Excitation functions for production of medically relevant radioisotopes in deuteron irradiations of Pr and Tm targets

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Abstract

Activation cross sections of deuteron induced reaction on monoisotopic Tm (169Tm) and Pr (141Pr) were measured by stacked foil experiments. First measurements are presented for direct or cumulative production of 141Nd, 140Nd, 139mNd, 142Pr, 139Ce, 137mCe and 140La. Thick target yields and practical batch activity are derived from the fitted excitation curves for the medically relevant 140Nd and 139Pr (as decay product of 139mNd). For reactions on Tm targets previous published results up to 20 MeV are extended and confirmed up to 40 MeV for 169Yb, 167,168Tm while high energy values are presented for the first time for 166Yb. Values for the thick target yield (TTY) of 169Yb production are derived and the discussion of different production routes is in agreement with previous published results. A comparison of experimental values with different model codes shows that the upgraded versions of the ALICE and EMPIRE codes can give a better description of (d,pxn) reaction than older ones.

Section snippets

Rationale

Radionuclides of rare earth elements are gaining importance as emerging therapeutic agents in nuclear medicine. They often are nearly pure Auger electron emitters that can deposit high local doses and their strikingly similar chemical properties as lanthanides make that they can be conjugated to biomolecules using a single class of chelates like DOTA [1]. Some (177Lu, 169Yb) have been included in an ongoing IAEA-CRP on “Production of Therapeutic Radioisotopes” [2] and 169Yb also attracted

Experimental

The well-known activation formula together with a standard stacked foil technique was used to determine the excitation functions of deuteron induced reactions on thin targets containing oxides of Tm and Pr. Irradiations were performed at the external beam lines of the Tohoku University (Sendai) and Vrije Universiteit Brussels (VUB) cyclotrons. The Tm2O3 (25–35 μm thick in Tm equivalent for Sendai experiments; 15–60 μm thick for VUB experiments, purity 99.9%) and Pr6O11 (250–60 μm thick in Pr

Data processing

Incident beam energy and number of particles on target were initially derived from accelerator settings and from the charge collected on the Faraday cup like target holders.

Adaptations to these values are made based on remeasurement of the cross sections of the monitor reactions natTi(d,x)48V and natAl(d,x)22,24Na over the whole energy range and comparison with recommended data. This results in a determination of incident energy and number of bombarding particles with reduced uncertainty and

Model calculations

In our previous works the codes ALICE-IPPE [18] and EMPIRE [19] have been used widely to describe lots of reaction cross sections induced by light charged particles. However, during the recent analyses of the (d,p) reactions on the isotopes 114Cd [20], 165Ho [21], 192Os [13] and some others we were confronted with large underestimations of the measured cross sections. We came to the conclusion that the experimentally observed cross sections can be described only by means of a simulation of

Results and discussion

As Pr and Tm are monoisotopic elements the derived values are physical reaction cross sections obtained by irradiation of natural targets.

Results are discussed for each activation product separately and details on the way of calculation or need for correction are given were necessary.

For the Pr targets a good correspondence between the experiments at the different accelerators is found and the results are presented as a single series for these activation products where overlap exits. Data are

Production of 141m+gNd

This radionuclide has a short lived metastate (T1/2 = 62 s, Iπ = 11/2) that shows a nearly 100% isomeric transition to the long-lived ground state (T1/2 = 2.49 h, Iπ = 3/2+). At time of measurement all the short lived metastable nuclei have decayed and contributed to the cumulative cross section for 141gNd that is presented here. The characteristic shape of a (d,2n) reaction with a high maximum of more than 1300 mb at 16.5 MeV is found. The longer waiting time for the Sendai high energy experiments did

Production of 169Yb

Our new results up to 38 MeV are a good extension of the values for the 169Tm(d,2n) reaction measured earlier up to 21 MeV and discussed in detail in [10]. All experimental data are compared with calculations in Fig. 5. The new data show a slowly decreasing cross section above 20 MeV with a value around 100 mb. The EMPIRE-D results reproduce the high energy data quite reasonably, but they give a too high value for the near-maximum cross section. On the other hand, the ALICE-D results agree better

Thick target yields and comparison of production routes

From a fit to our experimental excitation function, integral yields were calculated for 140Nd, 139mNd and 169Yb in function of incident energies up to 40 MeV (Fig. 8).

It is clear that to benefit fully from the high cross sections over a rather wide energy range for 140Nd production (Fig. 2) dedicated “high” energy deuteron beams have to be developed. If incident energy of at least 40 MeV would be available, physical yields of 217 MBq/μAh are attainable with a thick target degrading the beam to 16 

Conclusion

By correlating the measured excitation functions to assessment of monitor reactions over the whole energy region we can present here for the first time reliable values for the cross sections of Nd, Pr and Ce radioisotopes produced directly or in cumulative processes by deuterons induced reactions on monoisotopic Pr. The high cross sections values (above 1000 mb) of the (d,3n) and (d,4n) reactions allow efficient production of medically relevant radioisotopes 140Nd and 139Pr (indirect route from

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