will i require a collimator for investigation of gamma attenuation or the same experimental set up can be performed for both gamma attenuation and radioactivity check .I am using gamma ray spectrometer us using Nal(Tl) detector
Technically yes, but the chances are your spectrometry software could be doing some pulse filtering, i.e. dropping some counts in favor of better resolution. It may be possible to turn off the filtering.
Absolute activity is always tricky, you need a calibrated source, preferably of the same energy.