Fluorescent Response of NaI(Tl) and CsI(Tl) Crystals to Heavy Ions of Energies 1 to 300 keV

Abstract

The relative fluorescence output of freshly cleaved NaI(Tl) single crystals under bombardment with ions of mass from 1 to 133 amu was determined as a function of particle kinetic energy, in keV, as follows: H and D from 1 to 100; He from 10 to 100; Li from 20 to 100; C from 10 to 200; N from 10 to 200; O. F, and Na from 20 to 200; Ne from 15 to 200; Al from 30 to 100; Ar and Xe from 30 to 200; I from 50 to 100; Kr from 60 to 200; and Cs from 50 to 200. Except for H, D, Ne, and Na ions, the pulse-height-versus-energy relation for each different-mass ion shows two distinct linear regions. The change in slope for ions of mass less than that of Na is negative; for ions of mass greater than that of Na the change in slope is positive. In the energy range used, the slope of the pulse-height-versus-energy relation decreases as the incident ion mass is increased. In CsI(Tl) the pulse-height-versus-energy relation is linear for all the different ions used. The ratio of pulse height from NaI(Tl) to that from CsI(Tl) for a given mass and energy ion was typically 2.3. However, because of the slope change for NaI(Tl) the ratio varied from 2.0 to 2.5. Except for a few unexplained instances, the resolution by NaI(Tl) is approximately proportional to $E^{-\frac{1}{2}}$. A theoretical calculation based on a model of energy transfer by the incident particles to the principal constituents of the crystal lattice is presented. In this model, the energy transfer is divided into light-producing atom-electron interactions and atom-atom interactions in which negligible light production occurs. The model can explain logically the decreasing scintillation efficiency at a given energy as the incident particle mass is increased. Theories of Bohr and of Lindhard, Scharff, and Schiott are coupled in the development of the model used. Universal pulse-height-versus-energy relations result.