RITESH KSHETRI

1
High spin structure of 35Cl and the sd-fp shell
gap

• RITESH KSHETRI
• Nuclear and Atomic Physics Division,
• Saha Institute Of Nuclear Physics, Kolkata, INDIA.

1. Introduction Motivation. 2. Experimental
Details. 3. Experimental Results Analysis of
experimental sidefeeding intensity pattern.
Lifetime analysis by using DSAM. DCO
Polarization measurements. Level scheme. 4.
Theoretical predictions with Shell Model. 5.
Summary Conclusion.
2
1. Introduction Motivation
Mass 40
region has many interesting features A) Nuclei
in the neighbourhood of doubly closed 40Ca

good
testing ground for shell model calculations
since they belong to sd-fp interface,
yield information on
effective nuclear interaction. B) Spectroscopy
of even doubly magic nuclei like 40Ca revealed
deformed states at low excitation energies
indicating that they can easily lose spherical
shape. C) Superdeformation has been seen very
recently in doubly magic NZ 40Ca and 36Ar. D)
Violation of Mirror Symmetry, Vanishing of N20
shell gap in neutron-rich nuclei also seen. E)
Nuclei in this mass region have low level
density. As a result, sidefeeding to a level may
be substantial which presents great problem for
measurement of lifetime of a level especially in
the singles mode.

• Mass 40 region - extensively studied by
  experiments during 70s with some limitations
• a) limitations regarding analysis of
  lifetime data,
  
• b) experimental limitations in
  sensitivity efficiency of detection systems.
• With these motivations, we studied the
  spectroscopic properties of nuclei in this mass
  region using
• a) detector system having a better
  resolving power (used Clover detectors which have
  high efficiency
• for high energy g-rays),
  
• b) used reverse kinematics for higher
  velocities to get distinct lineshapes,
  
  
• c) improved lifetime analysis
  techniques.
  
  

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2. Experimental Details

• (INGA set up at TIFR, Mumbai)
• (1) 28Si (70 MeV) 12C (50mg/cm2), (Au backed
  10mg/cm2)
• (2) 16O (40 MeV) 24Mg (500mg/cm2), (Thick Ni
  backed)
• Array of 8 Clover detectors
• Seven angles (30, 60, 65, 90, 105, 120,
  145)
• Fourteen NaI multiplicity detectors
• BARC-TIFR Pelletron

Indian National Gamma Array

• (INGA set up at NSC, New Delhi)
• (3) 28Si (88 MeV) 12C (50mg/cm2), (Au backed
  10mg/cm2)
• Array of 8 Clover detectors
• Two angles (80, 136)
• Ten-element charge particle detector array
• Four neutron detectors
• NSC Pelletron

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3(i). Analysis of side-feeding intensity
pattern

• Observed lifetime (?eff ) of a Nuclear Level
  includes
• effect of actual lifetime (? ) of the level
• contributions (?d) from discrete transitions from
  above
• contribution (?sf) due to side feeding.

If ?d ?sf ? ? ? ?eff are different If
?d or ?sf ? ? ? ?eff are different
depending on Id Isf
Measure ? Fractional side-feeding Intensity (
nsf )
nsf ( Iout Iin ) / Iout I3 (I1 I2) /
I3
Unknown Discrete Feeding
?d2
1
?d1
Direct Feeding
?sf
2
?
3
5
Formulation using the statistical model CASCADE
F. Puhlhofer, Nucl. Phys. A 280(1976)267
Input contains adopted level schemes of all
residue nuclei
CASCADE
Output contains for each residual nucleus
population distribution over a matrix in Eex J
For a residual nucleus using adopted level
schemes experimental branching ratios, we
distribute these theoretical populations and
calculate nsf of a level.
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Double peaked nature
? seems promising work still in progress
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3(ii). Lifetime Measurements using Doppler Shift
Attenuation Method

• Extracting t from GTA spectrum
• step 1 g1 gated, observed g ? ?eff measured
  
• (SF
  excluded, SF1 included).
• step 2 g gated, observed g1 ? ?deff
  measured
• 
  (SF1 included).
  
• step 3 from step1 step2 ? subtracted
  effect of SF1
• 
  ? ? extracted

SF1
td
SF
g1
t
t not effected by SF
g
Extracting t from GTB spectrum If step 1 is
not possible because of insufficient statistics
of g1 then putting gate on g2 gives limit
for lifetime
? lt ?eff
ta
g2
t effected by SF
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Lineshapes of 882 2179 in 1763 gate
3942
9/2
2179
2645
7/2
1185
1763
5/2
1763
0
3/2
9
Doppler-shifted spectra for 1059 1185 keV g-ray
t 26ps
7873
2466
5407
11/2-
1059
4347
9/2-
1185
3163
7/2-
3163
0
3/2
10
Lineshape of 3163 keV g-ray
4
t 3 -1 ps
4347
9/2-
1185
3163
7/2-
3163
0
3/2
nucl-ex/0507019
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Comparison with Single Particle Estimate

• Using Adopted value t 45.3 (6) ps,
• calculated B(M2)expt1 4.73 (24) ?o2fm2
• 18.1(9) times retarded value compared to the
  single particle estimate2 of 85.61 ?o2fm2 for a
  stretched M2 transition
• ?sp 2 ps.
•  
• Our measured value 341 ps.
• B(M2)expt 6834-39 ?o2fm2 0.8 Bsp
• 1. J.Keinonen et al. Phys. Rev. C 14 (1976) 160
• 2. Bohr Mottelson Vol.1, Pg 388

No M2 RETARDATION
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Results of Lifetime Measurements (tmean)
NNDC
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Estimation of normalization factor a(Eg)
3(iii). Polarisation Measurements
Nucl. Inst. Meth. A 491 (2002) 113
Measured Asymmetry values for different g
transitions
IPDCO measurements
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3(v).
Level Scheme of 35Cl
Ritesh Kshetri et. al., Nuclear Physics A 781
(2007) 277.
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4. Theoretical Predictions with Shell Model

• Large basis calculations performed with OXBASH.
• Inert Core 16O
• Interaction sdpfmw taken from WBMB sd-fp shell
  Hamiltonian.
• Valence particles 19 ? Unrestricted
  calculations difficult ? Use truncations.
  

Truncations
(Theory I) for positive parity states Valence
Space is 1d5/2, 1d3/2, 2s1/2.
(Theory III) for both positive negative parity
states Valence space is 1d3/2, 2s1/2, 1f7/2,
2p3/2 1d5/2 full 1p-1h taken for negative
parity states.
(Theory II) for negative parity states Only
1p-1h states taken in 1d5/2, 1d3/2, 2s1/2,
1f7/2, 1f5/2, 2p3/2, 2p1/2
7 valence nucleons
19 valence nucleons
Ground state Binding Energy prediction -215.433
MeV Experimental B.E. -215.321 MeV
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Theory I Valence Space is 1d5/2, 1d3/2, 2s1/2.
Theory II 1p-1h states in 1d5/2, 1d3/2, 2s1/2,
1f7/2, 1f5/2, 2p3/2, 2p1/2
After depressing f7/2 p3/2 spes by 1.5 MeV
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Theory III Valence Space is 1d3/2, 2s1/2, 1f7/2,
2p3/2, 1d5/2 full, (only 1p-1h states for
negative parity states).
After depressing f7/2 p3/2 spes by 4 MeV
19
a
20
6 NNDC
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5. Summary Conclusion

• 35Cl nucleus is studied using the gamma
  spectroscopic techniques.
• Analysis of side-feeding intensity seems
  promising and is in progress.
• Few new levels have been identified and spins
  parities have been assigned from DCO and
  polarization measurements. Mixing ratios of
  several transitions have been reported for the
  first time.
• Estimation of lifetimes and limits for them have
  been provided for a number of levels. Lifetimes
  of 6 new excited states have been estimated for
  the first time.
• Large basis shell model calculations were
  performed. Excitations to fp shell is seen to be
  essential to reproduce even the positive parity
  higher spin states. sd-fp shell gap has to be
  decreased to reproduce both parity levels. This
  may be an artifact of other effects connected to
  the Hamiltonian matrix elements and the
  particular truncation scheme involved.

Acknowledgements
We would like to thank Mr. Pradipta Kumar
Das for preparing the target and the accelerator
staff at the BARC-TIFR Pelletron Accelerator
Facility, Mumbai and the IUAC Pelletron
Accelerator Facility, New Delhi for their sincere
efforts in delivering the beams.
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Collaborators
Saha Institute of Nuclear Physics, Kolkata-700064
M. Saha Sarkar, Indrani Ray, P. Banerjee,
Rajarshi Raut, A. Goswami, J. M. Chatterjee, S.
Chattopadhay, U. Datta Pramanik, A. Mukherjee, C.
C. Dey, S. Bhattacharya, B. Dasmahapatra
Department of Physics, Bengal Engineering and
Science University, Howrah-711104
S. Sarkar
Department of Physics, Surendranath Evening
College, Kolkata-700009
Samit Bhowal
University of Calcutta, Kolkata-700009
G. Gangopadhyay
Anandamohan College, Kolkata-700009
P. Datta
Tata Institute of Fundamental Research,
Mumbai-400005
H. C. Jain
Nuclear Science Centre, New Delhi-110067
R. K. Bhowmik, S. Muralithar, R. P. Singh, R.
Kumar