STP Physical Sciences 2

Question 1

Which type of electromagnetic radiation has the shortest wavelength used in medical imaging?

Accepted Answer

X-ray

Answer

X-rays have wavelengths in the range of 0.01–10 nm, making them the shortest wavelength radiation routinely used in medical imaging.

Question 2

What is the half-life of a radioactive isotope if 75% of a sample decays in 20 years?

Accepted Answer

10 years

Answer

If 75% decays, 25% remains; since (1/2)^2 = 0.25, two half-lives have passed, so the half-life is 20/2 = 10 years.

Question 3

In a semiconductor detector for ionising radiation, what charge carriers are collected?

Accepted Answer

Electrons and holes

Answer

Ionising radiation creates electron-hole pairs in the semiconductor, and these charge carriers are swept to electrodes by an applied electric field.

Question 4

What physical principle underlies Magnetic Resonance Imaging (MRI)?

Accepted Answer

Nuclear spin precession in a magnetic field

Answer

MRI exploits the precession of hydrogen nuclei (protons) aligned in a strong magnetic field when perturbed by a radiofrequency pulse.

Question 5

Which law states that the pressure of a fixed mass of gas is inversely proportional to its volume at constant temperature?

Accepted Answer

Boyle's Law

Answer

Boyle's Law states that P ∝ 1/V at constant temperature, meaning pressure and volume are inversely proportional for an ideal gas.

(STP) Scientist Training Programme Practice Test