Recent progress on the study of nuclear fission using laser - PowerPoint PPT Presentation

FNPMLS Recent progress on the study of nuclear fission using laser spectroscopy Kieran Flanagan 6th Workshop on Nuclear Fission and Spectroscopy of Neutron-Rich Nuclei, Chamrousse, March 2017

Overview • Introduction to laser spectroscopy and considerations. • Highlights of recent work on heavy elements. • New technique developments • Shell structure studies on fission fragments • Summary and outlook

Atomic spectroscopy for nuclear physics Composition, shape and size F=J+I of the nuclear wave function, without introducing assumptions from nuclear models. Data using existing techniques • Production rates of 1 atom/second or lower and are not accessible with existing laser spectroscopy techniques in this region. • Goal region for CRIS (far from a complete survey)

Laser Spectroscopy Options • Choice of transition often lead by the constraints of the technique (sensitivity to HFS/IS fortuitous). • More atomic physics input is required. Resonance Ionization Fluorescence (trapped, collinear …) Wavelength A coefficient Dark states State selective Flux and Fluence Modulated Light V.N. Fedosseev, Yu. Kudryavtsev and V. I. Mishin Phys. Scr. B. Cheal, et al. Phys. Rev. Lett. 102 , 222501 (2009) 85, 058104 (2012) L. Vermeeren, et al. Phys. Rev. Lett. 68 , 1679 (1992)

Transition choice for experiment (example of francium) • Ionization potential measured • Atomic theory available for 7s 1/2 , 7p 1/2, and 7p 3/2 • HFS A(7p 3/2 )/(8p 3/2 ) ~3.0 B(8p 3/2 )/(7p 3/2 ) = ~3.4 355 nm • Lifetime for 7p 3/2 = 21 ns and 8p 3/2 = 85 ns • But 355 nm • 355+355 nm will non-resonantly ionization and add significant background. • Multi-step scheme looked risky (original plan was to Doppler tune) 355 nm • Two step resonant ionization scheme • nm resonant step 717.986 nm • nm non-resonant step A. Voss et al., Phys. Rev. Lett. 111, 122501 (2013) 7s 1/2 - 7p 3/2 transition in 204-206 Fr A. Voss et al., Phys. Rev. C. 91, 044307 (2015)

Status of Laser spectroscopy: 2016 Gaps in knowledge Production of exotic nuclei Atomic physics Rare isotopes will require more sensitive techniques P. Campbell, I.D. Moore, M.R. Pearson, Progress in Particle and Nuclear Physics 86 (2016) p127

Heavy nuclei Structure relevant to beta-delayed fission 202-206,214, 218,219,229,231 Fr 179-185 Tl 197,198,203,205,207,209,211,217 At 191-210,216,218 Po Fission 182-189 Pb 177-182 Au 239-244 Pu Region of octupole deformation N=104 Permanent static ground state deformation Shape Coexistence Intruder States

In-source laser spectroscopy B. A. Marsh et al., 20013 EMIS conference, NIM B317, p.550 (2013) WM: A.Andreyev et al, Phys. Rev. Lett 105, 252502 (2010) MR-ToF MS : R. N. Wolf et al, NIM, A686, 82 (2012)

Summary of charge radii in the Pb region At? Hg? Au? Andrei Andreyev

Collinear resonance ionization spectroscopy • 1982: Outline of method proposed by Yu. A. Kudriavtsev and V. S. Letokhov, Appl. Phys. B29 219 (1982) Promised • σ for non resonant ionization = 1-10 • High resolution, Mb (10 7 -10 8 smaller than allowed • High efficiency atomic transitions) • Requires high power pulsed lasers • Low background • ns pulses & 10 Hz – 10 kHz • Flight time through interaction region ~5-10 μ s (depends on space and budget). • Necessitates 100-200 kHz repetition rate laser or an ion trap. • UHV in the interaction to avoid collisional ionization <10 -8 mbar Laser Laser • Ion optic tuning required when Doppler scanning

CRIS laser laboratory Sep 2015 • Increasing the available wavelengths we can produce for RIS schemes • M2 Ti:Sa laser and frequency-doubling cavity • Matisse dye laser and frequency-doubling cavity • Industrial Nd:YAG laser and (injection-seeded) Ti:Sa cavities • 200 Hz Nd:YAG laser and pulsed-dye laser • Fibre-couple or mirror-couple light downstairs to the beam line

CRIS laser laboratory April 2016 • Increasing the available wavelengths we can produce for RIS schemes • M2 Ti:Sa laser and frequency-doubling cavity • Matisse dye laser and frequency-doubling cavity • Industrial Nd:YAG laser and (injection-seeded) Ti:Sa cavities • 200 Hz Nd:YAG laser and pulsed-dye laser • Fibre-couple or mirror-couple light downstairs to the beam line 14

CRIS laser laboratory • Increasing the available wavelengths we can produce for RIS schemes • M2 Ti:Sa laser and frequency-doubling cavity • Matisse dye laser and frequency-doubling cavity • Industrial Nd:YAG laser and (injection-seeded) Ti:Sa cavities • 200 Hz Nd:YAG laser and pulsed-dye laser • Fibre-couple or mirror-couple light downstairs to the beam line 15

Status 2012 • Francium run completed 202-231 Fr • RILIS Narrowband laser used 1.5 GHz linewidth • ~1% total experimental efficiency estimated from 202,218,219 Fr • Non-resonant ionization efficiency ~0.0003%. Background rate 0.002 counts/s 202 Fr (arising from 202 Tl). • At 9x10 -9 mbar, 1pA of contaminant isobar reduced to 18cps • Laser on/off 218 Fr alpha detection>330 (expected 3000-4000)

High resolution CRIS: 2015 • First experiments used a I 1.5 GHz laser system. E • New method of chopped CW laser spectroscopy: 20(1) MHz linewidth. • Separating pulses reduces coherent effects. • Same rate on 219 Fr in CRIS 2012 narrow linewidth mode. CRIS 2014 R. de Groote Phys Rev. Lett. 115 (13), 132501 (2015)

Comparison • Factor of 75 improvement in linewidth 219 Fr 2012 219 Fr 2014 Γ =1.5 GHz Γ =20 MHz 206 Fr 2012 206 Fr 2014 R. de Groote Phys Rev. Lett. 115 (13), 132501 (2015) K.M. Lynch Phys Rev C 93 (1), 014319 (2016)

Laser spectroscopy of shortlived Exploratory check to during a break while high resolution laser system was “optimised”

Fortuitous Spectroscopy • Low resolution (RILIS) 1.5 GHz • Started scan on resonance (based on a best guess). • Alpha identification on resonance confirmed ground state • Change in bunching rate from 200 Hz to 100 Hz also used. • Most exotic N=127 isotone measured. • 5ms represents the shortest lived isotope measured with laser spectroscopy on-line. GJ Farooq-Smith et al, Physical Review C 94 (5), 054305 (2016)

Departure from lead δ <r 2 > • Amazing overlap with the δ <r 2 > trend in lead down to N=122. • Typically nuclei are deformed yet the extended region around 208 Pb remains spherical • Transition after 205,206 Fr • Q moments suggest small change in static deformation. • Can still describe magnetic moments with using single particle coupling rules. • Suggestive of a ‘soft’ spherical potential and zero point oscillations about the minimum.

Shell evolution of fission fragments: Ni region • Nucleon-nucleon interaction: single-particle energies evolve as function of nucleons in an orbit • Away from stability, this can lead to (dis)appearance of shell closures • Cu chain: Z =29: probe for the magicity of Z =28 and N =28,40,50 ν in g 9/2 T. Otsuka et al, PRL 104 , 012501 (2010) K.T. Flanagan et al, PRL 103 , 142501, 2009

Moments of neutron-rich Cu (Z=29) Magnetic moments: sensitive to details of wave function π f 5/2 vg 9/2 Does the deviation increase? Can we verify this prediction? • Theoretical reproduction: excitation across Z =28 required • Moment confirms dominant π f 5/2 contribution for 72,74 Cu 23

Moments of neutron-rich Cu (Z=29) Quadrupole moments: collectivity and deformation Will the reproduction continue for A > 75? • Quadrupole moment is sensitive to E2 transitions • Neutrons across N=50 (not included in jj44b/JUN45) not required up to A=75

Initial Copper experiment • Low efficiency and high background only allowed us to reach 71 Cu. IP • Choice of scheme was sub optimal • Utilized novel frequency chopping method with a strong transition (lifetime < 10 ns) 266 nm – Achieved resolution of 75 MHz P 3/2 71 Cu Ex 1 324 nm S 1/2 GS 25

Second Attempt • Laser ionization scheme: 249 nm + 314 nm • Laser system: injection locked pulsed ti:sapphire laser (Jyvaskyla/Mainz) and pulsed dye laser • High efficiency (total ε ~1%) • High resolution (70 MHz linewidth) • High background suppression 26

Experimental spectra • Data on 63-78 Cu • 80hrs total of data taking 78 Cu Count rate (Hz) Frequency detuning from centroid (MHz) 27

Future work at CRIS • Extending laser measurements to 79 Cu. • Neutron-rich indium up to 134 In (later neutron deficient down to 100 In) • Extending measurements in francium to 201 Fr, 203m Fr, 214m Fr • 52,53 K and testing the N=32,34 shell closures. • Neutron deficient tin towards 100 Sn • Decay spectroscopy of 80m Ga and laser spectroscopy of 83 Ga • High resolution measurements of neutron deficient Po

Summary • In source laser spectroscopy is in a harvesting period (70 new measurements in the last decade). • In-source measurements have now extended measurements in Au,Hg and At (and recently Bi) • CRIS has now demonstrated ultra-high resolution combined with high efficiency laser spectroscopy across the nuclear chart.