GAMMA-RAY PRODUCTION IN THE 58 Ni( 16 O,X) NUCLEAR REACTION

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1 NUCLEAR PHYSICS GAMMA-RAY PRODUCTION IN THE 58 Ni( 6 O,X) NUCLEAR REACTION A. MIHAILESCU,2, GH. CATA-DANIL 2,3 Department of Physics, Faculty of Chemistry, University of Bucharest, Bd. Elisabeta 4 2, Bucharest, Romania alex2hr@yahoo.com 2 Department of Physics, University Politehnica of Bucharest, Splaiul Independentei 33, Bucharest, Romania 3 Horia Hulubei-National Institute for Physics and Nuclear Engineering, P.O. Box MG6, Bucharest, Romania Received August 2, 2009 Residuals of the fusion-evaporation reactions following the bombardment of a 58 Ni target with 6 O beams at energies ranging from 40.0 to 60.0 MeV have been studied at the Bucharest FN Tandem Van de Graaff accelerator. Measured γ ray energies along with the corresponding relative intensities are reported in the present work. From the observed γ yields were determined relative cross sections of the open channels and a comparison is performed with the predictions resulted from statistical model calculations in the Hauser-Feshbach formalism. The computer codes CASCADE and PACE 4 were employed for numerical calculations. Key words: Heavy ion reactions, gamma ray production, compound nucleus, excitation function.. INTRODUCTION Nuclear gamma lines produced by bombarding medium mass targets with accelerated ions carry important information on the nuclear structure of the populated nuclei and provide nuclear data for different applications like medical radioisotopes production or reactors design. From the gamma yields, can be inferred cross section values for the open reaction channels. The information on the excitation functions of residual nuclei are also important for testing statistical model calculation in order to understand the reaction mecanism and to estimate the radionuclide impurities in a compound target. In heavy-ion fusion reactions many studies concentrated on the energy, angular momentum and charge distribution of reaction s products and considerable interest was given to the study of complete fusion (CF) and incomplete fusion (ICF) which are the dominant reaction mechanisms [] below beam energies of 0 MeV/nucleon. In complete fusion reaction process of the projectile with the target, the highly excited nuclear sistem decays Rom. Journ. Phys., Vol. 55, Nos. 7 8, P. 3, Bucharest, 200

2 2 Gamma-ray production in the 58 Ni( 6 O,X) nuclear reaction 73 by evaporation of low energy nucleons and alpha particles during thermalization at the equilibrum stage while in the incomplete fusion a part of the projectile fuses with the target nucleus and the remaining part moves in the forward direction at almost the same velocity but with an incomplete linear momentum transfer [2]. The first goal of the present paper is to report the gamma ray yield following the bombardment of a 58 Ni target with an 6 O ion beam in the energy range MeV. From these yields were extracted relative excitation functions for the cross sections. Previous experiments involving the same reaction partners but with different incident energy range were reported in Ref. [3,4,5]. The second goal was to compare the experimental results with detalied compound nucleus (CN) calculations. Measured excitation funcions (EFs) were compared to predictions for the statistical decay of a compound nuclear system. The computer code PACE 4 [6] based on the Monte-Carlo simulation and the deterministic code CASCADE [7] were employed to evaluate the cross sections for evaporation residues formed in the CF reaction process of the projectile with the target. The experiment has been performed at the Bucharest Tandem Van de Graaff electrostatic accelerator. Gamma rays were recorded with a high resolution, high efficiency GeHP detector coupled to a PC based multichannel analyzer. Peak deconvolution from the recorded spectra was performed with LEONE package [8] and efficiency corrections were performed with standard calibration sources. This allowed a precise determination of the gamma intensities in a relative scale by correcting gamma peak areas with detector efficiency and normalizing to the integrated charge. 2. EXPERIMENT The 58 Ni target was bombarded with an 6 O beam in the range energy of MeV with a step of 5 MeV, the irradiation times were about 2 hours each run. Beam intensities of the order of 0 na produced counting rates in the detector below. KHz, acceptable for the 30% efficiency GeHP γ ray spectrometer employed in our measurement. The gamma ray spectra were recorded at 0 0 and 55 0 relative to the incident beam. The energy calibration of the spectrometer was performed with a 52 Eu gamma radioactive source, covering the energy range from 22 kev to 408 kev. Pulseheight analysis was done on 4096 channels, fine enough to exploit the 2.0 kev energy resolution (at MeV) of the gamma detector. In order to estimate the relative efficiency calibration curve of the spectrometer, we measured the photopeak areas of the 52 (i) Eu γ lines ( ), and after correcting for their intensities we finally fit the set A γ

3 74 A. Mihailescu, Gh. Cata-Danil 3 of points with an analytical function. The experimental data for the relative efficiency presented in Figure have been obtained with the relation where () i () () A i / I i γ γ ε = () () i A γ represents the area of the photopeak with energy E (i) and () i I γ is the relative intensity of the γ ray with energy E (i) of the 52 Eu source (Ref. [9]). The efficiency data were interpolated by the curve given by, ε = ε 0 + ε exp( E / t) (2) where the parameters ε 0, ε and t were obtained by the best fit to be ε 0 =2.92, t=232.5 kev, and ε = Detector efficiency θ= ε Energy(keV) Fig. Efficiency curve for the employed 30% Ge(HP) detector placed at an angle of 55 0 relative to the incident beam. Gamma ray energies and relative intensities standards of 52 Eu were used. Error bars are of the order of symbol size. Beam current was integrated in a Faraday cup and the electrical charge was digitized with an ORTEC charge integrator/digitizer. Measured gamma spectra were normalized to these values. The target used in this experiment was a selfsupported film of 58 Ni with a thickness of.8 mg/cm SPECTRA ANALYSIS The first run was set up at the energy beam of 50.0 MeV, this projectile-target combination producing a fairly high number of gamma-ray lines with an important

4 4 Gamma-ray production in the 58 Ni( 6 O,X) nuclear reaction 75 contribution of fusion-evaporation reaction to the residual nucleus. The gamma ray relative intensities were calculated by correcting the peak areas with the efficiency values from Figure. In Table are presented the relative intensities normalized to the value of the 862. kev gamma-ray transition (I ). Table Observed gamma rays induced by 50.0 MeV 6 O beam on the 58 Ni target, arranged in order of increasing transition energy (E γ ). Intensities (Iγ) are normalized to the one of the 862. kev transition (I ). Energy values were extracted from the calibrated spectra, with the uncertainty of 0. kev Eγ(keV) Iγ Eγ(keV) Iγ Eγ(keV) Iγ () () (2) () () (2) (5) () (3) (5) () (3) (2) () (2) (2) () (4) () () (2) () () (2) () () (2) () () (2) () () (2) () (2) (2) Figure 2 shows a typical gamma ray spectrum obtained with the GeHP detector placed at an angle of 55 0 relative to the incident beam when the nickel target was bombarded with a 50.0 MeV 6 O beam. In the energy range from 30 kev to about 700 kev several gamma lines with relative intensities varying with the beam energy were observed. At the bombarding energies near threshold used in this experiment the dominant reaction mechanism is compound nucleus [0]. The compound nucleus 74 Kr formed in the reaction decays into several reaction channels, with relative intensities changing with incident beam energy. The appropriate model describing this processus is the fusion-evaporation [].

5 76 A. Mihailescu, Gh. Cata-Danil 5 Counts O+ 58 Ni Detector angle: 55 0 Energy: 50.0 MeV Energy(keV) Counts bkg O+ 58 Ni Detector angle: 55 0 Energy: 50.0 MeV Energy(keV) Fig. 2 Gamma ray spectra obtained for a 50.0 MeV 6 O beam incident on 58 Ni target. The energies indicated on photopeaks are obtained in the present experiment. 4. CROSS SECTION CALCULATIONS FOR THE 58 Ni( 6 O,X) REACTIONS The Coulomb barrier-value calculated in the laboratory system for the 58 Ni( 6 O,X) reaction is MeV [2]. At the beam energies used in the present work the nuclear reactions evolve dominantly by compound nucleus mechanism. In the present study we employed the Hauser-Feshbach formalism [3] of the CN. Numerical estimations of the cross sections are obtained by using two computer codes CASCADE and PACE 4. The optical model parameters for the emitted particles (neutrons, protons and alphas) and the average gamma transition strength are taken from the systematics [4, 5, 6]. The statistical computer code CASCADE assumes the reaction to occur in two steps. First the formation of compound nucleus and then the statistical decay of

6 6 Gamma-ray production in the 58 Ni( 6 O,X) nuclear reaction 77 the equilibrated system so it does not consider the possibility of incomplete fusion nor the pre-equilibrum emission of nucleons from the composite system. The decay probabilities are determined by the level densities of the daughter nuclei and the barrier penetrabilities for various channels. Fermi gas model is used for calculating the level densities for the product nuclei. The partial cross section for the formation of the compound nucleus of spin J and parity Π, from a projectile and a target nucleus of spins J p and J T respectively, at centre of mass energy E is given by 2 2J + ( Π ) = π Jp+ JT J+ S σ J, TL ( E) (3) (2Jp )(2JT ) S= Jp JT L= J S where T L are the transmission coefficients dependent on energy, L the orbital S = J + J is the channel spin. angular momentum L, and ( p T ) + + The transmission coefficients mainly affect the lower energy part of the particle spectrum. In the standard application of CASCADE the transmission coefficients are derived using optical model parameters for inverse fusion reactions. The total fusion cross section for the maximum angular momentum L c of the compound nucleus is given by L L c 2 σ = π (2L+ ) T ( E) (4) L= 0 The level density formula implies a yrast line 2 J( J + ) Erot ( J) = + (5) 2I where is pairing energy which determines the zero point of the effective excitation energy and I = (2/5)mr 2 is the effective moment of inertia, r = r 0 A /3 is the radius of spherical nucleus. The level density formula for a given angular momentum (J), and excitation energy (E) independent of parity π is given by L /2 ( ( ) ) 2 3/2 / 2 2 2J + ρ( EJ, ) = a exp 2 ae t EJ ( ) (6) 2 2 I ( E t E( J)) where a is the level density parameter which determines the energy dependence and t is the thermodynamic temperature. The code PACE 4 is a modified version of JULIAN the Hillman-Eyal evaporation code. PACE (Projection Angular-momentum Coupled Evaporation) is a Monte-Carlo code coupling angular momentum. The deexcitation process is followed by a Monte-Carlo procedure. The results presented in the present work were obtained using 0 5 deexcitation cascades. Cross sections have been computed for the the range energy up to 60.0 MeV, obtaining a so-called excitation function for the cross section, quantity that

7 78 A. Mihailescu, Gh. Cata-Danil 7 indicates the expected yields for different reaction channels as a function of the incident energy. CASCADE 58 Ni( 6 O,xpynzα)R 00 0 Br Se 7 Se 7 As 69 As As Ge 66 Ge 65 Ga 0. PACE 4 58 Ni( 6 O,xpynzα)R Br Se 7 Se 7 As 69 As As Ge 66 Ge 65 Ga 0.0 Fig. 3 Comparative cross section values for the reaction channels calculated with the CASCADE and PACE 4 codes.

8 8 Gamma-ray production in the 58 Ni( 6 O,X) nuclear reaction 79 Figure 3 shows EFs calculations performed by CASCADE and PACE 4 codes by using default parameters of the codes. Both of them are predicting the strongest reactions as being the 2p channel in the range of 40 53MeV and the 3p channel in the MeV range. The second strong channel reaction is predicted slightly different. The CASCADE code predicts the 3p channel up to 53 MeV whereas PACE 4 predicts the αp channel over the same energy region. For both of the computer codes the strongest reaction channel 2p switches with the other strong channels at bombarding energy values within the MeV region. In the following section theoretical calculations are compared to the experimental values obtained in the present work on a relative scale. No atempt was made in the present study to determine absolute cross sections due to the high uncertainity in the absolute values of the integrated charge and target thickness. 5. RESULTS AND DISCUSSIONS The EFs for Se, 7 As, Br, 7 Se and Ge isotopes produced through the xp, nxp, and α2p channels of the 6 O+ 58 Ni compound system are displayed in Figure 4 (see below). Over much of the region studied the 2p channel is dominant and its excitation function has been determined by the yield of 454.6, 862., and kev γ-rays of Se. The 3p channel is also large over much of the energy region. Its excitation function has been determined from the yield of the 47.2, 30.5, 73.9, and kev γ-rays of 7 As. The np channel was found quite large over the energy region. Its excitation function has been determined from the yield of the 239.3, 270.2, and 66.6 kev γ-rays of Br. The α2p channel has been determined from the yield of 05.7 and kev γ-rays of Ge. The n2p channel has been determined from the yield of kev γ-ray of 7 Se. a b 00 0 Detector 0 0 Se Cascade Se Pace keV 774.7keV 830.0keV 862.keV 00 0 Detector 55 0 Se Cascade Se Pace keV 774.7keV 830.0keV 862.keV 0.

9 0 A. Mihailescu, Gh. Cata-Danil 9 c 000 d 00 0 Detector As Cascade 7 As Pace keV 000.3keV 47.2keV 975.8keV 30.5keV 00 0 Detector As Cascade 7 As Pace keV 000.3keV 47.2keV 975.8keV 30.5keV e f Detector 0 0 Br Cascade Br Pace keV 270.2keV 353.3keV 66.6keV Detector 55 0 Br Cascade Br Pace keV 270.2keV 353.3keV 66.6keV 000 g 000 h Detector 0 0 Ge Cascade Ge Pace keV 634.0keV 0 0. Detector 55 0 Ge Cascade Ge Pace keV 634.0keV

10 0 Gamma-ray production in the 58 Ni( 6 O,X) nuclear reaction 000 i 000 j 00 0 Detector Se Cascade 7 Se Pace keV 00 0 Detector Se Cascade 7 Se Pace keV Fig. 4 Comparison of cross section excitation functions for Se, 7 As, Br, Ge and 7 Se gamma ray lines. Filled symbols: computer codes calculations. Open symbols: experimental data. Reaction cross sections were calculated and experimental data were normalized to the theoretical values derived from CASCADE code at the beam energy of 50.0 MeV. It can be observed that EFs for the evaporation residues aforementioned are in fair agreement with the theoretical CASCADE and PACE 4 predictions. This is quite expected as most of evaporation residues are associated with n and p emission, hence they can only be produced through the complete fusion (CF) reaction process. In Figures 4 a, b can be observed that both CASCADE and PACE 4 predict similar behaviours for 2p reaction channel ( Se residuals). Description of the experimental data is accurate for 50.0, 55.0 MeV. At 60.0 MeV CASCADE calculations are little closer to the experimental data. At 40.0 MeV both calculations overestimate the experimental data. In Figures 4 c, d in the lower region of the energy (40.0 MeV) CASCADE underestimate the experimental points while in the upper part of the energy region ( MeV) both CASCADE and PACE 4 predict similar behaviour for the 3p reaction channel ( 7 As residuals). In Figures 4 e, f can be observed that both CASCADE and PACE 4 predict similar behaviours for np reaction channel ( Br residuals). Description of the experimental data is accurate for MeV. At 40.0 MeV PACE 4 calculations underestimate the experimental data. In Figures 4 g, h over much of the region studied ( MeV) CASCADE code shows a better prediction than PACE 4. The latter code calculations underestimate much the experimental data at 45.0 MeV. The disagreement between experimental and calculated data at the lowest bombarding energy (40.0 MeV) for the Ge residuals is associated with α particles emission. Thus, the production of Ge may be taking place through a process other than/or along with the complete fusion, i.e. the incomplete fusion reaction (ICF) could occur.

11 2 A. Mihailescu, Gh. Cata-Danil In Figure 4 i, j one notice a better prediction given by the CASCADE code. Over much of the region studied CASCADE increases by one order of magnitude while PACE 4 increases by three orders of magnitude. Other small disagreements may be due to statistical uncertainities in the CASCADE and PACE 4 calculations; overall the agreement is considered to be quite satisfactory. 6. CONCLUSIONS Gamma rays from the 6 O+ 58 Ni compound system reaction have been measured by high resolution gamma ray spectroscopy. Tables with measured gamma-ray energies and intensities are reported. Model estimations of the cross-section for the reaction channels has been performed with a statistical model. The EFs of residual nuclei have been investigated by γ-ray spectroscopy and were compared to predictions from evaporation codes CASCADE and PACE 4. A satisfactory agreement is generaly found for the evaporations associated with n and p emissions. A small disagreement occurs due the underestimations of α emission channel compared to channels involving nucleon evaporation only. Although similar projectile-target systems have been studied in the past, more experimental data are required to have a better insight into the reaction mechanisms. Acknowledgements. The authors wish to express their appreciations to the IFIN-HH Tandem accelerator staff for providing experimental facilities. Technical support with the experiment has been kindly offered by the nuclear structure group from DFN-IFIN. REFERENCES. P.E. Hodgson, E. Gadioli, E. Ggadioli Erba, Introductory Nuclear Physics (Oxford Science Publications, New York, 997). 2. L.F. Canto, R. Donangelo, Lia M. de Matos, M.S. Hussein, P. Lotti, Complete and incomplete fusion in heavy ion collisions, Physical Revue C, vol. 58, issue 2, p. 07 7(998). 3. R.L. Robinson, H.J. Kim, J.L. Ford Jr., Absolute cross sections for the 58,60 Ni( 6 O,X) reactions, Physical Revue C, vol. 9, issue 4, p (04/974). 4. J.J. Simpson, P.O. Tjom, I. Espe, G.B. Hagemann, B. Herskind, M. Neiman, Decay of the compound nucleus 74 Kr formed in the reaction 6 O+ 58 Ni, Nuclear Physics A, vol. 287, issue 2, p (09/977). 5. N. Keeley, J.S. Lilley, J.X. Wei, M. Dasgupta, D.J. Hinde, J.R. Leigh, J.C. Mein, C.R. Morton, H. Timmers, N. Rowley, Fusion excitation function measurements for the 6 O+ 58 Ni and 6 O+ 62 Ni systems, Nuclear Physics A, vol. 628, issue, p. 6(0/998). 6. A. Gavron, Statistical model calculations in heavy ion reactions, Physical Revue C, vol. 2, issue, p (0/980). 7. F. Pulhofer, Nuclear Physics A, vol. 280, p. 267 (977). 8. IKP, Koln and DFN, IFIN-HH, Bucharest Package.

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