Yields and cross sections of the 241Am(γ, n)240mfAm and 243Am(γ, n)242mf-Am reactions in the E1 giant resonance region

The ratio of delayed to prompt fission yields has been measured for ²⁴¹Am and ²⁴³Am samples irradiated by a bremsstrahlung photon pulsed beam over the energy range E_γmax = 10–80 MeV. The relative probabilities of ^240mfAm and ^242mfAm isomer production and the cross sections of the ²⁴¹Am(γ, n)^240mfAm and ^243Am(γ, n)^242mfAm reactions were deduced from the yields measured in the E1 giant resonance region.     

Multimode approach to 241Am and 237Np fission induced by 660-MeV protons

The results obtained by measuring cross sections for the formation of fragments originating from ²⁴¹Am and ²³⁷Np fission induced by 660-MeV protons are presented. The charge and mass distributions of fragments are analyzed within the multimode-fission model, symmetric and asymmetric fission channels being separated. The contributions of various fission components are estimated, and the fission cross sections for the ²⁴¹Am and ²³⁷Np nuclei are calculated along with the fissilities of these nuclei.     

Untersuchung der Kernphotoreaktionen12C(γ,n),12C(γ, 2n),39K(γ,n) und40Ca(γ,np) bis zur Mesonenschwelle

In order to obtain data on the photon absorption process between the giant resonance and the meson threshold the cross sections of the reactions¹²C(γ,n)¹¹C,¹²C(γ, 2n)¹⁰C,³⁹K(γ,n)³⁸K^g, and⁴⁰Ca(γ,np)³⁸K^g have been determined by the analysis of yield curves at the 140 MeV electron synchrotron of the PTB. Though the (γ,n) cross sections grow small with increasing photon energy they are different from zero up to energies of 60 MeV and above. The cross section of the reaction¹²C(γ, 2n) is extremely small; its highest value amounts to 0.15% of the highest value of the¹²C(γ,n) reaction. The measured⁴⁰Ca(γ,np) cross section is of the order predicted by the naive quasi-deuteron model. The integrated cross sections of the above reactions up to 140MeV are 85±7,0.90±0.10, 139±16, and 76±7MeVmb respectively.     

The study of the (n, α) and (n, γ α) reactions on143Nd

Partial cross sections have been measured for the¹⁴³Nd(n _th,α)¹⁴⁰Ce reaction and also for the two-step process¹⁴³Nd(n _th,γα)¹⁴⁰Ce. The results of the present work confirm previous data, but due to lower background, better statistics and resolution improved precision has been obtained. The double-humped structure in the observed (n, γ α) spectrum can still be explained through the Porter-Thomas fluctuations of the reduced partialγ andα widths.     

The (γ, p n) and (γ, 3p3n) reactions in40Ca at intermediate energies

The yields of the reactions⁴⁰Ca(γ, p n)^38g K and⁴⁰Ca(γ, 3p 3n)^34m Cl have been measured by the activation method in the energy range 80–800 MeV. From the measured yields the cross sections are deduced. The experimental cross sections are compared to calculations with a cascade-evaporation model for photo-induced reactions.     

Investigation of the formation of residual nuclei from radioactive 237Np and 241Am targets in reactions with 660-MeV protons

The burning of radioactive waste is investigated. Targets from ²⁴¹Am and ²³⁷Np were irradiated with 0.66-GeV proton beams. The cross sections for the formation of 60 and 80 residual nuclei from ²³⁷Np and ²⁴¹Am are determined. The experimental results are compared with the theoretical cross sections calculated by the cascade-evaporation model.     

Experimentelle Untersuchung zur Fluktuation der Wirkungsquerschnitte für die Reaktionen F19(n,2n)F18, Al27(n,α)Na24 und Cu65(n,2n)Cu64

Using the activation method investigations on the fluctuation of the total reaction cross sections for the reactions F¹⁹(n, 2n)F¹⁸, Al²⁷(n, α)Na²⁴ and Cu⁶⁵(n, 2n)Cu⁶⁴ were performed in the 14 MeV region under similar conditions as in the experiments reported byStrohal et al. and byCsikai on the same subject. In disagreement to the results of these authors no considerable fluctuation amplitudes were found.     

Intermediate structure in the cross section for the subbarrier fission of 241Am

The cross section for the neutron-induced fission of ²⁴¹Am at neutron energies between 0.2 eV and 10 keV was measured by means of a lead slowing-down spectrometer (LSDS-100). The parameters of the intermediate structure in the behavior of the fission strength function were estimated.     

Study of the excitation functions of27Al(n, α)24Na,27Al(n,p)27Mg and28Si(n,p)28Al reactions

Cross sections for the reactions²⁷Al(n, p)²⁷Mg and²⁸Si(n,p)²⁸Al were measured by activation method between 13.40 and 14.83 MeV neutron energy. An accuracy of about 4% was achieved using the²⁷Al(n, α)²⁴Na reaction as a reference at 14.1 MeV where the relative excitation function has also been measured. Results obtained were compared to a recent compilation and that calculated by the Hauser-Feshbach model. Using the back-shifted level density formula and taking into account the contribution of the separated levels, the calculations were extended to the energy range from the threshold to 18 MeV. A structure was observed in the²⁷Al(n, α)²⁴Na reaction cross section curve around 14 MeV neutron energy.     

The mechanism for pair production in the α-Decay of241Am

The mechanism for pair production in the α-decay of²⁴¹Am is discussed. The pair creation is dominantly involved with the dynamics of the α-decay process and contributions from²³⁷Np levels with energies >2mc ² are relatively small. One of the authors (B.A.L.) would like to thank the Natural Sciences and Engineering Council of Canada for financial support.     

Bestimmung des Kompoundkern- und Stripping-Anteils der (d,p)- und (d,n)-Reaktionen an Zr94 und Zr96

An experiment has been accomplished which gives information on the reaction mechanism of the (Zr^94,96+d)-reactions. Total cross sections and excitation functions up to 11,8 MeV have been measured for the reactions Zr⁹⁴ (d, p), Zr⁹⁴ (d, n), Zr⁹⁶(d, p), Zr⁹⁶(d, n) and Zr⁹⁶(d, 2n) by the activation method. The results are compared with cross sections calculated according to the statistical theory of nuclear reactions. It is shown that the (d, p) -reactions proceed almost completely by the stripping-mechanism. The (d, n)- and (d, 2n)-excitation functions on the other hand are in good agreement with the predictions of the statistical theory and one can conclude that the compound nucleus mechanism is the dominating process.     

Fission fragment energy correlation measurements for the sub-barrier (nth, f) of 241Am and 243Am

Fission-fragment mass and kinetic-energy distributions and their correlations have been studied for the sub-barrier fission of the odd-odd ²⁴²Am and ²⁴⁴Am nuclei resulting from thermal neutron capture in ²⁴¹Am and ²⁴³Am. Unwanted events were eliminated by a coherence test based on the time of flight. The ²⁴³Am mass distribution is more asymmetric and shows structures at μ_H ≈ 139 and 144, compared with that for ²⁴¹Am which is smooth and structureless. The structure at μ_H ≈ 144 is caused by deformed neutron shells at N ≈ 88 in the heavy fragment and $\text{N ≧ 60}$ in the light fragment. While the 〈E_K〉 at symmetry for ²⁴¹Am is ≈ 19 MeV higher than that for ²³⁵U, it is shifted upwards only ≈ 6.5–10 MeV in the other mass regions. However, 〈E_K〉 at symmetry for ²⁴³Am is ≈ 6 MeV lower than for ²⁴¹Am and this decrease tapers down to ≈ 1 MeV for μ_H > 135. These data show a decrease in the total fragment deformation for ²⁴¹Am at symmetry as predicted by calculations. However, the ²⁴³Am data show a sudden change back to higher deformation.     

Angular anisotropy of the alpha emission from241Am nuclei oriented by magnetic hyperfine interaction

Angular anisotropy of the alpha emission from²⁴¹Am nuclei oriented by magnetic hyperfine interaction at low temperatures in ZrFe₂-host have been observed. The sign of the anisotropy indicates unambiguously the enhanced alpha emission from the poles of²⁴¹Am nucleus. The value of magnetic hyperfine field on²⁴¹Am nuclei have been estimated from the temperature dependence of the anisotropy.     

Investigation of (n, 2n) reactions by the evaporation process

The (n, 2n) cross sections for about 80 nuclei ranging fromA=45 to 209 were calculated using the evaporation model. The calculation was performed for incident energiesE _n which satisfy the conditionU _R =E _n +Q _n ,2n=6±1 MeV. For 11 nuclei in the above mass region, (n, 2n) excitation functions were calculated from threshold to 20 MeV. In all calculations a single set of input parameters was consistently used. The influence of the level-density parameter and neutron cross sections on the evaporation model was tested. All calculated results were compared with experimental data. The systematic discrepancy between experimental data and evaporation calculations leads to the necessity of introducing the preequilibrium emission mechanism.     

Isomeric cross section ratio for110Pd(n,2n)109Pd reaction at 14.7 MeV

Reaction¹¹⁰Pd(n, 2n)¹⁰⁹Pd has been investigated at 14.7 MeV neutrons. Cross sections for the formation of^109m Pd and^109g Pd were measured. Experimental value of the isomeric cross section ratio has been compared with the theoretical calculation performed with the Huizenga and Vandenbosch method.     

Bestimmung der Halbwertszeit von108m Ag über (n, 2n)-Querschnitte des Silbers

The half-life of^108m Ag was determined as (310±132) years by measuring the absolute activity of a sample containing a known number of^108m Ag nuclei. This could be done by evaluating the cross section for the production of the isomerσ ¹⁰⁹Ag(n, 2n)^108mAg=(670±266) mb. From the measured half-life a logft-value of 8.9 for the electron capture decay of the isomer to the 1770 keV state of¹⁰⁸Pd and a transition probability of (1.0±0.4) · 10^?2 Weisskopf units for the M4 transition to the 2^?state of¹⁰⁸Ag were derived. The cross section for thermal neutrons¹⁰⁷Ag(n, γ)^108mAg could be calculated to be (7.5±3.2) b by using a related measurement from the literature.     

Non statistical reaction contributions in (α, n) excitation functions

The excitation function²⁴Mg(α, n)²⁷Si has been measured from threshold to 27 MeV excitation energy. A survey is given of the experimental excitation functions¹²C(α, n)¹⁵O,¹⁶O(α, n)¹⁹Ne,²⁰Ne(α, n)²³Mg and²⁴Mg(α, n)²⁷Si and HF calculations have been made for them. The agreement is good within a factor of 2. The deviations of the experimental data from the pure statistical cross sections show common structures which can be interpreted as direct contributions.     

Measurement of 232Th(n, γ) and 232Th({n}, {2n}) cross-sections at neutron energies of 13.5, 15.5 and 17.28?MeV using neutron activation techniques

The ²³²Th(n, ??) reaction cross-section at average neutron energies of 13.5, 15.5 and 17.28?MeV from the ⁷Li(p, n) reaction has been determined for the first time using activation and off-line ??-ray spectrometric technique. The ²³²Th(n, 2n) cross-section at 17.28?MeV neutron energy has also been determined using the same technique. The experimentally determined ²³²Th(n, ??) and ²³²Th(n, 2n) reaction cross-sections from the present work were compared with the evaluated data of ENDF/BVII and JENDL-4.0 and were found to be in good agreement. The present data, along with literature data in a wide range of neutron energies, were interpreted in terms of competition between ²³²Th(n, ??), (n, f), (n, nf) and (n, xn) reaction channels. The ²³²Th(n, ??) and ²³²Th(n, 2n) reaction cross-sections were also calculated theoretically using the TALYS 1.2 computer code and were found to be in good agreement with the experimental data from the present work but were slightly higher than the literature data at lower neutron energies.     

Photofission of americium isotopes in the energy range 6–12 MeV

By means of the reference method, the cross sections for the fission of the ²⁴¹Am, ^242m Am, and ²⁴³Am isotopes were measured at the microtron of the Institute of Physics and Power Engineering (Obninsk). These measurements, which employed the cross section for ²³⁸U photofission as a reference, covered the energy range 6–12 MeV scanned with a variable step of 50 to 200 keV. Data on ^242m Am photofission in the energy range 6–12 MeV and ²⁴³Am photofission in the region between 6 and 7 MeV were obtained for the first time. New results for ²⁴¹Am reveal that the cross sections for ²⁴¹Am photofission from previous studies of the present author were exaggerated. The new results for this isotope comply well with data of other authors. It is not confirmed that the fissility of ²⁴¹Am is less than the fissility of ²⁴³Am. The energy dependences of the fissilities of the americium isotopes from photofission data are compared with those that were obtained for these fissilities from data on direct reactions like ²⁴⁰Pu(³He, df)²⁴¹Am. The results of this comparison show that the observed fission thresholds and the plateaulike dependences at energies above 7.5 MeV from the two types of studies comply well. The present data show evidence that, in the energy region around 6 MeV, the photofission cross section has a maximum, which is associated, in all probability, with the low-energy resonance structure in the cross section for dipole photoabsorption.     

The (γ, n) reaction on 16O between giant resonance and pion threshold

A new TOF facility has been built for measurements of differential (γ, n) cross sections to discrete final states of light nuclei in the photon energy range between giant resonance and pion threshold. The observed neutron angle θ_n can continuously be varied between 0° and 150°, and additionally measurements at 175° and 180° are possible. Differential cross sections for the reaction ¹⁶O(γ, n₀)¹⁵O are presented for E_γ = 60 MeV (40° ? θ_n ? 149°) and for θ_n = 90° (60 MeV ? E_γ ? 160 MeV). The results, combined with the corresponding (γ, p₀) cross sections, indicate an absorption mechanism of high energy photons by neutron-proton pairs.