Properties of Ionising Radiation
This topic covers the distinct properties of alpha, beta, and gamma radiation, focusing on how they interact with matter and are affected by electric and magnetic fields. Understanding these characteristics is crucial for evaluating their practical applications and associated safety hazards.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Penetrating power follows the order: Gamma > Beta > Alpha. Gamma requires thick lead for significant shielding, beta is stopped by thin aluminium, and alpha is stopped by paper or skin.
- Ionising ability is the inverse of penetrating power: Alpha > Beta > Gamma. Alpha particles are highly ionising due to their large mass and charge, causing significant damage over a short range.
- In electric and magnetic fields, charged particles (alpha and beta) are deflected, while neutral gamma rays are not. They deflect in opposite directions due to their opposite charges (+2e for alpha, -1e for beta).
- Beta particles are deflected much more than alpha particles in the same field. This is primarily due to the beta particle's vastly smaller mass.
- Background radiation is a constant, low-level exposure from natural sources (like radon gas, cosmic rays) and artificial sources (like medical imaging).
- The hazards of radiation stem from its ability to ionise atoms in living cells, potentially causing mutations. Applications leverage specific properties, e.g., alpha in smoke detectors (short range) and gamma for sterilisation (high penetration).
› Why does this happen?
Why ionising and penetrating power are opposites
Think of the types of radiation travelling through a material like different objects trying to get through a dense forest.
An alpha particle is like a slow, wide truck: its large size and +2 charge mean it's almost guaranteed to hit and interact with the atoms it passes. Each interaction (knocking an electron off, or 'ionisation') uses up some of its energy. Because it interacts so often, it loses all its energy very quickly and stops within a few centimetres of air. So, it's highly ionising over a short range but has very low penetrating power.
A beta particle is like a fast-moving motorbike: its small size and -1 charge mean it interacts less frequently than an alpha particle. It still causes ionisation, but it can travel further through the 'forest' of atoms before it has lost all its energy. This gives it medium ionising ability and medium penetrating power.
A gamma ray is like a ghost: it has no mass and no charge. It can pass straight through most atoms without any interaction at all. Only a direct 'hit' on a particle within an atom will stop it. Because these interactions are very rare, gamma rays travel a long way, causing little ionisation along their path. This gives them very high penetrating power but low ionising ability.
Why particles deflect differently in electric & magnetic fields
Deflection is caused by forces. Electric and magnetic fields only exert forces on charged particles; since gamma rays are uncharged, they are unaffected and travel straight.
Alpha and beta particles are charged, so they feel a force. The direction of this force depends on the sign of their charge. Since alpha particles are positive (+2) and beta particles are negative (-1), they are pushed in opposite directions.
The amount of deflection depends on both the particle's mass and the size of the force on it. A particle with a large mass is much harder to push off course than a light one. A beta particle is over 7000 times less massive than an alpha particle. Although the alpha particle's +2 charge means it feels a stronger force than the beta's -1 charge, its enormous mass makes it far more difficult to deflect. The beta particle's tiny mass is the deciding factor, so its path is bent much more.
Formulae
Use Fleming's Left-Hand Rule for magnetic deflection. To determine the direction of the force, and hence deflection, on a charged particle moving through a magnetic field. Note: For beta particles (electrons), conventional current (the 'I' finger) is in the opposite direction to their velocity.
Definitions
- Ionising Radiation
- Any radiation with sufficient energy to knock electrons out of atoms or molecules, thereby creating ions.
- Alpha Particle (α)
- A particle composed of two protons and two neutrons, identical to a Helium-4 nucleus, with a charge of +2e.
- Beta Particle (β)
- A high-energy, high-speed electron (or positron) emitted from the nucleus during radioactive decay, with a charge of -e (or +e).
- Gamma Ray (γ)
- A packet of high-energy electromagnetic radiation (a photon) emitted from the nucleus, with no mass or charge.
- Background Radiation
- The ubiquitous ionising radiation present in the environment from both natural and artificial sources.
Worked example
An experiment measures the radiation from a sample using a Geiger-Müller tube. The background radiation is found to be 40 counts per minute. With the sample in place, the count rate is 840 counts per minute. When a 1 mm thick sheet of aluminium is placed between the sample and the tube, the rate drops to 290 counts per minute. When the aluminium is replaced by a 5 cm thick block of lead, the count rate is 40 counts per minute. What types of radiation does the source emit?
- 1
First, correct all readings for background radiation by subtracting 40 counts/min.
The total emission from the source is 840 - 40 = 800 counts/min.
- 2
The lead block reduces the count rate to the background level (40 counts/min), meaning the source's contribution becomes 40 - 40 = 0.
This shows the lead blocks all radiation from the source.
- 3
The aluminium sheet reduces the count rate from 840 to 290.
The radiation from the source is reduced from 800 counts/min to 290 - 40 = 250 counts/min.
This means the aluminium blocked 800 - 250 = 550 counts/min of radiation.
This blockage is characteristic of beta particles (and would also stop any alpha particles, if present).
- 4
After passing through the aluminium, there are still 250 counts/min of radiation from the source.
This radiation penetrates aluminium but is stopped by lead.
This is characteristic of gamma radiation.
- 5
The experiment does not use a paper shield, so we cannot definitively confirm or rule out alpha radiation.
However, the data strongly supports the presence of beta (stopped by aluminium) and gamma (penetrated aluminium but stopped by lead).
Answer: Beta and Gamma radiation.
Common mistakes
- ×Forgetting the inverse relationship: highly ionising radiation (alpha) is the least penetrating, and weakly ionising radiation (gamma) is the most penetrating.
- ×Confusing the extent of deflection in fields. Students may assume alpha particles are deflected more due to their greater charge (+2e vs -1e), forgetting that their much larger mass (~8000 times greater) is the dominant factor, causing them to be deflected far less than beta particles.
- ×Failing to subtract background radiation from experimental data before drawing conclusions about which types of radiation are present.
- ×Applying Fleming's Left-Hand Rule for beta particles without reversing the direction of conventional current relative to the electron's velocity.
No-calculator tips
- ✓In deflection problems, remember that mass dominates. The alpha particle is a 'heavy truck' and the beta particle is a 'bicycle'; the truck is much harder to turn, even with a stronger push (charge).
- ✓For penetration experiments, work with the net counts from the source by subtracting the background count first. This simplifies the ratios and makes it easier to see what proportion of the radiation is being blocked by each material.
- ✓Think of ionisation as a series of collisions. A particle that is very effective at colliding (alpha) will lose its energy and stop quickly. A particle that is poor at colliding (gamma) will travel a long way before it happens to lose all its energy.