DECK 3 — IONIZATION CHAMBERS

What is an ionization chamber?

An ionization chamber is a gas-filled detector that collects ion pairs produced by radiation without gas multiplication.

What determines the signal size in an ionization chamber?

The signal size is determined solely by the number of ion pairs created by the radiation.

Why do ionization chambers operate in current mode?

They produce small signals that are easier to measure as an average current rather than individual pulses.

Why do ionization chambers have poor energy resolution?

They create few ion pairs, leading to large statistical fluctuations in signal size.

What is the saturation region of an ionization chamber?

The saturation region is the voltage range where all ion pairs are collected and the output current is constant.

Why does the current become constant in the saturation region?

All ion pairs are collected before recombination can occur.

What is recombination in an ionization chamber?

Recombination occurs when electrons and ions reattach before reaching the electrodes.

Why does low voltage increase recombination?

Low electric field strength causes slow drift velocities, allowing ions and electrons to recombine.

How does increasing voltage reduce recombination?

Higher voltages increase drift velocity, reducing the chance of ions and electrons recombining.

Why do ionization chambers not produce gas multiplication?

The electric field is intentionally kept too low for electrons to gain enough energy to ionize more gas molecules.

What is columnar recombination?

Columnar recombination occurs when dense ionization tracks cause ions to recombine before drifting away.

What is general (volume) recombination?

General recombination occurs uniformly in the gas when ions and electrons meet before collection.

Why does an ionization chamber measure dose effectively?

Ionization current is proportional to the total energy deposited in the gas.

Why are ionization chambers used for high radiation fields?

They do not saturate or suffer from dead time effects at high fluxes.

What is the role of electrode geometry in ionization chambers?

Electrode geometry determines electric field uniformity and collection efficiency.

Why do ions drift much slower than electrons?

Ions are thousands of times more massive than electrons, resulting in much slower drift speeds.

How does slow ion motion affect the signal?

Slow ion motion broadens the current pulse, making individual events indistinguishable.

What is the typical W-value for gases in ion chambers?

The W-value for gases is around 25–35 eV per ion pair.

Why does the ion chamber output depend on W-value?

W-value determines how many ion pairs are produced for a given deposited energy.

Why is an ion chamber unable to perform spectroscopy?

The lack of gas multiplication and small charge signal prevent energy discrimination.

Why are sealed ion chambers convenient?

They require no gas flow and maintain stable gas density.

What limits the smallest detectable signal in an ion chamber?

Electronic noise often masks very small ionization currents.

Why is a stable high-voltage supply important for ion chambers?

Voltage variations change collection efficiency and affect dose accuracy.

Why are ion chambers preferred for radiation protection surveys?

They measure true exposure or dose rates with minimal energy dependence.

Why is the ion chamber region below saturation unsuitable for measurements?

Recombination makes the output current nonlinear and unstable.

What happens if voltage is increased above saturation?

The chamber may enter the proportional region and begin gas multiplication.

Why are ion chambers insensitive to pulse pile-up?

They do not detect individual pulses, only the average current.

How do pressure and gas density affect ion chambers?

Higher gas density increases ionization and improves sensitivity.

Why are chambers often filled with air or argon?

These gases are stable, inexpensive, and provide predictable ionization behavior.

What is the purpose of a vented ion chamber?

A vented chamber equalizes internal pressure with ambient air to maintain calibration.

What happens if humidity enters a vented ion chamber?

Humidity can cause leakage currents and distort measurement accuracy.