Photographic film
- Photographic film darkens when ionising radiation hits it, just as it darkens when visible light hits it
- The more radiation absorbed, the darker the film when developed
- Radiation workers (radiographers, nuclear-medicine staff, lab technicians) wear film badges with several layers of absorber inside them:
- paper to filter out alpha
- aluminium to slow beta
- lead and copper windows to gauge how much gamma got through
- The patchwork of darkening on the developed film tells the health-and-safety team how much of each kind of radiation the wearer was exposed to over the badge's wearing period
Geiger–Müller tube (GM tube / Geiger counter)
- A Geiger–Müller tube is a sealed glass tube filled with low-pressure gas. A thin mica window at one end lets in alpha, beta and gamma (alpha only if the window is thin enough)
- Whenever a radioactive particle enters the tube, it ionises the gas inside, producing a short electrical pulse that the connected counter records as a single "count"
- The counter either makes an audible click for every pulse or shows a digital count rate display
- Count rate is just counts divided by time. Convert to counts per second (cps, or Hz) by dividing the per-minute total by 60
Example — a GM tube registers 240 counts in 2.0 minutes. Calculate the count rate in counts per second.
- Convert time to seconds: 2.0 min × 60 = 120 s
- Count rate = 240 / 120 = 2.0 counts per second (2.0 Hz)
Background radiation
- Some level of radioactivity is always present in the surroundings, even with no laboratory sources nearby. This is called background radiation and it has to be subtracted from any measurement to find the true contribution of the source
- The biggest sources of natural background radiation:
- Radon gas seeping from rocks and building materials. Radon is an alpha emitter; it accumulates in basements and poorly ventilated buildings and is the single largest contributor in most homes
- Cosmic rays from the Sun and from beyond the Solar System, which strike the upper atmosphere and produce showers of high-energy particles that reach the ground
- Foods like bananas (potassium-40), Brazil nuts, and root vegetables naturally contain trace radioactive isotopes
- Carbon-14 present in all living tissue
- Artificial (human-made) sources are smaller in everyday life but include:
- Medical procedures (X-rays, CT scans, radioactive tracers, radiotherapy)
- Nuclear waste from power stations and weapons programmes
- Fallout from past nuclear-weapons testing or accidents (Chernobyl, Fukushima)
- Background level varies with location (higher at altitude because of more cosmic rays; higher in areas of granite-rich rock because of more radon) and with time (small spikes during severe space weather)
Subtracting background from a measurement
- The correct workflow for a radiation experiment:
- With no source present, take a long background count (several minutes)
- Calculate the background count rate
- With the source in place, take the apparent count
- The corrected count rate is the apparent rate minus the background rate
Example — with no source present a GM tube reads 30 counts per minute. With a source 3 cm away, the GM tube reads 285 counts per minute. Calculate the corrected count rate from the source.
- Corrected count rate = apparent − background = 285 − 30 = 255 counts per minute
- In counts per second: 255 / 60 = 4.25 cps
Improving the accuracy
- Background levels fluctuate randomly, so a one-minute reading is noisy. To get a more reliable corrected rate:
- Take readings over a long period: counting for 10 minutes and dividing gives a more stable rate than counting for 30 s
- Repeat the experiment and average the results
- Use a strong source so the source's contribution dominates the background fluctuation