J. M. Khan

OXYGEN SURFACE-DENSITY MEASUREMENTS BASED ON CHARACTERISTIC X-RAY PRODUCTION BY 100-keV PROTONS.

The oxygen‐K x‐ray yield from known thicknesses of aluminum oxide (1–70 μg O/cm2), bombarded by 100‐keV protons, indicates that oxygen surface density may be measured over the range from 50 to 0.004 μg/cm2. At the lower limit, background is 50% of the signal. In thin oxide layers where proton energy loss and x‐ray absorption are negligible, the x‐ray yield I (x rays/proton) is related to the oxygen surface density t (μg/cm2) by I=(2.9×10−6)t.

Proposed Method for Microgram Surface Density Measurements by Observation of Proton‐Produced X Rays

X‐ray production by proton bombardment may be used to determine the surface density of thin films. It is desirable that the energy loss of the protons in the film be small and that the self‐absorption of the produced x‐ray be negligible. To illustrate the method, thin surface films of aluminum are discussed. The aluminum x‐ray yield (x rays/proton) as a function of surface density is presented for the range 0 to 115 μg/cm2 and at an incident proton energy of 100 keV. Under the conditions of a 10‐μA proton current incident on a thick target, a typical detection system would record 10 000 counts/sec with a counter signal‐to‐noise ratio of 104. At a signal‐to‐noise ratio as low as unity, the calculated surface density is 0.005 μg/cm2 or approximately 1014 atoms/cm2. Details of the calculation are presented, along with a table of eleven elements to which the method is currently applicable.

Effect of Thin Carbonaceous Films on 500‐keV Helium Ion Sputtering of Copper

A radioactive tracer technique capable of measuring less than 10−10 g of copper has been developed. A 500‐keV magnetically analyzed beam of helium ions is used to observe the sputtering yield as a function of carbonaceous film thickness over a copper target. It is shown that an amorphous film of the order of 2×1017 atoms/cm2 reduces the sputtering yield of copper by a factor of 100. A first collision model of sputtered atom penetration of the film is in good agreement with the data and yields a copper—carbon cross section of 3×10−17 cm2. A clean surface sputtering yield of 2×10−2 atoms/ion is obtained by extrapolation of the data and is consistent with the yield of 800‐keV D+ sputtering of copper reported by Kaminsky [Phys. Rev. 126, 1267 (1962)]. It is postulated that ordering of the amorphous film occurs for areal densities greater than 2×10+17 atoms/cm2 on the basis that the rate of reduction of copper yield with increasing film thickness undergoes a sharp decrease at that point.

A Small Bragg Diffraction X‐Ray Spectrometer

A small, easily constructed, and high efficiency Bragg angle x‐ray spectrometer has been developed. Employing either a lead stearate or a KAP crystal, it has been used to measure x rays in the 150–1000 eV region from ion‐atom collisions. At the carbon‐K energy (277 eV), it has an FWHM of 14 eV and an efficiency of 10−2 counts per incident x ray. Electronics and controls for fully automatic scans are described.

X‐Ray Study of Proton‐Induced Disorder in Cu3Au

Measurement of fundamental and superlattice x‐ray diffraction lines as a function of integrated proton‐bombardment current has been used to determine induced disorder in the surface layers of fully ordered polycrystalline targets of Cu3Au. It is shown that a proton range for induced disorder and the number of disordered atoms created per proton can be inferred from the data.