Werner Rom

14C dating with the bomb peak: An application to forensic medicine

Abstract Samples originating from the time period after 1950 can be radiocarbon dated utilising the 14C bomb peak as a calibration curve. The applicability of “radiocarbon dating” of recent organic human material for the determination of the time of death of humans was tested. The radiocarbon results from hair and lipid samples from individuals with known date of death were compared with the results from two individuals with unknown time of death. An estimate of the year of death for the unknowns could be derived by this way. Due to the long turnover time of collagen in human bones it is not possible to use the radiocarbon content of bone collagen for a reliable estimate. In order to study the time dependence of the collagen turnover we tested “soft” chemical methods for the isolation of collagen from the bone matrix. First radiocarbon results of this investigation are presented.

The use of Bayesian statistics for 14C dates of chronologically ordered samples : A critical analysis

Bayesian mathematics provides a tool for combining radiocarbon dating results on findings from an archaeo- logical context with independent archaeological information such as the chronological order, which may be inferred from stratigraphy. The goal is to arrive at both a more precise and a more accurate date. However, by means of simulated measure- ments we will show that specific assumptions about prior probabilities—implemented in calibration programs and not evident to the user —may create artifacts. This may result in dates with higher precision but lower accuracy, and which are no longer in agreement with the true ages of the findings.

Pushing the precision limit of 14C AMS

High precision for radiocarbon cannot be reached without profound insight into the various sources of uncertainty which only can be obtained from systematic investigations. In this paper, we present a whole series of investigations where in some cases (super 16) O: (super 17) O: (super 18) O served as a substitute for (super 12) C: (super 13) C: (super 14) C. This circumvents the disadvantages of event counting, providing more precise results in a much shorter time. As expected, not a single effect but a combination of many effects of similar importance were found to be limiting the precision. We will discuss the influence of machine tuning and stability, isotope fractionation, beam current, space charge effects, sputter target geometry, and cratering. Refined measurement and data evaluation procedures allow one to overcome several of these limitations. Systematic measurements on FIRI-D wood show that a measurement precision of + or -20 (super 14) C yr (1 sigma ) can be achieved for single-sputter targets.

VERA: A new AMS facility in Vienna

Abstract The basic features of VERA, a new AMS facility based on a 3-MV Pelletron tandem accelerator installed at the University of Vienna, are presented.

First performance tests of VERA

VERA is a new 3-MV Pelletron tandem AMS facility in Vienna, which was installed during the last months of 1995. This report will discuss the performance characteristics of the facility established during the test operating phase and present first measurements of 14C standards.

Systematic Investigations of 14C Measurements at the Vienna Environmental Research Accelerator

A newly operating accelerator mass spectrometry (AMS) facility such as VERA has to go through an extensive testing phase in order to establish optimal conditions for (super 14) C measurements, especially in the field of archaeological samples where an overall precision of 0.5% is desirable. We discuss the results of our measurements at the milligram carbon level as they relate to long-term stability, reproducibility, precision and isotope fractionation.

Extension of the measuring capabilities at VERA

Our standard setup used for accelerator mass spectrometry (AMS) with 14C was modified for measuring various other radionuclides. The injection and detection systems were modified to suit the particular isotope investigated. 10B, the stable isobar of 10Be, is stopped in a gas absorber in front of a surface barrier detector. 26Al is measured similar to 14C, except for a modification of the timing for the sequential isotope injection. For 129I, a time-of-flight setup was built to separate 129I from 127I. For heavier rare radionuclides, the mass resolution of the injector system was improved. We also improved our capabilities in measuring carbon samples having very low mass. We describe two different methods to prepare and to measure sub-milligram 14C samples.

Detailed analysis of the isotopic composition of CO and characterization of the air masses arriving at Mount Sonnblick (Austrian Alps)

Air sampling for analysis of CO and its isotopic composition (13C, 18O, and 14C) has been performed at the alpine station Sonnblick (47°N, 13°E, 3106 m above sea level) since September 1996. A high degree of variability is observed, which is due to the wide variation in the origin of air masses sampled. On the basis of the CO and isotope results, a classification of the different samples is performed. Other data such as 7Be, O3, relative humidity, and back trajectories are used to give additional information about the air mass origin. Background values, representative of the NH midlatitudes free troposphere, are observed about 50% of the time and are used to define seasonal cycles. CO and its isotopes show a minimum in summer and a maximum in winter with extreme values of 90 and 160 ppb for CO, −30 and −25‰ for δ13C, 0 and 8‰ for δ18O, and 8 and 20 molecules cm−3 STP for 14CO. CO and stable isotope data are compared with results from a three-dimensional model (TM2). Generally good agreement supports the CO, δ13CO, and δC18O source/sink distributions inferred by the model. According to model calculations, fossil fuel combustion contributes 35% in summer and 50% in winter of total CO for such a midlatitude location. Other categories of sampled air are “subtropical,” “polluted,” and “stratospheric” and are observed 24%, 18%, and 4% of the time. Corresponding signatures of CO and its isotopic variations are presented, and some specific events are discussed.

Methodological aspects of atmospheric 14CO measurements with AMS

Abstract This paper examines special features arising in the measurement of atmospheric 14 C monoxide. A detailed error analysis of the 14 CO method including sampling, chemical processing and AMS measurement of samples, standards and blanks is given. Problems regarding the term “percent Modern Carbon (pMC)” are discussed, and the conversion of pMC values to atmospheric 14 CO concentrations is explained.

Automated evaluation of 14C AMS measurements

Abstract The huge amount of raw data collected during routine 14 C AMS measurements requires sophisticated processing tools to guarantee the quality and reliability of the resulting radiocarbon dates. This paper discusses the automatic evaluation system, that is in use and under continuous development at the Vienna Environmental Research Accelerator (VERA) laboratory. It includes a calibration program which is able to handle the bomb-peak. The flexibility of the system allows its use for other rare isotopes also.