Consideration of the initial build up of dose-rate with depth in a slice of thermoluminescent material irradiated by a ₉₀SR-₉₀Y beta source suggests that the presently accepted ratio between the dose rate to fine grains (1-8mum) and that to 100 mum grains is incorrect. Recalibration experiments using accurately known gamma irradiations indicate that the correct ratio is 1. 03 ± 0.03; this is consistent with build-up data. -- AATA