Joint ICTP-IAEA Workshop on Nuclear Structure Decay Data: Theory and Evaluation August Experimental Nuclear Physics: Part 2

Transcription

1 Joint ICTP-IAEA Workshop on Nuclear Structure Decay Data: Theory and Evaluation 6-17 August 2012 Experimental Nuclear Physics: Part 2 E. Ricard McCutchan Brookhaven National Lab. USA

2 Experimental Nuclear Physics: Part II E.A. McCutchan National Nuclear Data Center Brookhaven Nation Laboratory

3 Measuring Lifetimes Nitty gritty details of a lifetime experiment

4 A huge range to cover And others : Coulomb excitation NRF Direct width measurements Inelastic electron scattering Mossbauer technique GRID

5 Lifetime Why do we care?? B(E) or B(M) Overlap of initial and final state wavefunctions If B(E2) is big, is small (i.e. fast = collective); If B(E2) is small, is big (i.e. slow = isomer)

6 Systematics of B(E2)s

7 Measuring long lifetimes Time range : seconds to years Observe the activity, A, as a function of time +/- Count s or s or s following decay

8 Can reach incredible precision T 1/2 = (5) seconds T 1/2 ~ 0.012%

9 Electronic measurements Time range : seconds to few ps Beam on Start Beam off Collect Beam on Re start PROMPT gammas Isomeric state DELAYED Gammas Log N PROMPT Log N DELAYED

10 Electronic measurements Time range : seconds to few ps 1 ns 800 ns PROMPT gammas Isomeric state Shortest T 1/2 : limited by width of beam Longest T 1/2 : limited by time between pulses DELAYED Gammas Singles : beam--t Coinc : beam---t

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12 Recoil Distance Doppler Shift Method Beam Target E sh E sh E o E Stopper Lifetimes of ~ 1 to 1000 ps o (RDM) v ( 1 cos ) c Counts / 0.5 kev Counts / 0.5 kev Counts / 0.5 kev m m Hf : shifted unshifted 200 m Energy (kev)

13 New Yale Plunger Device motor crystal target stopper beam NYPD micrometer Developed by A. Dewald for RDDS method Piezo electric motor & crystal to move foils Capacitive feedback loop maintains distances to μm μm

14 A model for phase transitions Vibrator Transitional Rotor V() V() V() F. Iachello, Phys. Rev. Lett. 85, 3580 (2000); 87, (2001). Critical Point New analytical solution X(5)

15 Lifetimes in 162 Yb and 166 Hf Yrast level lifetimes for 4 +, 6 +, and 8 + states Most values consistent with previous measurements Significant reduction in uncertainty B(E2; J J-2) / B(E2, ) Yb Vibrator X(5) Rotor J 166 Hf Vibrator X(5) Rotor J

16 A variation on RDM Target Degrader Beam v 1 v 2 E sh E o To recoil separator

17 Doppler Shift Attenuation Method (DSAM) v E Eo[ 1 cos( )] c Intensity Intermediate 0 E o unshifted E full shift Energy Thin target with thick backing to slow/stop recoils Line shape depends on nuclear lifetime Short lifetime: Full shift Long lifetime: No shift

18 Musical medley of 10 Be measurements Physics questions we want to address How to improve over existing measurements Technological advances Re thinking the technique Experimental results What we ve learned

19 # 1 Question in nuclear physics What are the forces between protons and neutrons that bind nuclei?? Up till about 10 years ago, we had no idea UNTIL Pieper and Wiringa (ANL) 18

20 Ab-initio Greens Functional Monte Carlo Calculations Pieper and Wiringa (ANL) Described in NPA751, 516c (2005) 19

21 NN Potential A number on the market: Argonne V18, CD Bonn, Nijmegen Argonne v18 two body potential: H K i v ij i i 18 Operators ~40 Parameters Fit to ~4300 NN Scattering data j v ij v R ij EM 1- short range Problem : Binding energies of most light nuclei too small R.B. Wiringa et al., Phys. Rev. C 51, 38 (1995). 20

22 Three body forces From Wikipedia: A three-body force is a force that does not exist in a system of two objects but appears in a three-body system. Very Basic Example Object = People Force = Emotions 3-body force = Jealousy 21

23 Three body forces in nuclei Extra interactions in nuclear matter when nucleons are packed close together V ijk V V 2 3 ijk ijk V R ijk Urbana Potentials: a + V R Illinios Potentials: a + b + c + d + V R Fujita Miyazawa Dominant Term Steven C. Pieper et al., Phys. Rev. C 64, (2001). Coupling constants from fitting binding energies of light nuclei ~4 parameters 22

24 23 Neutron Stars

25 Can we figure out what s going on inside? Need to know the Nuclear Equation of State How much energy does it cost to squeeze nuclear matter? This density dependence is CRITICALLY dependent on the THREE BODY forces, as the more you crush. Energy? then closer then nucleons get and the more chance of 3 body interactions. Density, AND We can only learn about the 3 body forces by studying nuclei on earth.

26 A=10 nuclei provide a sensitive test of GFMC Can lifetime measurements probe the effects of 3 body forces? S.C. Pieper, K. Varga and R.B. Wiringa, Phys. Rev. C 66, (2002). 25

27 B(E2) s: The rate of emitting gamma rays The problem is..the radiation comes out VERY fast. Units of time: 1ms 10 3 s 1s 1ns 10 6 s 10 9 s B(E2) ~ 1 1ps s 1fs s 10 Be lifetimes are fs 26 How can you measure femtosecond type lifetimes? No clock runs evenly remotely that fast. THE ONLY MECHANISM THE NUCLEAR TRAIN WRECK!!!

28 Doppler Shift Attenuation Method (DSAM) v E Eo[ 1 cos( )] c Intensity 0 Intermediate E o unshifted E full shift Energy Thin target with thick backing to slow/stop recoils Line shape depends on nuclear lifetime Short lifetime: Full shift Long lifetime: No shift

29 Previously measured lifetimes in 10 Be All DSAM measurements 9 10 Be( d, p) Be Evaluated = 180 (17) fs E.K. Warburton et al., Phys. Rev. 129, 2180 (1963). G.C. Morrison et al., - unpublished. E.K. Warburton et al., Phys. Rev. 148, 1072 (1966). T.R. Fisher et al., Phys. Rev. 176, 1130 (1968). T.R. Fisher et al., Phys. Rev. 176, 1130 (1968).

30 Key component : Stopping powers Huge deceleration!! a = g!!! At high velocities: Electronic stopping At low velocities: Electronic stopping Nuclear stopping Ions see mainly a cloud of electrons Scattering is small Pure Coulomb, easy to calculate Scattering off of target nuclei Large deviations from trajectory Not well understood

31 Stopping powers Reaction 7 Li( 7 Li, ) 10 Be E Li = 10 MeV Large positive Q value 10 Be (g.s) ~18 MeV high velocity recoils

32 Key component :Recoil velocity vector θ v E Eo[ 1 cos( )] c Need to know SLOW Ions 1 MeV 10 Be Li Au FAST Ions α 10 Be 15 MeV 10 Be Unique selection of recoil velocity Li Au J.F. Ziegler, J.P. Biersack and M.D. Ziegler

33 Gamma ray and recoil detection Gammasphere Fragment Mass Analyzer (FMA) 100 Germanium detectors Almost 4 coverage Vacuum recoil separator 2 Electric 1 Magnetic Dipole Selects based on A/Q Rejects beam on level 32

34 Gammasphere + FMA : The ideal tools 2 body kinematics 7 Li( 7 Li, ) 10 Be High recoil velocity Well defined recoil angle θ Q1 Q2 FMA Q3 Q4 Ion chamber ED1 MD1 ED2 PPAC High efficiency Large number of angles 10 Be * (3.3) = 16.5 MeV 10 Be * (5.9) = 15.0 MeV Control of state population

35 The old way Backing stops all recoils 10 Be 11 B What we call a brick target 10 Be β = 0.055

36 And the new way 3 mg/cm 2 Cu backing ΔE E tot in Ion Chamber 35

37 And the new way /

38 Centroid Shift Analysis 16 Angle Rings in Gammasphere Determine peak centroid in each ring Fit with: E E o cos <1% in 37

39 Results for 3369keV J=2 state Targets of 7 Li and 7 LiF Backings of Gold and Cu 38

40 Results for 3369keV J=2 state Targets of 7 Li and 7 LiF Backings of Gold and Cu 39

41 Results for 3369keV J=2 state Targets of 7 Li and 7 LiF Backings of Gold and Cu 40

42 2 + states in 10 Be First 2 + state NNDC transition strength 205( 5) ( 7 stat ) sys fs B 2 4 ( E2; ) 10.5(10) e fm Revised transition strength B 2 4 ( E2; ) 9.2(3) e fm Ab initio transition strength B 2 4 ( E2; ) 8.8(2) e fm Second 2 + state 59(10) B fs 2 4 ( E2; ) 0.11(2) e fm B(E2) ~ 1 41

43 Comparison of different potentials Energy (MeV) 42

44 43 Comparison of different potentials

45 Conclusions In Experiment Precise measurement of the first 2 + state lifetime in 10 Be. Reliable <5% DSAM IS possible. In Theory We have a powerful tool for investigating nuclear structure. Three body forces play a critical role: in determining total binding energy AND in the fine details of the wavefunctions. Electromagnetic transition rates ARE very discriminating! 44