Gamma rays from resonance neutron capture in 115In

Gamma rays in the range 5.4–6.7 MeV have been studied for 31 s-wave neutron resonances of ¹¹⁵In, selected by time of flight in the range 3–430 eV. In a subsidiary experiment, spin J = 5 has been assigned to 17 and J = 4 to 14 resonances by measuring intensity variations of some strong low-energy transitions. The reduced widths averaged over all initial states of the same spin have been estimated for 41 primary transitions: these values have provided information on the spin and parity of the corresponding ¹¹⁶In final states. Overall mean values of E1 and M1 radiative strength have been calculated. The width distribution has been fitted with a χ² function with ν = 1.10^+0.27_?0.09 degrees of freedom for M1 and ν = 1.42 ^{+ 0.014}_?0.08 for E1 radiation. An estimate of the spin cut-off parameter σ = 3.6_?0.4^{+ 0.8} has been derived. A non-statistical effect already evidenced in previous measurements has been confirmed, consisting of a strong modulation of the radiative strength against resonance energy, correlated also with the local neutron strength function. In addition, it has now been shown that this structure is due to E1 radiation only.     

Resonance neutron capture gamma rays in 177Hf

Primary capture γ-rays have been studied for 38 ¹⁷⁷Hf neutron resonances with energies in the range 1–165 eV. Intensities were measured for 29 transitions ending at states with an excitation energy in ¹⁷⁸Hf up to 2050 keV. The analysis was facilitated by the previous knowledge of the spin and parity of all neutron resonances and of most low-lying states. For nine final levels, which had not previously been seen, information on J and π was deduced from the corresponding average intensities. The distribution of partial widths was fitted with a χ² function with ν = 1.38_?0.13^+0.18 degrees of freedom for E1 radiation and ν = 1.5_?0.40^+0.60 for M1 radiation. The average El reduced photon strength was found to be $\text{S}{}_{\mathrm{<mtext>El</mtext>}}\text{= 〈Γ}{}_{\mathrm{\gamma itij}}\text{/DE}{}_{\mathrm{\gamma }}{}^5\text{〉A}{}^{\mathrm{<mtext>?8</mtext><mtext>3</mtext>}}\text{= (4.8 ± 1.0) × 10}{}^{\mathrm{?15}}\text{MeV}{}^{\mathrm{?5}}$ and the ratio between El and Ml intensities equal to 5.5 ± 1.4. A comparison of this value for the El strength with those reported for other nuclei with A$\text{\$}\text{?}$= 100 showed that the intensities follow the A-dependence predicted by the Brink-Axel model. A non-statistical effect was observed, consisting of an enhancement of El transition probalilities to K = 2, 3 final states as compared to K = 0, 4 states.     

Correlation between energies and angles of α-particles emitted in thermal-neutron fission of U

The energy distribution and yields of the α-particles emitted in the thermal-neutron fission of ²³⁵U were measured with the same detector system for the cases when the average angles between fragments and α-particles were 90°, 46°, 27° and 11°. The data were analysed by the Monte Carlo method to take into account the effects of the finite size of the source and the various detectors, and the following results were obtained: (i) Even at small angles with respect to the fission axis the yield of the α-particles is about 2–3 % of the yield at 90°, and does not go to zero as would be expected for a true Gaussian angular distribution of a variance which fits the data around 90°. (ii) At these angles where most of the α-particle yield belongs to the non-Gaussian component, the α-particle energy distribution has a significantly higher most probable energy but the FWHM of the energy distribution is not significantly different, (iii) The rms width σ_θ of the Gaussian angular distribution is found to increase for very high (E_α > 20 MeV) α-particle energies and also to a lesser extent for very low (E_α < 15 MeV) energies. The origin of the yield of the α-particles at small angles, and the dependence of the rms width σ_θ on the energy are discussed.     

Gamma rays from thermal neutron capture in gold

The thermal column of the 10 kW University of Maryland pool reactor was used as the neutron source for this nuclear spectroscopy study. A 40 cm³ lithium drifted germanium crystal was the gamma (γ) ray detector used to measure the high energy prompt photon emissions from thermal neutron capture. The energy region was from 2.6–7.0 MeV. New capture gamma ray lines were observed.     

Two-hole strengths in K from the K(He, αK reaction

States in ³⁸K up to 7.13 MeV excitation have been studied using the 21 MeV ³He⁺⁺ beam of a tandem Van de Graaff and the ³⁹K(³He, α) reaction. Charged particles were analysed by a split-pole magnetic spectrograph equipped with a telescope arrangement of proportional counters for position and energy-loss measurement.     

Gamma-ray spectra and level structure of Cr from thermal neutron capture in Cr

Gamma-ray measurements have been made of the ⁵⁴Cr(n, γ)⁵⁵ Cr reaction at the Livermore reactor. Spectra were taken with Ge(Li) Compton-suppression and pair-coincidence spectrometers. A total of 83 observed γ-rays are attributed to capture in ⁵⁴Cr, of which 26 are assigned to specific transitions among 10 inferred levels in ⁵⁵Cr. The neutron binding energy is determined to be 6246.3 ± 0.4 keV.     

Angular distributions of γ-rays from fast neutron capture in strontium and yttrium

Gamma-ray spectra from neutron capture in natural samples of strontium and yttrium have been recorded at various angles with respect to the direction of the incident neutron flux. Angular yields have been observed at six neutron energies in the range 7 to 11 MeV using time-of-flight techniques to improve the signal-to-background ratio. The γ-radiation was detected by a large NaI(Tl) crystal placed in a heavy radiation shield. Certain combinations of Legendre polynomial coefficients were extracted for transitions to low-lying single-particle states ($\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ and $\text{3}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) in the final nuclei. The energy dependence of the angular distribution coefficients indicates interference between the electric dipole amplitude and amplitudes of opposite parity. The results are compared with theoretical calculations based on the direct-semidirect model.     

Delayed gamma rays from muon capture on 207Pb

We have measured the energies and intensities of delayed γ-rays from ^207,201Tl following muon capture on ²⁰⁷Pb. Probabilities for the emission of 0–6 neutrons have been deduced. Measured and predicted neutron multiplicities (based on an impulse approximation model) are in reasonable agreement for multiplicities of 2, 3, 4 and 5. Experimental values for high and low multiplicities are not accounted for by the theory. High angular momentum states involving the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton hole are observed in several of the thallium daughters. Using a simple shell-model approach, we have correlated μ-capture on the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton shell with the energy spectrum of directly emitted neutrons.     

Spallation reactions in isotopically enriched targets

Yields of ⁴²K and ⁴³K have been measured in spallation reactions at 590 MeV and at 7 GeV in isotopically enriched targets from ⁵⁸Ni to ⁶⁸Zn. The results are correlated with other recent measurements in the development of a new empirical spallation-yield formula which takes into account the isotopic composition of the target. A practical example of the use of this formula is discussed.     

Resonance neutron capture in silicon

Gamma ray spectra from neutron capture by natural silicon have been measured for resonances at 31.7, 38.8, 55.9 and 67.7 keV. Absolute partial radiative widths have been obtained with the 35 keV s-wave resonance in aluminium as a reference standard. Strong single particle effects were observed in the final state correlation. These cannot be accounted for by the valence model of neutron capture. A different single particle mechanism must therefore occur at these energies.     

Gamma decay of low-lying states in S and Ar

Decay modes and lifetimes of low-lying levels in ³⁰S and ³⁴Ar were investigated by the Doppler-shift attenuation method using the ²⁸Si(³He, n)³⁰S and ³²S(³He, n)³⁴Ar reactions. The lifetimes measured in ³⁰S were (in fs): τ_m(2210) = 310 ± 75, τ_m(3402) = 160 ± 45 and τ_m(3664) > 1400; and in ³⁴Ar: τ_m(2090) = 150 ± 50, τ_m(3286) = 120 ± 65 and τ_m(4513) = 300 ± 80, also in fs. These results are compared with theoretical calculations. A comparison of the E2 decay strengths in masses 30 and 34 is also made to test the isospin dependence of electro-magnetic transitions.     

Resonant neutron capture in 40Ca

The neutron capture cross section of ⁴⁰Ca has been measured with ≈ 0.2 % energy resolution below E_n = 300 keV. Resonance parameters have been extracted for many new p- and d-wave resonances. Gamma-ray spectra were also measured following capture in one doublet and two resolved resonances below 50 keV. Strong feeding of low-lying p-wave levels was observed in all cases. Calculations showed that valence transitions were inadequate to account for the observed dominance of these transitions and a further mechanism is required. The average resonance parameters obtained from the data are as follows: 〈D〉 = 37 + 4keV, 10⁴S₁ = 0.16 ± 0.05, 10⁴S₂ = 2.0 ± 0.7. The average radiative widths and standard deviations of their distributions were found to be strongly l-dependent as follows: 〈Γ_γ〉_s = 1.5 ± 0.9 eV, 〈Γ_γ〉_p = 0.36 ± 0.09 eV and 〈Γ_γ〉_d = 0.7 ± 0.4 eV.     

Resonance neutron capture in 56Fe

The neutron capture cross section of ⁵⁶Fe has been measured with 0.2–0.3% energy resolution from 2.5 keV up to the inelastic neutron scattering threshold. Results are compared with recent total cross-section data and average parameters are derived for s-, p- and d-wave resonances. The low correlation coefficient observed between the s-wave reduced neutron and radiative widths is consistent with the minor contribution of the valence capture mechanism as calculated in the framework of the optical model. Broad E1 and M1 doorway states for s-, p- and d-wave resonances are postulated to explain the cross-section data and γ-ray spectra up to 1 MeV.     

Properties of nuclear levels excited by neutron capture γ-rays from cobalt

Neutron capture γ-rays from cobalt have been used to photoexcite nuclear levels in the 5–8 MeV region. The decay properties of the 7491 keV level in ⁵⁵Mn and the 6877 keV level in ¹⁴²Nd were studied in detail. Total and partial radiative widths of nuclear levels in several isotopes were determined using nuclear self-absorption, temperature variation, and absolute scattering cross section measurements. The total radiative widths were found to be of the same magnitude as those of unbound levels populated in neutron resonances. The spins and parities of some resonance levels were determined by carrying out angular distribution and polarization measurements, respectively.     

Gamma-rays following muon capture

The energy and intensity of γ-rays following capture of muons by Si, Ti, Mn, Fe, Co, Ni, Y, Ag and Au were measured with a Ge(Li) detector. In most cases it was possible to identify the final nucleus from the energy of the γ-ray observed. In general, this showed that one or more neutrons were emitted in the initial stages of de-excitation. In the case of capture by Fe and Ni the evidence pointed to a mode of de-excitation by the emission of a proton and a neutron and in Ti by γ-rays alone. In these measurements it was possible to account for as much as 60% of the muons captured in Co, less in the other elements, and only 15 % in Si.     

Gamma rays and neutrons following preequilibrium 165Ho(p, xcnypγ) reactions at Ep = 60 MeV

Neutrons and γ-rays following 60 MeV proton bombardment of ¹⁶⁵Ho were measured in coincidence with discrete γ-rays characteristic of the reaction channels (residual nuclei). The cross sections for the (p, xcnγ) reactions with x = 2–6, the γ-ray multiplicities, and the energy and angular distributions of the emitted neutrons were analyzed in terms of the preequilibrium and equilibrium deexcitation processes. Characteristic behaviours of the preequilibrium process were found in the (p, 2nγ) and (p, 4nγ) reactions where the sum E_T, = ∑^xE_i of the energies E_i of the emitted neutrons was large, while those of the equilibrium process were typical for the (p, 6ny) reaction with small E_T. The reactions are well reproduced by the expression σ. ≈0.35∑_xσ (2, x?2) + 0.4∑_xσ(1, χ?1)+ 0.25∑_xσ(0, x), where σ(n_p, n_c) stands for emission of n_p neutrons at the preequilibrium stage followed by evaporation of n_c neutrons at the equilibrium stage.     

Total cross-section measurements for the production of nuclear gamma rays from light nuclei by low-energy deuterons

The thick target yields of the reactions ⁶Li, ⁹Be, ¹⁰B(d, nγ) for specific final nucleus γ-rays have been measured between deuteron bombarding energies of 48 and 170 keV. The measured thick target yields are used, together with recently published values of the stopping power of low-energy deuterons in matter, to infer the total reaction cross sections for the production of the specific γ-rays between deuteron energies of 65 and 160 keV. The cross sections are compared to appropriate Coulomb barrier penetration probabilities and the astrophysical functions S(E) are deduced.     

Non-statistical effects in neutron capture γ-ray spectra

A search for non-statistical effects in γ-ray spectra following neutron capture has been made for incident neutron energies in the range 40 keV to 1 MeV. The experiments were performed using a 20 cm × 15 cm Nal detector; thirty elements ranging from calcium to uranium were examined. Marked non-statistical effects were found in the spectra from elements in the mass regions where the 3s, 3p and 4s neutron strength functions maximize. Results from the first two regions could often be explained in terms of enhanced transition strengths to final states with a strong single-particle nature.     

Resonance neutron capture in 52Cr

Neutron capture γ-ray measurements have been performed upon a natural sample of Cr. Twenty-six γ-rays were observed from the 1626 eV resonance of the ⁵²Cr(n, γ)⁵³Cr reaction, and twenty-four of them were assigned to a level scheme. A value of $\text{3}\text{2}$^? was determined for the spin-parity of this resonance. The neutron separation energy, derived from a separate thermal measurement with an enriched (99.9 %) ⁵²Cr sample was determined to be 7939.1 ± 0.2 keV. The high (n, γ) (d, p) correlation found for thermal capture is absent for the p-resonance, however the γ-ray intensities from thermal and resonance capture are correlated, with r = 0.86_?0.11^+0.06.