1) The probability that the particle will decay after having already decayed once is proportional as follows: R ~ e^(-t). Therefore the number of counts which are directly related to this probability will also exhibit this exponential behaviour.
2) Detector 2 is most likely at an angle of 90 degrees relative to detector 1. Detector 3, on the other hand, most likely has a relative anhle of 180 degrees with detector 1.