Radiochim. Acta 90, 1–5 (2002)  by Oldenbourg Wissenschaftsverlag, München

Cross section data for the production of the positron emitting 90Nb via the 90Zr(p, n)-reaction

By S. Busse1,2,F.Rösch1,∗ andS.M.Qaim2

1 Institut für Kernchemie, Johannes Gutenberg-Universität, D-55128 Mainz, Germany 2 Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

(Received April 19, 2001; accepted in revised form July 16, 2001)

Positron emitter 90Nb / Nuclear reaction / (α, 3n)-reactions on natural . A medium-sized cy- Excitation function / Calculated integral yield / clotron would allow the production of 90Nb via the (p, n)-, Experimental thick target yield / Radionuclidic impurities (d, 2n)- or the (3He, 2n)-process. In fact even a small-sized

cyclotron (Ep ≤ 16 MeV) should to sufficient quanti- ties of the radioisotope via the (p, n)-reaction. A few stud- Summary. The radioisotope 90Nb decays with a positron ies have shown that the (d, 2n)-reaction requires a deuteron + branching of 53% and a relatively low β -energy of energy of about 16 MeV [5–7], the (3He, 2n)-process a 3He- = . = . . Emean 0 66 MeV and Emax 1 5 MeV. Its half-life of 14 6h energy of ≥ 30 MeV and the (α, 3n)-reaction an α-particle makes it especially promising for quantitative investigation energy of ≥ 45 MeV [8]. Furthermore, the systematics of of biological processes with slow distribution kinetics using (3He, 2n)- and (p, n)-reactions suggest that the production positron emission tomography. To optimise its production, 90 the excitation functions of 90Zr(p, xn)-processes were studied yield of Nb should be higher in the latter process. With the common availability of dedicated cyclotrons for produc- over the proton energy range of 7.5 to 19 MeV via the + stacked-foil technique using both natZr and 99.22% enriched ing short-lived β -emitters, the (p, n)-reaction represents 90 90 90 ZrO2 as targets. Thick target yields of Nb were calculated an advantageous route for production of Nb. Some cross from the measured excitation functions and were verified section data on the natZr(p, xn)-reactions using thick tar- experimentally. The optimum energy range for the produc- get irradiations with high initial proton energy have already tion of 90Nb via the 90Zr(p, n)-process was found to be been reported in the literature [9, 10]. However, those ex- = → 90 /µ Ep 17 7 MeV, with a yield of 600 MBq Nb Ah.The periments were not designed to determine cross sections in 90 yield and radionuclidic purity of Nb over the energy range of the low energy region relevant to the production of 90Nb, E = 17.6 → 8.1 MeV were determined experimentally using p i.e.atE < 20 MeV. The data may have large errors because natZr. At 4 h after EOB the yield of 90Nb was found to be p 290 MBq/µA h and its radionuclidic purity ≥ 95%. large foil-stacks with high incident proton energies (for ex- ample, Ep = 70 → 10 MeV) were used. The aim of this study was to investigate proton induced nat 90 Introduction reactions on Zr and Zr and to measure the experimen- tal production yield and radionuclidic purity of the desired The radioisotope 90Nb is a positron emitter with a positron product. + branching of 53% and a relatively low β -energy of Emean = 0.66 MeV and Emax = 1.5 MeV. Its half-life of 14.6h ren- ders it especially promising for quantitative investigation Experimental of slow biological processes, in particular those involving labeled peptides and proteins, via positron emission tomog- Cross sections were measured as a function of incident pro- raphy. Recently, the radiochemical separation of no-carrier- ton energy using the conventional stacked-foil technique, as added 90Nb from macroscopic targets and first described earlier, cf. [11, 12]. Thick target yields were deter- approaches to introduce no-carrier-added 90Nb into pep- mined using thicker targets. Some of the salient features of tidic tracers such as the octreotide derivative DFO-succinyl- the present experiments are given below. (D)Phe1-octreotide (SDZ 216-927) using the bifunctional ligand desferrioxamine have been described [1, 2]. 90Nb was originally identified via the (d, 2n) reaction Target material on 90Zr [3]. High-purity 90Nb was first obtained via the For irradiations with incident proton energies up to 15 MeV, 93Nb(p, 4n)90Mo → 90Nb process [4]. For the production of i.e. the onset of the 91Zr(p, 2n)90Nb-process, natZr was used 90Nb today, several nuclear reactions seem to be reasonable: as target material in the form of 10 µm or 250 µm thick foils. the (p, n)- or (d, 2n)-process on 90Zr and the (3He,2n)- or Commercially available natZr metal foils were used as sup- plied by Goodfellow Metals. The isotopic composition of *Author for correspondence (E-mail: [email protected]). natural Zr is: 90Zr (51.45%), 91Zr (11.32%), 92Zr (17.19%), 2 S. Busse, F. Rösch and S. M. Qaim

94Zr (17.28%), 96Zr (2.76%). The chemical purity was spec- Measurement of radioactivity ified by the supplier as 99.8+%. The remaining typical im- The absolute radioactivity was determined by γ-ray spec- purities in natZr (in ppm) were: Nb (50), Hf (250), Fe (200), trometry using either a Ge(Li) or a HPGe detector coupled to Cr (200), Al (40), Ti (20), Ni (20), Cu (20), V (20), W (200), an Ortec (Spectrum ACE) 4 K MCA plug-in card. The card rare earths (50). was connected to an IBM-compatible PC-AT. The peak area For irradiations at proton energies above 12 MeV, iso-  analysis was done using the software Gamma Vision 4.10. topically enriched 90Zr was used. To prepare 90Zr samples The detector counting efficiencies for different photon en- commercially available metal dioxide powder from Cam- ergies and counting distances were determined using cali- pro Scientific was used. The isotopic composition in this brated standard sources (errors < 3%) obtained from PTB case was: 90Zr (99.22%), 91Zr (0.39%), 92Zr (0.29%), 94Zr Braunschweig and Amersham International. (0.15%), 96Zr (0.01%). The content of impurities (in ppm) The radioactivity was measured non-destructively, i.e. was specified as: Fe (50), Cr (60), Al (30), Ti (20), Ni the 90Zr samples on Cu backing foils together with the Al (< 20), Cu (50). covers were counted without a mechanical removal of the covering foil or chemical treatment of the target material. Sample preparation With respect to cross section measurements, counting was In the case of natZr, samples for cross section measurements performed only over two days after each irradiation, and at- were prepared by cutting pieces of 13 mm diameter out of tention was paid exclusively to the short-lived Nb a10µm thick metal foil. For thick target yield measure- 90Nb, 89Nb and 89mNb. Apart from the activity of the men- ments, discs of 250 µm thickness were cut out of a large- tioned isotopes the activity of the monitor reaction products sized metal foil. 62Zn and 63Zn was also determined. The decay data used in Thin samples of enriched 90Zr were prepared by means the γ-ray spectroscopic analysis of the radioisotopes inves- of a special sedimentation technique. Details of this pro- tigated were taken from Ref. [16] and are summarized in cedure were described in [13]. In brief, a suspension of Table 1. 90 5–6 mg of very fine ZrO2 powder in 100 µl of water-free chloroform in an Eppendorf vessel was prepared by ac- Calculation of cross sections and errors tion of an ultrasonic bath for 3 min. The suspension was transferred to a cylindrical polytetrafluoroethylene (PTFE) The count rates were corrected for pile-up losses as well vessel (10 mm diameter) with a 13 mm diameter as for γ-ray intensities and the efficiencies of the detectors. foil (25 µm thick) as removable bottom. The chloroform of The uncertainties due to random coincidences were kept the suspension was allowed to evaporate slowly. The cop- small by choosing a sample to detector distance such that per foil and the empty PTFE cylinder were then carefully the dead time was < 7%. Since the distance between the separated from each other. A homogeneous and mechan- sample and the detector was always > 10 cm, the correc- 90 ically stable ZrO2 sample of thickness in the order of tion for real coincidence losses was negligible. The cross 5–6 mg/cm2 was thus obtained on the surface of the copper sections were calculated using the well-known activation backing foil. To provide mechanical stability to the sedi- formula. The overall uncertainty in each cross section was mented layers a thin adhesive polymer film (< 1mg/cm2) obtained by taking the square root of the sum of the squares was sprayed over the samples. Furthermore, to avoid con- of the individual uncertainties, which were considered to be tamination, the surface of each sample was covered with as follows: a10µm thick Al foil, having the same diameter of 13 mm as target mass (∼ 2%), the Cu backing foil. inhomogeneity in target thickness (5%–10%), Irradiations and beam current monitoring For cross section measurements, several stacks, each con- nat 90 Table 1. Decay data of the product nuclei used in the present study, taining five or six Zr foils or six ZrO2 sedimented sam- cf. [16]. ples, were irradiated at the Jülich compact cyclotron CV 28, each for 30 min at a beam current of 100 nA. The pri- Nuclide Mode of T1/2 Eγ γ-abundance mary proton energies used were 20, 16 and 12 MeV. For the decay (%) [keV] [%] measurement of thick target yields and isotopic impurities a 30 min irradiation at 1 µA was carried out using a stack 89mNb β+ (81) 66.0 min 588.099.5 . . of three 13 mm ∅ natZr foils, each 250 µm thick. Each stack EC (19) 507 4850 µ 89Nb β+ (75) 2.0 h 1833.43.2 contained a Cu monitor foil (25 m thick) on the front side. . . > EC (25) 1627 030 For incident proton energies 14 MeV, it was thus pos- 920.51.4 sible to control the proton energy experimentally via the 90mNb IT (100) 18.8 s 122.464.2 σ /σ + energy dependent ratio (p,2n) (p,n) of the monitor reactions 90Nb β (53) 14.6 h 1129.192.0 63Cu(p, 2n)62Zn and 63Cu(p, n)63Zn, cf. [14]. The proton EC (47) 2319.182.8 beam current was measured directly using a Faraday cup as 141.269.0 well as indirectly via the 63Cu(p, n)63Zn reaction, cf. [15]. 63Zn β+ (93) 38.1 min 669.88.4 . . At low intensities the proton flux measured via the two tech- EC (7) 962 666 62 . . . niques differed considerably. We therefore put more reliance Zn EC (93.1) 9 23 h 548 4152 β+ (6.9) 596.725.7 on the monitor reaction. Cross section data for the production of the positron emitting niobium isotope 90Nb 3

statistical errors (2%–10%; for energy points near the Detailed cross sections of the 90Zr(p, n)90Nb-reaction threshold up to 20%), over the most relevant proton energy region of 7.5 to 19 MeV detector efficiency and sample-detector geometry (∼ 6%), have been measured for the first time in this work. Up to decay data errors (< 3%), 15 MeV, natZr was used as target material, and correction bombarding proton beam intensity (8%–10%). for isotopic composition was made. Beyond 15 MeV, use of 90 For the 90Zr(p, n)-reaction the overall error in cross sec- enriched Zr was necessary, because of increasing contri- 91 , 90 tion amounts to 15 to 20%. The primary proton energy had bution of the Zr(p 2n)-process to the formation of Nb. ± . Between 12 and 15 MeV, a few measurements were carried an error of 0 2 MeV. The proton energy degradation in nat 90 90 out using both Zr and Zr to check the consistency of each stack foil or ZrO2 sample was calculated and its half- value was combined with the uncertainty of the primary results. energy. The uncertainty in the effective energy of the front Our cross section data are shown as a function of pro- foil was estimated to be 0.3 MeV. Due to propagation of ton energy in Fig. 1 together with the results of an earlier 90 , error it increased to 0.7 MeV in the sixth foil of a stack. study [9]. The Zr(p n)-reaction leading to the formation of 90Nb has a threshold of about 7 MeV and reaches a max- imum cross section of about 820 mb at 13.5MeV.These Results and discussion results show considerable deviations from the data reported by Kondratev et al. [9], who gave a maximum cross section Cross section data of 661 mb at a somewhat higher energy of about 17 MeV. The measured cross sections of the 90Zr(p, n)90Nb, 90Zr However, as mentioned above, those experiments were not (p, 2n)89mNb and 90Zr(p, 2n)89Nb reactions are given in designed to measure accurate cross sections in the low en- Table 2. The data for 90Nb describe the cumulative formation ergy region. Considering the (p, n)-excitation functions for cross sections of the 14.6 h isotope, since the 18.8 s isomeric some other target nuclei in this mass region investigated state 90mNb decays 100% by IT to 90Nb. Because of the short in recent years, e.g. 85Rb [17], 86Sr [13] and 94Mo [18], half-life of 90mNb, a short cooling time of only a couple of our data appear to be more precise than the values given minutes was sufficient to allow complete decay of 90mNb be- in Ref. [9]. fore the γ-ray measurements. The data given for 89mNb and At proton energies higher than 17 MeV the 90Zr(p, 2n)- 89Nb describe the direct production cross sections, i.e. with- process also occurs, leading to the isotopes 89mNb and 89Nb. out involving the decay of any precursor. Those results are also shown in Fig. 1.

Table 2. Cross sections for the nat 90 formation of 90Nb, 89mNb and Target Thickness Ep Zr ZrO2 89Nb in proton induced nuclear [ / 2] [ ] σ(90 ) a σ(90 ) a σ(89m ) b σ(89m ) b reactions on natZr and on highly- material mg cm MeV Nb Nb Nb Nb enriched 90Zr. [mb] [mb] [mb] [mb]

90 ZrO2 5.9 19.0 ± 0.3 − 243 ± 41 67 ± 15 91 ± 40 90 ZrO2 5.6 18.4 ± 0.3 − 271 ± 43 43 ± 10 64 ± 32 90 ZrO2 5.6 17.8 ± 0.3 − 352 ± 63 23 ± 6 43 ± 26 90 ZrO2 5.0 17.2 ± 0.3 − 460 ± 86 9 ± 3 90 ZrO2 5.5 16.6 ± 0.3 − 556 ± 93 90 ZrO2 5.7 16.0 ± 0.3 − 617 ± 98 90 ZrO2 5.9 15.5 ± 0.3 − 743 ± 111 natZr 7.5 15.3 ± 0.3 859 ± 123 − 90 ZrO2 5.6 14.9 ± 0.3 − 791 ± 144 natZr 7.2 14.3 ± 0.4 816 ± 148 − 90 ZrO2 5.6 14.2 ± 0.4 − 771 ± 125 90 ZrO2 5.0 13.5 ± 0.3 876 ± 152 natZr 7.2 13.5 ± 0.4 797 ± 125 natZr 7.5 13.2 ± 0.4 809 ± 121 90 ZrO2 5.5 12.8 ± 0.4 − 826 ± 132 natZr 7.2 12.6 ± 0.5 749 ± 137 natZr 7.2 12.3 ± 0.4 763 ± 131 90 ZrO2 5.6 12.0 ± 0.6 − 808 ± 125 natZr 7.1 11.6 ± 0.6 728 ± 111 natZr 7.2 11.3 ± 0.4 724 ± 129 natZr 7.6 11.1 ± 0.3 640 ± 115 natZr 7.2 10.6 ± 0.7 647 ± 110 natZr 7.2 10.3 ± 0.5 650 ± 127 natZr 7.6 10.0 ± 0.4 590 ± 105 natZr 7.1 9.1 ± 0.6 524 ± 95 natZr 7.4 8.8 ± 0.4 408 ± 105 natZr 7.2 7.9 ± 0.7 292 ± 70 natZr 7.5 7.5 ± 0.5 132 ± 31

a: Cumulative cross section b: Independent formation cross section 4 S. Busse, F. Rösch and S. M. Qaim

Fig. 2. Thick target yields of 90Nb, 89mNb and 89Nb calculated from the Fig. 1. Excitation functions of 90Zr(p, xn)-processes leading to the for- excitation functions measured in this work (cf. Fig. 1). mation of 90Nb, 89mNb and 89Nb. The values for 90Nb describe the cumulative formation cross section. Error bars are also shown. The solid curve is an eye-guide through our data. A few available points for experimental values are given together with the calculated 90Nb from the literature [9] are also shown. ones. There is a good agreement between the experimen- tal and calculated yield data of 90Nb for proton energies < 15 MeV, i.e. when only the (p, n)-reaction occurs. For

Theoretical integral yields Ep > 15 MeV the experimental values are slightly higher due to some contribution of the 91Zr(p, 2n)90Nb-process to the The differential and integral yields of the niobium ra- formation of 90Nb. The last two columns give the calculated dioisotopes at EOB formed in the two nuclear processes, 90 90 , yields of Nb if 100% enriched Zr in metallic form or as viz 90Zr(p, n)90Nb and 90Zr(p, 2n)89m 89Nb, were calcu- 90ZrO would be used. Evidently, those yields are higher. lated from the experimentally measured excitation functions 2 (Fig. 1) and the stopping powers, assuming a 1 h irradiation at 1 µA. The stopping powers were calculated on the basis Impurities of the Bethe-Bloch equation which, using the formalism of The radionuclidic impurities of some concern in the pro- Williamson et al. [19], has been transformed to a computer duction of 90Nb from highly enriched 90Zr are the Nb iso- program “STACK” at Jülich. The integral yields are given 89m 90 topes 89m and 89, and their Zr and Y decay products Zr in Fig. 2. Up to about 17 MeV, Nb is the only product. 89 89m , (T / = 4.2min), Zr (T / = 78.4h)and Y(T / = 16 s), The formation of 89m 89Nb becomes relevant only at proton 1 2 1 2 1 2 resulting from the 90Zr(p, 2n)-reaction at high incident pro- energies > 17 MeV. jectile energies. The production of these impurities can be From the excitation function (Fig. 1) and the yield curve avoided by limiting the incident proton energy to below (Fig. 2) it is evident that the optimum energy range for the , 90 = → the threshold energy of the (p 2n)-reaction, however, with production of Nb is Ep 17 7 MeV. For this energy 90 90 a somewhat lower Nb yield. Because of the relatively short range the integral yield amounts to 600 MBq Nb/µAh. , half-lives of 89m 89Nb as compared to 90Nb, the level of these At a somewhat higher proton energy, e.g.19MeV,some , impurities decreases with time; consequently a simultaneous 89m 89Nb will also be formed. However, because of the longer production of these short-lived isotopes can be tolerated to half-life of 90Nb, the ratio of the product nuclide 90Nb to the 89m,89 a certain extent. In addition to the above mentioned im- radionuclidic impurties Nb would become better with , , purities some other isotopic Nb-impurities like 91m 92m 96Nb extended irradiation times. may also occur. Their levels depend on the isotopic abun- dances of 91,92,96Zr in the enriched 90Zr sample. Addition- Experimental thick target yields ally, 90Zr(p,α)87Yand90Zr(p, pn)89Zr reactions may occur. Using three natZr foils of 250 µm thickness each, thick tar- These are, however, of no great significance since Zr and Y get yields were determined experimentally. In Table 3 the can be chemically separated from the radioniobium.

Table3. Experimental and calculated thick target yields Yield of 90Nb [MBq/µAh] from various targets of 90Nb for a 30 min irradi- nat nat nat 90 90 ation of Zr with a 1 µApro- Foil No. EP [MeV] Zr Zr (cal.) via Zr-metal (cal.) ZrO2 (cal.) ton beam and respective cal- (exp.) 90Zr(p, n)-reaction (100% enriched) (100% enriched) culated yields for natZr and 90 100% enriched Zr-metal as . → . 90 1 17 6 15 0 128 89 173 110 well as ZrO2 . 2 15.0 → 11.9 120 123 239 152 3 11.9 → 8.1 82 95 184 118 2 + 3 15.0 → 8.1 202 218 423 270 1 + 2 + 3 17.6 → 8.1 330 307 596 380 Cross section data for the production of the positron emitting niobium isotope 90Nb 5

Table 4. Measured levels 89m a 89 a 90 91m 92m 95m 95 96 of radioniobium impuri- Foil Ep Nb Nb Nb Nb Nb Nb Nb Nb [ ] ...... ties (1 a > T1/2 > 10 min) No. MeV (1 1h) (20h) (146 h) (62 d) (10 2d) (361 d) (35 d) (23 4h) after a 30 min irradiation nat of Zr as percent of total 1 + 2 + 3 17.6 → 8.1 1.28 0.33 96.15 0.10 0.92 0.34 0.12 0.76 radioniobium activity for 2 + 3 15.0 → 8.1 −−96.91 0.10 1.42 0.36 0.12 1.10 different proton energy 3 11.9 → 8.1 −−94.96 0.08 2.39 0.33 0.09 2.15 ranges (values refer to 4 h after EOB). a: Calculated values

As far as the production of 90Nb from natZr is con- 5. Mercader, R. C., Caracoche, M. C., Mocoroa, A. B.: Excitation cerned, the typical composition of radioniobium found after functions for the production of 90Nb and 88Y by irradiation of a 30 min irradiation of natZr is given in Table 4. The values zirconium with deuterons. Z. Physik 255, 103 (1972). γ 6. Wasilevsky, C., Dos Santos, F., Herreros Usher, O., Nassiff, S. J.: were obtained via a -ray spectrometric analysis of all the Excitation functions and thick target yields for deuteron induced radioisotopes at 4 h post EOB and are given as % of the total reactions on zirconium. Radiochem. Radioanalyt. Lett. 32, 127 niobium activity. (1978). A of 4 h after EOB was chosen as a realistic 7. Gonchar, A. V., Kondratev, S. N., Lobach, Yu. N., Nevskii, S. V., time with respect to the separation of 90Nb from a Zr target Skylyarenko, V. D., Tokarevskii, V. V.: Formation of long-lived ra- 90 dionuclides by irradiation of zirconium with deuterons. Atomnaja and the synthesis of a potential Nb-radiopharmaceutical Energija 75, 205 (1993). for nuclear medical application. As can be concluded 8. Smend, F., Weirauch, W., Schmidt-Ott, W.-D., Flammersfeld, A.: from Table 4, the 90Nb activity is almost constant at about Cross sections and isomeric ratios for the reactions 89Y(α, 3n) 90gNb and 89Y(α, 3n)90mNb. Z. Physik 207, 28 (1967). 95%–97% over the investigated energy range of Ep = . → . 92m 9. Kondratev, S. N., Kuzmenko, V. A., Lobach, Yu. N., Prokopen- 17 6 8 1 MeV. The levels of the activities of Nb and ko, V. S., Sklyarenko, V. D., Tokarevskii, V. V.: Radionuclide pro- 96 90 Nb are not very high and the 95% purity of Nb may duction cross sections up to 70 MeV by interaction of protons with be acceptable for some (animal) nuclear medical appli- zirconium nuclides. Atomnaja Energija 71, 325 (1991). cations. However, it is recommended that the radiation 10. Michel, R., Bodemann, R., Busemann, H., Daunke, R., Glo- dose from those three impurities should be estimated prior ris, M., Lange, H.-J., Klug, B., Krins, A., Leya, I., Lüpke, M., Neumann, S., Reinhardt, H., Schnatz-Büttgen, M., Herpers, U., to large scale production and application in humans of Schiekel, Th., Sudbrock, F., Homqvist, B., Conde,´ H., Malm- 90Nb produced from natZr. With 100% enriched 90Zr tar- borg, P., Suter, M., Dittrich-Hannen, B., Kubik, P.-W., Synal, gets, however, the yield of 90Nb in the energy ranges of H.-A., Filges, D.: Cross sections for the production of residual E = 17.6 → 8.1MeVand E = 15.0 → 8.1 MeV could be nuclides by low- and medium-energy protons from the target p p elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, increased by a factor of 1.81 and 1.94, respectively, and the Y, Zr, Nb, Ba and Au. Nucl. Instrum. 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