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.     

Calculated 118Te production rates from enriched 122,123Te targets

The ¹¹⁸Te/¹¹⁸Sb generator has potential application for positron emission tomography. In this study, we modelled the production of the ¹¹⁸Te precursor via proton activation of ¹²²Te and ¹²³Te targets. ¹¹⁸Te excitation functions were calculated for the (p, xn) and (p, pxn) production channels using a compound nucleus/statistical evaporation approach. These calculations indicate that ¹¹⁸Te can be produced in GBq (100 mCi) quantities by the irradiation of a thin (0.4 g/cm²) enriched ¹²²Te target with 62 MeV protons. This corresponds to a cumulative ¹¹⁸Te production rate in the 62-60 MeV proton energy region of 16 GBq/C (1.6 mCi/μA-h). In this proton energy range, ¹¹⁸Te production is maximized while the production of such contaminant nuclides as ¹¹⁹Te is relatively low. In view of the sizable ¹¹⁸Te production rate estimate, we are proceeding with the development of optimal Te/Sb separation chemistry applicable to a generator system.     

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 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.     

Differential solubility labelling and purification of ortho-iodo-hippuric acid with 123I

A simple method is described for the production of high quality clinical ¹²³I hippuran at TRIUMF. The first feature is that solid ortho-iodo-hippuric acid (IHA) is heated with aqueous [¹²³I]iodide creating a single phase where labelling occurs. When subsequently cooled to room temperature, IHA crystallizes. The second feature is employed at this stage. A density gradient is created with NaCl to remove the supernatant fluid from the IHA crystals. The IHA is then neutralized for pharmaceutical use. The product purity is consistantly >99% by TLC and by HPLC analysis that is described. The initial tag is typically 96% and the useful recovery is 85% by this process. Iodine-123 has been used up to 100 mCi per batch from the 500 MeV ¹³³Cs (p, 2p, 9n) ¹²³Xe reaction and from the ¹²⁴Xe (p, 2n) ¹²³Xe reaction.     

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.

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 124,125I were calculated. The energy range Ed = 14 --> 10 MeV appears to be the best compromise between 124I-yield and 1251-impurity. The calculated 124I-yield amounts to 17.5 MBq/microA 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.     

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.     

Study on the decay of iodine-123

Following the decay of a sample of ¹²³I, it was found after about two weeks the decay no longer corresponds to a half life of 13.21 h, but to a longer half life equal to that of ^123mTe.After a chemical separation of iodine/tellurium, and comparing the measured activities with a reference solution, the branching ratio of ¹²³I/^123mTe was found to be (7.7 ± 0.2) 10⁻⁵.     

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.     

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.     

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%.     

Production of Curie quantities of high purity I-123 with 15 MeV protons

Using the technology described for electrodeposition of elemental tellurium, it is possible to bombard a target with 2 kW of proton beam power without significant loss of radio iodine. If Te-123 enriched to 96% is used as target material, the I-123 yield would be about 1 Curie with a 0.23% I-124 impurity for a 2-h bomardment with 130 muA of 15 MeV protons. The same I-124 impurity level can be maintained with a lower enrichment of Te-123 by reducing the proton energy to 11.5 meV, but with a consequent reduction in I-123 yield.     

Production of curie quantities of high purity I-123 with 15 MeV protons

Using the technology described for electrodeposition of elemental tellurium, it is possible to bombard a target with 2 kW of proton beam power without significant loss of radio iodine. If Te-123 enriched to 96% is used as target material, the I-123 yield would be about 1 Curie with a 0.23% I-124 impurity for a 2-h bombardment with 130 A of 15 MeV protons. The same I-124 impurity level can be maintained with a lower enrichment of Te-123 by reducing the proton energy to 11.5 meV, but with a consequent reduction in I-123 yield..     

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.     

Assessment of the short-lived non-pure positron-emitting nuclide 120I for PET imaging

Purpose The non-pure positron-emitting iodine isotope ¹²⁰I (T _1/2=81 min) is a short-lived alternative to ¹²⁴I. ¹²⁰I has a positron abundance more than twice that of ¹²⁴I and a maximum positron energy of 4 MeV. This study was undertaken to evaluate and characterise the qualitative and quantitative PET imaging of ¹²⁰I.Methods ¹²⁰I was produced via the ¹²⁰Te(p,n) reaction on highly enriched ¹²⁰Te. The measurements were done with the Siemens scanner HR+ and the 2D PET scanner GE PC4096+. A cylinder containing three cold inserts and a phantom resembling a human brain slice were used to evaluate half-life, positron abundance and background correction. To analyse the image resolution, a 1-mm tube placed in water was filled with ¹²⁰I and ¹⁸F. Comparisons with ¹⁸F, ¹²⁴I and ¹²³I (measured with SPECT) were made using the Hoffman 3D brain phantom.Results The half-life of 81.1 min was reproduced by the PET measurements. The PET-based positron abundance ranged from 47.9% to 55.0%. The reconstructed image resolution found with the HR+ was 5.4 mm FWHM (12.3 mm FWTM), in contrast to 4.6 mm (8.6 mm) when using ¹⁸F. Erroneous positive and negative numbers of radioactivity found in the cold inserts became nearly zero when the background of γ-coincidences was corrected for. Images of the Hoffman phantom were inferior to those obtained when ¹⁸F or ¹²⁴I was applied but superior to the ¹²³I-SPECT images.Conclusion Our data show that ¹²⁰I of high radionuclidic purity can be regarded as a suitable nuclide for the PET imaging of radioiodine-labelled pharmaceuticals.     

Cross sections of natSb(p,x) reactions for 30–46 MeV protons

In order to optimize the production of ¹¹⁸Te in thick targets for use in a ¹¹⁸Te/¹¹⁸Sb radionuclide generator, the excitation function for the ¹²¹Sb(p,4n)¹¹⁸Te reaction has been measured for 30–46 MeV protons. The excitation functions for the competing reactions ^natSb(p,xn)^119mTe, ^natSb(p,xn)¹¹⁹Te, ^natSb(p,xn)^121mTe, ^natSb(p,pxn)^120mSb and ¹²³Sb(p,pn)¹²²Sb have also been determined using stacked foil techniques. The ¹²¹Sb(p,4n)¹¹⁸Te reaction cross section maximum was found to be 480 mbarn at 44 MeV. In order to minimize the ^{119m + 119}Te interference a minimum proton beam energy of 40 MeV is required. The cross section results are compared with published data and with calculated excitation functions.     

Nuclear data relevant to the production of I via the Te(p,n)-Process at aSmall-Sized Cyclotron

Excitation functions of the nuclear reactions ¹²⁰Te(p,xn)^119,120m,gI were measured for the first time from their respective thresholds up to 35 MeV. Use was made of the stacked-foil technique. Thin samples were prepared by electrolytic deposition of highly enriched ¹²⁰Te (99.0%) on Ti-backing. Integralyields of ^119,120m,gI were calculated from the measured cross section data. For the production of the medically interesting β⁺-emitting radioisotope ^120gI (T_1/2=1.35 h) the energy range E_p=16→9 MeV appears to be optimum, the thick target yield of ^120gI amounting to 2.3 GBq (62 mCi)/μAh and the ^120mI and ¹¹⁹I impurity levels to 4.8 and 4.4%, respectively. A comparison of the ¹²²Te(p,3n)- and ¹²⁰Te(p,n)-processes for the production of ^120gI is given. Despite the higher cost of the target material, the ¹²⁰Te(p,n)-process is superior since a small-sized cyclotron is adequate and the radionuclidic quality of the product is better.     

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.     

Cross section measurements of proton and deuteron induced reactions on natural europium and yields of SPECT-relevant radioisotopes of gadolinium

The existing cross section data of the ^natEu(d,x) and ^natEu(p,x) reactions relevant for the production of ^147,149Gd were expanded up to 70.9 MeV and 44.8 MeV, respectively. Integral yields of radiogadolinium were calculated, showing production rates higher than for the earlier proposed irradiation of highly enriched ¹⁴⁴Sm with α- or ³He-particles. The formation of radioisotopic impurities like ¹⁵¹Gd (T_1/2=124 d) and ¹⁵³Gd (T_1/2=240 d) was below 5%. Production of ^147,149Gd using enriched europium is also discussed.     

(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.