Reduced Afterglow Codoped CsI:Tl for High-Energy Imaging

CsI activated with thallium is one of the most important scintillators in the radiation detection field. With one of the highest conversion efficiencies (60000 photons/MeV) of all scintillators in current use, along with its high density and effective atomic number, excellent light transport properties, broad commercial availability, and low cost, it has become the detector of choice for a wide variety of applications, especially medical diagnostics. Now, it has become particularly attractive for radiographic scanning and active interrogation of cargo in transit as well. It does have one drawback, however, in its relatively long-lived afterglow, which restricts its use in high-countrate applications, causing reconstruction artifacts in CT and reduced contrast and image blurring in high-speed radiographic scanning and imaging. In previous work, we have already demonstrated that this afterglow can be substantially suppressed at low energies (<;500 keV); here, we have extended our development to demonstrate afterglow suppression at higher energies as well. Our approach, which involves codoping of the material with small amounts of Sm
²⁺
or Eu
²⁺
, has been found to be effective at energies as high as the 7 MeV relevant for typical cargo inspection systems. The improved CsI:Tl scintillator is a straightforward replacement of the standard material, increasing contrast and steel penetration while allowing dual-energy performance, thus enabling reduced scan times and increased speeds for the same performance level. This in turn leads to greater throughput efficiency at ports and borders (where such imaging often creates choke points in traffic flow), as well as more efficient detection of contraband such as explosives, drugs, weapons, and special nuclear materials. We have also produced CsI:Tl,Sm in the form of microcolumnar films, where it exhibits the same afterglow reduction as in the single crystal form. We currently have the capability to produce such films up to 800 μm thick and up to 45 × 45 cm
²
in area, and expect yet further development in the future. This provides a combination of high-resolution, highframe-rate, and large-area capabilities not hitherto available for imaging applications.