We present results from high resolution ( R \simeq 28000 ) spectra of six high-redshift QSOs taken at the ESO NTT telescope that allow the detailed study of the Lyman- \alpha population in the redshift interval z = 2.8 - 4.1 .

The typical Doppler parameters found for the Lyman- \alpha lines lie in the interval b = 20 \div 30 km s ^ { -1 } , corresponding to temperatures T > 24000 K , with a fraction of the order 15 % in the range 10 \leq b \leq 20 km s ^ { -1 } .
These values are still consistent with models of low density , highly ionized clouds .

The observed redshift and column density distributions obtained from these spectra and from the observations of 4 additional QSOs taken in the literature allow an accurate estimate of the proximity effect from a relatively large Lyman- \alpha sample ( more than 1100 lines with \log N _ { HI } \geq 13.3 ) in the redshift interval z = 1.7 - 4.1 .

A Maximum Likelihood analysis has been applied to estimate simultaneously the best fit parameters of the Lyman- \alpha statistics and of the UV background .
After correcting for the blanketing of weak lines , we confirm that the column density distribution is best represented by a double power-law with a break at \log N _ { HI } \simeq 14 , with a slope \beta _ { s } = 1.8 for higher column densities and a flatter slope \beta _ { f } = 1.4 below the break .

A value J _ { LL } = 5 \pm 1 \times 10 ^ { -22 } erg cm ^ { -2 } s ^ { -1 } Hz ^ { -1 } sr ^ { -1 } is derived for the UV background in the redshift interval z = 1.7 - 4.1 , consistent with the predicted QSO contribution .
No evidence is found for redshift evolution of the UVB in the same redshift interval .

The comoving volume density distributions of protogalactic damped systems , Lyman Limit systems and Lyman- \alpha clouds with \log N _ { HI } \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { % $ > $ } } 14 and radii R \simeq 200 kpc are found to be similar , suggesting a possible common association with galaxies .