Summary

My research grant will end tomorrow... So now I try to overview the achievements of my work in this period.

The period of my grant started in September 2013. It seems to be a long time since then (16 months), but for a researcher, and mostly for an experimentalist, time is going very fast. 16 months in experimental nuclear physics is especially a short term: sometimes we need to wait a whole year to get a beamtime at an accelerator research facility to perform our experiment, and if something happens to this quite complicated machine, we have to wait another year... Something the same happened to us a year ago in Munich as I wrote in a previous entry.

Despite of the technical problems I think this period was very successful. My research work was very productive, which is best represented by my six scientific publications in high-rank journals. I feel that I could also fulfil the requirements of dissemination as well with a couple of informative manuscripts for the open public and with this blog. Besides, I was involved "seriously" in teaching activities of the University of Debrecen. At the Informatics Faculty I was the lecturer of a course with the title "Artificial Intelligence" which is a basic course for engineers. I have also started a new special course "Experimental Planning and Data Analysis".

But the most important achievements of the research grant period is collaboration agreement and research project contract that was signed on 14th of December 2014 by MTA Atomki and ELI-NP. This contract is based on the research project that was initiated and elaborated by my research group here at MTA Atomki.  I will be one of the principal investigators of the research project. I think this research project contract both ensures our scientific contribution in the EU funded ELI-NP project in a long term and also grant a nice research fund for our scientific work.

As a summary I think that this research grant gave me an excellent support for my research and also for all the other activities during this period. This grant has a large contribution in the realization of the collaboration between ELI-NP and MTA Atomki. The benefit of having a formal collaboration with world-leading research facility is obvious....

The fission barrier of Neptunium-238

In a recent entry I have already introduced our measurement on Neptunium-238 and the first steps of the data analysis. In the last couple of weeks I could proceed so now I want to write some details about it

a) Fission probability of Neptunium-238 in the function of excitation energy. b) Sharp resonances in the fission probability and its rotational structure

In Figure (a) the data points of our measurement are indicated by blue open triangles, and open circles represent a previous, low resolution measurement. The continuous red line indicates the result of our model calculations, in which we altered the free parameters (actually the fission barrier parameters) until the theoretical line fits to the data points. As a result we got a quite good description with a triple-humped fission barrier having barrier heights of 5.5 MeV, 6.2 MeV and 6.3 MeV.

 

This result shows that the structure of Neptunium-238 and the previously examined Protactinium-232 (both odd-odd nucleus) is quite similar. The fission resonance structure which is shown in Figure (b) also supports this similarity: these resonances are in the same energy region as in the case of Protactinium-232. Actually, this means that their 3rd minimum in the potential barrier have the same depth.

The manuscript on these results have already been accepted by Acta Physica Polonica B for publication, and will be published expectedly in April.

 

Design and simulation

As I presented in previous entries, we develop two different detector arrays for our research projects at ELI-NP. The first step of the development is planning, which means we sketch our idea on paper according to the requirements and the scientific goals of the project. We have to decide the dimensions, the geometry and the materials. During the planning we also have take our financial possibilities into consideration. It also a great advantage if the tolerance of the mechanical and electric parts allows to use the infrastructure of our institute (e.g.: the mechanical and electronic workshop).

A very important step is to estimate the response of the detector to the investigated phenomenon and also to estimate the possible background signals having particular physical conditions. For these estimations we perform simulations with a software environment developed at CERN (GEANT4).

It is especially important to perform such simulations for the Bragg ionization chamber. For this we have to know all the relevant environmental conditions of the experiments. The time structure of the photon beam seems to be particularly important. And we are going to have serious technological challenges due to the time structure of the beam at ELI-NP: photons will come in little bunches and not as a continuous beam. We are going to have 100 big bunches in one second each having 32 small bunches. The time interval of one small bunch is 1-2 picosecond (!!), in which we get 100000 photons. The problem is obvious: reactions induced by individual photons will "overlap" since the time resolution of our detector limited resulting in a distortion of the energy measurement. This background we have to minimize.

According to our very recent GEANT4 simulations we can minimize the effect down to 1% of the effective signal if we design a chamber taking some consideration on the materials and dimensions. For example we have to use entrance and outgoing window on the wall of the chamber for the beam covered by a very thin (Kapton) foil in order to minimize the scattering of photons.

Foton optics at very high energy

In experimental nuclear physics quite often we need to localize very precisely the position of the nuclear reaction within the target, or, equivalently, the place of origin of the emitted reaction products. For that it is elementary to reduce the size of the target and the beam diameter as much as possible. The later can be done by focusing the beam.

At a conventional particle accelerator the charged particle beam can be focused on the target by quadrupole magnets. Submillimeter beam diamater can be easily achieved. For the focusing of photon beams one can use optical lenses: the focal distance of the lense is determined by the index of refraction of its material (n), its thickness at the middle (d) and the two radius of curvature (R1,R2). However, the index of refraction depends on the photon energy: n(E) = 1 − δ(E) + iβ(E), where iβ(E) stands for the absorption, and δ(E) drops very fast with energy! So at very high energy the focal distance (f) is divergent: for X-rays typically δ=10^-5 - 10⁻⁶, and in the γ energy region δ → 0. By increasing the number of lenses f can be reduced since f=n/(2Nd), where N is the number of lenses (see Figure). But for the energetic γ photons it does not help...

Array of lenses for focusing X-rays

Very recently a series of experiment performed by a group of physicists from the Ludwig Maximilians University and from Institut Laue-Langevin showed signatures on a deviation from the above mentioned tendency. The index of refraction can be different from 1 even for γphotons! A possible explanation is based on the so-called Delbrück scattering, which means an interaction between the photons and the electron-positron pairs those appear in the vicinity of nucleus with high atomic numbers.

We hope that in the near future already at ELI-NP the γ beam can be focused by using this effect with lenses having high atomic numbers (e.g. gold lenses).

Conference in Zakopne

I am going to participate the fourth time now at the International Nuclear Physics Conference in Zakopene (Poland), which has been organized every second year. Conferences, organized by the Polish physical society, are always pretty good due to the Polish hospitality (especially towards the Hungarians) and the high scientific quality. From these the Zakopane Conference is one of the best: the site is located in the middle of the High Tatras... Participants and organizers are neither hiding one of main goal of the conference: hiking at the mountains discovering the beauty of the nature.

At the conference I am going to give a presentation about the topic and the scientific results discussed in my last entry. Before the conference I have lot of questions to myself: who will be there? In which field they are professionals at? Can I give a presentation that can be understood by everyone? Will be any relevant questions, comments, remarks after the talk?

Anyway, the weather seems to be perfect so my hiking boot goes to my bag...

Summertime...

... in our research institute is usually the time for maintenance so a researcher must have a quite long break in his experimental work. This is the time for a little rest, but also for have an overview on the latest publications in the field, and for having a "look" at the data of previous experiments...

Very recently we performed an experiment at the Tandem Accelerator of the Technical University (Munich). In this experiment we measured the fission probability of 238Np as a function of the excitation energy by employing the 237Np(d,pf) reaction. However, the data analysis of this experiment is complicated by the very large level density which originates from the odd-odd nature of this nucleus: it has an odd number of neutrons and protons. Now, I have the time for the careful analysis of the data of this experiment...

Fission probability of 238Np as a function of excitation energy (blue spectrum), and a zoomed spectrum on the individual resonances.

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!