Angular momentum distribution for the formation of evaporation residues in fusion of 19F with 184W near the Coulomb barrier

Nuclear Physics A 850 (2011) 22–33 www.elsevier.com/locate/nuclphysa

Angular momentum distribution for the formation of evaporation residues in fusion of 19F with 184W near the Coulomb barrier S. Nath a,b,∗ , J. Gehlot a , E. Prasad c , Jhilam Sadhukhan d , P.D. Shidling e,1 , N. Madhavan a , S. Muralithar a , K.S. Golda a , A. Jhingan a , T. Varughese a , P.V. Madhusudhana Rao b , A.K. Sinha f , Santanu Pal d a Inter University Accelerator Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110067, India b Department of Nuclear Physics, Andhra University, Visakhapatnam 503003, India c Department of Physics, Calicut University, Calicut 673635, India d Physics Group, Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700064, India e Department of Physics, Karnatak University, Dharwad 580003, India f UGC-DAE Consortium for Scientific Research, Kolkata Centre, 3/LB8, Bidhan Nagar, Kolkata 700098, India

Received 27 October 2010; received in revised form 2 December 2010; accepted 13 December 2010 Available online 18 December 2010

Abstract We present γ -ray multiplicity distributions for the formation of evaporation residues in the fusion reaction 19 F + 184 W → 203 Bi 83 120 at beam energies in the range of 90–110 MeV. The measurements were carried

out using a 14 element BGO detector array and the Heavy Ion Reaction Analyzer at the Inter University Accelerator Centre. The data have been unfolded to obtain angular momentum distributions with inputs from the statistical model calculation. Comparison with another neighboring system, viz. 19 F + 175 Lu → 194 Hg 114 with nearly similar entrance-channel mass asymmetry, hints at the depletion of higher angular 80 momenta after crossing of the Z = 82 shell in the compound nucleus. © 2010 Elsevier B.V. All rights reserved.

* Corresponding author at: Inter University Accelerator Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi

110067, India. Tel.: +91 11 26893955; fax: +91 11 26893666. E-mail address: [email protected] (S. Nath). 1 Present address: Cyclotron Institute, Texas A&M University, College Station, TX 77843-3366, USA. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.12.004

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Keywords: N UCLEAR REACTIONS 184 W(19 F, X), E = 89.2, 94.2, 99.2, 104.3, 109.3 MeV; measured γ -folds in coincidence with evaporation residues, using BGO array and Heavy Ion Reaction Analyzer; deduced γ -multiplicities; calculated angular momenta with statistical model inputs. Comparison with 175 Lu(19 F, X) data

1. Introduction Angular momentum distribution in fusion reactions has mostly been studied for mediumheavy compound systems [1]. The studies have primarily aimed for a detailed understanding of the fusion mechanism below the Coulomb barrier. Enhancement of sub-barrier fusion cross sections by several orders of magnitude can be explained rather successfully by coupled-channels calculation. However, fusion cross section being the zeroth moment of the angular momentum distribution, different theoretical approaches can yield similar fusion excitation functions. Angular momentum distribution is more sensitive to the ingredients of the theoretical models than the fusion cross section itself. Experimental determination of the angular momentum distribution thus provides more stringent test for the theories aimed at explaining fusion mechanism below the barrier. For heavy compound systems formed in fusion reactions, fission becomes a significant exit channel. Especially for the production of the heaviest elements, the survival of the evaporation residue (ER) crucially depends on the competition between fission and particle evaporation. In this context, it is interesting to study the limits of angular momenta which lead to the formation of ERs. It has been pointed out by Ackermann [2] that higher partial waves are important for a better understanding of the survival of the compound nucleus (CN) in the form of ER. A significant part of the excitation energy, in these cases, is stored in rotation. For nuclei stabilized by their shell structure, shell correction energies of the order of a few MeV can be felt by the CN and lead to enhanced survival probabilities. Investigation of ER-gated CN angular momentum distribution for fissile systems may reveal such an effect if present. Only a handful of studies have been carried out so far in the compound mass region A ∼ 200 and above [3,4]. We have measured the γ -fold distribution in coincidence with ERs for the fusion reaction 19 F + 184 W → 203 Bi. We describe the experimental procedure in Section 2. Detailed account of the estimation of angular momentum distribution is presented in Section 3 followed by a discussion in Section 4. We summarize the results and conclude in Section 5. 2. The experiment The experiment was performed at the 15UD Pelletron accelerator facility of the Inter University Accelerator Centre (IUAC), New Delhi. A schematic of the experimental set up is shown in Fig. 1. A pulsed 19 F beam with 4 µs pulse separation was bombarded onto a 210 µg/cm2 184 W target [5] with 110 µg/cm2 carbon backing. ER measurements were performed at beam energies (Elab ) of 89.2, 94.2, 99.2, 104.3 and 109.3 MeV. ERs were separated from intense primary beam background by the Heavy Ion Reaction Analyzer (HIRA) [6] and transported to its focal plane (FP). The HIRA was operated at 0◦ with respect to the beam with full acceptance (10 msr). Two silicon detectors were installed inside the sliding-seal target chamber at ±25◦ to measure Rutherford-scattered beam particles. A 35 µg/cm2 carbon foil was placed 10 cm downstream of the target to reset the charge state of ERs. At the FP of the HIRA, a two-dimensional position-sensitive silicon detector with an active area of 50 mm × 50 mm was used to detect ERs. Since the system is very asymmetric (entrance-

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Fig. 1. Schematic of the experimental set up. The 14 element BGO array, in two parts, is shown on the left (side view). The HIRA is shown on the right (top view). Q, ED, M, and MD stand for magnetic quadrupole, electrostatic dipole, magnetic multipole, and magnetic dipole, respectively.

Fig. 2. Separation of the ERs from scattered beam particles and their unambiguous identification on the basis of energy and TOF for the reaction 19 F + 184 W at Elab = 104.3 MeV. The plot has been produced using ROOT [7]. |A −A |

channel mass asymmetry, η = App +Att = 0.813; Ap and At are projectile and target masses, respectively), scattered beam background at the FP was expected to be insignificant. Nevertheless, time of flight (TOF) of the slowly moving ERs over the flight path of the HIRA (8.82 m) was recorded for further cleaning of the background. The ERs were unambiguously identified in the two-dimensional plot of energy vs TOF. Fig. 2 shows the ERs forming a single group at Elab = 104.3 MeV. A multiplicity filter, consisting of 14 Bismuth Germanate (Bi4 Ge3 O12 , in short BGO) crystals, was placed around the target to detect γ -rays emitted by the ERs. The multiplicity filter had two groups with seven 38 mm × 75 mm hexagonal BGO element in each group. The two groups of BGO crystals, one above and one below the target, were mounted in a close geometry (at a distance of 24 mm from the target). The central detector of each group was pulled outward to ensure uniform efficiency for all the detector elements. The multiplicity filter covered 48% of the total 4π solid angle. The target was tilted at 45◦ with respect to the beam direction and about a horizontal axis to rule out attenuation of γ -rays in the target ladder. For constructing the γ -fold distribution, 14 time-to-digital converter (TDC) signals were recorded with ER arrival time at the FP as the common start and individual BGO timing signals, suitably delayed, as the stop. The BGO detectors were gain-matched

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Fig. 3. Ungated and ER-gated γ -fold spectra, measured at Elab = 99.2 MeV for the reaction 19 F + 184 W using a 14 element BGO detector array.

and the threshold for each detector was kept at ∼ 150 keV, using standard γ -ray calibration sources. Analysis of data has been performed using the data analysis code CANDLE [8]. 3. Data analysis and results 3.1. Construction of γ -fold distribution We generated the γ -fold distribution offline, from the 14 TDC signals, using CANDLE. The γ -fold (p) for each event was given by the number of BGO detectors having the timing signals within the prompt peaks in the TDC spectra. The raw γ -fold spectrum was gated with the energy versus TOF spectrum (shown in Fig. 2 for a particular Elab ) at each Elab for removing the background. The ungated and the ER-gated γ -fold spectra for Elab = 99.2 MeV are shown in Fig. 3. One can notice that the gating is particularly effective for lower γ -folds, as those are heavily contaminated by γ -rays from the non-ER events. The direct identification of ERs, thus ensures unbiased and efficient determination of ER γ -folds, as was pointed out in Ref. [9]. Experimental γ -fold distributions for all Elab are shown in the left panel of Fig. 4. 3.2. Calculation of moments of γ -multiplicity distributions Often the γ -multiplicity probability distribution is characterized by its moments [10]. The first three moments viz. the mean, the variance and the skewness are of special significance, which are measures of central tendency, dispersion and degree of departure from symmetry, respectively. We followed the formalism presented in Ref. [11] to unfold the ER-gated γ -fold distribution into the moments of the γ -multiplicity distribution. Here we outline the method, in brief. Let N be the number of identical detectors in the array and the efficiency of each is Ω. Then the probability that a cascade of Mγ uncorrelated γ -rays will cause the number of p detectors to fire is given by     p M M p  N 1 − (N − l)Ω γ (−1)p−l (1) PNpγ = l p where

N  p

l=0

denotes the number of ways p combinations are possible out of N numbers.

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Fig. 4. The best fit to the experimentally observed γ -fold distributions are shown in the left panel. The right panel shows the corresponding γ -multiplicity distribution approximated by a Fermi function.

Let the probability of observing a p-fold coincidence be denoted by P (p). It is related to the γ -multiplicity probability distribution P˜ (Mγ ) by P (p) =

∞ 

M PNpγ (Ω)P˜ (Mγ )

(2)

Mγ =0

where the summation on the right-hand side runs upto the maximum value of Mγ , say Mγ max , in practice. P (p) can explicitly be expanded in terms of the factorial moments  fold-probabilities

 MThe γ m! = M (M − 1) · · · (Mγ − m + 1), as given below γ γ m P (p) =

 ∞    M γ

m=0

m

 m! Apm (Ω)

(3)

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Table 1 Experimental raw moments (μ 1 , μ 2 and μ 3 ) of the γ -multiplicity distribution. The errors are indicative of ±5% uncertainty in determining the absolute efficiency of the BGO detectors. Elab (MeV) 89.2

Mγ 

Mγ2 

Mγ3 

+0.9 16.1−0.8

348.4+36.8 −31.6

+1499.2 9373.1−1231.8

+1.0 17.4−0.8 +0.8 14.6−0.7 +0.9 16.5−0.8 +0.8 15.1−0.7

94.2 99.2 104.3 109.3

369.8+39 −33.6 260.2+27.3 −23.5 330.9+34.8 −30.0 287.0+30.2 −26.0

+1474.3 9244.6−1211.5

5424.8+857.8 −705.5

+1215.7 7649.8−999.4 +1025.9 6463.4−843.5

where

    p (−1)m N  p−l p (N − l)m Ω m . (−1) Apm (Ω) = l p m!

(4)

l=0

Eq. (3) is a matrix equation in which the matrix A is (N +1)×∞ dimensional and its elements below the diagonal in the left (N + 1) × (N + 1) block vanish. Solving for the lowest factorial moments (upto N ) by inverting the left (N + 1) × (N + 1) block of A, we get    N Mγ m! = A−1 (5) mp P (p). m p=0

The matrix A−1 is the response matrix of the array which is dependent on the number of detectors (N ) and the efficiency of each detector (Ω) in the array. The factorial moments can be used to find raw moments i.e. moments about the origin. The n-th raw moment is given by μ n =

n 

S(n, i)μ {n}

(6)

i=1

where μ {n} is the n-th factorial moment and S(n, i) is the Stirling number of the second kind given by   i 1 i j n , 0  i  n. S(n, i) = (−1)i−j (7) j i! j =0

The raw moments can be further used to find central moments i.e. moments about the mean   n   i i n μ n−i μ 1 . (−1) (8) μn = i i=0

We calculated the first, the second and the third raw moments of the γ -multiplicity distribution, P˜ (Mγ ), from the experimentally observed fold distributions. These are given in Table 1 as a function of Elab . 3.3. Conversion to γ -multiplicity distribution The experimental γ -fold distribution contains only a limited information about the actual γ multiplicity distribution, P˜ (Mγ ). Therefore, P˜ (Mγ ) can only be estimated in an approximate

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way. The fastest and the easiest method of reconstructing a distribution is by parameter fitting, assuming a priori a simple shape described by a standard mathematical function. Two common prescriptions used in the literature earlier are: (a) skewed Gaussian shape for P˜ (Mγ ) [12] and (b) P˜ (Mγ ) described by a Fermi function [3]. Here we follow the latter approach and assume P˜ (Mγ ) to have the form P˜ (Mγ ) =

2Mγ + 1 1 + exp(

Mγ −Mγ 0 Mγ )

(9)

where Mγ 0 is the mean multiplicity and Mγ is the diffuseness in the multiplicity distribution. We recall Eq. (2) which relates P˜ (Mγ ) to the experimentally observed fold distribution P (p). We varied Mγ 0 and Mγ at each Elab to obtain the best fit for P (p). Fitted fold-distributions and the corresponding γ -multiplicity distributions are shown in Fig. 4. 3.4. Transformation to angular momentum distribution Our aim is to find the angular momentum distribution (in other words, partial cross sections σl ) of the CN leading to formation of ERs. This is often accomplished by a simple relation between the mean angular momentum deposited in the compound system, lCN , and the mean number of γ -rays emitted in the subsequent decay cascade, Mγ , [1]:   (10) lCN  = a Mγ  − b where a denotes the average multipolarity of the stretched transitions and b accounts for the average number of statistical γ -ray transitions. We can construct a more generalized relation between Mγ  and lCN  [13] by recalling the physics of the CN decay. The final stage of deexcitation of the CN is a cascade of ‘yrast’ or ‘nonstatistical’ γ -rays, which carry away angular momentum but essentially no excitation energy. Prior to this, there is also the emission of a few ‘statistical’ γ -rays in each cascade, carrying off thermal energy but not much angular momentum. Thus, measuring the number of γ -rays (Mγ ), one can deduce the angular momentum of the ER before the γ -ray cascade occurred. This angular momentum can, in turn, be related to the entrance-channel angular momentum, lCN , by adding back the angular momentum removed via particle evaporation. Angular momentum carried off by the evaporated particles and the statistical γ -rays can be estimated from the statistical model. The generalized relation including the angular momentum carried off by neutrons and light charged particles is of the form:

lCN  = Mγ − Mγ S  lγ N S  + Mγ S lγ S  + Mn ln  + Mp lp  + Mα lα .

(11)

Here Mγ − Mγ S , Mγ S , Mn , Mp  and Mα  are the average multiplicities of nonstatistical γ -rays, statistical γ -rays and evaporated neutrons, protons and α-particles, respectively. lγ N S , lγ S , ln , lp  and lα  represent the average amount of angular momentum carried off by each type of transition. The values of Mγ S , lγ S , Mn , ln , Mp , lp , Mα  and lα  were estimated from statistical model calculation, the summary of which is given in Table 2. We notice that angular momenta carried off by statistical γ -rays and evaporated protons and α-particles are insignificant for the present system. Therefore, we rewrite Eq. (11) as follows: lCN  Mγ lγ N S  + Mn ln .

(12)

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Table 2 Summary of statistical model calculation. Capture cross sections (σcap ) were calculated by CCFULL. Elab (MeV)

σcap (mb)

σER (mb)

σfis (mb)

Mγ S 

lγ S  (h¯ )

Mn 

ln  (h¯ )

Mp 

lp  (h¯ )

Mα 

lα  (h¯ )

89.2 94.2 99.2 104.3 109.3

184.7 377.0 556.2 705.6 835.4

139.0 235.8 281.0 264.8 226.5

45.7 141.2 275.2 440.8 608.9

0.14 0.09 0.13 0.27 0.30

1.0 1.0 1.0 1.0 1.0

4.88 5.32 5.58 5.76 5.96

1.28 1.28 1.30 1.33 1.35

0.03 0.05 0.08 0.10 0.14

0.28 0.28 0.28 0.29 0.29

0.01 0.01 0.02 0.04 0.05

2.25 2.19 2.25 2.69 2.82

The first term is closely linked to the experimental data and accounts for most of the angular momentum removed from the CN. The remaining amount of angular momentum is accounted for by the second term. Consequently, the deduced lCN -values are not much affected by the results of the statistical model calculation. The average angular momentum removed by non-statistical γ -rays, lγ N S , depends on the multipolarity of the transitions. For the systems in which ERs are good rotors, one can assume that the deexcitation cascade proceeds exclusively by stretched E2 transitions and lγ N S  2. We assumed that lγ N S  = 1.1 for the present system. This assumption, in addition to the non-zero ground state spin and the presence of isomeric states in the ERs [14], add to the uncertainty in the deduced lCN -values. Fig. 5 shows the experimental angular momentum distribution together with the results of statistical model calculation. 3.5. Statistical model calculation We reported ER cross section measurement for 19 F + 184 W at beam energies in the range of 80–130 MeV in our earlier work [14]. We refer to the same for a detailed account of statistical model calculation [16] for this system. Here, we highlight the method followed for γ and particle evaporations from the CN. The angular momentum distribution of the fused CN, which is used as an input to the statistical model calculation, is generated by the coupled-channels code CCFULL. For each event neutron, proton, α and γ evaporations are considered as the competing exit channels with fission. Out of these a particular channel is selected through Monte Carlo sampling. In case of evaporation, the kinetic energy and the angular momentum of the emitted particle are also calculated with Monte Carlo sampling. Then the CN excitation energy and angular momentum are adjusted accordingly using the vector rule for the addition of angular momenta. Only the giant dipole resonance (GDR) γ -emission is considered in case of γ -evaporation. So the magnitude of the angular momentum taken away by a statistical γ -ray is always unity. For particle evaporation [17], the magnitude of the angular momentum of the emitted particle is given by, lν = lmax R where R is a uniform random number between 0 and 1 and   1 3.4 2 lmax = 0.187(2μ ) R + 1

2 for neutrons and  1 1 V 2 lmax = 0.187(2μ ) 2 1 − R

(13)

(14)

(15)

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Fig. 5. Measured angular momentum distributions for the formation of ERs in the reaction 19 F + 184 W at five laboratory energies. Capture cross sections from CCFULL [15] and angular momentum distributions calculated using statistical model are also shown.

for proton and α-particles. The radius R is defined by 1  1 R = 1.21 (A − Aν ) 3 + Aν3

(16)

and the potential by (Z − Zν )Zν Kν V= MeV, (17) R + 1.6 where Kν = 1.32 for α-particles and 1.15 for protons. In the above equations , Aν and Zν are the energy, mass and charge of the emitted particle, respectively. After emission of a particle, the angular momentum of the daughter nucleus is given by  (18) ld = lp2 + lν2 − 2lp lν cos θ

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where lp is the angular momentum of the parent nucleus and θ is the angle between lp and lν selected randomly. In a single event, the sampling process is repeated until fission occurs or an ER is formed. 4. Discussion We have assumed a priori shape (Fermi function) for the γ -multiplicity distribution because of simplicity and ease of computation. Other methods had been reported in literature, e.g. iterative Monte Carlo procedure [18]. We point out that the differences between various methods are most likely to be washed out during Mγ → lCN transformation for the present system, given the uncertainties involved. One can observe the mismatch between the experimental σl and the theoretical σl in Fig. 5. The primary cause for the same is the fact that we have replaced the average values lCN  and Mγ  in Eq. (12) by the variables lCN and Mγ , respectively, for Mγ → lCN transformation. Also, the various evaporation channels are not uniformly distributed over the whole range of angular momentum, as was pointed out in Ref. [19]. Therefore, the apparent deficit of yield at low values of lCN is an artifact caused by the limitations in the process of Mγ → lCN transformation. Halbert et al. [13] had suggested an approximate compensation by an extrapolation on the low-lCN side of each σl distribution. The extrapolation could be done under the assumption that the transmission coefficient Tl is constant from lCN = 0 to the point of tangency just below the peak of σl . Our choice of lγ N S  = 1.1 is guided by the facts that (a) many of the known γ -transitions in the dominant ERs (199,198,197 Bi) are of M1-type [20] and (b) this value results in good agreement between the extents of the measured and the calculated σl distributions at the higher end. Further measurements in this compound mass region and detailed structure information of the ERs will make the choice more reliable. The CN formed in the present experiment lies above the Z = 82 shell. We compare the angular momentum distribution of our system with the same for the system 19 F + 175 Lu → 194 Hg [3] in which the CN lies below the Z = 82 shell. Both systems have nearly similar entrance-channel mass asymmetry. We observe that population of higher l-values in our system are less compared to the same in the system in Ref. [3], at similar Elab . This observation is highlighted in Fig. 6, in c.m. ∼ 1.3 are compared (VCoul. which angular momentum distributions for the two systems at VECoul. is the Coulomb barrier). In our earlier work [14], we had compared ER excitation functions for different reactions with nearly similar entrance-channel mass asymmetry, across the Z = 82 shell. We required an enhancement of the fission barrier, Bf (l = 0), in our statistical model calculation, for the system forming the CN with Z = 82, to account for the measured ER cross sections. We had interpreted this as the manifestation of extra stability against fission provided by the Z = 82 shell closure. The present measurement seems to endorse our earlier observation about the onset of instability in the CN after crossing of the Z = 82 shell. Although, we note that the methods followed for Mγ → lCN transformation are different for the two cases. 5. Summary and conclusions We measured ER-gated γ -ray multiplicity distribution for the system 19 F + 184 W at five laboratory energies near the Coulomb barrier. We unfolded the data to obtain CN angular momentum distribution for the formation of ERs and compared our results with the system 19 F + 175 Lu. We observe that higher angular momenta are somewhat depleted in our system compared to

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Fig. 6. Comparison of measured angular momentum distributions between the systems 19 F + 175 Lu and 19 F + 184 W at similar Elab .

the same in the other system. This observation appears to bring out the stabilizing effect of the Z = 82 shell against fission. Further measurements of ER-gated CN angular momentum distribution are clearly needed around the Z = 82 shell closure in the CN. Also, identical unfolding method should be followed while comparing between different systems. Acknowledgements We thank Mr. Ranjeet, Dr. J.J. Das and Prof. R. Singh for their support during the experiment, the Pelletron crew of IUAC for providing beam of excellent quality and Dr. A. Roy for useful discussions. References [1] R. Vandenbosch, Annu. Rev. Nucl. Part. Sci. 42 (1992) 447. [2] D. Ackermann, Yad. Fiz. 66 (2003) 1150. [3] S.K. Hui, C.R. Bhuinya, A.K. Ganguly, N. Madhavan, J.J. Das, P. Sugathan, D.O. Kataria, S. Muralithar, Lagy T. Baby, Vandana Tripathi, Akhil Jhingan, A.K. Sinha, P.V. Madhusudhana Rao, N.V.S.V. Prasad, A.M. Vinodkumar, R. Singh, M. Thoennessen, G. Gervais, Phys. Rev. C 62 (2000) 054604. [4] P.D. Shidling, N. Madhavan, V.S. Ramamurthy, S. Nath, N.M. Badiger, S. Pal, A.K. Sinha, A. Jhingan, S. Muralithar, P. Sugathan, S. Kailas, B.R. Behera, R. Singh, K.M. Varier, M.C. Radhakrishna, Phys. Lett. B 670 (2008) 99. [5] P.D. Shidling, S.R. Abhilash, D. Kabiraj, N. Madhavan, Nucl. Instr. Meth. A 590 (2008) 79. [6] A.K. Sinha, N. Madhavan, J.J. Das, P. Sugathan, D.O. Kataria, A.P. Patro, G.K. Mehta, Nucl. Instr. Meth. A 339 (1994) 543. [7] http://root.cern.ch/drupal/. [8] E.T. Subramaniam, B.P. Ajith Kumar, R.K. Bhowmik, Collection and Analysis of Nuclear Data using Linux nEtwork (unpublished). [9] A.H. Wuosmaa, R.R. Betts, B.B. Back, M.P. Carpenter, H. Esbensen, P.B. Fernandez, B.G. Glagola, Th. Happ, R.V.F. Janssens, T.L. Khoo, E.F. Moore, F. Scarlassara, Ph. Benet, Phys. Lett. B 263 (1991) 23. [10] B.K. Nayak, R.K. Choudhury, L.M. Pant, D.M. Nadkarni, S.S. Kapoor, Phys. Rev. C 52 (1995) 3081. [11] S.Y. Van Der Werf, Nucl. Instr. Meth. 153 (1978) 221. [12] D.L. Hillis, J.D. Garrett, O. Christensen, B. Fernandez, G.B. Hagemann, B. Herskind, B.B. Back, F. Folkmannet, Nucl. Phys. A 325 (1979) 216. [13] M.L. Halbert, J.R. Beene, D.C. Hensley, K. Honkanen, T.M. Semkow, V. Abenante, D.G. Sarantites, Z. Li, Phys. Rev. C 40 (1989) 2558. [14] S. Nath, P.V. Madhusudhana Rao, S. Pal, J. Gehlot, E. Prasad, G. Mohanto, S. Kalkal, J. Sadhukhan, P.D. Shidling, K.S. Golda, A. Jhingan, N. Madhavan, S. Muralithar, A.K. Sinha, Phys. Rev. C 81 (2010) 064601.

S. Nath et al. / Nuclear Physics A 850 (2011) 22–33

[15] [16] [17] [18]

33

K. Hagino, N. Rowley, A.T. Kruppa, Comput. Phys. Commun. 123 (1999) 143. G. Chaudhuri, S. Pal, Phys. Rev. C 65 (2002) 054612. M. Blann, Phys. Rev. C 21 (1980) 1770. M. Jääskeläinen, D.G. Sarantites, R. Woodward, F.A. Dilmanian, J.T. Hood, R. Jääskeläinen, D.C. Hensley, M.L. Halbert, J.H. Barker, Nucl. Instr. Meth. 204 (1983) 385. [19] D. Ackermann, B.B. Back, R.R. Betts, M. Carpenter, L. Corradi, S.M. Fischer, R. Ganz, S. Gil, G. Hackman, D.J. Hofman, R.V.F. Janssens, T.L. Khoo, G. Montagnoli, V. Nanal, F. Scarlassara, M. Schlapp, D. Seweryniak, A.M. Stefanini, A.H. Wuosmaa, Nucl. Phys. A 630 (1998) 442c. [20] http://www.nndc.bnl.gov/.