Tritium measurement

Morgenstern, U. Taylor, C.B. 2005. Low-level tritium measurement using electrolytic enrichment and LSC. Proc. Int. Symp. Quality Assurance for Analytical Methods in Isotope Hydrology. International Atomic Energy Agency. 

It should be emphasized that there are a number of factors in the public health aspects of tritium which are not well established and whose discussion is beyond the scope of this paper. Studies should continue to develop more sensitive techniques for tritium measurement and to increase present knowledge in this field of radiological health. 

The counting of tritium in water is a special problem about which much has been written. Current methods for assay of tritium in water have a range of 0.1 -5000 TU, where a tritium unit (TU) has the value of 7.2 dpm/L. The most desirable feature of a tritium measuring system is that it be capable of measuring a large number of samples rapidly, simply, and cheaply as possible with an uncertainty of +10% or better. It is generally more important to assay 100 samples with an uncertainty of +10% than to assay 10 samples with an uncertainty of +3%. 

The vials that contain the sample, blank, and the diluted standard solution are counted according to the protocol for operating the liquid scintillation counter for tritium measurement. The vials are placed in holders in a chain that moves first to a darkened holding area for decay of luminescence and... 

Low tritium concentrations are determined on samples that have been preconcentrated electrolytically, necessitating samples of about 300 ml. To allow for repeated measurements, samples of 11 should be collected for tritium measurements. 

Figure 10.10 provides data on repeated tritium measurements in a well and in the adjacent Mohawk River. What hydrological conclusions can be drawn The variations in tritium concentrations in the well followed variations in the river, as shown in Fig. 10.10. Hence, the river is recharging the well. The time lag in the well s response can be used to calculate recharge velocities. The data reveal piston flow of the recharge water, with little smoothing by dispersion (Fig. 10.10).

Fig. 10.10 Repeated tritium measurements in a well and in the adjacent Mohawk River, New York, USA. The river was monitored for tritium by beta decay counting, and the results are expressed in pc/1 (1CT12 curie/1). (From Winslow et al., 1965.)...

These examples demonstrate the importance of tritium measurements in every 14C study. In the Paris basin a relatively large number of tritium measurements were done and, except for the four samples mentioned, no tritium was detected in the samples assigned to the confined aquifer. 

Tritium as a low energy beta-emitter is most conveniently measured by liquid scintillation counting. In general, the tritium measurement is the most complicated of all the radionuclide labels. The difficulties are related to the character of the sample which is tritium labeled. Tritium labeling is not suitable in all cases ir. which the system yields high chemiluminiscence the samples are coloured or turbid. Recently, however, new types of liquid scintillation counters appeared which make it possible to count tritium even under such complicated conditions (Beckman, Packard, Kontron, Berthold). 

Tritium measurements are frequently used to calculate recharge rates, rates or directions of subsurface flow, and residence times. For these purposes, the seasonal, yearly, and spatial variations in the tritium content of precipitation must be accurately assessed. This is difficult to do because of the limited data available, especially before the 1960s. For a careful discussion of how to calculate the input concentration at a specific location, see Michel (1989) and Plummer et al. (1993). Several different approaches (e.g., piston-flow, reservoir, compartment, and advective-dispersive models) to modeling tritium concentrations in groundwater are discussed by Plummer et al. (1993). The narrower topic of using environmental isotopes to determine residence time is discussed briefly below. 

All tritium measurements were performed using a 1220 liquid scintillation counter (Wallac Quantulus ). 8ml of aqueous sample were mixed with 12ml Gold Star (Meridian) scintillation cocktail in a 22ml polythene vial. The counter was routinely calibrated for using a traceable tritiated water standard. In this paper, all uncertainties are quoted at the 90% confidence level except for RO water. 

IAEA, Low level tritium measurement, IAEA-TECDOC-246, International Atomic Energy Agency, Vienna (1981). 

Eichinger, L. Forster, M. Rast, H. Rauert, W. Wolf, M.. nLaw-Level Tritium Measurement, IAEA, Vienna, TECDOC 246,1980, pp43-64. Eichinger, L. Rauert, W. Salvamoser, J. Wolf, M. Radiocarbon 22/2, 1980, pp417-427. 

Sea and the Levantine Basin. The deep waters of these basins used to be very homogenous in temperature and salinity displaying only small horizontal gradients (Fig. 4.2 Schlitzer etal., 1991). Roether and Schlitzer (1991) computed an annual mean formation rate of 0.3 Sv for LDW formed in the Adriatic. Using freon and tritium measurements, Roether etal. (1996) calculated a mean residence time of LDW in the eastern basin which was 50-80 years. 

NCRP 1976a. National Council on Radiation Protection and Measurements, Report No. 47. Tritium Measurement Techniques. Bethesda, MD NCRP. 

Incorporation of leucine into the C isoprene units of echinulin (112) through catabolism to mevalonate is thought to proceed via acetoacetate cf. ref. 9. Loss of C-2 of leucine is required in this pathway, which is supported by loss of label from [2- C, 5- H]leucine on incorporation into echinulin (also some loss of tritium, measured in another experiment). ... 

The tritium measurement protocol Is relatively simplistic and consists of neutralization of the sample, single plate distillation, removal of an appropriate size aliquot (usually eight mL.) via reproducible automatic pipets and the addition of 15 mL. of dark adapted scintillation cocktail under Incandescent lighting conditions. After shaking to ensure a uniform gel, the sample is allowed to settle and dark adapt for up to twenty but no more than sixty minutes. The liquid scintillation unit efficiency Is determined dally on a previously prepared standard and background measurements are determined at least dally. Quench corrections are not applied to the system due to the lack of an external standards ratio capability and an effort to minimize the amount of hazardous waste which would be generated if an Internal standards approach were adopted. 

Reinig WC, Hutchinson JMR, Koranda JJ, Moghissi AA, Osbourne RV Ostlund HG (1976) Tritium measurement techniques, NCRP report 47. National Council on diation Protection and Measurements, Washington DC... 

Since 1952, most of the tritium measured in the atmosphere originates from thermonuclear explosions. Like hydrogen, deuterium and tritium also exhibit molecular isomerism. Because of the important differences between the relative atomic masses of the three isotopes, their physical properties (e.g., density, enthalpy of vaporization) differ greatly. This allows an easier isotopic separation than for any other element. Several separation processes are used for the enrichment and separation of hydrogen isotopes. Most of these processes use isotopic exchange reactions (e.g., H D-H O or NH3-HD) and to a lesser extent fractional distillation and water electrolysis (e.g., Norway, Canada). 

---