Excitation functions of 125Te(p, xn)-reactions from their respective thresholds up to 100 MeV with special reference to the production of 124I.

Excitation functions of the nuclear reactions 125Te(p, xn) (119,120m, 120g, 121,122,123,124,125)I were measured for the first time from their respective thresholds up to 100 MeV using the stacked-foil technique. Thin samples were prepared by electrolytic deposition of 98.3% enriched 125Te on Ti-backing. In addition to experimental studies, excitation functions were calculated by the modified hybrid model code ALICE-IPPE. The experimental and theoretical data generally showed good agreement. From the measured cross section data, integral yields of (123,124,125)I were calculated. The energy range Ep 21 --> 15 MeV appears to be very suitable for the production of the medically interesting radionuclide 124I (T(1/2) = 4.18 d; I(beta)+ = 25%). The thick target yield of 124I amounts to 81 MBq/microA h and the level of 125I-impurity to 0.9%. The 125Te(p,2n)124I reaction gives 124I yield about four times higher than the commonly used 124Te(p,n)124I and 124Te(d,2n)124I reactions. The proposed production energy range is too high for small cyclotrons but large quantities of 124I can be produced with medium-sized commercial machines.     

Preparation of 124I solutions after thermodistillation of irradiated 124TeO2 targets.

The 4.15-d radionuclide 124I is produced via the nuclear reaction 124Te(d, 2n) 124I by irradiation of 96% enriched 124TeO2 with 14 MeV deuterons, followed by thermodistillation. In order to minimise the loss of 124I, the quartz distillation tube was fitted to a stainless steel helix capillary trap directly behind the end of the furnace. Using this device, distillation yields of more than 80% were routinely obtained, and the activity was concentrated in markedly less than 100 microL solution. The 124I produced by this method proved to be useful for labelling proteins and IUdR.     

Optimization of 124I production via 124Te(p,n)124I reaction.

(124)I was produced, via (124)Te(p,n)(124)I reaction, in greater than 3.7GBq (100 mCi, EOB) amount by bombarding (124)TeO(2) targets at 24 microA current for about 8h. This was achieved by keeping the target at 37 degrees relative to the beam during irradiation, by sweeping the beam across the target and by keeping the incident energy of the proton at 14.1MeV. The time-averaged yield of our 8h run was 21.1 MBq/microAh (0.57 mCi/microAh), which was 90% of the theoretical yield calculated using thick target yield data obtained from the reported excitation function for the reaction. At the end of bombardment, the level of (125)I and (126)I impurities, co-produced with (124)I, were 0.03% and 0.007%, respectively.     

Excitation functions of 120Te(d,xn)121,120m,gI reactions from threshold up to 13.5 MeV: comparative studies on the production of 120gI.

Excitation functions of the nuclear reactions 120Te(d,xn)121,120m,gI were measured for the first time from their respective thresholds up to 13.5 MeV. Thin samples prepared by electrolytic deposition of 99.0% enriched 120Te on Ti-backing were used. Integral yields of 121,120m,gI were calculated from the measured cross section data. A comparison of the 122Te(p,3n)-, 120Te(p,n)- and 120Te(d,2n)-processes for the production of 120gI is given. The 120Te(d,2n)-process is unsuitable for production purposes since the yield of 120gI is very low and the level of 121I impurity very high. The choice lies either on the 122Te(p,3n)- or the 120Te(p,n)-reaction and is governed by the available proton energy and the financial resources for procuring the enriched target material.     

A new binary compound for the production of 124I via the 124Te(p,n)124I reaction.

The binary compound, aluminum telluride (Al(2)Te(3)), was investigated as a target material for the production of (124)I by way of the (124)Te(p,n)(124)I reaction on a low-energy cyclotron. The high melting point and formation of a glassy matrix upon heating provided a stable target material at irradiations up to 20 microA of 11 MeV protons. The 87% tellurium mass fraction and 95% iodine separation yield led to significantly higher quantities of iodine compared to traditional TeO(2)/6%Al(2)O(3) admixtures. Radiochemical analysis of distilled samples using ion chromatography showed that the product remained in the iodide form while supported in weak buffer solutions. Stable Te impurities in the radioiodine product were less than 0.5 microg following purification by ion exchange chromatography. Average thick target yields of 229+/-18 microCi/microAh were achieved, and typical production runs at 18 microA for three hours yielded 12 mCi at the end-of-bombardment. Total losses of the target material after each irradiation and distillation cycle were approximately 2%.     

Evaluation of excitation functions of proton and deuteron induced reactions on enriched tellurium isotopes with special relevance to the production of iodine-124

Cross-section data for the production of medically important radionuclide ¹²⁴I via five proton and deuteron induced reactions on enriched tellurium isotopes were evaluated. The nuclear model codes, STAPRE, EMPIRE and TALYS, were used for consistency checks of the experimental data. Recommended excitation functions were derived using a well-defined statistical procedure. Therefrom integral yields were calculated. The various production routes of ¹²⁴I were compared. Presently the ¹²⁴Te(p,n)¹²⁴I reaction is the method of choice; however, the ¹²⁵Te(p,2n)¹²⁴I reaction also appears to have great potential.     

Some optimisation studies relevant to the production of high-purity 124I and 120gI at a small-sized cyclotron.

Optimisation experiments on the production of the positron emitting radionuclides 124I(T(1/2) = 4.18d) and (120g)I (T(1/2) = 1.35 h) were carried out. The TeO(2)-target technology and dry distillation method of radioiodine separation were used. The removal of radioiodine was studied as a function of time and the loss of TeO(2) from the target as a function of oven temperature and time of distillation. A distillation time of 15 min at 750 degrees C was found to be ideal. Using a very pure source and comparing the intensities of the annihilation and X-ray radiation, a value of 22.0 +/- 0.5% for the beta(+) branching in 124I was obtained. Production of 124I was done using 200 mg/cm(2) targets of 99.8% enriched 124TeO(2) on Pt-backing, 16 MeV proton beam intensities of 10 microA, and irradiation times of about 8 h. The average yield of 124I at EOB was 470 MBq(12.7 mCi). At the time of application (about 70 h after EOB) the radionuclidic impurity 123I (T(1/2) = 13.2 h) was <1%. The levels of other impurities were negligible (126I < 0.0001%;125I = 0.01%). Special care was taken to determine the 125I impurity. For the production of (120g)I only a thin 30 mg target (on 0.5 cm(2) area) of 99.9% enriched 120TeO(2) was available. Irradiations were done with 16 MeV protons for 80 min at beam currents of 7 microA. The 120gI yield achieved at EOB was 700 MBq(19 mCi), and the only impurity detected was the isomeric state 120 mI(T(1/2) = 53 min) at a level of 4.0%. The radiochemical purity of both 124I and 120gI was checked via HPLC and TLC. The radioiodine collected in 0.02 M NaOH solution existed >98% as iodide. The amount of inactive Te found in radioiodine was <1 microg. High purity 124I and 120gI can thus be advantageously produced on a medium scale using the low-energy (p,n) reaction at a small-sized cyclotron.     

Excitation functions of proton induced nuclear reactions on natural tellurium and enriched 123Te: Production of 123I via the 123Te(p,n)123I-process at a low-energy cyclotron

Excitation functions were measured by the stacked-foil technique for (p, xn) reactions up to E_p = 20 MeV on natural tellurium and enriched ¹²³Te. Thick target yields were calculated for the formation of ¹²¹I, ¹²³I, ¹²⁴I, ¹²⁶I, ¹²⁸I and ¹³⁰I from natural tellurium, and ¹²²I and ¹²³I from enriched ¹²³Te. The optimum energy range for the production of ¹²³I via the ¹²³Te(p, n)¹²³I reaction is E_p = 14.5 → 11.0 MeV, and the theoretical thick target yield 4 mCi/μAh. The levels of the three major impurities ¹²⁴I, ¹²⁶I and ¹³⁰I are directly dependent on the ¹²⁴Te, ¹²⁶Te and ¹³⁰Te contents, respectively, in enriched ¹²³Te. The levels of impurities determined experimentally under high-current production conditions agree with those calculated from the cross section data. A comparison of the three direct methods of ¹²³I-production, viz. ¹²⁴Te(p, 2n)¹²³I, ¹²³Te(p, n)¹²³I and ¹²²Te(d, n)¹²³I, under optimum conditions for each reaction, is given. The yield and impurity-level data suggest that the ¹²³Te(p, n)¹²³I reaction has a great potential for production in a low-energy cyclotron, if highly enriched ¹²³Te (>91%) is used.     

Radioiodination of 1-(2-deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (dRFIB), a putative thymidine mimic nucleoside for cell proliferation studies

Iodine-124 was produced via the (124)Te(p,n)(124)I reaction by 15 MeV proton irradiation of an in-house solid mass tellurium dioxide target, using the Tübingen PETtrace (General Electric Medical Systems) cyclotron. 1-(2-Deoxy-beta-D-ribofuranosyl)-2,4-difluoro-5-iodobenzene (dRFIB), a stable, non-polar thymidine mimic nucleoside, was synthesized in 5 steps following a literature method, for radioiodination with [(124)I] iodide via isotope exchange in the presence of copper sulphate and ammonium sulphate in methanol-water. The radiolabelling procedure was optimized with respect to temperature, amount of dRFIB, amount of sodium hydroxide and reaction time, to produce radiochemical yields of up to 85% with a 1-h reaction at 140 degrees C. With routine I-124 production of 30 MBq/run, relatively high specific activities, approaching 100 MBq/mmol, can be expected. The activation energy for dRFIB radioiodination was calculated from temperature-time RCY data to be approximately 100 kJ/mol using no-carrier-added [(124)I]iodide.     

Excitation function of 122Te(d,n)123I nuclear reaction: production of 123I at a low energy cyclotron

The excitation function of the ¹²²Te(d, n)¹²³I nuclear reaction has been measured from threshold up to 21 MeV by the stacked foil irradiation technique. Good agreement was obtained with the results of the recent model calculations but an energy shift of 2 MeV to lower energy can be seen when comparing with cross section measured earlier. Integral yields have been deduced from the measured excitation function and have been compared with experimental thick target yields found in the literature. A comparison of the yields of the proton and deuteron induced reactions for production of ¹²³I is given.     

Evaluation of irradiation parameters of enriched (124)Xe target for (123)I production in cyclotrons.

The production of high-purity (123)I that utilizes an isotopically enriched (124)Xe target and bombardment with 30MeV protons, through the reactions (124)Xe (p, 2n) (123)Cs-->(123)Xe-->(123)I and (124)Xe (p, pn) (123)Xe-->(123)I, is described. The aim of this work was to improve the production parameters, such as (124)Xe load pressure, beam current, decay time and target heating to recover (123)I to obtain high-production (123)I yield at low cost.     

(3)He-particle-induced reactions on (nat)Sb for production of (124)I.

Excitation functions of the reactions (nat)Sb((3)He,xn)(124,123,121)I were measured from their respective thresholds up to 35 MeV, with particular emphasis on data for the production of the medically important radionuclide (124)I. The conventional stacked-foil technique was used. From the experimental data the theoretical yields of the three investigated radionuclides were calculated. The yield of (124)I over the energy range E9(30He) = 35 --> 13 MeV amounts to 0.95 MBq/microA h. The radionuclidic impurities are discussed. A comparison of (3)He- and alpha-particle-induced reactions on antimony for production of (124)I is given. The alpha-particle-induced reaction on enriched (121)Sb and the (3)He-particle-induced reaction on enriched (123)Sb would lead to comparable (124)I yields, but the level of impurities in the latter case would be somewhat higher.     

Standardization and half-life measurement of antimony-124

The half-life of ¹²⁴Sb has been measured using two solutions prepared from ¹²⁴Sb obtained from the nuclear reactions: ¹²⁴Sn(p,n)¹²⁴Sb (cyclotron) and ¹²³Sb(n,γ)¹²⁴Sb (reactor). These solutions were standardized by the 4πβ-γ-coincidence extrapolation method with a total uncertainty of 0.12%. A ¹²⁴Sb half-life of (60.11±0.07) days was measured.     

Excitation functions of (p, 2n) and (p,pn) reactions and differential and integral yields of 123I in proton induced nuclear reactions on highly enriched 124Xe

Excitation functions were measured for (p, 2n) and (p, pn) reactions on 99.9% enriched ¹²⁴Xe from threshold up to 44 MeV. The (p, 2n) reaction is much stronger than the (p, pn) channel; above 36 MeV, however, the two processes have almost equal cross sections. Differential yields of ¹²³I were measured experimentally as a function of proton energy and were also calculated from the excitation functions. Our experimental and theoretical yield data are consistent within 15%, but are lower by a factor of 2 than the literature experimental values. Our studies show that the optimum energy range for the production of ¹²³I is E_p = 29 → 23 MeV. The theoretically expected thick target yield of ¹²³I at 6.6 h after EOB is 11.2 mCi/μAh, and is in agreement with the high-current experimental production yields.     

Alpha-particle induced reactions on natSb and 121Sb with particular reference to the production of the medically interesting radionuclide 124I.

Excitation functions of the reactions (nat)Sb(alpha,xn)(123,124,125,126)I and (121)Sb(alpha,xn)(123,124)I were measured from their respective thresholds up to 26 MeV, with particular emphasis on data for the production of the medically important radionuclide (124)I. The conventional stacked-foil technique was used, and the samples for irradiation were prepared by a sedimentation process. The measured excitation curves were compared with the data available in the literature. From the experimental data the theoretical yields of the investigated radionuclides were calculated as a function of the alpha-particle energy. The calculated yield of (124)I from the (nat)Sb(alpha,xn)(124)I process over the energy range E(alpha) = 22-->13 MeV amounts to 1.02 MBq/microA x h and from the (121)Sb(alpha,n)(124)I reaction over the same energy range to 2.11 MBq/microA x h. The radionuclidic impurity levels are discussed. Use of (nat)Sb as target material would not lead to high-purity (124)I. Using highly enriched (121)Sb as target, production of (124)I of high radionuclidic purity is possible; the batch yield, however, is low.     

Study of the 192Os(d,2n) reaction for production of the therapeutic radionuclide 192Ir in no-carrier added form.

In the frame of an IAEA coordinated research project (CRP) on nuclear data for production of therapeutic radionuclides, the production of 192Ir via deuteron-induced reactions on enriched 192Os was investigated up to 21 MeV deuteron energy. Cross sections were measured using the conventional stacked-foil irradiation technique and high-resolution gamma-ray spectroscopy of the activation products. No earlier experimental data were found in the literature. The excitation functions of the 192Os(d,2n)192m1+gIr and 192Os(d,p)193Os reactions were compared with the results of nuclear model calculations using the standard and presently upgraded versions (D-version) of ALICE-IPPE, EMPIRE-II and GNASH codes, while for the (d,2n) channel the GNASH and EMPIRE-II codes reproduced the data in an acceptable way; in both cases for the (d,p) reaction the very large discrepancy observed between the experimental data and standard codes results is vanishing when the upgraded versions of ALICE and EMPIRE-II are used. A comparison of the reactor and cyclotron production routes of 192Ir is given.     

Production of high purity (94m)Tc for positron emission tomography studies

The radioisotope (94m)Tc (T(1/2) = 52.5 min; I(beta)+ = 72%; E(beta)+ = 2.47 MeV) is of considerable interest for quantitative biodistribution studies of Tc radiopharmaceuticals using positron emission tomography. The nuclear processes (94)Mo(p,n), (93)Nb((3)He, 2n), (92)Mo(alpha,pn), and (92)Mo(alpha,2n)(94)Ru --> (94m)Tc are of potential interest for the production of (94m)Tc. Detailed cross section and yield measurements showed that the (94)Mo(p,n)-reaction over the energy range E(p) = 13 --> 7 MeV is most suitable: The yield of (94m)Tc is high (2 GBq/muAh), the impurity level is low (provided highly-enriched (94)Mo is used as target material), and a small-sized cyclotron is adequate. Using conventional targetry, sufficient quantities of the radioisotope can be produced. The chemical processing methods employed include both solvent extraction and thermochromatography. The quality of the final product is discussed.     

A simple method to quantitate iodine-124 contamination in iodine-123 radiopharmaceuticals

Iodine-123 (123I) produced by the 124Te(p,2n)123I reaction contains several percent 124I radionuclidic contamination at the time of imaging. Since 124I degrades the quality of the images and causes unnecessary radiation absorbed dose to the patient, it is important to know the amount present in radiopharmaceuticals at the time of administration. A simple approach is described which uses a radionuclide dose calibrator and lead shield. The sample is assayed both shielded and unshielded and the ratio of readings depends uniquely upon the percent 124I present. The technique can be adopted for any type of dose calibrator, sample container, and Pb shield, but use of the numeric constants reported here should be restricted to the specified equipment.     

Performance of collimators used for tomographic imaging of I-123 contaminated with I-124

Iodine-123 prepared from the 124Te(p,2n)123I reaction is contaminated with between 3% to 5% I-124 when imaging is performed. The effects of such a mixture were evaluated for medium-energy and low energy general-purpose collimators on a commercially available rotating gamma camera equipped to perform tomography. The planar sensitivity for I-123 was less for the general-purpose collimator, varying between 0.84 and 0.85 in water relative to that measured for the medium energy-collimator. Counts due to scattering or septal penetration of I-124 photons were greater for the general-purpose collimator (36%) than for the medium-energy collimator (15%). Evaluation of the higher-frequency components of the modulation transfer functions confirmed that the low-energy general-purpose collimator is expected to offer significantly more contrast information at frequencies above 0.21 cycles/cm. This is expected to contribute to image quality when studies are performed with collimators of similar design.     

Excitation functions of Se(d,xn)Br reactions: Comparative evaluation of possible routes for the production of Br at a small cyclotron

Excitation functions were measured for the first time for ⁷⁴Se(d,n)⁷⁵Br and ⁷⁴Se(d,2n)^74mBr reactions from threshold to 23 MeV. Use was made of the stacked-foil technique, and thin samples were prepared by electrolytic deposition of 31.4% enriched ⁷⁴Se on Al-backing. Differential and integral yields of ^74mBr and ⁷⁵Br were calculated from the measured excitation functions. The optimum energy range for the production of ⁷⁵Br via the ⁷⁴Se(d,n)-process was found to be E_d = 12 → 8 MeV, with ⁷⁵Br-yield amounting to 509 MBq (13.75 mCi)/μAh and the ^74mBr impurity to 78Kr(p,)⁷⁵Br and ⁷⁴Se(d,n)⁷⁵Br, suggested for the production of ⁷⁵Br at a small cyclotron is given. The (d,n) reaction gives higher yield than the (p,) process and is preferable at cyclotrons with E_d 10 MeV. In general, at a small cyclotron the achievable batch yields of ⁷⁵Br via both the processes are limited.