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We analyze the X-ray spectra and their timing properties of the compact X-raybinary 4U~1820-30. We establish a number of spectral transitions in this sourceseen with BeppoSAX and RXTE. During the RXTE observations (1996 -- 2009), thesource were approximately 75% of its time in the soft state making the lowerbanana and upper banana transitions combined with long-term low-high statetransitions. We reveal that all of the X-ray spectra of 4U~1820-30 are fit by acomposition of a thermal (blackbody) component, a Comptonization component(COMPTB) and a Gaussian-line component. Thus using this spectral analysis wefind that the photon power-law index Gamma of the Comptonization component isalmost unchangeable (Gamma~2) while the electron temperature kT_e changes from2.9 to 21 keV during these spectral events. We also establish that for thesespectral events the normalization of COMPTB component (which is proportional tomass accretion rate, \dot M increases by factor 8 when kT_e decreases from 21keV to 2.9 keV. Before this index stability effect was also found analyzingX-ray data for Z-source GX 340+0 and for atolls, 4U~1728-34, GX~3+1. Thus, wecan suggest that this spectral stability property is a spectral signature of anaccreting neutron star source. On the other hand in a black hole binary Gammamonotonically increases with \dot M and ultimately its value saturates at large\dot M.

Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail