Internal dose assessment of 148Gd using isotope ratios of gamma-emitting 146Gd or 153Gd in accidently released spallation target particles

The pure alpha emitter 148Gd may have a significant radiological impact in terms of internal dose to exposed humans in case of accidental releases from a spallation source using a tungsten target, such as the one to be used in the European Spallation Source (ESS). In this work we aim to present an approach to indirectly estimate the whole-body burden of 148Gd and the associated committed effective dose in exposed humans, by means of high-resolution gamma spectrometry of the gamma-emitting radiogadolinium isotopes 146Gd and 153Gd that are accompanied by 148Gd generated from the operation of the tungsten target. Theoretical minimum detectable whole-body activity (MDA) and associated internal doses from 148Gd are calculated using a combination of existing biokinetic models and recent computer simulation studies on the generated isotope ratios of 146Gd/148Gd and 153Gd/148Gd in the ESS target. Of the two gamma-emitting gadolinium isotopes, 146Gd is initially the most sensitive indicator of the presence of 148Gd if whole-body counting is performed within a month after the release, using the twin photo peaks of 146Gd centered at 115.4 keV (MDA < 1 Bq for ingested 148Gd, and < 25 Bq for inhaled 148Gd). The corresponding minimum detectable committed effective doses will be less than 1 µSv for ingested 148Gd, but substantially higher for inhaled 148Gd (up to 0.3 mSv), depending on operation time of the target prior to the release. However, a few months after an atmospheric release, 153Gd becomes a much more sensitive indicator of body burdens of 148Gd, with a minimum detectable committed effective doses ranging from 18 to 77 µSv for chronic ingestion and between 0.65 to 2.7 mSv for acute inhalation in connection to the release. The main issue with this indirect method for 148Gd internal dose estimation, is whether the primary photon peaks from 146 and 153Gd can be detected undisturbed. Preliminary simulations show that nuclides such as 182Ta may potentially create perturbations that could impair this evaluation method, and which impact needs to be further studied in future safety assessments of accidental target releases.

The European Spallation Source (ESS), located north-east of the city of Lund in south-western Sweden, is designed to be the most powerful neutron source in the world using a 5 MW proton beam irradiation against a tungsten target 1-3 . An inevitable side effect of neutron generation during spallation reactions is the production of various radionuclides in the spallation source. During a 5 years operation of the ESS tungsten target, it is estimated that a number of gamma emitters will be produced, e.g. 187 W (> 10 16 Bq) and 172 Hf (> 10 15 Bq), as well the pure beta emitters such as 3 H (~ 10 15 Bq) and pure alpha emitters as 148 Gd (> 10 14 Bq) 2 . The national competent authority in Sweden regarding emergency preparedness (Swedish Radiation Safety Authority, SSM) commissioned ESS to elaborate potential technical scenarios that could lead to an atmospheric release of spallation source particles 3 . Of these scenarios SSM considered the one involving loss of cooling of the spallation source while the neutron production operates at full effect (5 MW), being the one that dimensions the local emergency planning zone. Within minutes of proton beam irradiation of the tungsten target, the temperature increase in Theoretical outline and methods Biokinetic model. ICRP has presented a systemic biokinetic model for gadolinium 10 . Like all rare earth metals, gadolinium has a low uptake into tissue when ingested. ICRP 141 12 presents a systemic biokinetic model for Gd and proposes a very low gastrointestinal (GI) uptake fraction (f 1 ) of 0.0005 based on literature surveys 13 . The internal doses incurred are hence estimated to be rather low in relation to the activity intake, compared with more easily incorporated radionuclides such as radiocaesium, which is associated with fission products released from nuclear accidents or nuclear weapons debris. However, given the average transit time of 36 h for foodstuffs through the GI tract (described by, e.g., ICRP 100 14 ), intakes of gadolinium by humans must still be considered, as the gadolinium isotopes may cause internal exposure during its passage time as well. In case of accidental releases from the tungsten target, the likely physiochemical form would be as particles, volatilized tungsten, and tungsten oxides 15,16 . When using that model, there appears to be a long time (< 1 y) before reaching the equilibrium whole-body stable gadolinium level at chronic intake. An ingestion of 1 Bq day −1 of any of the listed gadolinium isotopes listed in Table 1 corresponds to an infusion of 0.0005 Bq/day to systemic tissues. At 1 year after onset of the chronic intake of 1 Bq day −1 of a given gadolinium isotope, the systemic gadolinium content in a human will be 0.125 Bq, 0.025 Bq, and 0.075 Bq for 148 Gd, 146 Gd, and 153 Gd, respectively. Moreover, it is estimated that equilibrium in the whole-body content of stable gadolinium is not reached until after 30 years of constant intake. For 148 Gd, the equilibrium level is then estimated to be approximately 0.9 Bq per 1 Bq daily ingestion. Table 1. Alpha and photon energies (keV) and associated emission probability, n g , of the emission lines of gadolinium isotopes generated from proton irradiation of a W target 9 . www.nature.com/scientificreports/ For systemically incorporated Gd, a large fraction will be found in soft tissue (approximately 17.3%). However, a yet larger fraction (55.1%) will be found in the cortical bone surface. This will result in a nearly homogeneous distribution in the whole body. However, due to the extremely low GI uptake (f 1 = 0.0005 according to ICRP 141 12 ), it is not anticipated that the component of systemic uptake of gadolinium will be important in comparison to the fraction of gadolinium in the GI tract. Hence, for in vivo whole-body counting of gamma emitting gadolinium isotopes in subjects who have chronically ingested radiogadolinium, a measurement geometry assuming major uptake in the abdominal region is more appropriate (See section "Estimating 148 Gd whole-body burden and cumulative committed effective dose by means of high-resolution gamma spectrometry").
Relating 148 Gd body burden with activity ratios of released gamma emitting gadolinium isotopes. Some model derivations are needed to yield expressions that relate what is measurable by means of whole-body counting in a scenario with widespread release of radioactive gadolinium with a certain distribution between released gadolinium isotopes, 146 Gd, 148 Gd and 153 Gd, to incurred committed effective dose of exposed subjects from 148 Gd. Considering a passage time (residence time) in the GI tract of 36 h 14 , a chronic ingestion of 1 Bq/day of a given gadolinium isotope will lead to an equilibrium level of 1.5 Bq per Bq/day intake in the GI tract, if disregarding the low fraction (f 1 = 0.0005) that has been taken up systemically. Given the long physical half-life of 148 Gd (T½ = 76.4 y), this means that, after 1 year of chronic constant intake of 148 Gd, the expected gadolinium abundance in the GI content during passage will be more than one order of magnitude larger than the fraction of 148 Gd being incorporated systemically. Thus, in practice for protracted internal exposures, radioactive gadolinium will predominantly be found in the abdominal part of the body (Fig. 1, Right). It will also mean that, even for very long protracted intakes, the systemic gadolinium will only be a small fraction of the whole-body burden at any given time after the onset of the intake.
For short-lived gadolinium isotopes, the colon doses are more relevant over the long term compared with the dose to systemic tissue. The whole-body activity of a given gadolinium isotope, Q Gd , at any given time is the sum of the component in the GI contents, Q Gd,GIcont (t), and the systemically incorporated Gd, Q Gdsys (t): Using 14 for the systemic component and a 36 h retention time of the inert fraction of gadolinium in the GI tract, the gadolinium content as a function of time after a constant protracted daily intake, I Gd (Bq day −1 ), can be expressed in terms of intake normalized body content, q Gd (t): contents and systemic tissue, respectively, normalized against the daily protracted intake I Gd . For 148 Gd, the normalized body content q Gd-148 (t) as a function of time, t (d), after start of chronic ingestion of 1 Bq day −1 , can be expressed as follows: q Gd−148 (t) = 1 · t; if t < 1.5 d 1.5 + 0.93 · 1 + e − ln2 1821 ·t ; if t ≥ 1.5 d  www.nature.com/scientificreports/ Equation 3 is obtained by curve regression from the combination of the systemic biokinetic model in 12 and the ICRP colon model described in 14 . In the right from of Fig. 1 the plot of Eq. (3) is given for 146 Gd, 148 Gd and 153 Gd, respectively.
The whole-body activity of the gamma-emitting gadolinium isotopes 146 Gd and 153 Gd at a given time t after the onset of the chronic intake of 148 Gd, I Gd-148 , can be related to the whole-body activity of the alpha emitter 148 Gd with the corresponding retention of the accompanied gamma-emitting gadolinium isotopes 146 Gd and 153 Gd. Thus, for a scenario of intakes from a release containing a composition of different radio-gadolinium isotopes, the whole-body activity of 148 Gd, Q Gd-148 , can be expressed in terms of the corresponding whole-body activity of either of the gamma-emitting radionuclides 146 Gd or 153 Gd, using the following relationships: or where h 148/146 (t) is the time-dependent activity ratio between 146 and 148 Gd in the GI contents divided by the daily intake of 148 Gd, I Gd-148 (Bq day −1 ), and h 153/146 (t) is the corresponding activity ratio for 153 Gd and 148 Gd. Moreover, in analogy with the expression in Eq. (2), the term m 148/146 (t) in Eq. (4) is the activity ratio at time t between 146 and 148 Gd, divided by I Gd-148 in the systemic tissues, and m 153/146 (t) is the corresponding ratio for 153 Gd and 148 Gd. In turn, the expressions in Eqs. (3) and (4) can be rewritten as a time-dependent relationship between the whole-body burden of Q Gd-148 and Q Gd-146 and Q Gd-153 , respectively, in terms of the time-dependent scaling factors k 148/146 and k 148/153 , respectively. The purpose of the expressions in Eqs. (3) and (4) is thus to relate the whole-body burden of the alpha-emitting 148 Gd with quantities Q Gd-148 and Q Gd-153 , which are measurable by means of whole-body counting.
Since for a chronic ingestion of radiogadolinium Q Gi-cont > > Q syst , Eqs. (3) and (4) can virtually be rewritten as or The factors k 148/146 (t) and k 148/153 (t) are in turn given by the initial activity proportions at the start of the intake (i.e., at time t = 0). If the initial activity ratio between 148 and 146 Gd is denoted as a 146/148 , and the ratio between 148 and 153 Gd is denoted as a 153/148 , then the values of h 148/146 (t 0 ) and m 148/146 (t 0 ) will be equal to 1/a 146/148 , and the values of h 148/153 (t0) and m 148/153 (t 0 ) will be equal to 1/a 153/148 . Likewise, the factors k 148/146 (t) and k 148/153 (t) will then be equal to 1/a 146/148 and 1/a 153/148 , respectively, at t = t 0 . In this study, a 146/148 and a 153/148 are simulated using the FLUKA code 17,18 , where t 0 = time of the release to the environment. Assuming equal biochemical and biokinetic behavior for all radiogadolinium isotopes, the daily intake of 146 Gd and 153 Gd will then be I Gd-148 / a 148/146 and I Gd-148 /a 148/153 , respectively. In this computational study, no account of ecological turnover of dispersed gadolinium was considered, meaning that the effective ecological half-times of gadolinium isotopes are assumed to be equal to their corresponding physical half-lives.
Committed effective dose calculations. ICRP 7 provides data on committed effective dose coefficients per unit intake of gadolinium isotopes. The committed effective dose, E Gd-148 (mSv), incurred at time t (d) after the start of a chronic intake of 148 Gd, denoted as I Gd-148 (Bq day −1 ), can then be expressed as: where e Gd-148 (mSv Bq −1 ) is the committed effective dose coefficient taken from ICRP 119 7 for an adult person. The coefficient refers to the time-integrated effective dose incurred upon intake (ingestion or inhalation) of a radionuclide. For the alpha emitter, this coefficient is 5.5·10 -5 mSv Bq −1 for ingestion of 148 Gd, which is more than 50 times higher than for ingestion of 146 Gd and 200 times higher than for ingestion of 153 Gd. The corresponding formulae for 146 Gd and 153 Gd are Exploiting that I Gd-148 is equal to the ratio Q Gd-148 (t)/q Gd-148 (t), Eq. (8) can be expressed as Q Gd-148 in turn can be expressed through either Eqs. (3) or (4) by relating it to the corresponding whole-body activities of 146 Gd and 153 Gd, respectively. The cumulative committed effective dose as a function of time per a chronic daily intake of I Gd-148 (Bq day −1 ) can then be deduced by the following: www.nature.com/scientificreports/ Hence, by numerically computing the time-dependent ratios q Gd-148 , k 148/146 (t), and k 148/153 (t) using the ICRP models ICRP 100 14 , ICRP 119 7 , and ICRP 141 12 , the whole-body activity and associated cumulative committed effective dose from the alpha emitter 148 Gd can be related to the measurable quantities Q Gd-146 or Q Gd-153 for a given set of isotope release ratios, a 148/146 and a 148/153 , respectively. Inhalation of radiogadolinium. Acute intakes through inhalation can lead to significant proportions of systemic activities of 148 Gd, even after full excretion of the initial GI contents. It is assumed that inhalation of radiogadolinium is only relevant during the immediate phase after a release event. A varying amount of the inhaled 148 Gd will then be taken up into the systemic tissues depending on the absorption rate from respiratory tract to blood (ICRP 12 ). If inhaled in oxide form, most of the gadolinium will be confined to the lungs, even months after inhalation. However, when considering the total body burden of 148 Gd, Q Gd-148 (Bq), for an acute inhalation of 148 Gd, I inh,Gd-148 (Bq), the measured body burdens of 146 Gd or 153 Gd at time t after intake can then be expressed as where R(t) is the retention curve for 146 Gd (N.B. not decay corrected) upon inhalation of the radiogadolinium. To our knowledge, it is not well-known which particle diameter should be expected in different accident scenarios 13 . Given the lack of this knowledge, here we use the retention derived from the ICRP model 12 19 . The parameters correspond to a moderate rate of absorption (Type M) and to an activity median aerodynamic diameter (AMAD) particle size of 2.2 μm (ICRP 12 ).
Particle size is an important parameter affecting the dose calculations. The Swedish Radiation Safety Authority uses an AMAD of 1 μm in their dispersion and dose calculations for the boundary accident scenario (smaller particle sizes are not applicable in the dispersion model used by SSM) and has performed a sensitivity analysis for particles with an AMAD > 5 μm 3 . According to the bioassay software tool, IMBA (Integrated Modules for Bioassay Analysis 20 ), the particle size assumed here will yield a committed effective dose of 1.26·10 -5 Sv per unit inhaled Bq 148 Gd, which is about a factor of two less than that for a Class F (fast absorption rate) particle in the size range 1 to 5 μm but somewhat higher than the corresponding values for M Class particles in the same size range. The corresponding effective doses for 146 Gd and 153 Gd are orders of magnitude lower: 7.6 and 2.5 nSv Bq −1 , respectively.
Hence, I inh,Gd-148 can be deduced from a 146/148 , the R(t) function, and the measured whole-body burden of the gamma-emitting 146 Gd or 153 Gd. The corresponding committed effective dose from 148 Gd will then be where e Gd-148,inh is the dose coefficient computed by the software IMBA, given the retention functions and associated parameters mentioned previously. Thus, the committed effective dose, E Gd-148 , from an acute inhalation of 148 Gd could be estimated through a whole-body burden measurement of 146 Gd or 153 Gd.
Target and release activity ratios of 146 Gd, 148 Gd, and 153 Gd. Activity ratios a 146/148 and a 153/148 were evaluated using data obtained from simplified ESS target modeling of the radionuclide composition [2new]. All major components of the ESS target were included in the model with simplified geometries. The FLUKA code was used for calculations, as it is suitable for calculations of particle transport and interactions with matter using the Monte Carlo method 17,18 . We obtained about a factor of 2 higher absolute values of 148 Gd in comparison with other authors 21,22 , and these differences can be attributed to differences in spallation and nuclide evaporation models. Unfortunately, there are no experimental data yet to evaluate which of the predictions is more accurate regarding absolute values. Activity ratios a 146/148 and a 153/148 were calculated for different operation times and decay periods, up to 350 days after 5 years of target operation (designed lifetime of the target).
Estimating 148 Gd whole-body burden and cumulative committed effective dose by means of high-resolution gamma spectrometry. In combination with estimated activity ratios of 146 Gd, 148 Gd, and 153 Gd in the spallation target and the biokinetic models described in Eqs. (7) and (13), the minimum detectable activity (MDA) of the alpha emitter 148 Gd for a high-resolution whole-body counting system, consisting of a 123% high purity germanium (HPGe) described by Rääf et al. 8 , was calculated. The whole-body counter is calibrated for a uniform body distribution of gamma emitters, but in this study alternative uptake geometries were needed to better mimic the anticipated uptakes of subjects exposed to internal radiogadolinium contamination. Using the VMC in vivo tool (VMC 2018 23 ), the relative difference in the efficiency calibration of a HPGe whole-body counter between a uniform whole-body distribution of gamma emitters in the energy range 100 to 150 keV, and that of specific organ uptakes could be simulated. In this tool the geometry of lung uptake in male adult phantom was available and used here for acute inhalation of a gamma emitter, whereas an uptake in the liver in the same phantom was used to mimic the calibration factor for a whole-body counting with elevated uptake in the abdominal region. The calibration factors for the 123% HPGe system in the photon energy range of 100 to 150 keV (roughly encompassing the considered gamma lines of 146 Gd and 153 Gd given in Table 1) could then be corrected by a factor of 2 (± 10% k = 1) for abdominal region uptake and by a factor of 0.66 (± 10% k = 1) for lung uptake. The MDA Gd-148 value in combination with Eq. (14) could then be used to estimate the corre- www.nature.com/scientificreports/ sponding minimum detectable committed effective dose, MDD Gd-148 . The MDA and MDD values as a function of time of the after the release, for two different operation times (1 and 5 y) were explored. Finally, the potential perturbations from other gamma lines present will be discussed, based on simulations of gamma spectra.

Results and discussion
Simulated relative W-target inventories of radiogadolinium and assumed daily ingestion after a release. Simulated W-target activity ratios between 146 Gd and 148 Gd and between 153 Gd and 148 Gd, respectively, during operation of the ESS target are given in Fig. 2 (left). The corresponding activity ratios for dispersed W-target particles as a function of time after the release are plotted in Fig. 2 (right). The activity ratio values taken from the ESS Preliminary Safety Analysis Report (PSAR) 22 and SSM report on emergency preparedness planning around the facility 3 are also provided in Fig. 2. The ratios from those reports are higher, i.e., they predict relatively lower activities of 148 Gd in comparison with other gadolinium isotopes. Our predictions might be considered more conservative in terms of relative proportion of the alfa emitting gadolinium isotope, but experimental data are necessary to prove this hypothesis. The SSM report 3 also suggests that 148 Gd deposition on the ground might be monitored using the gamma-emitting 146 Gd, considering the activity ratio of these radionuclides.
Note that the abovementioned activity ratios will represent the initial release activity ratios, a 146/148 and a 153/148 , in the case of an accidental atmospheric release either during or after operation. The resulting daily ingestion of 146 Gd and 153 Gd normalized to that of 148 Gd, assuming only physical decay in the environment, is given in Fig. 3 for a number of different target operation times (1 to 5 y).   www.nature.com/scientificreports/ From these values, the resulting proportions (k 148/146 and k 148/153 ) between the whole-body activity of 148 Gd and the gamma emitters 146 Gd and 153 Gd can be computed for a 1 to 5 years operation time (Fig. 4). It can be seen that, after 1 year of continuous intake of gadolinium isotopes released from a 5 years operation W spallation target, the model predicts a body content of 8.8 Bq of 148 Gd for every Bq of 146 Gd in an adult person. The corresponding value for 153 Gd is much less, only a value of 0.21 (Bq Bq −1 ). The longer physical half-time of 153 Gd will outweigh its lower initial isotopic abundance in the aforementioned release event, and after about 40 days after a release from a 5 years operated W target, the body content of 153 Gd will be higher than that of 146 Gd. Figure 5 plots the corresponding cumulative committed effective dose as a function of time per unit wholebody activity of 146 Gd and 153 Gd, respectively, assuming a daily intake, I Gd-148 , of 1 Bq day −1 . From these plots, it can be seen that the model predicts a cumulative committed effective dose of 0.30 μSv from 148 Gd per detected activity (Bq) of 146 Gd in the whole body, if observed 1 years post release of the W target (5 years operation). For 153 Gd, this value is considerably lower: 7.1 nSv per unit observed whole-body activity (Bq) of 153 Gd. This implies that the detection of 153 Gd in vivo will, in theory, be a much more sensitive indicator of 148 Gd cumulative committed effective dose than 146 Gd when surveying potentially affected persons, already one-month post release from the W target.

Body burdens of
The relative contributions to the cumulative committed effective dose from 146,153 Gd and 148 Gd are given in Fig. 6. Only after some months after the start of the protracted radiogadolinium ingestion does the alpha emitter 148 Gd account for the larger part of the cumulative committed effective dose from the three major gadolinium isotopes. One year after the onset of the ingestion, the radionuclide will account for 89% of the cumulative effective dose incurred from the three major gadolinium isotopes for an adult.  www.nature.com/scientificreports/ Detection limits of whole-body burden and cumulative committed effective dose of radiogadolinium isotopes for a high-resolution whole-body counting system. For the 123% HPGe detector setup described by Rääf et al. 8 with a pulse acquisition time of 2400 s in a Palmer geometry, the estimated minimum detectable activity, MDA, of 146 Gd using the 114 + 115 keV and 154.6 keV gamma lines, and a correction factor for enhanced detectability in the abdominal region by a factor of 2.1 described previously, is estimated to be 6.3 and 12 Bq, respectively, for a homogeneous nuclide distribution in the abdominal region for a 70 kg person. For an activity ratio a 146/148 (t) of 21.0 (5 years operation tungsten target) at t = 0, this will give an MDA of 0.31 (using the 115.4 keV peak) and 0.58 Bq (using the 154.6 keV peak) for 148 Gd. The corresponding minimum detectable committed effective dose, MDD(t) = e Gd-148 ·t·MDA(t)/q Gd-148 (t), is 0.017 µSv and 0.032 µSv, respectively, for an acute ingestion just 1 day after the release ( Table 2). As the amount of 148 Gd is initially cumulated in the body according to Eq. (3), the detection level will decrease slightly with time; however, within one week, the physical decay of the tracing nuclide 146 Gd will instead lead to an exponentially increasing detection limit. Hence, the corresponding MDA and MDD values (when using the 115.4 keV peak of the 146 Gd isotope) for chronically exposed adults will become 56 Bq and 810 μSv after 1 year post release of a 5 years operated W particle release (Table 2). If instead 153 Gd (with either the gamma lines at 97.4 and 103.2 keV, respectively) is used as an indicator of body activity and cumulative committed effective dose of 148 Gd, the detection levels are initially higher than when using 146 Gd due to the relatively lower initial isotope ratio in the released W-target material (e.g., 42.5 vs. 101 for a target under 1 year operation). As mentioned previously, however, 1 year post release it is evident that 153 Gd will be a much more sensitive indicator for the presence of 148 Gd, with significantly lower MDA and MDD values compared with 146 Gd (Table 3).
For gadolinium in oxide form, up to 50% of inhaled radiogadolinium will be accumulated in the lungs ( 12 ; see also Fig. 7), and the measurement geometry in vivo would therefore be a torso geometry, as previously mentioned in the Section "Estimating 148 Gd whole-body burden and cumulative committed effective dose by means of high-resolution gamma spectrometry". This gives rise to a corresponding factor of 3.2 increase in MDA of the primary photon peaks in this energy region of 146 Gd and 153 Gd, compared with assuming an abdominal uptake, and thus a corresponding increase in the indirect determination of 148 Gd. From the results given in Table 2, it   www.nature.com/scientificreports/ can be seen that MDD values can be reasonably low (< 0.20 mSv) using high-resolution whole-body counting of 146 Gd as a trace nuclide for the internal dose of 148 Gd if measured within 30 days upon release, regardless of whether the uptake occurred through ingestion or inhalation. However, for longer monitoring delays, it appears that 153 Gd will be a much more sensitive indicator of inhaled 148 Gd, regardless of the operation history of a W target before release. Nevertheless, it will then not be plausible to determine committed effective doses from acute inhalations lower than about 3 mSv, even if using 153 Gd (Table 3).
Perturbations in whole-body gamma spectrometry of radiogadolinium. In addition to the theoretical detection limits, the presence of perturbing radionuclides must also be considered. A representative W-target particle was investigated that contained a radionuclide composition according to our calculations 2 . Monte Carlo N-Particle (MCNP) code simulations 25 of the emission spectrum from a representative W-target particle are shown in Fig. 8. It appears that a number of perturbing gamma emitters will be present, of which the 182 Ta (t ½ = 114.4 d) peaks at 152.5 keV (n g = 0.070) and 156.4 keV (n g = 0.027) will most definitely affect the 154.6 keV line of 146 Gd. Tantalum has uptake properties similar to those of gadolinium (ICRP 119), and inhalation or ingestion of gadolinium may be accompanied with corresponding intakes of 182 Ta. Ongoing work will shed light on the time window for in vivo determination of inhaled 146 Gd in lungs and of the various potential contributions to the internal dose from spallation source products.

Conclusions
A potential release of W-target material from a spallation source may lead to atmospheric dispersion of radioactive gadolinium which is continuously generated in the target during the spallation operation. According to ICRP, the predominant effective dose contribution of the gadolinium isotopes will be from 148 Gd due to its alpha emission and can in an accident scenario with atmospheric dispersion of the nuclides potentially lead to significant internal exposures through inhalation. A theoretical investigation has been done of a method to determine internal exposures from inhaled or ingested 148 Gd in affected subjects using in vivo whole-body counting in combination with pre-calculated activity ratios between the alpha emitter 148 Gd and the corresponding gammaemitting gadolinium isotopes 146 Gd and 153 Gd. 146 Gd will initially be the most sensitive indicator of the 148 Gd internal dose, but some months after a release event, 153 Gd will, in theory, be a much more sensitive 148 Gd dose indicator. For a 123% HPGe detector used in Palmer geometry, 1-year post release, in vivo detection of 153 Gd Table 3. Theoretical minimum detectable whole-body activity (MDA) and corresponding minimum detectable cumulated dose (MDD) using either the 97.4 keV or 103.2 keV photo peaks of 153 Gd as a marker for the whole-body activity of 148 Gd for chronic ingestion and for an acute inhalation for an adult male (AMAD = 2.2 μm).  www.nature.com/scientificreports/ can yield a minimum detectable cumulative committed effective dose from 148 Gd ranging from 18 to 77 μSv for ingested 148 Gd, and 0.64 to 2.7 mSv for acutely inhaled 148 Gd, depending on the operational age of the released spallation target material and on which gamma peak (97.4 or 103.2 keV) is used in the assessment. However, preliminary Monte Carlo simulations of particle emission spectra from a W target in a spallation source being operated for 5 years show that the 182 Ta peak may potentially perturb some of the investigated primary gamma lines from 146 Gd and 153 Gd. If that is the case, in vivo detection of gadolinium uptake can be made indirectly through the 146 Gd daughter, 146 Eu. This is to be investigated further in continued studies.
Received: 28 July 2020; Accepted: 10 November 2020 Figure 8. Simulated spectrum from a W spallation target, operated for 5 years. The emission spectrum refers to 50 days post end of operation.