8867 / 2027
Nuclear Physics overview

Lesson 6 of 9

Choose radiation for a task

  • I can discuss medical and industrial uses and hazards using both half-life and penetrating and ionising properties.Syllabus

    Syllabus 8867, 11(j). Discuss medical and industrial uses and hazards using both half-life and penetrating and ionising properties.

Make a guess. You are not marked.

Why is gamma radiation, not alpha, used to sterilise syringes in sealed packs?

  1. Gamma is more strongly ionising than alpha, so it kills more bacteria.
  2. Gamma passes through the packaging to reach the syringes inside.
  3. Alpha would make the syringes radioactive, but gamma does not.
Show the answer

Gamma passes through the packaging to reach the syringes inside.

Gamma is very penetrating, so it reaches every part of the sealed pack and kills bacteria there. Alpha would be stopped by the packaging.

A useful source must interact in the required way and remain suitable for the time involved. Explain both its penetration or ionisation and its half-life, using the stated task.

Measure sheet thickness

Place a radiation source and detector on opposite sides of a sheet. For a suitable beta source and sheet material, some particles are absorbed and some are transmitted. Greater thickness then produces a lower corrected detector rate when the geometry and other conditions stay fixed.

A source absorbed almost completely by even the thinnest sheet gives little useful contrast. A source that passes through all the relevant thicknesses almost unchanged also gives poor sensitivity. Choose radiation that is partially transmitted over the required thickness range.

The source half-life should be long enough for useful output over the operating and calibration period, or the expected decay must be allowed for. A falling rate caused by source decay must not be interpreted automatically as a thicker sheet.

A partially transmitting sheet allows a thickness comparison

A partially transmitting sheet allows a thickness comparisonTwo schematic comparison rows use the same source and detector positions, identical incident direction and the same sheet centre. The upper sheet has drawing width eight and the lower has width twenty-six; these are qualitative thicknesses, not material data. A beta source sends radiation through the sheet towards a detector. Under otherwise fixed conditions the thicker sheet gives a lower transmitted count rate. Both rows show some transmission; neither complete absorption nor almost unchanged transmission is the intended sensitive regime. This is a conceptual industrial gauge, not a handling procedure.Same source and detector geometryβThinner sheetHigher rateSourceβThicker sheetLower rateSourceBoth cases allow partial transmission.

Compare thickness only while the source output, detector geometry, sheet material and other relevant conditions are controlled. A falling source activity could also reduce the count, so it must not be mistaken for a thicker sheet.

The source is inside; the detector receives photons outside

The source is inside; the detector receives photons outsideA schematic body outline contains a small internal tracer marker. Four gamma-photon direction arrows begin at that marker and leave in different directions; one reaches the external detector on the right. The detector is physically outside the body, separated from its outer edge. The figure does not place the source in the detector, make all emissions travel towards it, or specify a clinical dose or organ-specific uptake. Body proportions, source size and arrow counts are schematic.DetectorTracerOnly some emitted photonsreach the detector.

The emitted photons must escape the tissue to be detected externally. Only part of the emission reaches and is registered by the detector. A suitable half-life must also fit preparation and measurement, alongside the stated chemical and biological requirements.

The thickness gauge compares partial transmission with fixed source and detector positions. The tracer view places the emitting material inside the body and the detector outside: photons must escape the tissue to be detected. Arrows identify radiation paths, not movement of the radioactive material.

Detect an internal tracer

For external detection of a tracer within the body, suitable emitted photons must penetrate tissue and reach the detector. The distribution of the tracer provides information about the stated organ or process; its chemical suitability also matters.

The half-life must allow preparation and measurement while avoiding unnecessarily prolonged radioactive activity. Physical half-life describes nuclear decay; it does not by itself give the time for biological clearance from the body.

Compare supplied candidate properties

Meet both detection and timing conditions

In a simplified selection, all other suitability conditions are supplied as equal. A detector outside the body must receive photons, and at least half the initial activity must remain after 60 min. Consider fictional candidates emitting alpha with a 2 h half-life, gamma with a 1 min half-life, or gamma with a 2 h half-life.

The gamma emitter with a 2 h half-life meets both conditions. After 60 min it retains 2-1/2, approximately 0.707, of its initial activity. The alpha choice fails the specified external photon-detection requirement; the 1 min gamma choice decays far too quickly for the stated time.

This comparison uses the supplied conditions. The longest half-life is not automatically best for every application.

Deliver radiation to a treatment target

In external treatment, sufficiently penetrating radiation can reach a target within tissue. Ionisation deposits energy and can damage cells, which is the intended effect in the target but also a concern for surrounding tissue. Direction and exposure distribution matter alongside radiation type.

The source must maintain suitable output over its operating period, with decay allowed for. A long half-life reduces the rate of output change but also means the source remains radioactive for longer. Half-life alone does not determine a treatment's suitability.

Sterilise items through packaging

Penetrating gamma radiation can reach microorganisms inside suitable final packaging. Ionisation damages them without requiring the package to be opened. The source must provide useful output over the processing interval, and its changing activity must be allowed for as it ages.

Explain the hazard in context

Ionisation can damage living cells and genetic material. Relevant factors include activity, emitted energy and radiation type, exposure time, distance, shielding, and whether radioactive material enters the body. Short alpha range can limit an external exposure while an internal alpha emitter deposits energy densely near its location.

Irradiation means exposure to radiation. Contamination means radioactive material is present on or inside an object. Ordinary irradiation does not necessarily make the object radioactive; do not confuse the radiation reaching an object with the source material moving onto it.

Check your understanding

A tracer inside a pipe must be detected from outside it, and at least half its initial activity must remain after 60 min. Which radiation and half-life meet both conditions?

  1. An alpha emitter with a 2 h half-life.
  2. A gamma emitter with a 1 min half-life.
  3. A gamma emitter with a 2 h half-life.
Show the answer

A gamma emitter with a 2 h half-life.

It supplies photons and retains 2^(-60/120), about 0.707, of its activity after 60 min. Gamma photons pass through the pipe wall to the detector, so both conditions are met.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Data-based question: The americium-241 smoke detector [15]

Many smoke detectors contain a tiny sample of americium-241 (proton number 95), which decays by alpha emission to neptunium-237 with a half-life of 432 years. The alpha particles ionise the air between two metal plates connected to a battery. The ions move to the plates, so a small current flows. When smoke enters, ions attach to the smoke particles and move much more slowly, the current falls, and the alarm sounds.

Table 1. Data for one detector
mass of americium-2410.30 µg
molar mass of americium-2410.241 kg mol-1
activity of source3.7 x 104 Bq
fraction of alpha particles that enter the air gap0.50
kinetic energy of each alpha particle5.5 MeV
average energy needed to make one ion pair in air35 eV
  1. (a) Write the nuclear equation for the decay of americium-241. [2]
  2. (b) Explain why an alpha emitter is used rather than a beta or gamma emitter. [2]
  3. (c) Calculate the number of americium-241 nuclei in the source. [2]
  4. (d) Calculate the number of ion pairs produced by one alpha particle that stops in the air gap. [1]
  5. (e) Calculate the maximum current in the air gap. [3]
  6. (f) State one assumption you made in (e). [1]
  7. (g) The maker says the source will not need replacing during the detector's 10-year life. Use the data to evaluate this claim. [3]
  8. (h) Even in clean air, the current fluctuates slightly about its mean value. Suggest why. [1]

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