recall that a tracer that decays by β+ decay is used in positron emission tomography (PET scanning)

Production and Use of X‑rays

1. How X‑rays are produced

X‑rays are generated when high‑energy electrons are decelerated or when inner‑shell electrons are removed from atoms. The two principal mechanisms are:

• Bremsstrahlung (braking radiation) – fast electrons are deflected by the electric field of nuclei, losing kinetic energy that is emitted as a continuous spectrum of X‑ray photons.
• Characteristic radiation – an incident electron ejects an inner‑shell electron; an outer‑shell electron then drops down to fill the vacancy, emitting an X‑ray photon with an energy equal to the difference between the two energy levels.

The energy of an emitted photon is given by the Planck relation:

$$E = hu$$

where $E$ is the photon energy, $h$ is Planck’s constant and $u$ is the frequency of the radiation.

2. Applications of X‑rays

Two major categories of application are:

1. Imaging – medical radiography, computed tomography (CT) and industrial non‑destructive testing.
2. Therapy – radiotherapy for cancer treatment, where high‑dose X‑rays are used to damage tumour DNA.

3. Positron Emission Tomography (PET) scanning

PET scanning relies on the detection of two 511 ke \cdot photons produced when a positron emitted by a radioactive tracer annihilates with an electron. The tracer must undergo β⁺ decay:

$$\ce{_{Z}^{A}X -> _{Z-1}^{A}Y + e^{+} + u_e}$$

The emitted positron travels a short distance in tissue, then annihilates with an electron, producing two photons travelling in (approximately) opposite directions. Coincidence detection of these photons allows reconstruction of the tracer distribution inside the body.

Key PET tracer

The most widely used PET tracer is fluorine‑18 labelled fluorodeoxyglucose (ⁱ⁸F‑FDG). It is a glucose analogue that participates in cellular metabolism, making it ideal for imaging metabolic activity.

Tracer	Radioisotope	Half‑life	Decay mode	Typical use
ⁱ⁸F‑FDG	Fluorine‑18	110 min	β⁺	Oncological imaging, brain metabolism
⁸⁸Y‑DOTATATE	Yttrium‑88	106 min	β⁺	Neuroendocrine tumour imaging
¹¹C‑PiB	Carbon‑11	20 min	β⁺	Alzheimer’s disease amyloid imaging

All PET tracers share the essential feature of undergoing β⁺ decay, providing the positrons required for annihilation photon production.

4. Summary of learning objective

Remember:

• A tracer that decays by β⁺ decay (e.g., ⁱ⁸F‑FDG) is the fundamental source of positrons in PET scanning.
• The positrons annihilate with electrons, producing two 511 ke \cdot X‑ray photons that are detected in coincidence.
• The spatial distribution of these events reflects the biochemical activity of the tracer, enabling functional imaging of the body.