ORNL actinide materials and a new detection system for superheavy nuclei

The actinide resources and production capabilities at Oak Ridge National Laboratory (ORNL) are reviewed, including potential electromagnetic separation of rare radioactive materials. The first experiments at the Dubna Gas Filled Recoil Separator (DGFRS) with a new digital detection system developed at ORNL and University of Tennessee Knoxville (UTK) are presented. These studies used 240Pu material provided by ORNL and mixed-Cf targets made at ORNL. The proposal to use an enriched 251Cf target and a large dose of 58Fe beam to reach the N = 184 shell closure and to observe new elements with Z = 124, 122 and 120 is discussed. 1. ORNL actinide resources Actinide materials originating from the ORNL’s High Flux Isotope Reactor (HFIR) and Radiochemical Engineering Development Center (REDC) have been used in experiments identifying nine superheavy elements, from Z = 104 to 106 and from Z = 113 to 118. The relatively high power of HFIR combined with its flux trap design provide an intense steady-state neutron flux of 2.4*1015 neutrons/s/cm2. This very high neutron flux is essential for actinide production. The adjacent REDC operates several hot cells capable of handling highly radioactive materials from HFIR. REDC provides unique capabilities for the efficient processing and separation of actinide materials including Z = 98 252Cf with a half-life of 2.6 years. 252Cf is produced in HFIR in hundred milligram quantities from Am and Cm feedstock material, mostly recoverable after production and separation cycles lasting from several months to two years. Z = 97 249Bk can be obtained as a bi-product of 252Cf production campaigns [1]. Currently, approximately 10 mg of 249Bk can be separated after a typical 252Cf campaign. Increasing the production of 249Bk requires a dedicated irradiation in HFIR using a suppressed thermal neutron flux tailored for 249Bk production [2]. Such an approach can increase 249Bk production by factors of two or more but is more costly since it is not compatible with standard 252Cf production campaigns. Actinide materials valuable for research on superheavy nuclei include highly enriched 237Np, 242Pu, 241,243Am, 244Cm, and 248Cm. Up to gram quantities of these isotopes are currently in actinide inventories at ORNL, with the exception of 248Cm where the current inventory is 80 mg. REDC is currently recovering about 10 mg/year of highly enriched 248Cm from old 252Cf sources. In addition, about 15 mg of mixed-Cf material, containing longer-lived 249Cf (50%), 250Cf (15%), and a Corresponding author: rykaczewskik@ornl.gov C © The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 131, 05005 (2016) DOI: 10.1051/epjconf/201613105005 Nobel Symposium NS160 – Chemistry and Physics of Heavy and Superheavy Elements Figure 1. Left panel: One of the mixed-Cf target sectors electrodeposited on Ti foils at ORNL’s REDC. Right panel: Twelve target sectors with required shielding arriving at JINR Dubna early 2015. 251Cf (35%), has recently been recovered from these old sources. In 2015, twelve target sectors containing mixed-Cf, with a typical thickness of about 0.33 mg/cm2 and 3-cm2 area, were electrodeposited at REDC on Ti-foils for a planned experiment at JINR to search for heavier isotopes of element 118 (see Fig. 1). The total content of 252Cf in all sectors was below two micrograms. However, with a high percentage of 250Cf (half-life T1/2 = 13 years), the neutron emission from the target material is still quite high, at the level of 2*107 neutrons per second into full solid angle. The isotope 251Cf (T1/2 = 898 years) is the most valuable component of the mixed-Cf target enabling the possible synthesis of new Z = 118 isotopes, two neutrons closer to the predicted N = 184 shell closure. Technically, it would be possible to mass separate tens of milligrams of 251Cf suitable for a target wheel containing nearly 100% enriched 251Cf. This would require an efficient electromagnetic separator dedicated to radioactive materials. A lower current prototype separator is currently operating at ORNL to separate stable isotopes. A similar separator optimized for actinide materials would be more than adequate for the needs of the superheavy element research community. 2. Experiments with the ORNL/UTK digital detection system at the DGFRS A new detection system for superheavy nuclei has been designed, built and commissioned at ORNL. The concept follows the experience gained during discovery research on proton and emitters at the Recoil Mass Separator at ORNL’s Holifield Radioactive Ion Beam Facility. The new detector system uses a very large Si-wafer, with 128 mm by 48 mm active area, processed to work as a Double-sided Silicon Strip Detector (DSSD) by Micron Semiconductor (BB-17 type, 300 microns thick). This detector is installed at the final focus of the DGFRS, to detect implanted nuclei and observe related and fission decays. It has a matching size silicon single veto detector and six single detectors of 120 mm by 60 mm area acting as an escape-catching silicon-detector box. This system has replaced the position sensitive silicon detectors used earlier at the DGFRS for studies of superheavy nuclei. All silicon detectors and two Dubna recoil transmission detectors Multi-Wire Proportional Chambers (MWPCs) are operated using Pixie 16 digital signal processing electronics manufactured by XIA [3], in parallel to the local analogue data acquisition system. The digital data acquisition system was developed at the UTK’s Digital Pulse Processing Laboratory, by R. Grzywacz and collaborators [4], and at ORNL, with contributions by K. Miernik (Oak Ridge/Warsaw) and N. Brewer (Oak Ridge). All single pulses are analysed on board the Pixie-16 module, while on-board recognized pile-up signals occurring within ten microseconds are recorded as digital


ORNL actinide resources
Actinide materials originating from the ORNL's High Flux Isotope Reactor (HFIR) and Radiochemical Engineering Development Center (REDC) have been used in experiments identifying nine superheavy elements, from Z = 104 to 106 and from Z = 113 to 118.The relatively high power of HFIR combined with its flux trap design provide an intense steady-state neutron flux of 2.4*10 15 neutrons/s/cm 2 .This very high neutron flux is essential for actinide production.The adjacent REDC operates several hot cells capable of handling highly radioactive materials from HFIR.REDC provides unique capabilities for the efficient processing and separation of actinide materials including Z = 98 252 Cf with a half-life of 2.6 years. 252Cf is produced in HFIR in hundred milligram quantities from Am and Cm feedstock material, mostly recoverable after production and separation cycles lasting from several months to two years.Z = 97 249 Bk can be obtained as a bi-product of 252 Cf production campaigns [1].Currently, approximately 10 mg of 249 Bk can be separated after a typical 252 Cf campaign.Increasing the production of 249 Bk requires a dedicated irradiation in HFIR using a suppressed thermal neutron flux tailored for 249 Bk production [2].Such an approach can increase 249 Bk production by factors of two or more but is more costly since it is not compatible with standard 252 Cf production campaigns.Actinide materials valuable for research on superheavy nuclei include highly enriched 237 Np, 242 Pu, 241,243 Am, 244 Cm, and 248 Cm.Up to gram quantities of these isotopes are currently in actinide inventories at ORNL, with the exception of 248 Cm where the current inventory is 80 mg.REDC is currently recovering about 10 mg/year of highly enriched 248 Cm from old 252 Cf sources.In addition, about 15 mg of mixed-Cf material, containing longer-lived 249 Cf (50%), 250 Cf (15%), and Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements 251 Cf (35%), has recently been recovered from these old sources.In 2015, twelve target sectors containing mixed-Cf, with a typical thickness of about 0.33 mg/cm 2 and 3-cm 2 area, were electrodeposited at REDC on Ti-foils for a planned experiment at JINR to search for heavier isotopes of element 118 (see Fig. 1).The total content of 252 Cf in all sectors was below two micrograms.However, with a high percentage of 250 Cf (half-life T 1/2 = 13 years), the neutron emission from the target material is still quite high, at the level of 2*10 7 neutrons per second into full solid angle.
The isotope 251 Cf (T 1/2 = 898 years) is the most valuable component of the mixed-Cf target enabling the possible synthesis of new Z = 118 isotopes, two neutrons closer to the predicted N = 184 shell closure.Technically, it would be possible to mass separate tens of milligrams of 251 Cf suitable for a target wheel containing nearly 100% enriched 251 Cf.This would require an efficient electromagnetic separator dedicated to radioactive materials.A lower current prototype separator is currently operating at ORNL to separate stable isotopes.A similar separator optimized for actinide materials would be more than adequate for the needs of the superheavy element research community.

Experiments with the ORNL/UTK digital detection system at the DGFRS
A new detection system for superheavy nuclei has been designed, built and commissioned at ORNL.The concept follows the experience gained during discovery research on proton and emitters at the Recoil Mass Separator at ORNL's Holifield Radioactive Ion Beam Facility.The new detector system uses a very large Si-wafer, with 128 mm by 48 mm active area, processed to work as a Double-sided Silicon Strip Detector (DSSD) by Micron Semiconductor (BB-17 type, 300 microns thick).This detector is installed at the final focus of the DGFRS, to detect implanted nuclei and observe related and fission decays.It has a matching size silicon single veto detector and six single detectors of 120 mm by 60 mm area acting as an escape-catching silicon-detector box.This system has replaced the position sensitive silicon detectors used earlier at the DGFRS for studies of superheavy nuclei.All silicon detectors and two Dubna recoil transmission detectors Multi-Wire Proportional Chambers (MWPCs) are operated using Pixie 16 digital signal processing electronics manufactured by XIA [3], in parallel to the local analogue data acquisition system.The digital data acquisition system was developed at the UTK's Digital Pulse Processing Laboratory, by R. Grzywacz and collaborators [4], and at ORNL, with contributions by K. Miernik (Oak Ridge/Warsaw) and N. Brewer (Oak Ridge).All single pulses are analysed on board the Pixie-16 module, while on-board recognized pile-up signals occurring within ten microseconds are recorded as digital Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements pulse images, with 10-ns time resolution, for initial on-line and further off-line analysis.An example of the capability of the new digital detection system is illustrated in Fig. 2.
The reaction between a Z = 70 natural ytterbium target and 48 Ca ions was used to calibrate the -energy scale of the silicon detectors.Short-lived Z = 90 thorium activities were produced and implanted in the DSSD, and their decay was recorded.This includes the direct observation of the implantation and decay of 219 Th activity having a 1-s half-life.The 219 Th decay curve was obtained through the analysis of recoil-decay signal traces.For the first time, the recoil implantation followed by a decay of such a short-lived activity was directly identified at the DGFRS.

Fusion-evaporation reactions between a 48 Ca beam and 239,240 Pu targets
The first experiment aiming at new superheavy nuclei and using the new digital detection system was performed at the DGFRS with a 48 Ca beam irradiating 239 Pu and 240 Pu targets (the latter material provided from ORNL).The spectroscopy results include the observation of three decay chains of 285 Fl including the previously not detected full energy of the first particle and evidence for the new even-even isotope 284 Fl undergoing fast spontaneous fission [5].The low cross sections detected in the experiments with 239,240 Pu targets compared to the larger values observed with 242,244 Pu targets indicate that the west shore of the Island of Stability has been identified, as illustrated in Fig. 3.

Irradiation of a mixed-Cf target with a 48 Ca beam
The irradiation of the ORNL mixed-Cf target with a 48 Ca beam started at the DGFRS at the beginning of October 2015.After nine days of bombardment at about 0.5 particle-A, a decay chain of the isotope 294 118 was observed through both analogue and digital data acquisitions [6].The observed decay properties were close to the previously identified four chains of this only known isotope of the heaviest known atomic element.Two decays followed by a Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements The dramatic drop of cross-section values was interpreted as being caused by the reduction of fission barriers with decreasing neutron number of the Fl compound nucleus [5].
spontaneous fission of 286 Fl were detected.The experiment continued with slowly increasing beam current, up to 0.7 particle-A.However, frequent checks of the target performance, through the observation of particles emitted from the target, began to show signs of target thickness increase, i.e., of a build-up of an extra layer of material at the target surface.Optical inspection of the targets indeed revealed a layer of material covering the electrodeposited mixed-Cf material.We are in the process of returning the mixed-Cf target to ORNL, for assessing the problem and refurbishing the target sectors.

Towards the predicted neutron shell closure at N = 184 and new elements
The properties of decays for nuclei produced in the reactions between radioactive actinide targets and a 48 Ca beam point clearly to the large enhancement of stability with increasing neutron number.See, e.g., reference [7].The largest neutron number reached so far is N = 177, in 293 Lv and 294 117 nuclei.While the spectroscopic evidence is in agreement with a predicted shell closure at N = 184, the experiment is far from this neutron number.However, use of heavier and more intense beams and the heaviest actinide targets might bring us closer to N = 184.For example, the reaction between a 58 Fe beam and a 251 Cf target could create the compound nucleus 309 124 in an excited state.The isotope 309 124 has N = 185 neutrons, i.e., one neutron added to the predicted spherical magic core.At known parts of the Segre chart such "core plus one particle" nuclei have a relatively small binding energy for this valence nucleon.One can expect that the evaporation of the 185 th neutron is highly probable.The resulting one-neutron evaporation product, the isotope 308 124 with N = 184 neutrons might have stability against particle evaporation and radioactive decay enhanced by its single-or even doubly-magic character.The decay chain starting from the 308 124 activity would reach the known 292 Lv isotope after four decays.The subsequent two particles of known properties will populate 284 Cn which undergoes spontaneous fission with about 90-ms half-life.It means there are several known spectroscopic signatures allowing Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements us to identify the decay of the new nucleus 308 124.The observation of the one-neutron evaporation channel in the 58 Fe+ 251 Cf reaction would constitute a revival of the "cold fusion" approach in the search for superheavy nuclei.Of course, the qualitative analysis presented here assumes that N = 184 is a good magic number for a new element with high atomic number, Z = 124.The HFB calculations summarized recently [8] support the magic character of N = 184 at Z = 124 and predict a very large fission barrier in this region of nuclei, of the order of 10 MeV.However, microscopic-macroscopic calculations [8] point to rather small fission barriers for the nuclei discussed, closer to 3 MeV, indicating that the magic character of N = 184 does not hold for such superheavy elements.As one can expect, the cross-section estimates vary dramatically with fission barrier parameter values.The very preliminary and rough estimates by Krystyna Wilczyńska indicate that the production yield for the xn-evaporation channels might be within experimental reach with the fission barrier parameter even slightly below 5 MeV [9].
However, the experimental attempt to identify the nucleus 308 124 and following decay products 304 122 and 300 120, i.e., three new elements including a N = 184 isotone, will be much better justified if an enriched 251 Cf target will be available.Therefore, having the actinide separation capability together with a new generation of accelerator laboratories can elevate superheavy element science from our current capabilities, for instance 294 118, toward further discoveries including several new elements and the magic 308 124 nucleus.

Figure 1 .
Figure 1.Left panel: One of the mixed-Cf target sectors electrodeposited on Ti foils at ORNL's REDC.Right panel: Twelve target sectors with required shielding arriving at JINR Dubna early 2015.

Figure 2 .
Figure 2. Energy spectrum of particles emitted from the fusion-evaporation products of the 48 Ca+ nat Yb reaction and measured using the new digital detection system at the DGFRS.The inset shows the decay pattern of particles emitted from a 1-s activity of 219 Th derived from collected traces of the DSSD preamplifier signals.

Figure 3 .
Figure 3.The cross section pattern observed for the Z = 114 Fl isotopes produced using a 48 Ca beam and four Pu targets, with mass A ranging from 239 to 244 (figure courtesy of Physical Review C).The dramatic drop of cross-section values was interpreted as being caused by the reduction of fission barriers with decreasing neutron number of the Fl compound nucleus[5].