NRRPT Radiation Fundamentals & Detection 4

Question 1

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

Accepted Answer

1.25 mCi

Answer

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

Question 2

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

Accepted Answer

Gas multiplication amplifying the primary ion signal

Answer

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

Question 3

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

Accepted Answer

Below about 100 keV

Answer

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

Question 4

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

Accepted Answer

Liquid scintillation counter

Answer

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

Question 5

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

Accepted Answer

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

Answer

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.

(NRRPT) National Registry of Radiation Protection Technicians Practice Test

(NRRPT) National Registry of Radiation Protection Technicians Practice Test

NRRPT Radiation Fundamentals & Detection 4