加速器质谱测量182Hf的效率研究

182Hf是长寿命放射性核素,其半衰期约为9×106 a. 测量环境样品中的182Hf对核天体物理学和核工程的研究具有重要的意义. 加速器质谱(AMS)作为一门超高灵敏核分析技术,可以测量样品中极其微量的182 Hf. 本工作在中国原子能科学研究院HI 13串列加速器核物理国家实验室的AMS装置上对182 Hf的AMS效率进行了测量,得到了初步的测量结果.     

New Neutron-Rich Nuclide 185Hf

New neutron-rich nuclide ¹⁸⁵Hf has been identified for the first time.The ¹⁸⁵Hf was produced via the ¹⁸⁶W(n,2p)¹⁸⁵Hf reaction by 14MeV neutron irradiation of natural metallic tungsten powder and separated by radiochemical methods.The identification of ¹⁸⁵Hf was based on the observation of decay γ-ray of its daughter ¹⁸⁵Ta.A half-life of 3.5±0.6min was determined for ¹⁸⁵Hf by fitting the measured growth and decay curve of the 177.59keV γ-ray which is the strongest decay γ-ray of ¹⁸⁵Ta.In addition,a new γ-ray of 164.5±0.5keV was found and assigned to ¹⁸⁵Hf.     

Perturbed Angular Correlation of the stretched cascade in the decay of 180mHf using a digital spectrometer

We report on the measurement of the nuclear quadrupole interaction (NQI) at Hf sites using the nuclear probe ^180mHf in HfF₄·HF·2H₂O at 300 K by exploiting all possible start quanta in the stretched cascade with a digital Time Differential Perturbed Angular Correlation (TDPAC) spectrometer. With conventional spectrometers, multiple prompt start signals would paralyze the router. The gain in coincidence rate is about a factor of 5 compared to a conventional spectrometer using a single start only. With multiple starts ^180mHf is a promising new isomeric nuclear probe in TDPAC experiments. As an additional feature we implemented the possibility to measure up to four cascades simultaneously in order to save data collection time or to measure isobaric contaminations like ^111mCd and ¹¹¹In.     

Calculation and Analysis of IXK/Iγ(343keV)Ratio and Absolute Intensitiy Calculation of Radiation Data for 175Hf Decay

The method of calculating I_XK/I_γ(343keV)ratio and the analysis and the comparision of experimental measurement for ¹⁷⁵Hf decay are introduced.Based on the data analysis,the emission probability of 343 keV γ-ray, P_γ(343keV)=0.869±0.004,is obtained.Absolute intensities of radiation data for ¹⁷⁵Hf decay are calculated and recommended.     

Theg-factor of the 2+ state of180W and quadrupole moment ratios observed for180W and180Hf by the Mössbauer technique

By Mössbauer absorption experiments the magnetic hyperfine splitting has been observed for the 2+ states of¹⁸⁰W and¹⁸²W in a tungsten iron alloy (3.6 at%W). Since theg-factor of the 2+ state of¹⁸²W is known the measured splitting of the¹⁸²W line could be used for the calibration of the magnetic hyperfine field and the measurement with¹⁸⁰W gave then for the unknowng ₂₊-factor of¹⁸⁰W: $$g_{2 + } (^{180} W) = 0.260 \pm 0.017.$$ By use of a WO₃ absorber the electric quadrupole splittings in the same states were measured. The ratio of the quadrupole moments was derived $$\frac{{Q_{2 + } (^{180} W)}}{{Q_{2 + } (^{182} W)}} = 0.983 \pm 0.022.$$ This ratio is somewhat smaller, but more accurate than the weighted means of previous results and in disagreement with the theoretical prediction. A similar measurement with¹⁷⁸Hf and¹⁸⁰Hf and a HfO₂ absorber gave $$\frac{{Q_{2 + } (^{178} Hf)}}{{Q_{2 + } (^{180} Hf)}} = 1.052 \pm 0.021.$$ This result is larger than the average of previous measurements and agrees with theory. The isomer shifts of the Mössbauer lines of¹⁸⁰W and¹⁸²W were measured for sources in a tantalum metal environment and for absorbers of metallic tungsten. Different signs were observed which indicate that the mean squared charge radius of the 2+ state of¹⁸²W is larger than that of the ground state whereas for¹⁸⁰W the ground state has the larger 〈r ²〉-value.     

Inner conversion process of electric dipole transitions in180Hf and182W

The conversion electron process of the retarded electric dipole transitions of 58keV in¹⁸⁰Hf and 67, 116, 152, 156 and 222 keV in¹⁸²W have been studied by means of high resolution double focusing beta-ray spectrometers. No penetration effects were found in the conversion process of the retarded 58 keV and 67, 116 and 222 keV E1 transitions. While the retarded 152 and 156 keV transitions in¹⁸²W show an anomalous internal conversion ratios compared with theoretical values. Implications of the results for current nuclear theories are discussed.     

New half-life measurement of 182Hf: improved chronometer for the early solar system

The decay of 182Hf, now extinct, into stable 182W has developed into an important chronometer for studying early solar system processes such as the accretion and differentiation of planetesimals and the formation of the Earth and the Moon. The only 182Hf half-life measurements available were performed 40 years ago and resulted in an imprecise half-life of (9+/-2)x10(6) yr. We redetermined the half-life by measuring the specific activity of 182Hf based on two independent methods, resulting in a value of t(1/2)(182Hf)=(8.90+/-0.09)x10(6) yr, in good agreement with the previous value, but with a 20 times smaller uncertainty. The greatly improved precision of this half-life now permits very precise intercalibration of the 182Hf-182W isotopic system with other chronometers.     

Excitation of deformation vibrations by radiationless muonic transitions

An investigation is carried out for selected doubly even nuclei in the rare-earth and actinide regions for possible resonances between pairs of muonic levels of the appropriate quantum numbers and certain nuclear collective states. Such resonances can lead to the excitation of the particular nuclear state involved, and could be used as an alternate means of nuclear excitation. Electric monopole excitations were found which can result in the excitation of the nuclear 0⁺ level of the β-band. The muonic states involved, however, are such that the probability of nuclear excitation may be small. An electric quadrupole resonance in ¹⁸²W involving the excitation of the 2⁺ state of the β-band is found to be a very likely candidate. Dipole and octupole resonances were also investigated; however no possible resonances were found. Also unsuccessful was the search for quadrupole excitations of the 2⁺ state in the γ-band. The matrix elements involved in the EO and E2 resonances are calculated, the nuclear contribution being computed using the nuclear collective model of Davydov and Chaban. The probability for nuclear excitation in the E2 resonance in ¹⁸²W is computed.     

Experimental Measurements for the QEC-Value of 183Os-Decay

Activities of ¹⁸³Os were produced by 39MeV α particles bombarding on a natural tungsten target.β⁺ spectrum with coincidence gate set on the strongest 382keV γ-rays of ¹⁸³Os decay was measured.The endpoint energy of the β⁺ spectrum was determined and the Q_EC value of ¹⁸³Os decay was extracted to be 2.24±0.10MeV which is in good agreement with the predicted value 2.30±0.10MeV on the basis of mass systematics.     

Total nuclear photoabsorption cross sections in the region 150 < A < 190

The curves of the total gamma-absorption cross sections (σ_tot) in the E1 giant resonance energy range for the nuclei ¹⁵⁴Sm, ¹⁵⁶Gd, ¹⁶⁵Ho, ¹⁶⁸Er, ¹⁷⁴Yb, ¹⁷⁸Hf, ¹⁸⁰Hf, ¹⁸¹Ta, ¹⁸²W, ¹⁸⁴W, ¹⁸⁶W and ¹⁹⁷Au have been measured using the absorption method. Parameters of the Lorentz curves fitting the measured cross sections σ_tot are given. Quadrupole moments (Q₀) and nuclear deformation parameters (β) were obtained.For deformed nuclei in the ～ 155 < A < ～ 180 region a violation of the correlation between giant resonance widths (Γ) and nuclear deformation parameters was found. Γ₁ and Γ₂, the widths of the resonances corresponding to vibrations of nucleons along and across the nuclear deformation axis, were observed to decrease with the increase of A which could be accounted for by the presence of an N = 108 subshell.     

The response of Mo to supernova neutrinos

Knowledge about nuclear responses to neutrinos is essential for both astrophysical applications and studies of neutrino properties. We perform in this paper calculations of the cross sections for neutral-current neutrino scattering off the odd A=95,97 Mo isotopes for energies appropriate for the detection of supernova neutrinos. Both the incoherent and coherent contributions to the cross sections are evaluated. The prominently contributing nuclear final states are identified and analysed. We employ the microscopic quasiparticle-phonon model (MQPM) to construct the wave functions of the initial and final nuclear states. The response of the aforementioned nuclei to supernova neutrinos are computed by folding the obtained cross sections with a two-parameter Fermi-Dirac distribution. The sensitivity of the calculated nuclear responses to the adopted supernova model is also discussed.     

Isospin Effect on Relative State Population Temperature in the Reactions 40Ar+ 112,124Sn at 30MeV/u

An array of 13 detector telescopes has been used for detecting small-angle particle-particle correlations in the reactions ⁴⁰Ar+^112,124Sn at 30MeV/u. The α-α correlation functions were extracted from two-particle coincident events. A three-body trajectory code MENEKA was used to calculate the background correlation function. The detection efficiency was calculated by using the Monte-Carlo method. After subtracting the background correlation and making the detection efficiency correction, the temperatures of 4.18±0.25 0.21 MeV and 4.10±0.22 0.20MeV are obtained from relative populations of ⁸Be states for the reaction systems with different isospin,⁴⁰Ar+¹¹²Sn and ⁴⁰Ar+¹²⁴Sn, respectively. From the relationship between the state population temperature and outgoing particle energies, it is found that the emission temperature decreases with increase of the particle energies for both studied reaction systems. For the neutron deficient system ⁴⁰Ar+¹¹²Sn the emission temperature is 5.13±0.30 0.26MeV for the lower emitting particle energies and 3.87±0.37 0.29MeV for the higher emitting particle energies. And for the neutron rich system ⁴⁰Ar+¹²⁴Sn the emission temperature is 5.39±0.30 0.26MeV for the lower emitting particle energies and 3.32± 0.28 0.23MeV for the higher emitting particle energies. This kind of isospin dependence is discussed.     

β-Delayed Proton Decay of 93Pd

The β-‘waiting point’nucleus ⁹³Pd has been produced via ⁵⁸Ni(⁴⁰Ca,3n2p) reaction. A helium-jet fast tape transport system was used in combination with p-γ coincidence measurement to identify the β-delayed proton precursor ⁹³ Pd, whose half-life has been measured to be 1.3±0.2s. The measured energy spectrum of β-delayed protons and relative branching ratios to the final states in the daughter nucleus ⁹²Ru are fitted by a revised statistic model calculation, and then the ground state spin of ⁹³Pd are assigned to be 9/2. Meanwhile, a nuclear energy-potential-surface (EPS) calculation is performed by using the Woods-Saxon Strutinsky method. The calculated result supports the ground state spin and parity of 9/2⁺ assignment.     

Branching Ratios of J/ψ→π03(π+π－)

The decay mode of J/ψ→π⁰3(π⁺π^－) was studied very little before. There are no informations except branching ratios for this channel in PDG book which based on less than 0.2×10⁶J/ψ events of MARKⅠ. Now, BESⅠ has a data sample of 7.8×10⁶J/ψ events, we can make more accurate results than MARKⅠ. Total of 3290±43 J/ψ→π⁰3(π⁺π^－) events including 1275±86 J/ψ→ω²(π⁺π^－) events are selected. J/ψ→π⁰3(π⁺π^－) and J/ψ→ω²(π⁺π^－) events have been generated with MC simulition for calculating events selection efficiency. BESⅠ result is: Br(J/ψ→π⁰3(π⁺π^－))=(2.52±0.06±0.43)% and Br(J/ψ→π⁰3(π⁺π^－))=(1.31±0.09±0.21)%. Meanwhile, we studied the invariant mass of 4π and ωππ to find interesting signals. It seems that there is a peak at about 1.74GeV in the invarianat mass spectrum of 4π, and a enhancement was also found at about 1.9GeV in the invariant mass spectrum of ωππ. It is not clear whether they are from resonances or background. Therefore, more work in detail is still going on.     

Goldanskii-karyagin effect at mössbauerE2-transitions

Investigations of anisotropic Debye-Waller factors have been carried out mainly using the M1-transitions of⁵⁷Fe and¹¹⁹Sn. For this case the theory of the Goldanskii Karyagin effect is well established. Mössbauer studies of electric quadrupole interactions in hafnium and tungsten compounds made it necessary to extend the theory to the more complex case of E2-transitions. As the investigated compounds show a strong axial anisotropy of the electric field gradient, a similar behaviour has to be expected in principle for the tensor of the mean-square displacements, which determines the Debye-Waller factor immediately. A fitting procedure taking into account an axial asymmetric Debye Waller factor is worked out and applied to measurements performed with several complex hafnium and tungsten fluorine compounds. With NaWO₂F₃ the quadrupole interactions of¹⁸²W,¹⁸³W, and¹⁸⁴W have been measured yieldingQ ^5/2+(¹⁸³W)/Q ²+(¹⁸²W)=0-86(6) andQ ²+(¹⁸⁴W)/Q ²+(¹⁸²W)=0.955 (3).     

The electric field gradient at Ta in HfB2

Using the time-differential perturbed-angular correlation technique, we have observed the nuclear electric quadrupole interaction at ¹⁸¹Ta (482 keV) as an impurity in the refractory compound HfB₂. The measured interaction frequency is ν_Q=730±5 MHz which corresponds to an electric field gradient of |eq|=(1·19±0·05)×10¹⁸ V/cm² at room temperature. By considering a second measurement at 4·2°K, which yields the same results as above, and by comparison with available results for Hf in HfB₂, we conclude that the d-electron density of states at the Fermi level is quite small in agreement with trends observed by others.     

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.     

Measurement of Michel Parameter in τ Decay

We have analysed the e⁺e^－ collision data collected with Beijing Spectrometer at the center-of-mass energy 4.03 GeV and selected e⁺e^－→τ^±τ⁺→e^±X⁺v's events, X^± required as μ^±、π^± and ρ^±. From fitting electron energy spectrum in τ→ev's decay, the measurement value of Michel parameter is given as ρ_τ→e=0.705, this result is in good agreement with the prediction of standrd Model.     

First Experimental Determination of Half-Life for New Nuclide 65Se

By using experimental data obtained from the measurement of beta-delayed proton decay of ⁶⁵Se produced in ⁴⁰Ca(²⁸Si,3n)reaction,the absolute detection efficiency of particle telescope for rotating radioactive source was imitatively calcu-lated.The half-life of ⁶⁵Se was thus accurately determined to be 9.6_-4.1^+5.3 ms.The β-delayed proton energy of ⁶⁵Se was 3.70±0.08MeV.The partial decay scheme of ⁶⁵Se β-delayed proton decay was revised and proposed.     

Analysis of the total activation cross section of all possible reactions producing the same radioactive nuclide for the same element

Firstly,according to the regulation of growth and decay of radioactive nuclides produced in reactions,a formula used to calculate the total activation cross section of all possible reactions producing the same radioactive nuclide for the same element is deduced,and it is pointed out that the activation formula given in two references is incorrect.Then,as an example,the so-called total activation cross section in one of the two references is analyzed and the correct results of the cross sections of 182W(n,p)182(m+g)Ta,183W(n,p)183Ta and 206Pb(n,a)203Hg induced by neutrons around 14 MeV calculated with the data given in the literature,the nuclear parameters and some evaluated values are given.Finally,the correct results are compared with other values collected in the literature.