low-Z Targets Research Articles

Extending the range of the time projection Compton spectrometer to lower energy

The Time Projection Compton Spectrometer (TPCS) has previously been used with a 12.7- mu m-thick polypropylene target to measure the time-integrated energy spectrum of intense flash X-ray sources. In the present work, the authors investigated the application of the TPCS to photoelectron spectrometry, substituting a 2- mu m thick gold target to improve device sensitivity to low-energy photons. Experiments with a gold target, a polypropylene target, and no target (background) were done at the Saturn pulsed X-ray source for various endpoint bremsstrahlung up to about 2 MV. Response functions were calculated using the TIGERP electron/photon Monte Carlo code. The signal from gold is larger and persists longer, as expected. The spectrum unfolded from the gold signals has problems, however. Alternative approaches to the unfold problem that combine data from both targets may prove more successful. If so, a future spectrometer could be designed to incorporate both low-Z and high-Z targets in a single instrument. >

Measurements of backscattered light from the interaction of 0.35 μm laser light with high-Z targets

Backscattered light at wavelengths near the incident laser frequency of 0.35 μm irradiated 0.4 μm thick gold targets has been spectrally and temporally resolved. At the start of the laser pulse, the wavelength of the backscattered light is longer than that of the incident laser, and the wavelength subsequently increases with time. The observed scaling of the backscatter level with the incident laser intensity, the bandwidth of the backscatter spectrum, and the absolute value of the backscatter flux, are adequately described by absolute growth of the stimulated Brillouin scatter instability from thermal noise, provided that realistic hot spots in the incident laser profile are taken into account. The differences between backscattered light from high-Z and low-Z targets are discussed.

Backscattered light near the incident laser wavelength from 0.35 μm irradiated long scale length plasmas

Backscattered light at wavelengths near the incident laser wavelength of 0.35 μm irradiated thin targets has been spectrally and temporally resolved. Low-Z targets show significant backscattered signals even after the foils burn through when the density gradient scale length is approximately 1000 times the laser wavelength. The results show the importance of considering seeding mechanisms in the plasma.

Theoretical studies on electron and radiation preheatings

In order to enhance the coupling efficiency, a low Z ablator is generally used for ICF targets. The ablator thickness is appropriately chosen so it is burned out by the end of a laser pulse. Then all of the implosion kinetic energy is contained in the DT fuel. However, a small amount of preheating degrades the compression in a hollow shell, DT fueled target implosion. In this paper, we investigate the preheating level of the fuel shell by Fokker–Planck simulations of the electron heat transport.From the analysis, it is found that a thick surface layer of a laser irradiated low-Z target is preheated by Maxwellian tail electrons which have a long mean free path. Hence, we propose that the target be precompressed by a tailored pulse, in order to increase the shell ρΔR at the laser peak.

Angular and energy distributions of δ-rays ejected from low-z molecular targets by incident protons and α particles

The double differential cross-section mixed treatment (DDCS-MT), which has been developed in order to calculate the relatively energetic δ-rays measured along 4 to 12 MeV α particle tracks recorded in ionographic emulsions of various compositions, reproduces quite rigorously experimental DDCS for medium energy protons and α particles crossing not only helium and argon, but also molecular gases like water vapour, hydrocarbons, etc. In the latter case, a special procedure is required to describe the molecular target in terms of atomic quantities, notably by taking into account the binding energies of the molecular orbitals, as well as the breaking up of the molecular orbitals in terms of atomic orbitals.

Effect of target metarial on absorption measurements at short laser wavelengths

We measure the effect of target material on absorption processes for short laser wavelengths ( λ = 0.53 μm and λ = 0.26 μm). The results show some increase of absorption with increasing Z-target at 0.53 μm, which can be attributed to the increase of inverse bremsstrahlung efficiency by analysing the data with a 1-D hydrodynamics code. At 0.26 μm, absorption rates are very high, ∽ (90 ± 5) %, even for low Z-targets at fluxes ∽5 × 10 14 W cm 2 .

Search for the density effect in inner-shell lonization by ultra relativistic electron impact

We have extended the measurements ofK-andL-shell ionization cross sections by electron impact into the ultra relativistic energy region, 0.9≦E≦2.0 GeV, in order to search for a saturation of the cross section. This phenomenon, which is due to the polarization of the target medium, is called density effect. It is predicted to occur at several hundred MeV impact energy and preferentially for lowZ target elements. Theoretical calculations are presented, based on the one-photon-exchange approximation. The absolute measurements of theK-andL-shell cross sections for Ni(K), Cu(K), Ag(K, L) and Au(L) performed at the 2.5 GeV electron synchrotron of the Bonn University, however, exhibit that the cross sections show no saturation but are still increasing. Furthermore, from theK X-ray yields, obtained at 0.9 and 2.0 GeV by bombarding the lowZ elements S, Ca, Mn, Ni and Ge, we obtain for the corresponding cross section ratio σ K (2GeV)/σ K (0.9GeV)=1.08±0.01 on the average. TheK X-ray yield of a composite Ca — Mn target amounts to $$[\sigma _K (Ca;2.0GeV)/\sigma _K (Mn;2.0GeV)]/[\sigma _K (Ca;0.9GeV/\sigma _K (Mn;0.9GeV)] = 0.99 \pm 0.02.$$ . All three results are in disagreement with theory. This severe discrepancy is discussed but the origin for it is not yet understood.

Energetic-distribution measurements of the atomic-field pair production process at low energy by using a streamer chamber

The differential cross-section dσ/dE+ for the atomic-field pair production in uranium (Z=92) and barium (Z=56) targets is studied. The incident-photon energy isk=2.62 MeV. The results concern the upper half-part (0.5<Y=(E+−1)/(k−2)<1.0) of the e+ energetic spectrum. The comparison with the theoretical results of Overbo, Mork, Olsen confirms the Coulomb correction values and the asymmetrical shape of the e+ spectrum provided for in calculations, but the measurements are not accurate enough for a comparison with the screened theory of Tseng, Pratt. From the experimental point of view, the pair events are registered in a streamer chamber; insulating UO2 and BaF2 targets are located inside, a monitoring target gives the electronical dead-time correction. A further experiment is performed with a low-Z target (Z≃5, Coulomb correction negligible) to make up the electronical and kinematical detection coefficients. About 8000 pair events have been registered on the whole; 3200 measured in the experimental targets.

Production of x rays beyond theK-series limit in gaseous and solid targets

X rays of energy beyond the K-series limit have been detected during the bombardment of various low-Z, solid and gaseous targets by C/sup +/, N/sup +/, and O/sup +/ ions of energy 0.75 to 1.9 MeV. K..cap alpha cap alpha.. x rays, most likely from the transition (1s/sup -2/2p/sup -1/)..-->..(2s/sup -1/2p/sup -2/), are emitted by both collision partners in gaseous targets. The low-resolution energy measurements are in good agreement with published calculations. The relative yield from the heavier collision partner is larger than simple vacancy-sharing models would indicate, but electron screening may account for the excess. Molecular-orbital (MO) continuum radiation completely obscures the Kaa lines from thin solid carbon targets. The exponential shape of the MO spectrum beyond the united-atom limit is in reasonable agreement with collision-broadened spontaneous emission.

Evidence of Anomalous Interaction between a Relativistic Electron Beam and a Solid Target

The dynamic behavior of the plasma produced from a thin foil target irradiated by a focused intense electron beam is observed optically by the shadowgraphy and interferometry. Evidence of anomalous stopping of intense electron beam on the surface of the low-Z target is shown experimentally.

Anomalous energy dissipation of intense focused electron beam in low atomic-number target

An energy distribution of transmitted electrons through thin-foil targets irradiated by a tightly pinched relativistic electron beam has been measured by a multichannel electrostatic particle energy analyzer. The experimental result indicates the existence of an anomalous energy dissipation of beam electron in low-z target.

Chemical effects in negative muon capture in some ionic and covalent solids and ionic aqueous solutions

Muonic x-ray spectra have been measured for target sequences including graphite, diamond, their boron-nitride counterparts, three organic compounds, ice and water, eight oxides of metals, alkali halides in aqueous solution and alkali chlorides in anhydrous crystalline form, and calcium and titanium hydrides. Comparisons are made of Lyman-series intensity patterns, and relative muon capture probabilities in the constituent elements of compounds are compared with ''Z-law'' relation. The Lyman intensities of the C and N in the low-Z targets are found to exhibit two kinds of pattern, one with relatively weak higher-series members characterizing the diamond and graphite structures, and one with stronger higher-series members characterizing the other (hydrogen-containing) compounds. There is some evidence that the latter intensity behavior applies to the aqueous ions as well. For the oxide sequence, increasing covalency appears to correlate with increasing muon capturing ability of the metal element and with increasing intensity of the higher Lyman members for both the metal element and the oxygen. Effective relative muon-capture cross sections have been obtained for aqueous alkali and halide ions. (AIP)

Energy Straggling Parameters from Single Event Spectra

The variance of an energy straggling distribution for thin absorbers is determined by the cross section for energy loss in single collisions. The ratio, δ2, of this variance and the mean energy loss is independent of the thickness of the absorber; it is also numerically equal to the quotient of the second by the first moment of the single collision spectra. Measured doubly differential cross sections for electron emission by energetic charged particles from low-Z molecular targets provide a major part of the primary data required for calculating δ2. The relationship between δ2 and experimentally measurable cross sections for single collision energy loss is developed. Values of δ2 calculated from measured single collision cross sections are compared with values obtained from the free electron model and experiment.

Detection of Low Energy X Rays with Si(Li) Detectors

The continuing improvement in energy resolution of semiconductor detector X-ray spectrometers has led to interest in the use of these devices at energies less than 2 keV. This is an energy region of potential analytical interest since the K X rays of several elements of biological and chemical importance occur at these energies. However, the low X-ray fluorescence yield in low-Z elements, combined with the absorption of low-energy X rays due to the entry window of the vacuum chanber and that of the detector, have made work in this energy region impossible with the standard semiconductor detector X-ray spectrometers. In the present work, the X-ray absorption path has been reduced to a minimum to permit low-energy X ray studies of excitation, entry window thickness, detector linearity and resolution. Using electron beam excitation on low-Z targets, we have performed measurements of characteristic K X rays of elements down to and including carbon (277 eV).