2.5.1.8 - Uranium Isotopes by Extractive Scintillation

Introduction

Several separation steps before the measurement of Uranium are necessary in order to avoid interferences from other Actinides [Prichard and Cox 1991]. Radon also interferes, as it is transferred into the organic phase and forms daughter nuclides.

Uranium may be complexed and extracted into the organic phase by di-ethylhexyl-phosphoric acid (HDEHP), tri-octyl-phosphinoxide (TOPO) or tri-butyl-phosphate (TBP).

We have applied a rapid procedure for Uranium in water by HDEHP extractive scintillation. The method has been applied for the determination of Depleted Uranium within the IAEA-Training course on „Depleted Uranium (DU) in the Environment“ [IAEA 2001]. Depleted Uranium has been used as penetrator during the war in Kosovo and Iraq.

The method is simple and fast, however, can only be used in absence of interfering radionuclides. Thorium, when present, has to be complexed by fluoride ions (for details see literature, e.g. in [GMELIN 1990], Th.

Materials and Equipment

• 2 M HNO₃

• Organic cocktail (Toluene Scint)

• HDEHP

• Separation funnel (125 mL)

• PSD LS counter

Procedure

(according to [Prichard and Cox 1991], see also [HIDEX 1998, 2000])

1. 100 mL of water sample are flushed with inert gas (Radon exhalation).

2. The sample is acidified with HNO₃ to pH 1.5.

3. In a separation funnel the aqueous phase is contacted with 20 mL Toluene Scint containing 60 g/L HDEHP by viguorous shaking.

4. The organic phase is transferred into a LS vial and stored 3 hours for the decay of Radon daughter products.

5. The measurement is done in the α-channel using PSD. 

Evaluation

The activity concentration A_C of Uranium is calculated from the net rate in the α-channel R_N by

whereas

R_N = Net rate (cps)

ε = Measuring efficiency

h = Extraction yield

V = Sample volume (mL)

Detection Limit (MDA): 20 mBq/L

Total analysis uncertainty: 5 % in absence of interfering radionuclides

The uncertainty budget is mainly caused by the extraction yield. For samples with different composition it should be quantified using a U-tracer (²³²U).

The extraction yield has been found to be > 90 %. Figure 23 shows a typical spectrum of natural Uranium containing both α-emitting isotopes ^238/234U in equilibrium. It can be seen that the energy spectrum in the α-channel allows predictions on the isotopic composition of Uranium. The method is suitable as well for in-situ measurements using the mobile Triathler device. Figure 15 (chapter 2.1.3.) presents the corresponding 2D PSD plot.

GMELIN 1990: Möbius S.: Analytical chemistry of Thorium; in: Gmelin Handbook of Inorganic Chemistry Thorium, Supplement Volume A5, pp 1-67, Springer Verlag, Berlin 1990

HIDEX 1998, 2000: Triathler applications and technical features: liquid scintillation determination of Uranium (²³⁹U/²³⁸U) from water by cocktail extraction; DOC 411-008 HIDEX Oy, Turku, Finland

IAEA 2001: Training Course on Depleted Uranium in the Environment; Karlsruhe und Seibersdorf 2001, International Atomic Energy Agency, Course Material on CD