Beta and isomer delayed spectroscopy of neutron-rich Dy to Os Nuclei: Mapping shape evolution and Quadrupole collectivity at large neutron excess. Paddy Regan Department of Physics, University of Surrey. Guildford, GU2 7XH, UK & Radioactivity Group National Physical Laboratory Teddington TW11 0LW, UK p.regan@surrey.ac.uk & paddy.regan@npl.co.uk

Decay spectroscopy. How quickly something decays…. … Decay spectroscopy. How quickly something decays…. …..tells you what’s going on inside…. 2008 1979 1988 1985

2+ 0+ Excitation energy (keV) PHR, Physics World, Nov. 2011, p37 Ground state Configuration. Spin/parity Ip=0+ ; Ex = 0 keV

Deformed region: Neutron-rich A～170 nuclei K quantum number and collective rotation: K isomers R I Change direction Spin selection … Yes K-selection … Sort of ! K hindered transitions Weisskopf hindrance Reduced hindrance The degree of K forbiddenness The identification and characterization of K-isomers provides information on Intrinsic orbits near the Fermi surface Pairing energies The degree of axial symmetry

Deformed region: Neutron-rich A～170 nuclei Kπ = 6+ isomers in N = 104 isotones 822 ns ? 104 F.R. Xu et al., PLB 435 (1998) 257 G.D. Dracoulis et al., PLB 635 (2006) 200 P.H. Regan et al., PRC 65 (2002) 037302

Deformed region: Neutron-rich A～170 nuclei Kπ = 8- isomers in N = 106 isotones and E1 decays 106 5.8 s ? F.R. Xu et al., PLB 435 (1998) 257 E1 reduced hindrances G.D. Dracoulis et al., PLB 635 (2006) 200 fν~100 from systematics assume Eγ = 150 keV T1/2 ~ 6.5 s G.D. Dracoulis et al., PRC 79 (2009) 061303(R) ⇒ compete with β decay?

While we were waiting for ~170Dy … N=100, 164Sm, 166Gd.

166Gd 164Sm

Deformation (inferred from E(2+) and E(4+) states) appears to reduce again after ‘local’ increase at N=100; possible evidence for localised N=100 ‘deformed shell closure’

The role of the hexacontatetrapole (β6)deformation Highest β6 predicted for ~164Sm (P. Moller et al.) Sm β6 Kπ=6- -> 5- ~150 keV effect Calc. by Honliang Liu

WAS3ABI & EURICA WAS3ABI: Wide-range Active Silicon-Strip Stopper Array for Beta and Ion detection Double-sided Silicon Strip Detectors 60 x 1 mm strips in x direction 40 x 1 mm strips in y direction EURICA: Euroball RIKEN Cluster Array for (ion correlated) gamma-ray measurements. 84 HPGe in 12 x 7 element CLUSTER dets. 18 LaBr3(Ce).

Decay spectroscopy of neutron-rich rare-earth isotopes around double mid-shell (report on RIBF beam-time, Nov. 2014) H. Watanabe, P.-A. Söderström, P.H. Regan, P. Walker Beihang/RIKEN/Surrey Main goals: Search for isomers in the 10 ns to 10 s range b-delayed g-ray spectra around mid-shell b-decay half-lives and the rare-earth peak 345 MeV/u U primary beam at 11 pnA Purified by BigRIPS and ZeroDegree Implanted in the WASABI active stopper Gamma rays detected by EURICA Outcome from three days of beam-time: More than five new isomers in key nuclei 15 new beta-delayed gamma spectra 15 new half-lives Capability to measure very long-lived isomers confirmed online with 174mEr (5.8 s)

170Dy104

Under review at Physics Letters B (2016) g-gated 170Dy isomer decay b-gated 170Tb decay 170Dy spectroscopy for first time by combining: b--delayed tagging on 170Tb105 (T1/2 ~ 1 s) mother nucleus and (ii) isomer spectroscopy of 170Dy (T1/2~1 ms) Kp=(6+) isomer

Decay and mixing with Kp=2+ ‘gamma’ band and Kp=6+ isomeric decay ‘reduces’ the Reduced hindrance Compared to Np.Nn Systematics for 170Dy.

172Dy106 : Past the mid-shell

First 172Dy level scheme constructed by (a) Isomer spectroscopy (by gating on conversion of 0.7s isomer in 172Dy (using WASABI; 172Dy gs half life measured to be ~3 s) (b) b- delayed spectroscopy by gating on 172Tb Mother nucleus (spectrum (k) on left).

g vibrational Kp=2+ band is particularly enhanced in 172Dy106 due to simultaneous excitation of several 2qp configurations close to the Fermi surface for N=106 which couple to give K=2 and induce non-axially symmetric, collective motion

Dy Preliminary Candidates for Kπ = 2+ and 2- states 168 66 102 66 102 Dy G.X. Zhang Beihang Univ. M. Asai et al., Phys. Rev. C 59, 3060 (1999) 4-; π3/2+[411]⊗ν5/2-[512] Candidates for Kπ = 2+ and 2- states M. Asai et al., PRC 59, 3060 (1999) Preliminary

168Tb:Gamma Spectroscopy

Future at GSI / FAIR

F. Browne, A. M. Bruce, T. Sumikama et al. , EURICA + FATIMA, Phys F. Browne, A.M.Bruce, T. Sumikama et al., EURICA + FATIMA, Phys. Lett. B (2015)

TRD for FATIMA for NUSTAR.

FATIMA for DESPEC FATIMA = FAst TIMing Array = State of the art array for precision measurements of nuclear structure in the most exotic and rare nuclei. 36 LaBr3(Ce) detectors. Energy resolution better than 3% at 1 MeV. Detection efficiency of ~ 5% Full-energy peak at 1 MeV. Excellent timing qualities (sub 100 ps). Use to measure lifetimes of excited nuclear states & provide precision tests of nuclear structure, uses a fully-digitised Data Acquisition System (CAEN 1 GHz digitizers).

Passive Stopper measurements: g-rays from isomer with T1/2 for 10 ns  1 ms. Active Stopper measurements: b -particles, i.c. electrons, T1/2 ms →mins

RISING Active Stopper Measurements Passive Stopper: g ray from isomer cascades with T1/2 ~ 10 ns  1 ms. Active Stopper measurements: b-particles, internal conversion electrons. T1/2 up to ~ minutes; associated with delayed g rays. 5 cm x 5 cm DSSSD (16 strips x 16 strips = 256 pixels) x 3 = 758 total pixels. See P.H. Regan et al., Int. Jour. Mod. Phys. E17 (2008) 8 ; R. Kumar NIM A598 (2009) 754

b--gated (fast-timing) W-Pt region at GSI/FAIR

188Ta →188W 190Ta →190W 192Ta →192W

New data points on R(4/2) for 190W and 1/E(2+) point to new sub-shell closure signature around Z~74 in neutron-rich nuclei (N >114). 1/E(2+)

RISING with 208Pb primary Beam and active stopper. Gate on 194Re decays to Populate states in 194Os (including second 0+ state).

Mapping the deformation shape changes? Experimental signatures? E(2+) of yrast sequence and evolution with (N,Z), valence product ? R(4/2) = E(4+)/E(2+) = evolution from 3.3 <–> 2.5 <- > 1.8? E(2+2), excitation energy of ‘gamma’ band; low-lying 2+2 states consistence with axial softness. E(0+2), excitation energy of ‘b’- bandhead, low-lying states consistent with (e.g.) prolate/oblate shape co-existence shape (e.g., around N=116,118, 190,2W and 192,4Os) T1/2 for Ip=2+ (and perhaps higher spin states?) to get magnitude of Qt and b deformation parameter. Decays and id of isomeric states to map (deformed) single-particle spectrum (prolate vs. oblate isomeric configurations for example?)

What the yrast Ip=2+ lifetime can give you ?

b--gated (fast-timing) W-Pt region at GSI/FAIR

What can we reach / get to now?

Sum of time differences between 143-keV and any higher lying feeding transition (assumes negligible half-life for intermediate states). T1/2=0.87(12) ns

Some quick revision on extracting (nuclear excited state) lifetimes… Assuming no background contribution, the experimentally measured, ‘delayed’ time distribution for a g-g-Dt measurement is given by: P(t’-t0) is the (Gaussian) prompt response function and l=1/t, where t is the mean lifetime of the intermediate state. See e.g., Z. Bay, Phys. Rev. 77 (1950) p419; T.D. Newton, Phys. Rev. 78 (1950) p490; J.M.Regis et al., EPJ Web of Conf. 93 (2015) 01014

Deconvolution and lineshapes? If the instrument time response function R(t) is Gaussian of width s, If the intermediate state decays with a mean lifetime t, then The deconvolution integral for a single state lifetime is given by (ignoring the normalisation coefficients). 1-erf(x) is the complementary error function of x.

Examples / Technical limits ? For a fixed source, lifetimes of the order of > 1 ns are ‘easy’ using LaBr3 coincidences, if you have enough counts. E.g. 152Eu source data using RoSPHERE.

Some comments on ‘at the edge’ timing measurements at FAIR. Problem 1 Rare, weakly produced final nuclei, few counts at focal plane Solution: clean by tagging and ion-beta-gating Timing correlations using Fast LaBr3 detectors; also (in some cases) allows ordering of gammas in a cascade below beta or isomer. Problem 2 Distributed source (across the focal plane) = spread in time of flight of gammas, focal plane distances could vary by ~ 10 cm (Dt = d/c ~330 ps spread for 10 cm). Solution, clean position tagging at focal plane and event-by-event correction (demonstrated at Jyvaskyla by Cullen et al., for 138Gd). Problem 3 Need a fast ‘start’ signal for the LaBr3(Ce). Solution(s) LaBr3(Ce) - LaBr3(Ce) coincidences can be ‘self-timed’ wrt each other, but AIDA not fast enough for electron start for b-g coincs. Add in a fast plastic start detector (a la EURICA ?).

192Os(18O,16O)194Os at IFIN-HH, Bucharest, T. Daniel, PhD thesis, U 192Os(18O,16O)194Os at IFIN-HH, Bucharest, T. Daniel, PhD thesis, U. Surrey Use 192Os unsafe Coulex channel as test of analysis and Internal time/energy response calibration.

194Os deformation by fast-timing

Current state of the art and predictions?