NMTCB Radiation Physics & Detection 4

Question 1

The photoelectric effect cross-section (τ) depends on atomic number Z approximately as:

Accepted Answer

τ ∝ Z⁴ to Z⁵ / E³·⁵

Answer

Photoelectric cross-section scales approximately as Z⁴⁻⁵/E³·⁵, which is why high-Z materials like lead are effective gamma ray shields at low energies.

Question 2

In liquid scintillation counting, chemical quenching reduces efficiency by:

Accepted Answer

Absorbing energy from excited scintillant molecules before photon emission occurs

Answer

Chemical quenching agents absorb energy from excited fluorescent molecules via collisional deactivation, reducing the number of light photons produced per decay event.

Question 3

The effective half-life (T_eff) of a radionuclide in a biological system is calculated as:

Accepted Answer

T_eff = (T_physical × T_biological) / (T_physical + T_biological)

Answer

The effective half-life combines physical decay and biological elimination as parallel first-order processes: 1/T_eff = 1/T_physical + 1/T_biological.

Question 4

Which statement correctly describes the Compton plateau in a pulse height spectrum?

Accepted Answer

A broad continuum of pulse heights below the photopeak from partial energy deposits in Compton interactions

Answer

Compton scattering results in scattered photons that may escape the detector, depositing only partial energy and creating a continuum of pulse heights below the photopeak.

Question 5

Fluorescence yield (ω) in X-ray emission describes the probability that a vacancy in an electron shell will result in:

Accepted Answer

Characteristic X-ray emission rather than Auger electron emission

Answer

Fluorescence yield ω = N_X-ray / (N_X-ray + N_Auger), representing the fraction of inner-shell vacancies that decay by emitting characteristic X-rays versus Auger electrons.

(NMTCB) Nuclear Medicine Technology Certification Board Practice Test

(NMTCB) Nuclear Medicine Technology Certification Board Practice Test

NMTCB Radiation Physics & Detection 4