[1] Univ Fed Rio Grande do Norte, Dept Phys, BR-59078970 Natal, RN, Brazil
[2] State Univ Rio Grande do Norte, Dept Phys, BR-59610210 Mossora, RN, Brazil
[3] State Univ Rio Grande do Norte, Dept Sci & Technol, BR-59610210 Mossora, RN, Brazil
[4] Fed Rural Univ Semiarid Reg, Dept Sci & Technol, BR-59515000 Angicos, RN, Brazil