I linked it in another comment, but I'll put it here as well - here's an IAEA report about an accident where a worker walked into an active irradiation room of a commercial irradiator, he was there only for about a minute total but had to leave because of intense pain which made him realize something is wrong. The total dose is estimated at 10-20Gy but it was still enough to make him very unwell almost immediately and caused death about a month later:

https://www-pub.iaea.org/MTCD/publications/PDF/Pub925_web.pd...