The variation of beta dose-rate with depth below the surface of the aluminum (equivalent to porcelain) was investigated. The variation is due to build-up and attenuation effects of beta ray in the aluminum. Table 1 and the curve in Fig. l show the beta dose distribution with depth as a result of competing build-up and attenuation effects. Initially both co-exist with build-up dominant so that the dose increases gradually to match a maximum value. Beyond this point the build-up is in saturation, the attenuation begins to dominate and the dose gradually decreases with eventually build-up being totally replaced by attenuation. It is concluded that the maximum dose-rate is 1.05 (1.049 rounded to two decimals becomes 1.05) relative to surface at a depth of 75µm, and therefore that the slice thickness of 200µm increment of average dose is only 3% and that for a thickness of 400µm there is a reduction by the same percentage. If a slice thickness of 300µm is used, the average dose is equal to surface dose. This suggests that for porcelain dating of 300µm thick slice build-up can be neglected and no correction factor is required. The average beta dose in a slice of any thickness can be obtained by Equation (11). -- ICCROM