Optical Absorption in TlCl: Pb Crystal Induced with Reactor and γ-Ray Irradiation at Low Temperatures

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TlCl crystals doped with Pb++ ions were irradiated in a reactor at low temperature and their absorption spectra were measured from 4.2 K to room temperature. Two prominent absorption bands were observed at 429 and 478 nm at 4.2 K which shifted to 426 and 465 nm at 77 K. They were quite similar to the bands which were observed in the undoped TlCl crystals irradiated in the reactor at low temperature. In the Pb–doped crystals more absorption bands were observed at 393 and 750 nm on warming to 77 K. The absorption of the undoped and Pb-doped crystals irradiated by Co60–γ rays at 4.2 or 77 K were also measured and were compared with the results of the reactor-irradiated crystals. Only in the Pb–doped crystals γ-irradiated at 77 K, the 426 and 393 nm bands were observed with the intense 750 nm band. An interpretation of the effects of Pb++ ions on the creation of the defects in the crystal by irradiation is presented.