The Effect of Neutron Irradiation on Remanent Magnetization in Multidomain Iron and Kamacite

Abstract

The effects of neutron irradiation on remanent magnetization in multidomain iron and kamacite (3 wt% Ni) samples of varying initial coercivities were investigated in order to determine whether cosmic ray exposure significantly affects the natural remanent magnetization (NMR) of lunar samples and chondritic meteorites. Irradiation to 1017 fast neutrons/cm2 increased bulk coercive force (He) by an average of 150. Annealing for 1 hour at 200-300C returned Hc to its pre-irradiation value. The dominant effect of irradiation on remanent magnetization was to decrease the total intensity by 5-45%. AF demagnetization spectra of both isothermal and thermal remanence before and after irradiation indicate that neutron bombardment slightly increases the resistance to AF demagnetization. No similar increase in resistance to thermal demagnetization was indicated. The changes in Hc and AF demagnetization spectra are attributed to radiation induced point defects rather than to pinning of domain walls by vacancy clusters and dislocations. The NRM in chondritic meteorites and most lunar samples is too stable to be accounted for by exposure of isothermal remanence to cosmic rays. Thus the NRM must be some form of thermal or thermochemical remanence.

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

“…), and may lead to thermal annealing in some cases (Rubin ). Irradiation by protons has been shown to modify the magnetic properties of kamacite (Butler and Cox ), and irradiation by neutrons leads to ordering of taenite into tetrataenite (Paulevé et al. ).…”

Attempted proton-irradiation induced magnetization

“…For instance, HENRY and SALKOWITZ (1958), BRODSKAYA (1958), SMITH (1967), and BUTLER and Cox (1971, 1974, have studied the effects of -1-ray and neutron irradiations on the magnetic properties of various samples. In the work of BRODSKAYA and that of BUTLER and Cox, evidence of demagnetization brought about by y-ray and neutron irradiation, With regard to the origin of magnetism in meteorites, though, it is of utmost importance whether irradiation may act as a mechanism for production of magnetization.…”