Triple-peaked scintillation pulse-height spectra for MeV axially channeled ions in NaI(Tl)

Previous experiments with aligned positive-ion beams on NaI(Tl) showed that the scintillation pulse-height spectrum exhibits two characteristic peaks: one due to those particles that are never channeled, and one at higher pulse height due to those particles that remain in a channeled trajectory throughout their range. This paper reports experiments with improved instrumental resolution. Pulse-height spectra for ⁴He, ⁷Li, ¹²C, ¹⁶O, and ³²S ions aligned along the <100> axis of NaI(Tl) were recorded in the MeV range. In certain cases a clearly defined intermediate third peak was observed. The third peak is attributed to a maximum in the dechanneling probability per unit length as a function of penetration depth. Data are analyzed in terms of a parameter describing the stability of the channeling trajectory.