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Instruments 2019, 3(1), 7; https://doi.org/10.3390/instruments3010007

Article
New Cross-Sections for natMo(α,x) Reactions and Medical 97Ru Production Estimations with Radionuclide Yield Calculator
1
Groupement d’Intérêt Public ARRONAX, 44817 Saint-Herblain CEDEX, France
2
Faculty of Physics, University of Warsaw, 02-093 Warszawa, Poland
3
Heavy Ion Laboratory, University of Warsaw, 02-093 Warszawa, Poland
4
Subatech, CNRS/IN2P3, IMT Atlantique, Université de Nantes, CS 20722 44307 Nantes CEDEX, France
*
Author to whom correspondence should be addressed.
Received: 17 December 2018 / Accepted: 18 January 2019 / Published: 22 January 2019

Abstract

:
The production of 97Ru, a potential Single Photon Emission Computed Tomography (SPECT) radioisotope, was studied at ARRONAX. The cross-section of natMo(α,x)97Ru reaction was investigated in the range of 40–67 MeV irradiating the natMo and Al stacked-foils. The activities of 97Ru and other radioactive contaminants were measured via gamma spectroscopy technique. A global good agreement is observed between obtained cross-section results, previously reported values and TENDL-2017 predictions. Additionally, Radionuclide Yield Calculator, a software that we made available for free, dedicated to quickly calculate yields and plan the irradiation for any radioisotope production, was introduced. The yield of investigated nuclear reactions indicated the feasibility of 97Ru production for medical applications with the use of α beam and Mo targets opening the way to a theranostic approach with 97Ru and 103Ru.
Keywords:
SPECT; cyclotron; medical radioisotope production; radioactive impurities; cross-section; stacked-foils; gamma spectroscopy; thick target yield; Radionuclide Yield Calculator

1. Introduction

The 97Ru radioisotope was first acknowledged as medically interesting in 1970 [1] and is even studied in recent measurements [2,3]. It has a half-life of 2.9 d allowing non-local production and emits low-energy high-intensity gamma lines (see Table 1) which have favorable characteristics for prolonged Single Photon Emission Computed Tomography (SPECT) examinations. It decays only by electron capture (EC) which lowers the contribution to the dose as compared to β+ decays. It has a theranostic matched pair in the form of 103Ru (T1/2 = 39.26 d) that decays to the short-lived Auger emitter 103mRh (T1/2 = 56.12 min), a promising gamma-free therapeutic agent. Moreover, ruthenium element has a rich chemistry associated with its various oxidation states (II, III, IV and VIII) and forms more stable compounds compared to the SPECT-standard 99mTc [4]. Many radioactive Ru-labeled compounds have been studied and found applications as summarized recently by [5], in particular as the chemotherapy agents [6,7].
Due to these interesting characteristics, many studies on production of 97Ru have been conducted. The reactor route via 96Ru(n,γ)97Ru was reported by [1] but it yields very low specific activity which may limit its use for some applications such as molecular imaging. To obtain high specific activity product, one can use charged projectile from accelerators. In case of cyclotron routes, the first and most used reaction is 103Rh(p,spall)97Ru with 200 MeV proton beam and natural rhodium target, as suggested by [8]. While producing high amount of activity of no-carrier-added (NCA) 97Ru, this method requires high energy protons but no details about the impurity levels were reported. Another reaction route is the 103Rh(p,x)97Ru reaction using 60 MeV proton beam [9]; the 97Ru production yield is very high but accompanied by Tc radioactive impurities which are difficult to discard even after the chemical separation step. A very feasible option is the 99Tc(p,3n)97Ru reaction suggested by [10] and studied later up to 100 MeV by [4,11,12] as it produces significant amounts of 97Ru with very small amount of radioactive impurities. However, the availability of 99Tc radioactive target is an issue. Later, experimental excitation functions were reported for natAg(p,x)97Ru up to 80 MeV by [13] and for natPd(p,x)97Ru up to 70 MeV by [14]. These two production routes have much smaller cross-section, hence 97Ru production would require long irradiation time and would contain a substantial amount of radioactive impurities. In case of deuteron beam, the available reaction 96Ru(d,x)97Ru studied by [15] is favorable but would produce low specific activity as the target material is an isotope of the nuclide of interest. Some groups have also investigated more exotic projectiles such as helium-3 through natMo(3He,x)97Ru [16], 93Nb(7Li,3n)97Ru [17] and 89Y(12C,p3n)97Ru [2,18]. In these cases, after chemical separation, low level of radioactive impurities can be achieved but the availability of these beams is scarce making these processes not suitable to launched clinical trials. Finally, the cross-sections for α-induced reactions on Mo were investigated by [19]. natMo(α,x)97Ru production and impurities up to 40 MeV were thoroughly studied in [3,20].
In this work, we investigate the optimization of natMo(α,x)97Ru production route and extend the available cross-section data to higher energy in coherence with commercially available cyclotrons, [21] which are able to deliver up to about 70 MeV alpha beam. We also report on the coproduction of the measured radioactive impurities (listed in Table 1) via natMo(α,x) and explore the possible commercial production of 97Ru with the α beam on Mo target using the software Radionuclide Yield Calculator (RYC) that we developed and made freely available to the community.

2. Materials and Methods

2.1. Stacked-Foils Irradiations

Three experiments were performed at the ARRONAX facility [23], irradiating stacked-foils targets in vacuum with α beam of 67.4(5) MeV for about 1 h with beam currents of 40–60 nA. The stacked-foils technique and set-up in our facility have been described most recently in [24,25,26]. A typical stacked-foil target consisted of an Al monitor foil (~10 μm thick) in front, followed by the set of multiple natMo foils (~10 μm thick) and Al degraders (50–500 μm thick), arranged alternately. The order of the foils in the stacks were planned so that each natMo foil is activated with a different energy, all covering the energy range from 40 MeV to 67 MeV in about 3 MeV intervals (the projectile stopping-power in the stacks was calculated using SRIM software [27]). Certain foils were also used as catchers of the recoil atoms.
All foils were purchased from the GoodFellow© company with a purity of 99% for Al and 99.9% for natMo. Each foil was weighed before irradiation using an accurate scale (10−5 g) and scanned for area determination, allowing the precise thickness calculation (assuming the homogeneity over the whole surface).
As recommended by the International Atomic Energy Agency [28], the activity of the 24Na radioisotope formed in Al monitor foil was used to calculate the beam current impinging the stack. Additionally, during the irradiations, the online beam current monitoring was performed using a Faraday’s Cup with an electron suppressor for precise measurement and located behind the stack. The two measurements were consistent with each other.

2.2. Gamma Spectroscopy and Data Analysis

After about 14 h of cooling time, the gamma ray spectra of irradiated samples were collected using a HPGe Canberra detector with efficiency 20% at 1.33 MeV equipped with low-background lead and copper shielding. Each foil was placed at a height of 19 cm from the detector to ensure the dead-time below 10%. The detector was calibrated in energy and efficiency at 19 cm with 57Co, 60Co and 152Eu calibrated sources from LEA-CERCA (France) prior to the measurements. Gamma spectra were recorded using the LVis software from Ortec© while the activity of the radionuclides produced at the End of Bombardment (EOB) were derived using the FitzPeaks Gamma Analysis and Calibration Software (JF Computing Services). For the identification and activity estimation we used the γ-line and associated branching presented in Table 1. Knowing the activity of each isotope and the thickness of the foil in which they were observed, it was possible to calculate their production cross-section σ with the following formula:
σ = A E O B   M   Z   e H   N A   I   ρ   x   ( 1 e x p { λ   t } )
where: AEOB—activity of the radioisotope at the EOB, M—atomic mass of the target, Z—ionization number of the projectile, e—elementary charge, H—enrichment and purity of the foil, NA—Avogadro’s number, I—beam current, ρ—target material density, x—thickness of the foil, λ—decay constant of the radioisotope, t—time of the irradiation. The similar formula, solved for I and with cross-section values from [28], was used to calculate the beam current from the monitor foils. The projectile energy in the middle of the foils was adopted to the corresponding cross-section value.
The errors of cross-section values were propagated from the uncertainty of thickness measurements (around 1%), uncertainty of the counts in the γ-line peaks in the spectroscopy measurements (around 5–10%) and the error of the calculated beam current (around 5–10%) while the corresponding energy errors were propagated with SRIM software [27] considering the beam energy straggling through the foils (the initial energy error estimated by the cyclotron operators was 0.5 MeV).
The obtained cross-section values are then compared with TENDL-2017 (TALYS-based evaluated nuclear data library) [29] and the experimental results from other research groups.

2.3. Radionuclide Yield Calculator

Given the cross-section values, one can calculate the Thick Target Yield (TTY) with the following formula [30,31]:
T T Y ( E ) = H   N A   λ Z   e   M   E m i n E m a x σ ( E ) d E / d x ( E ) d E
where: Emax and Emin—maximal and minimal energy of the projectile penetrating the target (in case of TTY, Emin ≤ reaction threshold), dE/dx—stopping-power of the projectile in the irradiated target. To facilitate this calculation for 97Ru, as well as any other radioisotope and cross-section, we developed a Radionuclide Yield Calculator, later named RYC.
RYC is graphical user interface software written in python programming language (version 2.7) [32] using the TKinter module and compiled with PyInstaller software (version 3.4) [33]. It uses the cross-section and basic target data inputs to instantly calculate TTY and activity produced in any irradiation scenario. Data points can be fitted using different type of function, gaussian-like and polynomial functions, using the least-squares method. Excitation functions from TENDL [29] can be easily imported to compare with experimental data and to look for potential radioactive impurities. RYC with its detailed documentation can be downloaded from the ARRONAX website [34]. In particular, RYC uses implemented SRIM module [27] for stopping-power calculation.
The validation of this software was performed using data from the literature. On Figure 1, we compare the RYC-calculated TTY with the values published by IAEA [28,35] based on the same cross-section values for 127I(p,3n)125Xe, 64Ni(d,2n)64Cu and 209Bi(α,2n)211At reactions on metallic targets. Data calculated by RYC are presented as points whereas the curve published by IAEA correspond to the lines. As can be seen, for the 3 types of projectiles and for the different target masses, a very good agreement is obtained. The same good results have been obtained for all our tests.

3. Results and Discussion

3.1. Cross-Section Measurements

On Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 we present the measured cross-sections for natMo(α,x) reactions producing the 97Ru radioisotope as well as observed radioactive impurities: 89gZr, 95gTc, 96gTc, and 99Mo. The contributing reactions forming these radioisotopes are shown in Table 1. The experimental data are compared with previous experiments reported in literature [3,19,20] and the values from TENDL-2017 library [29]. Measured cross-section values are also listed in Table 2 (with corresponding energy errors, not visible on the graphs).
In the case of 97Ru production (Figure 2), our measurements correspond well to the data at lower energies. Compared to the experimental data, TENDL shows similar structure but underestimates the cross-section by about 30 mb in the region 20–40 MeV. The subsequent fall of the excitation function and a bump seem to be shifted by 5–10 MeV with respect to the experimental data.
The 89gZr excitation function (Figure 3) is measured for the first time. The predictions of TENDL shows a similar trend as our measurements but with a slightly shifted toward lower energies (5 MeV).
For 95gTc (Figure 4), our measurements are consistent with the previously measured data at lower energies. The shape of TENDL calculations seems to be the same as obtained in the measurements, but again a shift in energy is observed. This shift is probably related to the code since 3 different sets of data acquired at different time, different laboratories and overlapping energy range are consistent with each other and shows the same shift with respect to TENDL.
The experimental data describes well the excitation function for 96totTc (Figure 5). Additionally, our measurements preserve the trend of the ones reported earlier for lower energies. Here we do not observe any shift with respect to the TENDL calculations, as seen in the previous reactions.
The experimental cross-sections for 99Mo production (Figure 6) are consistent. Our results show a continuous rise of the excitation function up to the maximum energy of our measurements. This is in obvious contrast to TENDL which predicts a maximum at around 30 MeV and then a decrease of the excitation function. Slight shift between TENDL and experimental results is observed at low energies.

3.2. Calculated Yield and Production

Using RYC, we calculated TTY for natMo(α,x)97Ru reaction on metallic natMo target, based on our cross-section measurements above 40 MeV and the values reported by [3,20] below 40 MeV (Figure 7). The TTY values for other radioisotopes were also calculated in a similar way (not shown) to estimate the radioactive impurities.
The obtained experimental TTY values for 97Ru and radioactive impurities were used to estimate the possible production of 97Ru (Table 3) with natMo target and for two energies: 30 MeV and 67 MeV, which are the most common in commercially available cyclotrons. The 97Ru production yields are 3.5 MBq/µAh and 20 MBq/µAh respectively. Although the yield is almost 6 times larger at 67 MeV than at 30 MeV, the latter energy of α beam offer for example the optimal production of 211At (summarized recently by [36]) and 43Sc [37] medical radioisotopes. Additionally, in Table 3 we show the possible production of 50 MBq as this amount was proven SPECT-applicable in several clinical trials [38]. We have also calculated the yield and the number of produced stable Ru atoms (96Ru, 98Ru, 99Ru, 100Ru, 101Ru, 102Ru) based on the TENDL cross-sections to estimate the specific activity (SA) of 97Ru. The SA presented here assumes 100% successful chemical extraction of Ru isotopes from Mo target at EOB and hence is just an estimation used to compare different production routes.
It is worth mentioning that from the diagnostics point of view, the most dangerous impurity is 103Ru. It is the only other radioactive Ru element with long half-life (T1/2 = 39.26 d), which will contribute to the patient’s dose via high-intensity gamma-line (497 keV with 90.9% intensity) and Auger electrons from its daughter (103mRh). During the irradiation of natMo with α beam it can be only formed via 100Mo(α,x) reactions marked as the red line on Figure 2. Its contribution is rather small in our energy range and its activity was below our detection limit but we address it nevertheless (based on the measurements of [3,20]).
For the completeness of this study, we show the alternative production of 97Ru with the use of 100% enriched 95Mo and 96Mo targets and α beams of 30–15 MeV and 67–15 MeV, respectively.
Further chemical separation would be required to extract Ru element from Mo target and separate it from formed radioactive and stable elements of Tc, Nb, and Zr. This can be done for example with either the solvent extraction or distillation methods with an efficacy better than 80% [16]. The SA should also be considered in further chemical research as each production route form additional stable atoms of Ru, which would chelate the labeling compound.

4. Conclusions and Summary

We have extended the available cross-section measurements of selected natMo(α,x) reactions up to 67 MeV. Our measurements preserve well the trend of the cross-section values reported previously below 40 MeV and are consistent in overlapping energy ranges. A reasonable agreement with TENDL is observed however in certain cases the shift of 5–10 MeV is visible with respect to the experimental data.
We have shown the feasibility of no-carrier-added 97Ru production with α beam up to 67 MeV and thick natMo targets. The impurity of the only long-lived radioactive Ru radioisotope (103Ru) is small, around 0.1%. An irradiation of 1 h with few µA α-beam should satisfy the need for SPECT imaging for the patient. Several doses could be produced with longer irradiations at higher currents or using enriched 95,96Mo targets which will substantially increase the produced activity and SA.
The use of RYC [34] to calculate TTY based on cross-section data was also demonstrated.

Author Contributions

M.S.: Investigation, formal analysis, software, writing—original draft, writing—review and editing; E.N.: Investigation, resources, writing—review and editing; A.G.: Investigation, resources, writing—review and editing; F.H.: Investigation, conceptualization, supervision, funding acquisition, writing—review and editing; T.M.: supervision, writing—review and editing.

Funding

The cyclotron Arronax is supported by CNRS, Inserm, INCa, the Nantes University, the Regional Council of Pays de la Loire, local authorities, the French government, and the European Union. This work has been, in part, supported by a grant from the French National Agency for Research called “Investissements d’Avenir”, Equipex Arronax-Plus noANR-11-EQPX-0004 and Labex IRON noANR-11-LABX-18-01.

Acknowledgments

The PhD cotutelle scholarship from French Government and 17th WTTC bursary for Mateusz Sitarz are acknowledged. Special thanks to RYC beta testers: Roberto Formento, Julio Panama and Katarzyna Szkliniarz.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Comparison of TTY for selected nuclear reactions on metallic targets calculated with RYC and adapted from [28,35] based on the same cross-section values from IAEA.
Figure 1. Comparison of TTY for selected nuclear reactions on metallic targets calculated with RYC and adapted from [28,35] based on the same cross-section values from IAEA.
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Figure 2. Measured cross-section for natMo(α,x)97Ru reaction compared with data available in the literature. The coproduction of 103Ru via 100Mo(α,n)103Ru and 100Mo(α,p)103Tc→103Ru reactions was not observed in the investigated energy range, but the cross-section for natMo(α,x)103Ru from TENDL-2017 is plotted (red line) to complement the discussion from the text.
Figure 2. Measured cross-section for natMo(α,x)97Ru reaction compared with data available in the literature. The coproduction of 103Ru via 100Mo(α,n)103Ru and 100Mo(α,p)103Tc→103Ru reactions was not observed in the investigated energy range, but the cross-section for natMo(α,x)103Ru from TENDL-2017 is plotted (red line) to complement the discussion from the text.
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Figure 3. Measured cross-section for natMo(α,x)89gZr reaction compared with TENDL.
Figure 3. Measured cross-section for natMo(α,x)89gZr reaction compared with TENDL.
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Figure 4. Measured cross-section for natMo(α,x)95gTc reaction compared with the literature.
Figure 4. Measured cross-section for natMo(α,x)95gTc reaction compared with the literature.
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Figure 5. Measured cross-section for natMo(α,x)96totTc reaction compared with the literature. This is a cumulative cross-section of natMo(α,x)96gTc and natMo(α,x)96mTc reactions.
Figure 5. Measured cross-section for natMo(α,x)96totTc reaction compared with the literature. This is a cumulative cross-section of natMo(α,x)96gTc and natMo(α,x)96mTc reactions.
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Figure 6. Measured cross-section for natMo(α,x)99Mo reaction compared with the literature.
Figure 6. Measured cross-section for natMo(α,x)99Mo reaction compared with the literature.
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Figure 7. TTY for 97Ru production via natMo(α,x) on metallic natMo target. The experimental curve (blue) is calculated using the cross-section from this work (above 40 MeV) and the data provided by [3,20] below 40 MeV.
Figure 7. TTY for 97Ru production via natMo(α,x) on metallic natMo target. The experimental curve (blue) is calculated using the cross-section from this work (above 40 MeV) and the data provided by [3,20] below 40 MeV.
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Table 1. Nuclear data [22] of 97Ru and observed radionuclidic contaminants as well as reactions contributing to their formation during the irradiation of natMo target*.
Table 1. Nuclear data [22] of 97Ru and observed radionuclidic contaminants as well as reactions contributing to their formation during the irradiation of natMo target*.
RadionuclideT1/2Decay Mode (%)γ-Lines [keV] and Intensities** (%)Contributing Reactions***Q-Value [MeV]
97Ru2.83 dEC (100)215.7 (85.8)
324.5 (10.8)
94Mo(α,n)97Ru−7.9
95Mo(α,2n)97Ru−15.3
96Mo(α,3n)97Ru−24.5
97Mo(α,4n)97Ru−31.3
98Mo(α,5n)97Ru−41.6
100Mo(α,7n)97Ru−54.1
89gZr78.4 hβ+ (23), EC (77)908.96 (100)92Mo(α,x)89totZr−16.7
94Mo(α,x)89totZr−14.0
95Mo(α,x)89totZr−21.4
96Mo(α,x)89totZr−30.6
97Mo(α,x)89totZr−37.4
98Mo(α,x)89totZr−46.0
100Mo(α,x)89totZr−60.2
92Mo(α,x)89totNb→89totZr−21.1
94Mo(α,x)89totNb→89totZr−38.9
95Mo(α,x)89totNb→89totZr−46.2
96Mo(α,x)89totNb→89totZr−55.4
97Mo(α,x)89totNb→89totZr−62.2
98Mo(α,x)89totNb→89totZr−70.9
100Mo(α,x)89totNb→89totZr−85.1
96gTc4.28 dEC (100)778.22 (100)
812.58 (82)
849.93 (98)
1126.97 (15.2)
94Mo(α,x)96totTc−13.3
95Mo(α,x)96totTc−14.4
96Mo(α,x)96totTc−23.7
97Mo(α,x)96totTc−30.4
98Mo(α,x)96totTc−39.0
100Mo(α,x)96totTc−53.3
99Mo65.9 hβ (100)140.51 (89.43)
739.50 (12.13)
97Mo(α,2p)99Mo−13.7
98Mo(α,x)99Mo−14.7
100Mo(α,x)99Mo−8.3
95gTc20.0 hEC (100)765.8 (93.82)92Mo(α,n)95gTc−5.7
94Mo(α,x)95gTc−14.9
95Mo(α,x)95gTc−22.3
96Mo(α,x)95gTc−31.4
97Mo(α,x)95gTc−38.3
98Mo(α,x)95gTc−46.9
100Mo(α,x)95gTc−61.1
92Mo(α,n)95Ru→95gTc−9.0
94Mo(α,3n)95Ru→95gTc−26.7
95Mo(α,4n)95Ru→95gTc−34.1
96Mo(α,5n)95Ru→95gTc−43.3
97Mo(α,6n)95Ru→95gTc−50.1
98Mo(α,7n)95Ru→95gTc−58.7
100Mo(α,9n)95Ru→95gTc−73.0
* natMo composition: 92Mo (14.6%), 94Mo (9.2%), 95Mo (15.9%), 96Mo (16.7%), 97Mo (9.6%), 98Mo (24.3%), 100Mo (9.7%); ** lines with less than 10% intensities are not included; *** “tot”—the reaction produces the radionuclide directly and via decay of its metastable state.
Table 2. Measured cross-sections for natMo(α,x) reactions (with the uncertainties in the parenthesis).
Table 2. Measured cross-sections for natMo(α,x) reactions (with the uncertainties in the parenthesis).
E [MeV]natMo(α,x) Cross-Section [mb]
97Ru89gZr95gTc96totTc99Mo
41.80(75)237(20)ND*81(11)73(7)7.5(1.0)
46.03(68)225(20)ND127(14)89(8)10.1(1.2)
50.00(64)199(18)ND163(17)100(9)11.4(1.3)
51.93(62)166(14)ND170(16)101(9)12.8(1.3)
55.30(60)159(13)3.6(9)177(17)109(9)13.5(1.4)
58.51(56)176(15)11.7(1.6)205(17)119(10)ND
59.97(55)176(15)18(2)174(24)116(10)14.0(1.5)
63.47(53)180(16)30(3)188(16)118(10)15.0(1.7)
66.84(50)173(14)40(3)203(17)122(10)15.6(1.3)
* ND = not detected.
Table 3. Estimation of 97Ru activity produced via the irradiation of natMo (based on experimental data) and enriched 95,96Mo targets (based on TENDL-2017 [29]) with α beam in two energy ranges. The list of radioactive impurities is narrowed down to the long-lived ones and shows their activity relative to activity of 97Ru at EOB.
Table 3. Estimation of 97Ru activity produced via the irradiation of natMo (based on experimental data) and enriched 95,96Mo targets (based on TENDL-2017 [29]) with α beam in two energy ranges. The list of radioactive impurities is narrowed down to the long-lived ones and shows their activity relative to activity of 97Ru at EOB.
α energy30–15 MeV67–15 MeV
targetnatMo95Mo (100%)natMo96Mo (100%)
thickness100 mg/cm2100 mg/cm2540 mg/cm2540 mg/cm2
97Ru yield3.5 MBq/µAh14 MBq/µAh20 MBq/µAh31 MBq/µAh
irradiation1 h, 15 µA1 h, 15 µA1 h, 2.5 µA1 h, 2.5 µA
97Ru AEOB50 MBq
(1.4 mCi)
200 MBq
(5.4 mCi)
50 MBq
(1.4 mCi)
80 MBq
(2.2 mCi)
SA at EOB350 GBq/µmol
(9 kCi/mmol)
1300 GBq/µmol
(36 kCi/mmol)
420 GBq/µmol
(11 kCi/mmol)
630 GBq/µmol
(17 kCi/mmol)
relative activity [%]97Ru100100100100
89gZr0030.04
95gTc951E−3200150
96gTc40.22534
103Ru0.1200.020
reference[3], [20]TENDL-2017[3], [20]
this work
TENDL-2017

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