Two-photon decay of $K$-shell vacancies in silver atoms

The spectral distributions for the two-photon decay modes of singly $K$-shell ionized silver atoms are determined by x-ray–x-ray coincidence measurements. $\mathrm{Ag}K$-shell vacancies were induced by nuclear electron capture decay of radioactive cadmium isotopes ${}^{109}\mathrm{Cd}$ and two-photon coincidences were taken back to back $(180°)$ and at a $90°$ opening angle for the emission. Each of the two-photon transitions from the $2s$, $3s$, and $3d$ states exhibits unique angular and spectral distributions. The measurements agree nicely with relativistic self-consistent field calculations of Tong et al. Our results also confirm and extend the earlier experimental data of Ilakovac and co-workers with improved accuracy.

Figure 1

Experimental arrangement.

Figure 2

Two-dimensional event plot for true two-photon coincidences. X-ray events registered in detector $B$ are plotted against the sum energy of the two photons coincidently registered in detectors $A$ and $B$ (cf. Fig. 1). The arrows point to the two-photon ridges.

Figure 3

Sum x-ray spectra at 5.5, 1.5, and $15.5\mathrm{keV}$ (horizontal cuts in Fig. 2).

Figure 4

Differential two-photon transition probabilities for back to back and $90°$ emission of the two photons. The full squares are the present results, the open circles the results of Ilakovac et al. [19], and the full lines give the prediction of Tong et al. [12].

Figure 5

Intensity ratio for $180°$:$90°$ two-photon emission: the lines are calculations according to Tong et al. [12].

Figure 6

Normalized rates (square of the reduced matrix elements) for two-photon emission; present results (full squares) compared with theory (solid lines) of Tong et al. [12].