Thermal Capture Cross Section Research Articles

Neutron detection with mercuric iodide

Mercuric iodide is a high-density high-Z semiconducting material useful for gamma ray detection. This makes it convertible to a thermal neutron detector by covering it with a boron-rich material and detecting the 478-keV gamma rays resulting from the /sup 10/B(n,/spl alpha/)/sup 7/Li/sup */ reaction. However, the 374 barn thermal capture cross section of /sup nat/Hg, makes the detector itself an attractive absorber, and this has been exploited previously. Since previous work indicates that there are no low-energy gamma rays emitted in coincidence with the 368-keV capture gamma from the dominant /sup 199/Hg(n,/spl gamma/)/sup 200/Hg reaction, only the 368-keV capture gamma is seen with any efficiency with a relatively thin (few millimeter) detector. In this paper, we report measurements of neutrons via capture reactions in a bare mercuric iodide crystal and a crystal covered in /sup 10/B-loaded epoxy. The covered detector is an improvement over the bare detector because the presence of both the 478and 368-keV gamma rays removes the ambiguity associated with the observation of only one of them. Pulse height spectra, obtained with and without lead and cadmium absorbers, showed the expected gamma rays and demonstrated that they were caused by neutrons.

Light-induced degradation in a-Si:H and its relation to defect creation

Abstract Light-induced degradation of the photoconductivity of hydrogenated amorphous silicon, a-Si:H, called the Staebler–Wronski effect (SWE) is caused by the creation of defects that act as recombination centers. The creation efficiency as well as its kinetics is essentially the same between 4 and 300 K despite drastic changes in the recombination processes. Defects are created with the same spin signature but with greatly different thermal stability and electron capture cross sections. There appears to be a strong link between the latter two properties. The creation-annealing hystereses of free carrier lifetime and defect concentration are explained. The SWE is reduced in high electric fields at 4.2 K suggesting the possible importance of non-radiative geminate recombinations. The validity of accepted rate equations for the SWE excited by cw and laser pulses is questioned. Evidence for long-lived (∼10 μs) precursors of SWE defect creation is presented. Problems with existing SWE models and understanding of defect capture properties are formulated.

Neutron Capture and Transmission Measurements and Resonance Parameter Analysis of Samarium

The purpose of the present work is to measure the neutron cross sections of samarium accurately. The most significant isotope is 149Sm, which has a large neutron absorption cross section at thermal energies and is a 235U fission product with a 1% yield. Its cross sections are thus of concern to reactor neutronics.Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute (RPI) LINAC facility using metallic and liquid Sm samples. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission total cross-section measurements were performed at 15- and 25-m flight stations with 6Li glass scintillation detectors. Resonance parameters were determined by a combined analysis of six experiments (three capture and three transmission) using the multilevel R-matrix Bayesian code SAMMY version M2.The significant features of this work are as follows. Dilute samples of samarium nitrate in deuterated water (D2O) were prepared to measure the strong resonances at 0.1 and 8 eV without saturation. Disk-shaped spectroscopic quartz cells were obtained with parallel inner surfaces to provide a uniform thickness of solution. The diluent feature of the SAMMY program was used to analyze these data. The SAMMY program also includes multiple-scattering corrections to capture yield data and resolution functions specific to the RPI facility.Resonance parameters for all stable isotopes of samarium were deduced for all resonances up to 30 eV. Thermal capture cross-section and capture resonance integral (RI) calculations were made using the resultant resonance parameters and were compared to results obtained using resonance parameters from ENDF/B-VI updated through release 3. Extending the definition of the capture RI to include the strong 0.1-eV resonance in 149Sm, present measurements agree within estimated uncertainties with ENDF/B-VI release 3. The thermal capture cross section was calculated from the present measurements of the resonance parameters and also agrees with ENDF within estimated uncertainties. The present measurements reduce the statistical uncertainties in resonance parameters compared to prior measurements.

PROMPT GAMMA RAYS FROM RADIATIVE CAPTURE OF THERMAL NEUTRONS BY ELEMENTS FROM HYDROGEN THROUGH ZINC

The energies and intensities of prompt gamma rays produced by the radiative capture of thermal neutrons have been compiled and evaluated for elements with atomic numbers Z=1–30. These elements are important for many applications, including well logging, planetary remote sensing, and other analytic and diagnostic applications. Experimental data for 70 isotopes from 1H to 68Zn were compiled and evaluated. Published thermal-capture cross sections were used to convert the isotopic gamma-ray intensities to elemental intensities. The evaluated energies and absolute intensities of these capture gamma rays are presented by element and a subset is sorted by gamma-ray energy. The literature has been covered to August 2000.

Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

Thermal admittance spectroscopy measurements at temperatures ranging from room temperature to 90 K are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structures. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back to back by a conduction channel formed by the GaN layer. The description explicitly includes the Mg-related acceptor level, with its temperature- and position-dependent incomplete occupation state, leading to a dynamic exchange with the valence band. It fully reproduces the variations with temperature of the capacitance–frequency and conductance over frequency curves, allowing to give for all temperature ranges the origin of the various contributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance–frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-related acceptor level governs the electrical behavior below 230 K. The existence of a second acceptor level with an activation energy of several tens of meV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical characteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the structure, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor level of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN junctions requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects.

Ultra-thin 242mAm fuel elements in nuclear reactors

There is a growing interest in using 242mAm as a nuclear fuel. The advantages of 242mAm as a nuclear fuel derive from the fact that 242mAm has the highest thermal fission cross section. The thermal capture cross section is relatively low and the number of neutrons per thermal fission is high. These nuclear properties make it possible to obtain nuclear criticality with ultra-thin fuel elements. The possibility of having ultra-thin fuel elements enables the use of these fission products directly, without the necessity of converting their energy to heat, as is done in conventional reactors. There are three options of using such highly energetic and highly ionized fission products. 1. Using the fission products themselves for ionic propulsion. 2. Using the fission products in an MHD generator, in order to obtain electricity directly. 3. Using the fission products to heat a gas up to a high temperature for propulsion purposes. In this work, we are not dealing with a specific reactor design, but only calculating the minimal fuel elements’ thickness and the energy of the fission products emerging from these fuel elements. It was found that it is possible to design a nuclear reactor with a fuel element of less than 1 μm of 242mAm. In such a fuel element, 90% of the fission products’ energy can escape.

The new prompt gamma-ray catalogue for PGAA

A new catalogue of subthermal neutron-induced prompt gamma rays has been created for 79 elements, from hydrogen to uranium (including fission), on the basis of recent measurements at the Budapest guided-neutron PGAA facility. New energy values have been measured using 35Cl neutron-capture gamma rays, while the gamma-ray production cross-sections have been determined with respect to the 1H thermal capture cross-section. The elemental data have been compared with thermal neutron-capture data for individual nuclides from the Evaluated Nuclear Structure Data File, ENSDF, hence isotope identifications could be made. The catalogue contains elemental spectra and a table with nearly 7000 gamma rays with relative intensity over 1% of the strongest line. The average accuracy is about 0.08 keV for energies and about 5% for cross-sections in the whole energy range, from about 40 keV to 11 MeV.

Deep-level transient spectroscopy in the depletion zone of boron-doped homoepitaxial diamond films

Abstract Investigation of deep levels in the depletion zone of boron-doped homoepitaxial diamond films is achieved from analysis of the transient responses of Schottky junctions. The active layer consists of a homoepitaxial film grown by microwave plasma decomposition of a very small percentage of B2H6 mixed with CH4 in hydrogen on synthetic Ib diamond substrates. Rectifying contacts are implemented with either aluminium, gold or erbium after suitable pre- and post-treatments. Reverse-current transients due to the thermal emission of holes from deep-level traps into the valence band are recorded and analysed with a Fourier transform deep-level transient spectroscopy (Fourier DLTS) procedure as a function of temperature up to 520°C. At each temperature, a thermal emission time constant is extracted. Arrhenius diagrams are deduced. A deep level at 1.40±0.07 eV above the valence band is detected in samples that have been submitted to a 800°C annealing in ultrahigh vacuum. Another deep level at 1.16±0.03 eV above the valence band exists only if an oxygen radio-frequency plasma treatment is applied to the sample before the Schottky contact deposition. Thermal capture cross-sections are also deduced.

Characterisation of deep level trap centres in 6H-SiC p-n junction diodes

Deep level transient spectroscopy (DLTS) measurements were performed on silicon carbide (6H-SiC) n +p −p + junction diodes in order to compare the electrical properties and quality of epitaxial layers in the implanted and the epitaxial emitter diodes, where the p −p + layers are the same. Four hole trap centres were detected on the implanted emitter diodes. Their thermal activation energy’s are 0.49, 0.6, 0.7 and 0.87 eV, respectively, referred to the valence band. The last three trap centres are also observed on the epitaxial emitter diodes. The origin of deep levels, with E a=0.7 and 0.87 eV, is still under investigation. The thermal activation energy and capture cross section (0.6 eV, 4.3×10 −15 cm 2) is in good agreement with values reported for the boron-related D-centre. The trap centre with E a=0.49 eV is associated to the ion-implantation process of the n + layer. Double deep level transient spectroscopy measurements (DDLTS) have been performed to accurately profile this last defect through the depletion region. Its trap concentration N T( x) as a function of the depletion region width indicates that this defect is located near the n +/p − junction.

Neutron capture cross section of44Ti

The cross section for thermal neutron capture by radioactive ${}^{44}\mathrm{Ti}$ was measured to be ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}=1.1\ifmmode\pm\else\textpm\fi{}0.2$ b. This result was shown to be in agreement with a direct capture calculation. Possible implications of the ${}^{44}\mathrm{Ti}(n,\ensuremath{\gamma})$ reaction in nucleosynthesis are discussed.

Impurity Levels in Sn-Doped GaSe Semiconductor

Deep electron trapping centers in tin-doped gallium selenide single crystals have been studied by Hall effect, space-charge-limited current and thermally stimulated current experiments. At room temperature the electrical properties are dominated by a donor center at about 0.52 eV below the conduction band. Moreover, two electron trapping centers have been detected at 0.36 and 0.53 eV and the corresponding thermal capture cross sections are evaluated. Finally, the possible nature of these centers is also discussed.

Computer controlled microwave transient photoconductivity for the nondestructive characterization of GaAs substrates

In this paper we describe a nondestructive, contactless and fast characterization method for GaAs high-resistivity wafer substrates. Our method can be referred to as computer-controlled microwave transient photoconductivity. This method is based on the microwave measuring at 36.4 GHz of transient photoconductivity exited by laser pulse. The automatic measurements and computer simulation of experimental data make it possible to separate the multiple-rate decay mechanisms of transient photoconductivity. Our setup enables measuring the relaxation times ranging from 10 −8 to 10 −4 s in the temperature range from 80 to 300 K. The temperature dependence analysis of the photoconductivity permits determination of the activation energy and thermal capture cross section in doped wafer substrates. To demonstrate the possibilities of the method, we present experimental data obtained on two GaAs substrates.

Metastable defects in as-grown and electron-irradiated

New metastable defects have been observed in Si-doped layers, grown by MOCVD and irradiated with 1 MeV electrons in the dose range at T = 300 K. The investigations have been carried out by DLTS after preliminary isochronous and isothermal annealing with applied reverse bias voltage and at U = 0. In as-grown AlGaAs the stable-state parameters with a thermal activation energy and electron capture cross section were identical to those of the E1 defect formed under irradiation of AlGaAs and identified as . The parameters of the metastable state were and . The kinetics of configurational transformations of the defect was first-order. The thermal activation energy and the pre-exponential factor were and for the defect transition and and for the reverse transformation. The metastable defect is supposed to include and, possibly, a donor impurity. In irradiated material a rise in the concentration of metastable defects has been observed. As in as-grown AlGaAs, the signature of the stable state was identical to that of the E1 defect, but the transformation mechanism was changed, which is probably connected with the formation of another pair . The value of may correspond in order of magnitude to an interstitial single-jump process.

Measurement of neutron capture on 48Ca at thermal and thermonuclear energies.

At the Karlsruhe pulsed 3.75 MV Van de Graaff accelerator the thermonuclear $^{48}\mathrm{Ca}$(n,\ensuremath{\gamma}${)}^{49}$Ca(8.72 min) cross section was measured by the fast cyclic activation technique via the 3084.4 keV \ensuremath{\gamma}-ray line of the $^{49}\mathrm{Ca}$ decay. Samples of ${\mathrm{CaCO}}_{3}$ enriched in $^{48}\mathrm{Ca}$ by 77.87% were irradiated between two gold foils which served as capture standards. The capture cross section was measured at the neutron energies 25, 151, 176, and 218 keV, respectively. Additionally, the thermal capture cross section was measured at the reactor BR1 in Mol, Belgium, via the prompt and decay \ensuremath{\gamma}-ray lines using the same target material. The $^{48}\mathrm{Ca}$(n,\ensuremath{\gamma}${)}^{49}$Ca cross section in the thermonuclear and thermal energy range has been calculated using the direct-capture model combined with folding potentials. The potential strengths are adjusted to the scattering length and the binding energies of the final states in $^{49}\mathrm{Ca}$. The small coherent elastic cross section of $^{48}\mathrm{Ca}$+n is explained through the nuclear Ramsauer effect. Spectroscopic factors of $^{49}\mathrm{Ca}$ have been extracted from the thermal capture cross section with better accuracy than from a recent (d,p) experiment. Within the uncertainties both results are in agreement. The nonresonant thermal and thermonuclear experimental data for this reaction can be reproduced using the direct-capture model. A possible interference with a resonant contribution is discussed. The neutron spectroscopic factors of $^{49}\mathrm{Ca}$ determined from shell-model calculations are compared with the values extracted from the experimental cross sections for $^{48}\mathrm{Ca}$(d,p${)}^{49}$Ca and $^{48}\mathrm{Ca}$(n,\ensuremath{\gamma}${)}^{49}$Ca. \textcopyright{} 1996 The American Physical Society.

Anomalous off-current mechanisms in N-channel poly-Si thin film transistors

The anomalous off-current (I off) in polysilicon thin film transistors (polysilicon TFTs) is one of the major problems preventing a wide use of these devices in active matrix liquid crystal displays. While previous investigations have focused on the temperature range above 300 K, in this study we have investigated the behaviour of I off over a wide range of temperatures, namely 180–400 K. The data have been analysed by combining 2D simulations and existing analytic models. By this approach we have identified a pure trap-to-band tunnelling mechanism in polysilicon TFTs and deduced, by a simple procedure, the physical constants. The temperature and bias dependence of the off-current has been explained quantitatively in terms of phonon-assisted tunnelling. The number of generating centres, the dominant trap energy and the thermal capture cross section are deduced from this analysis.

Temperature dependence of the capture cross section determined by DLTS on an MOS structure

An energy-resolved DLTS technique was used to determine the distributions of both interface trap density and thermal capture cross section along the bandgap of an MOS structure. The resulting experimental values are criticized and tested in comparison with those obtained from a conventional a.c. conductance technique applied on the same diode. In this respect, incomplete filling of traps has an important effect on the interface trap density distribution. Furthermore, experimental measurements indicate that the capture cross section strongly depends on temperature, which might be interpreted as being due to a multiphonon emission process.

Deep levels in indium selenide single crystals doped with iodine

Electron trapping levels in indium selenide single crystals doped with iodine have been investigated by deep-level-transient spectroscopy measurements. Two traps located at about 0.60 and 0.21 eV have been detected below the conduction band and the corresponding thermal capture cross sections have been evaluated. The first trap is present both in undoped and doped InSe crystals, whereas the second trap appears in doped samples.

154 Analysis of a possible cancer treatment based on the radioactive capture reactions provided by Sm, Gd, I-incorporated radiosensitizers

The boron neutron capture is a nuclear reaction which has been used in a suitable technique for cancer treatment. A radiosensitizer for this technique implies in a common amino acid with a boron atom incorporated A higher thermal capture cross section than one found for boron-l0 makes other special isotopes, such as samarium, gadolinium, or iodine which produces xenonium, good candidates to be also used for cancer therapy. However, in these cases, the radioactive capture (n, γ ) is the main reaction. The efficiency of the possible radiosensitizers carrying those elements needs to be evaluated through the dose deposition in the tumor region. A possible cancer treatment based on Sm, Gd, I-incorporated radiosensitizers is debated and compared with BNCT. The dose evaluation on simulated cases has been done. The perspective results on this technic show that low activity sources of neutrons can be satisfactorily used in order to produced a level of dose in the tumor region similar to the conventional radiotherapy however using high activity gamma sources.

Monte Carlo study of the statistics of electron capture by shallow donors in silicon at low temperatures.

A statistical study of electron capture by shallow donors in silicon without an electric field as a function of temperature and impurity concentration is presented. The Monte Carlo calculations (based on the cascade capture theory) of the times spent by carriers in the conduction band before falling into impurity centers (i.e., the capture times) show evidence of nonexponential capture kinetics behavior. This work has allowed us to interpret the slow time decay of the distributions of capture times as the capture time actually measured in experiments. We have achieved very good agreement between the capture cross sections calculated by our Monte Carlo simulation with the experimental ones and with a previous analytical model that provides asymptotic behaviors of the thermal capture cross section.

Three-Dimensional Benchmark Tests of ENDF/B-VI for Thermal Reactor Core

Critical experiments of several thermal reactor cores are analyzed with ENDF/B-VI to assess its applicability to light water reactor analyses. The benchmark tests chosen use the continuous energy Monte Carlo code MCNP with exact treatment of a three dimensional geometry, and a cross section library is prepared by using the NJOY91 code. Calculated keff's underestimate experimental values from 0.3 to 0.9%Δk, and differences between calculated and measured keff's linearly increase with 238U effective capture rate. With a view to investigating the differences, sensitivity analyses are performed by minor modification of the 238U capture cross section at resonance as well as thermal energy regions. As a result, calculated keff's agree well with experimental results by changing the 238U resonance parameters to those of Liou and Chrien below 120eV and decreasing the thermal capture cross section value by 2.3%. These modifications improve the keff's of the TRX-1 and TCA 3.00U cores by 0.8 and 0.3%Δk, respectively.