Bulletin of the American Physical Society
2019 Fall Meeting of the APS Division of Nuclear Physics
Volume 64, Number 12
Monday–Thursday, October 14–17, 2019; Crystal City, Virginia
Session RC: Mini-Symposium: Excited State Lifetime Measurements for Nuclear Structure and Astrophysics |
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Chair: Peter Bender, University of Massachusetts, Lowell Room: Salon 3 |
Thursday, October 17, 2019 8:30AM - 9:06AM |
RC.00001: Recent highlights from lifetime measurements with rare isotope beams: techniques and results Invited Speaker: Hironori Iwasaki Two of the compelling questions identified for nuclear science today are "How does subatomic matter organize itself and what phenomena emerge?" and "How did visible matter come into being and how does it evolve?". Excited-state lifetime measurements provide precise and model-independent nuclear structure data of key atomic nuclei to validate nuclear models and pin down scenarios of astrophysical processes. With the advent of advanced gamma-ray arrays such as GRETINA, new experimental techniques have been developed extending the reach of lifetime programs far from the valley of stability. This talk will provide a brief overview of new implementation of Doppler-shift techniques with rare isotope beams and present recent science highlights including findings on shape coexistence phenomena in neutron-rich nuclei, shape evolution along N=Z, and properties of drip-line nuclei. Then discussion will focus on electromagnetic responses of weakly-bound neutron-rich nuclei and a possible interplay between a nuclear halo and deformation. [Preview Abstract] |
Thursday, October 17, 2019 9:06AM - 9:18AM |
RC.00002: Influence of the neutron $pf$ shell on the structure of $^{28}$Mg Jonathan Williams Excited states in $^{28}$Mg were studied in an experiment at the ISAC-II facility at TRIUMF to investigate the lowering in energy of $pf$ intruder orbitals predicted near the $N=20$ 'island of inversion'. A $^{12}$C($^{18}$O,2p)$^{28}$Mg fusion-evaporation reaction was used to populate states at high excitation energy where the influence of intruder orbitals is expected. Data corresponding to $^{28}$Mg was extracted via time coincident identification of gamma rays using the TIGRESS array at ISAC-II and protons using a new CsI(Tl) scintillator array, part of the TIGRESS Integrated Plunger (TIP) infrastructure. Lifetime measurements of excited states were performed using the Doppler shift attenuation method (DSAM). Three new excited states of $^{28}$Mg were identified near its neutron separation energy. Multiple intruder state candidates were also observed, including an unusually long-lived state thought to decay by an M2 transition ($I^{\pi} = (0,4)^-$). The observed level energies are consistent with shell model calculations in the $sdpf$ shell, where negative parity levels arise from single neutron excitation to the $pf$ shell. Experimental results and their interpretation with respect to the lowering of intruder orbitals near the `island of inversion' will be discussed. [Preview Abstract] |
Thursday, October 17, 2019 9:18AM - 9:30AM |
RC.00003: Recoil distance method lifetime measurement of the $2^{+}_{1}$ state in $^{94}$Sr and implications for the structure of neutron-rich Sr isotopes Aaron Chester, Greg Hackman, Jack Henderson, Krzysztof Starosta, Philip Voss, Jonathan Williams The TIGRESS Integrated Plunger (TIP) device [1] has been constructed to enable Doppler-shift lifetime measurements at TRIUMF's ISAC-II facility. TIP was commissioned using a stable $^{84}$Kr beam by coupling the recoil distance method with unsafe Coulomb excitation in inverse kinematics [2]. A high-precision lifetime measurement of the $2^{+}_{1}$ state in $^{94}$Sr was performed using the same Coulex-RDM technique. The data set had low statistics due to the luminosity of the radioactive $^{94}$Sr beam. A lifetime of $\tau = 7.80^{+0.50}_{-0.40}~\text{(stat.)} \pm 0.04~\text{(sys.)}$ ps was determined by comparing experimental data to Geant4 simulations using a likelihood ratio $\chi^{2}$ method. The corresponding $B(E2;2^{+}_{1} \rightarrow 0^{+}_{1})$ value is approximately 25$\%$ larger than previously reported while the relative error has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with N$\leq$58 [3]. The experimental results, data analysis methods, and a comparison to existing theoretical models are presented. [1] Voss et al., NIM A 746 87--97 (2014). [2] Chester et al., NIM A 882 69--83 (2018). [3] Chester et al., PRC 96 011302(R) (2017). [Preview Abstract] |
Thursday, October 17, 2019 9:30AM - 9:42AM |
RC.00004: Probing large collectivity $^{32}$Mg with a recoil-distance lifetime measurement Robert Elder, H. Iwasaki, J. Ash, D. Bazin, P. C. Bender, T. Braunroth, B. A. Brown, C. Campbell, H. Crawford, B. Elman, A. Gade, M. Grinder, N. Kobayashi, B. Longfellow, A. O. Macchiavelli, T. Mijatovic, J. Pereira, A. Revel, D. Rhodes, J. Tostevin, D. Weisshaar The $^{32}$Mg isotope lies within the $N=20$ island of inversion where intruder configurations drive collective phenomena. However, there is limited information on the $B(E2)$ values in $^{32}$Mg which are key quantities to understand quadrupole collectivity and the role of intruder configuration mixing. To determine model-independent $B(E2)$ values, lifetime measurements of excited states are crucial. In $^{32}$Mg the prompt $2_1^+$ and $4_1^+$ states are expected to have lifetimes on the order of 1 to 10 ps, which is the typical range for well-established methods such as RDM and DSAM. The experiment was performed at the NSCL using the TRIPLEX, GRETINA, and S800 devices, enabling sensitive measurements of the lifetimes of these states. The results will be presented in the context of collective models in the island of inversion. [Preview Abstract] |
Thursday, October 17, 2019 9:42AM - 9:54AM |
RC.00005: Precision Lifetime Measurements of Excited States in $^{38}$Si M. Grinder, H. Iwasaki, R. Elder, J. Ash, A. Revel, N. Kobayashi, D. Bazin, J. Belarge, P. Bender, B. Elman, A. Gade, C. Loelius, B. Longfellow, E. Lunderberg, D. Weisshaar, K. Whitmore, T. Haylett, T. Mijatovic, A. Dewald, S. Heil, M. Mathy The nuclear shell structure significantly evolves in the neutron-rich region at the traditional magic numbers N=20 and 28, resulting in rapid shape transitions as predicted by shell model calculations. The energy ratios between the first 2$^{+}$ and 4$^{+}$ states in the even-even Si isotopes from N=20 to 28 suggest a variety of collectivity evolving from vibrational, to possible triaxial, to rotational modes. The systematic behavior of the level schemes along the Si isotopic chain suggests $^{38}$Si as the turning point in this transition. The lifetime measurement of $^{38}$Si was performed at the National Superconducting Cyclotron Laboratory based on the Recoil-Distance Method using the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA). The data are used to extract the B(E2) ratio which provides a useful measurement to assess the nature of collective modes. [Preview Abstract] |
Thursday, October 17, 2019 9:54AM - 10:06AM |
RC.00006: Fast-timing capabilities of GRIFFIN Bruno Olaizola GRIFFIN is the decay spectroscopy facility at TRIUMF-ISAC in Canada. The core of GRIFFIN is an array of 16 large-volume HPGe clover detectors. One of its main features is the variety of ancillary detectors that can be coupled to the main array. SCEPTAR for $\beta$-particle tagging, the 70 liquid scintillators of DESCANT for neutrons or PACES (5 Si(Li) detectors) with high-energy resolution for conversion electrons. A new ancillary array of 8 LaBr$_3$(Ce) detectors for $\gamma$-rays and a fast plastic-scintillator called Zero Degree for $\beta$-particles is optimized for fast-timing experiments with GRIFFIN. The 51 mm x 51 mm cylindrical LaBr$_3$(Ce) crystals are coupled to Hamamatsu R2083 PMT with an integrated pre-amplifier in the PMT base. Timing resolutions as good as FWHM$\sim 300$~ps and time-walks $\pm50$~ps have been obtained using a hybrid analogue and digital DAQ system. electronics. A set of BGO shields minimize the Compton background, one of the main limiting factors in lifetime measurement. The LaBr$_3$(Ce) array has been used in a number of experiments to date, measuring lifetimes down to $\tau\sim 20$~ps. The general fast timing capabilities of GRIFFIN will be presented and some of the measured lifetimes in $^{188-200}$Hg and A=$144-146$ Ba, La and Ce discussed. [Preview Abstract] |
Thursday, October 17, 2019 10:06AM - 10:18AM |
RC.00007: Short lifetime determination via GRETINA and CHICO2 Jing Li A deep inelastic experiment was performed at ATLAS with a 170-MeV $^{48}$Ca beam bombarding a 1 $mg/cm^2$ $^{70}$Zn target to investigate the shape-driving effects of the $\nu$g9/2 orbital in the region at N=40. The heavy ion counter CHICO2 was used to detect the target-like and projectile-like products simultaneously. The $\gamma$ rays, emitted in flight by both reaction products, were detected by GRETINA. The combination of GRETINA and CHICO2 allowed for a proper Doppler reconstruction. The thin target provides sensitivity for fast transitions. A new level was identified in $^{70}$Zn; its lifetime was determined by a new method. The same detector setup was used for a multi-step Coulomb excitation experiment with a 284-MeV $^{70}$Zn beam on a $^{208}$Pb target. The Coulomb-excitation analysis was performed using the coupled-channel least-square search code, GOSIA [1]. Most lifetimes of the excited states obtained in the present analysis agree well with the published data except for the $4_+^1$ state with a measured lifetime as 1.9(2) ps, which is much shorter than the reported value of 4.2(12) ps [2]. A detailed account of the technique and the results will be presented.[1] T. Czosnyka et al, Am. Phys. Soc 28(1983)745 [2]G. Gurdal et al., Nuclear Data Sheets, 136(2016)1-162. [Preview Abstract] |
Thursday, October 17, 2019 10:18AM - 10:30AM |
RC.00008: DSAM lifetime measurements in $^{7,8}$Li C. Morse, E.A. McCutchan, C.J. Lister, G.L. Wilson, G. Hackman, M. Bowry, R. Caballero-Folch, L.J. Evitts, A.B. Garnsworthy, J. Henderson, A. Kurkjian, J.P. Measures, M. Moukaddam, P. Ruotsalainen, J. Smallcombe, J.K. Smith, D. Southall, M. Williams, A.J. Mitchell, C.Y. Wu The lithium isotopic chain is an ideal laboratory for studying magnetic properties of nuclei. Recently, \textit{ab initio} theoretical calculations have investigated charge current distributions in light nuclei, which give rise to $M1$ transition rates and magnetic moments, and have achieved precision comparable to available experimental data. Therefore, new experimental efforts are necessary to provide input for the further development of nuclear models. We have performed new lifetime measurements of the bound excited states of $^{7,8}$Li in order to precisely determine the $M1$ transition strengths in these nuclei. The experiment was performed at TRIUMF using the TIGRESS array, populating excited states through $(d,p)$ reactions in inverse kinematics. The lifetimes were determined using the Doppler Shift Attenuation Method, and the results of these measurements will be discussed. [Preview Abstract] |
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