STUDY OF THE EVOLUTION OF OCTUPOLE COLLECTIVITY IN 217RA

University of the West of Scotland

29th July 2019 INPC 2019, Glasgow

James.Keatings@UWS.ac.uk

Octupole Deformation

Requires pairs of Δl = Δj = 3 with strong coupling to exist near Fermi level.

Octupole ‘magic numbers’ occur above closed shells, just before first orbital is filled.

Interesting region for this work is near Z = 88 (f7/2↔i13/2) and N = 134 (g9/2↔j15/2).

Nuclei normally don’t experience static octupole deformation, Coriolis force helps ‘stabilise’ deformation.

[1] P. A. Butler and W. Nazarewicz, Rev. Mod. Phys. 68, 349 (1996).

π

ν

2

Causes nuclei to undergo a reflection-asymmetric deformation (Jahn-Teller effect). Produces a ‘pear-like’ shape. Separation of centre of mass and centre of charge.

Octupole Deformation

[1] P. A. Butler and W. Nazarewicz, Rev. Mod. Phys. 68, 349 (1996).

3

f7/2

g9/2

Just beyond doubly magic Z = 82 and N

= 126 (208Pb).

Octupole ‘magic numbers’ in red.

On octupole magic number Z = 88, and between N = 134 (222Ra) and N = 126 (208Pb).

Nuclear Landscape 4

[3] Y. Itoh, Nucl. Phys. A 410, 156 (1983). [4] N. Schulz et al., Phys. Rev. Lett. 63, 2645 (1989). [5] K. Valli et al., Phys. Rev. C 1, 2115 (1970). [6] G. D. Dracoulis et al., J. Phys. G 17 (1991). [7] N. Roy et al., Nucl. Phys. A 426, 379 (1984).

Previous Work

216Ra

216Ra described well by single particle structure [3], 218Ra shows vibrational deformation [4].

Studied previously through α decay [5], 208Pb(12C,3n) and 208Pb(13C,4n) [6,7].

Positive “band” described as νg9/23, and negative

“band” as νg9/22j15/2.

Measured B(E1)/B(E2) values between these bands cannot be explained with shell model.

Shows both single particle structure and possible evidence of collective structure.

5

Previous Work

218Ra

[3] Y. Itoh, Nucl. Phys. A 410, 156 (1983). [4] N. Schulz et al., Phys. Rev. Lett. 63, 2645 (1989). [5] K. Valli et al., Phys. Rev. C 1, 2115 (1970). [6] G. D. Dracoulis et al., J. Phys. G 17 (1991). [7] N. Roy et al., Nucl. Phys. A 426, 379 (1984).

216Ra described well by single particle structure [3], 218Ra shows vibrational deformation [4].

Studied previously through α decay [5], 208Pb(12C,3n) and 208Pb(13C,4n) [6,7].

Positive “band” described as νg9/23, and negative

“band” as νg9/22j15/2.

Measured B(E1)/B(E2) values between these bands cannot be explained with shell model.

Shows both single particle structure and possible evidence of collective structure.

6

Previous Work

217Ra

[3] Y. Itoh, Nucl. Phys. A 410, 156 (1983). [4] N. Schulz et al., Phys. Rev. Lett. 63, 2645 (1989). [5] K. Valli et al., Phys. Rev. C 1, 2115 (1970). [6] G. D. Dracoulis et al., J. Phys. G 17 (1991). [7] N. Roy et al., Nucl. Phys. A 426, 379 (1984).

216Ra described well by single particle structure [3], 218Ra shows vibrational deformation [4].

Studied previously through α decay [5], 208Pb(12C,3n) and 208Pb(13C,4n) [6,7].

Positive “band” described as νg9/23, and negative

“band” as νg9/22j15/2.

Measured B(E1)/B(E2) values between these bands cannot be explained with shell model.

Shows both single particle structure and possible evidence of collective structure.

7

Experiment performed at the INFN LNL using Tandem-ALPI with Galileo, Euclides, Neutron Wall set up.

The nuclei of interest were produced via 208Pb(16O,α3n)217Ra.

1mg/cm2 target, 90 MeV beam, 10 pnA.

7 days of beam time for measurement.

INFN LNL - Overview 8

α3n is channel of interest.

Majority of events are fission (95%).

Need to filter out background.

Need to make use of particle gating.

208Pb(16O,α3n)217Ra Compound

Nucleus Nucleus of

Interest

[8] http://nucalf.physics.fsu.edu/~riley/gamma/

Fusion Evaporation 9

25 HPGe single crystal detectors at backwards angles, in 2π layout.

Detects γ rays from reaction products.

Compton suppressed.

Used in tandem with auxiliary detectors to produce particle gated matrices.

[9] J.J. Valiente-Dobón et al., LNL-INFN Ann. Rep. (2014).

INFN LNL - Galileo 10

[10] D. Testov et al., Eur Phys. J. A 55 (2019). 150 μm 1000 μm

110 E-δE Si detectors in near (81%) 4π array.

Used to detect light charged particles emitted during reaction.

Uses angular position to reconstruct event and calculate Doppler correction for γ rays.

INFN LNL - Euclides 11

E

α

p

[10] D. Testov et al., Eur Phys. J. A 55 (2019).

110 E-δE Si detectors in near (81%) 4π array.

Used to detect light charged particles emitted during reaction.

Uses angular position to reconstruct event and calculate Doppler correction for γ rays.

INFN LNL - Euclides 12

45 liquid scintillator detectors.

Detects evaporated neutrons from reaction products.

Distinguishes between n/γ through use of ToF/ZCO signals.

[11] Ö. Skeppstedt et al., NIM A 528, 741 (1999).

INFN LNL – Neutron Wall 13

Shape is distinctly different for 217Ra when compared to other nuclei which have been shown to have well developed octupole deformation.

+ΔE E

E

J+

J+2+ J+1-

= +ΔE

E

E

J-

J+2-

J+1+

=

Results – ΔE 17

Δix ≈ 3 vibrational

Δix ≈ 0 deformed

Alignment shows clear difference from well deformed Ra isotopes.

No structure in the evolution of alignment.

Clear difference from other vibrational nuclei.

= positive

= negative

Results – Alignment 18

Measured B(E1)/B(E2) values compared to previous results from [7] and values for newly observed levels. Weaker E1 suggests an enhancement of the octupole collectivity at higher excitation energies, opposite to alignment.

[7] N. Roy et al., Nucl. Phys. A 426, 379 (1984).

Results – B(E1)/B(E2) 19

Collaborators: J. F. Smith1, G. de Angelis2, D. Bazzacco3, A. Boso3, L. Capponi4, R. Chapman1, M. Chishti1, D. M. Cullen5,

D. O’Donnell1, L. P. Gaffney6, A. Goasduff2, F. Gramegna2, E. T. Gregor1, G. Jaworski2, P. R. John3, N. Kelly1, J. Kownacki7, S. Lenzi3, T. Marachi2, K. Mashtakov1, P. P. McKee1, R. Menegazzo3, V. Modamio2,

D. R. Napoli2, M. Palacz7, F. Recchia3, M. Siciliano2, E. Sivre3, M. Scheck1, M. Smolen1, P. Spagnoletti1, M. J. Taylor5, D. Testov2, C. Ur4, J. J. Valiente-Dobón2.

1 University of the West of Scotland, Paisley, PA1 2BE, UK.

2 INFN Laboratori Nazionali di Legnaro, Legnaro, Italy. 3 Dipartimento de Fisica and INFN, Sezione di Padova, Padova, Italy.

4 ELI-Nuclear Physics, IFIN-HH 077125 Bucharest-Magurele, Romania. 5 Schuster Laboratory, University of Manchester, M13 9PL, UK.

6ISOLDE, CERN, Geneva CH-1211, Switzerland. 7Heavy Ion Laboratory, University of Warsaw, 02-093 Warsaw, Poland.

217Ra is a transitional nucleus between the single particle and octupole deformed region of the nuclear chart with experimental signatures of both.

Experiment performed using fusion evaporation at INFN LNL with Galileo, Euclides, and Neutron Wall set up.

Observed new excited states in “octupole” band. New levels show signatures of both behaviours. Octupole deformation not proven as stabilised at higher ħ.

Summary & Acknowledgements 20