D. Phillips

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.

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.

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.

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.

The effect of anodic oxidation on near-surface deuterium in Zr-2.5 wt.% Nb

Abstract The effect of anodic oxidation on near-surface deuterium in thin ( ∼ 3.5 μm) foils of Zr-2.5 wt.% Nb has been studied with the 2 D( 3 He, p) 4 He nuclear reaction. To introduce deuterium into the foil, the native oxide was etched with a solution of 0.1% NH 4 HF 2 in D 2 O. The foils were anodized, in a compact cell in a vacuum chamber, to oxide thicknesses of 160, 200 and 280 nm and the remaining D concentration measured. The D content was also measured, in separate experiments, with the anodized surface towards the beam. Detailed analysis shows that the effect of oxidation is to return most of the D in the anodized region to the solution.

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.

3dπ-2pπ radial coupling in symmetric and near-symmetric heavy-ion-atom collisions

Presents new evidence for a sharing of projectile 2p vacancies in heavy-ion-atom collisions, Z1 approximately=Z2 approximately 35 and E/M<or approximately=0.5 MeV/amu. The sharing is effected by 3d pi -2p pi radial coupling and can be described through a function WL=(1+exp(1.79XL))-1, analogous to 2p sigma -1s sigma K-vacancy sharing. XL is a simple function of the projectile velocity and the 2p binding energies of the projectile and target atoms. The data may be used to show that the region of internuclear distances where 3d pi -2p pi radial coupling occurs is spatially distinct from the region of 2p pi -2p sigma rotational coupling.

Depth profiles of 35 keV 3He ions in metals

Abstract The depth profiles of 3 He implanted at 35 keV energy into Al, Ti, V, Ni, Cu, Zn, Zr, Nb, Ag, Sn, Ta, W, Au and Bi targets have been measured using the thermal neutron reaction 3 He(n, p) 3 H. The profiles obtained from the energy distribution of the emitted protons show a marked Z 2 -oscillation in both the most probable depth and depth straggling. The measured values are compared to Monte Carlo computer simulation results.

Search for pre-equilibrium stopping for 4He ions in thin carbon foils

Abstract Mertens and Krist have suggested that ions with atomic number Z1 > 1 suffer enhanced stopping during passage through the first few atom layers of solid targets. We have measured the energy losses of 1H and 4He projectiles in carbon targets with thicknesses of 4–80 μm cm−2 using a time-of-flight system. Our results suggest that there is not a significant pre-equilibrium effect for 4He ions at ∼ 620 keV, and that a measurement of the 4He energy loss is an acceptable technique for determining relative foil thicknesses for carbon.