EPR identification of defects responsible for thermoluminescence in Cu-doped lithium tetraborate (Li2B4O7) crystals

Authors

A. T. Brant, Air Force Institute of TechnologyFollow
D. A. Buchanan, Air Force Institute of Technology
J. W. McClory, Air Force Institute of Technology
P. A. Dowben, University of Nebraska - LincolnFollow
V. T. Adamiv, Institute of Physical Optics
Ya. V. Burak, Institute of Physical Optics
L. E. Halliburton, West Virginia University

Date of this Version

2013

Citation

Journal of Luminescence 139 (2013) 125–131

Comments

This document is a U.S. government work and is not subject to copyright in the United States.

http://dx.doi.org/10.1016/j.jlumin.2013.02.023

Abstract

Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200 ^◦C in copper-doped lithium tetraborate (Li₂B₄O₇) crystals. As-grown crystals have Cu⁺ and Cu²⁺ ions substituting for lithium and have Cu⁺ ions at interstitial sites. All of the substitutional Cu²⁺ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu²⁺ EPR spectrum when a hole is trapped by substitutional Cu⁺ ions that have no nearby defects. These two Cu²⁺ trapped-hole centers are referred to as Cu²⁺-V_Li and Cu²⁺_active, respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu⁰ atoms are produced when interstitial Cu⁺ ions trap electrons. They are observed with EPR and are labeled Cu⁰_A and Cu⁰_B. When an irradiated crystal is warmed from 25 to 150 ^◦C, the Cu²⁺_active centers have a partial decay step that correlates with the TL peak near 100 ^◦C. The concentrations of Cu⁰_A and Cu⁰_B centers, however, increase as the crystal is heated through this range. As the crystal is futher warmed between 150 and 250 ^◦C, the EPR signals from the Cu²⁺_active hole centers and Cu⁰_A and Cu⁰_B electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200 ^◦C.