Atoms and Nuclear Radiation

Alpha (α):

• Identity: helium-4 nucleus (2 protons + 2 neutrons).
• Charge: +2.
• Mass: 4 amu.
• Ionising power: VERY HIGH (large mass, large charge).
• Range in air: ~5 cm.
• Stopped by: paper, skin, a few cm of air.
• Penetration: low.

Beta (β⁻):

• Identity: high-speed electron emitted from the nucleus when a neutron decays into a proton + electron + antineutrino.
• Charge: −1.
• Mass: ~1/2000 amu (tiny).
• Ionising power: medium.
• Range in air: ~1 m.
• Stopped by: ~5 mm of aluminium.

Gamma (γ):

• Identity: high-energy electromagnetic radiation (photon), emitted from the nucleus.
• Charge: 0.
• Mass: 0.
• Ionising power: LOW.
• Range in air: many metres.
• Stopped by: ~5 cm of lead or thick concrete.

Neutron:

• Identity: free neutron from the nucleus.
• Charge: 0.
• Mass: 1 amu.
• Penetration: VERY HIGH.
• Stopped by: water or hydrogen-rich materials (paraffin, polythene).

Ionising power = how easily the radiation knocks electrons out of nearby atoms. Penetration = how far the radiation travels through materials.

These are INVERSELY related:

• α: very ionising (lots of charge, slow) — interacts strongly with first atoms → stops quickly.
• γ: barely ionising (no charge, fast photon) — passes through many atoms before interacting → far penetration.

Type	Ionising	Penetration
α	Very high	Very low
β	Medium	Medium
γ	Low	Very high
n	Low	Very high

Why ionising matters. Strongly ionising radiation (α) is more dangerous if INSIDE the body (lots of damage in one place). Less ionising but more penetrating radiation (γ) is more dangerous from OUTSIDE — it can reach internal organs.

When a nucleus emits an alpha particle (a He-4 nucleus):

• A decreases by 4 (4 nucleons leave).
• Z decreases by 2 (2 protons leave).

The daughter nucleus is a DIFFERENT element (Z changed).

General form: $^A_Z X \to {}^{A-4}_{Z-2} Y + {}^4_2 \text{He}$

Worked example. Uranium-238 alpha decays to thorium-234.

$^{238}_{92}\text{U} \to {}^{234}_{90}\text{Th} + {}^4_2 \text{He}$

Check: A: 238 = 234 + 4 ✓. Z: 92 = 90 + 2 ✓.

Worked example. Radium-226 → ? + alpha.

$^{226}_{88}\text{Ra} \to {}^{222}_{86}\text{Rn} + {}^4_2 \text{He}$

(Element with Z = 86 is radon.)

In beta-minus decay, a neutron in the nucleus turns into a proton, emitting an electron and an antineutrino. AQA only requires you to track the electron at GCSE.

• A stays the same (a neutron becomes a proton — total nucleon count unchanged).
• Z increases by 1 (proton count goes up by 1).
• An electron $^0_{-1}\text{e}$ is emitted.

General form: $^A_Z X \to {}^A_{Z+1} Y + {}^0_{-1} \text{e}$

Worked example. Carbon-14 → Nitrogen-14 + beta.

$^{14}_6 \text{C} \to {}^{14}_7 \text{N} + {}^0_{-1} \text{e}$

Check: A: 14 = 14 + 0 ✓. Z: 6 = 7 + (−1) ✓.

Worked example. Strontium-90 beta decays to yttrium-90.

$^{90}_{38} \text{Sr} \to {}^{90}_{39} \text{Y} + {}^0_{-1} \text{e}$

Gamma rays are emitted from an excited nucleus that needs to release surplus energy after another decay. Gamma alone:

• A unchanged.
• Z unchanged.

So you do NOT write gamma equations the same way — it's an additional emission after α or β, releasing energy without altering identity.

Half-life ($t_{1/2}$) is the time taken for HALF the unstable nuclei in a radioactive sample to decay — equivalently, the time for the count rate (or activity) to halve.

A graph of activity vs time shows an exponential decay curve. To find the half-life:

1. Read the initial activity $A_0$.
2. Find where the curve falls to $A_0 / 2$ and read off the time.
3. This time is $t_{1/2}$.

After $n$ half-lives, the activity has fallen to $(1/2)^n$ of its initial value:

• 1 half-life: ½ remaining
• 2 half-lives: ¼
• 3 half-lives: ⅛
• 4 half-lives: 1/16

Radioactive decay is a random process: for any individual nucleus you cannot predict WHEN it will decay. But with a sample of trillions of nuclei, the AVERAGE rate of decays is highly predictable — like a fair coin: any single flip is 50/50, but flipping millions averages out to almost exactly 50%.

The half-life is fixed for a given isotope and is unaffected by temperature, pressure, chemical environment etc.

(HT) Ratio calculation: If a sample starts with 1000 nuclei and has a half-life of 5 years, after 15 years:

• $n = 15 / 5 = 3$ half-lives.
• Fraction remaining = $(1/2)^3 = 1/8$.
• Nuclei left = $1000 \times 1/8 = 125$.
• Net decline as ratio: $1000 : 125 = 8 : 1$.

Irradiation is exposure to radiation from an EXTERNAL source. Once the source is removed (or you move away), exposure stops. Examples: airport X-ray scanner, hospital X-ray, gamma sterilisation of medical equipment.

Contamination is the presence of a radioactive substance ON or IN the person, on their clothes, in their food, in the environment. Contamination keeps emitting radiation continuously until decayed or removed. Examples: dust from a nuclear accident (Chernobyl 1986, Fukushima 2011), radioactive iodine in food/water, leaked tracer in a lab.

Feature	Irradiation	Contamination
Source location	External	On/in body
Stops when	You move away	Contaminant removed or decays
Cleaning needed?	No	Yes (decontamination)
Most dangerous type	γ (most penetrating)	α (highly ionising once inside)

Why is contamination usually worse? The radioactive material continues to irradiate the person from close range. Alpha emitters are particularly dangerous internally — their high ionisation causes localised cell damage.

For irradiation (external source):

• Distance — radiation intensity falls with distance.
• Time — minimise exposure time.
• Shielding — lead for γ; aluminium for β; nothing (or paper) for α.
• Wear lead aprons (medical X-ray operators) and use remote-handling tongs.

For contamination:

• Wear PPE (gloves, lab coat, goggles, mask).
• Handle sources with tongs/tweezers.
• No eating, drinking, or smoking in radiation labs.
• Wash hands after handling.
• Use sealed sources where possible.
• Designate 'hot' areas and check workers with a Geiger counter on exit.

Background radiation (covered in 4.4.3.1) is always present at low levels — small dose from natural sources.

When scientists publish new findings about radiation (e.g. health risks from mobile phones, effects of low-dose CT scans), the work goes through peer review:

1. The scientist submits their work to a journal.
2. Independent experts examine the methodology, data, and conclusions.
3. They may suggest changes or rejection.
4. If accepted, the work is published — but readers can still challenge it.

Peer review reduces the risk of bad science (e.g. cherry-picked data, sample bias, conflict of interest). It is essential for radiation claims because misleading information can cause public fear or false reassurance.

Without peer review, dubious claims (e.g. 'phones cause brain cancer', 'small radiation doses are healthy') could spread. Modern public health relies on peer-reviewed evidence such as the LNT (linear no-threshold) model and ALARA (As Low As Reasonably Achievable) principles.