[en] Cross sections for compound-nuclear reactions are required for many applications. The surrogate nuclear reactions method provides an indirect approach for determining cross sections for reactions on unstable isotopes, which are difficult or impossible to measure otherwise. Current implementations of the method provide useful cross sections for (n,f) reactions, but need to be improved upon for applications to capture reactions.

[en] Cross sections for compound-nuclear reactions play an important role in models of astrophysical environments and simulations of the nuclear fuel cycle. Providing reliable cross section data remains a formidable task, and direct measurements have to be complemented by theoretical predictions and indirect methods. The surrogate nuclear reactions method provides an indirect approach for determining cross sections for reactions on unstable isotopes, which are difficult or impossible to measure otherwise. Current implementations of the method provide useful cross sections for (n,f) reactions, but need to be improved upon for applications to capture reactions.

[en] The prospects for determining cross sections for compound-nuclear neutron-capture reactions from Surrogate measurements are investigated. Calculations as well as experimental results are presented that test the Weisskopf-Ewing approximation, which is employed in most analyses of Surrogate data. The method is applied to the ¹⁵⁵Gd(n,γ) reaction. It is concluded that, in general, one has to go beyond this approximation in order to obtain (n,γ) cross sections of sufficient accuracy for most astrophysical and nuclear-energy applications. (authors)

[en] The Surrogate nuclear reactions method, an indirect approach for determining cross sections for compound-nuclear reactions involving difficult-to-measure targets, is reviewed. Focusing on cross sections for neutron-induced reactions on actinides, we review the successes of past and present applications of the method and assess its uncertainties and limitations. The approximations used in the analyses of most experiments work reasonably well for (n,f) cross sections for neutron energies above 1-2 MeV, but lead to discrepancies for low-energy (n,f) reactions, as well as for (n,γ) applications. Correcting for some of the effects neglected in the approximate analyses leads to improved (n,f) results. We outline steps that will further improve the accuracy and reliability of the Surrogate method and extend its applicability to reactions that cannot be approached with the present implementation of the method.

[en] The extraction of ⁷³As, ⁷⁵Se, ^197mHg, ²¹²Pb and ²¹⁰Po on Eichrom pre-filter resin was studied in HCl and HNO₃ with batch uptake, kinetic and column experiments. There was significant extraction of mercury in HNO₃ and HCl, and polonium and selenium in HCl. The kinetics are sufficient to allow for retention on pre-filter columns, and separations of ⁷³As from ⁷⁵Se and ^197mHg, and ²¹²Pb from ²¹⁰Po are demonstrated. This retention is important for the development of chemical separations as pre-filter resin is commonly used alongside extraction chromatography resins but its potential for uptake of metal ions is not well characterized. (author)

[en] The main goal of the test measurement was to determine the feasibility of the ²⁴³Am(p,t) reaction as a surrogate for ²⁴⁰Am(n,f). No data cross section data exists for neutron induced reactions on ²⁴⁰Am; the half-life of this isotope is only 2.1 days making direct measurements difficult, if not impossible. The 48-hour experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in August 2011. A description of the experiment and results is given. The beam energy was initially chosen to be 39 MeV in order to measure an equivalent neutron energy range from 0 to 20 MeV. However, the proton beam was not stopped in the farady cup and the beam was deposited in the surrounding shielding material. The shielding material was not conductive, and a beam current, needed for proper tuning of the beam as well as experimental monitoring, could not be read. If the ²⁴⁰Am(n,f) surrogate experiment is to be run at LBNL, simple modifications to the beam collection site will need to be made. The beam energy was reduced to 29 MeV, which was within an energy regime of prior experiments and tuning conditions at STARS/LIBERACE. At this energy, the beam current was successfully tuned and measured. At 29 MeV, data was collected with both the ²⁴³Am and ²³⁸U targets. An example particle identification plot is shown in Fig. 1. The triton-fission coincidence rate for the ²⁴³Am target and ²³⁸U target were measured. Coincidence rates of 0.0233(1) cps and 0.150(6) cps were observed for the ²⁴³Am and ²³⁸U targets, respectively. The difference in count rate is largely attributed to the available target material - the ²³⁸U target contains approximately 7 times more atoms than the ²⁴³Am. A proton beam current of ∼0.7 nA was used for measurements on both targets. Assuming a full experimental run under similar conditions, an estimate for the run time needed was made. Figure 2 shows the number of days needed as a function of acceptable uncertainty for a measurement of 1-20 MeV equivalent neutron energy, binned into 200 keV increments. A 5% measurement will take 3 days for U, but 20 days for Am. It may be difficult to be the sole user of the LBNL cyclotron, or another facility, for such an extended period. However, a 10% measurement will take 19 hours for U, and 5 days for Am. Such a run period is more reasonable and will allow for the first ever measurement of the ²⁴⁰Am(n,f) cross section. We also anticipate 40% more beam time being available at Texas A and M Cyclotron Institute compared to LBNL in FY2012. The increased amount of beam time will allow us to accumulate better statistics then what would have been available at LBNL.

[en] The separation of zirconium and hafnium isotopes from the early actinides and rare earth elements (REE) with Eichrom's Pb resin has been studied. Batch studies were performed to characterize the behavior of actinium, thorium, zirconium, hafnium, lutetium, and yttrium on Pb resin from HCl solutions (0.001 M to 11 M). The early actinides and REE had no affinity for the resin at any concentration of HCl, but zirconium and hafnium showed a moderate uptake at high concentrations of HCl with a maximum extraction at 11 M HCl. Several column separations were tested, including with only tracer isotopes and with mass. Rapid, simple separations of zirconium from actinium, thorium, protactinium, and the REE with high yields and low elution volumes are presented with applications for tracer isotope production and fission product separations. The resin is less suitable for hafnium separations as hafnium tends to bleed off the resin even at high concentrations of HCl. (author)

[en] The uptake behavior of ⁷³As and ⁷⁵Se was studied in HCl and HNO₃-H₂O₂ solutions on commercial extraction chromatography resins (TEVA, TRU, DGA, and Pb resin). There was no uptake of ⁷³As or ⁷⁵Se from HNO₃ media. From HCl, there was ⁷⁵Se uptake at high concentrations on all the resins and no uptake of ⁷³As. Separations of ⁷⁵Se and ⁷³As on these resins have high yields and high radiopurity for ⁷³As, but limited recovery of ⁷⁵Se. TRU and TEVA resin may have potential for use in isotope generators as ⁷⁵Se can be retained for at least 26 days with repeated elutions. (author)

[en] The uptake of ⁷⁵Se and ⁷³As on trioctylamine-impregnated resin from HCl solutions was studied with batch uptake, kinetics, and column studies. Selenium-75 extracts well (D_w ∼ 900) at high HCl concentrations (≥ 10 M) with and no extraction of ⁷³As at any HCl concentration. The uptake kinetics were slow with low extraction (D_w < 100) for time periods ≤ 15 min and no clear equilibrium achieved even after 24 h. Column studies were performed to characterize the separation of ⁷⁵Se and ⁷³As; ⁷⁵Se is retained from conc. HCl while arsenic is eluted with high yields and high radiopurity. (author)

[en] The uptake behavior of ⁷³As and ⁷⁵Se was studied in HCl and HNO₃ media with CL resin, ethanethiol resin and dimethyl sulfide resin. In HCl solutions, there was high extraction of selenium on CL resin and ethanethiol resin but lower uptake on dimethyl sulfide resin; arsenic was only extracted by CL resin. From HNO₃, selenium was extracted only by ethanethiol resin and CL resin and there was no uptake of arsenic on any resin. While the sulfur-based resins have high selenium uptake and selectivity, column separations are challenging due to the chemical instability of these resins. (author)