With reference to the various processes called quenching in gas-filled detectors:  Describe what is meant by quenching in a proportional counter and give an example of a suitable gas that could be used.

1.
(a) With reference to the various processes called quenching in gas-filled detectors:

Describe what is meant by quenching in a proportional counter and give an example of a suitable gas that could be used (include an explanation as to why it is desirable to do this). [2 MARKS]

Describe what is meant by quenching in a Geiger-Muller counter and give an example of a suitable gas that could be used (include an explanation as to why it is important to do this). [2 MARKS]

Describe why the transfer of charge between positive ions and neutral molecules is vital in gas quenching inside a Geiger-Muller counter (include an explanation as to why this happens). [2 MARKS]

(b) With reference to a typical cylindrical proportional counter,

(c) Sketch a graph to illustrate the magnitude of the signal produced by the counter as a function of the applied bias voltage assuming the same amount of energy deposited at each voltage and showing the range from zero up to the electrical breakdown of the detector.

Carefully annotate your graph to make clear the important features. [2 MARKS]

(ii) State how the size of the signal changes in such a counter when the amount of energy deposited in the counter is changed, for the different voltage regions of your graph.
[3 MARKS]

(c) Calculate the magnitude in volts of the signal produced in a Geiger-Muller counter following the interaction of an X-ray of energy 8 keV in the counter gas, stating the assumptions that you need to make (assume that the detector has a capacitance of 50 pF) and then comment on the size of the signals if a gamma-ray of 800 keV interacted in the counter instead.
[5 MARKS]

(d) Calculate the current in nA that would be produced in a properly operating free-air ionization chamber with an active volume containing 2 g of air and exposed to a flux of radiation depositing a constant dose rate of 0.3 J/kg/h (0.3 Gy per hour) in the air. [4 MARKS]

2.

(a) Regarding semiconductor detectors:

Why is it that silicon semiconductor detectors can be operated at room temperature but germanium semiconductor detectors must be cooled to much lower temperatures (typically 77K to 177K) in order to operate correctly?

[2 MARKS]

(ii) Sketch the band structure of the energy levels in a typical semiconductor on a diagram with energy in the vertical direction, indicating the typical magnitude of the band gap and also the locations of typical donor levels and acceptor levels. [2 MARKS]

(b) With reference to silicon semiconductor detectors, Explain why the electric field in the region of a p-n junction reaches a maximum exactly at the position of the junction.
[2 MARKS]

 

Sketch a graph for a 500 mm thick silicon detector showing the thickness of the depletion layer as a function of the applied reverse bias voltage, from the situation of zero voltage to when the detector becomes fully depleted. You may neglect the thicknesses of the n+ layer and the naturally occurring depletion layer at the junction with the bulk p-type material.

[3 MARKS]

(iv) Sketch a simple circuit diagram showing how you would connect a voltage supply to a silicon detector made from an n+ layer on one side of a wafer of p-type material, being careful to indicate the polarity of the voltage (which side is positive and which side is grounded).
[2 MARKS]

(v)Estimate the magnitude in volts of the signal produced by an alpha-particle that deposits 5.5 MeV in the depletion region of a silicon detector (assume that the detector has a capacitance of 10 pF). [3 MARKS]

(c) Use a sketch to show the band structure in a typical insulator that has been modified to act as an inorganic scintillator (such as NaI(Tl) for example). Indicate on your diagram the typical magnitude of the band gap and show how the energy level structure is modified in a luminescence centre by the addition of an activator material. Describe how the energy of a charged particle moving through this material is converted into optical photons and comment on why this type of detector will have poorer energy resolution for detecting gamma-rays than a typical germanium detector would have. [4 MARKS]
3.
(a) The linear energy transfer coefficient, for gamma-ray photons interacting in matter is given by the expression

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