T.R. Ophel

Accelerator mass spectrometry of plutonium isotopes

The feasibility of measuring plutonium isotope ratios by accelerator mass spectrometry has been demonstrated. Measurements on a test sample of known composition and on a blank showed that isotope ratios could be determined quantitatively, and that the present limit of detection by AMS is ∼ 106 atoms of plutonium. For 239Pu, this limit is at least two orders of magnitude lower than that practicable by alpha-spectrometry. In addition, 240Pu239Pu ratios were measured for four samples for which the combined activity of the two isotopes had been determined previously by alpha-counting. All measurements of plutonium isotope ratios entailed injection of PuO− into the 14UD accelerator operating at 3.5 MV, gas stripping, and analysis of the 7+ charge state after acceleration. Plutonium ions at 28 MeV were detected in a longitudinal-field ionisation chamber with an energy resolution of 3%. Using uranium oxide as a surrogate for plutonium oxide, it was shown that UO− was the predominant negative ion and that the probability for its formation and extraction was 0.3%.

Accelerator mass spectrometry at the Australian National University's 14UD accelerator: experience and developments

Although the major emphasis of the joint ANU/ANSTO accelerator mass spectrometry program has been the measurement of 36Cl samples, both 14C and 10Be capabilities have been implemented recently on the 14UD accelerator. The new developments and operating experience are reviewed.

The chlorine-36 measurement program at the Australian National University

Abstract The chlorine-36 measurement system at the Australian National University is described and some early results are presented.

Simplifying position-sensitive gas-ionization detectors for heavy ion elastic recoil detection

Abstract Heavy ion elastic recoil detection is an effective tool for materials analysis. This has been facilitated by the development of novel detection systems, including large solid-angle gas-ionization detectors with position-sensitivity. With such detectors the exposure of the sample to the beam is reduced, however, data analysis can be complex. This paper reports on a detailed investigation of a detector design commonly used. The results of this work have led to suggestions for design-modifications, which considerably simplify detector operation and data analysis. The modifications have been implemented and tested. With the modified detector, the total energy information can be obtained by adding the signals from the anode electrodes, but also directly from a grid-electrode, obviating the need for the relative calibration of the anode electrodes. The subdivision of the energy loss electrode, together with a carefully chosen position of the entrance window to maintain optimum resolution for two ΔE signals, enable light and heavy ions to be resolved at the same gas pressure. The placement of a sawtooth electrode within the anode gives position information, which is linear and independent of atomic number and ion energy. Protons can be identified simultaneously with heavy ions by combining the information from the grid and residual energy signals, both amplified with high gain.

The ANU AMS system and research program

Abstract Since the previous conference, the Australian National University AMS system has undergone a series of refinements, and the research program conducted on the facility has expanded significantly. The system is now used for the measurement of the isotopes 10Be, 14C, 26Al, 36Cl, 59Ni and 129I. The research program includes projects in hydrology, geomorphology, environmental geochemistry and biomedicine. Chlorine-36 remains the principal isotope, but the demand for other heavy isotopes, especially 26Al, has burgeoned during 1992–93. This paper describes the recent advances with AMS measurements on the 14UD, and details the current status and future direction of the AMS research program.

The development of a facility for heavy-ion elastic recoil detection analysis at the Australian National University

Abstract The design of a facility at the Australian National University for heavy-ion elastic recoil detection analysis with minimal beam exposure is presented. The system is based on an established technique using a position-sensitive gas-ionisation detector with a large acceptance solid angle. The kinematic energy spread of the detected recoil ions is corrected. The accuracy of this correction is demonstrated. The capabilities of the new facility are discussed.

New design features of gas ionization detectors used for elastic recoil detection

Several alternative design features of large acceptance, gas ionization detectors have proven to be successful for application to elastic recoil detection analysis (ERDA). In particular, effects due to the distortion of the entrance field by a large area window have been eliminated in a simple fashion, to allow measurement of the initial rate of energy loss and to provide an energy- and species-independent cathode signal. No less importantly, use of a divided electrode in the anode plane has enabled a more straightforward means of determining the scattering angle that is required for kinematic corrections. An intermediate grid was found to provide a direct and true total energy signal, with only slightly diminished resolution compared with that of the summed total anode equivalent.

NON-LINEARITIES AND ENTRANCE WINDOW EFFECTS IN LARGE SOLID-ANGLE GAS IONIZATION DETECTORS

Abstract The performance of a large solid-angle, position-sensitive, gas ionization detector used for elastic recoil detection analysis with heavy ions has been investigated in detail. In particular, a front electrode in the anode, that was intended for measurements of the initial energy loss of incident heavy ions, proved especially valuable as a means of understanding the non-linearities inherent to the methods of spatial determination employed by a widely used type of detector, and the influence of the window on the collection of electrons at the anode. Alternative design features are proposed to remove the non-linearities and to allow the measurement of the energy loss near the detector entrance.

Simultaneous Hydrogen Detection with an ERD Gas Ionization Detector

Abstract A large solid-angle gas ionization detector is shown to be capable of simultaneous hydrogen detection during heavy-ion elastic recoil detection of heavier elements. Different modes of detection are possible depending on the specific application. These include a transmission-mode in which the primary ion energy and gas-pressure in the detector are optimized for heavy-ion detection and energy loss signals are recorded for each hydrogen recoil. Alternatively, the primary ion energy can be reduced and the gas pressure in the detector increased so that the recoiled protons are fully stopped in the detector. In this case, a conventional total energy signal can be recorded. The transmission mode is shown to be particularly suited to determining the total hydrogen content of thin films, where the emphasis is on optimum mass and depth resolution for the heavy elements, whilst the stopped mode allows simultaneous hydrogen profiling with reasonable depth resolution.

Characterization of silicon oxynitride films using ion beam analysis techniques

Abstract Heavy-ion elastic recoil detection analysis (HIERDA) is the ideal technique for quantitative analysis of silicon oxynitride films on silicon because of its unique ability to measure simultaneously all elements of interest (i.e., H, C, N, O and Si), thereby permitting key parameters such as the O/N-ratio to be determined in a single measurement. However, high-energy accelerators suitable for such HIERDA measurements are becoming much less readily available. Hence, the present paper investigates and calibrates an alternative IBA technique for simultaneous O, N and C analysis – namely, the use of (d,p) and (d,α) nuclear reactions. Under optimum analysis conditions (850 keV deuterons and 150° detector angle), the Si background level sets a lower detection limit of ∼1×10 16 nitrogen atoms/cm 2 and ∼3×10 15 oxygen atoms/cm 2 . H analysis is carried out separately, using low-energy ERDA and a 2 MeV 4 He beam. Absolute cross-sections have been obtained for each of the (d,p) and (d,α) groups. Comparison with data in the recent Handbook of Modern Ion Beam Materials Analysis shows reasonable agreement (10–15%) for the (d,p) reactions on oxygen and carbon. However, in the case of nitrogen, the measured cross-section values are ∼70% larger than the Handbook data. Several silicon oxynitride samples have been analyzed, first at UWO using 850 keV deuterons, and subsequently at ANU using HIERDA and a 200 MeV Au beam. The resulting O/N-ratios agree to within 10%. The relative importance of radiation damage effects is briefly discussed.