Challenging logic problem

I suggest to those who wish to solve the following problem:

There are 15 balls, 2 of them are radioactive. There is a device with a light bulb in which you can put any number of balls (even all fifteen), and which will show the presence of radiation. That is, if among the balls put into the device there is at least one radioactive - the light will come on, if not - it will not come on.
It is necessary to find 2 radioactive balls using the device no more than 7 times.