Production of neutron-rich nuclei approaching r-process by gamma-induced fission of 238U at ELI-NP

The investigation of neutron-rich exotic nuclei is crucial not only for nuclear physics but also for nuclear astrophysics. Experimentally, only few neutron-rich nuclei near the stability have been studied, however, most neutron-rich nuclei have not been measured due to their small production cross sections as well as short half-lives. At ELI-NP, gamma beams with high intensities will open new opportunities to investigate very neutron-rich fragments produced by photofission of 238U targets in a gas cell. Based on some simulations, a novel gas cell has been designed to produce, stop and extract 238U photofission fragments. The extraction time and efficiency of photofission fragments have been optimized by using SIMION simulations. According to these simulations, a high extraction efficiency and a short extraction time can be achieved for 238U photofission fragments in the gas cell, which will allow one to measure very neutron-rich fragments with short half-lives by using the IGISOL facility proposed at ELI-NP.


Introduction
The production and measurement of very neutron-rich nuclei are of significant importance for nuclear physics as well as the r-process in nuclear astrophysics. However, so far, only few neutron-rich nuclei near the stability line have been measured, while most neutron-rich exotic nuclei have not been experimentally investigated because of their small production cross sections and short halflives. Therefore, more experimental studies are required for these neutron-rich nuclei away from the stability line.
Very brilliant and intense gamma beams obtained by laser Compton backscattering at ELI-NP will provide new chances to investigate very neutron-rich nuclei, including those close to the r-process path, produced by photofission of 238 U targets placed at the center of a gas cell [1]. An IGISOL (Ion Guide Isotope Separator On-Line) facility with a gas cell is proposed at ELI-NP. This facility is mainly aimed at studying neutron-rich exotic nuclei, especially refractory isotopes [1][2][3], which are difficult to be investigated at traditional ISOL (Isotope Separator On-Line) facilities.
In this work, the production cross sections and rates of neutron-rich fragments produced by photofission of 238 U are reliably calculated for the planned IGISOL facility at ELI-NP. Furthermore, some SIMION simulations are performed for the optimization of the gas cell of the ⋆ e-mail: bo.mei@eli-np.ro ELI-NP IGISOL facility, used to produce and extract the neutron-rich fragments.

Calculations for the production of neutron-rich exotic nuclei at ELI-NP
Accurate calculations for production cross sections of fragments produced by 238 U photofission at low energies over the whole energy region of the giant dipole resonance (GDR) are required for estimating the production yields of neutron-rich fragments and optimizing nuclear physics experiments at the planned IGISOL facility at ELI-NP. The gamma beam used at this IGISOL facility will cover a wide energy range between about 10 and 18.5 MeV [1][2][3].
Recently, a reliable empirical parametrization, called GIF 238 U (Gamma Induced Fission of 238 U) [4], has been proposed for accurately calculating the production cross sections (rates) of neutron-rich fragments produced by photofission of 238 U at low energies (E γ < 30 MeV). This parametrization is based on some measured 238 U photofission data and has been validated by comparing with many elemental, mass, and isotopic yields measured in 238 U photofission experiments at different energies (see Ref. [4] for details). Figure 1 presents the production cross sections of various fragments produced by 238 U photofission at 14 MeV near the GDR peak, which are calculated by the GIF 238 U parametrization [4]. For many neutron-rich nuclei with

31
Z 62 and 80 A 160, which are on or close to the r-process path [5], their production cross sections of the order around 1 mb can be achieved by using 238 U photofission, and thus 238 U photofission is very suitable for producing many neutron-rich nuclei away from the stability. For example, production cross sections of 80−82 Ge and 85−87 Se around the closed shell N = 50 are roughly 0.17 and 1.1 mb, respectively, while production cross sections of 132 Sn and 134 Te around the closed shell N = 82 are about 0.66 and 6.4 mb, respectively.
The above cross sections calculated with the GIF 238 U parametrization can be used to estimate the production rates of neutron-rich nuclei produced by 238 U photofission at the ELI-NP IGISOL facility. As an example, production rates of 132 Sn and 134 Te in the 238 U targets with a total thickness of 251 µm are about 2×10 4 and 2×10 5 ions/s, respectively, according to calculations by the GIF 238 U parametrization for the IGISOL facility at ELI-NP. At the gamma production point, the total intensity of the gamma beam with a broad energy distribution between 10 and 18.5 MeV is conservatively estimated to be 5×10 10 γ/s for the above calculations of day-one experiments, while it can be improved to be around 1×10 12 γ/s for future experiments [1][2][3]. The 238 U target in the gas cell is placed around 7 m after this gamma production point.

Simulations for 238 U photofission fragments in the ELI-NP gas cell
According to the design of the ELI-NP IGISOL facility, the 238 U target is sliced into roughly 33 foils and tilted at 15 • for a high release rate of produced fragments [2,3]. The fragments released from 238 U foils are then stopped by the helium buffer gas filled in the gas cell. To maximize the rate of the released and stopped photofission fragments, the geometry of the gas cell for the ELI-NP IGISOL facility has been optimized by simulations using the Geant4 toolkit [6] (see Ref. [2] for detailed Geant4 simulations). After the stopping process, the stopped fragments are transported by the DC as well as RF fields to the extraction nozzle positioned at the center of the RF carpet, namely, the innermost ring electrode. Figure 2 shows the preliminary design for the gas cell of the planned IGISOL facility at ELI-NP. As shown in Fig. 2, the fragment extraction is performed in the perpendicular direction with respect to the gamma beam, which leads to a very short extraction path and time [7]. This perpendicular extraction method is particularly useful for the investigation of very exotic and short-lived nuclei. Typical trajectories of two fragments in the gas cell are shown with the black solid arrows in Fig. 2.
The DC and RF fields in the gas cell have been optimized by simulations using the SIMION software [8]. The DC field produced by the push-electrodes pushes the produced fragments towards the RF carpet. Voltages on the right and left-side rods are set in order to constrain fragment trajectories and prevent fragments from hitting the wall of the gas cell. The final position and velocity of stopped fragments in the stopping process are from Geant4 simulations [2] and are used as inputs for SIMION simulations. The DC drag field produced by the RF carpet is used to drag fragments from other positions to the center nozzle, where fragments are finally extracted, and the RF field of the carpet is applied to prevent the ions from hitting the RF carpet.
According to many SIMION simulations, an extraction efficiency of about 50% and a short extraction time (average time around 15 ms) can be reached for photofission fragments after the DC and RF fields have been optimized. Thus, the total rate of fragments extracted from the gas cell is around 5×10 5 ions/s. Final rates of 132 Sn and 134 Te extracted from the gas cell are about 1×10 3 and 1×10 4 ions/s, respectively. In these calculations, a release efficiency of about 15% is used for the produced fragments, according to Geant4 simulations in Ref. [2]. More details about the final design of the optimized gas cell will be discussed in future works.

Summary
In order to study neutron-rich exotic nuclei, an IGISOL facility including a gas cell is proposed at ELI-NP. The production yields (cross sections) of 238 U photofission fragments in the gas cell have been calculated by a reliable empirical parametrization GIF 238 U, which is based on 238 U photofission experimental data. Calculations indicate that a production rate between about 10 4 and 10 5 ions/s can be obtained for many neutron-rich nuclei with 31 Z 62 and 80 A 160. Furthermore, the preliminary design for the gas cell of the planned IGISOL facility at ELI-NP is discussed. Both the DC and RF fields have been optimized by using SIMION simulations. According to these SIMION simulations, a high extraction efficiency of about 50% and a short extraction time (average time around 15 ms) can be obtained for the optimized gas cell, which demonstrates that this gas cell is very suitable for the investigation of the short-lived neutron-rich nuclei. The total rate of all fragments extracted from the gas cell is estimated to be around 5×10 5 ions/s, while the rate of extracted isotopes is of the order around 10 3 ions/s for many neutron-rich ones.