Measurement of thermal neutron radiative capture cross-sections of the ¹⁴N and ¹⁹F by in-beam γ-spectroscopy method with reactor neutrons

Abstract

The cross-sections of the ¹⁴N(n,γ) ¹⁵N and ¹⁹F(n,γ) ²⁰F reactions were measured by γ-spectroscopy method with the target placed at the end of a neutron guide of the WWR-M reactor (PNPI, Gatchina). The neutron spectrum was measured by the TOF method. The maximum of the spectrum is located at 0.028(2) eV and FWHM is about 0.03 eV. Cross-section of radiative neutron capture in aluminium and absolute γ-intensities of the 1779 and 1633 keV γ-lines from beta decay of the ²⁸Al and ²⁰F, respectively, and the prompt 1678 and 1885 keV γ-lines from the ¹⁴N(n,γ) ¹⁵N reaction were taken as standards. The thermal neutron radiative cross-sections of the ¹⁴N and ¹⁹F were measured to be 0.0795(13) and 0.00950(15)b, respectively. New value of the γ-intensity of the 1678 keV γ-line from the ¹⁴N(n,γ) ¹⁵N reaction has been proposed.

Introduction

The determination of the neutron radiative capture cross-sections (NRCCS) by the measurement of intensities of prompt γ-rays from the (n,γ)-reaction has a high level of discrimination against impurity effects in objects which are available as chemical compounds only. It is just the case for nitrogen and fluorine. This method compares the (n,γ)-spectrum of the element with unknown cross-section with the (n,γ)-spectrum of a standard. Knowledge of the decay schemes for both nuclei is required to determine the NRCCS.

The thermal NRCCS of the ¹⁴N was measured by this method at McMaster University Reactor [1] and at Los Alamos Omega West Reactor [2]. Targets consisting of melamin were placed in the tangential facility with Maxwellian neutron spectrum [1] and in thermal column [2] of these reactors. The corresponding value of the ¹⁹F was measured at Los Alamos [3] with Teflon target in the same conditions as in Ref. [2]. These research groups used for the NRCCS's determination a considerable amount of ${\sigma }_{\gamma }$ and γ-intensity standards which was taken from a literature. It should be noted that the neutron spectrum in Los Alamos experiments had approximately Maxwellian form, with the most probable neutron velocity being 2400 m/s. Authors of McMaster and Los Alamos works presented the final NRCCS values reduced to the most probable neutron velocity $V_{\mathrm{n}}=2200\mathrm{m}/\mathrm{s}$ ($E_{\mathrm{n}}=0.0253\mathrm{eV}$).

Described in the present study is the application of this method for the ¹⁴N and ¹⁹F NRCCS measurements performed at the external thermal neutron beam of the WWR-M reactor (PNPI, Gatchina) with narrow, not Maxwellian form, neutron spectrum (maximum at $E_{\mathrm{n}}=0.028(2)\mathrm{eV}$, FWHM about 0.03 eV and the most probable neutron velocity $V_{\mathrm{n}}=2200\mathrm{m}/\mathrm{s}$). The NRCCS values were determined using significantly less number of standards than in above-mentioned works (Refs. [1], [2], [3]).