Radiation Measurement in Industrial Applications
Radiation in Industry: A Tool, Not a Threat
On a steel sheet production line, each sheet passes between a radiation source and a detector. If the sheet is thicker than specified, less radiation reaches the detector, triggering an immediate correction signal. This is radiation thickness gauging — non-contact, continuous, accurate to fractions of a millimeter. Radiation here is not a hazard — it is a precision engineering tool.
But radiation demands respect: understanding its types, measuring its doses, and following safety protocols. That is what this lesson covers.
Types of Radiation
Alpha Radiation (α)
Alpha particles are helium nuclei (two protons + two neutrons). Heavy and relatively slow:
- Very low penetrating power: stopped by a sheet of paper or a few centimeters of air
- Very high ionizing power: causes significant biological damage if inhaled or ingested
- Sources: Polonium-210, Radium-226
Beta Radiation (β)
Beta particles are high-energy electrons (or positrons):
- Moderate penetrating power: travel several meters in air, stopped by a thin aluminum sheet (a few millimeters)
- Moderate ionizing power
- Industrial sources: Strontium-90 (paper and plastic thickness gauging)
Gamma Radiation (γ)
High-energy electromagnetic waves — like X-rays but more energetic:
- Very high penetrating power: requires centimeters of lead or meters of concrete to stop
- Relatively low ionizing power per unit distance
- Industrial sources: Cesium-137, Cobalt-60
Radiation type comparison:
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus | Electron | Electromagnetic wave |
| Charge | +2 | -1 (or +1) | Neutral |
| Mass | Heavy (4 u) | Very light | Zero |
| Stopped by | Paper | Aluminum sheet | Thick lead / concrete |
| Ionization | Very high | Moderate | Low |
| Speed | ~5% of light speed | Up to 99% | Speed of light |
Industrial Applications of Radiation
Thickness Gauging
A radiation beam passes through the material while a detector on the opposite side measures residual intensity. The relationship follows the Beer-Lambert Law:
I = I0 * e^(-mu * x)
Where I0 = initial intensity, mu = linear attenuation coefficient, x = thickness.
- Thick steel sheets: gamma source (Co-60 or Cs-137)
- Thin plastic films: beta source (Sr-90)
- Coatings and paper: low-energy beta or X-rays
Level Measurement
A sealed tank containing hazardous chemicals cannot be opened for level measurement. The solution: a gamma source on one side and a detector on the opposite side. When liquid is between source and detector, it absorbs radiation and the detector signal drops. The measurement is entirely non-contact.
Density Measurement
In slurry pipelines at mining plants, a gamma source faces a detector across the pipe. Higher slurry density means more absorption and a weaker signal. This provides continuous, real-time density readings.
Industrial Radiography
Like medical X-rays, but for metals. Used to detect internal defects in welds and castings:
- Internal cracks
- Gas porosity
- Incomplete fusion in welds
Radiation Detection Instruments
Geiger-Mueller Counter
The most recognized radiation detector. Contains a gas-filled tube with two electrodes. When a radioactive particle enters:
- It ionizes gas atoms
- Free electrons accelerate toward the positive electrode
- An electrical discharge produces a countable pulse
- Each pulse represents one detected particle
Advantages: inexpensive, simple, rugged. Limitations: cannot distinguish radiation type or energy.
Scintillation Detector
A crystal (typically sodium iodide, NaI) emits light flashes when struck by gamma radiation. A photomultiplier tube (PMT) converts each flash into an electrical signal. Key advantage: measures radiation energy — can identify different isotopes.
Personal Dosimeter
Worn by every worker handling radiation sources:
| Type | Principle | Application |
|---|---|---|
| Film badge | Radiation darkens photographic film | Monthly monitoring |
| TLD (Thermoluminescent) | Crystal stores energy, releases as light when heated | Accurate and durable |
| Electronic (EPD) | Semiconductor detector | Real-time reading with alarm |
Radiation Units
| Quantity | SI Unit | Description |
|---|---|---|
| Activity | Becquerel (Bq) | One disintegration per second |
| Absorbed dose | Gray (Gy) | 1 joule per kilogram of tissue |
| Equivalent dose | Sievert (Sv) | Absorbed dose multiplied by quality factor |
Quality factor (Q):
- Gamma and beta: Q = 1
- Neutrons: Q = 5-20
- Alpha: Q = 20
This means 1 Gy of alpha radiation inside the body causes damage equivalent to 20 Gy of gamma radiation.
The ALARA Principle: The Golden Rule
ALARA = As Low As Reasonably Achievable. The goal is not zero radiation (impossible), but the lowest practical level considering cost and benefit.
Three tools for applying ALARA:
Time
Minimize exposure duration. Dose is directly proportional to time:
Dose = Dose rate * Time
If the dose rate is 2 mSv/h and the task takes 30 minutes: Dose = 1 mSv.
Distance
Move away from the source. Radiation intensity follows the inverse square law:
I1 / I2 = (d2 / d1)^2
Doubling the distance reduces intensity to one-quarter. Long-handled tools and remote controls significantly reduce exposure.
Shielding
Place absorbing material between the worker and the source:
- Gamma: lead (1-10 cm depending on energy), concrete (30-100 cm)
- Beta: aluminum or plastic (a few millimeters)
- Alpha: no external shielding needed — paper is sufficient
Occupational Dose Limits
| Category | Annual Limit |
|---|---|
| Radiation workers | 20 mSv (averaged over 5 years) |
| Eye lens | 20 mSv per year |
| Skin and extremities | 500 mSv per year |
| General public | 1 mSv per year |
For comparison, natural background radiation is approximately 2.4 mSv per year from natural sources (earth, cosmic rays, food).
Radiation Safety Procedures in Factories
Classification and labeling system:
- Sealed sources stored in lead containers marked with the trefoil radiation symbol
- Periodic inventory of every radiation source — any loss is reported immediately
- Controlled areas marked with warning signs
Emergency procedures:
- If a container is damaged: evacuate the area immediately
- Never touch a source with bare hands
- Notify the Radiation Protection Officer (RPO)
- Cordon off the area at a safe distance
Practical Example: Selecting a Source for Thickness Gauging
A factory produces 5 mm steel sheets. Continuous measurement with an accuracy of plus or minus 0.1 mm is required.
Design steps:
- Material: steel (high density, moderate thickness) — a gamma source is needed
- Isotope selection: Cs-137 (energy 662 keV, half-life 30 years — stable output)
- Position source and detector on opposite sides of the production line
- Calibration: measure intensity at known thicknesses (4.5, 5.0, 5.5 mm)
- Connect detector to PLC control system for real-time correction
Result: continuous measurement with an accuracy of plus or minus 0.05 mm, no production stoppage, no contact with hot material.
Industrial radiation — when understood and respected — offers measurement capabilities unavailable from any other technology. The engineer who masters its fundamentals possesses a powerful, precise, and safe tool.