An aluminum filter is used to filter out low energy x-rays from a polychromatic x-ray beam. if the filter thickness is such that 50% of 25 kev photons are filtered out (that is, the thickness is one half-value layer, how many of these filters would we need to stack together to filter out 99% of the 25 kev photons?

Athin walled concentric tube exchanger is used to cool engine oil from 160°c to 60°c with water that is available at 25°c acting as a coolant. the oil and water flow rates are each at 2 kg/s, and the diameter of the inner tube is 0.5 m and the corresponding value of the overall heat transfer coefficient is 250 w/m2. oc. how long must the heat exchanger be to accomplish the desired cooling? cpwater=4.187 kj/kg-candcpengine el=2.035 kj/kg·°c, oil . 120]