Know what is meant by background radiation

5.2.1 Detection of Radioactivity – Background Radiation

Objective

To understand what background radiation is, identify its sources, measure it with a detector‑counter system, and correct experimental count‑rates for its presence.

1. Definition of Background Radiation

Background radiation is the ionising radiation that is always present in the environment, even when no artificial radioactive source is nearby. It originates from natural processes (cosmic rays, terrestrial radionuclides, radon, etc.) and from man‑made activities, and it is constantly recorded by radiation‑detecting instruments.

2. Sources of Background Radiation (Cambridge IGCSE 0625)

The syllabus expects five natural sources to be named. They are listed below together with a brief description of each.

• Cosmic rays – high‑energy particles from outer space that interact with the atmosphere and reach the Earth’s surface.
• Terrestrial radiation from rocks and buildings – radionuclides such as ²³⁸U, ²³²Th and their decay products in the Earth’s crust, concrete, bricks, etc.
• Radon (²²²Rn) – a noble‑gas decay product of uranium that can accumulate in homes, especially basements.
• Food and drink – low levels of naturally occurring radionuclides (e.g., ⁴⁰K, ¹⁴C) ingested with the diet.
• Other natural sources – internal radiation from radionuclides in the human body and any additional natural contributors.

Man‑made contributions (medical imaging, industrial sources, fallout from nuclear tests or accidents) are also present but are not required for the core syllabus.

3. Measuring Background Radiation

A typical school set‑up uses a Geiger‑Müller (GM) tube (or another radiation detector) connected to a counter that records the number of ionising events per unit time.

1. Arrange the detector exactly as you would for any source measurement.
2. Ensure no known radioactive source is within a few metres.
3. Start the counter and record the number of counts for a fixed time (standard: 60 s = 1 min).
4. Repeat the measurement at least three times and calculate the average.
5. Express the result as a count‑rate:

   • counts per minute (cpm) – most school experiments use this.
   • counts per second (cps) – the syllabus also accepts this unit (1 cpm = 1/60 cps).

4. Typical Levels of Background Radiation

Background dose rates are usually quoted in microsieverts per hour (µSv h⁻¹). Values vary with altitude, geology and building construction.

5. Why Background Must Be Considered

• The detector records a constant count even when the test source is absent.
• Accurate activity measurements require subtraction of this background count to obtain the net count‑rate.
• Knowing the background level helps assess laboratory safety and ensures compliance with dose‑limit recommendations.

6. Correcting Measured Data for Background

The net (background‑corrected) count‑rate is obtained by simple subtraction:

R_net = R_total – R_background

Worked example (exact syllabus format)

• Measured total count‑rate for the source: 250 cpm
• Measured background count‑rate (average of three readings): 30 cpm
• Net count‑rate: 250 – 30 = 220 cpm

If the count‑rate is required in cps, divide by 60: 220 cpm ÷ 60 ≈ 3.7 cps.

7. Calibration – Converting Counts to Dose Rate (optional)

Many classroom GM tubes are supplied with a calibration factor such as:

1 cpm ≈ 0.005 µSv h⁻¹

Using the example above:

220 cpm × 0.005 µSv h⁻¹ ≈ 1.1 µSv h⁻¹

8. Counting Statistics and Uncertainty

• Radioactive decay follows a Poisson distribution. For a count N, the standard deviation σ = √N.
• Uncertainty in a count‑rate R (cpm) is σ_R = √N / t, where t is the counting time in minutes.
• Longer counting times reduce the relative uncertainty because σ / N ≈ 1 / √N.
• When subtracting background, propagate the uncertainties:

  σ_net = √(σ_total² + σ_background²)

9. Reducing the Effect of Background Radiation

• Shielding – surround the detector with lead, aluminium or concrete to block external photons.
• Low‑background environment – perform measurements in a basement, underground lab, or a purpose‑built low‑background room.
• Long counting times – increase the interval to 5 min or more for weak sources; statistical fluctuations become smaller.
• Geometrical consistency – keep the source at a fixed distance from the detector for all measurements.

10. Safety Limits for Students (AO1 – Effects of Ionising Radiation)

In a typical school laboratory the measured background (≈ 0.1 µSv h⁻¹) is a very small fraction of the annual limit, but students should still follow the three‑principle rule: time, distance, shielding.

11. Common Detectors Used in School Experiments (AO3 – Practical Skills)

• Geiger‑Müller (GM) tube – gas‑filled detector; each ionising event produces a short pulse. Simple, robust, good for β and γ detection.
• Scintillation counter – crystal or plastic scintillator coupled to a photomultiplier; higher efficiency, useful for low‑level γ measurements.
• Cloud chamber – visualises tracks of charged particles; mainly a qualitative tool.
• Ionisation chamber – measures current produced by ionising radiation; used for high‑activity sources (rare in schools).

All of these instruments produce a count‑rate that must be corrected for background as described in § 6.

12. Key Points to Remember

• Background radiation is ever‑present ionising radiation from natural (and some man‑made) sources.
• Five syllabus‑specific natural sources: cosmic rays, rocks/buildings, radon, food & drink, other natural sources.
• Typical dose rate ≈ 0.1 µSv h⁻¹; a common calibration is 1 cpm ≈ 0.005 µSv h⁻¹.
• Measure background with the detector‑counter system, average several readings, and subtract it from every total count‑rate.
• Use either cpm or cps for count‑rates; the syllabus accepts both.
• Apply Poisson statistics (σ = √N) and propagate uncertainties when subtracting background.
• Keep student exposure ≤ 0.1 mSv yr⁻¹ and follow time‑distance‑shielding principles.
• Know the basic operation of the detector you are using and record data consistently.