Thick Tungsten Target Research Articles

Emission of Characteristic L and K Radiation from Thick Tungsten Targets

A method is proposed for calculating L and K photon emission from thick tungsten targets bombarded by electrons accelerated by potentials of 12–300 kV. Electron energy losses, electron backscatter losses, and photon attenuation in the target are included. Agreement with measured K emission is obtained using the Arthurs-Moiseiwitsch ionization cross sections and an expression of the form CZ(E0−k) 1−exp(−3k/EK)(k/E0)1/3 1−exp(−E0/EK)to describe the bremsstrahlung energy distribution. Satisfactory agreement with L-emission measurements is obtained using the Mott-Massey cross-section formula with constants BLi=4 ELi, bLI=0.25×1.6, bLII=0.25×2.75, and bLIII=0.25×4.2. Indirect radiation contributes 54–82% of the total K emission and 5–8% of the total L emission. The Webster-Clark empirical relation Ii=Ci(U0−1)ni agrees with calculation for U0<3, with CK=5.1×1011 photon/sec mA sr, nK=1.67, CL=2.6×1011 photon/sec mA sr, and nL=1.5.

X-Ray Spectral Distributions from Thick Tungsten Targets in the Energy Range 12 To 300 Kv

Bremsstrahlung emission from four x-ray tubes operating at 12 to 300 kV was measured. Spectral distributions are given in terms of absolute-photon and energy fluxes. Silicon and germanium semiconductors, sodium iodide scintillators, and a xenon proportional counter were used to measure the spectra. Detector distortions were corrected by assuming an undistorted spectrum, distorting the spectrum with a Monte Carlo computer program, and comparing the results with measurement. The assumed undistorted spectrum was revised until the Monte Carlo calculation gave satisfactory agreement with spectral measurements from all four types of detectors. The effects on the spectra of varying the tube potential, tube current, target aperture, and detector aperture were investigated. Corrections for the intervening material and the solid angle subtended by the detector were applied to obtain the absolute-photon and energy flux from the target. Total fluxes in the L lines, K lines, and continuum are given. X-ray-production efficiencies varied from 0.026% at 12 kV to 1% at 300 kV. The constant of proportionality varied in the range (0.23 to 0.73) x 10-6 kV-1.

Bremsstrahlung and Photoneutrons from Thick Tungsten and Tantalum Targets

Monte Carlo calculations have been made of electron-photon cascades in thick tungsten targets bombarded by electrons with energies up to 60 MeV. The following information has been obtained: (1) the bremsstrahlung efficiency, (2) the angular distribution of the emitted bremsstrahlung intensity, (3) the spectra of the bremsstrahlung emitted in various directions, (4) the transmission of primary and secondary electrons through the target, (5) energy deposition as function of the depth in the target, (6) the differential photon track length distribution inside the target, and (7) the yield of photoneutrons. The paper also includes various comparisons with experimental data.

Differential Cross-Section Measurements of Thin-Target Bremsstrahlung Produced by 2.7- to 9.7-Mev Electrons

The electron beam removed from a 50-Mev betatron and a total absorption scintillation spectrometer containing a sodium iodide crystal 5 in. in diameter and 9 in. long were used for the measurement of brems-strahlung cross sections that are differential in photon energy and angle. Thin targets of beryllium, aluminum, and gold were bombarded by electrons with kinetic energies of 2.72, 4.54, and 9.66 Mev. The bremsstrahlung spectra from thick tungsten targets were also studied.

Thick Target Bremsstrahlung Spectra for 1.00-, 1.25-, and 1.40-Mev Electrons

The spectrum of radiation produced by 1.0-, 1.25-, and 1.40-Mev electrons incident on a thick tungsten target was measured at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} with the incident beam by a method involving the magnetic analysis of Compton electrons. The effects of electron scattering and energy loss in the target preclude any simple interpretation of this data to yield a differential bremsstrahlung cross section. However, an estimate of the spectra to be expected at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} was obtained by combining the Sauter expression for the bremsstrahlung cross section with the available information on electron scatter and energy loss in the target and backscatter from the target. The reliability of the estimate is limited because the Sauter formula was calculated by using the Born approximation, the electron scattering calculations are applicable to an infinite medium only, and the backscatter was estimated empirically from Bothe's experimental data which were obtained with lower energy electrons (370 kev). Furthermore electron energy straggling was neglected. Nevertheless, the predicted spectral shapes at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} and the relative intensities at these two angles are in qualitative agreement with the measured values. The absolute magnitudes of the measured intensities at both angles are about a factor of two greater than the predicted values.

Electron polarisation

Dirac’s modified wave equation which successfully accounts for many of the phenomena interpreted as due to the spin of orbital electrons, also predicts that the free electron should have a spin. On this basis each electron wave is characterised by a definite direction other than that of propagation, and an electron beam should be capable of exhibiting polarisation. A method for the production and detection of a polarised electron beam is suggested by the double scattering experiments for the production and detection of polarised X-rays, especially in view of the similarity between diffraction phenomena for electrons and electromagnetic waves. Double scattering experiments have been performed by a number of investi­gators and it has been well established that no observable effect is found with electrons having energies in the neighbourhood of 100 volts.* With very much higher energies, however, an asymmetry in the intensity distribution of the secondary scattering appears to exist, although the evidence is somewhat contradictory and incomplete. This paper gives an account of a double scattering experiment in which electrons were successively reflected at approxi­mately 90° from thick polycrystalline tungsten targets. The results extend the region in which the asymmetry in the secondary scattering is known to be less than 1 per cent, to electron energies of 10 kilovolts.