Investigation of the photofission of Thorium

As a first step of our experimental program, we now plan to perform high resolution transmission resonance spectroscopy of ²³²Th via photofission to resolve, for the first time, the isomeric shelf at lower (E=4-4.5 MeV) energies. After resolving the isomeric shelf into individual resonances, one has to identify and assign them to the corresponding minimum in the potential energy surface. One characteristic property of the octupole- and quadrupole-deformed 3^rd minimum is the alternating-parity bands (0⁺,1^-,2⁺,3^-,…), while in the 2^nd minimum, which is only quadrupole deformed, the mass symmetric and asymmetric bands separately appears with a structure of (0⁺,2⁺,4⁺,...) and (1^-,3^-,5^-,...). Since the sub-barrier photo-excitation is very spin-selective, we can expect to observe always doublets of 1^- and 2⁺ states with a spacing of about ~6 keV in the 3^rd minimum, providing the moment of inertia, while in the 2^nd minimum, completely independent 1^- and 2⁺ resonances are expected. This represents a very clear criterion to discriminate between resonances in the 2^nd and 3^rd minimum. Therefore, in these studies, the measurement of the fission fragment angular distributions is essential in order to identify the spins, the parity, and the K values of the resonances. We can exploit the fact that the electromagnetic interaction follows well-established selection rules, leading to the very spin-selective photofission (1^-,1⁺,2⁺) together with typical angular distributions due to the Bohr's picture of transmission resonances with well-defined values of spins.

We can also determine the spin-dependent inner barrier heights for dipole and quadrupole excitations E_A(1-) and E_A(2+), respectively, and the depth E_II of the 2^nd minimum from the SD states via level density arguments. Once E_A and E_II is known, the lifetime of the so far unobserved fission isomer in ²³²Th will be estimated for the first time. In addition, identifying the multi-phonon β vibrational excitation pattern over a wide energy range from the isomeric ground state to the region of the barrier top will provide valuable insight into the harmonicity of the 2^nd potential well.

As a long term perspective, the γ-decay in the 2^nd minimum of thorium and uranium isotopes with its predominant γ back-decay to the 1^st minimum can also be studied with high resolution. Therefore, it will become possible to measure the isomeric excitation energies with an unprecedented resolution (ΔE/E=10^-3). Measurements of the ground state of the 3^rd minimum via its γ decay will also be enabled. Moreover, a detailed γ spectroscopy of the HD potential minimum will be possible for the first time due to the enhanced E1 strengths in the higher-lying minima and due to the strong spin selectivity of the photo-nuclear reactions.

On the other hand, theoretical considerations predicted that the hyperdeformed minimum in a cluster description consists of a rather spherical ¹³²Sn-like component with magic neutron and proton numbers of N=82 and Z=50, respectively, complemented by an attached elongated second cluster of nuclei [6]. Since the fission mass distribution is distinctly determined by the configuration at the scission point, and the 3^rd minimum is very close to the scission configuration, we expect that the mass distributions originating from the 3^rd minimum exhibit a much more pronounced asymmetric mass distribution with a much larger peak-to-valley ratio as compared to fission from the 1^st minimum. However, this effect has never been observed due to the limited mass resolution (~5 amu) of the fission fragment detection systems and the large non-resonant fission background. Therefore, we plan to study the fission fragment mass distribution following the fission decay of the HD states of ²³²Th. The precise knowledge of the fission barrier parameters of ²³²Th, and more generally of the light actinides, are of great importance even for designing more efficient nuclear power plants (cross-section inputs for IV. generation reactors), for performing astrophysical network calculations (heavy element production in the r-process), and also for testing the available theoretical models. 
Moreover, the resonances of the 3^rd minimum are expected to have much larger population width than those from the 1^st minimum. The very large deformation causes the giant dipole resonance (GDR) to split up into two components, with a low-lying oscillation along the long symmetry axis with a typical excitation energy of E=4-5 MeV. The long-term idea towards an application of brilliant γ beams is to use selected transmission resonances in the 3^rd minimum to transmute minor actinides by irradiating radioactive waste containers with deeply penetrating γ beams tuned to these special resonance energies in such a way that fission of these potentially harmful radioactive waste components can be induced non-invasively!