There is a trade-off between the high stopping efficiency of a scintillator and high spatial resolution. High stopping efficiency requires a thick material, while high spatial resolution requires a thin material. The way to overcome this trade-off is by using structured scintillators that guide the scintillation light towards the photodetector.
Illustration of the trade-off between scintillator requirements for spatial resolution and sensitivity and the means to overcome it.
At RMD we have developed vapor-grown microcolumnar CsI:Tl, which has now become the standard for diagnostic digital radiography. We fabricate structured CsI:Tl screens as large as 47×47 cm2 size, and have extensive experience in micro-columnar growth of high resolution screens.
Large-format scintillators, suitable for X-ray imaging of an entire chest.
This scintillator material is grown in microcolumnar format in thickness up to several millimeters.
Scanning Electron Microscope micrograph of RMD’s microcolumnar CsI:Tl scintillator. Note the uniformity in microcolumn height and width. (Broken microcolumns are a result of sample preparation for SEM imaging.)
Dynamic imaging of a tungsten coded aperture mask after 50 ns excitation, using microcolumnar CsI:Tl films with and without Sm to suppress afterglow. Each frame lasts 30 ms, separated from the next by 1 ms. The x-ray pulse occurs during the first frame only, but the image in CsI:Tl persists even after ten frames. CsI:Tl,Sm shows hardly any residual image after the second frame, illustrating its superior performance.
Research is underway to improve the performance of CsI:Tl scintillator with regards to its afterglow and hysteresis by the addition of dopants such as samarium.
(Left) Hermetically-sealed LaBr3:Ce film measuring 7 cm in diameter and approaching 1 cm in thickness. (Center) A 6 mm thick LaBr3:Ce film deposited with the HWE System, demonstrating the desired, polycrystalline nature of the films. (Right) A thick LaBr3:Ce film approaching ~2cm in thickness
RMD has demonstrated the feasibility of fabricating polycrystalline and microcolumnar LaBr3:Ce scintillator films by co-evaporation of LaBr3 and CeBr3 onto suitable substrates. Films up to 1 cm have been successfully fabricated, composed of LaBr3:Ce and LaCl3:Ce scintillator materials.
(left) ZnTe:Se samples fabricated at RMD using vapor deposition techniques. (Center) ZnTe:O film deposited on a flexible substrate. (Right) SEM showing the microcolumnar nature of the ZnTe:O film.
RMD has also synthesized bright scintillators such as ZnSe:Te, ZnTe:O in microcolumnar format.
(Left) Photograph of a LiI:Eu film fabricated at RMD; 2.5 cm diameter, 1.2 mm thickness. (Right) SEM image showing the columnar nature of the LiI:Eu film.
For neutron imaging, we have fabricated LiI:Eu in microcolumnar format. This is an extremely hygroscopic material that has been successfully synthesized and hermetically sealed at RMD.