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Excitation functions of 124Te(d,xn)124,125I reactions from threshold up to 14 MeV: comparative evaluation of nuclear routes for the production of 124I
Applied Radiation and Isotopes 55 (2001) 303–308
Excitation functions of 124Te(d,xn)124,125I reactions from threshold up to 14 MeV: comparative evaluation of nuclear routes for the production of 124I Th. Bastian, H.H. Coenen, S.M. Qaim* Institut fur . Nuklearchemie, Forschungszentrum Julich . GmbH, D-52425 Julich, . Germany Received 12 February 2001; accepted 12 March 2001
Abstract Excitation functions of the nuclear reactions 124Te(d,xn)124,125I were measured from their respective thresholds up to 14.0 MeV via the stacked-foil technique. Thin samples were prepared by electrolytic deposition of 99.8% enriched 124Te on Ti-backing. The excitation function of the 124Te(d,n)125I reaction was measured for the first time. The present data for the 124Te(d,2n)124I reaction are by an order of magnitude higher than the literature experimental data but are in good agreement with the results of a hybrid model calculation. From the measured cross sections, integral yields of 124,125 I were calculated. The energy range Ed=14 ! 10 MeV appears to be the best compromise between 124I-yield and 125 I-impurity. The calculated 124I-yield amounts to 17.5 MBq/mA h and the 125I-impurity to 1.7%. A critical evaluation of the three nuclear routes for the production of 124I, viz. 124Te(d,2n)-, 124Te(p,n)- and 125Te(p,2n)-processes, is given. The reaction studied in this work proved to be least suitable. The 124Te(p,n)-reaction gives 124I of the highest radionuclidic purity, and a small-sized cyclotron is adequate for production purposes. The 125Te(p,2n)-reaction is more suitable at a medium-sized cyclotron: the yield of 124I is four times higher than in the other two reactions but the level of 0.9% 125I-impurity is relatively high. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Iodine-124; (d,xn)-reactions; Excitation function; Thick target yield; Radionuclidic impurity
1. Introduction The radioisotope 124I ðT1=2 ¼ 4:18 d; Ebþ ¼ 2:13 MeV; Ibþ ¼ 25%Þ is a very suitable radionuclide for both diagnostic and therapeutic use in nuclear medicine. A brief summary of the various applications of this radionuclide has been recently given (cf. Hohn et al., 2001, and references cited therein). 124I was first produced in MBq amounts via the 124Te(d,2n)124I process (cf. Sharma et al., 1988; Lambrecht et al., 1988; Firouzbakht et al., 1993, 1994; Weinreich and Knust, 1996) although the level of 125I-impurity was considered to be rather high. The process is still used in *Corresponding author. Tel.: +49-2461-61-3282; fax: +492461-61-2535. E-mail address: [email protected] (S.M. Qaim).
some laboratories (cf. Knust et al., 2000). In 1995, it was shown that the 124Te(p,n)124I reaction is superior to the 124 Te(d,2n)124I reaction (Scholten et al., 1995) as far as the 125I-impurity is concerned. The 124Te(p,n)-reaction has been used since then in several institutions (cf. Qaim et al., 1996, 2000; Eschmann et al., 1999; McCarthy et al., 2000; Brown et al., 2000; Sheh et al., 2000). Recently, detailed cross section data have also been reported for the 125Te(p,2n)124I reaction (Hohn et al., 2001). Thus now three reactions, viz. 124Te(d,2n)124I, 124 Te(p,n)124I and 125Te(p,2n)124I, are available for the production of 124I. The cross section data base for the latter two reactions is fairly strong. For the (d,2n) process, however, no reliable data are available. Firstly, the 124Te(d,n)125I reaction has not been investigated at all, mainly due to the difficulty in the quantitative measurement of the low energy (35.5 keV) g-ray, and
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secondly, the data for the 124Te(d,2n)124I process reported by Firouzbakht et al. (1993) appear to be wrong (cf. Scholten et al., 1997; Shubin, 2001). We therefore decided to investigate the 124Te(d,xn)124,125I processes in detail. On the basis of results obtained on the above mentioned three production routes, it should be possible to present a comparative evaluation of the various production methods.
2. Experimental Excitation functions were measured using the well known stacked-foil technique. Thin samples of highly enriched tellurium-124 (supplied by Chemotrade, . Dusseldorf, Germany) were produced by electrolytic deposition on 20, 25 and 30 mm thick titanium backing foils (cf. Scholten et al., 1989). The electrolytic mixture used contained enriched 124Te in a mixture of 70 vol% ethanol, 27 vol% benzene and 3 vol% HCl (cf. Hohn et al., 1998). The isotopic composition of the tellurium was 123Te (0.2%) and 124Te (99.8%). The quality of the tellurium deposit was visually controlled. Only foils with a homogeneous black deposit were used. The purity of the tellurium deposit was controlled via UV–VIS spectroscopy using a thiourea complex (cf. Nielsch and Giefer, 1957). Three spot checks were carried out. The purity was >99%. The deposits were covered by 10 mm thick Al-foils to protect them and to avoid any loss of tellurium or radioiodine. Four stacks, each with 5 to 10 tellurium samples and additional natTi-, natNi- and natFe-foils as beam monitors and energy degraders, were irradiated for about 30 min with deuterons at a beam current of about 100 nA and a primary energy of (14.06 0.2) MeV. The primary beam energy was measured via a time-of-flight method (Korm!any, 1994). The energy loss inside the stack was calculated with the program ‘‘Stack’’ which is based on a formula described by Williamson et al. (1966). All irradiations were done at the compact cyclotron CV 28 . of the FZ Julich. During irradiation the beam current was checked via a charge collector. For an exact determination of the beam current, three monitor reactions, viz. nat Ti(d,xn)48 V, natNi(d,xn)61Cu and natFe(d,xn)56Co, with well-evaluated excitation functions (cf. T!ark!anyi et al., 2001) were used. The values obtained via the three reactions agreed within 8%. The radioactivity of each irradiated sample and monitor foil was determined non-destructively by g-ray spectroscopy using high resolution HPGe detectors. Counting was done at a distance of 10 cm or more from the endcap of the detector to avoid coincidence losses. The efficiency of the detector was determined using standard sources from Amersham International and PTB Braunschweig, with errors 53%. Measurement of
48
V, 56Co and 61Cu activities in the monitor foils and of I in the 124Te-sample was performed about 5 h after the end of irradiation and was relatively straightforward. The measuring time was chosen to obtain counting statistics better than 1%. For measurement of 125I ðT1=2 ¼ 59:4 dÞ; emitting a soft g-ray of energy 35.5 keV, a special low-energy HPGe detector (with a Be-window) connected to very stable electronics was used. The efficiency of the detector was experimentally determined. Also the self-absorption in the Al-foil covering the 124Te-deposit was experimentally determined. For this purpose a 133Ba standard source (from Amersham International), which has a Kbtransition of energy 36.36–37.45 keV, was used. The count rates of various radioactive products were converted to decay rates using the efficiency of the detector and the decay data (cf. Firestone et al., 1996). From the decay rates and the measured beam currents the cross sections were calculated using the usual activation formula. The major errors were associated with the measurement of the beam current (8%) and the absolute activity of the product (7%). Other sources of uncertainties like thickness of Te-deposit, irradiation time or radioiodine loss were small. The total error in the measured cross section was obtained by combining all the individual errors in quadrature; in general it amounted to about 15%. The error in the primary deuteron energy was given as 200 keV. To obtain the error in the deuteron energy at each sample the calculation of the energy loss was performed for the upper and lower border of the primary deuteron energy. 124
3. Results and discussion 3.1. Cross section data The measured cross sections together with their estimated errors are given in Table 1. The results are plotted as a function of incident deuteron energy in Fig. 1. Evidently the (d,n) reaction has a considerably lower cross section than the (d,2n) process. This observation is in agreement with the general systematics of (d,xn) excitation functions (cf. Qaim and Probst, 1984). For comparison the results of the present study are shown together with the literature data in Fig. 2. The 124 Te(d,n)125I reaction has been investigated for the first time in this work. For the 124Te(d,2n)124I process, however, a nuclear model calculation (Shubin, 2001) and an experimental measurement (Firouzbakht et al., 1993) have been reported. Our data for the 124 Te(d,2n)124I reaction disagree with the experimental data of Firouzbakht et al. (1993) but agree with the model calculation of Shubin (2001).
T. Bastian et al. / Applied Radiation and Isotopes 55 (2001) 303–308 Table 1 Cross sections of
124
Te(d,n)125I and
124
Te(d,2n)124I reactions
Deuteron energy (MeV)
Cross section 124 Te(d,n)125I (mb)
5.8 0.4 6.2 0.4 6.8 0.3 6.8 0.3 7.0 0.3 7.6 0.3 7.7 0.3 8.3 0.3 8.6 0.3 9.0 0.3 9.5 0.3 9.6 0.3 9.7 0.3 10.3 0.3 10.6 0.3 11.0 0.2 11.0 0.2 11.5 0.2 11.8 0.2 11.9 0.2 12.4 0.2 12.6 0.2 13.0 0.2 13.9 0.2 14.1 0.2
34 5 63 10 144 22 177 27 125 19 201 30 193 29 161 24 201 30 286 43 243 37 237 36 170 26 256 38 179 27 194 29 141 21 131 20 127 19 131 20 122 18 135 20 110 16
305
Cross section Te(d,2n)124I (mb)
124
} } 71 61 79 12 125 19 112 17 247 37 428 64 326 49 496 74 420 63 510 77 638 96 565 85 455 68
Fig. 2. Comparison of available data for 124Te(d,xn)-processes. For the 124Te(d,n)125I reaction no data exist in the literature. For the 124Te(d,2n)124I reaction the results of a model calculation agree with our data but the experimental results given in the literature are by an order of magnitude smaller.
611 92 546 82 529 79 655 98 824 124 747 112 763 114
Fig. 3. Calculated integral yields of 125I and 124I based on the cross sections determined in this work. The yield of 124I reported by Firouzbakht et al. (1993) is also shown and agrees with our value wihin a factor of 2.
tions for this reaction below 14 MeV reported by Firouzbakht et al. (1993) are therefore also incorrect. 3.2. Yield and impurity 124 Fig. 1. Excitation functions of Te(d,n)125I and 124 124 Te(d,2n) I reactions measured in this work. The curves are eye-guides.
We performed an exact analysis of the 159 keV g-ray. Within the limits of errors this could be attributed to 47 Sc (from the activation of the backing material) and partly (51 Bq) to 123mTe (from the 124Te(d,t)123mTe reaction). In other words, 123I was not detected. This result is in good agreement with the threshold of the 124 Te(d,3n)123I reaction of 13.89 MeV. The cross sec-
Based on the data measured in this work, thick target yields of 124I and 125I were calculated. The results are plotted in Fig. 3 as a function of deuteron energy. The yield data for 124I reported by Firouzbakht et al. (1993) are also shown. The two sets of data agree within a factor of 2. Presumably Firouzbakht et al. determined the yield correctly but made some calculational error in converting it to cross section. For convenience, we give our numerical data on the yield of 124I and the percentage of 125I impurity in dependence of the primary deuteron energy and the leaving energy from a 124Te target in Table 2. The results
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Table 2 124 I-yield and
125
I-impurity (in %) for different primary and leaving energies of deuterons on
124
Te
Primary energy (MeV) 9
10 125
1538 1529 1242
11.4 3939 10.7 3930 7.5 3643 2402
Leaving energy I-yield I (MeV) (kBq/mA h) (%) 6 7 8 9 10 11 12 13
Table 3 Comparison of production routes of a
Nuclear reaction
11
124
124
I-yield (kBq/mA h)
12
13
14
125
124
I I-yield (%) (kBq/mA h)
125
124
I I-yield (%) kBq/mA h)
125
124
125
I I-yield (%) [kBq/mA h)
125 I 124I-yield I (%) (kBq/mA h) (%)
6.9 6.6 5.2 4.0
7263 7254 6967 5725 3324
5.0 4.8 4.0 3.3 2.7
11330 11321 11034 9792 7391 4067
3.9 3.7 3.2 2.7 2.2 1.8
16068 16059 15772 14530 12129 8805 4738
3.1 3.1 2.7 2.3 1.9 1.6 1.4
124
Te(d,2n) I Te(p,n)124I 125 Te(p,2n)124I 124
I
Suitable energy range (MeV) 14 ! 10 12 ! 8 21 ! 15
2.6 2.6 2.3 2.0 1.7 1.4 1.3 1.2
124
Calculated thick target yield of 124I (MBq/mA h]
Calculated impurity (%)b 123
125
} 1.0 7.4
1.7 5 0.1 0.9
I
124
21464 21455 21168 19926 17525 14201 10134 5396
17.5 16 81
I
a
Investigated using 99.8% enriched 124Te and 98.3% enriched 125Te (cf. Scholten et al., 1995; Hohn et al., 2001; this work). The levels of the impurities 126I, 128I and 130I would depend on the levels of the tellurium isotopes 126Te, 128Te and 130Te in the target material used in production runs. While using target samples of ultrahigh enrichment, those impurities should not occur. b
energy range the theoretical yield of 124I amounts to 17.5 MBq/mA h (473 mCi/mA h) with an 125I-impurity level of 1.7%. The practical production yields of 124I reported in the literature (cf. Lambrecht et al., 1988; Sharma et al., 1988; Weinreich and Knust, 1996) are more or less in agreement with the theoretical value. 3.3. Evaluation of production routes
Fig. 4. 125I-impurity (as % of 124I-activity) plotted as a function of the leaving deuteron energy. The numbers given on the curves are the primary deuteron energies in MeV. A beam with an incident energy of 14.0 MeV and target leaving energy of, for example, 12 MeV will lead to an 125I-impurity of 1.3%.
on the impurity are graphically shown in Fig. 4. From those data it is concluded that the most suitable energy range for the production of 124I via the 124Te(d,2n)reaction at our cyclotron is Ed=14 ! 10 MeV. Over this
The radionuclide 124I can be produced via 3He- and aparticle induced reactions on antimony or via proton and deuteron induced reactions on tellurium. Early investigations dealt with the a- or 3He-particle induced route (cf. Silvester et al., 1969; Watson et al., 1973; Calboreanu et al., 1982; Sharma et al., 1988), but the yields were very low. More interest therefore lies in proton and deuteron induced reactions on tellurium isotopes. In particular three routes, viz. 124Te(d,2n)124I, 124 Te(p,n)124I and 125Te(p,2n)124I, were investigated in detail. The results on the yield and purity of 124I are summarized in Table 3. The data were deduced from the excitation functions of the three routes measured recently (this work, Scholten et al., 1995; Hohn et al., 2001). It is evident that the 124Te(p,n)-route provides 124I of the highest purity. The 123I-impurity is of little
T. Bastian et al. / Applied Radiation and Isotopes 55 (2001) 303–308
significance since it decays out rather fast and the level of the 125I-impurity is negligibly low. The 124Te(d,2n)route leads to somewhat higher yield of 124I but the level of the 125I-impurity is high. The 125Te(p,2n)-reaction gives about four times higher yield than the other two processes, but the level of 125I-impurity is fairly high. The levels of other impurities like 126I, 128I, 130I, etc. in the three processes under consideration would depend on the levels of the tellurium isotopes 126Te, 128Te and 130 Te present in the enriched 124Te or 125Te target material. The 125I-impurity may become the limiting factor in the widespread use of 124I. Its long half-life (T12=59.4 d) causes extra radiation dose as well as regulatory problems in the case of clinical PET centres. The most suitable production route thus appears to be the 124 Te(p,n)124I reaction which can be utilized even at a low energy cyclotron. The yield is rather low and hence development of a high current target is called for. The 125 Te(p,2n)124I reaction appears to be the process of choice if a high intensity 30 MeV proton cyclotron is available. By virtue of the much higher yield, this route could lead to 124I quantities sufficient to meet the ever increasing demands. A careful check of the 125Iimpurity, however, is essential.
Acknowledgements We thank the crew of the compact cyclotron CV 28 for performing the irradiations and S. Spellerberg and G. Blessing for their experimental assistance.
References Brown, D.J., McKay, D.B., Coleman, J., Luthra, S.K., Brady, F., Waters, S.L., Pike, V.W., 2000. A facility for the safe recovery of high activities of iodine-124 produced by the 124 Te(p,n)124I reaction. Proc. Eighth International Workshop on Targetry and Target Chemistry, St. Louis, USA, June 1999, p. 134. Calboreanu, A., Pencea, C., Salagean, O., 1982. The effect of gamma de-excitation competition on the (a,n) and (a,2n) reactions on gold and antimony. Nucl. Phys. A 383, 251. Eschmann, S.M., Zagar, I., Thelen, M.H., Reischl, G., Lietzenmayer, R., Dohmen, B.M., Bilger, K., Machulla, H.-J., Bares, R., 1999. Untersuchungen zur Dosimetrie der . Radioiodtherapie benigner Schilddrusenerkrankungen mit. Nukleartels 124I-PET. Abstract, Deutsche Gesellschaft fur medizin (DGN), Ulm 1999. Nuklearmedizin 38, A97. Firestone, R.B., 1996. In: Shirley, V.S., Baglin, C.M., Chu, S.Y.F., Zipkin, J. (Eds.), Table of Isotopes, 8th Edition. Wiley, New York. Firouzbakht, M.L., Schlyer, D.J., Finn, R.D., Wolf, A.P., 1993. Iodine-124 production: excitation functions for the 124 Te(d,2n)124I and 124Te(d,3n)123I reactions from 7 to 24 MeV. Nucl. Instr. and Meth. B 79, 909.
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Firouzbakht, M.L., Schlyer, D.J., Wolf, A.P., 1994. The yield of 124I from different target materials in the 124Te(d,2n)124I reaction and an improved recovery method for 124Te. J. Lab. Compd. Radiopharm. 35, 257. Hohn, A., Coenen, H.H., Qaim, S.M., 1998. Nuclear data relevant to the production of 120gI via the 120Te(p,n)-process at a small-sized cyclotron. Appl. Radiat. Isot. 49, 1493. Hohn, A., Nortier, F.M., Scholten, B., van der Walt., T.N., Coenen, H.H., Qaim, S.M., 2001. Excitation functions of 125Te(p,xn)-reactions from their respective thresholds up to 100 MeV with special reference to the production of 124I. Appl. Radiat. Isot. 55, 149–156. Knust, E.J., Dutschka, K., Weinreich, R., 2000. Preparation of 124 I solutions after thermodistillation of irradiated 124TeO2 targets. Appl. Radiat. Isot. 52, 181. Korm!any, Z., 1994. A new method and apparatus for measuring the mean energy of cyclotron beams. Nucl. Instr. Methods A 337, 258. Lambrecht, R.M., Sajjad, M., Qureshi, M.A., Al-Yanbawi, S.J., 1988. Production of 124I. J. Radioanal. Nucl. Chem. Lett. 127, 143. McCarthy, T.J., Laforest, R., Downer, J.B., Lu, A.-R., Margenau, W.H., Hughey, B., Shefer, R.E., Klinkowskein, R.E., Welch, M.G., 2000. Investigation of I-124, Br-76 and Br-77 production using a small biomedical cyclotron}can induction furnaces help in the preparation and separation of targets? Proc. Eighth International Workshop on Targetry and Target Chemistry, St. Louis, USA, June 1999, p. 127. Nielsch, W., Giefer, L., 1957. Zur photometrischen Bestimmung von Tellur mit Thioharnstoff. Z. Analyt. Chem. 155, 401. Qaim, S.M., Probst, H.J., 1984. Excitation functions of deuteron induced nuclear reactions on vanadium with special reference to the production of 43K: Systematics of (d,xn) reaction cross sections relevant to the formation of highly neutron deficient radioisotopes. Radiochim. Acta 35, 11. Qaim, S.M., Blessing, G., T!ark!anyi, F., Lavi, N., Br.autigam, . W., Scholten, B., Stocklin, G., 1996. Production of longerlived positron emitters 73Se, 82mRb and 124I. In: Cornell, J.C., (Ed.), Proceedings of the 14th International Conference on Cyclotrons and their Applications, Cape Town, South Africa, October 1995, World Scientific, Singapore, p. 541. Qaim, S.M., Hohn, A., Nortier, F.M., Blessing, G., Schroeder, I.W., Scholten, B., van der Walt, T.N., Coenen, H.H., 2000. Production of 124I at small and medium sized cyclotrons. Proc. Eighth International Workshop on Targetry and Target Chemistry, St. Louis, USA, June 1999, p. 131. . Scholten, B., Qaim, S.M., Stocklin, G., 1989. Excitation functions of proton induced nuclear reactions on natural tellurium and enriched 123Te: Production of 123I via the 123 Te(p,n)123I-process at a low energy cyclotron. Appl. Radiat. Isot. 40, 127. Scholten, B., Kov!acs, Z., T!ark!anyi, F., Qaim, S.M., 1995. Excitation functions of 124Te(p,xn124,123I reactions from 6 to 31) MeV with special reference to the production of 124I at a small cyclotron. Appl. Radiat. Isot. 46, 255. Scholten, B., Tak!acs, S., Kov!acs, Z., T!ark!anyi, F., Qaim, S.M., 1997. Excitation functions of deuteron induced reactions on 123Te: Relevance to the production of 123I and 124I at low and medium sized cyclotrons. Appl. Radiat. Isot. 48, 267.
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Sharma, H.L., Zweit, J., Downey, S., Smith, A.M., Smith, A.G., 1988. Production of 124I for positron emission tomography. J. Lab. Comp. Radiopharm. 26, 165. Sheh, Y., Koziorowski, J., Balatoni, J., Lom, C., Dahl, J.R., Finn, R.D., 2000. Low energy cyclotron production and chemical separation of no carrier added iodine-124 from a reusable, enriched tellurium-124 dioxide/aluminum oxide solid solution target. Radiochim. Acta 88, 169. Shubin, Y.N., 2001. Model calculation and evaluation of nuclear data for medical radioisotope production. Radiochim. Acta., in press. Silvester, D.J., Sugden, J., Watson, I.A., 1969. Preparation of iodine-123 by a-particle bombardment on natural antimony. Radiochem. Radioanal. Lett. 2, 17. T!ark!anyi, F., Tak!acs, S., Gul, K., Hermanne, A., Mustafa, M.G., Nortier, N., Oblo$zinsk!y, P., Qaim, S.M.,
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Excitation functions of 124Te(d,xn)124,125I reactions from threshold up to 14 MeV: comparative evaluation of nuclear routes for the production of 124I Th. Bastian, H.H. Coenen, S.M. Qaim* Institut fur . Nuklearchemie, Forschungszentrum Julich . GmbH, D-52425 Julich, . Germany Received 12 February 2001; accepted 12 March 2001
Abstract Excitation functions of the nuclear reactions 124Te(d,xn)124,125I were measured from their respective thresholds up to 14.0 MeV via the stacked-foil technique. Thin samples were prepared by electrolytic deposition of 99.8% enriched 124Te on Ti-backing. The excitation function of the 124Te(d,n)125I reaction was measured for the first time. The present data for the 124Te(d,2n)124I reaction are by an order of magnitude higher than the literature experimental data but are in good agreement with the results of a hybrid model calculation. From the measured cross sections, integral yields of 124,125 I were calculated. The energy range Ed=14 ! 10 MeV appears to be the best compromise between 124I-yield and 125 I-impurity. The calculated 124I-yield amounts to 17.5 MBq/mA h and the 125I-impurity to 1.7%. A critical evaluation of the three nuclear routes for the production of 124I, viz. 124Te(d,2n)-, 124Te(p,n)- and 125Te(p,2n)-processes, is given. The reaction studied in this work proved to be least suitable. The 124Te(p,n)-reaction gives 124I of the highest radionuclidic purity, and a small-sized cyclotron is adequate for production purposes. The 125Te(p,2n)-reaction is more suitable at a medium-sized cyclotron: the yield of 124I is four times higher than in the other two reactions but the level of 0.9% 125I-impurity is relatively high. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Iodine-124; (d,xn)-reactions; Excitation function; Thick target yield; Radionuclidic impurity
1. Introduction The radioisotope 124I ðT1=2 ¼ 4:18 d; Ebþ ¼ 2:13 MeV; Ibþ ¼ 25%Þ is a very suitable radionuclide for both diagnostic and therapeutic use in nuclear medicine. A brief summary of the various applications of this radionuclide has been recently given (cf. Hohn et al., 2001, and references cited therein). 124I was first produced in MBq amounts via the 124Te(d,2n)124I process (cf. Sharma et al., 1988; Lambrecht et al., 1988; Firouzbakht et al., 1993, 1994; Weinreich and Knust, 1996) although the level of 125I-impurity was considered to be rather high. The process is still used in *Corresponding author. Tel.: +49-2461-61-3282; fax: +492461-61-2535. E-mail address: [email protected] (S.M. Qaim).
some laboratories (cf. Knust et al., 2000). In 1995, it was shown that the 124Te(p,n)124I reaction is superior to the 124 Te(d,2n)124I reaction (Scholten et al., 1995) as far as the 125I-impurity is concerned. The 124Te(p,n)-reaction has been used since then in several institutions (cf. Qaim et al., 1996, 2000; Eschmann et al., 1999; McCarthy et al., 2000; Brown et al., 2000; Sheh et al., 2000). Recently, detailed cross section data have also been reported for the 125Te(p,2n)124I reaction (Hohn et al., 2001). Thus now three reactions, viz. 124Te(d,2n)124I, 124 Te(p,n)124I and 125Te(p,2n)124I, are available for the production of 124I. The cross section data base for the latter two reactions is fairly strong. For the (d,2n) process, however, no reliable data are available. Firstly, the 124Te(d,n)125I reaction has not been investigated at all, mainly due to the difficulty in the quantitative measurement of the low energy (35.5 keV) g-ray, and
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secondly, the data for the 124Te(d,2n)124I process reported by Firouzbakht et al. (1993) appear to be wrong (cf. Scholten et al., 1997; Shubin, 2001). We therefore decided to investigate the 124Te(d,xn)124,125I processes in detail. On the basis of results obtained on the above mentioned three production routes, it should be possible to present a comparative evaluation of the various production methods.
2. Experimental Excitation functions were measured using the well known stacked-foil technique. Thin samples of highly enriched tellurium-124 (supplied by Chemotrade, . Dusseldorf, Germany) were produced by electrolytic deposition on 20, 25 and 30 mm thick titanium backing foils (cf. Scholten et al., 1989). The electrolytic mixture used contained enriched 124Te in a mixture of 70 vol% ethanol, 27 vol% benzene and 3 vol% HCl (cf. Hohn et al., 1998). The isotopic composition of the tellurium was 123Te (0.2%) and 124Te (99.8%). The quality of the tellurium deposit was visually controlled. Only foils with a homogeneous black deposit were used. The purity of the tellurium deposit was controlled via UV–VIS spectroscopy using a thiourea complex (cf. Nielsch and Giefer, 1957). Three spot checks were carried out. The purity was >99%. The deposits were covered by 10 mm thick Al-foils to protect them and to avoid any loss of tellurium or radioiodine. Four stacks, each with 5 to 10 tellurium samples and additional natTi-, natNi- and natFe-foils as beam monitors and energy degraders, were irradiated for about 30 min with deuterons at a beam current of about 100 nA and a primary energy of (14.06 0.2) MeV. The primary beam energy was measured via a time-of-flight method (Korm!any, 1994). The energy loss inside the stack was calculated with the program ‘‘Stack’’ which is based on a formula described by Williamson et al. (1966). All irradiations were done at the compact cyclotron CV 28 . of the FZ Julich. During irradiation the beam current was checked via a charge collector. For an exact determination of the beam current, three monitor reactions, viz. nat Ti(d,xn)48 V, natNi(d,xn)61Cu and natFe(d,xn)56Co, with well-evaluated excitation functions (cf. T!ark!anyi et al., 2001) were used. The values obtained via the three reactions agreed within 8%. The radioactivity of each irradiated sample and monitor foil was determined non-destructively by g-ray spectroscopy using high resolution HPGe detectors. Counting was done at a distance of 10 cm or more from the endcap of the detector to avoid coincidence losses. The efficiency of the detector was determined using standard sources from Amersham International and PTB Braunschweig, with errors 53%. Measurement of
48
V, 56Co and 61Cu activities in the monitor foils and of I in the 124Te-sample was performed about 5 h after the end of irradiation and was relatively straightforward. The measuring time was chosen to obtain counting statistics better than 1%. For measurement of 125I ðT1=2 ¼ 59:4 dÞ; emitting a soft g-ray of energy 35.5 keV, a special low-energy HPGe detector (with a Be-window) connected to very stable electronics was used. The efficiency of the detector was experimentally determined. Also the self-absorption in the Al-foil covering the 124Te-deposit was experimentally determined. For this purpose a 133Ba standard source (from Amersham International), which has a Kbtransition of energy 36.36–37.45 keV, was used. The count rates of various radioactive products were converted to decay rates using the efficiency of the detector and the decay data (cf. Firestone et al., 1996). From the decay rates and the measured beam currents the cross sections were calculated using the usual activation formula. The major errors were associated with the measurement of the beam current (8%) and the absolute activity of the product (7%). Other sources of uncertainties like thickness of Te-deposit, irradiation time or radioiodine loss were small. The total error in the measured cross section was obtained by combining all the individual errors in quadrature; in general it amounted to about 15%. The error in the primary deuteron energy was given as 200 keV. To obtain the error in the deuteron energy at each sample the calculation of the energy loss was performed for the upper and lower border of the primary deuteron energy. 124
3. Results and discussion 3.1. Cross section data The measured cross sections together with their estimated errors are given in Table 1. The results are plotted as a function of incident deuteron energy in Fig. 1. Evidently the (d,n) reaction has a considerably lower cross section than the (d,2n) process. This observation is in agreement with the general systematics of (d,xn) excitation functions (cf. Qaim and Probst, 1984). For comparison the results of the present study are shown together with the literature data in Fig. 2. The 124 Te(d,n)125I reaction has been investigated for the first time in this work. For the 124Te(d,2n)124I process, however, a nuclear model calculation (Shubin, 2001) and an experimental measurement (Firouzbakht et al., 1993) have been reported. Our data for the 124 Te(d,2n)124I reaction disagree with the experimental data of Firouzbakht et al. (1993) but agree with the model calculation of Shubin (2001).
T. Bastian et al. / Applied Radiation and Isotopes 55 (2001) 303–308 Table 1 Cross sections of
124
Te(d,n)125I and
124
Te(d,2n)124I reactions
Deuteron energy (MeV)
Cross section 124 Te(d,n)125I (mb)
5.8 0.4 6.2 0.4 6.8 0.3 6.8 0.3 7.0 0.3 7.6 0.3 7.7 0.3 8.3 0.3 8.6 0.3 9.0 0.3 9.5 0.3 9.6 0.3 9.7 0.3 10.3 0.3 10.6 0.3 11.0 0.2 11.0 0.2 11.5 0.2 11.8 0.2 11.9 0.2 12.4 0.2 12.6 0.2 13.0 0.2 13.9 0.2 14.1 0.2
34 5 63 10 144 22 177 27 125 19 201 30 193 29 161 24 201 30 286 43 243 37 237 36 170 26 256 38 179 27 194 29 141 21 131 20 127 19 131 20 122 18 135 20 110 16
305
Cross section Te(d,2n)124I (mb)
124
} } 71 61 79 12 125 19 112 17 247 37 428 64 326 49 496 74 420 63 510 77 638 96 565 85 455 68
Fig. 2. Comparison of available data for 124Te(d,xn)-processes. For the 124Te(d,n)125I reaction no data exist in the literature. For the 124Te(d,2n)124I reaction the results of a model calculation agree with our data but the experimental results given in the literature are by an order of magnitude smaller.
611 92 546 82 529 79 655 98 824 124 747 112 763 114
Fig. 3. Calculated integral yields of 125I and 124I based on the cross sections determined in this work. The yield of 124I reported by Firouzbakht et al. (1993) is also shown and agrees with our value wihin a factor of 2.
tions for this reaction below 14 MeV reported by Firouzbakht et al. (1993) are therefore also incorrect. 3.2. Yield and impurity 124 Fig. 1. Excitation functions of Te(d,n)125I and 124 124 Te(d,2n) I reactions measured in this work. The curves are eye-guides.
We performed an exact analysis of the 159 keV g-ray. Within the limits of errors this could be attributed to 47 Sc (from the activation of the backing material) and partly (51 Bq) to 123mTe (from the 124Te(d,t)123mTe reaction). In other words, 123I was not detected. This result is in good agreement with the threshold of the 124 Te(d,3n)123I reaction of 13.89 MeV. The cross sec-
Based on the data measured in this work, thick target yields of 124I and 125I were calculated. The results are plotted in Fig. 3 as a function of deuteron energy. The yield data for 124I reported by Firouzbakht et al. (1993) are also shown. The two sets of data agree within a factor of 2. Presumably Firouzbakht et al. determined the yield correctly but made some calculational error in converting it to cross section. For convenience, we give our numerical data on the yield of 124I and the percentage of 125I impurity in dependence of the primary deuteron energy and the leaving energy from a 124Te target in Table 2. The results
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Table 2 124 I-yield and
125
I-impurity (in %) for different primary and leaving energies of deuterons on
124
Te
Primary energy (MeV) 9
10 125
1538 1529 1242
11.4 3939 10.7 3930 7.5 3643 2402
Leaving energy I-yield I (MeV) (kBq/mA h) (%) 6 7 8 9 10 11 12 13
Table 3 Comparison of production routes of a
Nuclear reaction
11
124
124
I-yield (kBq/mA h)
12
13
14
125
124
I I-yield (%) (kBq/mA h)
125
124
I I-yield (%) kBq/mA h)
125
124
125
I I-yield (%) [kBq/mA h)
125 I 124I-yield I (%) (kBq/mA h) (%)
6.9 6.6 5.2 4.0
7263 7254 6967 5725 3324
5.0 4.8 4.0 3.3 2.7
11330 11321 11034 9792 7391 4067
3.9 3.7 3.2 2.7 2.2 1.8
16068 16059 15772 14530 12129 8805 4738
3.1 3.1 2.7 2.3 1.9 1.6 1.4
124
Te(d,2n) I Te(p,n)124I 125 Te(p,2n)124I 124
I
Suitable energy range (MeV) 14 ! 10 12 ! 8 21 ! 15
2.6 2.6 2.3 2.0 1.7 1.4 1.3 1.2
124
Calculated thick target yield of 124I (MBq/mA h]
Calculated impurity (%)b 123
125
} 1.0 7.4
1.7 5 0.1 0.9
I
124
21464 21455 21168 19926 17525 14201 10134 5396
17.5 16 81
I
a
Investigated using 99.8% enriched 124Te and 98.3% enriched 125Te (cf. Scholten et al., 1995; Hohn et al., 2001; this work). The levels of the impurities 126I, 128I and 130I would depend on the levels of the tellurium isotopes 126Te, 128Te and 130Te in the target material used in production runs. While using target samples of ultrahigh enrichment, those impurities should not occur. b
energy range the theoretical yield of 124I amounts to 17.5 MBq/mA h (473 mCi/mA h) with an 125I-impurity level of 1.7%. The practical production yields of 124I reported in the literature (cf. Lambrecht et al., 1988; Sharma et al., 1988; Weinreich and Knust, 1996) are more or less in agreement with the theoretical value. 3.3. Evaluation of production routes
Fig. 4. 125I-impurity (as % of 124I-activity) plotted as a function of the leaving deuteron energy. The numbers given on the curves are the primary deuteron energies in MeV. A beam with an incident energy of 14.0 MeV and target leaving energy of, for example, 12 MeV will lead to an 125I-impurity of 1.3%.
on the impurity are graphically shown in Fig. 4. From those data it is concluded that the most suitable energy range for the production of 124I via the 124Te(d,2n)reaction at our cyclotron is Ed=14 ! 10 MeV. Over this
The radionuclide 124I can be produced via 3He- and aparticle induced reactions on antimony or via proton and deuteron induced reactions on tellurium. Early investigations dealt with the a- or 3He-particle induced route (cf. Silvester et al., 1969; Watson et al., 1973; Calboreanu et al., 1982; Sharma et al., 1988), but the yields were very low. More interest therefore lies in proton and deuteron induced reactions on tellurium isotopes. In particular three routes, viz. 124Te(d,2n)124I, 124 Te(p,n)124I and 125Te(p,2n)124I, were investigated in detail. The results on the yield and purity of 124I are summarized in Table 3. The data were deduced from the excitation functions of the three routes measured recently (this work, Scholten et al., 1995; Hohn et al., 2001). It is evident that the 124Te(p,n)-route provides 124I of the highest purity. The 123I-impurity is of little
T. Bastian et al. / Applied Radiation and Isotopes 55 (2001) 303–308
significance since it decays out rather fast and the level of the 125I-impurity is negligibly low. The 124Te(d,2n)route leads to somewhat higher yield of 124I but the level of the 125I-impurity is high. The 125Te(p,2n)-reaction gives about four times higher yield than the other two processes, but the level of 125I-impurity is fairly high. The levels of other impurities like 126I, 128I, 130I, etc. in the three processes under consideration would depend on the levels of the tellurium isotopes 126Te, 128Te and 130 Te present in the enriched 124Te or 125Te target material. The 125I-impurity may become the limiting factor in the widespread use of 124I. Its long half-life (T12=59.4 d) causes extra radiation dose as well as regulatory problems in the case of clinical PET centres. The most suitable production route thus appears to be the 124 Te(p,n)124I reaction which can be utilized even at a low energy cyclotron. The yield is rather low and hence development of a high current target is called for. The 125 Te(p,2n)124I reaction appears to be the process of choice if a high intensity 30 MeV proton cyclotron is available. By virtue of the much higher yield, this route could lead to 124I quantities sufficient to meet the ever increasing demands. A careful check of the 125Iimpurity, however, is essential.
Acknowledgements We thank the crew of the compact cyclotron CV 28 for performing the irradiations and S. Spellerberg and G. Blessing for their experimental assistance.
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