New data on cross sections for partial and total photoneutron reactions on the isotopes 91,94 Zr
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CLEI Experiment
New Data on Cross Sections for Partial and Total Photoneutron Reactions on the Isotopes 91,94Zr V. V. Varlamov1)* , M. A. Makarov2), N. N. Peskov1) , and M. E. Stepanov1), 2) Received November 20, 2014
Abstract—Experimental data on 91,94 Zr photodisintegration that were obtained in a beam of quasimonoenergetic annihilation photons by the method of neutron multiplicity sorting are analyzed. It is found that the cross sections for the (γ, 1n), (γ, 2n), and (γ, 3n) reactions on both isotopes do not meet the objective data-reliability criteria formulated earlier. Within the experimental–theoretical method for evaluating partial-reaction cross sections that satisfy these criteria, new data on the cross sections for the aforementioned partial reactions, as well as for the (γ, sn) = (γ, 1n) + (γ, 2n) + (γ, 3n) + . . . total photoneutron reaction, are obtained for the isotopes 91,94 Zr. DOI: 10.1134/S106377881505018X
1. INTRODUCTION Experimental data on the photodisintegration of a large number of medium-mass and heavy nuclei (such as 90 Zr, 115 In, 112,114,116,117,118,119,120,122,124 Sn, 159 Tb, 181 Ta, and 197 Au) studied in beams of quasimonoenergetic annihilation photons in [1–5] by the method of photoneutron multiplicity sorting, the multiplicities of photoneutrons being determined from their measured kinetic energies were analyzed. It was found that, as a rule, the cross sections for the (γ, 1n), (γ, 2n) and (γ, 3n) partial reactions were determined with large systematic errors, so that their reliability may be questioned. The circumstances that are indicative of the presence of such errors are primarily the following: (i) In many regions of photon energies, the specially introduced neutron-multiplicity transition functions (1) Fi = σ(γ, in)/σ(γ, xn) = σ(γ, in)/σ[(γ, 1n) + 2σ(γ, 2n) + 3σ(γ, 3n) + . . .] exceed limits physically allowed for them by definition (1.00, 0.50, 0.33,. . . for, respectively, i = 1, 2, 3, . . . ). (ii) In the same regions of photon energies, the cross sections for the various reactions—mainly for (γ, 1n)—take negative values, which are forbidden physically. 1)
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia. 2) Faculty of Physics, Moscow State University, Moscow, 119991 Russia. * E-mail: [email protected]
It was found that values established for the systematic errors are associated with shortcomings in photoneutron-sorting methods used in the quoted experiments and based on the results of measurements of photoneutron kinetic energies. In order to obtain reaction cross sections that are free from the aforementioned problems and which satisfy criteria of data reliability, an experimental– theoretical method for evaluating them was proposed in [1, 2]. This method is based on employing, in the evaluation procedure, only experimental neutronproduction cross sections σ(γ, xn) ≈ σ(γ, 1n) + 2σ(γ, 2n) + 3σ(γ, 3n) + . . . ,
(2)
which are independent of problems of neutron multiplicity sorting, and underlying principles of the combined
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