Pressure dependence of secondary NIR scintillation in Ar and Ar/CF4

The variation with charge gain of the total number of photons emitted per electron, in the visible and near-infrared (NIR) regions (400 < lambda < 1000 mn), in Ar/CF4 mixtures, is investigated for CF4 concentrations below 15%. Measurements were performed in a uniform field configuration and results are presented for charge gains below 100, under X-ray excitation. The maximum number of photons emitted per drifting electron (0.7 and 0.2 ph/e(-) for Ar + 2% CF4 and Ar + 10% CF4, respectively) is obtained for a charge gain G similar to 4. The spectral distribution of the emitted light in Ar/CF4 mixtures is also analyzed. The pressure dependence (1-4 atm) of the secondary light output, in the visible and near infrared regions is studied as a function of the reduced electric field E/P for pure argon and Ar/CF4 mixtures. Above the charge multiplication threshold, the maximum number of photons emitted per electron decreases by a factor of about 1.5 when the pressure increases from 1 to 2 atms.