The unit for decay constant should be in per second. If you who want to put the examiner to test you may try giving it in per year without omitting the unit.But ...
Don't set a trap for yourself ! You use the decay constant in per year and use it to calculate the activity A = lamda N and give the unit as per second or Bq.
To prevent further traps set by yourself always use per second in whatever calculations. There is no point saving time and getting wrong answers. Take a longer route to get everything in seconds and be 100% sure of your own calculations.
