Styrofoam insert in an HDR calibration well chamber unit

I have seen in the past a question on the ABR Oral exams about what the styrofoam insert in an HDR calibration well chamber is for. FOr almost a year I've searched for a definite anser to this problem with no luck. I asked a friend who used to work at the RPC, and he said he thought it had to do with the strength of a 10 Ci Ir192 source being strong enought to rasie the temperature in the chamger and tus change its response. --But that was just his guess he told me.Finally, I found a real-live reference that gives a solid answer.AAPM Task Group-41 (page 77) states:"The well chambers designed for HDR sourcesexhibit a heat dependency, e.g., the 370.-GBq (10.0-Ci) sources canincrease the air temperature in the collection well if they remain therefor several minutes.72 Styrofoam thermal absorbers can be positionedaround the source holder to alleviate this problem."I believe this answer along with a mention of TG41 would suffice as an answer for an ABR examiner.