Introduction

The humans are often exposed to natural and artificial radionuclides, which can enter the human body through breathing, eating, and drinking. In the food chain, milk is one of the fundamental foodstuffs for humans, especially for infants, children and old people. In addition, milk is sensitive to be contaminated because some radionuclides such as 137Cs, 40K are easily transferred to the milk through the grass-cow-milk pathway [1]. Therefore, evaluating and monitoring the level of radionuclides in milk (fresh and powdered) play an important role in estimating the annual effective dose for the population.

The radionuclides in milk have received much attention from researchers in many parts of the world. In Nigeria, a report showed unusually high average activity concentrations of 40K compared to other reported values, reaching 831.6 ± 53.8 Bq/kg [2]. For powdered milk in Brazil, Melquiades & Appoloni [3] indicated that the activity of 40K (475 ± 12 for polly and 489 ± 13 for cativa) was usual for powdered milk, while the observed 137Cs and 232Th activity concentrations were comparatively small. The average activity concentrations of 40K were also monitored in fresh and powdered milk in Tehran-Iran [4]. Observed mean activity concentrations of radionuclides in imported infant powdered milk in Malaysia of 232Th, 226Ra, 137Cs, and 40K were 2.6 ± 2.3, 3.1 ± 1.8, 0.3 ± 0.2, and 99.1 ± 69.4 (Bq/kg), respectively [5]. Amongst these, the infant powdered milk from the Philippines had the lowest level of radioactivity, and the brand from Singapore showed the highest one [5]. In addition, the research also determined the mean annual effective dose due to consumption of powdered milk in Malaysia and found it to be 635 and 111 µSv/year for infants ≤ 1 and infants from 1 to 2 years old respectively [5]. In general, the radioactivity of fresh and powdered milk in many countries has been investigated and the effective dose has been calculated for various parts of the population, especially for infants. These investigations have shown that the level of radioactivity and effective dose can change an order of magnitude depending on the source of origin.

In Vietnam, the demand for milk is on the rise year by year. Most of the fresh milk brands consumed in Vietnam are domestic products. Although Vietnam has no nuclear power plant, the country is somewhat affected by the Fukushima nuclear disaster in 2011 (transfer by fallout and rainfall to surface soil) [6, 7]. In addition, there are many natural radioactive sources, such as mines, along the mainland of Vietnam, which contain natural, and long half-life artificial radionuclides such as 232Th, 226Ra, 137Cs, and 40K. Thus, the radionuclides may transfer from soil to plant (grass) and from grass and air to cow’s milk. These reasons could potentially lead to high artificial radionuclide activity concentrations in milk products produced by cows in Vietnam. These pathways are well described by many, such as the transfer of radionuclides from soil to plant (grass) [8,9,10,11] and from grass and air to cow’s milk [12,13,14]. However, so far there is no research nor monitoring on the radionuclides in milk consumed in Vietnam. This study deals with the investigation of the radioactive levels in the local fresh milks in Vietnam. The research focuses on the determination of radioactivity level in fresh milk using a high-resolution HPGe detector. The results of activity concentration measurements will be used to calculate the average annual effective doses for different age groups. The data reported here can be used to establish the baseline for natural and artificial radioactivity in milk as well as comparison for new data, when there is some event involving nuclear plants or exploration activities in radioactive bearing mines in Vietnam.

Materials and methods

Eight brands of the fresh milk consumed commonly in Vietnam were bought from local prestigious markets and were used to investigate their radionuclide contents, including 232Th, 226Ra, 137Cs and 40K. All these milk brands were produced from local dairy cow’s farms in Vietnam and were collected between 2018 and 2019. These brands are, namely: Vinamilk (VN); TH True Milk (TH); Moc Chau (MC); Da Lat Milk (DL); Dutch Lady (DuL); Vinamilk Organic (VO); Ba Vi (BV); and Nutifood (NT).

About three kilograms of each type of fresh milk (wet w.t) were taken for analysis. The fresh milk in the form of liquid was converted to powder and dried at 90 °C in a clean non-stick container to avoid radionuclide loss, contamination and to ensure that the moisture was completely removed. The obtained samples were weighted and packed in a plastic cylindrical beaker and sealed to prevent the escape of radon. The samples were left for at least 28 days to obtain the secular equilibrium between 226Ra and its daughters (mostly 214Bi and 214Pb) [15]. The weight of milk after drying (dry w.t) is shown in Table 1.

Table 1 The information of studied samples
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After equilibrium was reached, activity concentration measurements were performed using a high-resolution HPGe detector (Canberra-GC5019) with 30 % relative efficiency. The analysis was performed using Genie-2000 software. The detector’s energy resolution is 1.9 keV at 1.33 MeV of 60Co gamma-ray peak. To reduce the surrounding natural background radiation at the laboratory, the detector is shielded by a 15 cm thick lead cylinder.

The activity concentration of each sample was determined based on their respective gamma lines: 609.3 keV, 1120.3 keV and 1764.5 keV from 214Bi were used to determine the activity concentration of 226Ra, while that of 232Th were determined from the gamma lines of 911.2 keV and 969.0 keV from 228Ac and 583.0 keV and 2614.4 keV from 208Tl [15,16,17]. The radioactive equilibrium in the 232Th decay chain might not be present all the way to 232Th, just 228Ra, but similarly to the current literature, the values will be reported as 232Th. For 40K, its activity concentration was determined from its 1461 keV gamma line and 662 keV was used for 137Cs. The samples were counted for over 50 h to avoid the statistical counting error. The gamma spectrometer was calibrated using IAEA reference materials RGU, RGTH and RGK [15]. The quality control tests were carried out based on standard reference materials (IAEA-321 and IAEA-414, IAEA reference materials produced by the International Atomic Energy Agency). The results agreed well with the values of IAEA-321 and IAEA-414 samples (except for 232Th because of low activity) for 40K, 137Cs, 238U and 40K, 137Cs in 492 ± 25, 3.02 ± 0.10, 1.36 ± 0.21 and 71.9 ± 2.1, 544 ± 29 Bq/kg and the reference values 480 ± 22, 3.09 ± 0.07, 1.40 ± 0.36 and 72.6 ± 1.6, 552 ± 17 Bq/kg respectively. The self-gamma absorption difference resulting from the difference in density of the studied samples and standard ones were corrected for following the method described by Debertin [18] and Jodlowski [19]. To lessen the photoeffect absorption in the sample, all of the gamma lines used are higher than 500 keV. The detection limits for 232Th, 226Ra, 137Cs, and 40K were 0.56, 0.32, 0.11 and 2.1 Bq/kg, respectively.

The activity concentrations of 232Th, 226Ra, 137Cs, and 40K is calculated by the formula (1).

$${A}_{sp}=\frac{{N}_{sp}\times {M}_{st}\times {A}_{st}\times {C}_{i}}{{N}_{st}{\times M}_{sp}}$$
(1)

where Nsp, Msp, and Nst, Mst are the net intensity, mass of the measured sample and of standard sample respectively; Ast is activity concentration of standard sample; Ci is the corrected factor for the differences between the densities of the sample and standard sample.

The annual effective dose D (µSv/year) of radionuclides to individuals due to the consumption of milk is calculated based on the following equation below [20].

$$D=A\times I\times E$$
(2)

where A is the activity concentration of radionuclides in milk (Bq/kg); E is the dose conversion coefficient for the radionuclides due to ingestion (µSv/Bq) (Table 2) [21]; I is the annual intake of milk (kg/year) which depends on the age groups. In this study, the annual ingestion dose for three age groups was investigated, including infants; children; and adults. The average annual intakes of fresh milk are 14.8 kg, 13.6 kg and 13.0 kg (dry w.t) for infants, children, and adults respectively [22].

Table 2 Dose conversion factors for different age groups
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Results and discussions

The activity concentration of 232Th, 226Ra, 137Cs, and 40K in eight selected brands of fresh milk are presented in Table 3. The results showed that smallest activity concentration belongs to the artificial radionuclide, 137Cs, 0.13 ± 0.06 (Bq/kg) was found in VN milk and all other milk samples had activity concentrations below the detection limit (BDL). All results for this radionuclide are well below the guideline level [23]. The low 137Cs activity concentration in milk samples could be explained by low 137Cs activity concentration in soil which was reported to be 3.82 Bq/kg (average) in surface soil (0–20 cm from surface) [24] before the Fukushima accident; the low 137Cs activity concentration in water; the reduction due to the transfer processes from soil to grass, and from grass and water to cow milk. More recent data, albeit local data confirms the low 137Cs activity concentration in soil (0.17–5.28 Bq/kg at Luong My Farm in Hoa Binh Province) [25]. The major radionuclide is 40K in all brands, it shows the highest value among other radionuclides with a low standard deviation. The MC milk sample shows the highest activity concentration with 387 ± 7 Bq/kg of 40K and the highest activity concentration of 226Ra with 2.45 ± 0.24 Bq/kg. The lowest activity concentration of 40K and 226Ra are found in NT milk samples with 341 ± 6 and 1.45 ± 0.18 Bq/kg respectively. The relatively high activity concentration of 40K in milk samples could be justified because potassium is a highly mobile in the environment [26,27,28], it is one of the major radionuclide elements in soil, and it naturally poses a part of potassium, which is a major nutrient for plants, animals and humans [10, 11]. These reasons lead to high 40K activity concentration in comparison with other radionuclides due to the transfer process from soil to grass and from grass and water to cow milk. The highest and lowest activity concentrations of 232Th are observed in NT and BV samples with 1.21 ± 0.24 and 0.60 ± 0.19 Bq/kg, respectively. The activity concentration of 226Ra is two times greater than that of 232Th. In general, 40K is the dominant radionuclide in the studied samples. All of the radionuclides (232Th, 226Ra, 137Cs and 40K) are similar in value for the eight milk brands and are far less than the guidance level reported by the WHO [29].

Table 3 Activity concentrations of 232Th, 226Ra, 137Cs, and 40K in eight fresh milk brands in Vietnam
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The activity of concentrations of 232Th, 226Ra, 137Cs, and 40K in fresh milk found by this study are compared with reports by other authors from different countries in in Table 4. Therein, for the natural radionuclides, countries such as Saudi Arabia, Singapore, Indian, Malaysia, New Zealand, Thailand, Spain show higher results of 232Th, 226Ra activity concentrations, but lower 40K activity concentration compared to those in Vietnam. Other countries such as Jordan, Israel and Iran reported lower values of the 226Ra, 232Th activity concentrations, but higher value of 40K activity concentrations compared to those in Vietnam. On the other hand, there are two countries having values of activity concentrations higher than those in Vietnam with 23.1, 4.35, 832 (Bq/kg) for 226Ra, 232Th, 40K respectively in Nigeria and 1.6–3.7, 5.1–11.2, 475–489 (Bq/kg) for 232Th, 137Cs, and 40K respectively in Brazil. By contrast, Australia has activity concentrations lower than those in Vietnam. For artificial radionuclides, 137Cs activity concentration in the world is generally higher than that in Vietnam, except for Australia and Israel with 0.11 and BDL-0.08 (Bq/kg) respectively [5, 33]. The variations of activity concentrations of 232Th, 226Ra, 137Cs, and 40K in powdered milk in the world may be affected by the type of milk, which is related to the animal of origin, different studied breeds, and the pollution in the local environment that affects the concentration of 232Th, 226Ra, 137Cs, and 40K as well.

Table 4 Activity concentration of 232Th, 226Ra, 137Cs, and 40K in milk from previous investigations in different countries
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The calculated annual effective doses (AED) due to the ingestion of 232Th, 226Ra, 137Cs, and 40K in Vietnamese milk for different age groups are presented in Table 5. The results show that the annual effective dose due to the intake of radionuclides in this study (except for 137Cs) by infants is much higher than that of children and adults, especially the adults. For the 137Cs, the AEDs by infants, children and adults are almost the same. This result is similar to that of Ababneh and coworkers [1]. The research results also indicate that 40K has the highest contribution to the total annual effective dose due to the ingestion of fresh milk: 66.9 % for infants, 48.4 % for children and 66.1 % for adults. Since K is in homeostasis in the body, it might be argued, that the resulting dose from 40K should be disregarded from the excess dose. 232Th gives the second-highest contribution to the total effective dose: 22.9 % for infants; 35.7 % for children and 18.2 % for adults. On the other hand, the contribution of 137Cs to the total effective dose is insignificant for all age groups and accounts for the smallest proportions among the four radionuclides.

The total average annual effective doses due to the intake of 232Th, 226Ra, 137Cs, and 40K from Vietnamese fresh milk are 340.0, 136.9 and 45.7 µSv for infants, children, and adults in respectively (Table 5). This shows that infants have the highest risk factor compared to other age groups. The risk factor for infants is 2.4 and 7.4 times higher than that for children and adults, respectively. This is similar to the results reported by Ababneh et al. [1] for the intake of milk in Jordan. In general, the total average annual effective dose results for the three age groups in this study are within the typical worldwide range of annual effective dose (200–800 µSv) due to the ingestion of natural radiation sources [20]

Table 5 Annual effective dose (AED) for different age groups
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Conclusions

A systematic study of radioactivity concentration in Vietnamese fresh milk is presented for the first time. Activity concentrations of radionuclides (232Th, 226Ra, 137Cs, and 40K) in selected eight fresh milk brands consumed in Vietnam have been determined by a HPGe detector. The annual effective dose due to the ingestion of fresh milk in Vietnam was also calculated. Based on the analysis of results, the following conclusions can be drawn:

The artificial of 137Cs has a very minor presence, while 40K is present in all brands in two order of magnitude higher concentration than the other observed radionuclides. The activity concentrations of the four radionuclides in each fresh milk brand was quite similar. In general, the level of radioactivity in Vietnamese fresh milk brands was found to be similar to the worldwide level reported in the literature.

The 40K has the highest contribution to the total AED due to the ingestion of Vietnamese fresh milk, followed by 232Th, 226Ra, and 137Cs. The AED due to the intake of radionuclides in this study (except for 137Cs) for infants is much higher than that for children and adults. For 137Cs, the AEDs by infants, children and adults are almost the same. The 40K contribution to the total annual effective dose was 66.9 % for infants, 48.4 % for children and 66.1 % for adults, due to K being in homeostasis in the body it might be argued, that the AED is less by this amount.

The total average AEDs due to the intake of 232Th, 226Ra, 137Cs, and 40K from Vietnamese fresh milk for infants have the highest risk factor compared to other age groups.