Gradiometers
An important application of SQUID is Biomagnetism. In these applications, the magnetic fields strengths are very small, ranging from few tens of fT to some tens of pT in a frequency range of a few hundreds of Hz. Since the background magnetic noise is several order of magnitude greater than the signal to be detected, it is mandatory to employ both a magnetically shielded room consisting, and a properly designed sensor. In particular, the SQUID sensors are often designed in an axial gradiometer configuration in order to reject the background uniform noise. However, the interest in the measurement of the magnetic field tangential component has grown, due to the possibility to employ a first order planar SQUID gradiometer with a long baseline (distance between the centers of pick-up coils). These devices offer the advantage of both a very high intrinsic balance limited by the precision of the photolithographic techniques (1 part in 10000) and avoid the unreliable superconducting soldering, guaranteeing a better reliability during the thermal cycling
We have developed a niobium based dc-SQUID planar gradiometer with a long baseline (50 mm) where the pickup antenna consists of two integrated rectangular coils connected in series and magnetically coupled to a dc-SQUID in a double parallel washer configuration via two multiturn input coils. Designed to has a high intrinsic responsivity, the sensors shows, at liquid helium temperature, a white magnetic flux noise spectral density of few µΦ0 per band unit. The magnetic response to a current dipole has been calculated and the results have been compared with those of an analogous axial gradiometer showing that there is no significant difference. Due to their high intrinsic balance and good performances, planar gradiometers may be the elective sensors for biomagnetic application in a soft shielded environment.
Performance
Design
Main references:
Long baseline planar superconducting gradiometer for biomagnetic imaging
C. Granata, A. Vettoliere, C. Nappi, M. Lisitskiy, and M. Russo
Applied Physics Letters 95, 042502 (2009)
Modeled Performance of a Long Baseline Planar SQUID Gradiometer for Biomagnetism
Antonio Vettoliere, Carmine Granata, Sara Rombetto, and Maurizio Russo
IEEE Transaction on applied superconductivity 21, 383, (2011)