Gamma-ray competition with neutron emission in the decay of 137I

Gamma-rays with energies above the neutron binding energy in ¹³⁷Xe have been observed at 3907 and 3994 keV in the decay of ¹³⁷I. Their combined intensity amounts to about 25% of the delayed neutron intensity and demonstrates that γ-ray widths are an important factor in defining the magnitude and energy distribution of the delayed neutron spectrum.     

Verification of the Lane-Lynn theory of direct neutron capture

Quantitative and detailed comparison between the Lane-Lynn theory of direct capture of thermal neutrons and experimental values shown exceptional agreement for a large body of data in the mass regions around 40 and 140. The ¹³⁶Xe(n,γ)¹³⁷Xe reaction is used to verify the theory. Other examples are also given to illustrate the ability of the theory to predict capture cross sections, coherent-scattering lengths, spins of final states, and (d,p) spectroscopic factors.     

Levels of188Os populated by thermal neutron capture

Direct and coincidence gamma spectroscopy has been applied to the reaction¹⁸⁷Os(n, γ)¹⁸⁸Os. A neutron binding energy of 7,989.3±0.3 keV is deduced and a level scheme is established. The experimental level energies andB(E2) ratios are compared to calculations in the framework of the Interacting Boson Model.     

Gas-cell-based setup for the production and study of neutron rich heavy nuclei

The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleosynthesis. This area of the nuclear map can be reached neither in fusion–fission reactions nor in fragmentation processes widely used nowadays for the production of exotic nuclei. A new way was recently proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be significantly high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of ¹³⁶Xe with ²⁰⁸Pb. A new setup is proposed to produce and study heavy neutron-rich nuclei located along the neutron closed shell N = 126.     

Fast neutron scattering from rubidium

A study was made of the (n, n), (n, n′), and (n, n′ γ) reactions which occur when a natural rubidium scattering sample is exposed to mono-energetic neutron fluxes of energies ranging from 120 to 1910 keV. Total cross sections, elastic scattering angular distributions and excitation functions for inelastic scattering were measured with a neutron time-of-flight spectrometer. Results are compared with Hauser-Feshbach theory. A 40 cc Ge(Li) detector was used in the time-gated mode to measure γ-rays from the (n, n′ γ) reaction. The latter measurements were used to refine and extend the energy level schemes derived from neutron spectroscopy. Several new levels were discovered in the low-lying (below 1900 keV) energy level spectra of⁸⁵Rb and⁸⁷Rb. Gamma decay schemes and branching ratios were determined for the low-lying levels of⁸⁵Rb and⁸⁷Rb.     

Neutron alignment at high spin in 156Dy

Utilizing the transient magnetic field interaction for nuclear recoils passing through an iron or a gadolinium foil, we have measured the precessions for discrete γ-ray transitions in the reaction ²⁴Mg(¹³⁶Xe,4n)¹⁵⁶Dy at 600 and 620 MeV incident beam energy. The average nuclear g-factors deduced at spins between 19 and 23? are in accord with one previous measurement and in contradiction with another. The present results imply the predominant population of neutron-aligned bands near the backbend in ¹⁵⁶Dy.     

Beta-delayed neutrons and high-energy gamma-rays from decay of137I

Beta-delayed neutrons and high-energyγ-rays following decay of¹³⁷I have been studied in order to characterizeβ-decay to and deexcitation of neutron-unbound levels in¹³⁷Xe. The totalγ-ray intensity observed accounts for at least 20% of theβ-intensity to levels aboveB _n . No evidence was found for neutron andγ-ray emission from the same levels. With the experimentalβ-strength function aboveB _n detailed tests were performed on the reproduction of the experimental neutron spectrum, the level density and Porter-Thomas fluctuations of the reducedβ-transition probabilities to highly excited states in¹³⁷Xe.     

Present status of the 129Xe comagnetometer development for neutron EDM measurement

A ¹²⁹Xe comagnetometer designed for the measurement of neutron electric dipole moment (nEDM) as precisely as 1 × 10^?27e cm is presented. Highly nuclear spin polarized ¹²⁹Xe are introduced into an EDM cell where the ¹²⁹Xe spin precession is detected by means of the two-photon transition. The geometric phase effect (GPE) which generates the false nEDM was quantitatively discussed and the systematic error of nEDM from the GPE was estimated considering the buffer-gas suppression due to Xe atomic collisions. Research and development are in progress to construct the ¹²⁹Xe comagnetometer with a field sensitivity of 0.3 fT. At present, about 70 % nuclear spin polarized ¹²⁹Xe atoms have been obtained in a spin exchange opitial pumping cell, that are in the process of being transferred into the EDM cell via a cold trap.     

Pairing-energy coefficients of neutron-rich fragments in spallation reactions

The ratio of pairing-energy coefficient to temperature(a_p/T)of neutron-rich fragments produced in spallation reactions has been investigated by adopting an isobaric yield ratio method deduced in the framework of a modified Fisher model.A series of spallation reactions,0.5A and 1A Ge V~(208)Pb+p,1A Ge V~(238)U+p,0.5A Ge V~(136)Xe+d,0.2A,0.5A and 1A Ge V~(136)Xe+p,and~(56)Fe+p with incident energy ranging from 0.3A to 1.5A Ge V,has been analysed.An obvious odd-even staggering is shown in the fragments with small neutron excess(I≡N-Z),and in the relatively small-A fragments which have large I.The values of a_p/T for the fragments,with I from 0 to36,have been found to be in a range from-4 to 4,and most values of a_p/T fall in the range from-1 to 1.It is suggested that a small pairing-energy coefficient should be considered in predicting the cross sections of fragments in spallation reactions.It is also concluded that the method proposed in this article is not good for fragments with A/A_s85%(where A_(s )is the mass number of the spallation system).     

Levels in154Eu populated by thermal neutron capture

The level scheme of¹⁵⁴Eu has been investigated by means of thermal neutron captureγ-rays and conversion electrons. The high energyγ-ray spectrum from thermal neutron capture in enriched¹⁵³Eu has been studied in the energy range of 5700 to 6500 keV. Low energyγ radiation has been observed with Ge(Li) and Si(Li) detectors from 5 to 300 keV and conversion electrons have been measured from 15 to 300 keV. Low energy (n, γγ) coincidences and half lives of transitions have been measured. The data, combined with three very recent studies of the 8^? isomer decay in¹⁵⁴Eu has led to the construction of a level scheme with 12 excited levels. Nuclear Reaction¹⁵³Eu(n, γ),E _n =thermal, measuredE _γ ,I _γ ,E _ce ,I _ce ,γγ-coincidence, halflives,¹⁵⁴Eu deduced levels.     

A search for deeply bound pionic states of xenon produced in the 136Xe(d, 3He)135Xeπ-bound reaction

A search for deeply bound pionic states of xenon produced in the ¹³⁶Xe(d, ³He)¹³⁵Xe_π-bound reaction at E _d = 500 MeV is reported. The population of the 1s pionic-atom state of ¹³⁵Xe is observed on the predicted level of the cross section ～ 40 μb/sr. The binding energy of the 1s state, B = 2.9 ± 0.5 MeV, however, is lower than theoretically predicted.     

Fast neutron radiative capture in silicon

Using the²⁸Si(n, γ)²⁹Si reaction, transitions to the ground state and first excited state in²⁹Si have been studied in the neutron energy range 3–14 MeV with improved neutron energy resolution (of about 100 keV). The 90° cross sections show considerable structure in the entire neutron energy range. Comparison with theoretical calculations shows that compound-nucleus and direct-semidirect processes account for the non-resonant part (smoothly varying part) of the cross section. A microscopic model is, however, required to describe the resonance structure. Continuum shell-model calculations have proven to be a very promising means towards a better understanding of the capture process in, and below, the giant resonance region in light nuclei. The angular distributions of gamma rays in the neutron energy range 8–14 MeV indicate that the capture reaction is mainly of direct character and that the effect of interference between the electric dipole and isoscalar quadrupole resonance is weak.     

A precise PKPβ+ measurement at Z = 55: (I). The mass of 130Cs

The Q-value for the ¹²⁹Xe(³He, d)¹³⁰Cs reaction is measured to be +5 ± 20 keV. By combining this result with the known neutron separation energy of ¹³⁰Xe, we derive the total decay energy of ¹³⁰Cs

¹³⁰Xe to beQ_EC = 2974 ± 20 keV. This value agrees well with two previous positron end-point measurements but disagrees with the corresponding value derived from the 1977 atomic mass evaluation. This has significance in testing the accuracy of the calculated ratio for ¹³⁰Cs decay. The mass excess of ¹³⁰Cs is derived to be ?86908 ± 14 keV. An excited state in ¹³⁰Cs. was also identified at 121 ± 15 keV with J < 3 and positive parity. The Q-value for ¹³⁶Xe(³He, d)¹³⁷Cs was measured to be 1918 ± 12 keV.     

Investigation of background sources associated with the production of radioactive isotopes in the DEVIS experiment (double-beta decay)

Measurements performed in searches for the double-beta decay of ¹³⁶Xe showed an excess of events in exposures of a target enriched in the isotope ¹³⁶Xe in relation to exposures of a target depleted in this isotope. The activation of xenon in the detector used under the effect of background neutron and muon radiation and the possible contribution of these processes to the results of relevant measurements are studied.     

The effect of fast neutron irradiation on the optical modulation spectra of GaAs

The electroreflectance and wavelength-modulated reflectance spectra of GaAs were measured before and after several reactor irradiation periods. High resistivity n type GaAs crystals were irradiated at a temperature between 300 and 310 K up to a fast neutron fluence of 3.3 × 10¹⁷n/cm². The E₀ and E₀+Δ_o peaks shift nonlinearly toward lower energy, the change reaching a maximum value of about ? 50 meV at 10¹⁷n/cm². At this fluence an additional peak appears at 1.33 eV. The E₁ and E₁+Δ₁ peaks move almost linearly toward higher energy with increasing fast neutron fluence, the shift being about + 25 meV at 2 × 10¹⁷n/cm². The results are discussed taking into account infrared absorption measurements and the calculations made by McNichols, Hayes and Ginell concerning the metallic GaAs precipitates. The effect of possible internal stress produced by the fast neutron bombardment on the modulation spectra is also discussed.     

Neutron multiplicities in inelastic collisions of132Xe with197Au

Neutrons have been detected in coincidence with charged reaction products in inelastic scattering of 7.5 MeV/u¹³²Xe ions from¹⁹⁷Au. The deduced neutron multiplicities associated with the heavy and light fragment, respectively, are roughly proportional to the total kinetic energy loss, and their ratios are close to the Au-Xe mass ratio for allQ-values. These results and the measured neutron energy spectra are consistent with the assumptions of thermal equilibrium between the fragments at scission, and of neutron emission from fully accelerated fragments. For deep inelastic events, the measured absolute multiplicities are smaller than expected from statistical model calculations, but an effect due to pre-equilibrium emission of particles — as suggested by an earlier analysis of the present data [1] — cannot be definitely established.     

Rotational states in 151Nd populated through thermal neutron capture

The level scheme in the nucleus ¹⁵¹ Nd has been studied following neutron capture by observing the γ-rays and conversion electrons with curved-crystal spectrometers, a Ge(Li) detector and a β-spectrometer. A comprehensive level scheme up to ? 1 MeV was established. It is shown that the present level scheme energies differ from those previously found in a ¹⁵⁰Nd(d, p) reaction study by a systematic shift of 27 keV. The binding energy of the last neutron in ¹⁵¹Nd was deduced. For most levels spin and parity values are proposed and an identification of bands with Nilsson-model configurations is given.     

Strength functions of primary transitions following thermal neutron capture in strontium

The primaryE1,M1 andE2γ-radiation in^87,88,89Sr observed after thermal neutron capture was compared with the predictions of single particle and giant resonance models. The nuclei feature a wide range of neutron binding energies between 6.3 and 11.1 MeV, which makes a 5.5 MeV spectrum of primary transition energies available for investigation. The (n, γ) reaction was used to estimate the parameters of the spin-flip M1 giant resonance in strontium. The total energy weightedM1 strength of this resonance exceeds the results of shell model and random phase approximation calculations for⁹⁰Zr by a factor of 3–4. TheE1 strengths were found to agree with the established giant dipole resonance model. The few data on primaryE2 transitions do not allow to differentiate between the giant quadrupole resonance and the single particle models.     

Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesP _n of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. TheP _n-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of²³⁵U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.