Hi, I am having much trouble on this particular question. Here it is: Four “point” gamma ray sources are permanently implanted in tissue so that the sources are at the corners of a 1 cm X 1 cm square. Each source has an initial activity of 15 MBq (1 Bq = one decay per second), every decay produces a 30 keV gamma ray, and the half-life is 60 days. The dose rate from each source falls off with distance, r, according to Dose Rate = (Constant)(exp -ur)(r-2) where u = 0.25 cm-1. Calculate the total dose delivered by the implanted sources to a point at the centre of the square. Sketch the isodose distribution in the plane that contains all of the sources. Hint: Use conservation of energy to evaluate the constant in the equation above. Attempt at a solution: This question confuses me because there is no real time frame given. There is a half life, but I am unsure how to use it. Also, when evaluating the Constant, I do not get proper units. The way I have been looking at it is: constant = (15 x 10^6 decay/s)(30,000 eV)(1.602 x 10^-19 J/eV) which gives me J/s. I know this must be wrong, but do not know where to go from here. Any help?