Benchmark Data on the Transmutation of 129I, 139La and 237Np
- The reduction of long-lived nuclear waste -
W. Westmeier1,7 , R. Brandt1, E.-J. Langrock2, H. Robotham7, K. Siemon7, R. Odoj3,
V.M. Golovatyuk4, M.I. Krivopustov4, S.R. Hashemi-Nezhad5, M. Zamani6
Institut fłr Physikalische Chemie, Kernchemie und Makromolekulare Chemie,
Philipps-UniversitĒt, D 35032 Marburg (Germany)
2 Forschungsbłro Langrock, D 02977 Hoyerswerda (Germany)
Institut fłr Sicherheitsforschung und Reaktortechnik, Forschungszentrum Jłlich
GmbH, D 52425 Jłlich (Germany)
4 Joint Institute for Nuclear Research, 141980 Dubna (Russian Federation)
5 Dept. of High Energy Physics, University of Sydney, Sydney, NSW 2006 (Australia)
6 Physics Department, Aristotle University, GR 52124 Thessaloniki (Greece)
7 Dr. Westmeier GmbH, D 35085 Ebsdorfergrund, (Germany)
Transmutation was proposed  as a hypothetical means to reduce the amount of very long-lived radioactive waste from technological applications of nuclear fission. With the advent of new technologies this idea came closer to reality and high-precision experimental data are now required to check the feasibility of the concept.
Experiments were carried out with the GAMMA-2 target setup  at the NUCLOTRON accelerator using protons in the energy range from 0.53 GeV to 4.15 GeV. Fig.1 gives a schematic view of the GAMMA-2 experimental setup together with its beam monitoring system.
Figure 1: Schematic view of the GAMMA-2 setup. The target is composed of 20 lead disks with 8 cm diameter and 1 cm thickness, the paraffin moderator shell has 20 cm outer diameter, 6 cm thickness and 31 cm length. The Al- monitor contains a stack of three thin aluminium foils where the center foil is used. Polaroid films were used for beam alignment before
Five scintillation detectors C1 to C5 and a 1 g/cm2 PE target were used to monitor the beam. Aluminium activation foils were used to determine the integral proton fluence on the target. The Al monitor foil stack was placed approx. 60 cm upstream the Pb target in order to avoid activation from backwards emitted particles. In each experiment a stack of three Al foils with a thickness of 31 mm (1.883*1020 atoms*cm-2 ) was mounted in an aligned position with the target and perpendicular to the beam axis as shown in Fig. 1, and irradiated during the whole run. The beam intensity was determined via the 27Al(p,3pn)24Na reaction in the center foil.
Samples containing 1 gram of lanthanum each were placed on top of the target assembly at distances of 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm from the front side of the paraffin block, i.e. the first sample sits just above the location where the proton beam hits the Pb. B-values for each of the five samples (corrected for neutron anisotropy) were measured in every experiment. The B-value is an absolute cross section which is specific for each experimental setup and defined for the example nuclide 140La as :
B(140La) = Atoms of 140La produced in 1 gram of 139La sample by 1 primary proton
In order to compare neutron densities from various experiments we have calculated the integrated B(140La ) for 140La on the GAMMA-2 setup by fitting the five data points with a modified (skewed) Gaussian function. The function is used because it has a suitable shape and not because of any physical significance.
B-values for 140La along the top of the paraffin moderator in the irradiation with 0.53 GeV protons on the GAMMA-2 target. The distance d=0 cm corresponds to the upsteam end of the 20 cm long Pb target, i.e. the point of proton impact.
The fitted distributions quantify findings from earlier experiments [3,4] that the shapes of B-value distributions (i.e. the neutron densities over the target) are almost identical over the entire proton energy range studied. The maximum of the B-values is always found at about 10 cm downstream the beginning of the lead target and the widths of the distributions are essentially the same for each energy in the 0.53 GeV £ Ep £ 4.15 GeV range.
The integrated B(140La )Ůvalues divided by the proton beam energy EP are plotted in Fig. 3 as a function of proton energy EP. This picture shows the effectiveness of the GAMMA-2 setup for transmutation of 139La via neutron capture reactions. Thus, it also displays the effectiveness of the GAMMA-2 setup for the production of low-energy neutrons. It is interesting to note that the effectiveness of GAMMA-2, which has only 20 cm Pb target length, for low-energy neutron production is best at low proton energies.
Figure 3: Normalized B-values for 140La on the GAMMA-2 setup. The dotted line serves to guide the eye.
Uncorrected data points at 0.65 GeV, 1 GeV and 1.5 GeV proton energy show the necessity of the anisotropy correction of measured B-values.
In Figures 4 and 5 the corresponding functions of B-values/Ep are shown for the transmutation of 129I and 237Np. In these experiments samples of approx. 1g of radioactive target material, which was weld sealed into Al-containers, were exposed to the secondary neutron fluence on top of the paraffin moderator on the GAMMA-2 target setup.
Figure 4: Normalized B(130I)/Ep measured Figure 5: Normalized B(238Np)/Ep measured
on the GAMMA-2 setup on the GAMMA-2 setup
The lines in Figs. 4 and 5 serve to guide the eye. Considering results from Figures 3 to 5 it is clear that the transmutation effectiveness B/Ep (also called —neutron economyž ) on the GAMMA-2 target is always highest at low proton energy and gradually falls off with rising bombarding energy. This may favour the use of proton beam energies that are lower than it has been assumed in other design studies. Operating at lower energy would of course be commercially attractive. However, the gradual fall may be a consequence of the size of the target where the small diameter and short length do not allow the intra- and inter-nuclear cascades originating from incident protons to be completed. Further experiments shall answer that question very soon.
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