Two-proton radioactivity: an experimentalist’s view

Catalin Borcea CEN Bordeaux-Gradignan and

IFIN-HH

NANUF03, Bucharest September 7-12, 2003

• Two proton radioactivity – a new decay mode• Two-proton emission from light nuclei• Decay of 42Cr and 49Ni • Two-proton radioactivity of 45Fe• Future perspectives and developments

Stable nuclei

Stable and -unstable nucleiAll observed nuclei:- stable- unstable- particle unstable

Leaving the valley of stability….

One-proton radioactivity

Physics topics• test of nuclear mass models• nuclear structure beyond stability - single particle ordering and energy - j content of wavefunction - shape of mean-field potential• quantum tunneling through 3D barrier

Model calculationsto determine single-particle level sequence

Two types of 2p emission: - ground-state emission • long lived: 45Fe, 48Ni, 54Zn • short lived: 6Be, 12O, 16Ne, 19Mg

- emission from excited states • β delayed: 22Al, 31Ar, … • others: 14O, 18Ne, 17Ne

To be measured: • proton energies • proton-proton angle

Two-proton radioactivitySequential emission:

Three-body decay:

2He emission:

Two-proton radioactivitySequential emission:

Three-body decay:

2He emission:

Measurement of protonsand recoil

Two types of experiments:

- long-lived nuclei s) implantation and decay- short-lived nuclei ps) : complete kinematics experiments

Two proton emission

from light nuclei

Mass: A = 6 – 20

Z = 4 – 12 low Coulomb (+ centrifugal) barrier broad states - short half-lives T1/2 = 10-21s – 10-12s decay already in the target

complete-kinematics measurements behind production target

2p emission from 12O

Total decay energy:

Proton-proton energy:

Proton-proton angle:

R.A. Kryger et al., Phys. Rev. Lett. 74 (1995) 860

Data consistent with 3-body or sequential decay

seq. decay

2He decay

Experimental Procedure

24Mg fragmentation in SISSI at 95 MeV/uSelection with Alpha spectrometer : 17F, 18Ne, 20MgStripping reactions at entrance of SPEG

GANIL

Mass of 19Na

19Na 18Ne + p

Reasonable agreement with experiment and predictions

J. Cerny et al.:W. Benenson et al.:

Garvey-Kelson:Pape-Antony:

12.97(7) MeV12.93(1) MeV

12.88(20) MeV13.30(20) MeV

Mass excess: 13.09(5) MeV

MU

ST

MUST

SPEG

Target9Be

20Mg

19Na

18Ne

p

First mass measurement of 18Na

MU

ST

MUST

SPEG

Target9Be

20Mg

18Na

17Ne

p

18Na 17Ne + p

If assumed to be ground state:- huge Thomas-Ehrman shift 1. peak: 18Na* 17Ne*

2. peak: 18Na 17Ne: 25.04(12)MeV- problems with theoretical calculations

Mass excess: 24.19(5) MeV

Audi-Wapstra: Garvey-Kelson:Pape-Antony:

25.3(4) MeV25.4(2) MeV25.7(2) MeV

Theoretical calculation for 18Na

Main shell-model components:• 18Na (1-): ([(d5/2)3]3/2+[p1/2]) ([(d5/2)2]2+[s1/2][p1/2])• 18Na (2-): ([(d5/2)3]5/2+[p1/2]) ([(d5/2)2]2+[s1/2][p1/2])• 17Ne (1/2-): ([(d5/2)2]0+[p1/2])• 17Ne (3/2-): ([(d5/2)2]2+[p1/2])

Barrier-penetration predicts all excited states to decay to 17Ne ground state

Shell-model spectroscopic factors:

2p emission from 17Ne

15O+p

15O+2p

Good agreement with known separation energies and 16F

17.431

16F+p17Ne 15O+2p

17.968

18.25418.398

18.16118.39218.689

16.490

17.778

17Ne* 16F + p 15O + 2p

MUST

MUST

SPEG

Target9Be

18Ne15O

p

p17Ne*

Proton-proton angle distribution

Three-body center of mass frame: 2p emission from 17Ne*

2HeSequential / 3-body

2He (53%)Sequential (47%)Sum

Proton-proton angle distribution

Three-body center of mass frame: 2p emission from 17Ne*

• Agreement with MSU data : no angular correlation for Q2p =0.9 MeV states• Indication of angular correlation for higher lying states…..

New search for 2p emission in 16,17Ne

MU

ST

MUST

VAMOS

Target9Be

17Ne

16,17Ne

Mass

15F+p

14O+2p

14,15O

p

p

SPIRAL24 MeV/u2*104pps

16Ne23.99 MeVΓ = 120 keV

23.89 MeVΓ = 1.2 MeV

19.31 MeV

Conclusion:

2p emission from 19Mg 19Mg seems to be a good candidate• unexpected effect for 17Ne …. to be checked• determined masses of intermediate nucleus 18Na for • study of one- and two-proton emission from light proton-rich nuclei

Collaborators:

T. Zerguerras, Y. Blumenfeld, T. Suomijärvi, D. Beaumel, M. Fallot, I. Lhenry-Yvon, J.A. Scarpaci, A. Shrivastava

IPN OrsayB. Blank, M. Chartier, J. Giovinazzo

CEN Bordeaux-GradignanW. Mittig, P. Roussel-Chomaz, H. Savajols

GANIL CaenC. Jouanne, V. Lapoux

DAPNIA SaclayM. Thoennessen

Michigan State University

Two-proton emission

from medium-mass nuclei

Two-proton emission from 45Fe ground state

X

Decay scheme for 45Fe

Possibly very rich decay pattern

GANIL 1999

• The observation of doubly-magic 48Ni• Study of 42Cr, 49Ni, and 45Fe

Production of radioactive nuclei at GANIL

• 58Ni @ 75 MeV/A on nickel target• High primary beam intensity: 3-5A

Production and observation of 48Ni

10 parameters to identify:48Ni: 4 counts49Ni: 106 counts45Fe: 53 counts42Cr: 287 counts

Spectroscopic studies of 42Cr: a 2p candidate

Charged-particle spectrum:

Decay-time spectrum:

Two-proton ground-state emitter?

• -decay half-life• 1.9 MeV peak too high in energy Most likely -delayed decay

Spectroscopic studies of 49Ni: a 2p candidate

Charged-particle spectrum: Decay-time spectrum:

• -decay half-life• 3.8 MeV peak too high in energy Most likely -delayed decay

-decay half-life predictions:• Möller et al.: 5.0 ms• Hirsch et al.: (7.3-20.4) ms

Spectroscopic studies of 45Fe: a 2p candidate

Charged-particle spectrum: Decay-time spectrum:

Too low statistics to decide between 2p emission and decay

-decay half-life predictions:Brown: 15.5 msOrmand: 7 msHirsch et al.: (3 - 7) msTachibana et al.: 12 ms

Two experiments:• GANIL in July 2000• GSI in July 2001

Two-proton decay of 45Fe

GSI:• low production rate: 6 45Fe implantations in 6 days

• fast data acquisition based on XIA modules

GANIL:• high production rate: 15 45Fe per day

• standard data acquisition (CAMAC – VME)

Two-proton decay of 45Fe: GANIL data

J. Giovinazzo et al., PRL 89 (2002) 102501

• 58Ni @ 75 MeV/A on nickel target• High primary beam intensity: 3-5A

Detection setup:

Si(Li)

Two-proton radioactivity of 45Fe: GANIL data

Identification of22 45Fe implants

J. Giovinazzo et al., PRL 89 (2002) 102501

Identification of 45Fe:

Decay-time spectrum:

Two-proton radioactivity of 45Fe: GANIL data

Decay-energy spectrum: spectrum:

T1/2 = 16.7±7.0 ms

1.14 MeV peak

J. Giovinazzo et al., PRL89 (2002) 102501

Q value in agreement with predictions no pile up for peakhalf-life short enough for 2p emission data in agreement with daughter decay no ’s in coincidence with 1.14 MeV peak

Two-proton decay of 45Fe: GSI data

The FRS:

M. Pfützner et al., EPJA 14 (2002) 279

Primary beam

650 MeV/u 58Ni

Implantationset-up

Beam current(SEETRAM)

Target 4 g/cm2 Be

S2 degrader3.6 g/cm2 Al

TOF 1, 2, 3(scintillators)

B2B3

E(MUSIC)

IDENTIFICATION IN FLIGHT

B2 p/Z TOF 1,2,3 v

E Z

A/Z

S1 degrader3.2 g/cm2 Al

NaI barrel

Stack of Si detectors7 x 300 m

511 keVIdentified ions

511 keV

B1

B4

Trigger, E300 m Si

Two-proton decay of 45Fe: GSI data

6 implantation events identified

M. Pfützner et al., EPJA14 (2002) 279

A/Z

0 1 2 3 4 5 60Energy (MeV)

5 correlated decay events:• four 2p decays• one -delayed decay

Two-proton radioactivity of 45Fe

Q2p = 1.14 MeVT1/2 = 3.8 ms

Clear evidence of 2p radioactivity

No coincident beta or gamma ray withthe 1.14 MeV peak

Peak width: no pile-up

Observed energy in agreement withpredictions

Observation of daughter decays

Two-proton radioactivity of 45Fe: di-proton model

Experimental half-life: T1/2 = 3.8± ms2.00.8

Di-proton model half-life: 0.02 ms

+ SM spectroscopic factors: 0.12 ms

+ preformation factor ? + resonance energy ?

Lower limit for the half-lifeR-matrix model (Brown-Barker): S-wave p-p interaction : 4-40 ms

Two-proton radioactivity of 45Fe

L.V. Grigorenko et al.Nice agreement between exp. data and predictions for p wave protonsbut: f-wave protons expected…..

di-proton: - - - -3-body:

Conclusions

• clear evidence for 2p radioactivity of 45Fe: a new radioactivity

• other candidates: 48Ni, 54Zn• decay mechanism: 2He or 3-body• setup to measure p – p angle and individual energies TPC

Future studies

First observation of 55Zn and 56Zn

Expected production rate for 54Zn: 10-15 per day

New setup for 2p experiments on LISE3: TPC

identification

Drift of ionisationelectrons

emitter

protons

X-Y detector

Ele

ctri

c fi

eld

What information can bring the two-proton radioactivity:• masses of nuclei beyond drip line• pairing in nuclei• sequence of single-particle levels• j content of wavefunctions• deformation• complex tunneling process• …..

Physics case

CollaboratorsGSI:

M. Pfützner, R. Grzywacz, Z. Janas, J. KurcewiczUniversity of Warsaw

B. Blank, M. Chartier, J. Giovinazzo, A.-S. Lalleman, J.-C. ThomasCEN Bordeaux-Gradignan

E. Badura, H. Geissel, L.V. Grigorenko, M. Hellström, C. Mazzocchi, I. Mukha, G. Münzenberg, C. Plettner, E. Roeckl, R.S. Simon

GSI Darmstadt

M. StanoiuGANIL Caen

C. Bingham, K.P. RycaczewskiOak Ridge National Laboratory

K. SchmidtUniversity Of Edinburgh

GSI:

Collaborators

GANIL:

J. Giovinazzo, B. Blank, M. Chartier, S. Czajkowski, A. Fleury, M.J. Lopez Jimenez, M.S. Pravikoff, J.-C. Thomas

CEN Bordeaux-Gradignan

G. de France, F. de Oliveira Santos, M. Lewitowicz, V. Maslov, M. StanoiuGANIL Caen

C. BorceaIAP Bucharest

R. Grzywacz, Z. Janas, M. PfütznerUniversity of Warsaw

B.A. BrownNSCL, MSU East Lansing

GANIL: