B K Nayak

B N Joshi

B V John

D C Biswas

D Patel

G Prajapati

L S Danu

N N Deshmukh

P Prajapati

S Appannababu

S Mukherjee

S Mukhopadhyay

Shradha Dubey

V V Desai

Y K Gupta

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Proceedings of the DAE Symp. on Nucl. Phys. 57 (2012) 552
Available online at www.sympnp.org/proceedings
Elastic scattering angular distribution in the 7Li + 232Th reaction 
Shradha Dubey1, S. Mukherjee1, D. Patel1, N. N. Deshmukh1, S. Appannababu1, 
P. Prajapati1 ,Y.K. Gupta2, G. Prajapati2, V. V. Desai2,,    B. N. Joshi2, L.S. Danu2,                 
S.Mukhopadhyay2, B.V. John2, B.K. Nayak2,  D.C.Biswas2 
 
1Physics department, The M. S. University of Baroda, Vadodara-390002, India. 
2Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai – 400085, India. 
email: dcbiswas@barc.gov.in                                                                                      
 
Introduction 
Elastic scattering reactions are the 
simplest of all nuclear reactions. Precise 
measurements of the elastic angular 
distributions determine parameters of the real 
and imaginary parts of the nuclear interaction 
potential. From systematic analysis of elastic-
scattering measurements involving tightly 
bound nuclei, the so called “threshold 
anomaly” (TA) has been observed in a number 
of systems [1]. A characteristic localized peak 
in the real part and the corresponding decrease 
of the imaginary part of the potential are 
observed as the bombarding energy decreases 
towards the Coulomb barrier. This has been 
understood in terms of couplings of elastic 
channel to the direct reaction channels that 
generate an additional attractive real dynamic 
polarization potential. In an earlier work, it has 
been observed that in heavy ion induced 
reactions, the projectile structure plays an 
important role [2]. Particularly, in case of 
scattering of loosely bound projectiles a 
different type energy dependence from that of 
TA is observed, which has been known as 
`breakup threshold anomaly’ (BTA). In case of 
BTA, a repulsive real dynamical potential is 
generated due to couplings of breakup 
channels to the elastic scattering. There are 
some contradictory observations regarding 
BTA. For 7Li + 208Pb, TA has been observed, 
whereas for 6Li + 209Bi, the BTA has been 
observed [3]. Therefore, more measurements 
involving heavy targets and weakly bound 
projectile are required to understand the 
systematics of TA and BTA. 
In the present work, the results on 
investigation of elastic scattering for 7Li + 
232Th system have been reported through very 
precise and complete angular distribution 
measurements at energies from below the  
 
Coulomb barrier to approximately twice this 
value. The total reaction cross sections for this 
system have also been derived in order to 
investigate the role of breakup on the total 
reaction cross section.  
 Experimental details and results:                 
The experiment was performed using 7Li 
beam from BARC-TIFR Pelletron facility, 
Mumbai, India. The beam was bombarded on 
self supporting 232Th target of thickness 
1.6mg/cm2and the elastically scattered 7Li ions 
were detected by four silicon surface barrier 
detectors in ∆E-E telescopic arrangements. 
The telescopes used had a thickness (T1) with 
∆E=25 µm and E=300 µm (T2) with ∆E=15 
µm and E=1.5 mm (T3) with ∆E=15 µm and 
E=1.0 mm and (T4) with ∆E=15 µm and E=1.0 
mm. Two monitor detectors with thickness 
around 300 µm were used for absolute 
normalization and beam monitoring. The 
elastic scattering angular distribution 
measurements were carried out for different 
beam energies covering a wide range from 24 
to 44 MeV.  
. 
 
Fig.1: Typical  ∆Ε Vs E spectrum for            
7Li + 232Th system at θ lab= 60°. 
Proceedings of the DAE Symp. on Nucl. Phys. 57 (2012) 553
Available online at www.sympnp.org/proceedings
Fig.1 shows a typical two-dimensional 
∆E vurses E spectrum for the present system 
7Li + 232Th at a laboratary angle θlab=60ο. The 
elastic scattering angular distribution  
measurements have been carried out in a wide 
angular range from 250 to 1700. The ratios of 
elastic to the Rutherford scattering cross 
sections have been plotted as a function of 
scattering angle (θc.m.) for various bombarding 
energies as shown in Fig.2. 
                                                                
The optical model analysis of the elastic 
scattering data were performed using the 
SNOOPY8Q code [4]. In the fitting procedure 
the real and imaginary diffuseness parameters 
(ao and aw ) were kept fixed and only the 
strength of real and imaginary potential 
parameters (Vo and Ws) were varied to obtain 
the best-fit of the experimental data.  Over all, 
very good fits to the experimental data were 
obtained   at   all   energies as shown in. Fig.2. 
The values of the potential parameters for the 
best fit and the total reaction cross section are   
shown  in Table-I. The best fitted optical 
model parameters show significant energy 
dependece as reflected from Table-I. This 
significant energy dependence is a 
characteristic feature of the elastic scattering. 
More detailed analysis with different form of 
the potential is being carried out and the results 
will be presented in the symposium.                                   
This work is partially supported by a 
research project being financed by UGC-DAE-
CSR, kolkata.                                                                                              
θcm (deg)
20 40 60 80 100 120 140 160 180
σ e
l/σ
R
10-2
10-1
100
E=44MeV
E=40MeV
E=35MeV
E=32MeV  
E=30MeV
E=26MeV
E=24MeV
7Li  +232Th
 
Fig.2: Elastic scattering angular distribution 
for the 7Li + 232Th system at different beam               
energies. The solid lines are optical–model fits 
to the data using the SNOOPY8Q code.    
 
TABLE-I: Optical model parameters obtained by fitting to experiment elastic differential cross  
                   section data using the SNOOPY8Q code in 7Li + 232Th reaction.             
                    
Energy(MeV) Vo Ws ao aw σtot (mb) 
44 31.53 18.83 0.670 0.670 2110 
40 35.28 33.61 0.670 0.670 1639 
35 42.23 23.18 0.670 0.670 685.2 
32 79.67 35.15 0.670 0.670 364.4 
30 132.96 30.41 0.670 0.670 161.9 
26 70.00 38.50 0.670 0.670 4.196 
24 55.00 38.50 0.670 0.670 0.2499 
 
References 
[1] G.R. Satchler, Phys.Rep. 199,147(1991). 
[2] D.  C. Biwas, R. K. Choudhary, D.M. 
     Nadkarni, and V. S. Ramamurthy, 
     Phys. Rev. C 52, R2827 (1995). 
  [3] N.N. Deshmukh et al., Phys. Rev. C83, 
             024607 (2011). 
   [4] P. Schwandt, SNOOPY8Q Optical model 
          code, Indiana University, report, (1984).
 


Elastic scattering angular distribution in the 7 Li + 232 Th reaction

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Elastic scattering angular distribution in the 7 Li + 232 Th reaction

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