14C Dating with the Icels Liquid Scintillation Counting System Using Fixed-Energy Balance Counting Window Method

This article presents an application of a fixed-energy balance counting window in radiocarbon dating of geological peat samples. We determine a fixed-energy balance counting window with an inexpensive liquid scintillation counting ICELS system. We show long-term modern biosphere standard records that show stability sufficient for dating samples up to approximately 30,00014C yr BP. We then compare our results to ones obtained previously using a Quantulus 1220™.

14C Dating with the Icels Liquid Scintillation Counting System Using Fixed-Energy Balance Counting Window Method

This article presents an application of a fixed-energy balance counting window in radiocarbon dating of geological peat samples. We determine a fixed-energy balance counting window with an inexpensive liquid scintillation counting ICELS system. We show long-term modern biosphere standard records that show stability sufficient for dating samples up to approximately 30,000 14C yr BP. We then compare our results to ones obtained previously using a Quantulus 1220™.

The Use of Standard Alpha and Beta Surface Scintillation Contamination Monitors to Confirm the Contamination Fingerprint and to Check on Source Quality

Normally, beta and alpha surface contamination monitors are used with a simple counting threshold, i.e. any pulse over a predetermined amplitude is counted. This is very different from gamma monitoring, where the use of counting windows is very popular and the use of full multi-channel analysis is common. Many current surface contamination ratemeters have the capacity to drive dual phosphor detectors and can be set up to provide beta and alpha channels. Effectively, the beta channel is a counting window, i.e. all pulses which are bigger than the threshold and smaller than the alpha threshold are counted. Larger pulses go into the alpha channel. This paper addresses how this can be used with beta only and alpha only detectors to provide information on the source. The detector is set up conventionally to a defined point for the lowest beta energy anticipated. The instrument is then switched to alpha + beta mode and the alpha threshold set to 3 times the beta threshold. With this set up, the alpha to beta channel count rate ratio varies smoothly by a factor of 14 between Y-90 (Emax 2.27 MeV) and C-14 (Emax 0.16 MeV). Hence the instrument can be used to estimate the energy of an unknown beta contaminant or to confirm that a mixed beta fingerprint has essentially the same mix. The same approach can be used with alpha probes to confirm the source quality. The main worry with alpha monitoring is the surface condition. A poor surface condition will lead to a low count rate. Using the channel ratio method will identify grubby sources. The resulting ratio can be used either as a go/no trigger, i.e. any surface with a low ratio will be treated as untrustworthy, or alternatively the ratio can be used to correct the reading to give a better estimate of surface activity.

Balanced-Energy Counting Window for Stable Liquid Scintillation Radiocarbon Dating

This paper describes an optimal radiocarbon counting window for liquid scintillation (LS) 14C dating that secures for unquenched as well as for heavily quenched dating samples maximal stability of 14C counting efficiency and theoretically minimal quench correction. In high-precision dating, a balanced counting window with fixed channel limits is frequently used, where about 3% of the highest part of the 14C spectrum is sacrificed for high 14C counting stability. The stability is, however, diminished for quenched samples. Therefore, this window is here replaced by a balanced fixed-energy 14C counting window where the channel limits depend on the quench level. The LS system used must have a linear amplifier and a multichannel analyzer. All samples are measured at a fixed high voltage. For energy calibration and determination of the quench level, the channel number of the middle of the 59.5-keV peak from an external 241Am gamma source is determined before and after measuring each sample. This counting mode is valuable in high-precision dating. It could be widely applied if adapted to systems with a logarithmic amplifier, generally used in LS dating.

Determination of 90Sr/90Y in Wheat Grains, Soil, and Deposition Samples by TBP Extraction and Cerenkov Counting

Within the framework of radioecological studies, 90Sr was determined in wheat grains, soil, and deposition samples. The radiochemical purification of 90Y consisted of liquid-liquid extraction by tributyl phosphate (TBP), followed by hydroxide and oxalate precipitations and, if necessary, the removal of thorium by anion exchange chromatography. The procedure proved to be very robust and reliable, having yttrium yields of 92.7 ± 4.6% for 1-kg wheat samples, 90.9 ± 4.2% for 50-g soil samples, and 90.6 ± 3.2% for wet and dry deposition samples. 90Y was determined by Cerenkov counting and proportional counting. By optimizing the Cerenkov counting window, a figure of merit (FOM) of 4750 could be reached using a Quantulus™ 1220 system. Minimum detectable activities were in the range of 10 mBq.

Simultaneously Measuring 14C and Radon in Benzene Dating Samples

After benzene synthesis, radiocarbon dating samples are usually stored for 3–4 weeks before counting to allow an eventual radon contamination to decay to a negligible level. This paper presents a technique that can minimize, and often eliminate, this delay by using a simple single-phototube liquid scintillation counting system, specifically designed for 14C dating. Radon contamination is assessed by pulses of 214Po (a 222Rn decay product, half-life 0.16 μs), identified through pulse-time analysis. For each 214Po pulse, 0.49 beta particle pulses of 214Pb and 214Bi fall in the 14C counting window, and the 214Po pulses are used to correct the 14C count rate. A 14C sample (count rate 11.6 cpm) was measured continuously for 16 days. It was then doped with radon, which increased the first 24-hr count rate in the 14C channel by 3.8 cpm, and the sample was measured for 27 more days. Radon did not measurably affect the 14C-corrected count rate. Counting a sample for 2 min reveals whether it needs storing. If the radon concentration is low, the sample can be measured immediately without degrading accuracy.

Balanced Window Method in 14C Liquid Scintillation Counting

The authors present a detailed theoretical and experimental study of the liquid scintillation balanced counting method, widely used in radiocarbon dating, using a simple, laboratory-made system. A fixed counting window becomes a balanced window when the high voltage is set where the 14C count rate rises to a maximum. Using a measured 14C pulse height spectrum, we have calculated the lower and upper limits for 11 balanced windows of varying width and their respective counting efficiencies. Furthermore, we have studied: (1) theoretically and experimentally, the counting efficiency for up to a ±15% shift in pulse height from the balanced setting, (2) the change in pulse height due to temperature variations, (3) the long-time stability of the system, and (4) a method that allows a quick determination of the balance voltage for individual samples, using the Compton spectrum of 133Ba. The standard deviation for thirty 24-hr measuring periods for a 14C standard (190 Bq) was within the expected statistical standard error (0.03%).

Performance of the Packard Tri-Carb® 2770TR/SL Liquid Scintillation Analyzer for 14C Dating

We present results that demonstrate the potential of the Packard Tri-Carb® Model 2770TR/SL for radiocarbon dating. For 2 g of sample benzene, a stable background count rate of 0.307 cpm and a stable counting efficiency of 64.78% were determined using a 13–75 keV counting window. Changes to the mathematical routines for t-SIE (quench indicating parameter) calculation and a reduction in the activity of the external standard have enabled stability of the t-SIE to be achieved, and combined with the use of a suitable balance point counting window; all of these factors give the stability of performance required for 14C dating. Calculations based on the above parameters indicate that the limit of detection for 2 g samples, counted for 5000 min, is >48,900 yr bp. The great advantage of this system is that these data were acquired using inexpensive standard 7-mL low potassium borosilicate glass vials. Vial holders manufactured from BGO reduced the background to 0.15 cpm with a minimum effect on efficiency (64.46% for 13–75 keV). A similar calculation of the limit of detection gave >51,700 bp. The use of the BGO vial holders in other instruments employing time-resolved liquid scintillation counting (TR-LSC) (Models 2250CA and 2260XL) also brought about significant improvements in detection limits.

Liquid Scintillation Counter Characterization, Optimization and Benzene Purity Correction

In liquid scintillation counting (LSC), small variations in benzene purity can cause 14C pulse-height spectra to move with respect to the counting window. Thus, one must carefully monitor the purity of each benzene sample and apply corrections for spectral shifts. I describe here the techniques used at Queen's University Belfast for deriving correction factors for observed small variations in benzene purity. I also describe the methods used at our laboratory to fine-tune our Quantulus LS counters for high-precision dating. The tuning of the instruments minimizes the effect of fluctuations in gain that may occur during the long counting periods required for high-precision dating. Any remaining influences on efficiency owing to gain changes are corrected for, along with the purity correction, by continuous monitoring of the spectrum produced by the external source.

Radon Elimination During Benzene Preparation for Radiocarbon Dating by Liquid Scintillation Spectrometry

Radon gas is a serious contaminant in radiocarbon dating by radiometry. The low specific ionizations associated with the α-particle emitting radon and its β-particle emitting daughters overlap within the 14C counting window. Elimination of radon is therefore imperative for precise 14C age determinations. This paper deals with the sources and mechanism of incorporation of radon affecting 14C dating by liquid scintillation (LS) counting, and reviews conventional radon elimination practices in 14C laboratories. It demonstrates, based on rigorous multichannel and multiparameter α- and β-particle spectral analyses of some 1000 benzene samples, that parent radium is not present and that its daughter radon is quantitatively eliminated during dynamic vacuum recovery of benzene at −78°C. However, the radon-free benzene can be recontaminated by exposure to air containing traces of radon, such as is common in concrete or low-lying laboratories. The use of radon-free air, when exposing the benzene to the atmosphere, and the monitoring of radon counts from the environment and sample benzene in a fixed ‘radon window', are essential prerequisites to the quality control of 14C age determinations in very low background systems.