arXiv subjects

J. A. Rowlands

Publications and source records attributed to J. A. Rowlands.

6 recordsLinked to original sources

arXiv2017

A high-resolution CMOS imaging detector for the search of neutrinoless double $β$ decay in $^{82}$Se

We introduce high-resolution solid-state imaging detectors for the search of neutrinoless double $β$ decay. Based on the present literature, imaging devices from amorphous $^{82}$Se evaporated on a complementary metal-oxide-semiconductor (CMOS) active pixel array could have the energy and spatial resolution to produce two-dimensional images of ionizing tracks of utmost quality, effectively akin to an electronic bubble chamber in the double $β$ decay energy regime. Still to be experimentally demonstrated, a detector consisting of a large array of these devices could have very low backgrounds, possibly reaching $10^{-7}$/(kg y) in the neutrinoless decay region of interest (ROI), as it may be required for the full exploration of the neutrinoless double $β$ decay parameter space in the most unfavorable condition of a strongly quenched nucleon axial coupling constant.

physics.ins-det↗

arXiv2013

Origin of n- and p-type conductivity in undoped $α$-PbO: role of defects

The first principles calculations (GGA) have been applied to study the crystallographic defects in $α$-PbO in order to understand an origin of $n$- and $p$-type conductivity in otherwise undoped $α$-PbO. It was found that deposition in the oxygen-deficient environment to be defined in our simulations by the Pb-rich/O-poor limit stimulates a formation of the O vacancies and the Pb interstitials both to be characterized by quite low formation energies $\sim$ 1.0 eV. The O vacancy being occupied by two electrons shifts a balance of electrons and holes between these two defects to excess of electrons (four electrons against two holes) that causes the $n$-type doping. For the Pb-poor/O-rich limit, an excess of oxygen triggers a formation of the O interstitials characterized by such a low formation energy that spontaneous appearance of this defect is predicted. It is shown that the concentration of the O interstitials is able to reach the extreme magnitude equal to number of the possible defect sites ($\sim 10^{22}$cm$^{-3}$). The localized state formed by the O interstitial is occupied by two holes and because there are no other defects in reasonable concentration to balance the hole redundancy, $p$-type doping is induced.

cond-mat.mtrl-sci↗

arXiv2013

Self-passivation of vacancies in α-PbO

We introduce a self-passivation of single lead (Pb) and oxygen (O) vacancies in the α-PbO compound through formation of a Pb-O vacancy pair. The preferential mechanism for pair formation involves initial development of the single Pb vacancy which, by weakening the covalent bonding, sets up the crystal lattice for an appearance of the O vacancy. Binding of the Pb and O vacancies occurs through the ionization interactions. Since no dangling bonds appear at the Pb-O pair site, this defect has a minor effect on the electronic properties. In such, vacancy self-passivation offers a practical way to improve the transport properties in thermally grown PbO layers.

cond-mat.mtrl-sci↗

arXiv2013

Solid-State Charpak Detector with Unipolar and Ultrafast Time-Differential Pulse Response

For the first time we have fabricated a high granularity micropattern solid-state detector (MSSD) with the unipolar time-differential property for ultrafast pulse response. Optical, x-ray, and gamma-ray photoconductivity measurements prove the unipolar time-differential response with a speed improvement factor of ~400 for amorphous selenium MSSD. The significance of the results presented here is the ability to reach the intrinsic physical limit for detector pulse speed by transitioning from the transit-time-limited response, which depends on the bulk carrier transport mechanism, to range-limited response set by the spatial dispersion of the photo-induced carrier cloud. Furthermore, the combination of the time-differential and the avalanche-multiplication properties of the proposed micropattern solid-state detector can yield another great leap forward in the field of radiation detectors with applications ranging from high-energy, nuclear, and astro- physics, to industrial and medical diagnostics, and crystallography.

physics.ins-det↗

arXiv2012

Lead monoxide $α$-PbO: electronic properties and point defect formation

The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline $α$-PbO platelets and the formation of the native point defects in $α$-PbO crystal lattice are studied using first principles calculations. The $α$-PbO lattice consists of coupled layers interaction between which is too low to produce high efficiency interlayer charge transfer. In practice, the polycrystalline nature of $α$-PbO causes the formation of lattice defects in such a high concentration that defect-related conductivity becomes the dominant factor in the interlayer charge transition. We found that the formation energy for the O vacancies is low, such vacancies are occupied by two electrons in the zero charge state and tend to donate their electrons to the Pb vacancies that leads to ionization of both vacancies.The vacancies introduce localized states in the band gap which can affect charge transport. The O vacancy forms a defect state at 1.03 eV above the valence band which can act as a deep trap for electrons, while the Pb vacancy forms a shallow trap for holes located just 0.1 eV above the valence band. Charge de-trapping from O vacancies can be accounted for the experimentally found dark current decay in ITO/PbO/Au structures.

cond-mat.mtrl-sci↗

arXiv2012

Unipolar Time-Differential Pulse Response with a Solid-State Charpak Photoconductor

We demonstrate, for the first time, a high granularity multi-well solid-state detector (MWSD) with the unipolar time-differential property for ultrafast pulse response. Results show a speed improvement factor of up to ~400 for amorphous selenium MWSD. The significance of the results presented here is the ability to reach the intrinsic physical limit for detector pulse speed by transitioning from the slow transit-time-limited response which depends on the bulk carrier transport mechanism, to the ultrafast dispersion-limited response which depends on the spatial spreading of the collected carrier packet. Furthermore, the proposed structure is the practical approach for achieving stable avalanche multiplication gain in large-area direct radiation detectors.

physics.ins-det↗