Toward Scintillator High‐Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager ( SHARP ‐ AMFPI ): Initial fabrication and characterization

Journal Article · Mon Dec 18 00:00:00 EST 2017 · Medical Physics

DOI:https://doi.org/10.1002/mp.12693· OSTI ID:1414005

Scheuermann, James R. ^[1]; Howansky, Adrian ^[1]; Hansroul, Marc ^[2]; Léveillé, Sébastien ^[2]; Tanioka, Kenkichi ^[1]; Zhao, Wei ^[1]

---

1. Department of Radiology Stony Brook University Stony Brook NY 11794 USA
2. Analogic Canada Montreal QC H4R 2P1 Canada

Purpose We present the first prototype Scintillator High‐Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager ( SHARP ‐ AMFPI ). This detector includes a layer of avalanche amorphous Selenium (a‐Se) ( HARP ) as the photoconductor in an indirect detector to amplify the signal and reduce the effects of electronic noise to obtain quantum noise‐limited images for low‐dose applications. It is the first time avalanche a‐Se has been used in a solid‐state imaging device and poses as a possible solution to eliminate the effects of electronic noise, which is crucial for low‐dose imaging performance of AMFPI . Methods We successfully deposited a solid‐state HARP structure onto a 24 × 30 cm ² array of thin‐film transistors ( TFT array) with a pixel pitch of 85 μm. The HARP layer consists of 16 μm of a‐Se with a hole‐blocking and electron‐blocking layer to prevent charge injection from the high‐voltage bias and pixel electrodes, respectively. An electric field ( E _{S e} ) up to 105 V μm ⁻¹ was applied across the a‐Se layer without breakdown. A 150 μm thick‐structured CsI:Tl scintillator was used to form SHARP ‐ AMFPI . The x‐ray imaging performance is characterized using a 30 kV p Mo/Mo beam. We evaluate the spatial resolution, noise power, and detective quantum efficiency at zero frequency of the system with and without avalanche gain. The results are analyzed using cascaded linear system model ( CLSM ). Results An avalanche gain of 76 ± 5 was measured at E _{S e} = 105 V μm ⁻¹ . We demonstrate that avalanche gain can amplify the signal to overcome electronic noise. As avalanche gain is increased, image quality improves for a constant (0.76 mR ) exposure until electronic noise is overcome. Our system is currently limited by poor optical transparency of our high‐voltage electrode and long integrating time which results in dark current noise. These two effects cause high‐spatial frequency noise to dominate imaging performance. Conclusions We demonstrate the feasibility of a solid‐state HARP x‐ray imager and have fabricated the largest active area HARP sensor to date. Procedures to reduce secondary quantum and dark noise are outlined. Future work will improve optical coupling and charge transport which will allow for frequency DQE and temporal metrics to be obtained.

• Sponsoring Organization: USDOE
• OSTI ID: 1414005
• Journal Information: Medical Physics, Journal Name: Medical Physics Vol. 45 Journal Issue: 2; ISSN 0094-2405
• Publisher: Wiley Blackwell (John Wiley & Sons)
• Country of Publication: United States
• Language: English