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Year : 2020  |  Volume : 43  |  Issue : 3  |  Page : 134-139  

Assessment of 235U and 234U concentration in confiscated uranium samples by alpha- and gamma-ray spectrometry techniques

Radiation Safety System Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Submission12-Jun-2020
Date of Decision26-Jul-2020
Date of Acceptance23-Sep-2020
Date of Web Publication6-Jan-2021

Correspondence Address:
Amar Dutt Pant
Radiation Safety System Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/rpe.RPE_30_20

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Identification and assessment of the isotopic composition of confiscated uranium are of very much concern when it enters the public domain. This paper describes the methodology used for the estimation of 235U and 234U concentration in five uranium samples using alpha and gamma spectrometry, and the results are compared with that of mass spectrometry technique. The activity ratio of 238U to 235U is obtained from the measurement on the 185 keV gamma-line of 235U and 1001 keV gamma-line of 234mPa. The isotopic abundance of 235U in the confiscated sample using radioanalytical techniques was found in the range of 0.22%–0.24%.

Keywords: Alpha spectrometry, depleted uranium, gamma-ray spectrometry

How to cite this article:
Pant AD, Pillai AS, Krishnan N, Verma AK. Assessment of 235U and 234U concentration in confiscated uranium samples by alpha- and gamma-ray spectrometry techniques. Radiat Prot Environ 2020;43:134-9

How to cite this URL:
Pant AD, Pillai AS, Krishnan N, Verma AK. Assessment of 235U and 234U concentration in confiscated uranium samples by alpha- and gamma-ray spectrometry techniques. Radiat Prot Environ [serial online] 2020 [cited 2021 Jan 17];43:134-9. Available from: https://www.rpe.org.in/text.asp?2020/43/3/134/306276

  Introduction Top

Uranium present in nature consists of three isotopes 238U, 235U, and 234U having abundances (Abs) nearly 99.27%, 0.72%, and 0.0057%, respectively. The uranium having 235U concentration above 0.72% is classified as enriched and below is classified as depleted. The depleted uranium (DU) is produced as a by-product of the enrichment processes and spent fuel reprocessing. Due to high density (19.1 g/cm3) of uranium, DU has wide applications in many areas such as gamma radiation shielding, penetrating calibers, and counterweights in aircraft. Having such a diverse nonnuclear application, the occurrence of this material in the public domain is quite common and the determination of uranium isotopic content in such samples is very much important because of its radioactive nature. At present, various radioanalytical techniques can be used for determining the isotopic composition of uranium, e.g., mass spectrometry,[1],[2],[3],[4],[5] alpha spectrometry,[6],[7],[8],[9] and gamma spectrometry.[10],[11] Alpha and mass spectrometry techniques are very sensitive and can detect the isotopic composition of uranium up to ppb level. These sophisticated methods offer very low combined uncertainty but take more time for sample preparation and analysis. Alpha spectrometry system is less costly than a mass spectrometry system and commonly used for routine sample analysis in laboratories.

Gamma-ray spectrometry is a powerful nondestructive analytical tool to analyze gamma active samples both qualitatively and quantitatively.[12] High-purity germanium (HPGe) detectors are widely used for high-resolution gamma spectrometry measurements. Naturally occurring uranium isotope 238U emits low-energy gamma-rays, namely 49.5 keV (0.064%) and 113.5 keV (0.0112%) of very small Ab. It makes quantitative estimation difficult from direct measurement of these gamma-lines. To obtain exact information about the measurement of 238U by gamma-ray spectrometry, any one of the daughters of 238U must exist in equilibrium with 238U.[13] The secular equilibrium of 238U–234mPa will be attained after about 4 months in a chemically separated uranium sample because of the short half-life (24.1 days) of 234Th. In equilibrium matrix, the gamma energies 63.3 keV (3.7%) of 234Th and 766 keV (0.317%) and 1001 keV (0.842%) of 234mPa, daughters of 238U, have been considered for analytical purposes.[14] The prominent gamma energies emitted by 235U are 143.7 keV (10.96%), 163.3 keV (5.08%), 185.7 keV (57.2%), and 205.3 keV (5.01%) used for gamma-ray spectrometry purposes.

Alpha spectrometry technique is the common technique that detects low levels of uranium (below ng/L). The detection limit of the alpha spectrometric technique is 100–1000 times lower than of gamma spectrometry technique[7] as the emission probability of alpha-particles is much higher than that of the gamma-ray. The separation of uranium from the sample is achieved by passing through an ion-exchange column, and later, electrodeposition is carried out for sample preparation to get better resolution. The most prominent alpha-energies of 238U, 235U, and 234U are 4197 keV, 4390 keV, and 4774 keV, respectively.

The present work describes the methodology to determine the concentration of 235U in uranium samples using alpha and gamma spectrometry as radioanalytical techniques. The results obtained are compared with measurements carried out with a mass spectrometry technique.

  Materials and Methods Top

Gamma spectrometry

Gamma spectrometry measurements were carried out with HPGe-based high-resolution gamma spectrometry system. The detector is co-axial P-type germanium having a relative efficiency of 30% and resolution better than 2 keV for 1332 keV of 60Co. The electronic setup was completed with digital spectrometric device ORION multichannel analyzer, and an analysis of the spectral data was carried out with InterWinner 7.0 software. The detector is shielded with 3” lead on all sides to reduce natural background. The energy calibration was carried out using 661.62 keV of 137Cs and 1173.24 keV and 1332.46 keV of 60Co standard sources, and efficiency calibrations were carried out using 152Eu National Institute of Standards and Technology certified gamma-ray reference standards. The measured photopeak efficiencies of prominent and distinct gamma-lines were fitted into the following logarithmic empirical function[15] (Equation 1):

where is the full-energy peak efficiency at a given gamma-energy of and A1, A2, and A3 are regression coefficients obtained from the fitting of measured efficiency data with energy. The typical detector efficiency calibration curve for the experimental geometry is shown in [Figure 1]. Five metal samples of uranium, each having weight nearly 3 kg, confiscated from the public domain were analyzed for the present work. The fine fillings are obtained from these metal pieces by machining. The fillings are filled into standard calibrated geometry for which the empirical efficiency-energy function is known. These fine fillings have moderately low self-attenuation for low-energy gamma rays emitted from the sample. The net weights of these samples are in the range of 50–100 mg. The samples were packed in air-tight bags of 1” x 1” size for HPGe-based analysis.
Figure 1: Efficiency calibration curve for the high.purity germanium detector

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Alpha spectrometry

Alpha measurements were carried out in a 300 mm2 Passivated Ion-Implanted Planar Silicon (PIPS) detector-based alpha spectrometry system. The PIPS detectors have high resolution, good stability, and low alpha background. The detector efficiency is nearly 20% in the energy range from 4.0–8.0 MeV. It is estimated using the following Equation 2:

where η is counting efficiency of alpha spectrometry system, N is alpha counts for a given time, A is the radioactivity of the radionuclide, and t is the counting time. For better alpha resolution, the measurements were carried out in controlled vacuum chambers. The typical alpha resolution comes out in the range of 20–25 keV for 5486 keV of 241Am standard. The energy and efficiency calibration was carried out using 239Pu and 241Am standard sources.

The uranium samples of approximately 100 mg each were dissolved in conc. HNO3 and solution were heated to dryness. Then, a stock solution of samples was prepared in 1 mol/L HNO3.

8 mol/L HNO3 was added to the solution and heated to dryness, and then, 8 mol/L HCl was also added to the residue and heated to dryness. This procedure of 8 mol/L HCl was repeated twice to bring all uranium in chloride form. The final solution was taken up in 8 mol/L HCl for column separation. An anion-exchange resin of 2 g (Dowex 1 × 8) was soaked in de-ionized water for 2 h. The soaked resin was packed into the cylindrical glass column of dimension 20 cm × 0.8 cm and preconditioned with 8 mol/L HCl. Uranium sample solution was loaded on the column, and the flow rate was maintained at 1.5–1.8 mL/min. Uranium was stripped from the column with 0.1 mol/L HNO3. Uranium elution of all the samples was carried out in a duplicate manner. The eluted solution was used for the electrodeposition of uranium.

The solution was electroplated using (NH4) 2SO4 electrolyte. The electrolysis was carried out on a clean, dry, mirror polished small stainless steel disc with 3.0A constant current and 6V for 2 h 30 min. A few milliliters of NH4OH solution was added to the electrolyte before completion of electrolysis to make a deposit firmly stick on the disc.[16] The electroplated samples were washed with distilled water and ethyl alcohol. These samples were further dried under an infra-red lamp and heated to red hot in a Bunsen flame.

  Results and Discussion Top

Estimation of 235U concentration

The 185 keV gamma-line of 235U and 1001 keV gamma-line 234mPa had been selected for the estimation of 235U and 238U concentration, respectively. The gamma-ray spectra of a unit mass of natural uranium standard and uranium sample are shown in [Figure 2] and [Figure 3], respectively. From the figures, it can be inferred that an Ab of 185 keV of 235U is low in uranium samples as compared to standard natural uranium. The activity ratio and Ab ratio can be related as follows:
Figure 2: Gamma spectrum of standard natural uranium

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Figure 3: Gamma spectrum of uranium sample

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By substituting the known half-lives (T238U , T235U) for 238U and 235U, we can write:

Then, the Ab of 235U can be calculated particularly in natural and DU samples by assuming that the entire uranium mass is contributed from the isotope 238U and 235U only neglecting the small contribution (~0.0057%) from 234U. If X is the percentage Ab of 235U, then the percentage Ab of 238U will be 100-X. Then, by substituting the experimental value of activity ratio, we will get the Ab value X (% 235U) as:

where and are activity concentrations of 238U and 235U, respectively. All samples were analyzed by the gamma spectrometry system, and the activity ratio of 238U to 235U was calculated. Activity concentrations of 235U and 238U isotope in all five samples are shown in [Table 1]. The isotopic Ab of 235U obtained using gamma-ray and alpha spectrometry technique was compared with results from mass spectrometry technique (measurements were carried out at our centre's mass spectrometric laboratory and not in the author's laboratory) and is shown in [Table 2].
Table 1: Activity concentration of 235U and 238U from gamma spectrometry

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Table 2: Isotopic abundance of 235U using different techniques

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From [Table 2], it is observed that 235U percentage isotopic concentration is in the range of 0.22–0.24 which confirms a uranium sample as DU. Since no fission product and by-product actinide activity was found in the sample, it can be concluded that the sample is not from the reprocessing facility. 234U is having a very weak gamma emission which makes it difficult to assess the isotopic content of this radionuclide by gamma-ray spectrometry. The isotopic Ab of 234U was obtained using alpha and mass spectrometry. The specific activity of 234U is less in DU than natural uranium, which is seen in the alpha spectrum of DU. [Figure 4] and [Figure 5] represent the alpha spectra of natural uranium standard and uranium sample, respectively. The isotopic Ab of 234U obtained using alpha spectrometry and compared with the mass spectrometry technique is shown in [Table 3]. Alpha spectrometry technique is very efficient in the determination of 234U, 235U, and 238U Abs in the samples. The high-resolution gamma spectrometry adopted for the isotopic estimation of 235U in the uranium samples is rapid and nondestructive with reasonably good accuracy. The error in the measurement may be attributed to the assumption of ignoring the mass contribution of 234U in equation 5 and the use of low-abundant gamma energy for the calculation of 234mPa activity. This method may lead to some error for enriched uranium samples where 234U concentration is significant. The results obtained from the gamma-ray spectrometry measurement were in good agreement with that of alpha and mass spectrometry, thus confirming the adequacy of the measurement technique for the quick estimation of the isotopic composition of 235U in uranium samples nondestructively.
Figure 4: Alpha spectrum of standard natural uranium

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Figure 5: Alpha spectrum of one of the uranium samples

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Table 3: Comparison of 234U abundance using alpha and mass spectrometry

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  Conclusions Top

In the present work, the application of high-resolution gamma-ray spectrometry and alpha spectrometry has been established for the determination of the isotopic Ab of 235U and 234U in uranium samples confiscated from the public domain. The results obtained from our study have been compared with that from mass spectrometry techniques and are in good agreement. The analysis of samples by high-resolution gamma spectrometry confirmed the material as uranium due to the presence of gamma energies of 235U and 234mPa in the spectrum. The estimated isotopic Ab of 235U from the spectrum of the samples confirmed the samples as DU. The advantage of gamma spectrometry against other techniques is rapid, nondestructive, and relative simple to assess the uranium isotopic ratios in samples. This technique is very useful in obtaining the information regarding the type of radioactive material appearing in the public domain in emergency scenarios within a short period which enables the first responders to decide the action plan. For getting the full isotopic information, it is necessary to obtain the concentration of 234U. The concentration of 234U also varies during various nuclear fuel cycle operations but may not contribute significantly to the mass of total uranium. However, information regarding this isotope is very important in identifying the source of uranium material in nuclear forensic attribution purpose. As 234U is not having significant gamma emissions, the assessment of isotopic content of this nuclide is not possible by gamma spectrometry techniques, and hence, alpha spectrometric technique was used. The alpha spectrometry technique requires radiochemical treatment of the sample and electrodeposition before the measurement making it time-consuming. Practically, most of the time nondestructive analysis of 235U in the sample is used for the classification of uranium material. The assessment of 235U content and its isotopic Ab is very important for the classification of uranium material encountered in various stages of fuel cycle operation and nuclear forensic investigation point of view. The isotopic Ab of 235U in samples using both analytical techniques was found in the range of 0.22%–0.24%. The isotopic Ab of 234U using alpha and mass spectrometry techniques was found in the range of 0.0008%–0.0009%. The absence of any by-product actinides in the sample by alpha spectrometry measurement also confirms that the DU sample is not from the reprocessing facility. The sample may be the by-product of the normal enrichment process using natural uranium. For the comprehensive isotopic analysis of all isotopes of uranium, it is required to go for a combination of analytic techniques such as gamma spectrometry, alpha spectrometry, and mass spectrometry.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2], [Table 3]


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