1.3 - Separation of Alpha and Beta Gamma Radiation by Extractive Scintillators and Pulse Shape Discrimination

Extractive agents for radionuclide separation are nowadays available in the form of Extraction Chromatographic Columns (EICHROM, TRISKEM), selective Disk Filter Materials (3M Empore), as Extractive Scintillators in liquid form (3M EMPORE) and as Plastic Scintillator Microspheres (University of Barcelona).

The fundamentals of solvent extraction and extractive behaviour of important radionuclides are outlined in [Moebius and Moebius 2012], for extractive agents see table 6. The method of Extraction Chromatography is described in more detail in chapter 2.2.1.5. and 2.3.2., Plastic Scintillator Microspheres see 2.3.3.

A comprehensive part of analytical procedures, which are presented in this Handbook (e.g. for Ra, Rn, U, Sr, Ni, Fe and Pu) applies solvent extraction as central separation step.

In Extractive Scintillators, a selective extractant is mixed together with a scintillation cocktail. The Extractive Scintillator is contacted before the measurement with the aqueous analyte phase and takes over the function of separating and/or enriching of mostly selective α-emitters. Sample preparation for radionuclide separation and measurement is done in one single analytical step.

A combination of different extractive scintillators allows rapid procedures for the determination of Actinides in nuclear plants [Bickel et al. 1992], [Lauria et al. 1987], [Möbius and Yang 1989] and [Yang et al. 1990, 1991]. Procedures applying Extractive Scintillators for the natural radionuclides Rn, Ra, Pb and Po in aqueous environmental samples are outlined in more detail in our recent “Handbook of LSS“ [Moebius and Moebius 2012], see also [Möbius et al. 1993] and [Yang et al. 1992]. Commercially available Extractive Scintillators are compiled in table 2.

An analytical procedure for the determination of U-isotopes, based on a HDEHP based extractive scintillator is described in chapter 2.2.

Extractive scintillators as organic cocktails combine the advantage allowing additionally a good α/β-PSD-discrimination (fig. 6, 15, 16).

Table 2: Commercially available extractive scintillators (3M EMPORE)

The method of Pulse Shape Discrimination PSD applies the different time duration of α- and β/γ-pulses.

The high specific ionization of α-radiation causes an incomplete energy transfer to the scintillation molecules in a liquid solution. The line spectrum of α-emitters is shifted to lower channel numbers and the whole spectrum becomes broader. The energy spectrum of α-emitters consequently appears in the region of medium and high energetic β-emitters (fig. 13).

A mixture of radionuclides results therefore in a complex spectrum.

Two different regions can be observed, when the light photons being created by the scintillation process are analyzed in dependence on decay time. The main part of the light emission shows a fast exponential decay in the nanoseconds range, while the other component shows slower pulse decay in the range of tenth of microseconds because of triplet transitions. The distribution of these two components depends on the type of radiation. The higher the ionization density of the particle, the higher is the part of the slow component. Therefore, for α-radiation the slow component dominates, while for β- and γ-radiation the fast component is preferred.

The electronic Pulse Shape Discrimination makes use of setting time ramps for α-particles, registering only the 30 to 40 ns longer α-component in separate channels (fig.4). A proper PSD-level may be optimized by visualization of the pulses using a multi channel analyzer. A good α/β-discrimination implies that the pulses are not affected neither by the scintillation cocktail, nor by quenching effects or through the registering electronically device. Even though the method has been published already in 1986 [McDowell 1986, 1991, 1994], counters with satisfying results and β-interferences in the α-channel of less than 0.1 % are only in the last two decades commercially available. Pulses are analyzed both by pulse duration and by pulse height. The practical performance of α/β-discrimination in commercially available LS-counters varies according to the manufacturer.

Figure 4: Time duration of pulses for α- and β/γ- signals and representation in a MCA
Figure 5: α/β-PSD spectrum of an organic Rn containing sample

The counter calibration for α/β-discrimination is outlined in detail in chapter 2.1.3.

Bickel M., Möbius S., Kilian F. and Becker H. 1992: Investigations on a rapid method for the estimation of alpha activity content in nuclear power plant primary coolant; Radiochimica Acta 57 (1992) 141-151

Lauria D. C., Möbius S. and Keller C. 1987: Messung von a-Strahlern durch extrahierende Szintillatoren; Wiss. Abschlußber. 22. Internat. Seminar Univ. Karlsruhe, Karlsruhe: pp 55-66

McDowell W. J. 1986: Alpha counting and spectrometry using liquid scintillation methods; National Academy of Sciences – National Research Council, Nuclear Science Series; Radiochemistry Techniques. Report NAS-NS-3116, pp 88-89

McDowell W.J. and McDowell B.L. 1991: Liquid scintillation alpha spectrometry: a method for today and tomorrow; in: H. Ross et al. “Liquid Scintillation Counting and Organic Scintillators”, pp 105-122, Lewis Publishers, Chelsea, Michigan

McDowell W. J. and McDowell B. L. 1994: Liquid Scintillation Alpha Spectrometry, CRC Press, Boca Raton, Florida

Möbius S. and Yang D. 1989: Neue Wege zur Bestimmung von Alpha-Strahlern; GIT Zeitschrift für das Laboratorium 8 (1989) 1013-1014

Möbius S., Kamolchote P. and Roeksbutr W. 1993: Use of extractive scintillation and pulse shape analysis for environmental alpha-assay; The Science of the Total Environment 130/131 (1993) 467-471

Möbius S., Kamolchote P., Ramamonjisoa T. L. and Yang M. 1993: Rapid detemination of Ra, Rn, Pb and Po in water using extractive liquid scintillation; in: J.E.Noakes et al., “Proceedings Liquid Scintillation Spectrometry 1992”, pp 413-416, Radiocarbon 1993, Tucson

Moebius S. and Moebius T. L. 2012: Handbook of Liquid Scintillation Spectrometry, DGFS e.V. and Karlsruhe Institute of Technology, Karlsruhe 2012

Yang D., Zhu Y., Möbius S. and Keller C. 1990: Simultaneous determination of alpha and beta-emitting nuclides by liquid scintillation counting; J. Radioanal. Nucl. Chem., Letters 144(1) (1990) 63-71

Yang D., Zhu Y. and Möbius S. 1991: Rapid method for alpha counting with extractive scintillator and pulse shape analysis; J. Radioanal. Nucl. Chem., Articles 147(1) (1991) 177-189

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