Radiation Effects in Rare-Earth Permanent Magnets
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RADIATION EFFECTS IN RARE-EARTH PERMANENT MAGNETS R. Cost*, Los Alamos Chemical University,
R. D. Brown*, A. L. Giorgi*, and J. T. Stanley National Laboratory, Los Alamos, NM87545 Dept., Science Engineering and Materials Tempe, AZ 85287
Arizona
State
ABSTRACT Nd-Fe-B and Sm-Co permanent magnets have been irradiated with fission neutrons and gamma rays. Irradiated samples were periodically removed for Hysteresis room temperature measurements of the open-circuit remanence. loops were measured before and after irradiation. For neutron irradiation, two Nd-Fe-B magnets showed a rapid loss of remanence, while a third magnet from another manufacturer decayed more slowly, suggesting that the radiation hardness of Nd-Fe-B magnets may depend on microstructural details. Irradiation in the Omega West Reactor at Los Alamos with fast neutrons caused the fast-decay samples to have an Wtial 1oss of remanence of 1% for n/cm . Both SmCo 5 and Sm2 Co1 7 irradiation at 350 K to a fluence of 10 magnets showed excellent resistance to radiatifg-induied loss of remanence n/cm . Results for gamma for neutron irradiation to a fluence of 2.6x10 radiation showed no loss of remanence for a dose of about 49 Mrad using a Possible mechanisms for radiation-induced loss of magnetic Co source. properties are discussed. INTRODUCTION The use of permanent magnets in accelerators allows reduction of the weight compared to using electromagnets, both in the magnet itself and in the elimination of the power supply. To be used successfully in a location where scattered radiation is present, the permanent magnets must be capable of resisting decay in remanence due to this radiation. Radiation produced by proton accelerators operating at energies of 50 to 100 MeV is expected to consist of neutrons and gammas produced when the fringes of the accelerated A rough beam strike structures located physically near the beam line. calculation of the total fluence of1 5 neutrons expected for a 50 MeV n/cm over the operating lifetime accelerator gives a fluence of about 10 (approximately 200 h) of the accelerator. magnets in dipole and quadrupole Previous applications of permanent magnets have specified the use of Sm-Co material. Previous work [1-4J has shown that Sm2 Co1 7 magnets are the most resistant to radiation-induced decay of the Semaneyce, and that SmCo 5 magnets are also very resistant to doses in the 10 n/cm range. The recently developed Nd-Fe-B magnet family is of interest to accelerator designers due to the high remanence and intrinsic The sensitivity of these magnets to coercivity of these materials. temperature-produced changes in magnetic properties has led to concern over their stability in a radiation environment. As of 1986, the one study which had been done [41 on radiation effects to Nd-Fe-B magnets showed them to be orders of magnitude more sensitive to charged particle radiation than the We have previously reported some initial results of Sm-Co magnets. We found that while the decay of irradiations of Nd-Fe-B magnets [5]. remanence is more rapi
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