The imager presented in this paper has a special blocking structure that ensures very low dark current of less than 1
pA/mm2 even with a 20 V/μm electric field. Hence the electric field can be increased from the generally applied 10
V/μm to 20V/μm, this reduces the energy required to produce an electron hole (e-h) pair from 60 eV to about 36 eV at
the given (19.3 keV mean) mammo energy. Furthermore, with special doping and manufacturing processes this a-Se
layer is very stable in the 0-70 C° temperature range as demonstrated by Ogusu et al. .
A new 5 cm × 5 cm size TFT array was developed with 50 μm pixel size, specifically for testing the resolution of
photoconductor based imagers. The first new imager of this type had a 200 μm thick a-Se layer evaporated onto the
array. Its MTF, NPS, and DQE values were evaluated using 28kVp Mo anode x-ray source with a 0.03mm thick Mo and
an additional 2 mm thick Al filters. The MTF value is about 40% and 50% in x-and y directions at the Nyquist frequency
of 10 lp/mm. The low frequency DQE at 20 V/μm electrical field is ~70% at 151 μGy dose and drops only about 10%
when going down to 23 μGy.
This new array also has excellent lag properties. The measured first frame image lag at 20 V/μm is less than 1%. Such
low lag provides opportunities to use this material not only for mammography but also for breast tomosynthesis
applications. Breast phantom images demonstrate that even the smallest 0.13 mm calcifications are clearly visible with
this high-resolution imager.
Amorphous selenium (a-Se) is well known to provide superior spatial resolution and low dark current when used as a
direct conversion X-ray photoconductor in flat panel detectors (FPD). However, a-Se properties are also known to
fluctuate at higher environmental temperatures, so the temperature has to be carefully controlled. To overcome this
problem we developed a newly modified a-Se photoconductor with electrical and X-ray characteristics that remain
constant at temperatures up to 70 degrees C. On the other hand, in terms of a-Se dark current levels, the higher the
electrical field, the higher the dark current level. For this reason, conventional a-Se photoconductors are used at a
comparatively low electric field of 10 V/μm. We also investigated the electrical characteristics of film compositions
containing a-Se that provide high gain and low dark current. Experiments were made with sandwich cells and then with
CMOS (50 μm pixel pitch) readout panels. Our new a-Se photoconductor operated at 40 V/μm delivers sensitivity 3 to 4
times higher than the conventional a-Se operated at 10 V/μm, while keeping the dark current density at 5 pA/mm2. This
a-Se photoconductor will prove effective for low-dose X-ray imaging including mammography and tomosynthesis.
We enhanced the photoelectric conversion efficiency of red light in a 15-&mgr;m-thick HARP film without deteriorating
image pick-up characteristics or reliability. To achieve a higher photoelectric conversion efficiency for red light, we
designed a new film structure with an increased amount of doped Te, which has a narrower band gap than that of a-Se.
The thickness of the LiF-doped layer for trapping holes was increased from that of the conventional red-extended HARP
film in order to weaken the internal field that would otherwise be enhanced by trapped electrons in extra doped Te. The
new red-extended HARP film achieved a photoelectric conversion efficiency for red light of about 22.5% at a
wavelength of 620 nm, which is twice that of the conventional red-extended film. We confirmed an improvement in
signal to shot noise ratio of 3 dB and a dramatic improvement in color reproduction when we experimented with an
HDTV camera with a camera tube incorporating the new film.
We developed an ultrahigh-sensitivity camera tube with a 15-μm-thick high-gain avalanche rushing amorphous photoconductor (HARP) film and applied it to an HDTV camera. The camera, called the "New Super-HARP", can achieve about 30 times the sensitivity (62.5 lux, F10) of conventional HDTV CCD cameras. Furthermore, for slow-moving subjects, the camera can dramatically increase the sensitivity in the intermittent read-out mode (for an accumulation time of 4 seconds, about 240 times the sensitivity of a New Super-HARP camera under normal operations). The very-low-dark-current feature of the HARP film results in excellent video images without any fixed pattern noise. We investigated the relationship between the operating temperature of the film and the occurrence of highlight defects in 15-μm-thick HARP films when shooting fixed, strong spot-lights directly. We found that defects could be suppressed by shifting the operating temperature to 35°C from the conventional 25°C. Furthermore, we optimized the concentration of arsenic (As) doped in the film to improve the heat resistance so that the film could be used at temperatures as high as 35°C. Ultrahigh-sensitivity imaging technology using HARP has been attracting considerable interest from many fields outside of broadcasting, such as medicine, biology, and digital film production.
The New Super-HARP image sensor, which relies on avalanche multiplication in a photoconductive film made mainly of amorphous selenium, is ultra-high sensitive. The sensor has already ben used to film very dark scenes, however in such a situation, shot noise due to the quantum characteristics of light becomes a serious problem. Increasing the quantum efficiency of the image sensors can reduce shot noise. The quantum efficiency of the New Super-HARP sensor has been improved to obtain an even better picture. To increase the quantum efficiency for green incident-light two improvements have been made to the amorphous selenium film. The first is doping the film with a suitable amount of tellurium on its incident-light side. The sensor is reducing the lithium- fluoride-doped layer to about 60 percent of the thickness of the conventional film. The improved version of the New Super-HARP Film has higher quantum efficiency. Its quantum efficiency at a wavelength of 540 nm was evaluated to be double that of the conventional film. Shot noise is reduced by three dB, that is, the S/N is improved by three dB.