ABSTRACT. The minimum size of radiocarbon samples for which reliable results can be obtained in an accelerator mass spectrometry (AMS) measurement is in many cases limited by carbon contamination introduced during sample preparation (i.e. all physical and chemical steps to which samples were subjected, starting from sampling). Efforts to reduce the sample size limit down to a few g carbon require comprehensive systematic investigations to assess the amount of contamination and the process yields. We are introducing additional methods to speed up this process and to obtain more reliable results. A residual gas analyzer (RGA) is used to study combustion and graphitization reactions. We could optimize the reaction process at small CO 2 pressures and identify detrimental side reactions. Knowing the composition of the residual gas in a graphitization process allows a reliable judgment on the completeness of the reaction. Further, we use isotopically enriched 13 C (98% 13 C) as a test material to determine contamination levels. This offers significant advantages: 1) The measurement of 12 C/ 13 C in CO 2 is possible on-line with the RGA, which significantly reduces turnaround times compared to AMS measurements; 2) Both the reaction yield and the amount of contamination can be determined from a single test sample.The first applications of isotopically enriched 13 C and the RGA have revealed that our prototype setup has room for improvements via better hardware; however, significant improvements of our sample processing procedures were achieved, eventually arriving at an overall contamination level of 0.12 to 0.15 g C during sample preparation (i.e. freeze-drying, combustion, and graphitization) of g-sized samples in aqueous solution, with above 50% yield.

Measured differential elastic-scattering cross sections of 17.72-, 20.97-, and 23.72-MeV neutrons from liquid helium-4 were re-evaluated and were corrected for sample-size and multiple-scattering effects by means of a Monte Carlo technique implemented in a more recent code (MCNPX). Results indicate that earlier corrections via the code MAGGIE-2 overestimated the size of multiple-scattering effects by an order of magnitude. The corrected differential cross sections and Legendre coefficients obtained by least-squares fits are given.

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Correction for finite angular resolution in measurements of elastic scattering of neutrons from 3 He was found to be significant [1]. Therefore, the prematurely published data of reanalyzed measurements of elastic scattering of neutrons from 4 He are corrected for finite angular resolution. In the original experiment in question [2], a close geometry was chosen to result in high neutron flux incident upon the sample. That is, the distance of neutron source to center of the 4 He scatterer was 11.5 cm, resulting in a mean incoming angular spread of ±6.6 • . The opening angle of detected neutrons was 1.0 • . Because neutron yield as a function of angle is not linear, the effect of finite angular spread is a shift from geometric mean of the scattering angle. The correct angle can be obtained by weighting the yield distributions over the subtended solid angles. Incoming and outgoing angular resolution corrections are dealt with separately. Instead of shifting the angle, a correction of the yield at the mean angle was applied. These corrections were determined by simulations under very small acceptance angles and also under actual experimental geometries. The original modeling of the experiment by the Monte Carlo code accounted just for the angle dependence of the intensity of the incoming neutrons over the acceptance angle but not for the finite angular resolution resulting from the incoming angular spread. Besides, this time the correct incoming energy of 17.71 MeV is used throughout rather than 17.72 MeV, as done in the original paper. TABLE I. Differential cross cections for elastic neutron scattering from 4 He (angles in degrees, cross sections, and uncertainties in mb/sr) E n = 17.71 ± 0.06 MeV E n = 20.97 ± 0.05 MeV E n = 23.72 ± 0.04 MeV lab c.m. σ c.m. σ c.m. c.m. σ c.m. σ c.m. c.m. σ c.m. σ c.m.