Neutron-induced reaction cross sections measured at Triangle Universities Nuclear Laboratory John Kelley North Carolina State University and Triangle Universities Nuclear Laboratory Supported in part by the NNSA under the Stewardship Science Academic Alliances Program DOE grant #DE-PS52-05NA25930, and by DOE DE-FG02-97ER41033 (Duke) and DE-FG02-97ER41042 (NC-State).
Anton Tonchev Group Leader
Neutron Production DENIS Shielded Neutron Source Area Neutron production D(d,n) neutron source φ n = 3x10 4 n /(cm 2 sec) Tunable from 4 18 MeV Pulsed at 2.5 MHz
Neutron Collimator and Beam Profile Shielded Neutron Source Area Neutron profile scan d=215 cm d=427 cm d n 75 cm Scanning conditions step: 0.3 cm in and direction
NNSA Setup at TUNL 4 Clovers + BGO 2 Planars + BGO 10 kev < E γ < 10 MeV n-beam 20 0 < θ lab < 160 0 ε array = 1.4%@E γ = 1.33 MeV Capabilities γ-γ coincidence measurements Angular distribution measurements Lifetimes (by Doppler method) Excellent tool for precision neutron induced cross section measurements in the fast energy region (4 E n 18 MeV)
Beam Correlated Measurements TUNL pulsed n-beam Start Stop n-beam ~2ns ON / 400ns Off Counts 1800 1200 600 600 400 200 6000 400 200 Fe U * Fe U Fe U * * * * * * * Fe * * U * 200 400 600 80 0 G 200 400 600 80 0 G-BG TOF TOF Fe TOF * 200 400 600 800 Fe U * Fe 800 900 1000 1100 1200 1300 Energy (kev)
Selected Experiments Time Reaction Energies n [MeV] [MeV] May 2003 238 U(n,2n ) 237 U 6, 10 Aug 2004 238 U(n,2n ) 237 U 8, 10, 14 Feb 2005 90 Zr(n,n' ) 90 Zr 6 Feb 2005 89 Yb(n,n' ) 89 Yb 6 Feb 2005 112 Sn(n,n' ) 112 Sn 6.5, 7.5, 8.0 Feb 2005 124 Sn(n,n' ) 124 Sn 6.5, 7.5, 8.0 May 2005 235 U(n,n' ) 235 U 5 Jun 2005 235 U(n,2n' ) 234 U 12 Jun 2005 nat Hf(n,xn' ) 12 Jun 2005 16 O(n,n' ) 16 O 7 Jun 2005 12 C(n,n' ) 12 C 7 Dec 2005 235 U(n,2n' ) 234 U 12 Time Reaction Energies n [MeV] [MeV] Jan 2006 235 U(n,2n' ) 234 U 10, 8 Jan 2006 181 Ta(n,2n' ) 180 Ta 14.5 Feb 2006 140 Ce(n,2n' ) 139 Ce 14.5 Mar 2006 94 Zr(n,n' ) 5.0 Mar 2006 241 Am(n,2n) 8-14 Jun 2006 nat Cu, nat Pb(n,n ) 8,12 Oct 2006 235,238 U(n, 2n ) 10 Nov 2006 241 Am(n,2n) 8-16 Dec 2006 241 Am(n,2n) 8-16 Dec 2006 235 U(n, 2ng) 12 Jan 2007 235 U(n,2ng) 8 Feb 2007 235 U(n,2ng) 10 Collaborators from 2 National Labs and 3 Universities www.tunl.duke.edu/groups/nnsa
Participants
94 Zr(n,n γγ): Mixed symmetry States In early 2006 we collaborated with Steve Yates et al., from the University of Kentucky to carry out a study of mixed symmetry states in 94 Zr. In this experiment we utilized the high efficiency of our clover array, by measuring 94 Zr(n,n γ γ) two γ-ray coincidence data.
neutron-induced backgrounds for deep underground experiments Open questions that can be addressed in 2β2 decay studies : Are n Majorana particles? (self anti-particles) Is lepton number conserved? (Standard Model) Σ 2039 kev
neutron-induced backgrounds for deep underground experiments We are measuring partial cross sections for neutron-induced γ-ray transitions on Cu, Ge,, and Pb.
Precision Measurements of the (n,2nγ) Reaction Cross Sections on Actinides Goal: Improve partial cross section data of actinide nuclei for the Stockpile Stewardship Program Emphasis : The 239 Pu(n,2nγ) cross section Technique: High-resolution γ-ray spectroscopy measurements: in beam with pulsed and monoenergetic neutron beam off-line with DC neutron beam Modeling: Hauser-Feshbach theory (GNASH, EMPIRE)
Collaborators* TUNLke: Duke: A.S. Crowell B. Fallin C.R. Howell A. Hutchison A.P. Tonchev W. Tornow Univ. of N. Carolina: H.J. Karwowski C.T. Angell Livermore J.A. Becker D. Dashdorj * J. Kenneally R.A. Macri D. Shaughnessy M.A Stoyer Ch.Y. Wu Los Alamos E. Bond J. FitzPatrick R.S. Rundberg A. Slemmons D.J. Vieira J.B. Wilhelmy N. Carolina State: J.H. Kelley D. Dashdorj * NC A&T: R. S. Pedroni *(n,2n) measurements supported by the NNSA Stewardship Science Academic Alliance and by DOE/NNSA LANL & LLNL
Participants
First Results from 238 U(n,n γ) 238 Uat TUNL Partial Cross Sections at E n = 6 MeV Egamma Transition TUNL OakRidge 1) LANL 2) (kev) (mb) (mb) (mb) 635.2 1- --> 2+ 83 (4) 76(7) 67(2) 687.0 3- --> 2+ 86 (4) 80(7) 106(2) 885.5 (1-) --> 2+ 63 (4) 56(7) 79(2) 905.5 2- --> 2+ 34 (3) 33(5) 40(2) 957.3 3+ --> 4+ 50 (3) 45(5) 65(2) 1060.3 2+ 0+ 95 (4) 93(11) 116(3) 1) D.K. Olsen, et al., ORNL report TM-6832 1979. 2) N. Fotiades et al., PRC 69 (2004) 024601. A. Hutcheson et al. NIMB (2007) in press.
Neutron Induced Cross Section on 235 U E n = 10 MeV, Beam time = 87 h 234 U
241 Am(n,2n) Reaction Cross Section Measurements Improved 241 Am(n,2n) cross sections are needed for: Nuclear forensics A( 240 Am/ 241 Am) Analogous to P ( 238 Pu/ 239 Pu) Sensitive to high-energy neutrons Advance Fuel Cycle / new fast reactors Important in transmutation studies Goal: 5-10% measurement from threshold (6.7 MeV) to 14.5 MeV
241 Am(n,2n) 240 Am Measurements (n,2n) data for 241 Am around 14 MeV Energy dependence near 14 MeV No data below ~13 MeV Different evaluation
241 Am(n,2n) 240 Am Measurements (n,2n) data for 241 Am around 14 MeV Energy dependence near 14 MeV No data below ~13 MeV Different evaluations The recent data shows very high (n,2n) CS at E n 11MeV Phys. Rev. C 73, 067601 (2006) Need to extend the 241 Am(n,2n) 240 Am cross section to lower energies
241 Am(n,2n) 240 Am Measurements Activation technique: γ count 240 Am activity after irradiation of 241 Am trick: attenuate the low-energy γ-rays (59.5 kev) coming from 241 Am target using 3 mm Pb Irradiated target with mono-energetic neutrons produced by D(d,n) reaction TUNL E n = 7.6-14.5 MeV / φ n ~ 10 7 10 8 n/cm 2 -s ~1 mg/cm 2 241 Am target / 1 cm dia. sandwich targets on Pt backing Measure neutron fluence using a set of witness foils mounted before & after target 197 Au(n,2n) 196 Au (t 1/2 = 6.2 d), T= 8.1 MeV 27 Al(n,α) 24 Na (t 1/2 = 15 hr), T= 3.2 MeV 58 Ni(n,p) 58 Co (t 1/2 = 71 d), T= 0 MeV 58 Ni(n,2n) 57 Ni (t 1/2 = 36 hr), T= 12.4 MeV 987.8 kev 888.8 25.1(4)% 73.2(10)% 240 Am 99.6(11)% 4 + ; 141.8 2 + ; 42.9 240 Pu 0 + ; 0.0 kev 3 + ; 1030.6, T 1/2 = 1.32 ns
Activation Measurements at TUNL DENIS Neutron TOF area Preparation room Activation parameters φ n = 1x10 8 n /(cm 2 sec) ~ 1mg 241 Am targets 3 monitor foils (Al, Ni, Au) n-flux monitoring: 3 neutron detectors (0 0, ± 10 0 ) Irradiation setup
Activation Measurements at TUNL Irradiate highly enriched 241 Am targets Low-background counting system 60% HPGe detectors Genie 2000 spec. system
High-Resolution Gamma Ray Spectra Plastic ring Foil 8 Foil 9 Plastic ring 4 4 4 4 513 µg/cm 2 241 Am 0.5 mil Pt 0.5 mil Pt 519 µg/cm 2 241 Am Fabrication of 241 Am target (LANL and LLNL) 3 stippled targets (Evelyn Bond/LANL) 6 electroplated targets after additional chemical purification (Mark Stoyer/LLNL) ~ 2% statistical uncertainties Improved γ-spectra from the electroplated targets Need for nuclear radiochemists
First Results on 241 Am(n,2n) 240 Am Reaction Cross Section
First Results on 241 Am(n,2n) 240 Am Reaction Cross Section
First Results on 241 Am(n,2n) 240 Am Reaction Cross Section A. Tonchev et al. PRC in preparation.
Summary Intense, pulsed and quasi-monoenergetic neutron beams at TUNL provide capabilities for Nuclear Physics Research. Application motivated basic research 1. Ongoing analysis of 235,238 U(n,2nγ) (A. Hutcheson poster) 2. Cross section measurements of the 241 Am(n,2n) 240 Am reaction. Curiosity driven research 1. n-induced backgrounds in deep underground experiments 2. Nuclear Structure: 94Zr(n,n 2γ) Strong TUNL-LANL-LLNL Collaboration Effort
Extra Viewgraphs
Decay Scheme of Am Isotopes
240 Am Gamma-ray Signature - 889 kev line is partially contaminated by a weak 241 Am line, but the 988 kev line is clean - Measured 2000-3000 γ events after a 1-2 day irradiation and 1day count time - Measured 11 energies in March, Nov., Dec. 2006 runs
Neutron Monitors n-monitors / BCI n-flux fluctuation T act = 12.5 h
NNSA Synergistic Activities at TUNL NNSA (n,xnγ) activity: low-energy introduction to nuclear experimental techniques for the Research Experience for Undergraduates (REU) program at TUNL Average 8-10 REU students every summer For the past three years one or two has been involved in the TUNL NNSA project Diversification of the NNSA setup for basic research: nuclear structure nuclear astrophysics neutrino and dark matter