application/xmlHigh resolution measurement of the 208Pb(p,γ) capture reaction up to Eγ=19 MeVM LipoglavšekR.A BarkM BenatarE GueorguievaJ KauF KomatiP KwinanaJ.J LawrieG.K MabalaP MaineS MukherjeeS.M MullinsS MurrayN.J NcapayiR.T NewmanP VymersA LikarM VenceljT VidmarRadiative proton captureIn-beam spectroscopyDSD modelHPGe detectorPhysics Letters B 593 (2004) 61-65. doi:10.1016/j.physletb.2004.04.074journalPhysics Letters BCopyright © 2004 Elsevier B.V. All rights reserved.Elsevier B.V.0370-26935931-422 July 20042004-07-2261-65616510.1016/j.physletb.2004.04.074http://dx.doi.org/10.1016/j.physletb.2004.04.074doi:10.1016/j.physletb.2004.04.074http://vtw.elsevier.com/data/voc/oa/OpenAccessStatus#Full2014-01-01T00:14:32ZSCOAP3 - Sponsoring Consortium for Open Access Publishing in Particle Physicshttp://vtw.elsevier.com/data/voc/oa/SponsorType#FundingBodyhttp://creativecommons.org/licenses/by/3.0/JournalsS300.3PLB20937S0370-2693(04)00724-510.1016/j.physletb.2004.04.074Elsevier B.V.ExperimentsFig. 1Parts of the γ-ray energy spectra summed from 8 clover detectors. Spectra measured at 11.3 MeV (bottom) and 13.2 MeV (top) proton energy are shown. Peaks are marked with their respective transition final state and escape peaks are due to the missed detection of a 511 keV annihilation γ ray.Fig. 3Low-energy part of a γ-ray spectrum showing γ rays detected simultaneously with those populating the 1/2− state at 4418 keV.Fig. 2Partial level scheme of 209Bi showing states with the highest single-proton strengths and their decay patterns.Fig. 4Excitation functions for capture into different single proton states in 209Bi. The differential cross sections are one half of the summed cross sections at θ=135° and θ=90°. Data are from current experiment (•), from Ref. [8] (∘) for the f7/2, p3/2+f5/2 and h9/2 final states and from Ref. [19] (∘) for the i13/2 state. The results of calculation with the consistent DSD model are shown with solid line. The sharp features that are not accounted for by the model are due to analog resonance states.High resolution measurement of the 208Pb(p,γ) capture reaction up to Eγ=19 MeVMLipoglavšek1matej.lipoglavsek@ijs.siR.ABarkMBenatarEGueorguievaJKauFKomatiPKwinanaJ.JLawrieG.KMabalaPMaineSMukherjee2S.MMullinsSMurrayN.JNcapayiR.TNewmanPVymersiThemba LABS, 7129 Somerset West, South Africa1Present address: J. Stefan Institute, 1000 Ljubljana, Slovenia.2Present address: Physics Department, Faculty of Science, M.S. University of Baroda, Vadodara 390002, India.ALikarMVenceljTVidmarJ. Stefan Institute, 1000 Ljubljana, SloveniaEditor: J.P. SchifferAbstractHigh-energy γ rays produced by the 208Pb(p,γ) reaction were measured with the AFRODITE germanium detector array. The use of Ge instead of NaI detectors improved the energy resolution by more than a factor of 10 in the γ-ray energy range between 10 and 19 MeV. Proton captures into different single particles states in 209Bi were thus clearly separated, most notably for the i13/2 high-spin state. The measured excitation functions are in very good agreement with the predictions of the direct–semi-direct model, including captures to the i13/2 state, for which the previous agreement had been poor.PACS25.40.Lw24.30.Cz23.20.LvKeywordsRadiative proton captureIn-beam spectroscopyDSD modelHPGe detectorDetection of photons in the energy range between 10 MeV and 30 MeV with high resolution is quite rare. Most measurements have so far employed large volume inorganic scintillator detectors like NaI(Tl), BGO or BaF2 and large arrays of inorganic scintillators have been constructed [1]. A major drawback of these detectors compared to modern high resolution HPGe detection systems is their energy resolution that cannot be better than about 5%. On the other hand, the use of HPGe detectors has been limited to energies below 10 MeV [2] which is sufficient for the nuclear structure studies. It is our primary goal to show that the high resolution spectra of a superb quality can be acquired also for the high energy photons. Qualitatively new data were obtained in order to resolve a specific open question on proton capture process in the giant dipole resonance region.Recent experimental interest in proton capture reactions has mainly focused on measuring cross sections of astrophysical importance with radioactive ion beams [3,4]. At higher beam energies, the nucleon capture reactions to the single particle bound states yield high energy photons of several tens of MeV, usually detected using large NaI(Tl) crystals. In the giant dipole resonance region, the nucleon capture reactions can be theoretically described with the direct–semi-direct (DSD) model [5]. The consistent formulation of this model describes very well the excitation functions and γ-ray angular distributions for both proton [6] and neutron [7] captures over the whole nuclidic chart. The only reaction with worse agreement between experiment and theory as compared to other nuclei is proton capture in 208Pb, particularly for captures into the high spin h9/2 and i13/2 single proton states in 209Bi. This stimulated us to remeasure the 208Pb(p,γ) reaction cross sections using superior instrumentation compared to the previous measurements [8,9].The experiment was performed at iThemba LABS using proton beam from the Separated Sector Cyclotron [10]. The measurement was performed at seven different beam energies, namely, 11.3, 11.6, 11.9, 12.6, 13.2, 14.3 and 14.8 MeV. The beam energy was measured by the time-of-flight of the protons. The target consisted of an enriched 208Pb foil with a thickness of 1 mg/cm2 mounted on a 5 μg/cm2 thick carbon backing. The average beam intensity was 40 nA and about 30 hours of beam time were used for each beam energy. High-energy γ rays following proton capture were detected with the AFRODITE germanium detector array [11], which consisted of eight Ge clover detectors surrounded by the BGO anti-Compton shields. Each clover detector consists of four tapered Ge crystals packed in a common cryostat [12]. For our experiment, four clover detectors were placed at polar angle 90° and another four at 135° with respect to the beam axis. Standard pulse processing electronics were used and data were stored on magnetic tapes in list mode. Events were accepted if at least one of the 32 Ge crystals gave an energy signal higher than 3.2 MeV. Signals from all four crystals in a clover detector were added for each event during the data analysis. The beam current was monitored with a Faraday cup and integrated for each run. Due to the absence of high energy radioactive sources the full-energy-peak efficiency of the AFRODITE array was measured at a γ-ray energy of 15 MeV with the reaction of 66 MeV protons on a carbon target. A cross section of 1.0(2) mb was assumed for the calibration reaction [13], and an efficiency of 9(2)×10−4 was deduced for the whole array at this energy. Gamma-ray detection efficiencies at other energies were calculated with a Monte Carlo simulation using the GEANT code [14]. The simulation gave excellent agreement with the efficiency curve measured at low energies with radioactive sources, as well as with the measurement at 15 MeV. Energy calibration at high γ-ray energies was achieved by finding a maximum overlap of simulated and measured spectra. Data were analysed with MIDAS [15] and RADWARE software [16]. Further details about the experiment will be given in a forthcoming paper [17].Fig. 1 shows portions of γ-ray spectra recorded at 11.3 MeV and 13.2 MeV proton energy. It shows all BGO unsuppressed γ rays detected in eight clover detectors in the energy range between 10.5 and 15.5 MeV for the lower proton energy and 12.3 and 17.3 MeV for the higher proton energy. Spectral lines are marked with the single particle orbit of the captured proton (see Fig. 2) and the escape peaks are due to the missed detection of a positron annihilation γ ray. A typical energy resolution of about 50 keV was achieved for capture γ rays in the energy range between 10 and 19 MeV, the highest γ-ray energy measured in the experiment. Full width at half maxima for γ ray peaks from other reactions were even lower than 50 keV, but the capture γ-ray peaks were broadened partly by the energy loss of the beam in the target, Doppler broadening due to the finite opening angle of the detectors and mainly by the limited beam energy resolution. Despite this limitation, an improvement in resolution by more than a factor of 10 was achieved as compared to the previous measurements [8,9], enabling us to clearly distinguish captures into different single-proton states in 209Bi, which was previously impossible.Fig. 2 shows a partial level scheme of 209Bi with only the states carrying most of the single-proton strengths and their decay patterns. The level scheme was created by studying γ rays detected simultaneously with high energy capture γ rays. An example of such a γ-ray spectrum is presented in Fig. 3 showing all γ rays in coincidence with those populating the 1/2− state at 4.4 MeV. The γ-ray decay of both 1/2− states in Fig. 2 was observed for the first time in our experiment corroborating the previously tentative spin-parity assignments [18].The measured excitation functions for the population of five different single-proton states in 209Bi are plotted in Fig. 4, which shows differential cross sections as measured in our experiment with one half of the detectors at polar angle θ=135° and the other half at θ=90°. The results of the current experiment are marked with full circles and they show a reasonable agreement with previous measurements [8,9,19], marked with empty circles, for all states except for the i13/2 state below 13 MeV proton energy. We attribute this disagreement to the inability of the NaI detectors to clearly distinguish captures into different single proton states in 209Bi due to the limited energy resolution. It can be seen from the spectra in Ref. [8] that the strong peak due to transitions to the f7/2 state almost completely overlays the weak i13/2 peak. Our results for the p3/2 and f5/2 states were summed for comparison with Ref. [8], where these two states could not be separated. It should also be noted that the results from Ref. [8] for the f7/2, p3/2+f5/2 and h9/2 states were normalized in Fig. 4 to match a later publication of the same data [19]. In the present comparison, the same normalization factor was used for all four states.The results of calculations with the consistent DSD model [6] are also shown in Fig. 4. These calculations are the same as those reported in Ref. [6] except that a radius of the Woods–Saxon potential well of 1.21A1/3 fm was used instead of 1.17A1/3 fm. The larger radius is closer to the real proton radius, but its main advantage is that the same radius can be used also for the 142Ce(p,γ) 143Pr and 40Ca(γ,p) 39K reactions studied in Ref. [6]. All parameters of the DSD model are, therefore, the same for nuclei from very different parts of the nuclidic chart. The lower potential radius in Ref. [6] was imposed by the absolute normalization of the experimental cross sections given in Ref. [8]. According to this normalization the cross sections for the reported states were higher by a factor of about 2 compared with the present ones. The cross sections calculated by the DSD model are quite sensitive to the potential radius. In order to obtain a fit to the reported cross sections the radius of the potential was adjusted accordingly to a relatively low value. The normalization factor from Ref. [8] is inconsistent with a different normalization factor for the i13/2 and 2f7/2 states given in Ref. [19]. Compared with the present results we believe that the factor published later is the correct one and therefore the cross sections of Ref. [8] are now corrected.Spectroscopic factors of 1.0 were used in the calculation for all states except for the i13/2 state, where a spectroscopic factor of 0.8 was used (see Refs. [18,20,21] and references therein). The agreement between theory and experiment is now very good. Most notably, the new experimental data for the i13/2 state is now adequately explained by the theory. The experimental excitation function for this state could not be explained with earlier versions of the DSD model [22] and it was still poorly reproduced by the use of effective form factors [6].To conclude, we have measured capture γ rays with energies between 10 and 19 MeV with an energy resolution about 10 times better than any previous measurement in this energy range. 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