You do not have to stop for even a second. We do not want babies to get radioactivity, but we rarely hesitate to do radioactive scans on them. I think that after most radioactive scans, the mother can continue breastfeeding, but if she and her physician are truly concerned, waiting 2 half lives is enough, for a material such as technetium. Note that if the mother is getting the scan during the first few days after the baby's birth, the baby will get much less because the baby gets much less milk during this time. During this early period, I believe no interruption of breastfeeding is necessary or desirable.
Colostrum is desirable for the baby.
Only occasionally is a radioactive scan that urgent that it cannot be delayed for a few days. Thyroid scans are different. Radioactive iodine is concentrated in milk and will be ingested by the baby and it will go to his thyroid where it will stay for a long time. This is definitely of concern. No, because often the test does not need to be done.