Γ-ray Anisotropy Research Articles

NMR-ON of182Ta and183Ta in Fe

Nuclear magnetic resonance has been observed on radioactive182Ta and183Ta oriented at low temperature in an Fe host, by detection of the change in spatial anisotropy of γ-rays emitted during nuclear decay. By measuring the resonant frequencies of183Ta in four different applied magnetic fields the nuclear magnetic moment and hyperfine field have been deduced. These are: $$|\mu \left( {{}^{183}Ta; I = \tfrac{\user2{7}}{\user2{2}}} \right)| = 2.28(3)\mu _{\rm N} and B_{hf} \left( {Ta\underline {Fe} at 0 K} \right) = - 67.2(1.3)T$$ . The spin of the ground state of182Ta has been determined asI=3 by comparing resonance results with those obtained in a thermal equilibrium nuclear orientation study. The ratio of the resonant frequencies observed for182Ta and183Ta at one applied field value yields a magnetic moment for the former of $$|\mu \left( {{}^{182}Ta; I = \user2{3}} \right)| = 2.91(3)\mu _{\rm N} $$ . The spin lattice relaxation time for183TaFe (0.12 at% Ta) at 18 mK in an applied field of 0.5 T has been found to be 40(10) s.

The establishment of a temperature scale from 0.01 K to 0.05 K using noise and 60Co γ-ray anisotropy thermometers

We report here on our latest intercomparison of the Josephson junction noise thermometer and 60Co γ-ray anisotropy thermometer from 0.01 K to 0.05 K. These results show both thermometers are in agreement to better than 0.5%. This is an improvement over our previous data and is due mainly to better temperature control and advances in instrumentation.

NMR-on of182Ta and183Ta in Fe

Nuclear magnetic resonance has been observed on radioactive182Ta and183Ta oriented at low temperature in an Fe host, by detection of the change in spatial anisotropy of γ-rays emitted during nuclear decay. By measuring the resonant frequencies of183Ta in four different applied magnetic fields the nuclear magnetic moment and hyperfine field have been deduced. These are: $$|\mu \left( {{}^{183}Ta; I = \tfrac{\user2{7}}{\user2{2}}} \right)| = 2.28(3)\mu _{\rm N} and B_{hf} \left( {Ta\underline {Fe} at 0 K} \right) = - 67.2(1.3)T$$ . The spin of the ground state of182Ta has been determined asI=3 by comparing resonance results with those obtained in a thermal equilibrium nuclear orientation study. The ratio of the resonant frequencies observed for182Ta and183Ta at one applied field value yields a magnetic moment for the former of $$|\mu \left( {{}^{182}Ta; I = \user2{3}} \right)| = 2.91(3)\mu _{\rm N} $$ . The spin lattice relaxation time for183TaFe (0.12 at% Ta) at 18 mK in an applied field of 0.5 T has been found to be 40(10) s.

Nuclear orientation of recoil-implanted52Mn in Au down to 3 mK

Measurements of the γ-ray anisotropy of recoil-implanted52Mn ions in pure Au down to 3 mK indicate marked deviations from free-ion behavior in low applied fields. The effective hyperfine field that explains the anisotropy is found to decrease below 10 mK. Although this behavior could be a signature of a “bound” Kondon state with a lower effective hyperfine coupling constant, it is better explained as arising from a combination of Kondo and relaxation effects. The data indicate that the Mn local moment relaxation timeT1 is comparable to or larger than the Larmor precession time of the Mn nuclei at 3 mK. Other possible reasons for an attenuated γ-ray anisotropy, such as nuclear quadrupole and second-order crystal field effects, are also considered.

Thermometry in 3He

Some of the methods used in measuring the temperatures of the transitions in 3He are reviewed. Although noise thermometers, γ-ray anisotropy thermometers and even Pt NMR thermometers are satisfactory for furnishing temperature standards, they are not satisfactory secondary thermometers. The susceptibility of CMN and various 3He properties make more precise and quicker thermometric indications.

Nuclear orientation of105Rh and95Tc in iron

The temperature-dependent anisotropy of γ-rays following the decay of oriented95Tc and105Rh nuclei was studied with a Ge(Li) detector. Mixing coefficients of some γ-and preceding β-transitions, the spins of two intermediate levels, and the magnetic hyperfine splitting of the95Tc and105Rh ground states in an Fe host were measured. From the known hyperfine fields the following magnetic moments were deduced:

Nuclear orientation of106Rh in iron

The anisotropy of γ-rays from the decay of oriented106Rh nuclei was studied at temperatures between 14 and 100 mK. The ground state magnetic moment was determined to be |μ(1+)|=3.07(9) n.m. Incomplete orientation was observed which is assumed to result from relaxation during the 43 s lifetime of the decaying nucleus.

In-beam nuclear magnetic resonance in isomeric states

A comprehensive description is given of the γ-NMR-PAD method combined with pulsed beam techniques. Detailed studies of the resonance behaviour were performed as a function of various timing conditions, of the phase between the r.f. field and thecreation time of the nuclear states, and of the amplitude of the linearly polarized r.f. field. The following features were realized by experiments on the 20 ms state in71Ge and on the 119 μs state in78 Br, excited and oriented by nuclear reactions in liquid metallic targets: (i) The resonance behaviour was observed time-differentially, yielding accurate values of the r.f. amplitude acting at the nuclear site, and of the relaxation time. (ii) The observation of the perturbed γ-ray distribution in time windows can yield strongly structured resonances with considerably larger effects compared to time-integral resonances. The time window technique is well suited to determine the resonance point precisely, and further — in case of saturation — the relaxation time can be extracted. (iii) Absorptionand dispersion-like resonance structures were observed with fixed r.f. phases for selected time windows and phase positions, yielding the sign of the magnetic interaction and the sign of the γ-ray anisotropy. (iv) At r.f. amplitudes comparable to the static magnetic fields resonance shifts and multiple quantum effects caused by the “nonresonant” component of the r.f field were studied in γ-NMR-PAD. A comparison is given with the stroboscopic method, and some possible applications of the technique developed are discussed to observe hyperfine interactions of muons in non-metals.

Electric Quadrupole Interaction of 20F in a KZnF3 Single Crystal

Abstract Nuclear magnetic resonance measurements were carried out on neutron activated 20F(T1/2=11s) nuclei in a single crystal of KZnF3. The quadrupolar splitted NMR spectrum, detected via the 20F β-radiation asymmetry, could be observed using a radio frequency modulation technique. The quadrupole coupling constant was determined to e2 q Q/h= + (12.0 ± 1.5) MHz at room temperature. The sign of e2 q Q was obtained from a simultaneous γ-ray anisotropy measurement on the succeeding 20Ne transition. Utilising a calculated field gradient of the fluorine atom, an fQ = 4.6% is determined. This value is compared with literature data of similar compounds.

High-field spin–lattice relaxation of cobalt nuclear spins in iron determined by nuclear orientation

In order to resolve the discrepancy in the various estimates of the spin–lattice relaxation rate for cobalt spins in iron, we have remeasured the relaxation time for 60Co spins using both magnetic resonance on the oriented spin system and a non-resonant method in which the lattice temperature is changed rapidly by fast demagnetization. The latter technique yields a value for the relaxation rate (γN2T1T)−1 = (0.43 ± 0.01) × 10−7 s G2 K−1 in an applied field of 3.2 kOe. Based on experimental results and theory, a proposal is made that there exists a field-dependent contribution to the relaxation rate even in the magnetically saturated state. Agreement is then found with NMR measurements on 59Co in iron. The resonant nuclear orientation experiment gives a result which is in doubt because of lack of knowledge of the initial conditions produced after the resonant destruction of the γ-ray anisotropy.

Nuclear orientation of 2.3 d 198mAu

The magnetic hyperfine interaction of the T 1 2 = 2.3 d high-spin isomeric state of 198Au has been measured in host metals of iron and nickel at temperatures down to 4 mK using the thermal-equilibrium nuclear-orientation technique. The resulting value for the magnetic moment, μ = (+) 5.55±0.35 n.m., strongly supports an interpretation of this state as a [π h 11 2 ; ν i 13 2 ]12 − configuration analogous to 196mAu and 200mAu. The observed γ-ray anisotropies are in agreement with the recently proposed decay scheme of 198Au; for the E2/M1 mixing ratio of the 204 keV γ-transition they result in a value of δ = −0.10±0.05.

The electric field gradient in cadmium metal and the decay of 115Cd m

Using the anisotropy of γ-rays from 115Cd m nuclei oriented in a cadmium single crystal at low temperatures, the electric field gradient is measured as V zz =+(1.29±0.37) × 10 15esu/sm 3. Mixing ratios of several γ-rays are also reported

Nuclear orientation of 142Pr in Pr metal

The anisotropy of γ-rays from oriented 142Pr nuclei in Pr metal can be fitted to calculations utilizing electronic moments measured by neutron diffraction if it is assumed that zero-order matrix elements dominate the preceding first-forbidden β transition. The results select one of two possible solutions obtained in previous work.

Shaped scintillation detector systems for measurements of gamma-ray flux anisotropy

The detection efficiencies of cylindrical detectors for various gamma-ray photon angular distributions have been studied in the energy range from 0.10 MeV to 15 MeV. These studies indicate that simple detector systems on small satellites can be used to measure flux anisotropy of cosmic γ-rays and the angular distribution of albedo gamma rays produced in planetary atmospheres. The results indicate that flat cylindrical detectors are most suitable for measuring flux anisotropy because of their angular response function. A general method for calculating detection efficiencies for such detectors is presented.

Fission fragment γ-ray anisotropies

The anisotropy of fission fragment γ-rays in the reactions 239Pu(n th, f) and 233U(n th, f) have been measured. In this work, two γ energy bands were used to confirm the energy independence of the anisotropy. For the γ energy band of 0.49–0.65 MeV and the γ energy band of 0.83–0.92 MeV the anisotropies were found to be 0.18 ± 0.03, 0.13 ± 0.03 and 0.19 ± 0.03, 0.11 ± 0.03 for the 239Pu and 233U targets respectively. The data are corrected for the Doppler effect and the time of flight was large enough to eliminate the counting of neutrons. Comparisons with the theories of Nix and Swiatecki, and Strutinsky have been made and approximate correction for fragment rotation along fission axis made.

The hyperfine field of xenon nuclei at lattice sites in iron

The hyperfine field of xenon nuclei at lattice sites in iron has been determined from nuclear orientation experiments at temperatures between 11 mK and 80 mK obtained with a 3He- 4He dilution refrigerator. From the temperature dependence of the M4 γ-ray anisotropies of 133Xe m and 131Xe m implanted in iron by isotope separator, the fractions of xenon nuclei at high-field sites were determined, and a value H hf = 1510 ± 160 kG was deduced.

Nuclear quadrupole alignment of 180Hf m and 175Hf in hafnium metal

180Hf m and 175Hf nuclei have been aligned in the electric field gradient of single-crystal hafnium metal at temperatures down to 3 mK. The temperature dependence of γ-ray anisotropies yielded the electric quadrupole interaction energies for 180Hf m, e 2 qQ(8 −) = +(6.6±0.4) × 10 −8 erg, and for 175 Hf, e 2qQ( 5 2 −) = +(4.0 ± 0.5) × 10 −18 erg , as well as the E2 M1 mixing ratio of the 343 keV γ-ray transition of 175Lu, δ 343 = −0.27 ± 0.03.

Study of the magnetic dipole moments of the odd-mass tellurium isomers by nuclear orientation

The magnetic moments of 123mTe, 127mTe and 129mTe implanted by isotope separation in an iron lattice have been measured using the low-temperature nuclear orientation technique. The magnitudes of the magnetic dipole moments, extracted from the γ-ray anisotropy measurements, are: μ = −1.00(5) n.m. for 123m Te, μ = −0.91(5) n.m. for 127m Te and μ = −1.15(5) n.m. for 129mTe. The results for these 11 2 − isomers are compared to theoretical calculations by the spin polarization procedure using wave functions from the pairing-plus-quadrupole model.

Nuclear orientation of 110mAg in iron and nickel

The measurement of the hyperfine interactions at 110mAg nuclei in iron and nickel host lattices by nuclear magnetic resonance on oriented nuclei has enabled the γ-ray anisotropies measured by nuclear orientation to be interpreted in terms of the decay multipolarities. Results for M1/E2 mixing ratios are discussed in terms of deviations from a collective vibrational model for 110Cd.

The magnetic dipole moment of 55Co

The ground state g-factor for 55Co has been measured as ¦g¦= 1.378±0.001 by the technique of nuclear magnetic resonance on oriented nuclei. The temperature dependence of γ-ray anisotropy in the 55Fe daughter decay determines both the 1408 keV level spin and the 55Co ground state spin to be 7 2 , and yields values of mixing ratios in the 1037 keV β-transition and the 477 keV γ-transition. The configuration mixing model is used to discuss 1f 7 2 moment systematics.