W.N. Lennard

Nonlinear response of Si detectors for low-Z ions

We report measurements of the relative mean pulse height produced by 1H, 4He, 7Li and 16O ions in Si surface barrier detectors. The data are anomalous in that the pulse height for different ions of the same energy (after window and nonionizing losses are subtracted) increases with atomic number, contrary to observations for fission fragments. A simple model invoking a stopping power dependence of the energy required to create an electron-hole pair is consistent with all experimental data, suggesting that the response of Si detectors is nonlinear with particle energy.

Equilibrium charge distributions of ion beams exiting carbon foils

Abstract An electrostatic analyzer with a scintillation particle detector has been used to measure equilibrium charge distributions for 21 projectiles, 5 ⩽ Z 1 ⩽ 26, exiting thin carbon foils at low velocity, ν ⪅ ν 0 . The mean charge and distribution widths exhibit oscillatory behaviour as a function of projectile atomic number. Existing semi-empirical estimates cannot account for our observations.

Electronic stopping values for low velocity ions (9 ≤ Z1, ≤ 92) in carbon targets

We have measured the specific energy loss, ΔE/ΔX, for 23 projectiles in carbon targets at low velocity, v < v0. We have used a Monte Carlo calculation to determine the elastic energy loss appropriate to the experimental conditions. The electronic stopping values derived from our measurements show a marked oscillation in Z1, the projectile atomic number, and depend on the target thickness. Our data are compared to other measurements in the same velocity regime.

Energy-angle distribution measurements for 0.8v020Ne and 209Bi ions penetrating thin carbon foils

Energy-angle distributions have been measured for 0.8v0, (v0 = Bohr velocity) Ne and Bi ions penetrating through carbon foils. Comparing the results with a Monte Carlo computer simulation that included an angle dependence only for the elastic collisions, we have observed for Ne projectiles an angle-dependent inelastic loss which, for small angles, is much larger than the elastic contribution in the case of thin foils. In the case of Bi, the energy loss distribution is dominated by elastic collisions. The calculations of Meyer, Klein and Wedell, and Ellmer and Wedell cannot describe the experimental results. The multiple scattering distributions are in agreement with both analytical and Monte Carlo calculations.

Dependence of specific energy loss on foil thickness

Abstract We report on extensive measurements to determine the target thickness dependence of heavy ion energy loss, ( d E d χ ), for the Ne → C system. For ions transmitted with zero net deflection at velocities 0.75 ≤ υ υ 0 ≤ 1.05 , a significant dependence of d E d χ on foil thickness has been observed which, parametrized through the Meyer-Klein-Wedell theory, identifies an inelastic component in the energy loss for scattered particles. A discussion is given of some of the precautions that have proved necessary in applying time-of-flight techniques to these energy loss measurements.

Time-of-flight system for slow heavy ions

Abstract We have constructed a time-of-flight spectrometer which employs cylindrically symmetric elements to measure the velocity of slow heavy ions. Emphasis is placed n the efficiency of the microchannel plate detectors, and a detailed discussion of the absolute time calibration is given. The system has been used to measure energy losses for heavy ions and a comparison is made of the results with those obtained with a silicon detector.

Energy loss and energy loss straggling for heavy ions

Abstract Energy loss distributions are reported for low velocity (υ ∼ 0.8 υ0) ions 9 ⩽ Z1 ⩽ 20 after passage through thin C and Al targets. For carbon targets, the energy loss straggling is enhanced for Z1-values near the nodal point of the Z1-oscillation in stopping. A comparison of these data with ion-induced Auger electron yields for the same collision systems suggests a contribution from electron promotion. No such enhancement is observed for Al targets.

Problems of interpreting energy loss data for non-zero emergent angles

Abstract The energy-angle distribution has been measured for heavy projectiles (9 ⩽ Z 1 ⩽ 92) incident on thin carbon foils in the velocity region 0.4 ⩽ ν/ν 0 ⩽ 1 (ν 0 = e 2 / h ) . Using 20 Ne data as an example, we report results that demonstrate that the thickness dependence of the specific energy loss depends strongly on the emergent angle of the ions. This observation resolves apparent discrepancies in the literature.

Small angle energy loss measurements for 1H and 4He in Carbon

We have measured the increase in energy loss as a function of emergent angle for 1H and 4He ions traversing carbon foils. Our observations for energies E < 1 MeV are in agreement with theoretical predictions using the local density approximation for the impact parameter dependent inelastic energy loss.

Low Energy Stopping Powers Determined by Time of Flight Techniques

Stopping powers have been measured for all ions of 6<Zl<20 at velocities of 0.82 and 1.0 V0 (V0 ¿ ¿c = 0.219 cm ns-1) in five materials C, Al, Ni, Ag and Au with additional data in some cases to a velocity of ~ 2 V0. This paper will briefly describe the experimental method and give an overview of the results. More detailed descriptions with tables of experimental stopping powers and comparisons with other work will be published elsewhere. The topics discussed in this paper are the effects of multiple scattering in low energy measurements, Z2 dependence of the Z1 oscillation amplitude, and the velocity dependence of low energy stopping powers and its implications for the analysis of nuclear lifetime measurements based on the Doppler broadened line shape technique applied to low recoil velocity data.