Some watch dials are coated with a phosphor, like ZnS , and a polymer in which some of the first H atoms have been replaced by third H atoms, tritium. The phosphor emits light when struck by the beta particle from the tritium decay, causing the dials to glow in the dark. The half-life of tritium is 12.3 yr.
If the light given off is assumed to be directly proportional to the amount of tritium, by how many percents will a dial be dimmed in a watch that is 60 years old?