Speaker
Description
According to the Nuclear Data High Priority Request List published by the Nuclear Energy Agency (http://www.oecd-nea.org/dbdata/hprl/), values of cross sections used in nuclear technology should be known with uncertainties between 1% and 5%, while cross sections of key isotopes involved in s- and r-processes must be known with uncertainties of about 1%.
The uncertainty associated with the measured cross sections has two sources, data evaluation and experimental procedure. Experimental uncertainties are mainly generated by time resolution of experimental configurations (duration and speed of neutron burst, etc.), detectors and electronics [1], and by the targetry (defined as discipline of design, manufacture and characterization of targets) [2]. On the other end several measurements of important cross sections have been hindered by the lack of adequate amount of the relative target nuclide with the requested high chemically and isotopically purity.
The first part of this contribution focuses on the production and purification of some exotic radionuclides at the Paul Scherrer Institut, Switzerland. Some examples, to illustrate the importance of the target chemical purity for obtaining the envisaged measurement, are presented.
The main part of the talk centers on recent developments of novel methods for the production of homogeneous and uniform solid targets allowing obtaining cross sections with lower uncertainty. In fact, only if the target material is homogeneous and has uniform thickness, the incident particles will have the same probability of interaction with the target nuclei and the energy loss of all the detected particles, especially if charged, will be similar. This will allow measurements that are more precise.
New findings on the role of the solvent on Molecular Plating, one of the most used techniques for solid target production [3], are presented. These results show that less volatile solvent allow obtaining more uniform and thinner targets. Figure 1 illustrates three scanning electron microscopy imagines (x 600 magnification) of three different targets of holmium obtained with identical conditions in term of temperature and concentration, but using solvent with different vapor pressure, i.e. isopropanol (5.78 kPa), Isobutanol (1.53 kPa) and N,N-Dimethylformamide (0.44 kPa). It is possible to see that the material deposited from solvents with higher vapor pressure are characterized by a less homogeneous and island-like structure, with presence of cracks and peeling off the deposited material.
A still unsolved problem related with the Molecular Plating is the inevitable co-precipitation of organic compounds, which causes thickening and introduction of impurities into the deposited layer, enhancing the background during the cross section measurement.
The development of a new method, based on electrodeposition of exotic radionuclides from ionic liquid, is under development in our laboratories to deposit chemically pure elements avoiding any co-precipitation. Some preliminary results of this project are presented too.
Reference
[1] H. Postma, P. Schillebeeckx, Neutron Resonance Capture and Transmission Analysis, Encyclopedia of Analytical Chemistry, John Wiley & Sons, Ltd, 2006.
[2] P. Schillebeeckx, A. Borella, J.C. Drohe, R. Eykens, S. Kopecky, C. Massimi, L.C. Mihailescu, A. Moens, M. Moxon, R. Wynants, Target requirements for neutron-induced cross-section measurements in the resonance region, Nucl. Instr. Meth. Phys. Res. A, 613 (2010) 378.
[3] W. Parker, R. Falk, Molecular plating: A method for the electrolytic formation of thin inorganic films, Nucl. Instr. Meth., 16 (1962) 355.