-Strength Determination Using Total Absorption -Ray Spectroscopy

Transcription

1 -Strength Determination Using Total Absorption -Ray Spectroscopy J.L. Tain Instituto de Física Corpuscular C.S.I.C - Univ. Valencia

2 Existing TAS data (so far)

3 -strength S and -intensity I INTENSITY STRENGTH -decay S i Relation between S and I : f ( Q Ii E i ) T Experimental: Q -value Half-life T 1/2 Intensity I 1 2 s 1 Relation between B and S : S E x B i f D Theoretical: reduced transition probabilities B [g v2 /4] Large level densities: averaged quantities.

4 The TAS technique Total Absorption Spectroscopy is the best method to measure beta strengths in -decay for complex decay schemes Avoids the Pandemonium effect (Hardy et al, PLB136p331): misplacement of -intensity to lower E x when using high resolution (germanium) spectroscopy - coincidences Pandemonium effect is due to: Fragmentation of the intensity Limited efficiency of Ge detectors Likely to happen when: Number of allowed levels is large : large level densities, large Q-values -ray intensities level scheme + intensity balance Discrete levels!

5 Using large 4 scintillation detectors, aims to detect the full -ray cascade rather than individual -rays real TAS spectrum (deconvolution) strength ideal TAS - decay NOTE: The TAGS technique is not sensible to g.s. feed BUT a TAS + -detector can be used to obtain I g.s.

6 The Pandemonium effect in neutron-rich A~100 MD. Jordan, PhD Thesis 101 Nb 105 Mo 102 Tc 104 Tc 105 Tc 106 Tc 107 Tc

7 The Pandemonium effect in 150 Ho decay: CLUSTER-CUBE: 6 EUROBALL Clusters in cubic geometry CLUSTER: 7 Ge detectors, 60% each Efficiency T P CLUSTER-CUBE at GSI rays 295 levels S S HR β TAS β 0.4 Algora et al PRC50(2002) BUT: the TAGS technique does not replace the high resolution spectroscopy!

8 How do we extract the -intensity (strength) from TAS spectra? decay response Relation between TAS data and the -intensity distribution: R d i j I i R ij f i f Deconvolution algorithms (inverse problem): EM, ME & LR k j f k R j j 1 b jk k 0 g jk R R j : decay response for level j b jk : branching ratios jk, from known level scheme and nuclear statistical model g jk : -ray response jk, from Monte Carlo simulations k b jk b.r. (the response may contain also the -penetration, CE effect, isomer effect, ) -response: g jk

9 Requirements for reliable TAS result From the analysis point of view: Response must be accurately known: for all particles emitted: e-/e+, -ray, response should depend weakly on deexcitation branching ratios Solution of inverse problem must be stable NIM A430 (1999) 333 NIM A430 (1999) 488 NIM A571 (2007) 719 NIM A571 (2007) 728 From the experimental point of view: Spectrum must be clean: eliminate background and contaminations Production: clean (mass separators, traps, laser selective sources) or ion-per-ion identification Control of daughter activity Background reduction / measurement Spectrometer: highly efficient Spectrometer: good resolution Use of ancillary detectors

10 During the past few years we have undertaken a systematic investigation of systematic uncertainties associated with the analysis of TAS data: 1. Demonstration of the accuracy of Monte Carlo simulations to obtain the spectrometer response (Cano et al. NIMA430, p.333) Exp. MC 2. Accurate calculation of pulse pile-up which constitutes an intrinsic background close to the end point (Cano et al. NIMA430, p.488) Pile-up 24 Na

11 3. Investigation of the adequacy of several algorithms for the solution of the TAS inverse problem (Tain et al., NIMA 571, 728) LINEAR REGULARIZATION MAXIMUM ENTROPY EXPECTATION-MAXIMIZATION Result insensitive to algorithm parameters:, B, f (0), n iter, The two (three) algorithms agree within few % f LR method: polynomial smoothing T 1 T 1 T 1 R V R λb B R V d d d f ME method: entropy maximization R ( s) ij f j exp di R 2 i i k ( s1) 2 j ik f ( s) k EM method: Bayes Theorem R ( s) ( s1) 1 ij j i f j i ( s) Rij Rik fk i k f d : regularization parameter, B: regularization matrix, V d =[1/ i2 ]: covariance matrix of data

12 4. Investigation of the dependency of the result on the assumption about the cascade branching ratios (Tain et al., NIMA 571, 719) : w. realistic b.r. : w. unrealistic b.r. : reference Needs to know the true branching ratios and intensity use statistical nuclear model to create decay of fictitious nucleus S at high energies (level densities) and S is rather insensitive to b.r. Rebinning of: d R f R j j 1 b jk k 0 g jk R k w. realistic b.r. introduces non-negligible effect w. unrealistic b.r.

13 Total Absorption Spectrometer LBL GSI: Karny et al. NIM B (1997) 411 Main characteristics true 4 geometry Ø35cm35cm NaI well crystal + NaI plug ancillary detectors: Ge detector: X-rays (EC) and -rays Si detector (-particles) Si telescope (p- and -particles) E = 1 MeV E = 5 MeV P T P T

14 Does the size matter? 156 Tm -decay St. Petersburg TAS vs. LBL

15 An analysis example: 104 Tc decay Q = 5600 MeV, N allowed ~ 10 4 Known part

16 An accurate knowledge of the distribution of the -decay probability over the daughter-nucleus levels provides information for the understanding of the structure of nuclei of importance on its own or for other fields as astrophysics Basic process: simple and sensitive to the wave function Fermi / Gamow-Teller f or i 2 τ στ In general the bulk of the strength lies outside the Q window Exception: +/EC for A150, A100, NZ

17 2d3/2 0h11/2 152 Tm Gamow-Teller + resonance GSI, Gatchina, Valencia, Warsaw 1h9/2 1f7/2 152 Tm 9 + LBL GSI Odd-Odd N=83 nuclei above 146Gd Ho Ho D. Cano, PhD Thesis E. Nacher, PhD Thesis 148 Tb Tb

18 Determination of nuclear shapes from -strength distributions NZ nuclei with A (MeV) [404]9 /2 [211]1 /2 g p 38 f p s d 28 f 9/2 1/2 5/2 3/2 7/2 20 1/2 3/2 [413]7 /2 [303]5/2 [301]1/2 [312]3/2 [321]1 /2 [303]7/2 [440]1/2 [310]1/2 [312]5/ 2 [321]3 /2 [330]1/2 [200]1/2 [202]3 / [301]3 /2 [431]3/ [422]5/2 30 ΣB(GT) (g A 2 /4π) Theo.prolate Theo.oblate E.Nácher PRL 92 (2004) Oblate () Prolate 76 Sr Q EC Energy (MeV) () P.Sarriguren et al.npa 658 (1999)13 ISOLDE Madrid, Strasbourg, Surrey, Valencia Q β

19 Region of shape coexistence: neutron deficient Pb, Hg, Pt isotopes ISOLDE Recent measurement (Nov08) 192 Pb spherical using RILIS of 192,190,188 Pb 0.4 B(GT) Q EC oblate X-ray gated (EC-decay) LASER ON prolate LASER OFF (Tl contam.) E ex [MeV] A. Algora et al. Debrecen, Madrid, Surrey, Valencia

20 IGISOL separator + ion guide source: refractory elements TAS Univ. Jyvaskyla JYFLTRAP Penning trap: isotopic purification Mass scan Measurement of Nb, Mo and Tc isotopes for Reactor Decay Heat Valencia, Jyvaskyla, Debrecen, Gatchina, Surrey Measured TAS spectra

21 Improvement of reactor decay heat calculations based on evaluated data E γ H n i t N t I E E Eˆ E i1 i ln2 T i 1/2 i Q β 0 β x x β x de x Isotope Emiss. TAS ENDF/BVII Diff. [kev] [kev] [kev] 101 Nb beta 1797 (133) 1966 (307) -169 (7.1 s) gamma 445 (279) 270 (22) Tc beta 1935 (11) 1945 (16) -10 (5.28 s) gamma 106 (23) 81 (5) Tc beta 931 (10) 1595 (75) -664 (1098 s) gamma 3229 (24) 1890 (31) Tc beta 764 (81) 1310 (205) -546 (456 s) gamma 1825 (174) 665 (19) Mo beta 1076 (43) 1922 (122) -846 (35.6 s) gamma 2347 (91) 552 (24) Tc beta 1457 (30) 1906 (67) -449 (35.6 s) gamma 3132 (70) 2191 (51) Tc beta 1276 (212) 2054 (254) -778 (21.2 s) gamma 1795 (450) 515 (11) 1280 E -ray discrepancy A. Sonzogni et al

22 Reactor neutrino spectrum: neutrino oscillations and homeland security How well known is the reactor anti-neutrino spectrum? Data from Schreckenbach et al. (ILL- Grenoble) Summation calculations Fallot et al. Nantes, Valencia, Jyvaskyla, Surrey Proposal for measurement at JYFL: Penning trap & precision trap to separate isotopes and isomers 92,93 Rb, 95 Sr, 96 Y, 99 Y, 100m,100 Y, 100m Nb, 100 Nb, 138 I and 142 Cs

23 Comparison of average - and -energies from TAS (Greenwood et al. NIMA 390, 95) with direct measurements (Rudstam et al. ADNDT 45, 239) Consistency check for R: E γ E β E ν Q β Both measurements are subject to several sources of systematic error: No reason to believe Rudstam et al. superior to Greenwood et al.

24 Test of the CVC hypothesis and unitarity of CKM matrix Super-allowed decay T1 / 2 f ( QEC ) ft I Ft ft g V 1 R K R M C 2 F NS Ga N=Z odd-odd nuclei: 62 Ga, 66 As, 70 Br,, 94 Ag Use of TAS to detect high lying weak GT branches Hardy & Towner PRL88p Precision of 10-3! 99.86% Hyland, PRL97, Zn

25 (nearly) THE END

26 Proposal approved at JYFL-Jyvaskyla (L. Batist, unpublished) (Barcelona-Valencia-Madrid-Jyvaskyla-Debrecen- Gatchina-Surrey-Caen) Isotopes: 87,88 Br, 94,95 Rb, 137 I Production: U(p,f) or (d,f) Separation: IGISOL + Penning-Trap Detection: TAS + 4n + ntof (+ -det + Ge-det) Determine: I, P n, E n Interest for r-process Reactor decay-heat and reactivity Testing of principles TAS 4n ntof

27 DESPEC/HISPEC DESPEC experiment of the NUSTAR collaboration New TAS under development modules: cm cm 3 NaI(Tl) 3 LaBr 3 :Ce Fast ions active stopper: Stack of DSSSD Valencia, Madrid, Gatchina, Darmstadt, Debrecen, St. Petersburg, Surrey

28 At DESPEC there is a strong motivation to measure - - strength far from stability A particular challenge is the application of this technique at the neutron rich side, due to the beta delayed neutrons n The beta-delayed neutrons and the subsequently emitted gamma-rays (may) become a contamination source NE213 implantation moderator + 3 He count

29 147 Cs -decay -intensity distribution S n T 1/2 = s ; J = 3/2 + ; Q = 9.2 MeV 147 Ba S n = 4.45 MeV ; P n = 27.5 % 147 Ba known levels: 20 (E x < 0.5 MeV) (total: ) 146 Ba levels: up to MeV Q -ray spectrum n spectrum T = 0.97 FN = 0.24 n = 0.11