Radiation Detectors Questions and Answers

Answer: C

The junction diode is used in the surface barrier detector.

A surface barrier detector is a semiconductor detector: a thin gold layer evaporated on n-type silicon forms a rectifying junction. Under reverse bias the depletion region acts as the sensitive volume, and an incoming charged particle creates electron-hole pairs whose charge is collected, just as in a reverse-biased p-n junction diode.

The Geiger-Muller counter and the proportional counter are gas-filled detectors that work by ionisation of a gas between electrodes. A scintillation detector converts radiation into light in a crystal or liquid and uses a photomultiplier, not a junction, as the sensing element.

Answer: C

A photomultiplier detects feeble light because of the multiplication of secondary electrons.

A photon striking the photocathode releases a photoelectron, which is accelerated to the first dynode and knocks out several secondary electrons. Each of these is accelerated to the next dynode and multiplied again, so after about ten stages one photoelectron becomes a pulse of a million electrons.

The photons themselves are not multiplied, the shape of the photocathode only affects collection, and multiplying the output pulse would add nothing that is not already in the signal.

Answer: D

The overall amplification is about \(10^{6}\).

Each of the 10 stages multiplies the number of electrons by 4, so the gain is:

\[ G = 4^{10} = 1\,048\,576 \approx 10^{6} \]

The option 10 confuses the number of stages with the gain, and \(10^{3}\) or \(10^{4}\) would correspond to only 5 to 7 stages of gain 4.

Answer: D

A G.M. counter cannot measure energy because all the atoms of the gas are ionised irrespective of the energy of the incident radiation.

In the Geiger region the applied voltage is so high that any initial ionisation, however small, triggers an avalanche that spreads along the whole anode wire until it is stopped by the space charge. Every pulse therefore has nearly the same size, so the pulse carries no information about the energy deposited.

Recombination happens in the low-voltage region, not the Geiger region; window losses affect only weak radiations; and partial ionisation is what happens in proportional counters, which can measure energy.

Answer: C

In general, HPGe has high energy resolution but low efficiency compared with NaI(Tl).

In germanium only about 3 eV is needed to create an electron-hole pair, against roughly 100 eV per photoelectron in a NaI(Tl) scintillator and photomultiplier, so far more charge carriers are produced and statistical fluctuations are much smaller. A typical HPGe detector resolves about 2 keV at 1332 keV, against about 7% (some 90 keV) for NaI(Tl).

NaI(Tl) contains iodine (Z = 53, against germanium's 32), which absorbs gamma rays strongly by the photoelectric effect, and it is grown in much larger crystals, so a typical NaI(Tl) detector stops more of the incident gamma rays. Hence both 'higher' and 'the same' are wrong, and option D reverses the comparison.