Possible implications of the channeling effect in NaI(Tl) crystals

Abstract

The channeling effect of low energy ions along the crystallographic axes and planes of NaI(Tl) crystals is discussed in the framework of corollary investigations on WIMP Dark Matter candidates. In fact, the modeling of this existing effect implies a more complex evaluation of the luminosity yield for low energy recoiling Na and I ions. In the present paper related phenomenological arguments are developed and possible implications are discussed at some extent.

Figures (10)

• Figure 1: Simplified schema of the channeling effect in the NaI(Tl) lattice. The axial channeling occurs when the angle of the motion direction of an ion with the respect to the crystallographic axis is less than a characteristic angle, $\Psi_c$, depicted there (see for details Sec. 2). Two examples for channeled and unchanneled ions are also shown (dashed lines).
• Figure 2: Fraction of solid angle interested by both axial and planar channeling in NaI(Tl) crystals as a function of the energy of the recoiling nuclei, calculated according to the text. In these calculations just the lower index crystallographic axes and planes have been considered: for the axial channeling: $<100>$, $<110>$, $<111>$ and for the planar channeling: $\{100\}$, $\{110\}$, $\{111\}$.
• Figure 3: Examples of light responses in terms of keVee, $\frac{dN_A}{dE_{det}}(E_{det}|E_R)$, for Iodine recoils of 4 keV ($a$) and of 40 keV ($b$) and for Sodium recoils of 4 keV ($c$) and of 40 keV ($d$) in the modeling given in the text. In this calculation the quenching factors for Sodium and Iodine recoils in amorphous or out of channel NaI(Tl) are assumed at the mean values given in ref. Psd96. Just to emphasize the effect of the channeling, the broadening due to the energy resolution of the detector has not been included here. The peaks corresponding to fully channeled events ($q\sim1$) and to fully quenched events (broadened by the straggling) are well evident; in the middle events, which have been de-channeled at least once, are also visible. It is possible to note that e.g. in the case of Iodine recoils the fully channeled events are about 25% at 4 keV; this percentage becomes smaller, about 1% at 40 keV.
• Figure 4: Fraction of events with quenching factor $\simeq 1$, that is fully channeled events, as a function of the energy of the recoiling nuclei in NaI(Tl) crystals according to the modeling described in the text.
• Figure 5: Examples of neutron calibrations of NaI(Tl) detectors at low recoil energy. In particular, the energy responses of NaI(Tl) detectors to Sodium recoils of 10 keV (left panel) neut06 and of 50 keV (right panel) neut03 are shown; the peaks corresponding to the quenched events are well clear. The superimposed continuous curves have been calculated as those of Fig. \ref{['fg:response']}, obviously broadening them by the energy resolution of the corresponding detector. The fully channeled peaks ($q\sim1$), which contain in these cases only the 6% and 0.15% of the events respectively, are smeared out by the energy resolution and can just contribute to the higher energy tails in the energy spectra.