2.6.13 - Sample Preparation for Pu-241 Analysis (BioRad)

²⁴¹Pu as low energetic pure β-emitter (E_βmax 21 keV, T_1/2 14.4 a) was introduced into the environment mainly from atmospheric nuclear testing. Because of its α-decaying daughters ²⁴¹Am and ²³⁷Np, it is an important nuclide for environmental assessment. Due to the availability of Actinide-specific resins like TEVA, TRU and UTEVA, the simultaneous determination of several Actinides (Th, U, Pu, Am, Cm, Np) and other radionuclides (²²⁶Ra, ²¹⁰Pb, ¹³⁷Cs, ⁹⁰Sr) from a single sample is possible. For a comprehensive survey see [L’Annunciata 2012] pp 671.

A rather simple procedure applying an anion exchange resin in combination with a Sr SPEC column, enables the determination of Pu, ⁹⁰Sr, and ²⁴¹Am e.g. in soil samples with high chemical yields of more than 70 % [Lee et al. 2011]. The central procedure, which applies the anion exchange column BIO-RAD AG 1-X2, is described below (fig. 33). It has been used successfully for terrestrial samples, when ²³⁸Pu is absent [Eikenberg 2002]. The authors used a low level α/β-PSD LS counter for the measurement of both α- and β-emitting Pu-isotopes. The most optimized pure organic cocktail mixture, being able to still homogenize with the elute solution with still sufficient peak resolution between ²³⁶Pu and ²³⁹ Pu, was found to be a 2/1 Ultima Gold F/AB mixture. In such quench free samples and absence of ²³⁸Pu, a ²³⁶Pu standard can be used as yield tracer by deconvolution of the a-spectrum.

Materials and Equipment

(a) Pu isotopes in absence of ²³⁸Pu (e.g. terrestrial)

• 65 % HNO₃

• ²³⁶Pu yield tracer

• 30 % H₂O₂

• Anion exchanger (e.g. BIO-RAD AG 1-X2, 100-200 mesh)

• Gelating cocktails (e.g. Ultima Gold AB and F, 2:1)

Procedure

(Pu-Isotopes in absence of ²³⁸Pu, terrestrial) [Eikenberg 2002])

1. 30 g soil sample are leached duplicate in 25 mL 65 % HNO_3.

2. ²³⁶Pu yield tracer is added, followed by 65 % HNO₃ / 30 % H₂O_2.

3. The solution is passed through an anion exchanger with a very high affinity for Pu fixation (e.g. BIO-RAD AG 1-X2, see procedure 2.3.12.).

4. Interfering ions are removed with 20 mL each 8 M HNO₃ and 10 M HCl.

5. Pu is eluted with 20 mL of a solution of 9 M HCl + 0.1 M HI (the latter causes reduction to Pu(III)) into a beaker containing 2 mL H₂O, 1 mL 98% H₂SO₄ and 0.1 mL 1 M NaHSO₄ solution.

6. The eluted Pu fraction is evaporated to dryness.

7. The residue is dissolved in 1 mL H₂O and homogenized in a cocktail mixture, consisting of Ultima Gol AB and F (2:1).

8. The sample is counted by α/β-PSD. ^239,240Pu is evaluated from the a-channel and ²⁴¹Pu from a low energy β-channel.

9. The chemical yield (typically 70-80 %) is monitored by separating the a-peaks between the ²³⁶Pu tracer (5.77 MeV) and ^239,240Pu (5.16 MeV) with relatively good resolution (0.25 MeV FWHM) in a weakly quenching α/β-cocktail.

MDA for 30 g soil samples: 0.2 Bq/kg (2 h counting time) with α/β-PSD

L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3^rd Edition 2012, Elsevier

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

Lee M.H., Ahn H.J., Park J.H., Park Y.J. and Song K. 2011: Rapid sequential determination of Pu, ⁹⁰Sr and ²⁴¹Am nuclides in environmental samples using an anion exchange and Sr-SPEC resins; Appl. Radiat. Isot. 69 (2011) 295-298