2.6.14 - Pu-241 in Presence of Other Plutonium Isotopes (LS Alpha Spectrometry)

Introduction

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

• Ethanol

• α-spectrometer

• 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]

1. After leaching and separation by an anion exchange resin, the sample is evaporated to dryness in a crystallization dish.

2. After cooling down to room temperature, the residue is dissolved in 0.4 mL 4 M HNO₃ and then transferred into the electrodeposition cell

3. 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.

4. The solution is electrolyzed for 2 hours with a current of 300 mA.

5. 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.

6. The stainless steel platelet is removed from of the cell, rinsed with distilled water and then with ethanol.

7. After measuring the α-activity by an α-spectrometer, Pu is removed from the platelet carefully with some drops of conc. HNO₃ and H₂O₂.

8. The solution is evaporated to dryness and is then dissolved in 1 mL 0.1 M HCl.

9. 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.

Evaluation

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