Nuclear and Particle Detectors — DU B.Sc. (Hons) Physics Previous Year Question Papers

Attempt five questions in all. (a) Differentiate between mass and linear attenuation coefficients. (b) "The pulse height in an ionization chamber is proportional to the energy deposited". Justify the statement. (c) A GM tube has dead time r = 300 us. Observed rate = 2500 counts/s- Compute the true count rate. (d) Differentiate between absolute efficiency and intrinsic efficiency of a (e) How can a single channel analyzer be used to measure energy spectra? How is stopping power of a material related to the range and energy of an incident ionising radiation ? g) What is the basic working principle of Computed Tomography. 2. (a) Discuss any three of following interactions with the help of appropriate equations and examples: (i) Elastic Scattering (1z) Inelastic Scattering (iii) Radioactive Capture (U) Spallation reactions. (b) (i) Define the term "Bremsstrahlung" radiation. How does it differ from characteristic radiation ? (iz) In an X-ray machine, electrons are accelerated through a potential of 40 kV. Compute the cut-off wavelength and energy of the emitted photons. 3+3 A photon incident on an atom scatters off at an angle of 55° with an energy of 150 keV. Determine the initial energy of the photon and the energy of the scattered electron. (Cos 55° = 0.5736 and Sin 55° = 0.8191). 3+3 (a) Assume radiations of two different energies are incident on a gas detector, with the help of a diagram explain its response with increase in the bias voltage. 6 (b) Define dead time of a detector. Illustrate with the help of a diagram, the effect of dead time on the count rate in a system. (c) A typical gas filled detector has a capacitance of about 50 uti and charge collection time of the order of 1 us. If all the charge created by a 3 MeV particle is assumed to be collected then estimate the expected voltage and 6 current output. 4. (a) How does a multiwire proportional counter (MWPC) detect and locate the position of incident nuclear radiation ? (b) Explain the mechanism of conversion of ionising radiation energy into detectable light signals by an organic scintillator detector. (Use a molecular energy-level diagram). 6 (c). A germanium detector is illuminated by a 5 Co source producing E = 122 keV photons. Calculate : (i) the mean number of electron-hole pairs produced. (i) variance in number of pairs using the Fano factor. (il) percentage energy resolution. Given for Ge: average energy to produce one electron-hole pair, w = 2.96 eV and Fano factor, F = 0.13 2+2+2 5. (a) Using the schematic diagram of a photomultiplier tube, explain the role of the dynode ladder and photocathode in its working. 6 (b) Explain how the Li(n, a) reaction is used for detecting slow neutrons. 6 (c) A material having a work function of 2 eV is used to convert photons into electrons, which are then detected by a photomultiplier tube. Compute the maximum wavelength of the photons it can convert into electrons. 6