Germanium ionization detectors with sensitivities as low as 100 eVee (electron-equivalent energy) open new windows for studies on neutrino and dark matter physics. The relevant physics subjects are summarized. The detectors have to measure physics signals whose amplitude is comparable to that of pedestal electronic noise. To fully exploit this new detector technique, various experimental issues including quenching factors, energy reconstruction and calibration, signal triggering and selection as well as evaluation of their associated efficiencies have to be attended. The efforts and results of a research program to address these challenges are presented.
The p-type point-contact germanium detectors have been adopted for light dark matter WIMP searches and the studies of low energy neutrino physics. These detectors exhibit anomalous behavior to events located at the surface layer. The previous spectral shape method to identify these surface events from the bulk signals relies on spectral shape assumptions and the use of external calibration sources. We report an improved method in separating them by taking the ratios among different categories of in situ event samples as calibration sources. Data from CDEX-1 and TEXONO experiments are re-examined using the ratio method. Results are shown to be consistent with the spectral shape method.
InGaN/GaN
quantum dots (QD) in nanowires exhibit excellent optical
properties and are promising candidates for nanoscale optoelectronic
devices. However, a large amount of surface states would cause low
quantum efficiency more severely than bulk materials, through not
only nonradiative recombination centers but also upward band bending.
Therefore, it is necessary to control the band bending effect in order
to improve the quantum efficiency of QDs. In this work, quantitative
measurements are carried out by ultraviolet photoelectron spectroscopy
(UPS) to describe the band bending effect in InGaN/GaN QD in nanowires
coated with three different dielectric layers including SiN
x
, Al2O3, and SiO2. Furthermore, their passivation mechanisms are investigated by photoluminescence
(PL), time-resolved PL. Contrary to SiO2 passivation, the
SiN
x
and Al2O3 passivation
nanowires demonstrate notable improvements in emission intensity.
Most essentially, all experimental findings are consilience with the
physical model that the deposition of dielectric layers effectively
alter the surface states of nanowires resulting a weakening or strengthen
in band bending near the surface. Our systematic studies on passivation
of nanowires can provide strategies for optimizing the performance
of nanowire-based optoelectronic applications.
Feedback is a fundamental mechanism existing in the human visual system, but has not been explored deeply in designing computer vision algorithms. In this paper, we claim that feedback plays a critical role in understanding convolutional neural networks (CNNs), e.g., how a neuron in CNN describes an object's pattern, and how a collection of neurons form comprehensive perception to an object. To model the feedback in CNNs, we propose a novel model named Feedback CNN and develop two new processing algorithms, i.e., neural pathway pruning and pattern recovering. We have mathematically proven that the proposed method can reach local optimum. Note that Feedback CNN belongs to weakly supervised methods and can be trained only using category-level labels. But it possesses powerful capability to accurately localize and segment category-specific objects. We conduct extensive visualization analysis, and the results reveal the close relationship between neurons and object parts in Feedback CNN. Finally, we evaluate the proposed Feedback CNN over the tasks of weakly supervised object localization and segmentation, and the experimental results on ImageNet and Pascal VOC show that our method remarkably outperforms the state-of-the-art ones.
Neutron production in lead by cosmic muons has been studied with a Gadolinium doped liquid scintillator detector. The detector was installed next to the Muon-Induced Neutron Indirect Detection EXperiment (MINIDEX), permanently located in the Tübingen shallow underground laboratory where the mean muon energy is approximately 7 GeV. The MINIDEX plastic scintillators were used to tag muons; the neutrons were detected through neutron capture and neutron-induced nuclear recoil signals in the liquid scintillator detector. Results on the rates of observed neutron captures and nuclear recoils are presented and compared to predictions from GEANT4-9.6 and GEANT4-10.3. The predicted rates are significantly too low for both versions of GEANT4. For neutron capture events, the observation exceeds the predictions by factors of 1.65 ± 0.02 (stat.) ± 0.07 (syst.) and 2.58 ± 0.03 (stat.) ± 0.11 (syst.) for GEANT4-9.6 and GEANT4-10.3, respectively. For neutron nuclear recoil events, which require neutron energies above approximately 5 MeV, the factors are even larger, 2.22 ± 0.05 (stat.) ± 0.25 (syst.) and 3.76 ± 0.09 (stat.) ± 0.41 (syst.), respectively. Also presented is the first statistically significant measurement of the spectrum of neutrons induced by cosmic muons in lead between 5 and 40 MeV. It was obtained by unfolding the nuclear recoil spectrum. The observed neutron spectrum is harder than predicted by GEANT4. An investigation of the distribution of the time difference between muon tags and nuclear recoil signals confirms the validity of the unfolding procedure and shows that GEANT4 cannot properly describe the time distribution of nuclear recoil events. In general, the description of the data is worse for GEANT4-10.3 than for GEANT4-9.6.
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