Rapid determination of 237Np in soil samples by multi-collector inductively-coupled plasma mass spectrometry and gamma spectrometry

A radiochemical procedure is developed for the determination of 237Np in soil with multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) and gamma-spectrometry. 239Np (milked from 243Am) was used as an isotopic tracer for chemical yield determination. The neptunium in the soil is separated by thenoyl-trifluoracetone extraction from 1 M HNO3 solution after reducing Np to Np(IV) with ferrous sulfamate, and then purified with Dowex 1 × 2 anion exchange resin. 239Np in the resulting solution is measured with gamma-spectrometry for chemical yield determination while the 237Np is measured with MC-ICP-MS. Measurement results for soil samples are presented together with those for two reference samples. By comparing the determined value with the reference value of the 237Np activity concentration, the feasibility of the procedure was validated.


Introduction
Neptunium-237 (T 1/2 = 2.14 9 10 6 years) is released to the environment as a result of nuclear weapons tests, reactor accidents and nuclear fuel reprocessing. As a multivalent element, Np may be mobile in certain speciation and migrates into biosphere with underground water. When ingested by human being, Np accumulates in the liver and bones. Therefore, 237 Np is regarded as a highly radiologically toxic pollutant due to its alpha particle emission and long half-life. In order to assess its environmental risk and determine its origin, the quantification of 237 Np in soil is necessary. Due to the low concentration of 237 Np in the environment, preconcentration is usually required before it can be measured with alphaspectrometry or ICP-MS [1]. 236 Pu or 242 Pu is usually used as a yield tracer for 237 Np because there lack appropriate isotopic tracers for 237 Np yield determination [2][3][4]. Isotopes of Pu are not good tracers for Np because the two elements often fractionate during chemical processing. 235 Np is a potential tracer for 237 Np [5,6]. However, 235 Np free from contamination of 237 Np is not commercially available. 236 Np was utilized as the tracer by some researchers to assay 237 Np in environmental samples where measurement was carried out with mass spectrometry [7][8][9]. However, 236 Np is not easy to produce and still not available in pure form to most researchers [4]. 239 Np has been used as a yield tracer for chemical recovery determination of 237 Np in the environmental samples with alpha-spectrometry [6,[10][11][12][13]. However, the analysis of 237 Np with alpha-spectrometry usually costs too much time due to its low concentration. Such analytical approaches also require additional chemical operations, such as electrolytic deposition. MC-ICP-MS is particularly effective for measurement of long-lived actinide isotopes with lower specific activity, including 237 Np. The purpose of this paper is to develop a method for rapid determination of 237 Np using 239 Np as a yield tracer, in which MC-ICP-MS and gamma-spectrometer are employed to measure 237 Np and 239 Np, respectively.

Procedure for determination of 237 Np in soil
The new procedure for determination of 237 Np in environmental soil samples is based on thenoyl-trifluoroacetone (TTA) extraction combined with anion exchange chromatography as illustrated in Fig. 2 and described as following.
(1) The soil sample was pulverized and dried to constant weight in an oven at 110°C, and then homogenized with a spatula. (2) Add 1 g of homogenized soil to a clean beaker and ignite at 550°C in a furnace over night.

Purity check of 239 Np tracer
The 239 Np intended as a tracer for 237 Np was obtained from 243 Am as described above. In order to assure the suitability of the 239 Np as a tracer, its purity was checked with a HPGe gamma-spectrometer. Figure 3 shows the gammaray spectrum of 243 Am in equilibrium with 239 Np before separation. The peaks of both 239 Np (277.6 keV) and 243 Am (74.7 keV) appear clearly in the spectrum. However, when 239 Np was separated from 243 Am parent and checked with a HPGe gamma-spectrometer, only the peak of 239 Np can be seen in the spectrum and no lines for 243 Am are distinguishable in the spectrum (see Fig. 4). 239 Np is a short-lived b-emitter (T 1/2 = 2.355 days) while 243 Am is a long-lived a-emitter (T 1/2 = 7370 years). The activity equilibrium of this mother/daughter pair is reached after 23.6 days, about 10 half lives of 239 Np [14,15]. Therefore, 239 Np can be prepared from the same 243 Am solution again and again after the activity equilibrium of this mother/daughter pair is reached.

Validation of the analytical procedure
In order to validate the applicability of the analytical procedure to soil samples with complicated matrix, two reference soil samples R1 (1 g) and R2 (1 g), with known 237 Np activity concentration were analyzed for 237 Np concentration according to the procedure described above and the results are compared with the reference values in Table 1. The 237 Np activity concentration of R1 was 0.040 Bq/g, with a difference of -3.6 % compared to the reference value. The 237 Np activity concentration of R2 was 0.050 Bq/g, with a difference of 2.0 % compared to the reference value. It can be seen that the determined results are in good agreement with the reference data for both reference soils, suggesting that the new analytical method applies to soil sample very well. Application of the proposed procedure in practical environment samples assay To verify the feasibility of the proposed analytical procedure for practical environment samples, the 237 Np activity concentrations of the two real soil samples (namely, S1 and S2) were determined following the proposed procedure. The S1 and S2 surface soil samples (sandy soil) were collected from the northwest of China, near a nuclear facility site. The soils were ground and sieved to get a powder with particle diameters ranging from 74 to 149 lm.
The uniformity was checked with gamma spectrometric measurement.
After the separation procedure, the resulting solutions containing purified 237 Np and spiked 239 Np were transfered to clean tubes and measured with the gamma-spectrometer in the same geometry as the 239 Np tracer solution had been measured. By this way, the detection efficiency of the detector is not required to be calibrated because only relative counts in the same equipment are used in calculations. The 277.6 keV photopeak of 239 Np was chosen because of its relatively high intensity and its location in a comparatively flat baseline region of the spectrum. The yield of 239 Np (denoted Y) is calculated by means of Eq. (1).
where C s is the counts of 239 Np in the resulting solution, cps; C 0 is the counts of 239 Np in added tracer spike, cps; k is the the decay constant of 239 Np, s -1 ; t the time gap between the two measurements of 239 Np, s. The yields of 239 Np are listed in Table 2 together with the determined 237 Np concentration. It can be seen that although the 237 Np activity concentration of the two soil samples is very low, the reproducibility of the results is \6 %, which is rather good for environmental analysis. However, the chemical yield is a bit lower due to the complicated matrix of the soil.
Due to the low concentration of 237 Np in the soil samples, the measurement with alpha-spectrometry [13] may cost too much time (about 3-4 days) in order to have precise results. Besides, the matrix components, including

Conclusion
A new rapid separation method was developed for the determination of 237 Np in soil samples with MC-ICP-MS and gamma-spectrometry. There are two advantages of the present procedure. One is that it adopts gamma-emitting 239 Np instead of 236 Pu or 242 Pu as the tracer, and the other is that the yield of Np can be determined without relative efficiency calibration of gamma-spectrometer for 239 Np sources. The feasibility of the procedure was validated by analyzing the two reference soil samples with known 237 Np activity concentration.
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