Radiation hardness of superconducting magneto-resistive detectors at CN facility

E. Mezzetti1, R. Gerbaldo1, G. Ghigo1, L. Gozzelino1, F. Laviano1, G. Lopardo1, B. Minetti1, R. Cherubini2, S. Gerardi2

1 Politecnico di Torino and INFN, Torino, 2 INFN, Laboratori Nazionali di Legnaro

INTRODUCTION

The sensor safety under particle irradiation either in space or in large facilities such as novel accelerators or fusion machines presents a challenge putting into play new materials and new technological solutions. Sensors based on high-Tc superconductors play an interesting role in this framework, due to their ability to swallow huge defect quantities without breaking their superconductive response to external excitations. In this paper we report about “ex-situ” radiation damage properties of YBCO films after 3.5 MeV proton irradiation. The samples are either as grown films or films with confined nanostructuring by high energy heavy ions [1]. The ex-situ measurements imply nanostructure changes investigation, magneto-optical investigation under transport current [2] as well as transport characterization up to 4T. The work is in progress [3] with a contemporary upgrading of the experimental set-up in order to perform also “in-situ” measurements.

APPARATUS AND PROTOTYPE GEOMETRY

The experimental apparatus currently used for the proton irradiation of the Di.S.Co.L.I. (DIspositivi Superconduttori COntrollati con Litografia a fascio Ionico) magnetic field sensors consists of a cryostat with rotating sample-holder equipped by a nitrogen reservoir. A copper cold finger with large thermal capacity is thermally anchored to it. The sensor with the network for the remote control and measurement unit is put on an aluminum support, thermally anchored to the copper cold finger (Fig. 1). The shielding environment is not shown in the photos.

The prototype under characterization consists into a planar square-shaped meander of YBCO film (Fig. 2). Five alternate rows of heavy ion irradiated/virgin zones were carved into the long arms of it by means of chessboard shaped micrometer-size stainless-steel masks (lateral aperture: 250 µm wide) (Fig. 3). The 0.25 GeV gold-ions fluence was 2·1011 ion/cm2. The characterization of each virgin/irradiated strip provides the before-damage characterization of the prototype. The defect density is finely tuned by the high-energy heavy-ion (HEHI) fluence, as monitored by the online measurement of the HEHI beam current onto the sample holder.

FIG. 1: The experimental apparatus currently used for the proton irradiation of the Di.S.Co.L.I. magnetic field sensors.

FIG. 2: Arrays of local magnetic field detectors (superconducting YBCO film shaped as a meander).

RADIATION DAMAGE MEASUREMENTS

By means of 150 µm thick tantalum transversal thin plates (variable aperture) the heavy-ion affected zones that were exposed to 3.5 MeV proton irradiation, in order to evaluate “ex situ” the proton damage. The proton fluence was 2.15·1014 p/cm2. Typical results are shown in Fig. 4. It turns out, for what concerns permanent damage, that the proton dose, equivalent to about 70 years in space, does not affect the prototype units.

FIG. 3: Scheme of the superconducting YBCO film shaped as a meander. Each branch of the meander was nanostructured with confined 0.25 GeV Au-ions (at a fluence of 2·1011 ion/cm2) in order to create five microchannels 250 µm wide. Then the branches were irradiated with 3.5 MeV protons at a fluence of 2.15·1014 p/cm2.

CONCLUSIONS

The performed “ex-situ” measurements hint how radiation-hard the Di.S.Co.L.I. sensors are under proton irradiation. Further measurements concerning magnetic field range up to 4 T are planned. “In-situ” measurements are needed and will be directly performed under beam: the magneto-resistive behavior of the targets will be monitored in real-time for different temperatures and magnetic fields in order to evaluate transient damage thresholds, damage thresholds and, if reachable, shut down thresholds.

40 60 80 100 120 1400.0

0.2

0.4

0.6

0.8

1.0

Φ = 2.15·1014 p/cm2

B = 0 OeIbias = 10 μA

before 3.5 MeV proton irradiation after 3.5 MeV proton irradiation

R/R

150K

Temperature (K)

0.00.20.40.60.81.0

0 50 100 150 200 250 300

Magnetic field (Oe)

ΔR

/ R

300

Oe

FIG. 4: Normalized resistance as a function of the temperature of a typical branch of the meander irradiated with 3.5 MeV protons at a fluence of 2.15·1014 p/cm2 In inset magneto-resistance response obtained across the Au-ion irradiated region before and after the homogeneous proton irradiation.

ACKNOWLEDGEMENTS

We acknowledge the contribution of I.N.F.N. under the Di.S.Co.L.I. (DIspositivi Superconduttori COntrollati con Litografia a fascio Ionico) project.

[1] E.Mezzetti et al., Advances in Cryogenic Engineering Materials 52 (2006) 786. [2] F.Laviano, E.Mezzetti et al., Applied Physics Letters 89 (2006) 082514. [3] E.Mezzetti et al., “Di.S.Co.L.I. sensor for local magnetic field mapping”, LNL Annual Report 2006.