Although neutron capture by stable isotopes has been extensively measured, there are very few measurements on unstable isotopes. The intense neutron flux at the Manual Lujan Jr. Neutron Scattering Center at LANSCE enables us to measure capture on targets with masses of about 1 mg over the energy range from 1 eV to 100 keV. These measurements are important not only for understanding the basic physics, but also for calculations of stellar nucleosynthesis and Science-Based Stockpile Stewardship. Preliminary measurements on {sup 169}Tm and {sup 171}Tm have been made with deuterated benzene detectors. A new detector array at the Lujan center and a new radioactive isotope separator will combine to give Los Alamos a unique capability for making these measurements

Fowler, M.

Haight, R.

Miller, G.

Rundberg, R.

Ullmann, J.

Wilhelmy, J.

UNT Digital Library

LA4JR- 98+284Approvedhrpubfic release;distributions untimitedAuthor(s):Submifted to:NEUTRON CAPTURE MEASUREMENTS ON UNSTABLENUCLEI AT LANSCEJohn UllmannRobert HaightJerry WilhelmyMalcolm FowlerRobert RundbergGeoff Miller15th International Conference on the Applications of Acceleratorsin Research and Industry, Denton TXNovember 4-7, 1998Los AlamosNATIONAL LABORATORYLos Alamos National Laboratory, an affirmative actiordequal opportunity employer, is operated by the University of California for the U.S.Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Governmentretains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S.Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under theauspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher’s right topublisly as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness.Form 836 (10/96)DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade namer trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.Neutron Capture MeasurementsJ.L. Ullmann,on Unstable Nuclei at LANSCER.C. HaightLAN$CE-3, Los Alamos National Laboratory, LOSAlamos, NM 87545M.M. Fowler, G.G. Miller, R.S. Rundberg, J.B. WilhelmyCST-11, LOS Alamos Natwnal Laboratory, Los Alamos, NM 87545Although neutron capture by stable isotopes has been extensively measured, there are very few measurements onunstable isotopes. The intense neutron flux at the Manual Lujan Jr. Neutron Scattering Center at LANSCE enables usto measure capture on targets with masses of about 1 mg over the energy range from 1 eV to 100 keV. Thesemeasurements are important not only for understanding the basic physics, but also for calculations of stellarnucleosynthesis and Science-Based Stockpile Stewardship. Preliminary measurements on 16%m and 17‘Tm have beenmade with deuterated benzene detectors. A new detector array at the Lujan center and a new radioactive isotopeseparator will combine to give Los Alamos a unique capability for making these measurements.INTRODUCTIONNeutron capture is one of the first reactions studied innuclear physics, and measurements have been made onvirtually all stable targets. Most cross sections on unstabletargets have been inferred from theoretical calculations.This is a difficult reaction to calculate, however, becausedetails of the nuclear structure of the compound nucleusnear the neutron binding energy are important. Evidencefor this is the order of magnitude differences in thermalneutron capture cross sections for adjacent and near-adjacent nuclei.In addition to fundamental scientific curiosity, severalapplied programs have a need for neutron capture crosssections in the keV region. Several facilities at LosAlamos combine to provide a unique capability to makethese measurements. I will describe a new detector arraybeing designed and built at the Los Alamos NeutronScience Center (LANSCE) to measure capture on smallquantities of unstable and stable nuclei., on the order of amilligram, and present some preliminary measurements ofcapture on *71Tm.NEUTRON CAPTURE REVISITEDDefense ApplicationsOne of the main drivers for this renewed interest in capturereactions is in understanding nuclear device performance.The extensive data associated with 50 years of nucleartesting is being systematically reviewed using modemanalysis and computational techniques, with a goal ofunderstanding nuclear weapons physics. One of theprincipal methods of diagnosing device performance hasbeen through the use of radiochemical tracer isotopes. Thehigh neutron density during an explosion can result inmultiple reactions on a single nucleus, driving it far fromstability. In order to calculate the observed isotopic yields,accurate cross sections are reauired for the nuclei involved,many of which are unstable. as-Process NucleosynthesisA second area of interest is in studying the stellarsynthesis of elements heavier than iron. ~is ~ believed tooccur primarily through neutron capture and ~ decay.These reactions take place in a “r” (rapid) process thatoccurs in the explosive environment of supernovae, and ina “s” (slow) process of sequential capture along the valleyof ~ stability that occurs in the “asymptotic giant branch”stage of evolution of low to medium mass stars, or in redgiants. The overall mechanism of the s process appears tobe understood, and solar and stellar abundances are ingeneral well reproduced (1,2).The flow of the s-process is illustrated in Fig. 1, formasses near A = 151. Of particular interest is thebranching of the flow at unstable nuclei, where thecompetition between neutron capture and ~ decay can shedinformation on the stellar environment where the s processoccurred. If the ~ decay rate in the hot stellar environmentis the same as in our laboratory, the neutron density at thes-process site can be inferred from observed abundancesand the neutron capture cross section. The most accurateabundances to use are those of nuclei that are formed onlyby the s-process.I. ,. .......... ........-’..152 Gd 153 Gd241 d;.,., ... j Y- -151 Eu\ 152 h\13y.148 Sm 149 Sm 15QSm\\93yr > . IFIGURE 1. S-process flow near mass 151. The branching at‘51Sm is of particular interest. Heavy outlines indicate “s-only”isotopes, and double outlines indicate unstable isotopes.Neutron densities and stellar temperatures extractedfrom branch-point analysis are given in Ref. 1. Theyappear quite consistent with a 22Ne(u.,n) neutron source at30 keV and a neutron density of 4 x 108 cm-3. However,this consistency is based on calculated values for neutroncapture cross sections on unstable nuclei. A comparison ofcalculated (3) and measured (4) capture cross sections forthe stable Sm isotopes shows that the even-even isotopesare calculated to 10% while the even-odd isotopes showpoorer agreement. A minimum 25% uncertainty is oftenassumed for the calculated values of the capture crosssection, and clearly more accurate measurements areneeded.Interestingly, although the measured parameters pointto a 30 keV 22Ne(a,n) neutron source for the s-process,detailed stellar models favor the 13C(ct,n) reaction, with anaverage energy of 10 keV, as the main neutron producingreaction driving thes process (1). Models predict that the sprocess occurs during a 20-year pulse of 13C burningfollowed by a 1-year pulse of 22Ne burning, which altersthe observed abundances. Thus while cross sections mustbe measured over the entire 10 keV Maxwell velocitydistribution characteristic of 13Cburning, the effect of thisepisode on the observed stellar abundances must bedetermined through detailed stellar models.TransmutationA third area of growing interest is field of accelerator-driven transmutation technology, in which neutronsproduced by an accelerator-chiven spallation source areused to transmute long-lived radioactive waste fromreactors into shorter-lived, easier-to handle isotopes. Anumber of capture cross sections need to be accuratelyknown, including capture on fission fragments such as WCand 135CS,and on lesser actinides such as 234U and 242Pu.Although some measurements do exist, it is likely thatmore experiments will be required.LOS ALAMOS CAPABILITIESThe ability to make these measurements requires aconfluence of capabilities that exists only at Los Alamos.The first of these is the intense neutron spallation source ofthe Manual Lujan Jr. Neutron Scattering Center (5). Thehigh neutron fluence available in the critical keV regionallows us to make direct measurements on samples withmass 1 milligram or less. Next is the ability to safelyhandle and produce radioactive targets. Los AlamosNational Laboratory supplies many of theradiopharmaceutical isotopes distributed by the U.S.Department of Energy, and most of these are produced byat LANSCE by proton spallation on heavy targets. TheNuclear and Radiochemistry group at Los Alamos (CST-11) maintains hot cells for handling the production targetsand chemically separating isotopes of interes~In addition to chemical separation, a magnetic isotopeseparator, the Radioactive Species Isotope Separator(RSIS) has been specifically designed and built to separatehighly radioactive isotopes. The RSIS will be housedentirely in a hot cell. Test operation of the separator hasshown separation factors between adjacent isotopes ofabout 104. As an example, it will take about 1 to 3 weeksto produce 1 mg of 170Tm, depending on the efficiency ofthe ion source, which ranges from about 10 to 30% for Tm.Finally, a new detector array is being constructed. Thiswill be discussed further below.PRELIMINARY MEASUREMENTSA preliminary experiment to measure neutron capturecross sections on a radioactive target used the isotope171Tm, which has a half-life of 1.9 yr. This isotope waschosen for several reasons. First, milligram quantities ofisotonically pure *71Tm were produced by reactorirradiation of 17kr and chemical separation by high-performance liquid chromatography, without the use of anisotope separator. Second, the low-energy of the ,decays(97 keV B, 66 keV y) presented the lowest radiologicalhazard of the possible targets. The target was electroplatedon a 12.7 pm Be foil which had a 70 nm layer of titaniumvapor-deposited on the surface. This was covered by asecond 12.7 pm Be foil.The experiment was done on Fiight Path 4 of the LujanCenter at LANSCE, which views the “high intensity”water moderator. The detectors were two C6D,Sscintillators, each 5 inches diameter by 3 inches thick,mounted on an RCA 4522 phototube. The detectors weremounted inside a 30cm x 30 cm x 10 cm thick Pb shieldinside a larger polyethylene house. The flight path was8.08 m. The results of the measurement are shown in Fig.2. The results are compared to calculations(6) made withthe GNASH code, which underpredicts the data by afactor of 2 to 5. A target of stable “hin was alsomeasured, and the results agreed very well with the.GNASH calculations, which used parameters determinedfrom an earlier measurement of capture on ‘b9Tm byMacklin (7)..0.\1.E-WI I I I.1.E-02 I1 10 100 1000 IOOcO 1oOOOoNeutron Energy (eV)FIGURE 2. Cross section measured for the radioactive target171Tmcompared to the GNASH theoretical calculation (6).,In this configuration, the signal from the 171Tm wasonly about 10% above the background from a blank Be foilassembly. Comparison of the background to the count ratewith no foil indicates that neutron scattering wasresponsible for part of the gamma background. Thebackground will be reduced in future measurements bydecreasing the mass near the detector array and improvingthe shielding against neutrons from adjacent flight paths.DANCEThe results of the preliminary experiment wereencouraging and led to a proposal for a new detector arrayand flight path at the Lujan Center. The new detector willbe located at 20 m on Flight Path 14, which views thenewly installed “upper tier” water moderator. Thismoderator will have about half the flux of the high-intensity water moderator on Flight Path 4. However,planned improvements to the accelerator ion source andbeam delivery are expected to eventually put up to 200 @on the spallation target, about a 3X increase over thecurrent proton cument. By using different collimationwhich will view the entire moderator surface, the neutronflux at 20 m will actually be greater than on Flight Path 4at 8 m.In addition, a new detector array, the Device forAdvanced Neutron Capture Experiments (DANCE) isbeing designed, and prototype detectors and collimationwill be tested at LANSCE in fall, ’98. The array will be,by necessity, fast and highly segmented. A 2 Ci target,which was the activity of the 17*Tm target reported above,has 7.4 x 1010decays/second, or 740 decays in a 10 ns bin.To get one count per detector in a 10 ns period, assuming20% efficiency, would require 148 detectors. This countrate will present interesting challenges. A capture eventproduces about 3 to 10 gammas. The segmentation of thedetector array will enable multiplicity to be used to helpdiscriminate against decay gammas.The leading candidate for detector material is BaFz,which has a 0.6 ns component that peaks in the ultravioletat 220 nm. It also has a slow component that produces fivetimes as much light, so filters or phototubes with specialphotocathodes will be required. We note that scintillatorscontaining Cs or I have a large neutron capture crosssection even at 10 keV, and would therefore produce anunacceptable background. Neutron absorbers, usedsuccessfully by others, do not appear to be effective above1 keV (8).DISCUSSIONWe are designing and building a new detector array formeasuring neutron capture on small quantities of unstableisotopes. These measurements will provide valuabIe crosssections for applied programs and nuclear astrophysics. Apartial array is expected to be operational in 1999, and weare very excited about the prospects for physics with thisdetector.ACKNOWLEDGEMENTSWe would like to acknowledge P.E. Koehler forpioneering these measurements at LANSCE. This workwas supported by the USDOE Science-Based StockpileStewardship program, and made beneficial use of theneutron beams at LANSCE.REFERENCES1. Wallerstein, G., et al., Rev. Mod. Phys. 69,995 (1997).2. Kappeler, F., Beer, H., Wkshak, K., Rep. Prog. Phys. 52,945(1989). Kappeler, F., Gallino, R. Busso, M., Picchio, G.,Raiteri, C.M., Ap.J. 354,630 (1990).3. Toucan, K.A., et al., Phys. Rev. C 51, 1540 (1995).4, Wisshak, K., et al., Phys. Rev C 48, 1401 (1993).5. For a description of the Manual Lujan Jr. Neutron ScatteringCenter, see http//www.lansce.lanl. gov.6. Young, P.G., private communication.7, Macklin, R. L., Drake, D. M., Malanify, J.J., Arthur, E.D.,Young, P. G., Nucl. Sci. Erg. 82, 143( 1982).8. Wisshak, K., et al., “The Karlsruhe 4X BaF2 Detector,”KFK 4652, 1989.

Neutron Capture Measurements on Unstable Nuclei at LANSCE

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Neutron Capture Measurements on Unstable Nuclei at LANSCE

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