Prakash Chandra Rout Nuclear Physics Division Bhabha ...pell/talkpdf_webpage/pcrout_pic13.pdf ·...
Transcript of Prakash Chandra Rout Nuclear Physics Division Bhabha ...pell/talkpdf_webpage/pcrout_pic13.pdf ·...
Prakash Chandra Rout
Nuclear Physics Division
Bhabha Atomic Research Centre
Shell effect studies in level density
� Introduction
�Motivation
�Experimental details
�Data reduction and results
�Summary
Outline of the talk
Introduction:
Shell effect is a cornerstone of the mean field theory describing finite
fermionic systems
Filled proton and neutron shells with magic configuration gives an
extra stability with respect to that expected from the average
behaviour described by the liquid drop model
Physical phenomena influenced by shell effect • Fission isomers
• Super heavy elements
• Super-deformed nuclei
• New magic numbers in exotic nuclei
Shell effect also affect the nuclear level density
(a)Resonance spectra of slow neutrons
(b) Direct count of levels populated in charged particle reactions
such as (p, p'), (α,p), and so on
(c) Analysis of evaporation spectra
Main sources of evidence on NLD:
‘a’ increases linearly with the mass
number of the nucleus (~A/8.5 MeV-1),
there is a large departure from this
behaviour at shell closures.
a ~A/26 MeV-1 at A~208
Shell correction in level density
Measurement of damping of shell effect near doubly closed shell
nucleus 208Pb
Motivation
Long standing prediction ……….
Shell effect on the NLD parameter is expected to decrease
asymptotically to its liquid drop value at Ex >40 MeV
V. S. Ramamurthy, S. S. Kapoor, and S. K. Kataria PRL 25 (1970) 386
No direct experimental measurement, spanning a wide EX range,
has been ever reported on this very important subject
One way is to measure evaporation particle spectra populating
low excitation energies in residual nuclei in this heavy mass region
Shell effect on nuclear level density as a function of EX is expected
to be pronounced near the doubly shell-closed 208Pb
There is a difficulty
So, populating low EX in residual nuclei after particle evaporation
is practically impossible
EX in parent Compound Nucleus is too high because of
the Coulomb barrier in the entrance channel
Measured proton evaporation spectra in 10;11B+198Pt reactions and
extracted the NLD in 208Pb at an excitation energy ~ 50 MeV
Washing out of shell effect
M. Lunardon et al., Eur. Phys. J. A 13, 419 (2002).
One possible way is
The measurement of particle spectra following the
transfer reaction populating low EX in the intermediate parent
nucleus
Measurement of continuum γ-ray spectra following inelastic
scattering and transfer reactions.
3He induced inelastic scattering and single nucleon transfer reaction
to populate 205-208Pb and extracted the energy dependence of NLD
from the coincident spectrum up to EX ~ 6 MeV
E. Melby et al., PRL 83, 3150 (1999)
N. U. H. Syed et al., PRC 79, 024316 (2009)
Effect of shell correction on ‘a’ can be seen by comparing the
particle spectra in the nuclei both near and away from shell closure.
Triton transfer fusion reaction on 205Tl and 181Ta to populate the CN 208Pb and 184W respectively and measure evaporation neutrons
Reaction: 205Tl(7Li, α)208Pb* → n+207Pb*181Ta(7Li, α)184W* → n+183W*
Beam:
30 MeV 7Li (pulse width ~1.5 ns, pulse period ~ 107 ns)
Targets:205Tl (enriched >98%) and thickness 4.7 mg/cm2
natTa and thickness 3.7 mg/cm2, 12C , Ta2O5
Detectors: α α α α : 8 CsI(Tl) - 2.5×2.5×1 cm3 coupled to Si(PIN)
θlab ~ 126 – 155o
Neutron: Neutron detector array(15 bars , 1m × 6cm × 6cm)
D~ 1 m , θmean ~ 90o
Energy from TOF
Experimental Details
CsI array
Neutron detector array
Shadow
pyramid
Experimental set up205Tl(7Li,αααα)208Pb* →→→→ n+207Pb*
181Ta(7Li,αααα)184W* →→→→ n+183W*
� TOF = 0.5* (TL+ TR ) + offset
� Position α (TL- TR )
� Q = √ (QL* QR)
� Multiplicity (Mn=1)
� Energy and PSD of CsI(Tl) detector
Data reduction:
Neutron energy (MeV)
2 3 4 5 6 7 8 9
dσ
/dE
(ar
b.
unit
s)
10-4
10-3
10-2
10-1
100 Ι
ΙΙ
ΙΙΙ
Time (ns)
140 160 180 200 220 240
Co
un
ts
1
10
100
1000
n γ
En = 2.0 MeV 9.0 MeV
Measured neutron energy spectra
Ex= 22.7 MeV
20.8 MeV
18.9 MeV
Statistical Model Analysis
Statistical model (SM) analysis of the spectra was done using
CASCADE with the EX and J dependent NLD
dσ
/dE
(ar
b.
un
its)
10-4
10-3
10-2
10-1
100
101 data 205
Tl
∆S=2.2 MeV
∆S=13.1 MeV
Neutron energy (MeV)
2 3 4 5 6 7 8 910-5
10-4
10-3
10-2
10-1
100
101 data 181
Ta
∆S=2.2 MeV
∆S=13.1 MeV
Statistical Model Analysis
Statistical model (SM) calculation
ã = A/8.5 MeV-1
γ = 0.055 MeV-1
σα= 40 mb
shell correction energy ∆s = 13.1 MeV
( for 207Pb) fits the shape of neutron
spectrum for the Tl target while ∆s =2.2
MeV does not
Ex= 20.8 MeV
Ex= 20.6 MeV
Excitation energy dependence of the NLD parameter (Ignatyuk-1975)
Constraining all three parameters ( ã, ∆S and γ) is not possible from the data
addressing even a much wider excitation energy range.
Since the shell correction energy is known with a reasonably good accuracy
(within a few hundred keV )
Fixed ∆S and searched for an acceptable range of ã and γ
Shell correction energy for 207Pb and 206Pb are 13.1 and 11.7 MeV , respectively
dσ
/dE
(ar
b. u
nit
s)
10-4
10-3
10-2
10-1
100data
205Tl
γ = 0.035 MeV -1
γ = 0.060 MeV -1
γ = 0.075 MeV -1
Neutron energy (MeV)
2 3 4 5 6 7 8 9
1
2
3γ = 0.035 MeV-1
γ = 0.060 MeV-1
γ = 0.075 MeV-1
(a)
Rat
io
(b)
Extraction of damping parameter
A change in the ∆S value to 0.5 MeV
has <2% effect on the shape of the spectra
Statistical model fits for the central bin
for ã= A/8.5 MeV-1 and three γ values.
γ = 0.060 MeV-1 gives a good fit
δa (MeV)
7.5 8.0 8.5 9.0 9.5 10.0
γ (
MeV
-1 )
0.02
0.04
0.06
0.08
Allowed values of δδδδa & γ ?γ ?γ ?γ ?
Neutron resonance data,
γ= (0.079 ± 0.007) MeV-1
Mughabghab & Dunford
PRL 81 (1998) 4083
Statistical model calculation for all three energy bins:
δa (= A/ã) = 6.5 - 11.0 MeV & γ = 0.02 - 0.08 MeV-1
arXiv 1210:3213 [nucl-ex], accepted in PRL
Shell damping factor:
� Large shell correction (13.1 MeV for 207Pb and 11.7 MeV for 206Pb)
is required to explain the neutron spectra from 208Pb*
�An exclusion plot on level density parameter and damping factor
has been made
�Shell damping factor extracted from the present data
Summary
Possible improvement :
� For Charged particles Si CD or Strip detector
Response is linear
measure simultaneously all light charged particle
(handle on various reaction channels)
� Using liquid scintillator (LS) detectors ( ~ efficiency )
Pulse shape discrimination & TOF
(no ambiguity in n-γ discrimination)
Angular distribution
( contribution of non-compound nuclear component)
� Improvement of beam timing (sharper beam profile)
Collaborators:
D. R. Chakrabarty
V. M. Datar
Suresh Kumar
E. T. Mirgule
A. Mitra
V. Nanal
S. P. Behera
R. Kujur
Vivek Singh
(BARC-TIFR)
Acknowledgement:
S. S. KapoorNitali Dash
A. B. Parui
M. Pose &
Pelletron staff
Thank you for your kind attention
Energy dependent threshold Eth (En)E
th (
keV
)
En (MeV)
205Tlt
α7Li
Time (ns)
260 280 300 320 340 360 380 400 420
Co
un
ts /
0.5
ns
100
101
102
103
104
105
n
γ
γR