A study of the photoneutron contribution to the giant dipole resonance of nuclei in the 64 ≦ A ≦ 86 mass region

The partial photoneutron cross sections [σ(γ, n) + σ(γ, pn)] and σ(γ, 2n) of ⁶⁴Zn, Ga, ^70,72,74,76Ge, ⁷⁵As and ^76,78,80,82Se were measured with a monochromatic photon beam and a high efficiency neutron detecting system in the photon-energy range 8 ≦ E ≦ 30 Me V. Integrated cross sections are discussed. The evolution of the GDR as observed through the neutron exit channel only, is tentatively interpreted either in terms of isospin splitting or in terms of the dynamic collective model.     

The photonuclear cross sections of 48Ca

The sum of the photoneutron and photoproton cross sections of ⁴⁸Ca (⁴⁸Ca(γ, n + p)), has been deduced from an activation yield curve measurement using bremsstrahlung. In a separate experiment the ⁴⁸Ca(γ, p) and partial photoproton cross sections to the ground and first excited states of ⁴⁷K have been deduced from measured photoproton spectra recorded from the electro-disintegration of ⁴⁸Ca. The ⁴⁸Ca(γ, n) cross section is estimated from these data. A statistical decay model is used to estimate the ⁴⁸Ca(γ, 2n) cross section. These reported cross sections are used to determine the relative strengths and energy distributions of the isospin components of the GDR states, and to compare these with theoretical predictions.     

Cross sections and rates of the thermonuclear reactions 64Zn(p,γ)65Ga and 66Zn(p,γ)67Ga

The excitation functions for the reactions of radiative proton capture, ⁶⁴Zn(p,γ)⁶⁵Ga and ⁶⁶Zn(p,γ)⁶⁷Ga, which are of interest for stellar nucleosynthesis, have been measured in the range of incidentproton energies from 1 to 2.8 MeV. The astrophysical S factor and reaction rates have been derived from the reaction cross sections. The experimental results are compared with the predictions of the Hauser-Feshbach statistical theory.     

Intermediate structure of the giant dipole resonance in 54Fe and 56Fe

The cross sections for the (γ, n) reaction on ⁵⁴Fe and ⁵⁶Fe for the high-energy component of the neutron spectrum have been investigated using a neutron spectrometer and a synchrotron with a photon energy range from 5 to 26.3 MeV. Neutron energy spectra have been measured. It is shown that the intermediate structure in the (γ, n) reaction cross section is due to high-energy neutrons and correlates with the structure in the neutron spectrum.     

The polarization and differential cross section of fast neutrons scattered from 6Li and an R-function analysis of the interaction below 4 MeV

The analyzing power of ⁶Li for the elastic scattering of polarized neutrons with energies between 2 and 4 MeV, has been measured at six angles ranging from 25° to 150°. The polarized neutrons were generated from the reactions Pb(γ, n) and ¹²C(n, $\text{n}$), and their polarization was measured using the double-scattering method. The neutron energies were determined with a nanosecond timeof-flight spectrometer. In addition, the differential cross section was measured at three angles in the same energy range; the cross section was determined relative to the well-known n-¹²C cross section. The present results have been combined with existing data for the neutron total, (n, α), and differential elastic cross sections, in a reduced R-function analysis. Clear evidence of a p-wave triplet of shellmodel states emerges from the analysis.     

Absolute photofission cross section of 209Bi in the energy range from 40 to 65 MeV

The absolute photofission cross section of ²⁰⁹Bi has been measured with monoenergetic γ-radiation between 40 and 65 MeV photon energy. Cross-section data have been obtained with an accuracy between 9 and 20%. The experimental result is compared with the excitation function calculated on the basis of the statistical model. In order to reproduce the measured data on σ_γf, the (γ, n) cross section must decrease with increasing photon energy faster than the experimental total (γ, n) cross section. This behaviour can possibly be explained by the assumption that after photon absorption a compound nucleus is formed only for a small, and with photon energy decreasing, fraction of all decays.     

Der Kernphotoeffekt am3H

The nuclear photoeffect in tritium was investigated by irradiating a gaseous³H-target with bremsstrahlung of 32,5 MeV endpoint energy in the collimated beam of the Heidelberg betatron. Protons and deuterons from the reactions³H(γ, n)d and³H(γ, p)2n were measured independently at (90±15)° with respect to the γ-beam by adE/dx-E-semiconductor telescope. The cross section of the twobody disintegration³ H(γ, n)d is derived forγ-energies fromE _γ=18 to 31 MeV. The integrated cross section from threshold to 32 MeV is (11.8±1.4) MeVmb. The cross section of the threebody disintegration³ H(γ, 2n)p is obtained by making assumptions regarding the angular distribution and energy split up amongst the three disintegration products, which appear plausible on theoretical grounds. Within experimental errors the results reported here seem to be in agreement with a theoretical calculation byFetisov.     

Cross section of the reaction 6Li(p, γ)7Be

The cross section of the reaction ⁶Li(p, γ)⁷Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E_p from 200 keV to 1200 keV. At E_p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μb. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for ⁶Li(n, γ)⁷Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.     

Reliable cross sections of partial photoneutron reactions on 188,189Os isotopes free of neutron multiplicity sorting problems

Experimental data on the cross sections of partial photoneutron reactions, obtained for ^188,189Os isotopes using quasimonoenergetic annihilation photon beams and neutron multiplicity sorting method, are analyzed. Using special criteria (transition multiplicity functions F _i = σ(γ, in)/σ(γ, xn), the ratios of the cross section of the corresponding partial reaction to the total neutron yield reaction’s cross section σ(γ, xn) = σ(γ, 1n)+ 2σ(γ, 2n) + 3σ(γ, 3n) free of the problems associated with experimental neutron multiplicity sorting), it is demonstrated that the data contain significant systematic errors. New data are evaluated for cross sections of partial photoneutron reactions (γ, 1n), (γ, 2n), (γ, 3n) and total photoneutron reaction (γ, sn) for ^188,189Os isotopes within an experimental-theoretical approach proposed earlier. It is shown that the significant systematic errors in the experimental cross sections of partial reactions can be attributed to the ambiguity of the relation between the photoneutrons’ multiplicity and their kinetic energy.     

Kernphotoprozesse im Sauerstoff und ihre Bedeutung für die Kontrolle der Energie und des Photonenspektrums an Elektronenbeschleunigern

The yields of the photonuclear reactions¹⁶O(γ,n) and¹⁶O(γ, 2n) are measured with a newly constructed device for fluid samples. The well known structures of the (γ,n) cross section are used for the energy calibration of our accelerator. Though the yield should be strongly dependent on small changes of the bremsspectrum no difference was found, using a thin or a thick target. The conclusion is, that also the thick target is in its effect a thin target under the accelerator conditions. The (γ, 2n) cross section is measured for the first time between threshold and 120 MeV. It is unusually small compared to the (γ,n) cross section.     

Interaction of neutrons with zinc and its isotopes

Coherent neutron scattering lengths and total cross sections have been measured for elemental Zn,Zn-compounds and on isotopicly enriched samples for neutron energies from 0.5 meV up to 143 keV using different techniques. From the experimental data the following quantities were obtained:

the coherent scattering lengths (b in fm) of Zn (5.689±0.014);⁶⁴Zn (5.23±0.04);⁶⁶Zn (5.98±0.05);⁶⁷Zn (7.58±0.08;b ₊=9.4±0.5/5.8±0.5;b _?=5.0±0.7/10.1±0.7);⁶⁸Zn (6.04 ±0.03);

the potential scattering radii (R′ in fm) of Zn (6.2±0.2),⁶⁴Zn (6.0±0.3) and⁶⁶Zn (6.2 ±0.3);

the absorption cross sections (σ _γ at 0.025 eV in barn) of Zn (1.11±0.02);⁶⁴Zn (1.1 ±0.1) and⁶⁶Zn (0.62±0.06).

Derived quantities are the “zero energy” scattering cross sections (σ ₀ in barn) for Zn (4.128±0.010) and⁶⁷Zn (7.8±0.3) and the incoherent bound cross sections of Zn (0.061 ±0.023) and⁶⁷Zn (0.6±0.4). In the epithermal region the Zn-cross section can be described by introduction of two strong bound levels of⁶⁷Zn+n for which estimated parameters are given.     

The photodisintegration of 40Ar

High resolution measurements of the (γ, n), (γ, 2n), (γ, p), (γ, np) and (γ, 2p) cross sections of ⁴⁰Ar over a photon energy range of 10 to 28 MeV are reported. From this data, the total photon absorption cross section integrated to 26 MeV is found to be 434 ± 40 MeV mb. The results of a dynamic collective model (DCM) calculation compare favourably with the photoabsorption cross section, supporting the use of the DCM in this mass region. It is confirmed that isospin plays an important role in the decay of the ⁴⁰Ar giant dipole resonance.     

Large charge exchange corrections to direct photonuclear processes

The two-step charge exchange contribution from the process ¹⁶O(γ,p)¹⁵N(p,n)¹⁵O is calculated for the photonuclear reaction ¹⁶O(γ,n)¹⁵O. The total cross section, computed as a function of energy, is compared to the conventional, one-step direct cross section for Both E1 and E2 transitions. For each transition the two-step correction to the cross section is found to be sizable, approximately 50% of the one-step cross section.     

Photoproton cross section of 34S

Measurements of the ³⁴S(γ, p) and ³⁴S(γ, np) cross sections are reported. The ³⁴S(γ, p) cross section shows two regions of strength, at 17 and 22 MeV, consistent with that seen in the ³⁴S(γ, n) cross section.A particle-hole calculation is made for the dipole absorption cross section in ³⁴S. The photoproton and photoneutron cross sections calculated on the assumption of statistical decay of GDR states under the constraints of isospin selection rules, agree well with those seen experimentally.     

The absolute 14N(γ, n) reaction cross section

There exists significant disagreement in the magnitude of the low-energy region of the ¹⁴N(γ, n) cross section as measured by three different methods: direct neutron detection, induced activity, and neutron time of flight. This paper reports two measurements of this reaction cross section, one from threshold at 16.5 MeV using an activation method, and the other from threshold to 15.5 MeV by directly detecting the emitted neutrons. The results resolve the discrepancies.     

Measurement of the total cross section for 235U and 238U photofission in the δ-resonance region

The total cross section for the photoemission of fission fragments σ(γ, F) has been measured for ²³⁵U and ²³⁸U using tagged bremsstrahlung photons in the energy range 120–460 MeV. The fission fragments were detected in parallel plate avalanche detectors. The results are compared with other information on the photonuclear absorption cross section.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

Spectroscopic information from the63Cu(p, γ)64Zn reaction in the GDR region

The proton capture cross sections for the reaction⁶³Cu(p, γ_i)⁶⁴Zn leading to several resolved states in the⁶⁴Zn nucleus have been measured forE _p =6.5–11.0 MeV. The excitation functions have been reproduced by the combined CN and DSD models. The extracted spectroscopic factors compared with those derived from the stripping reaction are found to show a distinct correlation.     

A study of the photoneutron reactions in 10B and 11B

Absolute (γ, xn) cross sections for ¹⁰B and ¹¹B were measured up to 35 MeV using a quasi-monoenergetic photon beam from positron annihilation in flight. The experimental cross sections were compared with many-particle shell-model calculations and data from previous bremsstrahlung experiments. The ¹¹B photoneutron cross section is discussed with respect to possible isospin splitting. A systematic review of the photoneutron cross sections of the p-shell nuclei integrated up to about 30 MeV seems to show a strong influence of the α-cluster structure of the target nuclei.     

Measurement of the 50Cr(γ, n)49Cr cross section with monochromatic γ-rays

The ⁵⁰Cr(γ, n)⁴⁹Cr cross section has been measured over the energy range E_γ = 20.43 to E_γ = 22.22 MeV using monochromatic γ-rays from the ³H(p, γ)⁴He reaction and detecting the resulting ⁴⁹Cr positon activity. The γ-rays were monitored by a 12.7 cm × 15.2 cm NaI(Tl) crystal. The positon activity was determined by a coincidence detector consisting of two 7.62 cm × 7.62 cm NaI(Tl) crystals set on the annihilation radiation photopeaks. The γ-ray energy resolution was less than 110 keV over the whole energy range. Structure in the cross section was not observed.