Measurement of density inhomogeneities in HIPed beryllium and niobium for STEP test masses

We have determined the inhomogeneities in the density of prototype STEP test masses made from HIPed beryllium and niobium using the hydrostatic weighing method developed at BIPM. 13 samples are taken from a HIPed beryllium rod (approximately 140 mm long by 90 mm) and 24 samples from a niobium rod (approximately 225 mm long by 45 mm). The maximum relative difference of density is found to be approximately 240 ppm in the beryllium samples and 60 ppm in the niobium samples. Assuming that there is a 10 g helium bubble at 250 mm from the test masses acting as;a source, these inhomogeneities of density would produce a disturbance of Deltaa/a = (1.1 +/- 0.7) x 10(-18) in a beryllium outer test mass and (5.1 +/- 0.5) x 10(-20) in a niobium inner test mass, in the STEP experiment. The inhomogeneities of density in the HIPed beryllium rod appear to be too large for the STEP experiment, which aims at a sensitivity of one part in 10(18). We suggest that it is important to find a way to fabricate more homogeneous HIPed beryllium.