ESUH

1. Experimental method

When a Josephson junction is coupled with a radio wave of frequency f, the response of the supercurrent gives rise to constant-voltage Shapiro steps in the dc I-V curve at voltages

Vn = nhf/2e | (1)

n being an integer and the step amplitude hf/2e is, for example, 31mV if the radiation frequency is 15 GHz.

The goal of the experiment is to find how nearly equal are the voltages (1), produced by two junctions irradiated with the same radiofrequency f. By directly comparing the voltages across the two junctions, biased on the induced step of the same order n, one make a differential measurement connecting the junctions by a full superconducting loop and reading the current flowing in this loop by a SQUID (its input coil is superconducting). A difference DV produces a current I, in this no resistance circuit, according with the equation

where L is the circuit inductance. The apparatus stability allows integrating for a couple of days, this is the reason of the large sensitive to the voltage differences.

The helium bath temperature is stabilized within 10-4K. A mmetal shield, to reduce the earth’s field to less than 100 mG, surrounds the cryostat. The experiment is mounted inside a niobium box, made of two separate compartments, one holds the RF board with the junctions and a second (very well shielded from the other) holds the DC SQUID and the superconducting switch. The good quality of this RF shielding allows connecting the input coil of the SQUID directly inside the superconducting loop.

We use custom made silicon chips with two junctions; one junction electrode is connected to the electrode of the other junction by a superconducting niobium sputtered line and the other two electrodes are separately connected, by the same type of line, to two superconducting pads. The superconducting loop is closed connecting the SQUID input coil with these two pads. We polarize both junctions on the same step, opening the superconducting loop by a superconducting switch; the step comparison starts turning off the switch heater.

2. Results of special relevance for other scientific disciplines or of special impact for applied research and industrial applications.

A promising non-invasive attempt for iron determination in hemochromatosis or thalassemia patients is provided by biosusceptometry, which is essentially an assessment of the diamagnetic (mainly from water) and paramagnetic (iron) properties of tissues. We have developed a susceptometer, which has been utilized to measure in vivo iron overloads in the whole liver of rats, injected with subtoxic doses of iron dextran. After animal sacrificing, magnetic measurements have been complemented with iron quantification by chemical analysis on excised livers and other organs. Results of in vivo measurements of iron concentration are in agreement, within 20%, with those coming from chemical analysis. Extraction of iron overload contribution, from the rat signal, requires evaluating the signal to be assigned to the injected rat ideally deprived of the overload: the indetermination on the knowledge of this quantity is the true limit of the present measurement sensitivity. The developed instrument is capable of measuring susceptibility variations with an intrinsic instrumental sensitivity corresponding to 35 g/cc of ferritin-hemosiderin iron. Normal levels in human livers are about ten times higher. Additional considerations, moving from a smaller to a larger instrument for humans, allow anticipating the possibility to build a susceptometer suitable for a noninvasive assessment of iron overloads in hemochromatosis or thalassemia patients.

This work has been done also with the contribution of the “Associazione Veneta per la Lotta alla Talassemia”, the “Arcispedale Sant'Anna - Azienda Ospedaliera di Ferrara” and "Associazione Ligure Thalassemici", used to pay the salary of a young physicist working full time for this project. A patent has been requested from the Genoa University. The "Parco Scientifico e Tecnologico della Liguria" sponsored the design of a susceptometer suitable to measure the iron overload of Thalassemia patients; this work has been done also with the contribution of the INFN technical staff. The INFN CSN5 has recently accepted the proposal to build the designed susceptometer (experiment THALAS). The general director of the "E.O. Ospedali Galliera di Genova" wrote a letter with the interest to use this apparatus and offering the necessary facilities for its hospital installation.