Pall Theodorsson

Measurement of weak radioactivity

A historical introduction radioactivity cosmic radiation interaction of radiation with matter dectors sources of background and means of reducing it low-level counting systems coincidence counting systems specific problems in low-level counting - radon, 134Cs, 137Cs, 90Sr. (Part Contents).

A review of low-level tritium systems and sensitivity requirements

Data from three international intercomparison projects are used to study the state of the tritium low-level technique and to set a realistic sensitivity level that should be demanded in ultralow-level tritium work, mainly in hydrology and oceanography. Only two laboratories reach the required goal, a standard deviation < or = 0.03 TU for weak samples. The methods used are described and important parameters illustrated graphically. Possible improvements in the measuring techniques are discussed, including the design of better counting systems.

A new method for automatic measurement of low-level radon in water

Abstract A new single phototube liquid scintillation counting system for automatic determination of 222Rn in ground water for earthquake prediction is described. About 40% of radon is transferred from 1.0 L of water with a stream of air into 25 ml of toluene in a sample vial. Delayed coincidence counting of 214Bi and its decay product, 214Po reduces the background to 0.02 cpm. The 214Po detection efficiency is above 90%. Water containing 20 mBq/L of radon can be measured with 10% accuracy in 3 h.

A multicounter system for scanning ultra-low-level radiochromatograms

Abstract A multicounter system consisting of an integrated array of flow counters for the scanning of ultra-low-level radioactivity on paper and thin-layer chromatograms was developed. Experience with routine measurements over a prolonged period has proved the advantages of this system over other systems, such as liquid scintillation counting. A description of the flow counter system is given together with an outline of an electronic data acquisition system, and results are presented to demonstrate its features.

Norse settlement of Iceland — close to ad 700?

Radiocarbon age determination in the 1970s of some 40 archaeological samples strongly indicates a Norse settlement (landnam) in both Reykjavik and the Westman Islands, Iceland, at about AD 700. This early date has generally been rejected by Icelandic scientists because it contradicts the firmly established dating of the settlement (landnam) at about AD 870. In the present article a critical search for an error in the radiocarbon dating work resulted in no weakness being found in the dating material. The history of the landnam of Iceland is thus caught in a dilemma, one which also affects the history of neighbouring countries. A necessary programme for extensive and more precise radiocarbon dating and a critical re‐evaluation of arguments for the conventional date of settlement are outlined.

External guard counters for low-level counting systems

Abstract The high background reduction that has been obtained with a low-level Ge-crystal counting system with external, rather than internal, guard counters is analyzed It is shown that this arrangement eliminates not only the muon component of the cosmic rays, but also a large part of the secondary gamma radiation it produces in the passive shield and therefore gives a much higher background reduction than the conventional arrangement with the guard system inside the main passive shield. The implication of this result for low-level counting is discussed with special reference to gas proportional counting systems used for radiocarbon dating.

A SIMPLE, EXTREMELY STABLE SINGLE-TUBE LIQUID SCINTILLATION SYSTEM FOR RADIOCARBON DATING

This paper describes a simple and compact liquid scintillation radiocarbon dating system, ICELS, and demonstrates its long-term stability and reproducibility to a precision level rarely presented before, better than 0.04% (3 14C yr). Inexpensive systems of this kind may, in the future, help to meet increasing demand for high precision (16 to 20 14C yr) and strict quality control. ICELS comprises a compact detector unit, where a 3-mL dome-shaped vial, with an optimal light reflector, sits on the top of a vertical 30-mm photomultiplier tube. Sample changing is manual. The high voltage is set at the balance point for each sample, securing maximal counting stability. The quench correction method used (spectrum restoration) corrects with 0.04% precision for all parameters that can normally shift the 14C spectrum. For 3 mL of benzene at 71% 14C counting efficiency (recent carbon 23 cpm), the background is 1.72 cpm behind a 5-cm-thick shield of lead (27 kg) and 1.53 cpm behind 10 cm of lead. The background count rate corrected for atmospheric pressure variations was completely stable over 47- and 57-d testing periods for the 2 systems.

A compact automatic low-level liquid scintillation system for radon-in water measurement by pulse pair counting

A novel compact liquid scintillation counting system with automatic changing of seven samples, dedicated to the assay of low levels of 222Rn in water, is described. Its has a single phototube, and its operating mode is 222Rn series selective, as only the close pulse pairs given by 214Bi and its short lived decay product 214Po (half life 0.16 ms), are counted. Its 214Bi/214Po counting efficiency is 90.4% and the background, with an unshielded detector, is 3 pulse pairs per hour. 75% of the radon in 200 ml water samples is transferred to the scintillator (15 ml toluene) by circulating air, which is bubbled through the two liquids for 4 min. With a counting time of 3 h, the minimum detection level is 8 mBq l(-1). The method is also suitable for various environmental and geophysical studies of 222Rn and 226Ra.

The background of gas proportional counters

A comparison, made eight years ago, of a large number of low-level gas proportional counting systems used for the determination of 14C in radiocarbon dating showed that there can be a large difference in the background of apparently similar systems. This reflects the empirical state of the technique and shows the need for a more detailed and quantitative knowledge of background components. A quantitative evaluation of important background components is presented and the possibility of quantifying others is discussed as well as the advantage of an external guard counting system. Better knowledge of the background components should make it possible to reduce the background below the value of the best counters today.

Improved tritium counting through high electrolytic enrichment

Abstract The sensitivity of the measurements of tritium in natural water has been increased by using volume reduction factor as high as 300 in a single-stage electrolytic enrichment, instead of a factor of 10–20 as normally used. By continuously filling the electrolysis cell from a reservoir a reduction factor of 1500 can be attained in a single stage. Using this technique the simple Geiger counter gives all the desired sensitivity for hydrological studies. A comparison of the present measuring technique with the more important alternative methods is given.