Proton-induced fluorescence properties of terbium gallium garnet
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Birks and Black2^* showed experimentally in 1951 that the fluorescence efficiency of anthracene bombarded by alpha particles varies with total dose as 1
h
N
1+
(1)
1
The / in Eq. (1) represents the fluorescence intensity for a given fluence, /o is the initial intensity, TV is the total fluence, and Ny2 is the total fluence required to reduce the fluorescence light output to half its original value. Plotting (/o//) versus proton dose yields a line whose slope is (NmY1- The intercept of the linear Birks and Black fit is equal to one. Schulman et al.5 observed a similar effect when anthracene was exposed to gamma irradiation. Black6 observed little effect on a fluor exposed to electrons of the same energy, since 40 keV electrons generate only ionization damage with no atomic displacements. Northrop and Simpson7 found the fluorescence efficiency deteriorated similarly. Studies cited previously were performed using organic fluors. A similar relationship was found by Broser and Kallmann8 for inorganic fluors under alpha particle bombardment. This relation implies that irradiation-induced quenching centers compete with emission centers for absorbed energy. The Birks and Black relationship described the deterioration for all yttrium and gadolinium fluors tested by the authors.1'9"11 Analysis of the data determines whether the intensity loss of terbium gallium garnet also follows the Birks and Black empirical relationship. A white terbium gallium garnet (Tb 3 Ga 5 0 12 or TGG) crystal seed was obtained from Dr. Mark Randies of Litton Airtron, Incorporated. A small round slice of the crystal (approximately 0.5 in. in diameter) was attached to a clean polished aluminum turret face usJ. Mater. Res., Vol. 10, No. 8, Aug 1995 http://journals.cambridge.org
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ing high vacuum epoxy to minimize outgassing. The crystal piece was sufficiently thin to allow protons to pass completely through before stopping in the aluminum turret. Irradiation of the TGG sample was completed using a 3 MeV proton beam at the Alabama A&M University Pelletron accelerator laboratory. A detailed description of the Fluorescent Materials Test Chamber (FMTC) used to expose TGG can be found in Ref. 12. The FMTC consists of a 102 mm stainless steel cross in a vertical plane. A turbo pump located on the bottom arm of the cross provides a minimum pressure of 5 X 10"7 Torr at the sample irradiation position. The chamber is mounted on three jack screws to allow turret alignment. Alignment is determined by sighting the glowing TGG sample through a clear window behind the beam switching magnet. A 1.0 cm2 square beam impinged upon one face of a four-sided aluminum turret. Each face was tilted 45° to the incident beam. Emitted photons were detected by a telescope and a Jarrell Ash grating spectrometer coupled to a 1024 element linear silicon photodiode array. Wavelength calibration was completed using a hydrogen lamp using the individual spectral lines to fit a third-order curve of peak pixel locations versus wavelength. The silicon photodio
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