Techniques for obtaining neutron induced reaction cross sections at RIA

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1 Unclassified Techniques for obtaining neutron induced reaction cross sections at RIA L. A. Bernstein, L. Ahle LLNL American Chemical Society Meeting March 27, 2003 New Orleans, LA Unclassified This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

2 Reactions on unstable nuclei and radiochemistry 13.9 ms kev 48 m kev 13.37h kev ms kev s kev h 79.8 h d 86 Y 87 Y 88 Y 89 Y The Yttrium Reaction Network Most reactions in a radchem network take place on radioactive targets The vast majority of these cross sections are unmeasured (with some heroic exceptions) stable Many of the cross sections need to be measured well

3 Many neutron-induced reaction cross sections on radioactive nuclei will be determined indirectly 10 9 Direct Measurements Half Life (seconds) Now DANCE Now/Near Future Surrogate/Indirect Measurements Future Future This is where RIA holds the greatest promise 10-6 NIF Assumptions pps production rate n/s on target Z

4 Indirect Measurements Surrogate neutron-induced reaction measurements Turns a very hard to impossible measurement into a do-able experiment plus a model calculation Pioneered in the 70 s by Britt & Wilhelmy New LLNL results Improving reaction models using physical data measurements (not enough time) Level densities and γ-ray strength functions Latter is very important for (n,γ) reactions Discrete state spectroscopy (including spin and parity) Also helps to improve optical model calculations for surrogate cross sections.

5 Surrogate neutron-induced reactions using charged particle beams n 155 Gd α 157 Gd 3 He Neutron-induced reaction 156 Gd** Surrogate reaction P γ P 2n σ n,x = P x σ absorption ( 3 He, αγ) 156 Gd* P n ( 3 He, αnγ) ( 3 He, α2nγ) 154 Gd* (n,γ) 155 Gd* (n,n ) (n,2n) From Optical Model Calc.

6 Silicon Telescope Array for Reaction Studies coupled to GAMMASPHERE 157 Gd( 3 He,α / 3 He) Gd at E( 3 He) = 45 MeV 3 day run, Average Current = pna GAMMASPHERE STARS E-E Particle ID Charged particle E E First experiment completed: 4/02

7 Surrogate (n,γ) from STARS ( 3 He,α) data normalized using the α-particle spectrum Intensity (Arbitrary Units) STARS data ( 156 Gd 4 2) ENDF/STAPRE Absorbtion Cross Section Equivalent Neutron Energy (MeV) Excellent agreement with the measured (n,γ) cross section

8 Surrogate (n,n ), (n,2n) from STARS data compared to STAPRE calculations To compare STAPRE and STARS we use the formulas: σ (n,xn) /σ absorbtion =I γ (n,xn)/ {Σ parallel I γ (n,xn) + Σ parallel I γ (n,yn)} 1.2 σ (n,xn) /σ absorbtion = 1/{1+(Σ parallel I γ (n,yn)/ Σ parallel I γ (n,xn)} Surrogate (n,n')/{(n,n')+(n, γ)} Surrogate (n,2n)/{(n,2n)+(n,n')} ( Gd/ 155 Gd) ( 3 He, 3 He ) Equivalent Neutron Energy (MeV) ( Gd/ 156 Gd) ( 3 He,α) Equivalent Neutron Energy (MeV) Excellent agreement for both ( 3 He, 3 He ) and ( 3 He,α) reactions

9 Comparison of STAPRE * (n,2nγ)/[(n,2nγ)+(n,n γ)] and STARS ( 3 He,α2nγ yrast )/[( 3 He,αn γ non-coincident )+ ( 3 He,α2nγ yrast )] data Gd γ-ray/sum(155gd γ-rays) Equivalent Neutron Energy (MeV) NOT SCALED!!! 154Gd γ-ray/sum(155gd γ-rays) *Courtesy of R.D. Hoffman - NTM Group

10 Measuring level densities and γ-strength functions to improve reaction models - (A. Schiller) Technique developed at the University of Oslo Simultaneously measures both level density and γ-ray strength functions using light-ion induced reactions Academic Alliance Partners: NC State University γ-ray energy (MeV) γ-ray energy (MeV) Direct impact on (n,γ) rate

11 The Future: Surrogate measurements on prompt fission fragments The total production of a specific A-chain decay daughter ( 95 Zr) depends on the production and destruction of its (grand)parent nucleus ( 95 Sr) with neighboring A-chains: N( 95 Sr) ~ σ (n,2n) ( 96 Sr) + σ (n,γ) ( 94 Sr) - σ (n,2n) ( 95 Sr) - σ (n,γ) ( 95 Sr) These cross sections are impossible to measure directly due to the short lifetime of the nuclei involved (t 1/2 ( 95 Sr) = 25.1 s). Surrogate reactions on these nuclei are possible using radioactive beams in inverse kinematics (d( 95 Sr, 96 Sr)p). ORNL has these beams now (weak), but TRIUMF will have them at higher intensity after 2005 (ISAC-II) 96 Zr β 96 Y 95 Zr β β 96 Sr 95 Y β β 95 Sr 94 Y β 94 Sr Can be measured using 95 Sr RIB 94 Zr

12 Direct Neutron Measurements Radiochemical decay counting experiments (Ahle, McMahan talks) Very high target purity required (<1:10 9 product) Lifetime limits set by neutron beam intensity and production rate Prompt Measurements Direct γ-ray emission - DANCE (R. Rundberg talk) High γ-ray background limits these experiments to long lifetimes, low background Radioactive Targets (RaTs) Only approach for many (n,γ) cross sections Charged particle emission: (n,p xn), (n,α xn) Less sensitive to background, can be magnetically filtered Neutron emission: J. Frehaut type measurements An old idea revisited. Quite promising.

13 A large portion of all neutron-induced reactions have charged particles in the exit channel Charged-particle-out/Total Cross Section HMS-ALICE Calculations 1 n V n + 88 Y Neutron Energy (MeV)

14 Charged Particle Detector for (n,p xn) or (n, α xn) reactions Limited mass near target to lessen neutron scattering Use weaker magnetic fields and larger radii Limited (preferably no) substrate material in beam. Difference measurement required otherwise Some measurements without magnetic filters possible. Limited target γ-ray production Target Neutron Beam Uniform Baffle Magnetic Field Si particle Detector` β trajectory decay α trajectory Reaction p-, α trajectories Side View

15 (n,xp) and (n,xα) cross sections on radioactive targets (RaTs) using magnetic filters Most background from a RaT are either low-energy α-, β- or a wide range of γ-rays. Little background from high energy charged particles Magnetic fields can be used to measure chargedparticle-out channels from prompt beam-induced reactions with low target purity requirements (1:100) Example: a 87m Y (79.8 h) target for (n,np), (n,α) etc. 1 ma p run for 3 days on a 180 mg/cm 287 Sr target: 1.6 x atoms = µg = 13 Curies!!! Neutron beam of neutrons on target (see later) Total counts assuming 500 mb σ charged-particle = 2 x x 10 5 counts

16 The Charged Particle Spectrometer Equation of Motion for Protons E p = 1/2 M p v p 2 Radius of curvature: < 40 cm for evaporation neutrons. > 50 cm for preequilibrium neutrons. Smaller size leads to lower efficiency. Entire volume should be in vacuum. M p v p 2 /R = ev p B R = M p v p /eb Radius for 1 Tesla (cm) Nb(n,px) proton evaporation spectrum Radius for 1 Tesla (m) # of Protons (PACE) Proton Energy (MeV)

17 Prompt Neutron Measurements (J. Frehaut et al.,) Use Collimated Neutron Beam and Gd-imbued Scintillator Excellent efficiency (75%) No neutron background for nuclei w/o fission Collimators Scintillator Neutron Beam Flux Monitor High Z Β/γ-shield Scintillator Sample The Ahle source would have to be 5+ orders of magnitude more intense than the one used in the earlier measurements

18 The approach of choice for (n,γ) cross sections: Total γ-ray calorimetry (a la DANCE) A ball of BaF detectors + a neutron source Near 4π coverage (limits angular distribution effects). Excellent high-energy γ-ray efficiency. Large number of detectors to minimize pile-up from target decay. Requires that background from decay γ-rays doesn t swamp signal γ-ray detectors Neutron Beam Radioactive Target Ready for experiments

19 What RIBs and RaTs can do for radiochemical interpretation RIA (or NIF) Indirect Measurement Direct RaT or Indirect RIB Measurement Done (GEANIE + Prestwood et al.) 13.9 ms kev 48 m kev 13.37h kev ms kev s kev h 79.8 h d stable 86 Y 87 Y 88 Y 89 Y RIBs and RIA fills in the missing pieces

20 The Future: A=90 neutron monitor surrogate cross sections at Yale (Academic Alliance) # n-induced reaction Charged particle surrogate reaction Reason 1 2 n+ 89 Yb 90 Yb+x (Studied at GEANIE) 89 Y (d,px) 88 Sr ( 3 He,px) Best candidate reaction (same ν-doorway state, similar J). Different doorway state (d instead of p) Zr (d,px) See # 1. n+ 90 Zr 91 Zr+x 4 (Studied at GEANIE) 92 Zr ( 3 He, 4 He) Different angular momentum plus ν-hole instead of a ν-particle We are exploring this technique now

Harvesting Isotopes For Neutron Cross-section Measurements at RIA

Harvesting Isotopes For Neutron Cross-section Measurements at RIA Larry Ahle and Lee Bernstein Lawrence Livermore National Laboratory ACS Symposium on Radiochemistry at RIA New Orleans, CA March 27, 2003