Cascade Gamma Rays Research Articles

Perturbed angular correlations in 177Hf

Ions of 177Lu have been implanted in iron and nickel hosts at concentrations of about 1018/cm3. Perturbation of angular correlations in the gamma-ray cascade through the 113–keV level gives values for the internal fields on hafnium nuclei in these metals at room temperature and for this concentration of H(Fe) = −140 ± 10 kOe and H(Ni) = −60 ± 10 kOe. The rotation of the 250-keV level was also observed in the iron host. The nuclear g factor of this state is g = +0.26 ± 0.09.

Precision measurement of the energies of the gamma rays from 144Nd

The energies of the two cascade gamma rays emitted by 144Nd following β-decay of 144Pr have been measured to be 1489.14±0.07 keV and 696.48±0.09 keV. The energy of the cross-over gamma ray is found to be 2185.72±0.05 keV in agreement with that expected from the sum of the energies of the cascade gamma rays. The ratio of the intensities of the 696, 1489 and 2186 keV gamma rays are found to be (100): (17.7±0.5): (48±9).

Thermal neutron activation cross sections for the isomeric states in odd indium isotopes and the j-selection rule

The thermal neutron activation cross sections for the two isomeric states in 114In were measured using a 2π flow counter for the long-lived and the pulsed activation method for the short-lived isomer. The values obtained are σ act = 7.5±0.7 b for 114 m 1 In (49 d) in agreement with a value measured by Keisch, and σ act = 3.1 −0.7 +0.4 b for 114m 2In (42 msec). The isomeric cross-section ratios were analysed using the statistical approach of Huizenga and Vandenbosch. It follows that both isomeric states should have the same spin value 5, in spite of the fact that they are connected by a pure E3 transition. The same situation prevails in 116In. Possible explanations are (i) E1 and M2 transitions are so strongly hindered that they are in fact completely suppressed; and (ii) the statistical approach in evaluating isomeric cross-section ratios breaks gamma-ray down because of the channellizing action of the ( g 9 2 −1) p ( h 11 2 ) n multiplet which directs the gamma-ray cascade to its member with the lowest excitation energy (i.e. the isomeric state). Both explanations are founded on the action of the j-selection rule, which forbids transitions of low multipole order between the configurations [( g 9 2 −1) p( h 11 2 ) n]1 − … 10 − and [( g 9 2 −1) p( S 1 2 + d 3 2 ) n] 4 +, 5 + .

Linear polarization measurements

The use of the Compton polarimeter is discussed for the measurement of linear polarizations of gamma rays from aligned nuclear excited states. A set of formulas is given for calculating the polarization-correlation of any member of a multiple gamma-ray cascade and for calculating the Compton scattering asymmetry expected from a polarization measurement. Corrections for the finite geometry of the polarimeter are discussed and simple approximate formulas are given for calculating the geometrical correction coefficients. The results of exact numerical computer calculations for one sample geometry are compared with the results of the approximate formulas. The mechanical construction and electronic circuitry for a typical Compton polarimeter and the shape of the pulse height spectrum obtained by summing the pulses from the scattering and absorbing detectors are discussed.

An empirical method for determining the relative efficiency of a Ge(Li) gamma-ray detector

An empirical method for determining the full-energy peak efficiency between 0.2 and 3.0 MeV and the two-escape peak efficiency between 2 and 9 MeV of a Ge(Li) gamma-ray detector is described. The full-energy peak efficiency is determined with the use of several nuclides which emit gamma rays with accurately known intensity ratios and a linear least-squares fitting procedure. With the full-energy peak efficiency determined, a number of high and low energy thermal neutron capture gamma ray cascades with known relative intensities are then utilized in the determination of the two-escape peak efficiency.

Ge(Li)Na(Tl) angular correlation studies of gamma cascades in the decay of 119mTe

Angular correlation measurements have been performed with a Ge(Li)NaI(Tl) detector combination on several gamma-ray cascades present in the decay of 4.7 d 119mTe. Results obtained for the 153-1213, 942-270, 1095-270, 976-979, 976-979-270, 2089-270, 1137-270 and 912-153-1213 keV cascades are in substantial agreement with previous spin assignments which were based primarily on conversion electron studies. Possible multipolarity mixing ratios are suggested for the 153, 270, 912, 942, and 1213 keV transitions.

The half-lives of natural 176Lu and 180Ta

An extremely low-level gamma-ray spectrometer has been used to measure gamma rays associated with the decay of naturally occurring 176Lu and 180Ta. For the half-life of 176Lu, a new value of (5.0±0.3)×10 10 y was derived from single-crystal sum-coincidence spectra of high-purity lutetium oxide samples of natural isotopic abundance. A search was also made for possible cascade gamma rays following the electron capture of 180Ta in a purified tantalum oxide by means of the usual gamma-gamma coincidence technique. Although the obtained spectra were slightly complicated by the presence of radioactive impurities in the sample, some positive evidence for 180Ta decay was observed. The lower limit deduced for the half-life was (1.5±0.5)×10 13 y.

High energy gamma ray energy measurements

An easy method to measure gamma ray energies to about 0.003 MeV is suggested which utilizes the reaction Q-value, and demands consistency between the sum of the cascade gamma ray energies between two levels and the energy of the direct transition. This procedure is applied to the 0.9363-MeV resonance in the 27Al(p, γ) 28Si reaction.

Measurement of the g-factors of two levels of 131I

The g-factors of the 150 keV first excited state and a state at 1797 keV of 131I have been measured from the rotation ωτ of the angular correlation pattern in an external magnetic field. The intergral method was used and coincidence counting rates at 135° were recorded for two directions of the field. The gamma-ray cascades involved are 452–150 keV n the case of the 150 keV state and 102–200 keV for the 1797 keV state. The results are: ωτ=0.097±0.012 for a magnetic field of 13.2 kG and g=1.11±0.20 for the 150 keV state, G 2ωτ=0.073±0.016 for a magnetic field of 12.8 kG and g=0.16±0.05 for the 1797 keV state, In the latter measurement the attenuation coefficient due to time-dependent perturbations was seen to be G 2 = 0.9±0.1 by studying the anisotropy as a function of time by delayed coincidence technique.

STUDY OF SOME 20Ne LEVELS BY RADIATIVE CAPTURE

Four resonances in the reactions 16O(α, γ)20Ne and 4He(16O, γ)20Ne have been studied using gas targets of oxygen and helium. Resonances at 5.374, 6.930, and 7.940 MeV alpha-particle bombarding energy had been observed previously using the first reaction. In this experiment, an additional resonance was found at 5.075-MeV alpha-particle bombarding energy. This resonance level decays by a prominent cascade of gamma rays through the 4+, 4.25-MeV level in 20Ne and can be identified with a 6+, 8.79-MeV level in 20Ne observed previously in the 12C(12C, α)20Ne reaction. A comparison of the yield of the new resonance with one previously reported gives a value of ωγ = 1.8 ± 0.4 eV, which implies an E2 strength of 28 ± 6 Weisskopf units for the 6+ to 4+ transition.

Angular correlation of cascade gamma rays in 94Nb

The directional correlation and polarization correlation of the 703-871 keV gamma ray cascade following the decay of 20 000 y 94Nb have been measured. On the basis of these measurements the spin and parity of the 1574 keV level in 94Mo has been determined as 4 +.

Correlation function of A γ-γ cascade (0.34 MeV–1.38 MeV) on49Ti

The angular correlations of a cascade of capture gamma rays with energies of E1=0.34 MeV, E2=1.38 MeV on49Ti nuclei have been measured. The results confirm that it is a cascade of the type 1/2− (M1+E2) 3/2− (E2) 7/2− with a mixing parameter of the first transition δ=−0.1, or δ=+2.2.

Decay ofNi57and Excited Levels inCo57

The decay of ${\mathrm{Ni}}^{57}$ has been investigated in order to obtain detailed information about the level structure of the odd-even ${\mathrm{Co}}^{57}$. Lithium-drifted germanium detectors as well as standard scintillation-spectrometry techniques have been used. The levels at 1.46, 1.59, 1.75, and 2.62 MeV have been established with direct de-excitation to the ${\mathrm{Co}}^{57}$ ground state, in addition to the already known levels at energies of 1.37, 1.497, and 1.9 MeV. It has been confirmed that the 1.497-MeV level is the only one which decays by a (0.127-1.37)-MeV cascade of gamma rays. The various gamma-gamma coincidence studies have been performed; and based on these results, and on the ratios of electron capture to ${\ensuremath{\beta}}^{+}$ decay, a decay scheme of ${\mathrm{Ni}}^{57}$ is proposed, and the results discussed in the light of the Coriolis coupling model. The half-life of the 1.497-MeV level has been measured as (0.60\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec and an upper limit for the half-life of the 1.37-MeV level is given as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec.

An attempt to observe the directional correlation of the 1700–1077 keV gamma-ray cascade in 68Zn

Directional correlation measurements were made on the 1700–1077 keV gamma cascade in 68Zn using a scintillation coincidence spectrometer. Correlations were observed using different thickness of absorber in front of the counters in order to attenuate the effects of annihilation radiation on the correlation. The correlation in each case was expressed in the form W( θ) = 1+ A 2 P 2(cos θ)+ A 4 P 4(cos θ). The variations of A 2 and A 4 with absorber thickness and arguments based on beta-decay theory suggest that the spin sequence 2(1, 2) 2(2)0 is appropriate for the cascade. The spin and parity 2 + is tentatively assigned to a 2780±18 keV level in 68Zn.

Decay ofTb162and the Excited States ofDy162

Sources of 7.5-min ${\mathrm{Tb}}^{162}$ were produced by the ${\mathrm{Dy}}^{162}(n,p){\mathrm{Tb}}^{162}$ and ${\mathrm{Ho}}^{165}(n,\ensuremath{\alpha}){\mathrm{Tb}}^{162}$ reactions with 14.8-MeV neutrons. The decay properties of ${\mathrm{Tb}}^{162}$ were investigated with the help of standard scintillation-counter techniques. In the beta- and gamma-ray measurements, the beta groups with end-point energies of 1.72\ifmmode\pm\else\textpm\fi{}0.1 MeV (13%) and 1.45\ifmmode\pm\else\textpm\fi{}0.1 MeV (87%), and the gamma rays with energies of 45\ifmmode\pm\else\textpm\fi{}5, 80\ifmmode\pm\else\textpm\fi{}5, 130\ifmmode\pm\else\textpm\fi{}5, 185\ifmmode\pm\else\textpm\fi{}10, 195\ifmmode\pm\else\textpm\fi{}10, 265\ifmmode\pm\else\textpm\fi{}5, 390\ifmmode\pm\else\textpm\fi{}10, 630\ifmmode\pm\else\textpm\fi{}10, 690\ifmmode\pm\else\textpm\fi{}15, 810\ifmmode\pm\else\textpm\fi{}10, 880\ifmmode\pm\else\textpm\fi{}10, 890\ifmmode\pm\else\textpm\fi{}10, 1070\ifmmode\pm\else\textpm\fi{}15, and 1205\ifmmode\pm\else\textpm\fi{}15 keV, all decaying with a half-life of 7.5 min, have been observed. The sum-spectrum and coincidence studies show that the following are gamma-ray cascades: 130-265-810-80; 130-265-890; 130-265-625-185-89; 130-195-880-80; 130-195-690-185-80; 130-1070-80; 395-810-80; 395-890; and 1205-80 keV. The beta-gamma coincidences taking each of the 80-, 810-, and 890-keV gamma rays as a gate show both of the above beta groups; while in those taking the 190- and 265-keV gamma rays as a gate, only a single beta group with the end-point energy of 1.45\ifmmode\pm\else\textpm\fi{}0.1 MeV was observed. Based on these results, a decay scheme for ${\mathrm{Tb}}^{162}$ is proposed with levels in ${\mathrm{Dy}}^{162}$ at 890 (${2}^{+},k=2$), 924 (${8}^{+},k=0$), 960 (${3}^{+},k=2$), and 1155 keV (${2}^{\ensuremath{-}},k=2$) and is discussed in terms of the unified model.

Accurate determination of the internal conversion coefficients of cascade gamma rays

A new approach has been adopted to determine accurately the internal conversion coefficients of cascade transitions from angular correlation measurements.

Gamma-Ray Yields from Nuclear Reactions and Level Densities of Deformed Nuclei

Theoretical calculations of spin distributions of compound nuclei and subsequent neutron-evaporation products are made for 27-MeV alpha particles on $^{178}\mathrm{Hf}$, 52-MeV alpha particles on $^{180}\mathrm{Hf}$, and 56-MeV $^{11}\mathrm{B}$ ions on $^{165}\mathrm{Ho}$. The first two cases were chosen to match experimental work of Lark and Morinaga. We use our calculations as a basis for interpretation of their relative yield of $^{180m}\mathrm{W}$, the $J={8}^{\ensuremath{-}}$ isomer, and the $J={4}^{+}$ state of the ground rotational band. We find these yields consistent with the simple assumption that after the last neutron is evaporated to form the excited $^{180}\mathrm{W}$ nucleus all products with spin $J\ensuremath{\gtrsim}10$ feed into the ${8}^{\ensuremath{-}}$ isomer and those with spin $J\ensuremath{\lesssim}10$ contribute to the prompt radiation from the first $J={4}^{+}$ level. Possible relationships between the course of the gamma-ray cascade and the nature of the Nilsson orbitals nearest the Fermi surface are discussed. Finally, the importance of measuring gamma-ray angular distributions in addition to integrated yields is stressed, and formulas of possible value for the analysis of such angular distributions are collected and presented.

Total and photopeak efficiencies for a well type NaI(Tl) crystal for cylindrical sources

The total and photopeak efficiencies for a 6.5 cm × 6.5 cm well type NaI(Tl) crystal with 0.8 cm well dia. and 3.2 cm well depth have been determined. These efficiencies have been determined for cyclindrical monoenergetic sources of 0.5 cm dia. and various heights in steps of 0.5 cm up to 3.0 cm as a function of the gamma-ray energy from 0.14 MeV to 2.75 MeV, using standard gamma sources and various absorbers covering the whole periodic table. Formulae have been given to calculate the effective total and photopeak efficiencies in the case of gamma rays in cascade using the monoenergetic gamma ray efficiencies given in this paper.

Angular distributions of gamma rays in (n, n′ γ) reactions as a basis for nuclear level spins

A computer program TRANSEC based on Satchler theory has been developed for the theoretical calculation of the differential cross sections for the production of gamma rays in (n, n′ γ) reactions. The code is sufficiently versatile to take into account multiple exit channels, gamma-ray cascades, mixed multipolarities and an arbitrary number of sets of transmission coefficients. Comparisons between the experimental angular distributions and the theoretically calculated distributions lead to information regarding the level spins and the mixing ratios of the various gamma-ray transitions. The predictions of the Satchler theory are in excellent agreement with the experimental cross sections measured for the 24Mg, 26Mg, 27Al, 28Si, 40Ar, 40Ca, 54Fe, 56Fe, 59Co, 89Y and 208Pb nuclei. This method has been used for the assignment of level spins in the 27Al, 40Ar, 59Co and 89Y nuclei. The sensitivity of the distributions and the cross sections to the assumed parities of the nuclear levels has also been investigated.

A gamma-ray cascade model for the calculation of average ?-ray multiplicities and isomeric cross section ratios

The calculations of theγ-ray cascades following the decay of a compound state were carried out using all levels with known values of energy, spin and parity at low energy and statistical assumptions for the distribution of the nuclear levels in the energy range between the initially occupied compound state and the low-lying levels. Dipole and quadrupole transitions were taken into account. The model works without the introduction of an artificial average transition energy. The model was applied to the calculation of averageγ-ray multiplicities and of isomeric cross section ratios. The applications to the determination of level density parameters are discussed. It was shown that it is not allowed to neglect the contribution of quadrupole radiation in the cascade as was done in other models in the past. Only in the case of In115(n, γ) In116 enough data are available to determine statistical parameters. We get for the level density parametera=18.6 MeV−1, for the spin cut-off factorσ=4.8 and a contribution of about 5% quadrupole radiation in the cascade.