V.HENZL Nuclear Physics Institute ASCR, Řež, 25068, Czech Republic

1. Transmutation of 129I with high energy neutrons produced in spallation reactions induced by protons in massive target V.HENZL Nuclear Physics Institute ASCR, Řež, 25068, Czech Republic

2. Motivation • Iodine is one of the problematic fission products in burned-up radioactive waste; it is also biogenic. • Half-life of 129I is 1,57x107 years. • Very few data on 129I transmutation are known. The only experimentaly measured 129I(n,2n)128I reaction cross sections. • The integral experiments are needed for verification of evaluated data and computer codes.

3. Spalation target • A massive lead spalation target (50 cm long, 9.6 cm diameter) was irradiated by a proton beam accelerated by Synchrophasotron of LHE at JINR Dubna (Russian Federation) • No moderator surrounding the target was used • The energy of proton beam was Ep=2.5 GeV during the first irradiation and Ep=1.3 GeV during the second irradiation. • The total intensity of the proton beam was 4.07x1013 protons in irradiation with 2.5 GeV protons, res. 2.77x1013 protons in irradiation with 1.3 GeV protons, as deduced from the yield of 24Na formed in 27Al(p,3np)24Na in Al monitors placed in front of the target.

4. Spalation target • Iodine samples were placed above the target. Position of these samples wasdetermined according to changes in the neutron field around the target, using the results of simulations provided by LAHET(Bertini)+MCNP4B code. • Set of Al, Cu, Au and Pb foils was placed above and on the side of the target for the purpose of measurement of the neutron field surrounding the target. Additional monitoring foils were placed within the target to measure the intensity and spread of the thoroughgoing proton beam. • The intensity of the proton beam was measured by a set of monitoring foils placed in front of the target.

5. Iodine samples • 4 samples with isotopic composition 85% 129I + 15% 127I • Iodine in the form of NaI • Mass of iodine in each sample 0.5-1.0 g • Aluminum enclosure – 70g Al !!! • => three samples placed at 9th, 37th • and 47th centimeter of the target • for proton energy Ep=2.5 GeV • => one sample placed at 37th • centimeter of the target for • proton energy Ep=1.3 GeV

6. The neutron field • One of the aims of the experiment was to measure the characteristics of the neutron field => important for understanding of iodine transmutation behaviour. • For this purpose the activation method was used. The energy thresholds of activation reactions [(n,), (n,2n) etc.] are not the same. Therefore both energy profile and the intensity of neutron field can be reconstructed if corresponding cross sections (En) are known. • γ-decay of products of the activation can be easily detected and identified with use of large HPGe γ-spectrometers. • The neutron field has its maximal intensity in the area of the 11th cm ofthe target for Ep=2.5 GeV, res. in the area of the 9th cm of the target for Ep=1.3 GeV. • The total number of produced neutrons is proportional to the energy of primary protons.

7. Neutron background • When no moderator around the target is used, the number of neutrons slowed down and back-scattered on surrounding material (the so-called „neutron back-ground“) can be of same order as the number of low-energy neutrons produced directly in target by the spalation reactions. • Contribution of such neutron background to the actual yield of (n,) reactions depends on the position of the activation foils along the target and varies between 10% (for the maximal neutron field intensity around 10th cm of the target) and 50% (at the end of the target where the intensity of the primaryneutron field is the lowest) in our experiment.

8. Simulations of the neutron field • The energy profile of the neutron field is related to the position along the target and energy of primary protons. • Results of simulations show that the energetic spectrum of the neutron field should peak around1 MeV and then slowly decreases approx. as 1/E. (Peak around 1 MeV corresponds to the evaporation neutrons from residual nuclei.) • The ratios of the neutron spectra in different positions show the signifi-cant differences which influences the transmutation yields

9. Transmutation of iodine samples • In the experiment we have focused on the isotopes produced in (n,xn) reaction channels. The other reactions, for example (n,pxn) or (n,axn), are much less probable and their detection was mostly beyond the detection limits of our experiment.

10. Transmutation of iodine samples • The increasing relative yield of 124I and 123I toward the end of the target is the result of increasing neutron mean energy. • The increasing relative yield of 130I is due to the increasing significance of low energetic neutron background toward the end of the target. • Following picture shows the effect of the neutron background on the measured transmutation yields of 130I compared with yields measured on Au foils.

11. Transmutation of iodine samples • The simulation predicts that the share of neutrons with energy 10-20 MeV is higher at the 37th cm of the target irradiated by 2,5 GeV protons in comparison with second irradiation with 1,3 GeV protons. The data are in agreement with this predictions and demonstrate higher dominance of (n,2n) channel leading to the production of 128I for the 2nd iodine sample as compared with the 4th iodine sample. • The discrepancies of absolute experimental and simulated values may be due to the unaccurant hit of the target by the beam or the systematic errors of LAHET code.

12. Conclusion => dominance of the (n,2n) channel • If we presume the σ(E) of (n,γ) reactions to be same on both isotopes present in the iodine sample (127I and 129I) then the contribution of 127I to the total trans-mutation yield of 128I via (n,γ) reaction can be evaluated and makes app. 5-7%. This means that 93-95% of 128I is produced in (n,2n) reaction on 129I (n,2n) transmutational channel dominates over (n,γ) channel by factor ~3