question_answer 32)

(a) Obtain the de Broglie wavelength of a neutron of kinetic energy 150 e. As you have seen in Exercise 44, an electron beam of this energy is suitable for crystal diffraction experiments. Would a neutron beam of the same energy be equally suitable ? Explain, . (b) Obtain the de Broglie wavelength associated with thermal neutrons at room temperature (27°C). Hence explain why a fast neutron beam needs to be thermalised with the environment before it can be used for neutron diffraction experiments.  

Answer:

(a)                                                                                          Using or                                                 This wavelength is about hundred times smaller than the inter atomic separation of crystals. Thus, neutrons are not suitable for diffraction experiments in case of crystals. (b) Here    T = 27°C = 300 K   This wavelength is comparable to the inter atomic spacing of crystals. Therefore, thermal electrons are able to interact with the crystal. Since   , increasing the temperature, decreases their de Broglie wavelength and they become unsuitable for crystal diffraction. Thus, the fast beam of neutrons needs to be thermalised with the environment for neutron diffraction experiment.