Spectroscopic studies of neutron
deficient light nuclei
decay properties of 21Mg, 25Si and 26P nuclei
Jean Charles THOMAS
University of Leuven / IKS, Belgium
University of Bordeaux I / CENBG, France
PROCON 2003, Legnaro-Padova, Italy
February 12-15 ,2003
Decay properties of neutron deficient
light nuclei
 Global properties
• Short half-lives (ms)
• High Qb values
• Low Sp values
b-delayed charged particle emission
 b+ (C.E.) emission
• Selection rules:
• Fermi:
DT=0 ; DJ=0 ; pf = pi
• Gamow-Teller: DT=0±1; DJ=0±1 ; pf = pi
• Reduced transition probability:
T1 / 2
K
C
ft  f *


2
2 2
2
B.R .
B(F)  B(GT)
G V   GA 
21Mg, 25Si
and 26P nuclei
Tz = -2
14
22Si
13
Tz = -3/2
16
27S
32S
15
26P
31P
25Si
30Si
22Al
27Al
stable nuclei
12
20Mg 21Mg
26Mg
b emitters
b-a emitters
11
20Na
23Na
b-p emitters
b-2p emitters
Production of neutron deficient nuclei at GANIL
Production target
(fragmentation)
Fragment separation
magnetic dipoles
velocity filter
target
degrader (9Be)
Detection set-up
ion source
Accelerator
36Ar18+ @ 95 MeV/u
Detection set-up
Implantation
 Identification
b-(2)p radioactivity
• time of flight: E1D6, E2
• energy loss: E1D6 , E1, E2, E3
g detection
BEAM
 Spectroscopic study
• b-(2)p spectrum: E3
• b coincidence: E4
• g spectrum: germanium detector
b coincidence
 Acquisition trigger
• implantation: E1D6
• radioactivity: E2, E3, E4
Identification and counting rate : 26P example
Identification matrix (DE,T.o.F)
Counting rates
 Implantation:
 Contamination:
• 65 ions/s (26P)
• 300 ions/s (21Mg, 25Si)
• 10 % (for 26P)
• < 1 % (for 21Mg, 25Si)
Analysis of b-delayed proton spectra
b energy deposit in E3
Counts
Counts
b coincidence in E4
b coincidence with E4
E3 (keV)
Energy (keV)
Identification of g transitions
Counts
g spectrum in
26P decay
Energy (keV)
 Need for a good g detection efficiency
 Use of in-beam experimental results
26P g-decay
scheme
21Mg
Experiment
Theory
decay scheme
25Si
Experiment
Theory
decay scheme
26P
Experiment
Theory
decay scheme
Mirror asymmetry principle
 Charge independence hypothesis of nuclear interactions: symmetry of
analog b transitions
b+ : p → n + e + + 
E.C. : p + e- → n + 
b- : n → p + e- + 
ftn
ft+
n
p
 Isospin symmetry breaking: asymmetry in mirror b-decays

ft
ft
-
-1
p
Systematics of experimental  values (A40)
 = 4.8 (4) %
 Allowed Gamow-Teller
transitions (log(ft)<6)
 17 couples of nuclei
 46 mirror transitions
Average asymmetry  :
11 (1) % in the 1p shell (A<17)
0 (1) % in the (2s,1d) shell (17<A<40)
Mirror asymmetry in the b decay of
A=21 & A=25 nuclei
 (%)
Experiment
Theoretical calculations
N. A. Smirnova & C. Volpe
INC + HO
IC + WS
INC + WS
10 ± 70
+1.8
-2.7
-0.6
20 ± 30
-2.7
-1.4
-4.1
0 ± 40
+1.1
-2.1
-1.1
0 ± 20
-8.5
-2.7
-11.1
30 ± 40
-7.1
-3.3
-10.1
Spectroscopic studies of neutron deficient nuclei
 Suitability of the fragmentation production method associated with the
spectroscopic study of neutron deficient light nuclei
 good selectivity and production rates
 good agreement between experimental results and shell model calculations
(nuclear structure and b decay strength)
 access to decay properties of exotic nuclei (21Mg, 25Si, 26P, 22Al, 27S)
 complementarity with in-beam studies
Perspectives
 Fundamental symmetries:
• study of the mirror asymmetry phenomenon
• evaluation of the Coulomb correction in super-allowed Fermi b decays
Need for high precision experiments
 Rare decay modes: study of the 2p radioactivity
Collaboration
B. Blank, G. Canchel, S. Czajkowski, J. Giovinazzo, CENBG Bordeaux - France
L. Achouri, LPC Caen - France
J. Äystö, P. Dendoveen, J. Honkanen, J. Jokinen, University of Jyväskylä - Finland
R. Béraud, A. Ensallem, IPN Lyon - France
A. Laird, University of Edinburgh – United Kingdom
M. Lewitowicz, F. de Oliveira-Santos, M. Stanoiu, GANIL Caen – France
C. Longour, IReS Strasbourg – France
21Mg, 25Si
DM (MeV)
26P
and 26P nuclei
25Si
21Mg
Z
N
Half-life of 26P
implantation
radioactivity
counts
Time correlation procedure
E4 > 0
Counts
Correlation intervals
Energy (keV)
Trad (ms)
Analysing procedure
 Transition assignment (energy):
• detector calibration (g sources, known b-g and
b-p transitions)
 Measurement of absolute transition intensities:
Ig,p 
g,p
N g , p * E ff
* Cg,p
g,p
N impl
B.R.
• Ng and Np: number of counts in spectra
• Effg, Effp: detection efficiencies
 g detection: radioactive sources
 p detection: simulations, nuclei implantation depth
• Cg and Cp: corrections on Ng and Np
 g detection: acquisition triggering
 p detection: fitting procedure + coincidence condition
• Nimpl: number of implanted ions
Identification of b-(2)p transitions
Counts
Counts
g in coincidence with (2)p emitted from the I.A.S.
(2)p transition identification
via total decay energy
Energy (keV)
Energy (keV)
Mirror asymmetry sources
Origin and consequences of the isospin non-conservation
in nuclear interactions
Nature:
Effects:
• Coulomb effects
• second class currents in weak interaction
• nucleon-nucleon interaction description
• shell structure of nuclear states
• calculation of b-decay transition probabilities
• beyond the V-A model of b-decay theory
Coulomb effects
Mirror asymmetry in allowed Gamow-Teller transitions:

ft
ft

-
- 1
with M  
f  - i
2
f   i
2
- 1
 A   j1,j2 
j1 j2
MM
2
2
j2  j1
-1

• isospin configuration mixing
• radial overlap of nucleon wave functions:
j2  j1


  j1 j2  o R j1 ( r ) R j2 ( r )
“binding energy effects”
Binding energy effects
The last proton of the b+ emitting nucleus is less bound than
the last neutron of the b- emitting nucleus : Sp+ < Sn radial overlap mismatch of wave functions in the b+ decay:
+ < - i.e.  > 0
Systematic approach of binding energy effects
How to see the binding energy effects on the radial overlap of the nucleon
wave functions?
• Behaviour of  with the total angular
momentum of the emitting nucleus
 decreasing of  as Ji is increasing i.e. as
the centrifugal barrier is increasing
• Behaviour of  with the binding energy
difference of the initial and final nucleons
 increasing of  with R-/R+ where:
R- = Sn- - Sp- + E-*
R+ = Sp+ - Sn+ + E+*
i.e.   R-/R+  -/+