Simple source preparation, high counting efficiencies and the ability of simultaneous determination for α- and β-emitters by pulse shape discrimination favors liquid scintillation as potential counting method for 241Pu. After measuring the Pu sample by α-spectrometry, LSC is used for determining the 241Pu activity. The initial α-spectrometric measurement is required, when the measurement of all Pu isotopes is necessary. Additionally, it provides the chemical yield through the 236Pu tracer. It may be combined with α-spectrometry (procedure b) for the α-emitting Pu nuclides, but can be applied as well as single method, when 236Pu is introduced as yield tracer and sufficient α-resolution is provided (procedure a).
Materials and Equipment
- Crystallizing dish
- 4 M HNO3
- Electrolysis cell 300 mA, stainless steel platelet
- 4 % (NH4)2C2O4
- 25 % NH4OH
- Conc. HNO3 and H2O2
- 0.1 M HCl
- Gelating cocktail
Procedure: Pu Isotopes by Combined a– and Liquid Scintillation Spectrometry
for sample preparation see procedure 2.3.12. [Eikenberg 2002]; for electrolysis details see [Möbius 1988]
- After leaching and separation by an anion exchange resin, the sample is evaporated to dryness in a crystallization dish.
- After cooling down to room temperature, the residue is dissolved in 0.4 mL 4 M HNO3 and then transferred into the electrodeposition cell
- The dish is washed 3 times with 1 mL 4 % (NH4)2C2O4, each, and with 0.6 mL H2O respectively. All washing solutions are transferred into the electrolysis cell.
- The solution is electrolyzed for 2 hours with a current of 300 mA.
- Before switching off the current, 1 mL 25 % NH4OH is added. Electrolyzing is continued for 1 min, the solution discarded, the cell rinsed with distilled water, and only then the current is disconnected.
- The stainless steel platelet is removed from of the cell, rinsed with distilled water and then with ethanol.
- After measuring the α-activity by an α-spectrometer, Pu is removed from the platelet carefully with some drops of conc. HNO3 and H2O2.
- The solution is evaporated to dryness and is then dissolved in 1 mL 0.1 M HCl.
- This solution is mixed with scintillator for 241Pu measurement in an optimized α/β-PSD unit with respect to pulse shape and pulse height discrimination.
If Fe and Pt are also electroplated with Pu, an additional solvent extraction or ion exchange step is recommended.
Remark: The dissolution of deposited Pu may be omitted if the purified Pu solution is divided into two portions in order to separate α-spectrometry and LS measurement.
The activity concentration of the low energy β-emitter 241Pu is calculated from the LS measurement according to the general scheme by
The measuring efficiency ε should be slightly below 60 %, but must be controlled by quench correction. The chemical yield η is quantified by isotope dilution of 236Pu by α-spectrometric measurement in step (7).
The α-emitting Pu isotopes are counted in an α-window by α/β-PSD. As the lower α-part overlaps with the β-window, the reduced α-counting efficiency has to be taken into account.
Lower Limit of Detection
for α-emitters: 10 mBq per sample
for β-emitters (241Pu): 50 mBq per sample
Eikenberg J., Bajo S., Ruethi M., Gann C., Beer H. and Butterweck. G. 2002: A rapid procedure for determining 239+240Pu and 241Pu in environmental samples using a/b LSC; in: S. Möbius et al. “LSC2001 Advances in Liquid Scintillation Spectrometry”, pp 351-362, Radiocarbon 2002, Tucson
Möbius S. 1988: Experiments for training in nuclear and radiochemistry; Report KfK 3920, Kernforschungszentrum Karlsruhe, Karlsruhe 1988