Spectral Capability Evaluation of Thick Needlelike CsI(Tl) for Scintillator Directly Coupled Charge-Coupled Device

Abstract

We report on a new type of a photon-counting device, scintillator directly coupled charge-coupled device (SD-CCD), possessing a high spatial resolution and a moderate spectral resolution for 0.3–100 keV X-rays. The SD-CCD consists of an X-ray charge-coupled device (CCD) and CsI(Tl) as the scintillator. The CsI(Tl) is directly coupled to the surface of the CCD wafer. Soft X-rays incident to the SD-CCD are absorbed in the CCD, whereas hard X-rays are absorbed in the CsI(Tl) and generate visible photons that are also detected by the CCD. The CsI(Tl) employed in the SD-CCD has a needlelike structure with a diameter of about 5 µm, which reduces the lateral spread of visible photons. Because previous SD-CCDs employed a needlelike CsI(Tl) of 100 µm thickness, we developed an SD-CCD that has a needlelike CsI(Tl) of 300 µm thickness directly deposited on a CCD. We measured the spectral resolution of the SD-CCD, to be (68±4)% at 59.5 keV, which was a factor of 2.3 worse than that of the previous best SD-CCD. To ascertain the problem of degradation of spectral capability, we ran a Monte-Carlo simulation using DETECT2000 software. We found that visible photons in the needlelike CsI(Tl) reflect diffusely on the surface of the needle and the number of visible photons detected varies according to the X-ray point of interaction, resulting in the degradation of spectral capability. We discuss a count-measure to this problem.