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

Motivation Iodine is one of the problematic fission products in burned-up radioactive waste; it is also biogenic. Half-life of 129 I is 1,57x10 7 years. Very few data on 129 I transmutation are known. The only experimentaly measured 129 I(n,2n) 128 I reaction cross sections. The integral experiments are needed for verification of evaluated data and computer codes.

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 E p =2.5 GeV during the first irradiation and E p =1.3 GeV during the second irradiation. The total intensity of the proton beam was 4.07x10 13 protons in irradiation with 2.5 GeV protons, res. 2.77x10 13 protons in irradiation with 1.3 GeV protons, as deduced from the yield of 24 Na formed in 27 Al(p,3np) 24 Na in Al monitors placed in front of the target.

Spalation target Iodine samples were placed above the target. Position of these samples was determined 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.

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

The neutron field 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  (E n ) are known. γ-decay of products of the activation can be easily detected and identified with use of large HPGe γ-spectrometers. One of the aims of the experiment was to measure the characteristics of the neutron field => important for understanding of iodine transmutation behaviour. The neutron field has its maximal intensity in the area of the 11 th cm of the target for E p =2.5 GeV, res. in the area of the 9 th cm of the target for E p =1.3 GeV. The total number of produced neutrons is proportional to the energy of primary protons.

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 primary neutron field is the lowest) in our experiment.

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

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,  xn), are much less probable and their detection was mostly beyond the detection limits of our experiment. isotopeT 1/2 production reaction possible interference 130 I12,36 h (n,  ) I24,99 m (n,2n)+(n,  ) I13,11 d(n,4n)+(n,2n) 126 Sb (T 1/2 =12,5 d) 124 I4,176 d(n,6n)+(n,4n) 124 Sb (T 1/2 =60 d) 123 I13,27 h(n,7n)+(n,5n) 123 Te (T 1/2 =120 d) 121 I2,12 h(n,9n)+(n,7n) 121m Te(T 1/2 =154 d) 120 I81 m(n,10n)+(n,8n)-

The increasing relative yield of 124 I and 123 I toward the end of the target is the result of increasing neutron mean energy. The increasing relative yield of 130 I 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 130 I compared with yields measured on Au foils. Transmutation of iodine samples

The simulation predicts that the share of neutrons with energy MeV is higher at the 37 th 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 128 I for the 2 nd iodine sample as compared with the 4 th 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.

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