An ionizing radiation source is required for experiments in radiation biology and radiotherapy. A (?-ray) Co-60 irradiator is relative expensive (cost up to \$300,000) and the safety may also be an issue in its use. A x-ray source costs less, but its biological efficiency (and of course the price) will depend on the radiation energy: for 160, 225 and 320 kV x-ray irradiators (the voltage is applied to the anode of a x-ray tube), their prices will approximately be \$100,000, \$140,000 and \$160,000, respectively. If you plan to do a project in cancer research, which requires to irradiate tumor cells both in vitro cancer cell lines and in vivo mice. Usually, the tumor site grown in mice for the experiment is within a few cm (e.g., less than 5 cm). Given that you have a total budget of \$170,000 for all the experimental expenses, which ionizing irradiator would you purchase? [Hint: you might have to use the knowledge you learnt in Chapters 1-3, considering the production of high-energy x-rays from x-ray tubes, the interaction processes of ionizing photons in matter, secondary particle energy spectra, the LETs, depth dose curves and the RBEs, etc.] (20 marks)
