Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). ...However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd
Zn
Te
Se
ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.
HERAFitter Alekhin, S; Behnke, O; Belov, P ...
The European physical journal. C, Particles and fields,
07/2015, Letnik:
75, Številka:
7
Journal Article
Recenzirano
Odprti dostop
HERAFitter is an open-source package that provides a framework for the determination of the parton distribution functions (PDFs) of the proton and for many different kinds of analyses in Quantum ...Chromodynamics (QCD). It encodes results from a wide range of experimental measurements in lepton-proton deep inelastic scattering and proton-proton (proton-antiproton) collisions at hadron colliders. These are complemented with a variety of theoretical options for calculating PDF-dependent cross section predictions corresponding to the measurements. The framework covers a large number of the existing methods and schemes used for PDF determination. The data and theoretical predictions are brought together through numerous methodological options for carrying out PDF fits and plotting tools to help to visualise the results. While primarily based on the approach of collinear factorisation, HERAFitter also provides facilities for fits of dipole models and transverse-momentum dependent PDFs. The package can be used to study the impact of new precise measurements from hadron colliders. This paper describes the general structure of HERAFitter and its wide choice of options.
We emphasize an improvement of the surface processing procedures for cadmium zinc telluride (CZT) detectors, which is one of the principal problems limiting the technology. A rough surface enhances ...the leakage current into the medium, creating additional trapping centers and thereby degrading the detector’s performance. Mechanical polishing followed by chemical treatment yields smoother surfaces as required, but chemical treatment, especially with bromine-based solutions, induces unwanted surface features, increases the surface conductivity, and generates chemical species that alter the material’s surface and interfacial properties. It is essential to avoid such adverse consequences of surface etching in the manufacturing of highly efficient radiation detectors. We approached the problem of processing the crystals’ surfaces by using two different solutions (a low-concentration bromine-based etchant mixture in conjunction with a surface-passivation reagent and a non-bromine-based etchant). The chemomechanical treatment yielded smooth nonconductive surfaces with fewer detrimental features, therefore allowing us to fabricate better devices. We determined the surface roughness using atomic force microscopy and optical profilometry (OP). We analyzed the surface structure, orientations of the crystals, and formation of chemical species by x-ray photoelectron spectroscopy techniques and delineated their effects on the devices’ electrical properties and performance. Our experimental data revealed that our new chemical etching process produced nonconductive surfaces with fewer surface defects and so improved the detectors’ charge transport and efficiency. We detail the results of our new etchants and compare them with those for conventional Br-methanol etchants.
We investigate the impact of displaced heavy-quark matching scales in a global fit. The heavy-quark matching scale
μ
m
determines at which energy scale
μ
the QCD theory transitions from
N
F
to
N
F
+
...1
in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling
α
S
(
μ
)
. We study the variation of the matching scales, and their impact on a global PDF fit of the combined HERA data. As the choice of the matching scale
μ
m
effectively is a choice of scheme, this represents a theoretical uncertainty; ideally, we would like to see minimal dependence on this parameter. For the transition across the charm quark (from
N
F
=
3
to 4), we find a large
μ
m
=
μ
c
dependence of the global fit
χ
2
at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from
N
F
=
4
to 5), we have a reduced
μ
m
=
μ
b
dependence of the
χ
2
at both NLO and NNLO as compared to the charm. This feature is now implemented in xFitter 2.0.0, an open source QCD fit framework.
Nuclear detectors for x-ray and gamma-ray spectroscopy and imaging are a vital tool in many homeland security, medical imaging, astrophysics and other applications. Most of these applications require ...room-temperature operation due to the operational constraints imposed by a cryogenic cooling system. CdZnTe (CZT) has been the main material with the desired detection properties, and CZT crystals have been used commercially for three decades. However, CdZnTe still suffers from long-standing issues of high densities of performance-limiting intrinsic defects such as Te inclusions and networks of dislocation walls (sub-grain boundaries). A recently invented new quaternary material CdZnTeSe showed excellent material properties for radiation detection. The material was found to be free from dislocation networks, possess reduced Te inclusions, and have better compositional homogeneity. Virtual Frisch grid detectors were fabricated from crystals taken from a CdZnTeSe ingot that was grown by the traveling heater method. The detectors were fabricated from an as-grown ingot, bypassing the post-growth annealing process commonly practiced for industrial-grade CZT. The performances of the detectors were studied with different Frisch grid lengths using an amplifier shaping time ranging from 1–6 µs. The detectors showed high-quality spectroscopic performance with an as-measured energy resolution of ~1.1% at 662 keV for an optimum Frisch grid length of 3 mm. The charge collection was observed to enhance for longer Frisch grids.
We grew Cd1-xMnxTe crystals with a nominal Mn concentration of 5% by the vertical Bridgman growth technique. The compositional variation along the length of the grown ingot was studied by powder ...X-ray diffraction. The composition was found to be uniform along the growth direction. The achieved resistivity was 1-2.5 x1010 ohm-cm with a mobility-lifetime (μτ) product value for electrons of ∼1.7x10-3 cm2/V. An energy resolution of ∼7.5% at 662 keV was achieved for a 9-mm long Frisch grid detector fabricated from an ingot grown using as-received starting materials.
CdMnTe offers several potential advantages over CdZnTe as a room- temperature gamma-ray detector, but many drawbacks in its growth process impede the production of large, defect-free single crystals ...with high electrical resistivity and high electron lifetimes. Here, we report our findings of the defects in several vanadium-doped as-grown as well as annealed Cd
1−
x
Mn
x
Te crystals, using etch pit techniques. We carefully selected single crystals from the raw wafer to fabricate and test as a gamma-ray detector. We describe the quality of the processed Cd
1−
x
Mn
x
Te surfaces, and compare them with similarly treated CdZnTe crystals. We discuss the characterization experiments aimed at clarifying the electrical properties of fabricated detectors, and evaluate their performance as gamma-ray spectrometers.
The local impurity distribution in Te inclusions of CdZnTe (CZT) crystal was investigated by the time-of-flight secondary ion mass spectrometry (Tof-SIMS) technique. Direct evidence of impurity ...gettering in Te inclusions has been observed for the first time. The impurity gettering in Te inclusions originated from the diffusion mechanism during crystal growth and segregation mechanism during crystal cooling. This phenomenon is meaningful, because it reveals how Te inclusions affect CZT properties and provides a possible approach to reduce the impurities in CZT by the way of removing Te inclusions.
The concentration and size distribution of Te inclusions/precipitates in CZT are key factors in a device's performance. High concentrations can degrade the performance drastically, especially for ...long drift-length devices (more than about 10-mm thick). Here, we extend our previous findings on the concentration and size distribution of Te inclusions/precipitates in CZT wafers grown by the THM technique, by considering the rate of cooling of the ingots. We measured their distribution along the diameter of the wafers in a conventional slow-cooled and a fast-cooled ingot. The overall average concentration of Te inclusions/precipitates for the slow-cooled sample was less than 1×10
5
cm
−3, attesting to their suitability for fabricating thick radiation-detection devices.
► THM growth of CdZnTe and Te inclusion/precipitates mapping was carried out. ► Mapping of Te inclusions/precipitates of as-grown CZT by THM, for the first time. ► The size, distribution, and concentrations were less/comparable to the commercial CZT. ► Effect of the rate of cooling on concentration of Te inclusions/precipitates was studied.
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