires about 1.8 to 4.0 for single-film technique with an X-ray source and 2.0 to 4.0 with a gamma source, measured in the area of interest, with IQI and penetrameter holes read inside that band. Use the numbers the stem or the procedure prints. Density is not set by kV alone.

The quantity that sets how many photons arrive, at a fixed energy, geometry, and screen, is the exposure product:

• X-ray: mA · minutes (sometimes mAs). 10 mA × 2 min = 5 mA × 4 min = 20 mA·min.
• Gamma: Ci · minutes or GBq · minutes, after decay correction. 40 Ci × 3 min = 20 Ci × 6 min = 120 Ci·min.

More product → more photons → higher density (darker film), until the film saturates. Less product → light, underexposed film, unread IQI holes, and a "clean" weld that is only clean because nothing was there to see.

Energy and exposure are not interchangeable labels:

A Level II who answers "the film is too light, so raise kV" has reached for the contrast knob to fix a quantity problem. The first correction for a uniformly light film that already uses the specified energy is more mA·min or Ci·min, or a shorter SFD if Ug still passes. A uniformly dark film is less product, not a mystery about Ir-192 half-life unless you forgot to decay-correct and overshot.

Reciprocity, conceptually

Reciprocity means density depends on the product of intensity and time, not on how you split the product. In the industrial film-plus-lead-screen regime the general exam uses, reciprocity is treated as good:

• 8 mA × 2.5 min = 4 mA × 5.0 min at the same kV, SFD, and screens.
• 50 Ci × 2 min = 25 Ci × 4 min of the same isotope at the same SFD.

Reciprocity failure — density that does not track the product — is a real photographic effect at extreme times or with fluorescent screens (light exposure of silver halide). It is not a license to ignore the exposure chart on an ordinary 2-minute Ir-192 shot with lead screens. If a stem mentions fluorescent screens and odd density at very long or very short times, reciprocity failure is the vocabulary word. If the stem is a standard lead-screen weld, treat mA × t (or Ci × t) as interchangeable.

Screens: lead intensify, fluorescent rare

Industrial film is a thin emulsion. X-ray and gamma photons mostly fly through it. Intensifying screens standing in intimate contact with the film convert some of that radiation into something the emulsion absorbs better.

Lead screens

Lead foil screens (or lead-backed tantalum in some high-energy work) are the industrial default.

• Photons eject photoelectrons and Compton electrons from the lead. Those electrons expose the film much more efficiently than the photons would have. That is intensification — you get density with less mA·min or Ci·min.
• Lead also filters softer scatter more than it filters the primary beam, so subject contrast often improves compared with bare film.
• Typical shop pair, stated as industry practice the procedure may tighten: a thinner front screen (beam side) and a thicker back screen. Many Ir-192 and kV procedures use on the order of 0.005 in (0.13 mm) front and 0.010 in (0.25 mm) back. Co-60 procedures often use thicker front foil because the beam is harder. Quote the sheet, not a memory of one trailer.
• Screens must be clean, flat, and in contact. A gap is a Ug-like blur (poor contact unsharpness). Scratches and oxidized foil print as false density.

Lead screens do not turn the beam into visible light. That is the other family.

Fluorescent (salt) screens

Fluorescent screens — historically calcium tungstate or similar phosphors — convert X-rays to visible light, which then exposes the film. Intensification factors can be large, so exposure time collapses. The cost is screen unsharpness: light spreads in the phosphor, definition falls, and fine IQI holes and tight cracks suffer. Modern industrial weld film radiography almost never uses fluorescent screens for interpretative ASME or ASTM work. They appear on the exam as the "rare, high-speed, poor-definition" option, and in some non-film or special low-output situations the procedure would have to justify.

Do not invert the pair. Lead = electrons + scatter filter + good definition. Fluorescent = light + speed + worse definition. Medical-style "rare-earth screens" language is not the default industrial weld answer.

Filters

A filter is metal in the beam, usually at the tube port, sometimes at the part.

• Copper, brass, or aluminum filters at the window eat the softest bremsstrahlung. The beam hardens, useful penetration rises a little, and the soft photons that would otherwise scatter in the part never start.
• Lead diaphragms and collimators are not "filters" in the spectrum sense; they shape the field so you do not irradiate a square metre of plate to image a 150 mm weld.
• Filters reduce subject contrast slightly (harder beam) and reduce scatter fog. On a thick, scattery part that trade is often a net gain in radiographic contrast.
• Gamma sources are already line spectra. You do not copper-filter Co-60 the way you filter a 200 kV continuum. You still collimate.

A Level II who stacks random sheet lead on the tube "for more contrast" is usually hardening the beam and cutting output. The technique sheet names the filter, if any.

Choosing X-ray, Ir-192, or Co-60 for thickness

Source selection is a thickness-and-contrast decision constrained by access, safety, and the written procedure. Typical industrial practice ranges — not ASNT-published exam secrets — look like this; the governing code table or the employer's technique sheet is the authority if they disagree.

Selenium-75 sits softer than Ir-192 and is used in some plants for thinner pipe with better contrast. Mention it if a stem names it; do not replace Ir-192/Co-60 as the pair you must know cold.

Decision rules the written exam likes:

• If the part is thin and contrast-critical (small porosity, tight incomplete fusion on a 10 mm weld), prefer X-ray at the lowest practical kV, or Ir-192 only if the procedure says the isotope is allowed at that thickness.
• If the part is medium field pipe and the bay has no tube, Ir-192 is the expected projector.
• If the part is thick and Ir-192 will not reach density without an absurd time or a Ug-destroying SOD, Co-60 (or a high-energy X-ray / linac) is the penetration tool.
• Never choose Co-60 on thin steel "because it is hotter." Hotter means more output and the wrong energy.
• Activity (curies) is not a reason to change isotope. More curies of Ir-192 are still Ir-192 energies.

Exposure charts

An exposure chart (technique chart) is an empirical family of curves for one film class, one screen pair, one SFD, one target density, and either a set of kV curves or one isotope.

How to read one:

1. Find thickness of steel (or the material the chart is built on) on the horizontal axis. Use the actual penetrated thickness — a double-wall shot is not a single-wall number.
2. Rise to the kV curve or the isotope curve the procedure allows.
3. Read mA·min or Ci·min on the vertical axis.
4. Convert to time: t = (mA·min) / mA, or t = (Ci·min) / (decay-corrected curies).
5. Apply inverse square if the chart SFD is not the setup SFD.
6. Apply a material factor only if the chart and the procedure give one (aluminum versus steel, Inconel versus steel). Do not invent a factor.

Gamma charts go stale every day for Ir-192. The chart assumes a stated activity on a stated date, or it is plotted in Ci·min so you supply today's activity. A Level II using last month's 100 Ci column on a 70 Ci source will underexpose.

Charts are built at a stated density (often 2.0 or 2.5). If the procedure wants a denser image, you need more product. Some shops keep a small density-correction table (a factor of about 1.3–1.5 in exposure to move a few tenths of density, depending on film). Again: use the factor the sheet prints.

The procedure and the technique sheet win

Everything in 10.1–10.3 is there so you can understand the sheet, not so you can override it.

A complete industrial technique, as ASME V / ASTM E1032 expect the procedure to control, names at least:

• Method (X-ray or specific isotope)
• Energy or kV range allowed for that thickness
• Film class / system class, screens, and filter
• SFD or maximum Ug
• IQI type, size, and source-side (or authorized film-side) placement
• Density range
• Geometric arrangement (single-wall, double-wall single-image, elliptical, panoramic)
• Processing or digital technique if it is an essential variable

If the sheet says Ir-192, 0.13 mm front lead, 0.25 mm back lead, 600 mm SFD, wire IQI source side, density 2.0–3.5, a Level II does not:

• Swap in Co-60 to finish before lunch.
• Pull the front screen because "bare film looks contrastier today."
• Walk SFD in to 200 mm without checking Ug and without a revised sheet.
• Park the IQI on the cassette for prettier wires.
• Skip decay correction because the source "is still pretty hot."

The general exam rewards the person who can explain why that sheet was written (contrast, density, Ug, scatter). The specific exam rewards the person who follows it. Topic 1 is the "why."

Realistic exam scenarios

A 12 mm groove weld is specified at 220 kV, lead screens, 36 in SFD. Production wants Co-60 at 14 in to cut time. Correct action: stay on the sheet. Co-60 will penetrate, flatten subject contrast, and the short SFD may fail Ug. Inverse square is not permission to change isotope.

A film of a 50 mm plate at 250 kV is paper-white in the area of interest; the IQI outline is barely visible. Energy may be too low for that thickness, or mA·min may be far too small. Check the chart. If the chart for 50 mm at 250 kV already demands an impractical time, the technique problem is penetration (raise kV within the allowed range, or change source as the procedure allows), not a longer coffee break at 250 kV. If the chart said 8 mA·min and someone shot 1 mA·min, the problem is density, not energy.

A Co-60 shot of a 20 mm weld shows density 2.8 but the 2T hole is a gray smudge and lack of fusion is barely modeled. Density is fine; subject contrast and possibly Ug are not. The energy is too high for the thickness, or geometry is soft. More Ci·min will only make a darker flat image.

An exposure chart at 24 in SFD calls for 12 Ci·min. Today's source is 24 Ci. Time = 12/24 = 0.50 min at 24 in. The setup is actually 36 in. New product = 12 × (36/24)² = 27 Ci·min. Time = 27/24 = 1.13 min. That is chart plus inverse square plus today's curies — the whole technique-factor skill in one paragraph.

A fluorescent-screen cassette is offered for an ASME weld "because the source is weak." Unless the procedure authorizes that system, refuse it. Lead screens and the specified film are the definition the IQI was qualified with.

What technique-factor items are really testing

If density is wrong and energy is already specified, move mA·min or Ci·min (and check decay and SFD). If the image is flat on a thin part, energy is probably too high. If nothing gets through a thick part, energy is probably too low. Lead screens intensify and filter; fluorescent screens blur. Source choice follows thickness and the sheet. Reciprocity lets you trade mA against time or curies against time. And when the stem prints a technique sheet, that document is the answer, not a creative harder beam.