The build-up and decay of radioactivity in arsenic caused by reaction with slow neutronsrnwill be studied. In particular, we will be able to measure the radioisotoperncapture cross section of the sample, using a GM-counter on the decay of radioactivityrndata taken. We will also a measurement of the half life of the sample and we willrnshow that the activity level reaches an asymptotic value using slow neutron activationrnanalysis technique