Uses of X-rays and Gamma Rays

An X-ray image is essentially a "shadow" picture created by the differential absorption of these waves:

• Bones: These are dense and contain elements with high atomic numbers, like calcium. They cause heavy absorption, meaning the wave's energy is transferred to the atoms and stopped. Because the waves do not reach the detector plate behind the patient, these areas appear white or light on the negative image.
• Soft tissue: Muscle, fat, and organs have a much lower density. They mostly allow the transmission of X-rays, letting the waves pass straight through to the detector. These areas appear dark or black.

Because soft tissues do not naturally absorb many X-rays, doctors sometimes use a contrast medium (like a barium meal or an iodine injection). This temporarily increases the absorption in a specific organ, making it visible on the scan.

Worked Example: Comparing Radiation Doses

A standard chest X-ray delivers a radiation dose of $0.1\text{ mSv}$. A CT scan delivers a dose of $10\text{ mSv}$. How many chest X-rays equal the radiation dose of one CT scan?

Step 1: Identify the relationship.

• Total dose from CT scan $\div$ dose per chest X-ray = number of X-rays

Step 2: Substitute the values.

• $10 \div 0.1$

Step 3: Calculate the final answer.

• $= 100$ chest X-rays

X-Rays in Airport Security

Every time you pass through airport security, you rely on high-energy waves to prove your luggage is safe. Airport scanners use X-rays operating at high energies (typically 140 to 160 kilovolt peak) to penetrate thick, dense suitcases.

The scanning process works step-by-step:

1. An X-ray tube emits a beam of radiation towards the luggage.
2. The waves easily penetrate and transmit through low-density organic materials like clothing, paper, and food.
3. The waves are absorbed by high-density metallic objects, such as keys, coins, or hidden weapons.
4. A detector beneath the luggage captures the transmitted waves.
5. A computer translates this into an image. Modern dual-energy scanners colour-code the image, often displaying organic materials as orange and inorganic/metallic materials as blue or green.

Luggage can be safely X-rayed because it is inanimate. Humans are generally scanned with non-ionising alternatives instead of X-rays, because excessive exposure to ionising radiation can cause cell mutations and cancer.

Gamma Rays for Sterilisation

The same radiation capable of causing genetic mutations is also used to preserve the strawberries in your fridge and clean medical tools. Sterilisation is the process of killing all living microorganisms (like bacteria, fungi, and parasites) to make an object safe.

This is done through irradiation, which involves exposing the object to a radioactive source, typically Cobalt-60 (chosen for its relatively long half-life of 5.27 years, meaning it doesn't need replacing frequently). Gamma rays are the most penetrating type of radiation. This is a crucial advantage because items can be sealed in airtight plastic or foil packaging before being irradiated. The gamma rays pass straight through the packaging, ionising the DNA of any trapped microbes and destroying them, ensuring no re-contamination can occur.

This method is ideal for heat-sensitive medical equipment (like plastic syringes) that would melt in an autoclave. It is also used to preserve food by killing decay-causing bacteria (like Salmonella) and slowing down the ripening process. Crucially, exposing these items to gamma rays does not cause contamination; the food or equipment does not become radioactive itself.

Gamma Rays in Medical Diagnosis

You can easily block alpha particles with paper, but you need thick lead or concrete to stop the gamma rays used in hospitals. This incredible penetrating power makes gamma radiation ideal for diagnosing illnesses from the inside out.

A medical tracer is a radioactive isotope attached to a chemical and either injected or swallowed by the patient. As the tracer moves through the body (for example, pooling in a specific organ), it emits radiation that is detected by a gamma camera outside the body. Gamma emitters like Technetium-99m are strictly used because:

• They are highly penetrating and can easily escape the body to reach the detector.
• They are weakly ionising compared to alpha or beta particles, minimising cellular damage to the patient.
• They have a short half-life (e.g., 6 hours), ensuring the radioactivity drops to safe levels quickly after the procedure.

Another diagnostic tool is the PET scan (Positron Emission Tomography). This uses a tracer that emits positrons. When a positron collides with an electron inside the body, they annihilate each other, releasing two gamma-ray photons travelling in exactly opposite directions, which the scanner detects to build a 3D image.

Worked Example: Tracer Decay

A medical tracer has an initial activity of $800\text{ MBq}$ and a half-life of 6 hours. What will its activity be after 24 hours?

Step 1: Calculate the number of half-lives.

• $24\text{ hours} \div 6\text{ hours} = 4\text{ half-lives}$

Step 2: Halve the initial activity four times.

• After 1 half-life (6 hours): $800 \div 2 = 400\text{ MBq}$
• After 2 half-lives (12 hours): $400 \div 2 = 200\text{ MBq}$
• After 3 half-lives (18 hours): $200 \div 2 = 100\text{ MBq}$
• After 4 half-lives (24 hours): $100 \div 2 = 50\text{ MBq}$

Step 3: State final answer.

• Activity $= 50\text{ MBq}$

Gamma Rays in Cancer Treatment

How do doctors destroy a deeply buried tumour without destroying the healthy organs surrounding it? They use carefully controlled radiotherapy, which uses high doses of ionising radiation to damage the DNA of cancer cells, preventing them from dividing or killing them outright.

In external radiotherapy, a high-intensity gamma beam is fired at the patient. To protect healthy tissue, the radiation source is rotated around the patient's body in a circular path, with the tumour at the exact centre. This ensures the tumour receives a massive, continuous dose of radiation, while any single area of surrounding healthy tissue only receives a brief, low dose.

Alternatively, internal radiotherapy involves placing radioactive pellets or liquids directly inside the body, right next to the tumour. Because radiotherapy uses highly penetrating and ionising waves, medical staff must protect themselves by operating the machinery remotely, standing behind concrete walls, or using lead shielding.