congratulations tristan and co-authors on your published paper in the sensors special
issue advanced materials and technologies for radiation detectors. tristian’s paper is titled
“dark current modeling for a polyimide—amorphous lead oxide-based direct conversion x-
ray detector”.

abstract

the reduction of the dark current (dc) to a tolerable level in amorphous selenium (a-se) x-ray
photoconductors was one of the key factors that led to the successful commercialization of a-se-
based direct conversion flat panel x-ray imagers (fpxis) and their widespread clinical use. here,
we discuss the origin of dc in another x-ray photoconductive structure that utilizes amorphous
lead oxide (a-pbo) as an x-ray-to-charge transducer and polyimide (pi) as a blocking layer. the
transient dc in a pi/a-pbo detector is measured at different applied electric fields (5–20 v/μm).
the experimental results are used to develop a theoretical model describing the electric field-
dependent transient behavior of dc. the results of the dc kinetics modeling show that the dc,
shortly after the bias application, is primarily controlled by the injection of holes from the
positively biased electrode and gradually decays with time to a steady-state value. dc decays by
the overarching mechanism of an electric field redistribution, caused by the accumulation of
trapped holes in deep localized states within the bulk of pi. thermal generation and subsequent
multiple-trapping (mt) controlled transport of holes within the a-pbo layer governs the steady-
state value at all the applied fields investigated here, except for the largest applied field of 20 v/μm.

this suggests that a thicker layer of pi would be more optimal to suppress dc in the pi/a-
pbo detector presented here. the model can be used to find an approximate optimal thickness of
pi for future iterations of pi/a-pbo detectors without the need for time and labor-intensive
experimental trial and error. in addition, we show that accounting for the field-induced charge
carrier release from traps, enhanced by charge hopping transitions between the traps, yields an
excellent fit between the experimental and simulated results, thus, clarifying the dynamic process
of reaching a steady-state occupancy level of the deep localized states in the pi. practically, the
electric field redistribution causes the internal field to increase in magnitude in the a-pbo layer,
thus improving charge collection efficiency and temporal performance over time, as confirmed
by experimental results. the electric field redistribution can be implemented as a warm-up time
for a-pbo-based detectors.

continue reading the article: https://doi.org/10.3390/s22155829

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oleksandr grynko and tristen thibault travelled to san diego, united states to present their research at the 2022 spie physics of medical imaging conference.

the post oleksandr grynko and tristan thibault present at 2022 spie physics of medical imaging conference appeared first on rezniklab and posted by pavlo karasyuk.

oleksandr grynko with co-authors: tristen thibault, emma pineau, and alla reznik
published a paper titled “the x-ray sensitivity of an amorphous lead oxide photoconductor” in
special issue: sensors and x-ray detectors. congratulations on the published paper!

abstract

the photoconductor layer is an important component of direct conversion flat panel x-ray
imagers (fpxi); thus, it should be carefully selected to meet the requirements for the x-ray
imaging detector, and its properties should be clearly understood to develop the most optimal
detector design. currently, amorphous selenium (a-se) is the only photoconductor utilized in
commercial direct conversion fpxis for low-energy mammographic imaging, but it is not
practically feasible for higher-energy diagnostic imaging. amorphous lead oxide (a-pbo)
photoconductor is considered as a replacement to a-se in radiography, fluoroscopy, and
tomosynthesis applications. in this work, we investigated the x-ray sensitivity of a-pbo, one of
the most important parameters for x-ray photoconductors, and examined the underlying
mechanisms responsible for charge generation and recombination. the x-ray sensitivity in terms
of electron–hole pair creation energy, w±, was measured in a range of electric fields, x-ray
energies, and exposure levels. w± decreases with the electric field and x-ray energy, saturating
at 18–31 ev/ehp, depending on the energy of x-rays, but increases with the exposure rate. the
peculiar dependencies of w± on these parameters lead to a conclusion that, at electric fields
relevant to detector operation (~10 v/μm), the columnar recombination and the bulk
recombination mechanisms interplay in the a-pbo photoconductor.

continue reading the article: https://doi.org/10.3390/s21217321

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oleksandr grynko with co-authors: tristen thibault, emma pineau, and alla reznik
published a paper titled “engineering of a blocking layer structure for low-lag operation of
the a-pbo-based x-ray detector” in ieee transactions on electron devices (volume: 68,
issue: 5, may 2021). congratulations on the published paper!

abstract

direct conversion flat panel detectors are of great significance to the field of medical x-ray
imaging since they offer imaging performance and diagnostic capabilities not achievable with
other methods. currently, mammographic direct conversion detectors employ a layer of
amorphous selenium (a-se) photoconductor. although its properties ideally fit the requirements
of mammography, where “soft” x-rays are used, a-se cannot be used in high-energy x-ray
procedures. to extend the diagnostic capabilities of the direct conversion detectors, amorphous
lead oxide (a-pbo) is proposed as an alternative photoconductor. it is a high effective atomic
number material and thus has a higher x-ray stopping power over the wide x-ray energy range.
a-pbo is, therefore, a suitable candidate for applications in radiography, fluoroscopy, and digital
tomosynthesis. here, we report on the development of a blocking structure with a polyimide (pi)
layer needed to maintain low dark current at high electric fields. we demonstrate that a 1- μm-
thick pi blocking layer allows the operation of the detector at strong electric fields (≥10 v/ μm)
while suppressing the dark current to an innocuous level (<; 1 pa/mm 2 ). it also improves
temporal performance by reducing signal lag. no ghosting effect was observed at exposure rates
up to 1 r/s; however, at high radiation levels, the detector's sensitivity degraded. this
degradation is not permanent as the detector restores its original sensitivity after several hours of
rest in the dark without bias applied.

continue reading the article: https://doi.org/10.1109/ted.2021.3067616

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oleksandr grynko with co-authors: tristen thibault, emma pineau, gytis juska and alla
reznik, published the paper “bilayer lead oxide x-ray photoconductor for lag-free operation” in
the scientific reports – nature. congratulations oleksandr on this published paper.

abstract

polycrystalline lead oxide (poly-pbo) was considered one of the most promising
photoconductors for the direct conversion x-ray medical imaging detectors due to its previous
success in optical imaging, i.e., as an optical target in so-called plumbicon video pick-up tubes.
however, a signal lag which accompanies x-ray excitation, makes poly-pbo inapplicable as an
x-ray-to-charge transducer in real-time x-ray imaging. in contrast, the recently synthesized
amorphous lead oxide (a-pbo) photoconductor is essentially lag-free. here, we report on our
approach to a pbo detector where a thin layer of a-pbo is combined with a thick layer of poly-
pbo for lag-free operation. in the presented a-pbo/poly-pbo bilayer structure, the poly-pbo
layer serves as an x-ray-to-charge transducer while the a-pbo acts as a lag prevention layer. the
hole mobility in the a-pbo/poly-pbo bilayer structure was measured by photo-charge extraction
by linearly increasing voltage technique at different temperatures and electric fields to
investigate charge transport properties. it was found that the hole mobility is similar to that in a-
se—currently the only commercially viable photoconductor for the direct conversion x-ray
detectors. evaluation of the x-ray temporal performance demonstrated complete suppression of
signal lag, allowing operation of the a-pbo/poly-pbo detector in real-time imaging.

continue reading the article: https://doi.org/10.1038/s41598-020-77050-w

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