Post on 20-Jan-2016
description
Proton and Two-Proton Decayof a High-Spin Isomer in 94Ag
Ernst ROECKL
GSI Darmstadt and Warsaw University
• Fusion-evaporation reactions 58Ni(40Ca,p3n)94Ag
• Isotope separation on-line
• Detectors:
› Silicon strip detectors: ß particles, protons
› Germanium detectors: γ rays
Experimental Techniques
Properties of the (21+) isomer of 94Agdeduced from - and -proton studies
E* = 6.7(5) MeV
T1/2 = 0.39(4) s
fusion-evaporation = 350 pb *)
*) Further parameters characterizing the experiment: 40Ca intensity 75 particle-nA, 58Ni-target thickness
3 mg/cm2, ion-source efficiency 10%, intensity of the
94Ag (21+) beam 10 atoms/min, measuring time 80 h
I = (21+)
C. Plettner et al., Nucl. Phys. A 733, 20 (2004)I. Mukha et al., Phys. Rev. C 70, 044311 (2004)
(21+) isomer in 94Ag: What was known by end of 2003
Population 0+ ground-state of 94Ag (T1/2 ~ 29 ms, A. Stolz, Ph D Thesis (2000)): not observed in fusion-evaporation reactions !
Comparison with shell-model predictions (up to 4p-4h excitations): inversion of 17+
and 21+ levels in 94Ag !
0.39(4) s
0.61(2) s
Beta and -delayed proton emission studied by using a multiple -ray trigger !
N=Z=47
One-proton decay of the (21+) isomer of 94Ag
Search by measuring coincidences between charged particles and - cascades in 93Pd
Experiment:I. Mukha et al., Phys. Rev. Lett. 95, 022501 (2005)
Theory:WKB estimate of protonwidths à la Gamow
Si-- coincidence, gates on 93Pd lines *)
Si singles spectrumscaled down by a
factor 240
1.01(3) MeV
0.79(3) MeV
*) 93Pd -rays identified by C. Rusu et al., Phys. Rev. C 69, 024307 (2004)
Data on the one-proton decayof the (21+) isomer of 94Ag
Ep (MeV)
______
0.79(3)
1.01(3)
Intensityper 100 decays_______
1.9(3)
2.2(4)
T1/2(exp)
(s)
______
21(6)
18(4)
L
___
(4)
(3,5)
T1/2(WKB) /T1/2
(exp)
_______________
110-6 (L=4)
310-7 (L=5)
Deformation effect?
Ii If
__________
(21+) (33/2+)
(21+) (33/2–,35/2–)
Two-proton decay of the (21+) isomer of 94Ag
Search by measuring coincidences between charged particles and - cascades in 92Rh
Experiment:I. Mukha et al., accepted for publication in Nature
Theory:Three-body breakup model,L. Grigorenko et al., Phys.Rev. C 68, 054005 (2003)
Si-Si-- coincidence, gates “shifted off”
the 92Rh lines
Si-Si-- coincidence, gates on 92Rh lines *)
*) 92Rh -rays identified by D. Kastet al., Z. Phys. 356, 363 (1997)
1.88(9) MeV
Experimental data on the two-proton decayof the (21+) isomer of 94Ag
E2p = 1.88(9) MeV
… and correlation data ...
I = 0.5(3) % per decay
T1/2 = 0.39(4)/0.5(3)% = 80(50) s
Ii If : (21+) (13+,15+)
L = (6,8)
Relative energy spectra ...
Predictions of thebreak-up model
Experiment
Expected for sequential 2p decay
… for proton-protoncorrelations
… for proton-92Rhcorrelations
Expected for isotropic 2p decay
Di-proton (2He) emission
a = (long-to-short axis ratio)-1/2
Simultaneous emissionof 2 protons
Simultaneous emissionof 2 protons
Summary and Outlook
The proton decay of the (21+) isomer of 94Ag has unprecedented features including
• discovery of one- and two-proton radioactivity from one and the same nuclear state,
• unambiguous identification of one- and two-proton by measuring coincidences with characteristic rays,
• new kind of emission of protons from the poles of a prolate-deformed nucleus, tentatively deduced from correlation data.
It is mandatory to try to improve the proton-proton correlation data with respect to statistics and angular resolution: Where can such a measurement be done ?
… if you want to see the field of exotic-beam research‘in full glory’, please feel invited to the following reading
The Euroschool Lectures on Physics with Exotic Beams, Vol. 1Lecture Notes in Physics Vol. 651 (2004), J. Al-Khalili and E. R. (Eds.)
Table of Contents: ‘The Why andHow...’ (M. Huyse), ‘Practioner’sShell Model’ (H.Grawe), ‘Halo Nuclei’(J. Al-Khalili), ‘In-flight Separation’(D.J. Morrissey, B.R. Sherrill),‘Stored Exotic nuclei’ (F. Bosch) …‘Precision Weak-interaction Studies’(N. Severjins), ‘Nuclear Astrophysics’(K. Langanke, F.-K. Thielemann, M.Wiescher), ‘Medical Applications ofAccelerated Ions’ (W. Kraft-Weyrather)
www.springeronline.com/3-540-22399-1
Si-Si-- coincidence, gates on 93Pd lines *)
Singles spectrum scaleddown by a factor 240
1.01(3) MeV
0.79(3) MeV
“ gate”
All spectra
gated by 93Pd - rays *)
*) 93Pd -rays identified by C. Rusu et al., Phys. Rev. C 69, 024307 (2004)
L=(4)
L=(3,5)
1.4 - 1.6 MeV
1.8 - 1.95 MeV
ESi1+ ESi2 gates:
Si-Si-- coincidence, gates “shifted off”
the 92Rh lines
Si-Si-- coincidence, gates on 92Rh lines *)1.88(9) MeV
92Rh
94Pd
*) 92Rh -rays identified by D. Kastet al., Z. Phys. 356, 363 (1997)
This work
Estimated by extrapolatingsystematical trends(G. Audi et al., Nucl. Phys.A 729, 337 (2003))
What this talk is about
1. In the odd-odd nucleus 94Ag an isomer has been observed to have the following unique properties:
• excitation energy of 6.7 MeV,• tentative spin/parity assignment of (21+),• half-life of 0.4 s,• one-proton and two-proton radioactivity
2. The one-proton decay is strongly hindered.
3. The two-proton decay is strongly enhanced.
Array of three silicon-strip detectors
– High resolution and low energy-threshold for detecting charged particles (positrons, protons)– High granularity*) position information
I. Mukha et al., Nucl. Phys. A 745, 9285 (2004)
*) Only 2 segments per detector used in this work