Page 65 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
62-63
Figure 2
148gPm and 148mPm produced by neutron activation of the 147Pm target are identified by their characteristic gamma rays. The decay scheme (left) with the relevant transitions (energies in units of
keV) and the measured gamma-ray spectrum of the 147Pm target after activation recorded by a HPGe detector (right), show the above-mentioned gamma rays plus additional ones from activated contaminants (marked in red).
While these neutron ‘activation’ experiments have been carried out for most of the stable isotopes on the chart of nuclides and can be performed at medium-size accelerator facilities, it is particularly challenging to perform them for the most interesting cases: the above-mentioned branching points in which the target nucleus is radioactive. To do so, one must first artificially produce the atoms of the isotope of interest, which will necessarily be a small amount. Next, one must make a suitable (high purity) target and then perform the neutron irradiation with an epithermal neutron beam of high intensity to make up for the limited number of atoms available.
In the study reported here, close to the core of the ILL reactor (at the V4 position) we inserted 98.2 mg of Nd,
in the form of a 146Nd2O3 enriched to 98.8 %, over a period of 54 days. Following each neutron capture a nucleus of 147Nd was formed, which decayed with a half-life of 11 days to 147Pm—the isotope of astrophysics interest. The irradiated material was then shipped to the PSI in Switzerland, where it was dissolved and chemically purified to make a high-purity and high-quality 147Pm target suitable for an activation experiment. Even though the target contained just 56 μg, it represents the highest mass of 147Pm put together to date for this purpose. In order to then produce enough activity from neutron capture, the irradiation of the target was performed at the LiLiT facility in Israel, featuring the highest intensity beam of epithermal neutrons in the world. At the LiLiT, a radio-frequency, superconducting linear accelerator delivers a proton beam of 1.93 MeV with a current of about 1.5 mA that hits a liquid lithium target, producing neutrons with an energy distribution equivalent to that of the s-process that occurs in
the 147Pm target. The activity induced by neutron capture was then assessed by studying the characteristic g-ray emission of 148Pm from its ground and metastable states, with quite different half-lives of 5.4 days and 41 days, respectively. The data analysis produced a MACS value of 826(57) mb at 30 keV, as the sum of the partial cross section values 469(50) and 357(27) mb, respectively.
The success of the experiment described briefly above could only be achieved thanks to the close collaboration
of three major players in the nuclear physics community: the ILL reactor, the PSI radiochemistry lab and the SARAF accelerator facility hosting LiLiT, co-ordinated by the Universidad de Sevilla and aided by the Hebrew University in Jerusalem. The result of this work is now in the hands of the astrophysics community studying the nucleosynthesis
of elements. However, more interesting results are on their way as a result of this collaboration established for this study. The 147Pm target has also been used to identify individual epithermal neutron resonances in an experiment performed at the CERN n_TOF facility, while part of the repurified 147Pm enabled high-resolution laser spectroscopy studies to be performed at Mainz University. This resulted in the first experimental determination of the ionisation potential of the chemical element promethium (which does not possess any stable isotope), filling the last remaining gap in the periodic table for this fundamental property—a timely occurrence for the 150th anniversary of the periodic table of the elements. Moreover, radioactive targets of 171Tm and 204Tl have already been produced at the ILL and used for irradiations at the LiLiT and the CERN n_TOF facility; new targets of 163Ho and 79Se are planned in the coming years.
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