Medium Energy Range Research Articles

Simulation and experimental results for the detection of conversion electrons with gas proportional scintillation counters

The application of gas proportional scintillation counters (GPSC) to the detection and identification of conversion electrons in the medium energy range is investigated. Experimental and Monte Carlo results are presented for the response of a xenon GPSC, filled at atmospheric pressure, to the decay of a 109 Cd source. This source emits 88.0 keV γ-rays, e L=84.6 keV and e K=62.5 keV conversion electrons, as well as fluorescence X-rays and Auger electrons. Good agreement is found between the measured and the calculated energy spectra. The response to higher-energy electrons is investigated by Monte Carlo simulation, by considering a hypothetical GPSC filled with xenon at 10 atm and doped with the 133 m Xe metastable isotope. The calculated energy spectra for the absorption of the 133 m Xe 233.2 keV γ-rays, e K=198.6 keV and e L=228.4 keV conversion electrons, as well as fluorescence X-rays and Auger electrons, are presented and discussed.

Direct capture of protons in the medium-energy range

Direct capture of protons to the bound states in 12C and 13N is treated using a previously suggested consistent approach to the capture reactions. In this approach, the initial state is represented by an optical model wavefunction and the final state by a single-particle wavefunction bound to the nuclear potential. The influence of the complicated configurations involved in the process is taken into account by employing a consistency requirement for the electromagnetic transition operator. The approach is tested against the capture of medium-energy protons by the light nuclei, where the direct process strongly dominates. The quality of the results obtained proves the validity of the consistency principle even at medium energies.

Surface-Induced Dissociation of Small Carbon Cluster Negative Ions (Cn-, n = 5−12): Correlation between the Dissociation Patterns and Stability of Fragment Ion−Neutral Pairs

We report detailed study of surface-induced dissociation (SID) of small carbon cluster negative ions (Cn-, n = 5−12) in collision with Si substrates. Fragment mass spectra have been observed in the collision energy range of 0−10 eV/atom. Neutral C3 loss occurred at collision energies lower than 2 eV/ atom. In the medium energy range (2−6 eV/atom), a marked even−odd difference in the dissociation has been observed: even fragment ions were predominantly generated from incident odd clusters, whereas both even and odd fragment ions from incident even clusters. These results are qualitatively in good agreement with the formation energies of fragment ion−neutral pairs calculated using the experimentally determined binding energies of ions and neutrals. At elevated collision energies (6−10 eV /atom), further fragmentation and electron detachment from fragment ions occurred and smaller cluster ions with even n were preferentially generated.

Correction factors for the NMi free-air ionization chamber for medium-energy x-rays calculated with the Monte Carlo method

A new method is described for the determination of x-ray quality dependent correction factors for free-air ionization chambers. The method is based on weighting correction factors for mono-energetic photons, which are calculated using the Monte Carlo method, with measured air kerma spectra. With this method, correction factors for electron loss, scatter inside the chamber and transmission through the diaphragm and front wall have been calculated for the NMi free-air chamber for medium-energy x-rays for a wide range of x-ray qualities in use at NMi. The newly obtained correction factors were compared with the values in use at present, which are based on interpolation of experimental data for a specific set of x-ray qualities. For x-ray qualities which are similar to this specific set, the agreement between the correction factors determined with the new method and those based on the experimental data is better than 0.1%, except for heavily filtered x-rays generated at 250 kV. For x-ray qualities dissimilar to the specific set, differences up to 0.4% exist, which can be explained by uncertainties in the interpolation procedure of the experimental data. Since the new method does not depend on experimental data for a specific set of x-ray qualities, the new method allows for a more flexible use of the free-air chamber as a primary standard for air kerma for any x-ray quality in the medium-energy x-ray range.

An overview of the bipartition model for charged particle transport

This paper systematically summarizes the progress to date and developments of the bipartition model, including its basic ideas and concepts, main mathematical formulae and numerical methods, and its relevant applications. The model has been extended from a transport theory for electrons in the medium energy range to a unified and accurate transport theory of charged particles, covering ion transport, atomic cascade collisions, particle emission from a surface including sputtering and the penetration of electrons in the radiation therapy energy range. The thirty figures in total give an extensive comparison between the results given by using the bipartition model and the data taken from other theories and experimental measurements, which shows that the bipartition model is an accurate, powerful and unified transport theory of charged particles. Some research being done using the bipartition model is mentioned briefly at the end of this paper.

Electron beam facility in polymer research: radiation induced functionalization of polytetrafluoroethylene

The main field of electron irradiation processing applications is polymeric material. A universal electron beam irradiation facility for research and service in the institute of polymer research was demonstrated. The industrial electron beam accelerator works in the medium energy range from 0.6 to 1.5 MeV. One of our research activities using the electron beam accelerator is the radiation-induced functionalization of polytetrafluoroethylene (PTFE). The changes in chemical structure of PTFE irradiated in ammonia, air, and vacuum were investigated by infrared spectrometry.

Crystallographic azimuthal-angle dependence of the neutral fraction of hydrogen emerging from GaAs (1 1 0)

Emission-angle dependence of the neutral fraction of hydrogen emerging from GaAs (1 1 0) has been measured in a medium energy range between 50 and 450 keV. The angular dependences of the neutral fraction show not only sharp peaks but also broad minima. The sharp peaks are located at blocking directions and can be attributable to the channeling of the scattered beam. The broad minima observed in the azimuthal dependence can be attributed to a surface structural effect as previously found in the case of NiSi 2 (1 1 1). A numerical calculation including the impact-parameter-dependent charge-exchange probabilities can reproduce the main features of the experimental angular dependences of the neutral fraction of hydrogen emerging from GaAs (1 1 0) surface to the random directions.

Influence of Si doping level on the Raman and IR reflectivity spectra and optical absorption spectrum of GaN

The optical absorption α( h ν) and Raman and Infra Red (IR) spectra of Si doped GaN layers deposited on sapphire through buffer layers have been recorded for electron concentrations from 5×10 17 to 5×10 19 cm −3. The α( h ν) values deduced from photothermal deflection spectroscopy (0.5–3.5 eV) and IR absorption (0.15–0.5 eV) vary between 50 and 10 4 cm −1 showing doping dependant free electron absorption at low energy, doping independant band gap at high energy, and slowly doping dependant defect absorption in the medium energy range. In our micro Raman geometry, maxima appear or can be deduced near the frequency expected for either the A 1(LO −) or the A 1(LO +) modes split from the A 1(LO) mode by plasmon phonon interaction. There is a large systematic evolution in the expected way for the IR reflectivity.

Capacitive Radio Frequency Discharge Plasma Containing Negative Ions

The parallel-plate RF discharge in oxygen has been investigated in the pressure between 10 -3 and 1 Torr. The discharge mechanism is considered by comparing the electron energy distribution f ( E ) in O 2 and N 2 discharges. In O 2 , the low energy part of f ( E ) becomes deficient; a broad peak of thermal electrons appears to be shifted into the medium energy range; and the population of a higher energy group of electrons coming from the cathode sheath is less than that of N 2 discharge. These are related to the negative ion formation in O 2 . Negative ions in the discharge in O 2 are measured not only by the probe but also by using the laser photodetachment technique. The result of measurements have revealed that optimum RF power and pressure to make the negative ion density maximum exist while positive ions increases with the RF power.

A low emittance cw-electron injector for the ELBE accelerator

For applications in nuclear and radiation physics there is an increasing demand for electron beams with superior brightness in the medium energy range. This work presents the injector design for the superconducting accelerator ELBE [1]. The injector consists of a thermionic rf-gun, working at a frequency of 1.3 GHz in cw-mode. The superconducting section is based on the technology established at TESLA [2], modified for cw operation. The accelerator will deliver a 20 MeV beam with an rms energy spread of 10 keV and an average current of up to 1 mA. The normalised transverse emittance remains 1 π mm mrad. The arrangement and operating parameters of the accelerator components were settled by numerical optimization considering longitudinal beam dynamics. The results led to a nonlinear bunching scheme, which compensates the high longitudinal emittance from the thermionic rf-gun.

Two-body density contribution to nucleus-nucleus total reaction cross section

Using the effective profile expansion for the Glauber elastic S-matrix for nucleus-nucleus scattering in which the leading term is the generally employed optical limit model and the other terms depend successively upon two-body, three-body, etc. densities of the colliding nuclei, we calculate total reaction cross sections for a few heavy-ion systems in the medium energy range, up to the two-body density term in the expansion. We find that the effect of the two-body density term on the calculated cross section depends considerably upon the incident energy per nucleon and, in general, it is not insignificant in some energy regions.

Activation of space-borne bismuth germanate /spl gamma/-ray detectors

Accurate knowledge of the sources of detector background is essential in /spl gamma/-ray astronomy for correct data interpretation, and for optimising detector design and operation. In the low-to medium-energy range, a major source of background arises from radioactivity that is induced in the detector medium itself by nuclear spallation and neutron capture. Results are presented from the Shuttle Activation Monitor flown in the Space Shuttle middeck area on two missions during August 1989 and September 1991. This experiment has been valuable in the study of induced radioactivity in BGO scintillators resulting from bombardment by cosmic-rays, trapped protons, and their secondaries created by interactions in a heavy spacecraft. Data from mono-energetic proton irradiation experiments have allowed investigation of activation effects in BGO for less complicated irradiation conditions, and the results from these experiments are also presented. Detailed simulations have been performed to determine BGO detector activation and response, and these take into consideration the effects of shielding by the spacecraft structure. The results compare very well to the experimental data, allowing features in the measured spectra to be identified. They also highlight the importance of shielding both in reducing activation by trapped protons, and enhancing cosmic-ray induced activation due to secondaries.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarisation effects in electron transfer reactions with laser excited Na(3P) at medium energies

We report new experimental data for the investigation of the role of electronic orbital alignment and orientation in charge transfer processes, in the medium energy range where the collision velocityv c and the velocity of the active electronv e are of the same order of magnitude. The results obtained for the H 2 + -Na(3p) and He+-Na(3p) collisions are discussed in comparison with the experimental and theoretical findings obtained for the H+-Na(3p) system. Recent time-of-flight measurements for charge transfer in Li+-Na(3s and 3p) collisions are also presented.

Beam position monitor electronics at the cooler synchrotron COSY Julich

COSY Julich, presently in the commissioning phase, accelerates/stores protons in the medium energy range (40 MeV/spl divide/2.5 GeV). The non-beam disturbing diagnostic methods and devices are important tools to optimise the closed orbit, as well as to measure the position and the dynamic behaviour of the beam for alignment and subsequent regular operation. The paper describes the beam position monitor electronics. The monitor is of particular importance among the applied beam diagnostic devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron and positron backscattering in the medium-energy range.

Experimental data for positron and electron backscattering from thick solids are presented as a function of incident and outgoing angles and energies, and of target atomic number. A full description of the experimental features and limitations is presented. The data are compared with Monte Carlo calculated backscattering distributions, and in all cases the agreement is extremely good. Specularlike scattering for positrons incident at oblique angles is observed and explained in terms of the strength of the elastic-scattering interaction.

EXOSAT X-ray spectra of quasars

view Abstract Citations (93) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS EXOSAT X-Ray Spectra of Quasars Comastri, Andrea ; Setti, Giancarlo ; Zamorani, Giovanni ; Elvis, Martin ; Giommi, Paolo ; Wilkes, Belinda J. ; McDowell, Jonathan C. Abstract We have measured the X-ray spectra in the 0.1-10 keV energy range for 17 AGNs (mostly PG quasars) using the EXOSAT Low and Medium Energy arrays. The sample has been selected on the basis of soft X-ray flux and includes all the AGNs in the 1981 Zamorani et al. and 1986 Tananbaum et al. publications with an Einstein flux greater than 6x10^-12^ ergs cm^-2^ s^-1^. For each object we have determined the best-fit spectral parameters in the medium-energy range (2-10 keV), i.e., the power-law slope, the normalization, and the absorbing column density. In this energy range, all the spectra are well described by a single power law model, with a wide distribution of energy spectral indices in the range 0.4 < α < 1.3. The average spectral index is <α> = 0.89 +/- 0.06, but most of the objects are distributed around α ~ 1.0. The overall distribution of spectral indices is significantly displaced toward steeper spectra, when compared to that found in the same energy band by Turner & Pounds, for a large sample of hard X-ray-selected Seyfert galaxies. This difference can be understood in terms of the different selection criteria of the two samples (soft versus hard X-ray selection), and it is concluded that the true distribution of hard X-ray spectral indices in AGNs is probably wider than previously suggested. Inclusion of lower energy data (0.1-2.0 keV) shows the presence of significant soft excess in six out of 17 cases. This soft excess emission can be modeled by either a steep power law with α ranging from 2.7 to 4.2 or the high-energy tail of a blackbody with temperatures in the range 40-80 eV. The intersection of this soft excess emission with the hard X-ray power law is in the energy interval 0.3-0.75 keV, in the source rest frame. A correlation analysis performed on our data set does not reveal any correlation between the medium-energy spectral properties and other physical parameters of the sources. However, fits performed in the energy range 0.1-1.0 keV (comparable with the Einstein IPC energy range) show a significant correlation between radio-loudness and X-ray spectral properties, with the radio-loud objects having on average, flatter slopes than the radio-quiet ones, in agreement with the findings of Wilkes & Elvis. The fact that the same correlation is not seen in our data at higher energy seems to be explained by a correlation between presence and strength of soft excess and "radio-quietness": the objects with significant soft excess emission tend to be the most radio-quiet quasars in our sample. Publication: The Astrophysical Journal Pub Date: January 1992 DOI: 10.1086/170851 Bibcode: 1992ApJ...384...62C Keywords: Exosat Satellite; Quasars; X Ray Spectra; Galactic Nuclei; Seyfert Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: QUASARS: GENERAL; GALAXIES: SEYFERT; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (17) NED (17)

Few body physics at new proton cooler rings

With the new accelerator techniques of phase space cooling it became possible to do experiments with high quality beams on internal targets and on external targets after superslow extraction. Three cooler rings are working or under construction in Bloomington, Uppsala and Jülich which will make possible a new generation of few body experiments in the medium energy range.

Recent developments in ion implantation accelerators

Ion implantation technology plays an important role in different fields of materials modification and has become a well established industrial technique. In semiconductor device fabrication more than 1000 ion implanters operating in the low- and medium-energy range (≤, 400 keV) are working worldwide. There is also an increasing interest in the improvement of the surface properties of metals, ceramics, and polymers by means of ion implantation. For some of the newer applications the required penetration depth is large, up to several μm, corresponding to an ion beam energy of several MeV. Furthermore, certain processes involve implant doses up to 10 18 ions/cm 2, which implies that high currents are needed as well. Both requirements — high ion current and high energy — are difficult to fulfill simultaneously by the commonly used static machines. Modem rf linacs combine strong radial focusing with efficient acceleration, which allows for high currents and high energy. In this paper recent developments in this field will be discussed.

Electron scattering by atomic chains: Multiple-scattering effects.

Multiple-scattering effects of electrons traveling along atomic chains are shown to be important and useful for surface structure determination, especially in the medium-energy range (200\char21{}1000 eV). This applies to techniques based on diffraction of elastic electrons, such as medium-energy electron diffraction, as well as to techniques based on the angle-resolved detection of ``secondary'' electrons such as photoelectrons, Auger electrons, and other electrons. Two new methods for computing multiple-scattering amplitudes for chains are derived and used to show how multiple scattering along chains of atoms produces focusing, defocusing, amplification, and layer-dependent enhancements that can be put to effective use in structure determination. Strong forward-focusing peaks along internuclear axes are dominant under many conditions: this is a single-scattering effect. However, multiple scattering can totally defocus such forward-focused peaks, while other interference effects produce additional scattering peaks that do not have the direction of an internuclear axis. In addition, it follows that surface composition analysis by conventional Auger-electron spectroscopy can be quite sensitive to the incident direction of the electron beam used to produce Auger emission.

Transport of low- and medium-energy electron and ion beams in seawater and its vapors.

A general theory of stopping power for electrons and ions in a target medium (gas, liquid, or thin solid foil) containing neutral as well as positive and negative ions is developed. The approach is quite accurate in the low- and medium-energy range (less than 1 MeV), where the Bethe stopping-power cross section is inaccurate. The energy transfer during individual collisions is treated by the binary-encounter approximation. The theory is applied to determine the stopping power of seawater, whose major components are H2O molecules and Na(+) and Cl(-) ions. The stopping-power cross sections for low- and medium-energy protons in water show excellent agreement with existing measurements.