12 (a) Explain what is meant by the binding energy of a nucleus. (b) The following nuclear reaction takes place: ²³⁵U + ⁰n → ¹⁴⁴Cs + ^xRb + y⁰n (c) Yttrium-90 decays into zirconium-90, a stable isotope. A sample initially consists of pure yttrium-90. Calculate the time, in days, when the ratio of the number of yttrium-90 nuclei to the number of zirconium-90 nuclei would be 2.0. The half-life of yttrium-90 is 2.7 days.

1. (a)Explain what is meant by the binding energy of a nucleus. [2][2]
2. (b)(i)Determine the values of x and y. x = ............................................................................ y = ............................................................................ [1][1]
3. (b)(ii)State the name of this type of nuclear reaction. [1][1]
4. (b)(iii)Compare the binding energy per nucleon of uranium-235 with the binding energy per nucleon of caesium-144. [1][1]
5. (c)Calculate the time, in days, when the ratio of the number of yttrium-90 nuclei to the number of zirconium-90 nuclei would be 2.0. The half-life of yttrium-90 is 2.7 days. time = ................................................................ days [3][3]