1. Transmutation of 129I with high energy neutrons produced in spallation reactions induced by protons in massive target V. HENZL1,*, D. HENZLOVA1, A. KUGLER1, V. WAGNER1, J.ADAM2, P. CALOUN2, V.G. KALINNIKOV2, M.I. KRIVOPUSTOV2, A.V. PAVLIOUK2, V.I. STEGAJLOV2, V.M. TSOUPKO-SITNIKOV2, W. WESTMEIER3, 1 Nuclear Physics Institute ASCR, Řež, 25068, Czech Republic 2Joint Institute of Nuclear Research, Dubna, Russia 3Philipps-University, Marburg, Germany
Spallation 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.
A massive lead spallation 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.
Iodine samples
Detection of neutron field
4 samples with isotopic com-position 85% 129I + 15% 127I
Iodine in the form of NaI
Mass of iodine in each sample 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
The activation method was used to measure the intensity and main characteristics of the neutron field.
γ-decay of products of such activation can be easily detected and identified with use of large HPGe γ-spectrometers.
Set of 25 activation detectors (Al+Cu+Au of 2.0x2.0 cm format) was placed along the top of the target with the step of 2 cm. Similar set was placed on the side of the target that we could monitor the asymmetry of the neutron field.
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.
Neutron background
Simulations of the neutron field
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.
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 around MeV and then slowly decreases approx. as 1/E. (Peak around 1 MeV corresponds to the evapo-ration neutrons from residual nuclei.)
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, as shown below.
The ratios of the neutron spectra in different position show the significant differen-ce between them which influences the transmu-tation yields
The background of high-energy neutrons was found to be experimentally negligible.

2. Intensity and energetic profile of neutron field
The intensity of the neutron field is directly related to the production rate of the radioactive isotopes from the activated foils.
Main results of the activation foils analysis can be summarized as follows:
The neutron field has its maximal intensity in the area of the 11th cm of the 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.
> for example: 24Na in Al foils, 194Au and 198Au in Au foils
Experimental results of 24Na production in Al foils show that our neutron field is strongly asymmetric. That is due to the proton beam hit cm above the center of the front plane of the lead target. Hit of the beam can be deduced from the activation yields in the front monitor sets, which were divided into smaller sections.
Transmutation of iodine samples
The most important of the detected and identified transmutational products and their experimental yields in 10-6 atoms.g-1.proton-1 :
isotope
T1/2
production reaction
possible interference
130I
12,36 h
(n,) -
128I
24,99 m
(n,2n)+(n,)
126I
13,11 d
(n,4n)+(n,2n)
126Sb (T1/2=12,5 d)
124I
4,176 d
(n,6n)+(n,4n)
124Sb (T1/2=60 d)
123I
13,27 h
(n,7n)+(n,5n)
123Te (T1/2=120 d)
121I
2,12 h
(n,9n)+(n,7n)
121mTe(T1/2=154 d)
120I
81 m
(n,10n)+(n,8n)
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.
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 transmutation 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
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.
(n,2n) transmutational channel dominates over (n,γ) channel by factor ~3
The simulation predicts that the share of neutrons with energy MeV is bigger 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 compare with the 4th iodine sample.