Search

The $\ensuremath{\beta}$ decay of the isomeric and ground state of $^{50}\mathrm{Sc}$ to the semimagic nucleus $_{22}^{50}\mathrm{Ti}_{28}$ has been studied using a $^{50}\mathrm{Ca}$ beam delivered to the GRIFFIN $\ensuremath{\gamma}$-ray spectrometer at the TRIUMF-ISAC facility. $\ensuremath{\beta}$-decay branching ratios are reported to 16 excited states with a total of 38 $\ensuremath{\gamma}$-ray transitions linking them. These new data significantly expand the information available over previous studies. Relative intensities are measured to less than 0.001 $%$ that of the strongest transition with the majority of $\ensuremath{\gamma}$-ray transitions observed here in $\ensuremath{\beta}$ decay for the first time. The data are compared with shell-model calculations utilizing both phenomenologically derived interactions employed in the $pf$ shell as well as a state-of-the-art ab initio-based interaction built in the valence-space in-medium similarity renormalization-group framework.

The electronic structure of Al(n)X (n = 1-6; X = As, Sb) clusters has been investigated using a synergistic approach combining negative ion photoelectron spectroscopy and first principles electronic structure calculations. It is shown that Al(3)X and Al(5)X exhibit enhanced energetic stability, as evidenced from calculated removal and embedding energies as well as chemical stability manifested through a large gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the stabilities of these species are derived from different mechanisms. Al(3)As and Al(3)Sb, with HOMO-LUMO gaps of 1.86 and 1.73 eV, respectively, are shown to have planar geometries where the p orbitals combine to form one pi and two sigma aromatic orbitals reminiscent of conventional all-metal aromatic species. Al(5)As and Al(5)Sb, with 20 valence electrons, possess a closed electronic shell (1s(2), 1p(6), 1d(10), 2s(2)) within a jellium framework and have HOMO-LUMO gaps of 1.12 and 1.17 eV, respectively.

An experimental and theoretical study of bismuth-doped aluminum clusters in the gas phase has revealed two particularly stable clusters, namely, Al(3)Bi and Al(5)Bi. We show that their electronic structure can be understood in terms of the aromatic and "Jellium" models, respectively. Negative ion photodetachment spectra provide a fingerprint of the electronic states in Al(n)Bi(-) (n = 1-5) anions, while theoretical investigations reveal the nature of the electronic orbitals involved. Together, the findings reveal that the all-metal Al(3)Bi cluster with 14 valence electrons is a cyclic, planar structure with a calculated large ionization potential of 7.08 eV, a low electron affinity of 1.41 eV, and a large gap of 1.69 eV between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap). The Al(3)Bi cluster has molecular orbitals reminiscent of aromatic systems and is a neutral cluster with no need for counterion or ligand support. A slightly larger cluster, Al(5)Bi, has 20 valence electrons and is another highly stable compact structure with a calculated large ionization potential of 6.51 eV and a large HOMO-LUMO gap of 1.15 eV. This cluster's stability is rooted in a Jellium electronic shell closing. The formation of stable species using aromatic bonding allows us to extend the idea of cluster-assembled materials built out of stable clusters with Jellium shell closings (superatoms) to include ones involving aromatic building blocks.

A cryogenic horizontal single loop target has been designed, built, tested and operated for the G 0 experiment in Hall C at Jefferson Lab. The target cell is 20 cm long, the loop volume is 6.5 l and the target operates with the cryogenic pump fully immersed in the fluid. The target has been designed to operate at 30 Hz rotational pump speed with either liquid hydrogen or liquid deuterium. The high-power heat exchanger is able to remove 1000 W of heat from the liquid hydrogen, while the nominal electron beam with current of 40 μ A and energy of 3 GeV deposits about 320 W of heat into the liquid. The increase in the systematic uncertainty due to the liquid hydrogen target is negligible on the scale of a parity violation experiment. The global normalized yield reduction for 40 μ A beam is about 1.5% and the target density fluctuations contribute less than 238 ppm (parts per million) to the total asymmetry width, typically about 1200 ppm, in a Q 2 bin.

A 150 cm long liquid hydrogen target has been built for the SLAC End Station A E158 experiment. The target loop volume is 55 liters, and the maximum target heat load deposited by the electron beam is {approx} 700 W. The liquid hydrogen density fluctuation with full beam current (120 Hz repetition rate, 6 x 10{sup 11} electrons/spill) on target is well below 10{sup -4} level, which fulfills the requirement for a precision measurement of the weak mixing angle in the polarized electron-electron scattering process.

Developments in spin-polarized internal targets for storage rings have permitted measurements of 197 MeV polarized protons scattering from vector polarized deuterons. This work presents measurements of the polarization observables A_y, iT_11, and C_y,y in proton-deuteron elastic scattering. When compared to calculations with and without three-nucleon forces, the measurements indicate that three-nucleon forces make a significant contribution to the observables. This work indicates that three-body forces derived from static nuclear properties appear to be crucial to the description of dynamical properties., 8 pages 2 figures Latex, submitted to Phys. Rev. Letters

A piezo-electrically controlled optical correction system was successfully used to reduce the helicity-correlated pulse-to-pulse position differences of a laser spot to better than ±100 nm at a pulse rate of 600 Hz. Using a simple feedback algorithm, average position differences of Δ x =−3.5±4.2 nm and Δ y =2.6±6.6 nm were obtained over a 6 h period. This optical correction system was successfully used in the polarized electron source at the Bates Linear Accelerator Center to stabilize the position of the electron beam during the recent SAMPLE experiment. Because this experiment measures a parity violating signal at the 10−6 level, it is sensitive to systematic effects which are correlated with the helicity of the incident electrons. One potentially large systematic effect is the helicity-correlated motion of the incident electron beam. By using this optical correction system, electron beam position differences at the location of the experiment were routinely kept well below ±100 nm, with averages over the entire two month run of Δ x =6.4±2.3 nm and Δ y =−3.5±2.0 nm.

The HERMES experiment is investigating the spin structure of the proton and neutron via deep-inelastic scattering of polarized positrons from polarized nuclear targets. The polarized positrons are provided by the HERA positron storage ring at DESY, Hamburg, Germany. The targets are pure internal gas targets. Data acquisition began in 1995, utilizing a polarized 3 He internal gas target to study the spin structure of the neutron. The target gas was polarized using the metastability-exchange optical-pumping technique and then injected into a cryogenically cooled target cell. The target was designed to operate with either longitudinal or transverse directions of polarization. Operating conditions included polarizations of up to 54% and target thicknesses of 1×10 15 nucleons/cm 2 . In this paper the HERMES polarized 3 He internal gas target is described in detail.

We present the results of the first direct measurement of nuclear polarization in an optically pumped spin-exchange polarized deuterium target. Measurements of the nuclear tensor polarization p zz are made in a storage cell as a function of flow of deuterium nuclei. For these measurements, the optical pumping was performed on atoms in magnetic fields of 600 and 3600 G. These measurements verify that the spin-exchange method is capable of producing polarized deuterium nuclei without the use of rf transitions. The electronic polarization, P e , is also measured. The P e and p zz measurements at 600 G indicate that the system is in spin-temperature equilibrium, whereas the 3600 G measurements show signatures of non-equilibrium effects. P e is also measured with the system operated as a source of polarized atoms, with no storage cell. These measurements are compared with previous measurements using a similar source.

The (e,e'p) reaction was studies on targets of C, Fe, and Au at momentum transfers squared Q{sup 2} of 0.6, 1.3, 1.8 and 3.3 GeV{sup 2} in a region of kinematics dominated by quasifree electron-proton scattering. Missing energy and missing momentum distributions are reasonably well described by plane wave impulse approximation calculations with Q{sup 2} and A dependent corrections that measure the attenuation of the final state protons.

The inclusive A(e,e′) cross section for x≃1 was measured on ^2H, C, Fe, and Au for momentum transfers Q^2 from 1 to 6.8 (GeV/c)^2. The scaling behavior of the data was examined in the region of transition from y scaling to x scaling. Throughout this transitional region, the data exhibit ξ scaling, reminiscent of the Bloom-Gilman duality seen in free nucleon scattering.

A laser-driven source of spin-polarized hydrogen (H) and deuterium (D) that relies on the technique of optical pumping spin exchange has been constructed. In this source, H or D atoms and potassium atoms flow continuously through a drifilm-coated spin-exchange cell where potassium atoms are optically pumped with circularly-polarized laser light in a high magnetic field. The H or D atoms become polarized through spin-exchange collisions with polarized potassium atoms. High electron polarization ( ≈ 80%) has been measured for H and D atoms at flow rates ≈ 2 × 1017 atoms/s. Lower polarization values are measured for flow rates exceeding 1 × 1018 atoms/s. In this paper, we describe the performance of the laser-driven source as a function of H and D atomic flow rate, magnetic field strength, alkali density and pump-laser power. Polarization measurements as a function of flow rate and magnetic field suggest that, despite a high magnetic field, atoms within the optical-pumping spin-exchange apparatus evolve to spin-temperature equilibrium which results in direct polarization of the H and D nuclei.

We report measurements of spin correlations and analyzing powers in $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}(\stackrel{\ensuremath{\rightarrow}}{p}, 2p)$ and $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}(\stackrel{\ensuremath{\rightarrow}}{p}, pn)$ quasielastic scattering as a function of momentum transfer and missing momentum at 197 MeV using a polarized internal target at the Indiana University Cyclotron Facility Cooler Ring. At sufficiently high momentum transfer we find $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}(\stackrel{\ensuremath{\rightarrow}}{p}, pn)$ spin observables are in good agreement with free $p\ensuremath{-}n$ scattering observables, and therefore that $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}$ can serve as a good polarized neutron target. The extracted polarizations of nucleons in $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}$ at low missing momentum are consistent with Faddeev calculations.

We describe the first experiment to use a polarized internal gas target and polarized beam in a storage ring. A laser optically pumped polarized He internal gas target has been used with circulating beams of 197-414 MeV polarized protons to carry out an extensive set of measurements of spin dependent scattering. A large acceptance non-magnetic detector system consisting of wie-chambers, scintillators and microstrip detectors was used to detect protons, neutrons, deuterons, and He-3 nuclei from the beam-target interaction. It is demonstrated that these techniques result in low backgrounds (< 1%) due to scattering from species other than the polarized target gas and allow detection of low energy recoiling nuclei. Specific issues such as interfacing the experiment to the storage ring and monitoring the luminosity and polarizations are discussed in detail.

The electron polarization of hydrogen (H) and deuterium (D) atoms in a spin-exchange optical-pumping apparatus has been measured as a function of H (D) density and magnetic-field strength. The atomic density was varied between 4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ and 2.6\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms/${\mathrm{cm}}^{3}$ and the magnetic-field strength from 120 to 400 mT. The results suggest that the atoms approach spin-temperature equilibrium even though the magnetic fields are large compared to the critical fields of the atoms.

We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized [sup 3]He target. The neutron magnetic form factor [ital G][sup [ital n]][sub [ital M]] has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of [ital G][sup [ital n]][sub [ital M]] at [ital Q][sup 2]=0.19 (GeV/[ital c])[sup 2] agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering.

The cross section for quasielastic ^(12)C(e,e’p) scattering has been measured at momentum transfer Q^2=1, 3, 5, and 6.8 (GeV/c)^2. The results are consistent with scattering from a single nucleon as the dominant process. The nuclear transparency is obtained and compared with theoretical calculations that incorporate color transparency effects. No significant rise of the transparency with Q^2 is observed.

The differential cross section for the reaction 2H(γ,p)n has been measured at several center-of-mass angles ranging from 50° to 143° for photon energies between 0.8 and 1.8 GeV. The experiment was performed at the SLAC-NPAS facility with the use of the 1.6 GeV/c spectrometer to detect the high energy protons produced by a bremsstrahlung beam directed at a liquid deuterium target. Contributions from concurrent disintegration by the residual electron beam were determined by measuring the proton yield without the Cu photon radiator. At angles not very far from 90°, the energy dependence of the cross sections is consistent with predictions of scaling using counting rules for constituent quarks. At least one theoretical calculation based on a meson-baryon picture of the reaction is able to reproduce the magnitude and energy dependence of the 90° cross section. The angular distribution exhibits a large enhancement at backward angles at the higher energies.

Measurements of the spin-dependent asymmetry in scattering longitudinally polarized electrons from polarized He-3 at quasielastic kinematics are reported. The measurements were made at two kinematics and spin angles, one sensitive to the helicity-dependent transverse-longitudinal interference response function, R(TL''), and the other to the helicity-dependent transverse response function, R(T''). For the experiment a metastability exchange optically pumped polarized He-3 target was used; a general discussion of the technique used to polarize the He-3, along with the details of the design and operation of the target system, are presented here. A comparison is made of the world''s data on the He-3 over arrow pointing right(e over arrow pointing right, e'') quasielastic asymmetry with several theoretical predictions, including calculations that use the plane wave impulse approximation and a fully spin-dependent spectral function. There is good agreement between data and theory at the current level of experimental precision.

We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized 3He gas target. The asymmetry is measured at kinematics sensitive to the transverse-longitudinal response function RTL(Q2,ω). The value of the neutron electric form factor GEn(Q2=0.16 (GeV/c2))=+0.070±0.100±0.035 is extracted from the asymmetry using a Faddeev calculation of the 3He wave function.

We present a measurement of the spin-dependent cross sections for the \vec{^3He}(\vec{e},e')X} reaction in the quasielastic and resonance regions at four-momentum transfer 0.1 < Q^2< 0.9 GeV^2. The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt--Cottingham and extended GDH sum rules for the first time. Impulse approximation and exact three-body Faddeev calculations are also compared to the data in the quasielastic region., Final version

We have computed radiative equilibrium models for the gas in the circumstellar envelope surrounding the hot, classical Be star $\gamma $Cassiopeia. This calculation is performed using a code that incorporates a number of improvements over previous treatments of the disk's thermal structure by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their relevant ions. Thus, for the first time, the thermal structure of a Be disk is computed for a gas with a solar chemical composition as opposed to assuming a pure hydrogen envelope. We compare the predicted average disk temperature, the total energy loss in H$\alpha$, and the near-IR excess with observations and find that all can be accounted for by a disk that is in vertical hydrostatic equilibrium with a density in the equatorial plane of $\rho(R)\approx 3$ to $5\cdot 10^{-11} (R/R_*)^{-2.5} \rm g cm^{-3}$. We also discuss the changes in the disk's thermal structure that result from the additional heating and cooling processes available to a gas with a solar chemical composition over those available to a pure hydrogen plasma., Comment: 11 pages, 8 figures high resolution figures available at http://inverse.astro.uwo.ca/sig_jon07.html

This chapter studies the ionization and electron detachment dynamics of several transition metal–carbon clusters with time-resolved multiphoton ionization mass spectrometry and anion photoelectron spectroscopy. Although nanosecond experiments on metallocarbohedrenes (Met-Car) clusters exhibit significant delayed ionization, prompt ionization is preferential with femtosecond ionization. Photodetachment experiments on anionic vanadium carbides provide insights into the growth mechanism of Met-Cars and other large metal–carbon networks. A connection is made among VC 2 , V 2 C 3 , and V 4 C 6 based on the electron affinities that are determined. These clusters display the lowest electron affinities of all clusters considered in the chapter. A similar effect was observed for the Met-Car with respect to similar-sized metal–carbon clusters. The influence of laser vaporization conditions on the geometric and electronic properties of the clusters generated is investigated.

We present measurements of the ratio of the proton elastic electromagnetic form factors, μ_pG_(Ep)/G_(Mp). The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. [Phys. Rev. Lett. 84, 1398 (2000)], that the form factor ratio decreases significantly from unity above Q^2=1 GeV^2.

A separation of the longitudinal and transverse 12 C(e, e8p) cross sections in the quasifree region has been performed in parallel kinematics at Q 2 of 0.64 and 1.8 GeV 2 for initial proton momentum ,80 MeV. The separated transverse and longitudinal spectral functions at Q 2 50.64 GeV 2 show significant differences for missing energy between 25 and 60 MeV indicating a breakdown in the single nucleon knockout picture. The transverse spectral functions exhibit definite momentum transfer dependence.

A laser driven source of polarized hydrogen and deuterium has been installed in the Cooler Ring at the Indiana University Cyclotron Facility. Polarized nuclei from the source are injected into a storage tube, and the resulting target has been used in a scattering experiment with 200 MeV proton beam. This paper discusses the performance of the source, including measurements of atomic fraction and electron polarization.

We present the results of a measurement of the spin-dependent asymmetry in 3→(e→,e′) inelastic scattering at kinematics on the low-energy transfer side of the quasielastic peak, including the region near the breakup threshold. Comparison with existing calculations based upon the plane wave impulse approximation shows significant deviation between the data and the model near the breakup threshold. Good agreement between data and theory is seen at higher energy transfer.

Cross sections are presented for the reaction 2H(e,e′p)n for momentum transfers in the range 1.2≤Q^2≤6.8 (GeV/c)^2 and for missing momenta from 0 to 250 MeV/c. The longitudinal-transverse interference structure function has been separated at Q^2 = 1.2 (GeV/c)^2. The observables are compared to calculations performed in nonrelativistic and relativistic frameworks. The data are best described by a fully relativistic calculation.

An optical pumping technique used in a laser‐driven source of polarized hydrogen or deuterium is described and measurements of the achievable polarization as a function of flow for both hydrogen and deuterium are presented. Ongoing tests of the feasibility of using a laser‐driven polarized deuterium source coupled to a storage cell as a polarized internal target, including direct measurements of the nuclear polarization, are discussed.

A series of experiments with an internal tensor polarized deuterium target in the 2 GeV VEPP‐3 electron storage ring in Novosibirsk is being performed. This paper describes the first results for a measurement of the tensor asymmetry in the quasi‐elastic (e,e′p) reaction. The found results are compared to a non‐relativistic calculation by Arenhovel and Leidemann, including meson‐exchange currents, isobar admixtures, and final‐state interactions.

Measurements of analyzing powers and spin correlation parameters for 3He(p↘,2p) and 3H↘e(p↘,pn) quasielastic scattering have been carried out using a polarized internal 3He target and a stored, polarized proton beam. They were made over a wide kinematic range with a proton energy of 197 MeV and a large acceptance detector system which allowed the simultaneous measurement of both reactions. At momentum transfers sufficiently high to minimize rescattering effects, the (p,pn) results are in agreement with PWIA calculations. The (p,2p) results were found to be less sensitive to kinematic conditions and are also in agreement with calculations.

The results of a measurement of the spin-dependent asymmetry in quasielastic scattering of longitudinally polarized electron from polarized 3He target are eported. The neutron magnetic form factor at Q2=0.19 (GeV/c)2 has been extracted from the measured asymmetry and is reported in this thesis work. The experiment was performed at the MIT-Bates Linear Accelerator Center with an incident electron beam energy of 370 MeV. Two magnetic spectrometers, MEPS and OHIPS, were used to detect the scattered electrons independently, with MEPS at the kinematics maximally sensitive to the spin-dependent transverse response function, RT', at Q2=0.19 (GeV/c)2 and OHIPS at the kinematics maximally sensitive to the spin-dependent transverse-longitudinal response function, RTL', at Q2=0.14 (GeV/c)2. The elastic asymmetry and the quasielastic asymmetry ATL' in the threshold region for three-body breakup have also been measured in OHIPS and reported. A cryogenic polarized 3He gas target using a metastability-exchange optical pumping technique was employed in this experiment. The target was operated at 13K during the experiment with an average target thickness of 3.3x10(18)cm(-2) MEPS and 1.6x10(19)cm(-2) (OHIPS). The average target polarization with 25 µA average beam current was over 38%. A GaAs source produced the polarized electrons and a Wien spin rotator was employed to rotate the spin to give longitudinally polarized electrons at the 3He target. The average beam polarization for the experiment was 36.5%. The measured 3He elastic asymmetry agrees very well with the calculated values using the 3He form factors measured by Rosenbluth separation. The measured spin-dependent transverse asymmetry, AT', which is expected to be sensitive to the neutron magnetic form factor is in good agreement with recent PWIA calculations using a spin-dependent spectral function. The neutron magnetic form factor at Q2=0.19(GeV/c)2 has been extracted from the measured asymmetry based on the recent PWIA calculations. The extracted GnM value agrees very well with the standard dipole parametrization at this Q2. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. The uncertainty of the extracted GnM2 is dominated by the statistical uncertainty and the uncertainty due to model dependence is found to be comparatively small. The measured ATL' in the threshold region has shown very large positive asymmetry close to the three-body breakup threshold. To interpret the AT' data, a complete calculation of the 3He asymmetry ATL' in the elastic-threshold region using a spin-dependent spectral function with final state interactions (FSI) and meson exchange currents (MEC) taken into account is probably necessary.

We report the first measurement of analyzing powers and spin correlation parameters using a storage ring with both beam and internal target polarized. Spin observables were measured for elastic scattering of 45 and 198 MeV protons from polarized [sup 3]He nuclei in a new laser-pumped internal gas target at the Indiana University Cyclotron Facility Cooler Ring. Scattered protons and recoil [sup 3]He nuclei were detected in coincidence with large acceptance plastic scintillators and silicon detectors. The internal-target technique demonstrated in this experiment has broad applicability to the measurement of spin-dependent scattering in nuclear and particle physics.

Recent results from a laser‐driven source of polarized hydrogen (H) and deuterium (D) are presented. The performance of the source is described as a function of atomic flow rate and magnetic field. The data suggest that because atomic densities in the source are high, the system can approach spin‐temperature equilibrium although applied magnetic fields are much larger than the critical field of the atoms. We also observe the potassium contamination in the source emittance can be reduced to a negligible amount using a teflon‐lined transport tube.

The question of whether nuclei are polarized as a result of H–H(D–D) spin‐exchange collisions within the relatively dense gas of a laser‐driven source of polarized hydrogen (deuterium) can be addressed directly by measuring the nuclear polarization of atoms from the source. The feasibility of using a polarimeter based on the D+T→n+4He reaction to measure the tensor polarization of deuterium in an internal target fed by the laser‐driven source has been tested. The device and the measurements necessary to test the spin‐exchange polarization theory are described.

Electron traps in bulk n‐type Hg0.7Cd0.3Te were investigated near the HgCdTe/SiO2 interface by use of deep level transient spectroscopy on metal‐insulator‐semiconductor structures [D. V. Lang, J. Appl. Phys. 45, 3022 (1974)]. Three electron traps are found with activation energies (relative to the conduction band edge) of 0.12, 0.172, and 0.079 eV, and corresponding capture cross sections of 3.5×10−18, 1.1×10−16, and 1.2×10−18 cm2. Depth profiles from the surface to ∼1.0 μm show the concentration of the 0.172‐eV trap to be uniform while the 0.12‐eV trap shows a strong depth dependence. It is undetectable at the surface, rising in concentration to twice its bulk value at ∼0.5 μm depth then falling to a value comparable with that of the 0.172‐eV trap in the bulk (1 μm). Divalent trapping behavior has also been detected. In this case, the deeper state significantly depopulates, enabling a shallower state to then depopulate [J. S. Blakemore, Semiconductor Statistics (Pergamon, London, 1962), p. 156].

We report the application of diode admittance spectroscopy in characterizing traps in Hg1−xCdxTe. Measurements performed on n+‐p junction photodiodes have identified a hole trap located 0.16 eV above the valence band in x=0.305 liquid‐phase‐epitaxy‐grown material and a single hole trap located 0.046 eV above the valence band in x=0.219 bulk‐grown material. Measurements of trap density and majority‐carrier capture cross section have also been carried out, with results suggesting hole capture at a neutral trapping center. Trap energies determined by the admittance spectroscopy technique were found to be in good agreement with lifetime‐versus‐temperature data analyzed in terms of a single Shockley‐Read recombination center.

Trapping effects in p‐type Hg1−xCdx Te (0.2

Deep‐level defects play an important role in determining minority carrier lifetimes and noise in Hg1−xCdxTe devices. Previous work on deep‐level defects in Hg1−xCdxTe has been meager because there have not been good experimental methods available to observe or characterize them. Deep‐level transient spectroscopy has now begun to provide information on concentrations, energy levels, and capture cross sections for defects in Hg1−xCdxTe. This paper reviews the DLTS results to date on nonstoichiometric (Hg lean) and copper‐doped material. In both cases, trends are beginning to appear for the trap levels as a function of x. Nonstoichiometric material commonly shows generation‐recombination centers at Ev+0.4 Egap and Ev+0.75 Egap. Copper‐doped material commonly shows two levels pinned to the valence band at about Ev+0.06 eV and Ev+0.15 eV. The capture cross sections and concentrations measured by DLTS have been used to calculate the effect these centers have on minority carrier lifetime and noise (R0A). In most...

Gold is a fast‐diffusing acceptor in HgCdTe. It has been used as a p‐type dopant and as a metal contact. This paper reports the presence of deep levels in gold‐doped x=0.48 Hg1−xCdxTe. Two types of spectra were observed; each had energy levels between Ev +1/4 Egap and Ev +1/2 Egap. The capture cross sections for these centers have been determined and for most of the levels they involve small hole capture cross sections, 10−17 to 10−21 cm2, and moderate electron capture cross sections, 10−15 to 10−16 cm2. The hole capture cross sections for these centers had large negative activation energies indicative of hole capture at a positive center with a Coulomb repulsive barrier. The corresponding electron capture is then at Coulombically attractive positive centers. The situation described requires that the DLTS activation energies be corrected for the temperature dependence of the hole capture cross sections and for the Poole–Frenkel effect for electron capture. The energy levels and capture cross sections repo...

Demonstration that luminescence in irradiated silicon consists of a spectral group between 0.80 and 1.0 eV which seems to be independent of impurities, while a lower energy group between 0.60 and 0.80 eV is seen only in pulled crystals. The small halfwidth and temperature dependence of the sharp zero-phonon lines observed in these spectra indicate that the luminescence arises from a bound-to-bound transition. A model is proposed for the transition mechanism. Stress data taken on the 0.79-eV zero-phonon line in pulled crystals can be fit by either a tetragonal 100 (in brackets) defect symmetry or by conduction-band splitting effects. It is suggested that the 0.79-eV zero-phonon line and the 0.60- to 0.80-eV spectral group arise from the EPR G-15 center. Stress data on a zero-phonon line at 0.97 eV associated with the 0.80- to 1.0-eV spectral group can be explained by a trigonal 111 (in brackets) defect. The divacancy is tentatively suggested as responsible for this luminescence spectra.