Usrp source & gr.file_sink

Hi everybody!
I’m trying to collect data from usrp using
gr.file_sink(gr.sizeof_gr_complex,“filename”) but I’ve some difficulty.
In my top block there is a trasmitter section that ends with an
usrp_sink, and a receiver section in which data caming from usrp_source
are stored in a file, of a given size, through file_sink block. I need
to do more than one acquisition so I tried to use a for cicle in the
main to store and rename files. Something like this:

if name == ‘main’:

tb = top_block()
acq = (1,10)
for x in acq:
os.rename(“file_a.dat”, “file_a_%u” % (x))
os.rename(“file_b.dat”, “file_b_%u” % (x))

In first acquisition everything is ok, files are stored and renamed,
but, after that, the receiver shut down and, at second iteration, no
more files are collected. Can anybody explain me why tb.start() doesn’t
turn on the receiver again? Where I’m wrong?
Thanks a lot