Radiation Fundamentals & Detection Flashcards

A sealed radioactive source contains 10 mCi of Cs-137 (T½ = 30 years). Approximately how much activity remains after 90 years?

Answer: 1.25 mCi

90 years = 3 half-lives; 10 × (1/2)³ = 10/8 = 1.25 mCi remaining.

In a proportional counter, what is the primary advantage over an ionization chamber?

Answer: Gas multiplication amplifying the primary ion signal

Proportional counters operate in the gas multiplication region where avalanche ionization amplifies the primary signal by factors of 10³–10⁵.

The photoelectric effect is the dominant photon interaction in tissue at energies:

Answer: Below about 100 keV

Photoelectric absorption dominates at low photon energies (below ~100 keV in tissue) and is highly dependent on atomic number (Z³).

Which detector is best suited for measuring low-energy beta emitters like tritium (H-3)?

Answer: Liquid scintillation counter

Liquid scintillation counting dissolves the sample in scintillant fluid, allowing nearly 100% geometric efficiency for weak beta emitters like tritium.

What is the significance of the 'dead time' in a radiation detector?

Answer: The period after each pulse during which new events cannot be recorded

Dead time is the minimum time that must separate two events for both to be detected; counts are lost during this interval at high count rates.

Which nuclear decay mode is characterized by emission of radiation from nuclear isomers (metastable states)?

Answer: Isomeric transition (IT)

Isomeric transition occurs when a metastable nuclear isomer (indicated by 'm') decays to a lower energy state, typically emitting a gamma ray.

The Compton edge in a gamma-ray spectrum represents:

Answer: The maximum energy transferred to the recoil electron in Compton scattering

The Compton edge is the maximum recoil electron energy, occurring when the photon is backscattered 180° and transfers maximum energy to the electron.