Development of the scintillator-deposited charge-coupled device and application for the balloon-borne experiment, SUMIT

We report on a new photon-counting detector possessing unprecedented spatial resolution and moderate spectral resolution for 0.5-100 keV X-rays. It consists of an X-ray charge-coupled device (CCD) and a scintillator. The scintillator is directly coupled to the back surface of the X-ray CCD. Low-energy X-rays below 10keV can be directly detected by the CCD. The majority of hard X-rays above 10keV pass through the CCD but can be absorbed by the scintillator, generating visible photons. We employ the needlelike CsI(Tl) in order to prevent the lateral spread of visible photons. We performed the Monte Carlo simulation with DETECT2000 both to maximize the number of visible photons detected by the CCD and to minimize the lateral spread of visible photons on the CCD. We then fabricated the optimized needlelike CsI(Tl) with 300 μm thick and coupled it on the front surface of the back-illuminated (BI) CCD. The high detection efficiency of BI CCDs in the visible band enables us to collect visible photons emitted from the CsI(Tl) efficiently, leading to the moderate spectral resolution of 30% at 59.5 keV combined with the high detection efficiency for hard X-rays. We plan to perform the hard X-ray imaging balloon-borne experiment, SUMIT, in autumn of 2006 at Brazil. We also describe the details about the balloon system of the SD-CCD.