A. Calderón and R. A. Muñoz Hernández CICATA-IPN, Legaria 694 Col. Irrigación, México City, México. G. Peña Rodriguez * ESFM-IPN., 07738 México City, México..A. Cruz-Orea and F. Sánchez Sinencio CINVESTAV-IPN. Apdo. Postal 14-740, 07000 México City, México

We present a study of the photoacoustical signal obtained for a heat transmission configuration in order to determine the validity range of the modulation frequency f in the thermally thick regime, in which is usually obtained the thermal diffusivity mainly in materials with low values of .. We show for f > (/2)² f_c , where f_c (=/l²) is the modulation frequency at which the thermal diffusion length matches the sample thickness l, the photoacoustic phase signal can be written closely in a linear dependence with , having a relative error smaller that 0.5 %. In this form the photoacoustical signal have a behavior like the semi-infinite solid that is characteristic of the thermally thick regime. This study was complemented with experimental verification in several samples using the open photoacoustic cell technique.

*Permanent address: Depto. de Física UFPS, A. A. 1055, Cúcuta, Colombia.