Determination of the neutron binding energy of the delayed neutron emitter137Xe

Abstract

The neutron binding energy of 237Xe has been deduced to be 4025.2 _+ 0.6 keV from a study of the 136Xe(n, 7) 137Xe reaction. The importance of a precise value for this quantity is due to the fact that an accurate determination of binding energies of delayed neutron emitters is possible only for STKr and t37Xe, neighbouring stable isotopes. Nuclear reaction. 136Xe(n, 7), enriched target; measured Ev, I~, deduced neutron binding energy, Ge(Li) detector.Much experimental effort has recently been concentrated on studies of finer details in the process of delayed neutron emission. A pronounced line structure has been observed in high resolution measurements [1 3] of delayed neutron spectra, particularly in the case where the final nucleus possesses a closed neutron shell. In these cases the density of the unbound levels, strongly populated via/%decay, is low in the neutron emitting nucleus. This permits interpretation of the lines in the delayed neutron spectra as individual neutron transitions from unbound initial levels to low-lying levels of the final nucleus. Details such as 7-rays following neutron emission [4] and 7-rays competing with the neutron emission [-5, 6] can be studied and compared with the results of delayed neutron spectroscopy. For this comparison, it is essential to know the neutron binding energy with high precision. This is because the B, value provides the zero point energy of the delayed neutron transitions to the ground state of the final nucleus. Only two delayed neutron emitters, STKr and 137Xe are close to stability, permitting precision determinations of the neutron binding energies using (d,p) or (n,7) reactions. The binding energy of 87Krhas recently been determined [-7] with a precision of one keV. On the other hand, the values of the neutron binding energy of 137Xe, 3860_+20 keV [8] and 4043+60keV [9], obtained through (d, p) spectroscopy, are unprecise and do not agree. It was therefore necessary to make an improved determination of the neutron binding energy of 137Xe ' Using the 136Xe(n, 7)~37Xe reaction the precision is only limited by the calibration accuracy of the Ge(Li) detector. The neutron capture cross-section of 136Xe is with 0.16b [10], relatively small, which made necessary the use of a high neutron flux. Therefore the experiment was carried out at one of the thermal neutron guides at the ILL high flux reactor in Grenoble. The very pure thermal neutron beam had a flux of about 109 n/cm 2-s. The beam was collimated to a diameter of 2 cm. A sample of Xe gas, enriched to 91.2 % in 136Xe, was contained in a "bubble" with dimensions of 3 cmx 3 cmx 1 cm, made of 4 mg/cm 2 A1 foil and placed in the neutron beam. Thinner A1 foil was found not to be leak proof. One can estimate that about 30 mg of the gas and 25 mg of A1 was exposed to the full neutron intensity. The gas container was viewed by a 80 cm 3 Ge(Li) detector through a 15 cm long lead collimator. Although much work was spent on reducing the background due to scattered neutrons, a considerable 7-ray intensity in the rec...