A CRYOSTAT FOR MEASURING LOW-FIELD FLUX-CREEP

We describe a SQUID-based magnetometer for measuring low field flux cr eep (LFFC) over the temperature range of 2-7.5 K. Temperatures below 4 .2 K are achieved by Dumping on a reservoir of liquid helium; the uppe r temperature limit is imposed by the SQUID magnetometer. Valid LFFC m easurements to 10(5) seconds in niobium at 4.2 K require temperature r egulation of the sample to within 1 mu K. The white noise floor of the thermal system has been measured at 3-4 mu K rms/root Hz, and the inc reased low frequency noise limits the temperature regulation to somewh at better than 1 mu K at 10(-5) Hz. Preliminary measurements of LFFC m ade using this magnetometer are presented.