Capture Γ-rays Research Articles

Width correlations and intermediate structure in the n-γ spectra of Au, Sb, I and Tb

A search for non-statistical effects in the resonance neutron capture γ-ray spectra of Au, Sb, I and Tb is made through observation of the correlation between neutron and partial radiation widths, and through detection of intermediate structure in the intensity of high energy γ-rays, when plotted as a function of the neutron resonance energy. The integrated intensity method, which is used in the present measurements, is shown to be attractive for a faster mapping of width correlations. A high correlation of 0.96 for Tb and the zero values for Au and I are in agreement with the doorway state hypothesis of Lane. The 159Tb data show non-statistical behaviour, indicating the existence of a narrow intermediate structure with a width of about 40 eV, together with a high value for the correlation coefficient. No evidence for such structure is found in the earlier measurements of similar type on 115In and 187Re when the recently established run statistic is applied to the data. From several experimental facts it is concluded that the neutron capture decay process in Au is in complete agreement with the statistical model, in contrast to the earlier 2p-1h doorway hypothesis for this nucleus.

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 transitions between low-lying levels in 119Sn

We have carried out measurements on the decay of 119In isomers and the 118Sn(n, γ) reaction to supplement Coulomb excitation measurements on 119Sn. In addition to the 311.39 keV isomeric transition in 119In, we observed 13 γ-rays in 119Sn from the decay of the 2 min and 18 min 119In isomers. These γ-rays have been incorporated into a level scheme of 119Sn with levels at 0, 23.867, 89.54, 787.01, 920.5, 921.4, 1089.5, 1187.76, 1249.67, 1304.44 and 1354 keV. Conclusive evidence for the existence of a 920.5–921.4 keV, 3 2 +- 5 2 + level doublet was obtained from capture γ-ray measurements of resonance energy neutrons.

A study of the 59Co(n, γ) reaction with polarized neutrons and polarized nuclei

Neutron capture γ-rays following capture of polarized neutrons in polarized 59Co nuclei were measured in the direction of nuclear orientation and perpendicular to this direction with parallel and antiparallel polarization directions of neutrons and nuclei. A hexagonal single crystal of cobalt and a Co-8%Fe single crystal were used as samples. These were cooled to about 0.05 K with a 3He- 4He dilution refrigerator. The polarized neutron beam was obtained by Bragg reflection from a magnetized Co-Fe crystal. The ratios of capture in states with the two possible spins of the compound nucleus were measured for 19 primary transitions. Spins of several final levels were deduced from the anisotropies of primary transitions and also from the measured intensities of low-energy transitions.

A comparison of gamma-ray resonance scattering techniques for borehole analysis

The various γ-ray resonance scattering methods have been studied for possible applications to in-situ borehole analysis. The methods studied use (1) radioisotope sources for which recoil energy losses are compensated by mechanical motion, heating or recoil from preceding decay (in solid, liquid and gaseous sources), (2) nuclear reactions induced by α-particles or accelerators, (3) bremsstrahlung and Compton scatter radiation and (4) neutron capture γ-rays. The techniques most suited to borehole analysis are those using radioisotope sources. Of these techniques, the gaseous source method is the most promising. The other methods are not suitable for borehole analysis because of either low resonance count rates, the need to use accelerators or low signal to background ratios.

Low-energy γ-rays and spins of 235U neutron resonances

Low-energy prompt γ-ray spectra in the range 95–670 keV have been measured for forty-one 235U neutron resonances selected by time-of-flight in the neutron energy range 1.5–58 eV. Discrimination against the natural γ-activity of 235U has been obtained by means of a coincidence technique. The spectra show a very complicated structure owing to a few capture and many fission γ-rays. The ratios of the intensities of the 160.3 keV and 642.4 keV capture γ-rays have been used for spin assignments of fourteen resonances with small fission widths. Some conclusions are drawn about the spin dependence of average fission widths, anisotropy of fission fragments and symmetric fission yields.

A fast time analyzer embodying a high speed plated wire memory

A fast time analyzer operating in a multiscaling mode with 256 channels and 25 ns minimum channel width was constructed by employing the high speed plated wire memory with a 175 ns cycle time for the purpose of its application to pulsed neutron experiments. The characteristic features and details of construction and circuitry of the time analyzer are presented. To test the performance several experiments were carried out. These include measurements of the shapes of the pulsed neutron bursts and measurements of time distributions of thermal neutron capture γ-rays of various foils in a graphite pile.

Utilization of 11.4 MeV photons from the 59Ni(n,γ) 60Ni reaction for scattering experiments

A new reaction, 59Ni(n,γ)Ni 60, producing the highest energy of all known useful capture γ-rays is utilised for photon scattering experiments in the energy region of the giant dipole resonance. The γ line energies of the ground state transitions of the 59Ni(n, γ), 61Ni(n,γ) and 63Ni(n, γ) reactions were measured and found to be 11387.5 ± 1.7 keV, 10596.2 ± 3.7 keV and 9657.4 ± 2.8 keV respectively. A comparison is made of the intensity of the 11387.5 keV γ line with that obtained from other (n, γ) sources.

16O(α, γ) 20Ne capture γ-ray measurements on the excited Kπ = 0 + bands in 20Ne near 7 MeV

Radiative widths have been measured for the E α = 3074 keV ( J π = 0 +), 3363 keV ( J π = 2 +), 3872 keV ( J π = 2 +) and 5368 keV ( J π = 4 + z) resonances in the 16O(α,γ) 20Ne reaction, corresponding to level energies at 7191±3, 7420±1, 7826±3 and 9029±3 keV respectively. The radiative widths for decay to the 2 +, first excited state and the 0 +, ground state of 20Ne are: 7191 → 1633 (4.35±0.75 meV), 7420 → 1633 (29±4 meV), 7420 → 0 (≦3.0±0.6 meV), 7826 → 1633 (11.7±1.6 meV), 7826 → 0 (57±7 meV) and 9029 → 1633 (340±42 meV). The following upper limits on the decay widths to the 4 +, 4246 keV level were also obtained: 7420 → 4246 (< 2.2 meV), 7826 → 4246 (< 2 meV) and 9029 → 4246 (< 6.8 meV). The experimentally known interband electromagnetic matrix elements are compared with nuclear model predictions. The excitation energy region from 10.2 to 10.8 MeV in 20Ne has been searched for a possible E x → E(4 +, 4246 keV) transition and an upper limit of ωγ < 0.6 eV can be placed on such a transition.

Efficiency determination for some standard type Ge(Li) detectors for gamma-rays in the energy range from 0.04 to 11 MeV

Absolute full energy and double escape peak efficiencies of 38 and 31 cm 3 true coaxial and of 5.8 cm 3 planar Ge(Li) detectors have been determined for γ-rays from 0.04 to 11 MeV in an arrangement typical for γγ-coincidence experiments with thermal neutron capture γ-rays from an external target. The results are compared with theoretical predictions. Experimental results for the ratios of double escape, single escape and full energy peak areas against the γ-ray energy are presented. The measurements have been performed with the γ-rays from the radioactive sources 153Gd, 169Yb and 154Eu and from the thermal neutron capture in 1H, 12C, 14N, 39K and 53Cr. To check the efficiency curves obtained in the present experiment, the relative intensities of some γ-ray transitions in the reactions 39K (n, γ) 40K and 45Sc(n, γ) 46Sc are compared with the results of other authors.

P-wave neutron resonances in 203Tl and 205Tl

Neutron-transmission measurements on a thick sample of normal thallium reveal weak resonances at 38.0 and 44.6 eV. Neutron-capture capture γ-ray measurements by Bartholomew et al. show that the 39 eV resonance belongs to 203Tl and the 45 eV resonance to 205Tl. Parameters for the new resonances are obtained by curve-fitting the transmission data. The small values of the neutron widths indicate that both resonances are almost surely excited by p-waves.

Circular polarization of neutron capture γ-rays from Mn, Ni, Ga and W

Capture γ-ray circular polarization measurements on the Mn(n, γ), Ni(n, γ), Ga(n, γ) and W(n, γ) reactions with polarized neutrons resulted in unambiguous spin assignments for 20 levels, 10 of which confirmed previous assignments. For another 20 levels the number of possible spin values has been reduced.

A two-dimensional coincidence apparatus for γγ-angular correlation measurements after thermal neutron capture

A coincidence apparatus for angular correlation measurements on thermal neutron capture γ-rays is described. The output signals of two Ge(Li)-detectors are digitized by two 4K-ADC's. The address pairs of coincident events are recorded on magnetic tape. In this way the angular distributions of all γ-cascades of the decaying nucleus can be measured in a single run together with the coincidence background.

Circular polarization of neutron capture γ-rays from Al, Ar and Ca

Measurements of the circular polarization of γ-rays from the capture of polarized thermal neutrons in natural Al, Ar and Ca result in new spin assignments to the following bound states: 28Al(3.47 MeV), J π = 4 -; 28Al(3.59), J π = 4 −(2) −; 41Ar(2.95), J π = 3 2 − J π = 3 2 − . The 41Ca levels at 3.61 and 4.75 MeV, for which conflicting spin values have been reported previously, both have J π = 1 2 − . The spins of 10 other bound states are confirmed. The M1 + E2 ground state transition from the capturing state in 28Al has an amplitude mixing ratio of δ = +1.00±0.14.

Measurement of the linear polarization of low-energy capture γ-rays from the 35Cl(n, γ) reaction

The 0.79 and 1.17 MeV levels of 36C1 and the associated γ-transitions following thermal neutron capture have been studied. From measurements of the correlation between direction and linear polarization of γ-rays, the following spins and E2/M1 mixing ratios δ 1 (for primary transitions) and δ 2 (for secondary transitions) have been found: for the 0.79 MeV level J φ = 3 +, δ 1 = 0.210±0.004, δ 2 = -1.1±0.4 and for the 1.17 MeV level J φ = 1 +, δ 1, = -0.47±0.10, δ 2 = 0.32±0.06.

The reaction 23Na(α, γ)27Al

Resonance strengths and decay schemes have been determined for twenty 23Na(α,γ)27Al resonances in the range Eα = 2.0–3.3 MeV. Spectroscopy of bound states is possible, even at relatively weak resonances, through the use of Ge(Li)γ-ray detectors. The reaction preferentially populates high-spin states. Calibration with neutron capture γ-rays leads to precision values for the excitation energies. New levels are reported at Ex = 6512.0 ± 1.5, 7443.7 ± 1.5 and 7661 ± 2 keV. The reaction Q-value is 10087.7 ± 1.3 keV. In addition, a survey is given of the 23Na(σ, γ)27Al resonances in the range Eα = 3.3–4.4 MeV.

Neutron capture γ-rays from 192Ir and 194Ir

The reactions 191Ir(n,γ) 192Ir and 193Ir(n, γ) 194Ir with reactor neutrons have been investigated in order to get information on the nuclei 192Ir and 194Ir. The high-energy γ-ray spectra measured with a Ge(Li)-NaI(Tl) pair spectrometer give 123 transitions in 192Ir and 106 transitions in 194Ir leading to levels between the ground states and 1700 keV of excitation energy. The neutron binding energy of 194Ir is B n = 6066.1 ± 1.5 keV. The 192Ir ground state transition cannot be measured because of its high multipolarity. There is evidence that the highest energy transition goes to the 58 keV 1 + excited state. The neutron binding energy is then B n = 6198.7 ± 2.0 keV. The proposed level schemes are based on these binding energy values. Some additional information has been extracted on the 32 ms isomeric state in 194Ir. Low-energy spectra up to 500 keV have been measured with single Ge(Li) detectors. The 192Ir high-energy spectrum taken with a cadmium-filtered neutron beam shows large intensity changes compared to the thermal-capture spectrum.

Thermal neutron capture cross sections in calcium

Neutron capture γ-ray spectroscopy studies of calcium isotopes have been analyzed to yield relative thermal neutron cross sections. These results have been combined with direct measurements using a Moxon-Rae type detector in order to obtain absolute values of all the stable calcium isotope cross sections. The cross sections for the stable isotopes of calcium are (inb): σ40 = 0.40 ± 0.04, σ42 = 0.65 ± 0.07, σ43 = 6.2 ± 0.6, σ44 = 1.03 ± 0.09, σ46 = 0.70 ± 0.11, σ48 = 1.09 ±0 .14, σnat. = 0.43 ± 0.04.

Level structure of 182Ta

The level structure of 182Ta has been studied from the decay of 182Hf and 182mTa and from the prompt γ-rays emitted following the capture of both thermal and 2 keV neutrons in 181Ta. These data establish the existence of 46 excited states below 1 MeV in 182Ta. Excited states are populated at 97, 114 and 270 keV in the decay of 182Hf and at 16, 163, 334 and 519 keV in the decay of 182mTa. Thirty seven of these observed states have been assigned as members of 15 rotational bands. The following levels have been interpreted as members of the bands with K π = 2 −, 3 −, 4 − and 5 − which result from the coupling of the proton orbital 7 2 + [404] p with the neutron Orbitals 1 2 − [510] n and 3 2 − [512] n : O( I π = 3 −), 97.80 (4 −), 114.33 (4 −), 173.4 (5 −), 237.27 (5 −), 270.41 (2 −), 292.97 (5 −), 317 (6 −), 360.53 (3 −), 479.8 (4 −), 488 (6 −) and 628.4 keV (5 −). States interpreted as members of the bands resulting from coupling of 9 2 − [514] p to the neutron orbitals 1 2 − [510] n and 3 2 − [512] n are: 16.5 (5 +), 150.4 (4 +), 163.3 (6 +), 250.2 (3 +), 331.3 (5 +), 334.8 (7 +), 397.3 (6 +) and 505.4 keV (5 +) . Other observed levels have been assigned to bands with configurations involving the orbitals 5 2 + [402] p , 11 2 + [615] n and 7 2 − [503] n . Four levels have been assigned as members of the { 7 2 + [404] p } , 7 2 − [503] n , K π = 0 − band ; the odd-even shift in this band is 54 keV. In all cases where the band heads from the two possible orientations of the intrinsic spins of the odd particles have been identified, the band corresponding to Σ = 1 lies lower by ≈ 100 keV than that for which Σ = 0. The absolute transition probabilities for the K forbidden E3 and M4 transitions from 182mTa are in reasonable agreement with those predicted from a Coriolis mixing calculation. No difference is observed in the strength of the primary capture γ-rays populating excited neutron and excited proton states in 182Ta. The neutron binding energy in 182Ta has been determined to be 6062.9±0.5 keV.

Neutron capture γ-ray techniques for iron and gold in mixtures

The feasibility of utilizing prompt γ-rays from thermal neutron capture as a non-destructive technique for the identification and estimation of gold in mixtures has been studied. Thermal neutrons from the University of Maryland reactor and a 40-cm 3 lithium-drifted germanium detector were applied with promising results. The high-energy capture γ-ray spectra for mixtures of gold and abundant elements, such as silicon, iron, calcium and aluminum were measured. A 'Capture γ-Ray Sensitivity Index' is introduced to provide an indication of relative sensitivity for detection of elements by this technique.