Femtosecond laser ablation of cadmium tungstate for scintillator arrays

S Richards,M.A Baker,M.D Wilson,A Lohstroh,P Seller

doi:10.1016/j.optlaseng.2016.03.004

S Richards, M.A Baker + Show 3 more

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https://doi.org/10.1016/j.optlaseng.2016.03.004

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Abstract

Ultrafast pulsed laser ablation has been investigated as a technique to machine CdWO4 single crystal scintillator and segment it into small blocks with the aim of fabricating a 2D high energy X-ray imaging array. Cadmium tungstate (CdWO4) is a brittle transparent scintillator used for the detection of high energy X-rays and γ-rays. A 6W Yb:KGW Pharos-SP pulsed laser of wavelength 1028nm was used with a tuneable pulse duration of 10ps to 190fs, repetition rate of up to 600kHz and pulse energies of up to 1mJ was employed. The effect of varying the pulse duration, pulse energy, pulse overlap and scan pattern on the laser induced damage to the crystals was investigated. A pulse duration of ≥500fs was found to induce substantial cracking in the material. The laser induced damage was minimised using the following operating parameters: a pulse duration of 190fs, fluence of 15.3Jcm−2 and employing a serpentine scan pattern with a normalised pulse overlap of 0.8. The surface of the ablated surfaces was studied using scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Ablation products were found to contain cadmium tungstate together with different cadmium and tungsten oxides. These laser ablation products could be removed using an ammonium hydroxide treatment.

Highlights

Cadmium tungstate is a commonly used material for the detection of X-rays and γ-rays, due to its high density of 7.9 g cmÀ3 and average high atomic number [1]
The volume was determined for each ablated hole using the atomic force microscopy (AFM) datasystem and an average taken of both measurements
The 3 highest pulse energy holes ablated with 10 pulses were too deep to be measured by the AFM

Summary

Introduction

Cadmium tungstate is a commonly used material for the detection of X-rays and γ-rays, due to its high density of 7.9 g cmÀ3 and average high atomic number [1] It converts incident ionising radiation into visible photons which are isotropically emitted from the point of radiation absorption. Any decrease in spatial resolution, typically associated with an increase in thickness, can be prevented using a material which is optically highly reflective between each segment. The manufacture of these arrays is typically undertaken by hand where scintillator crystals are mechanically cut, polished and assembled into large arrays. Laser micromachining can n Corresponding author at: STFC-Rutherford Appleton Laboratory, Harwell, Oxfordshire OX11 0QX, United Kingdom

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