Measurement of naturally occurring radioactive materials, ²³⁸U and ²³²Th: anomalies in photopeak selection

Abstract

There are more than 200 photopeaks of various daughter radionuclides of ²³⁸U and ²³²Th series, some of which have been randomly used for quantitative measurement of U/Th in natural samples. It has been observed that arbitrariness in photopeak selection may fail to stipulate statistically consistent data. This paper judiciously selects set of three photopeaks from each series whose respective averages could present statistically reliable measurement of ²³⁸U and ²³²Th based on minimum relative standard deviation (RSD) under the selected photopeaks. RSD is also proposed as an important parameter in NORM measurement.

Introduction

The ubiquitous natural background radiation felt on the Earth is mainly due to terrestrial and cosmic radiation [1]. Long-lived, primordial naturally occurring radionuclides or NORMs like ²³⁸U (T _1/2 = 4.468 × 10⁹a), ²³⁵U (T _1/2 = 7.04 × 10⁸a), ²³²Th (T _1/2 = 1.40 × 10¹⁰a) and ⁴⁰K (T _1/2 = 1.248 × 10⁹a) have geological presence since formation of the Earth [2]. They along with their daughter products (²²⁶Ra, ²¹²Pb, ²¹²Bi, ²²⁸Ac, ²¹⁰Pb, ²⁰⁸Tl, etc.) are prime contributors of background radiation. The global mean of ²³⁸U, ²³²Th and ⁴⁰K in terrestrial system reported are 35, 45 and 420 Bq/kg respectively [3]. The enhanced concentration of natural and anthropogenic radionuclides resulting due to human activities like mining, refining, nuclear experiments, etc., is termed as technologically enhanced naturally occurring radioactive materials or TeNORMs [4]. It could be further stated that nuclear weapon testing (1960–1970), Chernobyl accident (1986) and recent Fukushima-Daichii accident (2011) have made significant contribution to the global inventory of anthropogenic radionuclides.

There are several reports on measurement of NORMs (²³⁸U, ²³²Th and ⁴⁰K) all over the globe. These measurements have come out from laboratories with moderate experimental facilities as well as from renowned laboratories equipped with state-of-art detectors. The sample size for NORM measurement generally varied in the reported works from 20 to 50 g, which was further normalized to Bq/kg. The estimated radioactivity level of ²³⁸U and ²³²Th in such sample could be around only 1–2 Bq. Therefore slight discrepancy in measurement would reflect in terms of high uncertainty in the final normalized value. Low-level radiation measurement requires selection of high efficiency detector, accurate energy and efficiency calibration, optimum counting time, proper selection of photopeaks, etc. The literature review reveals that researchers in many cases have arbitrarily fixed the above-mentioned experimental parameters. In the present work we have discussed about proper selection of photopeaks from the daughter radionuclides of ²³⁸U and ²³²Th series to get reliable estimate of uranium and thorium present at ultra-low level concentration in natural matrices.

Different investigators have measured activity of ²³⁸U and ²³²Th by selecting different photopeaks; most of them selected multiple gamma-peaks from different daughter radionuclides of the corresponding series, and presented the average value of the activity of ²³⁸U and ²³²Th. Even when multiple photopeaks were used, different groups selected different sets of photopeaks (not necessarily the most intense peaks). In Table 1, we list the sets of photopeaks taken by various research groups to measure ²³⁸U and ²³²Th activity [1, 3, 5–18, 20–50]. A careful look to this table shows some interesting and apparently illogical choice of photopeaks. Few of them are illustrated here. Mohapatra et al. [1], Sartandel et al. [5], Wang et al. [6], Al-Sharkawy et al. [7], Chowdhury et al. [8], Janković et al. [9] and Song et al. [10], have considered low intensity (4.8 % only) 63.29 keV (²³⁴Th) photopeak for ²³⁸U activity measurement. Chowdhury et al. [8], Janković et al. [9], Mahur et al. [11] have included very low intensity (0.65 % only) 1001.03 keV photopeak of ²³⁴Pa along with other peaks to measure ²³⁸U. Many authors [12–18] have considered 186.21 keV photopeak of ²²⁶Ra, member of ²³⁸U decay series, to measure ²³⁸U. However, this photopeak may have significant interference from ²³⁵U, which could be as high as 11.4 % [19], therefore should be avoided otherwise correction for ²³⁵U should be made. Al-Sharkawy et al. [7], have selected both 92.38 and 92.8 keV photopeaks for ²³⁸U measurement. Both of these photopeaks have low intensities (2.81 and 2.77 % respectively). They also reported that they have measured using 50 % p-type HPGe detector, which normally will be unable to resolve these photopeaks. Similarly for ²³²Th measurement many authors [7, 14, 16, 20] have measured 964.76 keV (4.99 %) and 974.2 keV (0.05 %) photopeaks, both from ²²⁸Ac. These peaks are situated on the shoulder and on the trail of 968.97 (15.8 %) keV photopeak respectively and therefore difficult to have statistically reliable area count.

Table 1 List of photopeaks taken by different groups of researchers for measurement of ²³⁸U and ²³²Th activity

The pertinent question therefore boils down to which photopeaks are preferable for low-level measurement? In this paper we made an attempt towards optimization of NORM measurement (²³⁸U and ²³²Th) with respect to selection of photopeaks from different daughter radionuclides of ²³⁸U and ²³²Th decay series. To the best of our knowledge, despite large number of measurements on NORM reported in literature, this type of detailed analysis has been attempted for the first time.

Initial screening of photopeaks

In Table 2, we list the gamma energies of different daughter radionuclides of ²³⁸U, ²³²Th and major photopeaks of ²³⁵U. As ultra-low level activities are measured in NORM measurement, we excluded the photopeaks having intensities less than 2 % in ²³⁸U and ²³²Th series. However, some of them are even listed in the table, if frequently taken by different research groups (e.g., 1001.00 keV photopeak of ²³⁴Pa having intensity 0.65 %). Also we have excluded all the photopeaks of ²¹⁰Tl and ²⁰⁶Tl, which belong to ²³⁸U series. The reason of exclusion is extremely low population from their parent radionuclides, e.g. ²¹⁴Bi decays to ²¹⁰Tl with branching ratio 0.02 % only. Similarly, ²¹⁰Bi decays to ²⁰⁶Tl by emitting α-particle with only 1.3 × 10⁻⁴ % probability. We have assigned a score to each photopeak listed in Table 2, 0 or 1 where 0 denotes unsuitability of the gamma line for quantification of the parent radionuclide of the series; whereas the score 1 denotes the suitability of the gamma line based on the preliminary observation. The reason for assigning 0 is based on either very low intensity in the specific energy region or possibility of overlapping with the neighboring photopeaks either from the same series or from inter-series interference. While overlapping with another photopeak is considered, it is assumed HPGe detectors are used for NORM measurement that have generally 2–3 keV resolution in the higher energy region and ~1–2 keV in the lower energy region. All the photopeaks from ²³⁵U series have been assigned score zero because of its very low natural abundance, 0.7204 %. However, they have been included in the table to show possible interference to the radionuclides, like 185.71 keV interferring with 186.21 keV ²²⁶Ra photopeak and 351.06 keV interferring with 351.93 keV ²¹⁴Pb photopeak. From the preliminary screening it is revealed that only 14 photopeaks from ²³⁸U series, and 9 photopeaks from ²³²Th series qualify for quantitative measurements of low-level NORMs. Tables 3 and 4 represent these useful photopeaks as deduced from Table 2 for measurement of the activity of uranium and thorium respectively. Rest of the investigation has been carried out using only the useful photopeaks.

Table 2 List of different photopeaks of the radionuclides belonging to natural decay series and their suitability for measurement

Table 3 Photopeaks of ²³⁸U series suitable for measurement of ²³⁸U after preliminary screening

Table 4 Photopeaks of ²³²Th series suitable for measurement of ²³²Th after preliminary screening

Experimental

Four soil samples were collected from different parts of India, e.g., from Sundarban region (SB1, SB2) and from Punjab state (PU1 and PU2). It is noteworthy to mention Sundarban is world’s largest mangrove ecosystem known for its luxuriant floral-faunal diversity. The samples were air-dried until moisture was driven out and then further pulverized in grinder to obtain homogenized form. Each of the pulverized samples were weighed to 50 g, hermetically sealed in leak-proof petri-plates and kept aside for 40 days to ensure the state of secular equilibrium. The dimension of the petri-plates as well as that of the soil samples was 7.5 cm diameter and 1.1 cm height. In addition to four test samples, four standards (two each of ²³⁸U and ²³²Th) were also prepared. For preparation of two ²³⁸U standards (2 and 5 dps), weighed amount of IAEA Uranium Ore (Pitchblende); S-8 standard (0.35 and 0.14 g correspond to 5 and 2 dps respectively) was taken in leak-proof petri-plate. For ²³²Th standards (2 and 5 dps), weighed amount of thorium acetate, [Th(CH₃COO)₄] (0.995 and 2.49 mg correspond to 2 and 5 dps respectively) was taken in leak-proof petri-plate. To maintain the geometry at par with the test samples, all the four standard samples were mixed thoroughly with silica gel to attain the total weight of 50 g, equivalent to the sample size. The petri-plates were also hermetically sealed for 40 days to establish the secular equilibrium between the parent and daughter isotopes. One of the two standards (2 dps) was used as standard for all measurements in both cases of ²³⁸U and ²³²Th. The other one (5 dps) was used as sample of known activity (SU for ²³⁸U and STh for ²³²Th) to validate the result.

All samples and standards were measured for 75000 s using reverse electrode coaxial high-purity Germanium (HPGe) detector with 50 % relative efficiency and FWHM (full width at half maxima) of 3.3 and 0.96 keV respectively at 1.33 MeV and 122 keV. Shielding of this detector had CANBERRA model 747 lead shield with 9.5 mm thick low carbon outer jacket, 10 cm thick low background lead as bulk shield, also graded lining of 1 mm tin and 1.6 mm copper preventing the interference by lead X-rays [3]. Samples were kept at 1 cm distance from top of central HPGe detector. Energy calibration was performed using single elemental standards or point sources of ¹³³Ba, ⁶⁰Co, ¹³⁷Cs and ¹⁵²Eu. Count of 50 g silica gel was taken also for 75,000 s in a similar petri-plate. This was considered as background spectrum. This background spectrum was stripped from all sample and standard spectra. Analysis of the obtained gamma-spectra was done using GENIE 2K software, also procured from CANBERRA.

Result and discussion

In principle, the activities of ²²⁶Ra, ²¹⁴Pb and ²¹⁴Bi, (all of them are member of ²³⁸U series) should be same as they are in secular equilibrium. But in practice slight difference is always observed between the measured activities of different isotopes or even in between the different peaks of same isotope. We have measured activities of ²³⁸U for all four samples SB1, SB2, PU1, PU2 and 5 dps test sample (SU) using 2 dps standard for all the photopeaks listed in Table 3 and tabulated the activity values in Table 5. It is clear from Table 5 that still some of the photopeaks do not qualify for quantitative measurement of ²³⁸U. These photopeaks are 131.3 keV (²³⁴Pa), 152.7 keV (²³⁴Pa), 733.4 keV (²³⁴Pa), 768.4 keV (²¹⁴Bi), 831.5 keV (²³⁴Pa) and 946 keV (²³⁴Pa). These peaks give either too low or too high value, as compared to other photopeaks. There may be multiple reasons for the disqualification of these photopeaks, such as low intensity and overlapping with other low abundance nearby photopeaks, location of the peak at the Compton edge of other photopeak, etc. Therefore we have not considered these photopeaks suitable for quantitative analysis of U content from natural samples and deleted in the next stage of selection of photopeaks. In Table 6 we have listed photopeaks of ²³⁸U series still suitable for analysis of uranium content. Now the pertinent question is whether all the photopeaks listed in Table 6 have same merit? More elaborately, whether one can take average of all the photopeaks listed in Table 6 to report uranium content of the sample or one can take arbitrarily average of activities obtained from few of these photopeaks? To answer these questions, we go back to bottom part of Table 5, wherein we have calculated the activity of U in samples SB1, SB2, PU1, PU2 by taking average of activities under various combinations of photopeaks and also calculated relative standard deviation (${\rm RSD} = \frac{\rm standard\, deviation}{\rm mean \,value} \times 100$) of the activities obtained in different photopeaks. The RSD values varied from 2.8 % to as high as 76.8 %. The RSD value need to be as low as possible to get the best estimate using set of good photopeak combinations. Table 5 suggests that average of activity calculated from 295.22, 351.93 and 609.3 keV gives minimum RSD value and therefore can be used to report uranium content of natural samples in a statistically reliable manner.

Table 5 Calculated activity of different daughter radionuclides of ²³⁸U under different photopeaks using 2 dps as U standard (SU)

Table 6 Photopeaks of ²³⁸U series suitable for quantitative analysis of ²³⁸U

To further validate our result, we have calculated the activity of our four test samples with different combinations of photopeaks as taken by different researchers in Table 7. Only those results have been taken into account where the researchers selected three or more photopeaks. In some cases the RSD was even close to 100 %. For example, the RSD was ~100 % for all the samples, when photopeaks were selected as per Jankovic et al. [9] (entry no 5 in Table 7). This is because along with two good peaks they also selected two very low intensity peaks, 63.29 and 1001.03 keV. Only two groups of researchers, Aytekin et al. [21], and Alfonoso et al. [22] selected the photopeaks as proposed by us (295.22, 351.93 and 609.3 keV). However, these authors never mentioned the reason for choosing such photopeaks and therefore their selection can be considered “accidentally right selection.” The RSD value was found to be minimal for these photopeaks compared to any other entry in the table, which corroborates and strongly validates our recommended approach.

Table 7 Activity of ²³⁸U obtained for four test samples with different sets of photopeaks as taken by different researchers (see Table 1)

The same approach has been resorted to for the quantification of ²³²Th in all four samples by measuring activity under different photopeaks listed in Table 4. In all measurements 2 dps ²³²Th standard was used. Also a 5 dps ²³²Th (STh) sample was taken as known strength. All such results have been tabulated in Table 8. The RSD values between various sets of photopeaks are closer in ²³²Th series when compared to that of ²³⁸U series. The 277.35 keV photopeak from ²⁰⁸Tl gave very low activity for all four samples. However, all other photopeaks provided more or less acceptable results. Therefore, using the same analogy as that of uranium, we have listed acceptable photopeaks of ²³²Th series in Table 9, which indicates 8 numbers of photopeaks might be suitable for ²³²Th analysis. However, the same questions arise again. Whether all of these photopeaks have same merit? Whether one can choose any number of photopeaks from Table 9, and report the mean as ²³²Th content in the sample? To answer this question, we have shown few combinations at the bottom of Table 8, with RSD for each combination. The minimum and maximum RSD amongst different combination was 1.7 and 52.6 % respectively. However, average of activity obtained from 338.32, 583.19 and 911.20 keV yielded minimum RSD value, and hence recommended as the best combination of photo-peaks to measure ²³²Th.

Table 8 Calculated activity of different daughter radionuclides of ²³²Th under different photopeaks using 2 dps Th standard (STh)

Table 9 Photopeaks of ²³²Th series suitable for quantitative analysis of ²³²Th

Again to validate our approach for ²³²Th series, we have tabulated the activity of four test samples SB1, SB2, PU1, PU2 with different combinations of photopeaks of ²³²Th series as taken by different researchers in Table 10. The results are more consistent vis-a-vis the U series, but as high as 127 % RSD was observed in particular combination of photopeaks. Again, brilliantly Aytekin et al. [21] reported natural radioactivity in Black sea region of Turkey using the photopeaks as proposed by us (338.32, 583.19, and 911.20 keV), and have the lowest RSD compared to any other entry in the table, further corroborating and validating our proposed approach for measurement of low level environmental radioactivity.

Table 10 Activity of ²³²Th obtained for four test samples with different sets of photopeaks as taken by different researchers (see Table 1)

In Table 11, we list our final proposed recommendations related to the appropriate selection of photopeaks from ²³⁸U series and ²³²Th series for carrying out statistically reliable quantitative measurement of NORMs like ²³⁸U and ²³²Th. It should however be kept in mind that this recommendation should not be treated as the ultimate one as the role of detector used, sample size, counting time etc. still needs to be further investigated. However, the above discussion advocates to take at least three photopeaks for quantitative measurement of low-level ²³⁸U/²³²Th, especially in natural samples and the best combination of photopeaks is that one where minimal RSD value is obtained.

Table 11 Final recommended photopeaks of ²³⁸U and ²³²Th series suitable for low-level radioactivity measurement

Conclusion

Measurement of naturally occurring radionuclide materials (NORMs) is becoming increasingly important in the present world scenario. In conclusion it can be stated that the present work is the first attempt to systematically investigate the contribution of photopeaks and has come out with a prescription to get a better and statistically reliable estimate of activity/concentration of radionuclides while carrying out low-level radioactivity measurements. It would be interesting to extend the work further to understand the role of parameters like nature of detector, sample size, counting time etc. in the study of environmental radioactivity. This paper also states that RSD between different photopeaks is one of the important criteria to impose restrictions on the arbitrariness on choice of photopeaks for quantitative measurement of low-level ²³⁸U or ²³²Th in natural samples.