DECK 4 — PROPORTIONAL COUNTERS

What is a proportional counter?

A proportional counter is a gas-filled detector that amplifies primary ionization through controlled avalanche multiplication.

What causes gas multiplication in a proportional counter?

Gas multiplication occurs when electrons gain enough energy between collisions to ionize additional gas molecules.

What is the Townsend avalanche?

The Townsend avalanche is a chain reaction of ionizations caused by high-energy electrons near the anode wire.

Why is the avalanche confined near the anode wire?

The electric field is strongest near the thin anode wire, creating conditions for multiplication only in that region.

What determines the gas multiplication factor (M)?

M depends on electric field strength, gas type, pressure, and distance to the anode.

What defines the proportional region of detector operation?

The proportional region is where pulse height is proportional to the number of primary ion pairs created.

Why does the proportional region allow energy discrimination?

Pulse amplitude reflects the energy deposited by the radiation.

Why do proportional counters have better resolution than GM tubes?

They preserve information about primary ionization rather than producing identical pulses.

Why do proportional counters have worse resolution than semiconductors?

Gas multiplication introduces statistical fluctuations and fewer total charge carriers are produced.

Why does pulse height increase with voltage in the proportional region?

Higher voltage increases avalanche size while still maintaining proportionality.

What happens in the limited proportional region?

The avalanche becomes too large and the pulse no longer scales linearly with primary ionization.

What physical effect causes limited proportionality?

Space charge from positive ions reduces the electric field near the anode, distorting gain.

What happens when voltage is raised above the proportional region?

The detector enters the Geiger region where pulses become identical in size.

Why is the anode wire very thin in proportional counters?

A thin wire creates a strong electric field needed for avalanches to occur.

How does gas composition affect proportional counters?

Gas type influences ionization potential, multiplication stability, and quenching properties.

Why are noble gases commonly used in proportional counters?

Noble gases have stable ionization properties and produce efficient signals.

What is the purpose of a quench gas in proportional operation?

A quench gas absorbs UV photons and limits secondary avalanches.

Why do proportional counters detect alpha particles efficiently?

Alphas create large primary ionization, producing strong proportional pulses.

Why can proportional counters distinguish between alpha and beta radiation?

Alpha particles deposit more energy, resulting in larger pulse amplitudes.

Why is pulse height analysis useful with proportional counters?

It allows measurement of energy spectra for low-energy photons and charged particles.

How does detector geometry affect electric field strength?

The field depends on the ratio of anode wire radius to cathode radius and applied voltage.

Why is uniform field distribution important in proportional counters?

Uniform fields ensure consistent gas multiplication along the wire.

What causes afterpulses in proportional counters?

UV photons or ion feedback can trigger secondary avalanches.

Why is stable high-voltage important for proportional counters?

Voltage fluctuations change multiplication factor and distort energy calibration.

What is the relation between primary ion pairs and pulse amplitude?

Pulse amplitude is proportional to the number of primary ion pairs multiplied by gain.

Why does gas pressure affect proportional operation?

Higher pressure increases collision frequency, reducing avalanche size.

What is the maximum sustainable count rate for proportional counters?

Limited by ion drift times and recovery of the electric field.

Why are proportional counters used for x-ray spectroscopy in the keV range?

They have adequate resolution and efficiency for low-energy photons.

Why are proportional counters less suitable for high gamma energies?

High-energy photons interact infrequently in the gas, reducing efficiency.

Why does ion space charge reduce proportionality at high count rates?

Positive ions accumulate near the anode, reducing the electric field strength.

What is the main advantage of proportional counters over GM tubes?

They provide pulse height information for energy discrimination.