The Neutron Long Counter NERO for studies of Beta-delayed Neutron Emission

Transcription

1 The Neutron Long Counter NERO for studies of Beta-delayed Neutron Emission Jorge Pereira National Superconducting Cyclotron Laboratory (NSCL/MSU) Joint Institute for Nuclear Astrophysics (JINA)

2 Why do we care about β-delayed neutron emission? Direct impact in Heavy-Element Nucleosynthesis Models (r-process) also Nuclear Structure Probes

3 β-decay properties in the r-process β-delayed neutron-emission probabilities (P n ) : What is the path followed by matter flow after freeze-out (Abundance pattern post freeze-out) Additional source of late neutrons Half-lives of r-process nuclei: What are the bottle-necks of matter flow? Abundance pattern prior freeze-out T 1/2 and P n (gross β-decay properties): First insights into shell structure via β-decay strength function (Deformation, nucleon-nucleon interaction, new magic numbers, etc )

4 Measurements of β-delayed neutron-emission probabilities at NSCL

5 Measurement of β-delayed with a Neutron Long Counter

6 Design of NERO detector: Minimize Background & Maximize Efficiency

7 Design of NERO detector: Minimum Background & Maximum Efficiency β neuron background: Spurious coincidences β + neutrons β background + REAL neutrons β background + Neutron background REAL β decay + Neutron background Neutron background origin: Electronic noise Gamma rays Cosmic rays Fragmentation reactions β background origin: REAL β decays from implants mother Light particles (t, Li, )

8 Design of NERO detector: Minimum Background & Maximum Efficiency β neuron background: Spurious coincidences β + neutrons β background + REAL neutrons β background + Neutron background REAL β decay + Neutron background Neutron background origin: Electronic noise Gamma rays Cosmic rays Fragmentation reactions β background origin: REAL β decays from implants mother Light particles (t, Li, ) Large segmented implantation detector DSSD 40 x independent detectors Reduction of background ~ 1600 x number of gas counters

9 Design of NERO detector: Minimum Background & Maximum Efficiency Large segmented implantation detector Large NERO cylindrical cavity to accommodate DSSD Reduce neutron efficiency MCNP simulations to maximize efficiency Energy-independent Efficiency

10 Final design Polyethylene Moderator BF 3 Proportional Counters (44) 3 He Proportional Counters (16)

11 NERO neutron efficiency MNCP MNCP scaled Innermost Ring Intermediate Ring External Ring 11 B(α,n) 51 V(p,n) 13 C(α,n) 252 Cf

12 Beta Counting System + NERO NERO Beta Counting System (BCS) J. Pereira et al., 618, 275 NIMA (2010) J.J. Prisciandaro et al., NIMA 466, 492 (2001)

13 Results and discussions

14 Sn In Cd Ag Pd Rh Ru Tc Mo Nb Zr Y Sr Rb Kr Br Se As Ge Ga Zn Cu Ni Co Fe Z N NSCL β-decay r-process campaigns (NSCL/Mainz/Notre Dame/Maryland) J. Pereira, H. Hennrich et al. M. Quinn, J. Pereira et al. P. Hosmer, H. Schatz, et al. F. Montes, H. Schatz, et al. A=80 Xe I Te Sb g 9/2 d 5/2 s 1/2 g 7/2 d 3/2 h 11/2 K.-L. Kratz (private communication) Ba A=110 g 9/2 p 1/2 p 3/2 f 5/2 f 7/2 A=130 n n = cm -3

15 P n values of A 110 r-process nuclei 104 Y FRDM/QRPA 2003 Pereira et al. PRC 79, (2009) P. Sarriguren and J. Pereira, PRC 81, (2010) FRDM/QRPA (triaxiality) 109 Mo 110 Mo

16 T 1/2 of A 110 r-process nuclei 105 Y 106 Zr 107 Zr FRDM/QRPA 2003 Pereira et al. PRC 79, (2009) P. Sarriguren and J. Pereira, PRC 81, (2010) FRDM/QRPA (triaxiality) 111 Mo

17 T 1/2 and P n around the 120 Rh F. Montes et al., PRC73, (2006)

18 T 1/2 and P n around the 78 Ni waiting-point ( Co, Ni, Cu, Zn, Ga) Shell-model calculations: GT operator renormalized New data by 0.37 instead of Pfeiffer et al., PNE (2002) Limitation of the model space that excluded f 7/2? Hosmer et al., PRC 82, (2010)

19 Summary P n values and β-decay half-lives are crucial to model the synthesis of Heavy Elements ; they are also sensitive nuclear structure probes The neutron detector NERO was designed for measuring P n values. Its performance combines low background rates with high neutron efficiencies ~40% Measured P n values were used to investigate deformation of nuclei around 106 Zr (triaxiality, weakly-deformed intruder states) Measured P n values of nuclei around 78 Ni may indicate sensitivity of shell-model calculations to model space (inclusion of f 7/2 ). Results for Cu isotopes may indicate the inversion of π2p 3/2 and π1f 7/2 MORE

20 Thanks to: NSCL/MSU: Paul Mantica, Hendrik Schatz, Paul Hosmer, Giuseppe Lorusso, Marcelo Del Santo, Fernando Montes, Linda Schnorrenberger Univ. Notre Dame: Ani Aprahamian, Joachim Görres, Matt Quinn, Michael Wiescher, Andreas Wöhr Mainz Univ.: Karl-Ludwig Kratz, Clemens Herlitzius, Bernd Pfeiffer, Florian Schertz Los Alamos NL: Peter Möller, Peter Santi Univ. Maryland: Bill Walters Pacific Northwest NL: Paul Reeder

21 Back up slides

22 PRE freeze-out N=126 POST freeze-out Mass known Half-life known Terra Incognita Abundance pattern pre freeze-out S n : r-process path (Nuclei involved) T 1/2 : time scale + bottle necks N=82 (Pearson, et al. 1996) N=50 Abundance pattern post freeze-out P n : decay path towards stability K.-L. Kratz et al., Ap. J. 203, 216 (1993)

23 Nuclear structure probes Sensitive to integral structure via Strength function S β weighted by Fermi-function f ~(Q β -E) 5 Q 0 β Structure of daughter at low energies Qβ 1 S T β(e) 1/2 and f(z,r,q P n βnuclear-structure E) de = T probes. 1 2 = T Sβ(E) Cheaper f(z,r,qβ E) than de γ-spectroscopy 1/2 Structure of daughter at energies ~S n / Pn S 9 Increase T 1/2 20 Increase P n (saturation limit) For neutron-rich 104 Y nuclei, 104 Zr+e Q - β is large: P. Möller (private communication) P n =5.2%, T 1/2 =43 ms Deformed n 104 Y 104 Zr+e - P n =100%, T 1/2 =389 ms ot 1/2 is mostly sensitive to S β near the ground-state, only op n is mostly sensitive to S β at higher energies (~S n ) (emphasize importance of forbidden transitions) Spherical

24 Production and separation of nuclei (e.g. A1900 separator at NSCL) Ion Source K500 N3 vault K1200 Im2 A1900 Production of nuclei by fragmentation (inverse kinematics) Separation and identification of exotic beam: A1900 Exotic beam Implantation station (in the N3 vault)

25 Correlation of implanted nuclei and β decays (β-decay end-station) Implantation DSSD: Fit (mother, daughter, granddaughter, background) x-y position (pixel), time T 1/2 Veto light particles from A Zr Decay DSSD: x-y position (pixel), time Silicon PIN Stack 4 x Si PIN DSSD (40 40) 6 x SSSD (16) Ge

26 Processing of signals Moderation time Multi-hit TDC (trigger-gate mode)

27 Processing of signals 5 min 252 Cf source 1 h experiment data 12 h background 3 He Proportional Counters 3 He + n 3 H + p Q=0.764 MeV 10 B+n 7 Li + α Q=2.792 MeV 7 Li * + α Q=2.310 MeV BF 3 Proportional Counters

28 β-decaying nuclei β-delayed neutron-emitter nuclei ffββdetermination of β-delayed neutrons-emission probabilities (P n ) Pereira et al. PRC 79, (2009) N B Pn = N εβnβnβn N P n = N B βnβn CεN ffβ C λd P λ = λm t m C 1 1 λd λm λd ffβnn eεn ( λ ) d t e

29 Detection of β-delayed neutrons: Analysis β-decaying nuclei β-delayed neutron-emitter nuclei Pereira et a. PRC 79, (2009) 11 B(α,n) 51 V(p,n) 13 C(α,n) 252 Cf

30 A=110 r-process nuclei Epicenter of A=110 r-process nuclei: 110 Zr 70 Standard structure N=70 around N=50/82 mid-shell Possible existence of a spherical double semi-magic 110 Zr Zr 70 is expected to be highly prolate, β 2 ( 104 Zr) ~ 0.4, [Urban et al. NPA 689 (2001)] Dowaczewski et al. PRL 1994 CONSEQUENCES in the r-process: Direct: Change S n r-process path (synthesized nuclei) 70 Direct: Reduce T 1/2, P n (factors ~6) Indirect: Weakening of N=82 shell (shell quenching) 40

31 A=110 r-process nuclei A~115 abundance trough predicted in r-process calculations Quenching of N=82 OR/AND Astrophysical Conditions? Abundances (Si 10 6 ) Classic model. Different Nuclear Physics ETFSI-Q ETFSI-1 C. Freiburghaus et al., ApJ516, 381 (1999) Mass number Same Nuclear Physics H. Schatz (private communication) Mass number High S bubble Classic model

32 Nuclear deformation from β-decay properties: QRPA vs. deformation Macro-Microscopic FRDM/QRPA model 1. Calculation of ground-state masses and deformation parameters FRDM + Strutinsky microscopic corrections (Shell + Pairing) 2. Use deformation parameters to determine single-particle levels φ (folded-yukawa + Lipkin-Nogami) 3. Calculate Gamow-Teller β-strength function using QRPA equations with residual interaction V GT =2χ GT :β 1- β 1 4. First-forbidden transitions from statistical gross theory P. Möller et al., NPA 1992; ADNDT 1995, 1997; PRC2003

33 Gross β-decay properties used as nuclear structure probes Gross β-decay properties are sensitive to f ( Q ) 5 nuclear structure at different energy regimes β E Q β T1/2 Sβ(E) f(z,r,qβ E) de 0 Low energies P n T 1/2 Q β S n S β (E) f(z,r,q Energies above S n β E) de Dobaczewski et al., PRL72 (1994) 981 B. Pfeiffer et al., NPA693 (2001) 282

34 QRPA calculated T 1/2 and P n around 106 Zr For 104 Zr 64 : ε 2 ~ 0.25 inferred from QRPA Calculations Lower than ε 2 ~ 0.4 obtained from γ-spectroscopy [Urban et al., NPA 689, 605 (2001)] Presence of weakly-deformed intruder states? Already seen ~ Zr [Lhersonneau et al., PRC49, 1379 (1994)] P. Sarriguren and J. Pereira, PRC 81, (2010)

35 Intruder weakly-deformed configurations Shape coexistence? QRPA does not include shape coexistence Low ε 2 values from QRPA may be really an average deformation = Coexistence of prolate ground state + weakly deformed intruder configurations at N=64,65!!! spherical 110 Zr 70? Urban et al., NPA 689, 605 (2001)

36 Predicted intruder spherical configurations with HFB + Goriely effective interaction S.Hilaire and M.Girod Could these configurations be investigated with Selfconsistent QRPA calculations (P. Sarriguren et al.)?

37 R-process nuclei around the 78 Ni waiting-point

38 T 1/2 and P n around the 78 Ni waiting-point ( Co, Ni, Cu, Zn, Ga) Visible effects in calculations of r-process abundances (110 ms) QRPA97 T 1/2 78 Ni (477 ms) Hosmer et al., PRC 82, (2010)

39 T 1/2 and P n around the 78 Ni waiting-point Wigner et al., PRL102, (2009) New data Pfeiffer et al., PNE (2002) QRPA-97 Möller et al., PRC (1997) CQRPA: I.N. Borsov, PRC (2005) CQRPA: πf 5/2 πp 3/2 Hosmer et al., PRC 82, (2010) πp 1/2 πf 5/2 πp 3/2 5/2-1/2-3/ (7/2 - ) (7/2 - ) (5/2 - ) (3/2 - ) (7/2 - ) (7/2 - ) (5/2 - ) (3/2 - ) 1 0 E ex (MeV) Evidence for 5/2 ground state for 75 Cu, 77 Cu (Walters, Flanagan private communication) 69 Cu Cu Cu 44 67,69 Cu: B. Zeidman et al. (1978) 71 Cu: R. Grzywacz et al. (1998) 69,71,73 Cu: S. Franchoo et al., (1998, 2001)

40 R-process nuclei around the 78 Ni waiting-point Wigner et al., PRL102, (2009) Underpredicted values (CQRPA and FRDM+QRPA) may be an indication for inversion of the π2p 3/2 and π1f 7/2 orbitals CQRPA + inverted orbitals improves the results (what is the effect in T 1/2?) FRDM+QRPA + inverted orbitals improves P n BUT degrades T 1/2

41 R-process nuclei around New P n value of 120 Rh affects 119 Sn/ 120 Sn ratio by 20% 120 Rh is ~ 3p and 3n away r-process path More P n values are necessary!