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12-16 September 2005
University of Liverpool
Europe/Zurich timezone

Optimization of Direct-Scintillator-Deposited Charge-Coupled

15 Sep 2005, 10:30
45m
University of Liverpool

University of Liverpool

Greenbank Conference Park
Board: P21
Contributed Poster Applications in Astronomy and Astrophysics P : Coffee and Poster Session

Speaker

Mr Noriaka Tawa (Osaka University)

Description

We developed a new photon-counting device for X-ray in the 0.1-100 keV energy range. The new device is an X-ray charge-coupled device (CCD) on which scintillator is directly deposited. It is called a scintillator-deposited CCD (SD-CCD). Low energy X-rays (0.1-10 keV) can be directly detected by the CCD while high energy X-rays (10-100 keV) pass through it into the scintillator where they generate hundreds or thousands of visible light photons. Their number is proportional to the incident X-ray energy. These photons can be detected by the CCD, a fact that enables it to effectively detect X-rays in the 0.1- 100 keV energy range. In order to achieve a high energy resolution, it is important that the number of visible light photons detected by the CCD increase. There are two types of CCD: a front-illuminated (FI) CCD and a backside-illuminated (BI) CCD. The BI CCD has higher sensitivity at low energy as well as visible light photons than the FI CCD, so we employed the BI CCD. As scintillator, we selected CsI(Tl). CsI(Tl) possesses the highest light yield among scintillators, and the light yield is above 60 photons/keV at -60oC. The emission spectrum of CsI(Tl) ranges between 350 and 700nm with a maximum at 550 nm. The detection efficiency of the FI CCD is of 20% while the BI CCD is of 85% at 550 nm. We fabricated two types of SD-CCD. One is coupled CsI(Tl) to the front side of the BI CCD with an optical cement, and the other one is directly deposited CsI(Tl) to the front side of the BI CCD. We suppose that depositing SD-CCD is less photons loss between CsI(Tl) and CCD than coupling SDCCD. But the coupling SD-CCD is higher energy resolution than the depositing SD-CCD. The FWHM energy resolution at 59.5 keV of the coupling SD-CCD is (26±1% whereas the depositing SD-CCD is (38±3)%. In order to optimize the structure of SD-CCD, we employed the Monte- Carlo simulation software, DETECT2000. Since the CsI(Tl) crystals of the SD-CCDs were needlelike structure, it prevents the lateral spread of visible light photons. We simulated light transport in the needlelike CsI(Tl) and the non-needlelike CsI(Tl). In the needlelike CsI(Tl), the spread of visible light photons on the CCD is 60 μm at FWHM while it is 270 μm in the nonneedlelike CsI(Tl). Therefore, the number of photons detected in one pixel (24x24 μm2) of the SD-CCD employing non-needlelike CsI(Tl) is about half that of the SD-CCD employing needlelike CsI(Tl). This simulation is consistent with experimental result obtained with SD-CCD.

Primary author

Mr Noriaka Tawa (Osaka University)

Presentation materials