Volume SIA, number 2 PHYSICS LETTERS 10 February 1975

EFFECT OF LOW TEMPERATURE NEUTRON IRRADIATION ON

SUPERCONDUCTING PROPERTIES OF Nb, Sn

M. SOELL Max-PIanck-Institut fiir Plasmaphysik, 8046 Garching, W.-Germany

H. BAUER* Instftut fair Angewvlndte Physik der Unive-rsit~t Giessen, 63 Giessen, W.-Gernmy

K. BOENING PhyJtk-Department Techn. Universitbt Miinchen, 8046 Garching, W.-Gwmany

and

R. BETT AERE, Harwell, Didcot, Oxfordshire, US.

Received 27 January 1975

NbaSn diffusion wires were irradiated with fast neutrons at 4.6 K. Exposure to 3.9 X 10’s n/cm2 caused an in- crease in the critical current density and a decrease in the transition temperature.

In future fusion reactors, magnetic fields for plasma confinement will be produced by supercon- ducting magnet systems. This will involve the super- conducting material being exposed to fast neutron ir- radiations [l] at low temperature, which will produce lattice defects, thus causing changes in the properties of the superconductor [2]. Nb$n seems to be a promising material for this application because of its high critical data. Since room temperature irradiations [3-71 can not give conclusive information about the behaviour of Nb3 Sn in the operating conditions of a fusion reactor, low temperature irradiations with fast neutrons at 4.6 K were carried out in the low tempera- ture irradiation facility of the research reactor at Garching [8].

Nb,Sn diffusion wires made by vapour phase diffu- sion (95O’C - 24 h) of tin into niobium wires (0.3 mm dia.) were used. This material is characterized by a critical temperature T, of 17.95 K and a typical criti- cal current density& of 3 X 10s A/cm2 at 4.2 K and in a magnetic field of 5 T, which compares with (6-10) X 10s A/cm2 in current commercial Nb3Sn [9]

*Work supported by the Deutsche Forschungsmeinschaft.

The critical current density was determined from current voltage characteristics by a voltage criterion of 1 PV along 10 mm length of sample. The sample temperature was determined by non-irradiated cop per-constantan thermocouples and carbon resistors; the critical temperature was obtained in this way us- ing the sample resistivity transition from the super- conducting to the normal state. After irradiation the samples were moved from the irradiation position in the reactor core (irradiation temperature 4.6 K) into the test cryostat without warming up. Annealing ex- periments were carried out at different temperatures up to 250 K for 10 minutes, respectively.

The main results are shown in fig. 1 and 2. Expo sure to 3.9 X 1018 fast neutrons/cm2 (E>O.l MeV) caused anje enhancement of a factor of about 2.5 and a T, reduction of (0.8 f 0.1) K. An additional small T, decrease is produced by annealing (fig. 2). This behaviour can be explained by assuming that the larger insterstitial or vacancy clusters, which are pro- duced during annealing after irradiation, have a stronger infhrence on the critical temperature T,

than the more isolated Frenkel pairs in the displace- ment cascades produced during low temperature irra- diation [lo]. The observed changes of T, after room

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Volume 51A, number 2 PHYSICS LETTERS 10 February 1975

IO I I I I I

lO’A/cd NblSn Diffusion Wire

9- 0 8 A

s- 0

2-

I -

before lrrodiotion

after Irrod. with 3.9 . IO” Fort Neutckd

Anneoled after Irradiation : 100 K - 10 min

Annealed ofter Irradiation :ZSOK -10 min

0’ I I I 1 I J 0 I 2 3 L 5 T6

Magnetic Field

Fig. 1. Critical current density in transverse magnetic fields for NbsSn diffusion wires before and after low temperature (4.6 K) irradiation with fast neutrons and after subsequent anneaBng.

temperature irradiations [6,7] confnm this assump tion. Some indications have been observed that the annealing behaviour of je as measured at constant field H are somewhat different at different tempera- tures in the range between 6-12 K. Comparison with results of room temperature hradiations on the same type of samples [7] shows that up to this neutron fluence the effects are of the same order of magnitude However, more detailed measurements and low tem- perature irradiations with higher neutron doses have

Nb,Sn Diffusion Wire

before Irradiation

after Irradiation with

3.9. IO” Fast Neutr. /cm2

Annealed after Irradiation:

250 K - IO min

0 L-’ Jl I .A. A J 16.5 17.0 17.5 K 16.0

Temperature

Fig. 2. Transition curves for NbsSn diffusion wires before and after low temperature (4.6 K) irradiation with fast neu- trons and after subsequent annealing.

to be done in order to get a more complete picture about the response of Nb,Sn to irradiation damage.

We would like to thank the Deutsche Forschungs- gemeinschaft for fmancial support and Prof. Saur and Dipl. Ing. Schmitter for their interest in this work.

[l] GM. McCracken and S. Blow, CL&R 120 (1972). [2] M. Soell, IPP 41104 (1972). [ 31 J.P. McEvoy, Jr., RF. Decell and R.L. Novak, AppL

Phys. Lett. 4 (1964) 43. [4] P. Swartz, H.R. Hart, Jr. and RL Fleischer, AppL Phys.

Lett. 4 (1964) 71. [S] G.W. Cullen and R.L. Novak, J. AppL Phys. 37 (1966)

3348. [6] R. Bett, Cryogenics 14 (1974) 361. [7] H. Batter, E.J. Saur and D.G. Schweitzer, J. Low Temp.

Phys. 9 (1975) 314, to be published. [8] R. Doll, H. Meissner, N. Riehl, W. SchiBing and F.

Schmeissner, Z. Angew. Phys. 17 (1964) 321. 191 CH. Rosner, AppL Superc. Conf. Annapolis 1972.

[lo] A. Seeger, D. Schumacher, W. SchBBng and J. Diehl, Vacancies and interstitials in metals, Amsterdam (1970).

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