The results are presented of an investigation comparing the performance of commercially available liquid scintillation spectrometers claiming 'low-level' radioisotope detection abilities. Determination of ¹⁴C at naturally occurring concentrations was carried out in both old and new technology liquid scintillation counters using a ¹⁴C labelled benzene sample with butyl-PBD as scintillant. The signal to noise ratio and the ¹⁴C detection efficiency were evaluated.
The results show a wide range of merit for radiocarbon dating of so called 'low-lever instruments

Calf, G.

Harkness, D.

Hogg, Alan G.

Kaihola, L.

Polach, H.

Robertson, S.

Applied Radiation and Isotopes

Research Commons@Waikato

Preprint, Nuclear Geophysics, 1988, v. 2, no. 2. Performance of New Technology Liquid Scintillation Counters for 14C Dating H. POLACH1, G. CALF2, D. HARKNESS3, A. HOGG4, L. KAIHOLA 5and S. ROBERTSQN1 1 Australian National University. Canberra 2601, Australia, 2Australian Nuclear Science and Technology Organisation, Lucas Height 2234, Australia, 3NERC Radiocarbon Laboratory, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, Scotland, 4University of Waikato, Hamilton, New Zealand, swallac Oy, 20101 Tur1<u 10, Rnland, The results are presented of an investigation comparing the performance of commercially available liquid scintillation spectrometers claiming 'low-level' radioisotope detection abilities. Determination of 14c at naturally occurring concentrations was carried out in both old and new technology liquid scintillation counters using a 14c labelled benzene sample with butyl-PBD as scintillant. The signal to noise ratio and the 14c detection efficiency were evaluated. The results show a wide range of merit for radiocarbon dating of so called 'low-lever instruments. Introduction Radiocarbon dating is a geochronological research tool used commonly in archaeology, geosciences, climatic, floral and faunal history, oceanography, hydrology and environmental sciences such as pollution studies, volcanology and earthquake prediction. There ·are some 117 registered Radiocarbon D�ting Laboratories (1) and at least half as many unregistered, all meeting the large demand for 14c assays. Two techniques of 14c radiometric determinations are practiced today: gas proportional counting as reviewed by Mook (2) and liquid scintillation counting as reviewed by Polach (3J. In recent times, thanks to the generally good performance of commercially available liquid scintillation spectrometers, laboratories using these spectrometers dominate. It has thus become pertinent to evaluate the performance of the newly available liquid scintillation spectrometers claiming 'low-level' detection ability and to compare them with the old technology counters. 8/11/87 Experimental Apparatus and 14c standard. Polach et al p 2 Two commercially available liquid scintillation spectrometers claim 'low-level' performance, defined as high net isotope signal represented by efficiency (E) and very low background (B). resulting in a large Figure of Merit (FM = E2fB). These are the Packard 2000 CA/LL (USA) and the Pharmacia-Wallac Ouantulus (Finland) liquid scintillation spectrometers. To monitor their performance two standards were prepared and distributed by the Australian National University, ANU-Radiocarbon Dating Research Laboratory: a labelled 14c benzene sample and a 14c free benzene-background sample, both with butyl-PBD scintillant at 15 g per litre (g/l). The 14c concentration of the labelled standard corresponded to 25.7 ± 0 .2 dpmlg benzene. This activity is equivalent to 205.6% Modern, where Modern is represented by 95% of the 14c activity of the National Bureau of Standards (Washington DC) oxalic acid radiocarbon dating international standard (4) (0.950x = No cpm in the tables and figures that follow). Each participant used the counters available to him (old or new), optimised to manufacturer specification for low-level detection of 14c and reported the observed net 14c and background counts for 3, 7 and 15 ml of the suppfied standard solutions. Because Packard recommends a 6 g/L butyl-PBD concentration the Packard users tested the standard as suppfied as well as diluted with analytical reagent grade benzene (14c free) to achieve the reoommended scintillant ooncentration. Results MaAufacturers of nuclear counters prefer to express the performance of their equipment in terms of the Figure of Merit (FM) given by E2fB, whilst users involved in radiocarbon dating prefer a factor of Merit (fM) given by Norfs(S). Both terms are given in the table of results (Table 1) but the interpretation of data is based on the factor of Merit. The participants supplied data relating to counting vials (size, volume and grams of sample) and the observed results as net cpm and background. The interpretative data (%E, No cpm, fM and FM) was calculated at the Australian National University, as none of the participants were advised of the precise radiocarbon concentration of the standards supplied. The data as tabulated therefore enables a relative intercomparison of counting systems. Two types of counting vials were used: the standard low potassium glass, - 20 ml, liquid scintillation counting vial as recommended by Packard, and the Teflon vials, Australian Nuclear Science and Technology Organisation (ANSTO) (5) and Austrafian National University (6) as recommended by Pharmacia-Wallac. Four types of counting environments are represented: 1) surface location with some attenuation of cosmic radiations by ceilings of buildings (Calf and Harkness), 2) surface location with attenuation of cosmic radiation by 1.5 m of concrete on the sides and above the equipment (Polach), 3) surface location in cosmic ray shielded and y-ray free environments (low-level lab, Lola, Kaihola(7l), and 4) the laboratory in the Warragamba dam wall, with very high cosmic shielding (8). Anowanceswill be made for these environmental factors when interpreting the results. 8/11/07 Polach et al p 3 Interpretation of results The detection efficiency values range from a low 42.8 to a high 84.9 %E and the factor of Meritvalues range from a low 15 to a high 172 fM. The old technology counters, represented here by the Beckman 7500, LKB-Wallac 1215 Rackbeta-K and Packard 3330, give values for E of 70.4 to n.6% which is as expected. The new technology counters contrast in that the Packard-2000 CNLL efficiency values range from a low 51.7 and do not exceed 57.1% at manufacturer recommended butyl-PBD concentrations, whilst the Pharmacia-Wallac Ouantulus gives systematically high values for E at 70.0 to 84.9 %. The lowest backgrounds are achieved in the Wallac low-level laboratory location by the Ouantulus. This is as expected as both cosmic and environmental y-rays have been attenuated in this special environment. The excellent background achieved by the old Packard 3330 located at the Warragamba dam (ANSTO) confirms the merit of cosmic and y-ray shielding. The laboratory at the Australian National University does not perform the same function. With 1.5 m of ordinary concrete it attenuates the soft, therefore variable component of cosmic rays, without affecting the hard muon flux. They-rays from the concrete aggregate also contribute to the observed background. The design aims were to achieve a constant not a low background. Tests have shown that the background value is the same as at any other surface location at the Australian National University, albeit free of externally induced variations or Interference. The values of B achieved by the old technology counters (Beckman and LKB) are therefore representative, at 3 to 4 cpm, of that type of technology which contrasts with the Quantulus at 0.14 to 1.1 cpm. The new Packard 2000 CML gave results for B ranging from 0.73 to 3.5 cpm. The relative merit of radiocarbon dating systems is given by plotting (Fig 1) the derived radiocarbon dating reference standard values (No cpm, from Table 1) against the factor of Meritvalues (9). The countrate of No (Y axis) relates to the ultimate precision that can be achieved by a counting system I vial/ sample size configuration. In practice the counting error of a sample, whose countrate is close to the modem reference standard, is primarily given by the Poisson error of the accumulated counts in a given time. The background, and thus the environment, play only a minor role <10). The value of No is both sample size and efficiency related; therefore the precision of an age determination, for a given counting time, increases with both sample size and counting efficiency. The factor of Merit is plotted on the X axis and by definition takes account of the values No and B. The derived values are thus directly related to the maximum age that can be determined (5) and are dependent on the counting efficiency of systems, their background and hence the environment in which the counters are placed. The higher the value of fM the higher is the ultimate age resolution and hence the greater the maximum determinable age. The three shaded areas in Fig. 1 represent the three counting systems we wish to compare. From left to right: K and Bn are the old counters; H and Care the only marginally better new Packard; and Q is the significantly better new Pharmacia-Wallac Quantulus; all placed in virtually the same environment. An alternative to the above presentation is to calculate (based on the cpm values of No and B, Table 1) the errors associated with the age determinations and the radiocarbon dating age limits and to plot the results as in Fig. 2. 8/11/87 Polach et al p 4 Selected from Table 1 were, for purposes of direct comparison in Fig 2, results relating to the 3 ml sample size. The error was calculated using a 1000 minute counting error for standard (No) and background (B) and a 3000 min error for the sample, as is the practice at the Austratian National University (10>. The age determination limits were calculated using the internationally accepted 2 <1detection criterion (4). It is seen that the Packard 2000 CML results give a slightly higher error (due to their lower efficiency) and slightly better old age limit (due to their lower background) than the 'Old technology' counters. Only the Pharmacia-Wallac Quantulus in a normal environment. or any liquid scintillation or gas proportional counter placed in an underground laboratory (as demonstrated by the Warragamba experiment), has achieved a higher precision and greater age resolution. Conclusion The results of the intercomparison of liquid scintillation 14c counting systems indicate that, in terms ofeffectiveness for radiocarbon dating{ there is a need to maintain high efficiency whilst significantly reducing the background. Background reduction can be achieved by placing any �-particle counter in a deep underground/ under water/ low y- and cosmic-ray radiation environment. The results further indicate that the term 'low-level liquid scintillation spectrometer'does not by itself guarantee improved 14c resolution as compared to those liquid scintillation counters that make no such claims. This papergives unambiguous parameters enabling the assessment of the merit of any counting system for radiocarbon dating. References 1 . Radiocarbon, American Journal of Science, 1956 to present. 2. Mook w. G. Radiocarbon 25,475 (1983).3. Polach H. A. Radiocarbon 29, 1 (1987).4. Stuiver M. and Polach H. A. Radiocarbon 19,355 (1977).5. Calf G. E. and Polach H. A. In Liquid Scintillation Counting: Recent Advances (Eds Stanley P. E.and Scoggins B. A.) pp. 223-234 (Academic Press, London, 1974).6. Eolach H., Gower J., Kojola H. and Heinonen A. In Advances in Scintillation Counting (EdsMcQuarrie S. A., Ediss C. and Wiebe L.) pp. 508-522 (Univ. Alberta Press, Edmonton, 1983).7. Kaihola L., Kojola H. and Kananen R. Nucl. Instr. & Meth. B, 509 (1986).8. Calf G. E. and Airey P. L. In Archaeometry: an Australasian Perspective (Eds Ambrose W. andDuerden P.) pp. 351- 356 (Australian National University, Canberra 2601, Australia, (1982).9. Polach H. A. Nucl. Instr. & Meth. B (1987, in press).10. Gupta S. K. and Polach H. A. Radiocarbon Dating Practices at the Australian National University(Radiocarbon Dating Research, Garran 2605, Australia, 1985).8/11/87 E C. 0 0 z 100 50 Polach et al p 5 -o- 15ml -+- 7ml .... 5ml -+- 3ml O-t-.......-....... ---.--...----........ ---.�----........ ---.-.-�------.......-i 0 50 100 150 factor of Merit No/ ./e 200 Fig. 1. Comparative merit of liquid scintillation spectrometers for 14c dating. Represented are: 'Old technology', Beckman 7500 (Bn) and LKB-Wallac 1215K (K), left shade, teflon vials (t); 'New technology' Packard 2000 CA/LL (C and H), centre shade - glass vials (g); 'New technology' Pharmacia-Wallac Ouantulus at the Australian National Unh,:ersity location (0), right shade and the Low-level Laboratory at the Wallac Oy location (Lola), both teflon vials; and the 'Old technology' Packard 3330 at the Warragamba dam low-level laboratory (W both (t) and (g) vials). The merit of a liquid scintillation counter for radiocarbon dating is based on: (a) the countrate for the radiocarbon reference standard (No cpm), and (b) the value of the factor of Merit. The higher the values of (a) and (b), the better the counter. 8/11/87 -tn to,,. � � 1000 C) ca C 0 -e ca () 0 i5 100 e . 5to,,. E to,,. w .. · .. Polach et al p 6.... �.. .  ./.... .... ... �···..... . .. ...... Old technology� .• ·······....... /........ .-.Packard .,.... L ··· ............... , .... ..................... Quantulus ············· ....................... ... ........ ........... 10-+-�--�--�--�--�--�--�--�--�--�--�� 0 10000 20000 30000 40000 50000 Radiocarbon ages In years Fig. 2. Achievable resolution of liquid scintillation spectrometers in terms of the radiocarbon dating error and age resolution: Selected for comparison purposes were the results (Table 1) relating to 3 ml of the same sample. Thus, the performance of various counters can be assessed on equalterms. The 'Old technology' and the Pharmacia-Wallac Ouantulus are represented by the dotted lines. The Packard 2000 CML (solid 6ne) crosses but nevertheless closely follows, in terms of resolution for radiocarbon dating, the performance of the 'Old technology' counters. Table 1. Performance of selected liquid scintillation counters for radiocarbon dating • Name Vial b-PBD benzene Net count B count Efficiency No (95% Ox) factor of M Fig of M counter type ml g/L ml g cpm cpm O/o cpm .Nor/a e21B CALF glass 20 15 15 13.115 166.0 2.85 49.2 80.74 48 850 Packard-2000 20 6 15 13.167 183.0 3.48 54.0 89.01 48 840 CA/LL 20 15 7 6.125 78.0 1.58 49.5 37.94 30 1550 20 6 7 6.162 84.0 1.87 53.0 40.86 30 1500 20 6 5 4.445 62.0 1.55 54.2 30.16 24 1900 20 15 3 2.634 29.0 0.73 42.8 14.11 17 2510 20 6 3 2.631 35.0 1.19 51.7 17.02 16 2250 CALF glass 20 15 15 13.115 241.7 1.79 71.7 117.56 88 2870 Packard-3330 20 15 7 6.125 110.9 1.21 70.4 53.94 49 4100 in 20 15 3 2.634 48.5 0.96 71.6 23.59 24 5340 Warragant>a dam Tef/ANSTO 5 15 5 4.394 84.3 0.48 74.6 41.00 59 11590 3 15 3 2.656 49.8 0.31 72.9 24.22 44 17140 POLACH Tef/Cu 15 15 15 13.191 288.2 1.10 84.9 140.18 134 6560 Phannacia-Wallac 7 15 7 6.155 121.3 0.44 76.6 59.00 89 13340 Ouantulus 3 15 3 2.641 51.9 0.21 76.4 25.25 55 27820 Beckman 7500 Tef/Cu 7 15 5 4.395 82.1 3.92 72.6 39.94 20 1350 LKB-Wallac 'K' Tef/Cu 3 15 3 2.641 52.7 3.02 77.6 25.63 15 1990 HARKNESS glass 20 7 15 13.124 195.2 3.16 57.9 94.95 53 1060 Packard-2000 20 15 7 6.121 84.8 1.70 53.9 41.22 32 1710 CA/LL 20 15 3 2.642 37.7 1.27 55.5 18.32 16 2420 7 6.5 7 6.171 90.5 2.02 57.1 44.00 31 1620 7 15 3 2.641 36.8 1.01 54.3 17.91 18 2910 KAIHOLA Tef/Cu 15 15 15 13.185 0.52 75.0 123.80 172 10820 Quantulus 7 15 7 6.150 0.27 73.0 56.20 108 19740 -0 in 'Lola' 3 15 3 2.640 0.14 70.0 23.10 62 35000 :t> ANSTO = Australian Nuclear Science and Technology Organisation; B = Background; M = Merit; Lola= Low-level Laboratory, Wallac Oy; Tef/Cu = TeflorvCopper rt' cu � -0 " 

Performance of new technology liquid scintillation counters for 14C dating

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Performance of new technology liquid scintillation counters for 14C dating

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