First Evidence for Neutrinoless Double Beta Decay

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with oscillation experiments. Recent analysis of the most sensitive experiment since nine years—the HEIDELBERG-MOSCOW experiment in Gran-Sasso—yields a first indication for the neutrinoless decay mode. This result is the first evidence for lepton number violation and proves the neutrino to be a Majorana particle. We give the present status of the analysis in this report. It excludes several of the neutrino mass scenarios allowed from present neutrino oscillation experiments—only degenerate scenarios and those with inverse mass hierarchy survive. This result allows neutrinos to still play an important role as dark matter in the Universe. To improve the accuracy of the present result, considerably enlarged experiments are required, such as GENIUS. A GENIUS Test Facility has been funded and will come into operation by early 2003.     

无中微子双β衰变相关的中微子势（英文）

介绍了在双β衰变中的闭合近似下的原子核矩阵元,并在此基础上研究了无中微子双β衰变的中微子势部分及统计性质。分析结果显示,费米型和伽莫夫-泰勒型矩阵元部分贡献了几乎相等的正值,并且费米型部分一般比伽莫夫-泰勒型部分的贡献稍大,而张量部分的贡献虽小但不可忽略,其中有少量矩阵元为负。阐明了无中微子双β衰变的中微子势中费米型、伽莫夫-泰勒型及张量部分大于零矩阵元的关联。该统计结果粗略地揭示了这些组份对原子核矩阵元的贡献。Nuclear matrix element in double beta decay under the closure approximation is outlined, in which neutrino potential for neutrinoless double beta decay is studied with focusing on its statistical property. It is shown from the analysis that Fermi and Gamow-Teller parts provide almost the same positive values with the Fermi part slightly larger than the Gamow-Teller part in general, while the tensor part includes small but non-negligible positive and negative values. Positive correlation of the values between Fermi, Gamow-Teller, and tensor parts has been clarified. The statistics provides a gross view of understanding amplitude of constitutional components of the nuclear matrix element.     

Neutrinoless Double Beta Decay: Status of Evidence

The present experimental status in the search for neutrinoless double beta decay is reviewed, with emphasis on the first indication for neutrinoless double beta decay found in the HEIDELBERG-MOSCOW experiment, giving first evidence for lepton number violation and a Majorana nature of the neutrinos. Future perspectives of the field are briefly outlined.     

用反应48Ca+254Es合成119号元素的可能性研究

利用最近发展起来的两步模型分析重离子核反应48Ca+254Es 的熔合过程。在这个模型中,熔合过程被分为前后独立的两个过程,即粘连过程和形成过程。本研究组计算了对应的粘连概率和形成概率,并结合适用于蒸发过程的统计蒸发模型,计算了该反应合成119 号超重元素的剩余截面,即当Elab = 250:2 MeV时,得到最大剩余截面为0.7 pb。Two-step model is adopted to analyze the fusion process of heavy-ion reaction 48Ca+254 Es. Based on this model, the fusion process is divided into two consecutive steps, i.e., the sticking step and the formation step, and corresponding sticking probability and formation probability are calculated. Combining the statistical evaporation model for the evaporation stage, the maximum evaporation residue cross section for reaction 48Ca+254 Es is 0.7 pb at 4n, Elab =250.2 MeV.     

Radial Expansion Flow in Reactions of 40Ca+58Fe and 40Ca+58Ni

By means of using an isospin-dependent Boltzmann-Langevin equation which includes isospin-dependent symmetry energy, Coulomb energy, isospin-dependent nucleon-nucleon cross sections, Pauli blocking, and initialization, the radial expansion flow of reaction systems ⁴⁰Ca+⁵⁸Ni and ⁴⁰Ca+⁵⁸Fe at 53, 100, 150, and 200 MeV/u in the central collisions were studied. It has shown that the more neutron rich system exhibits smaller radial expansion flow. It was found that the neutron rich system had smaller threshold energy which may provide a new method to determine the isospin dependent nuclear equation of state from calculated result and linear fitting result.     

最小seesaw模型对氚β衰变和无中微子双β衰变的限制

在最小seesaw模型下计算了氚β衰变的有效质量〈m〉_e以及无中微子双β衰变的有效质量〈m〉_ee. 利用最新的中微子振荡数据, 在正质量等级情况下得到了0.00424eV≤〈m〉_e≤0.0116eV和0.00031eV≤〈m〉_ee0.0052eV; 如果中微子的质量谱是倒质量等级情况, 能够得到0.0398eV≤〈m〉_e≤0.0571eV和0.0090eV≤〈m〉_ee≤0.0571eV.最后还讨论了最小的中微子混合角和Majorana CP破坏位相对〈m〉_ee的影响.     

Neutrinoless Double Beta Decay-A New Formalism

A new method-the operator expansion method is used in neutrinoless double beta decay processes. Both the neutrino mass and mixing of right-handed leptonic current are included. It is shown that for nuclear neutrinoless double beta decay in 2n mechanism, there appear some new terms besides the terms given by the conventional treatment based on closure approximation. The ββ with a Majoron emission is also discussed.     

Dynamic Model Study of Fusion Reactions for 40,48Ca+90,96Zr

Based on the Improved Quantum Molecular Dynamics Model the fusion reactions of ^40,48Ca+^90,96Zr are studied by making a more rigorous treatment of the initial condition. The study shows that:(1) the calculated fusion cross sections for all four reactions of ^40,48Ca+^90,96Zr are in good agreement with experiment data; (2) the sub-barrier fusion for the neutron-rich reaction ⁴⁰Ca+⁹⁶Zr is ubstantially enhanced as compared with the other three reactions. In order to understand the reason why the sub-barrier fusion of the neutron-rich reaction ⁴⁰Ca+⁹⁶Zr is enhanced as compared with the non-neutron-rich reaction ⁴⁰Ca+⁹⁶Zr and that of the neutron-rich reaction ⁴⁸Ca+⁹⁶Zr is not enhanced, the further investigations are carried out, those are: the dynamic fusion barriers for four reactions, the relation between dynamic fusion barrier and nucleon transfer and the relation between nucleon transfer and corresponding reaction Q value. It has been found that positive Q value leads to the strong nucleon transfer which reduces the dynamic fusion barrier and enhances the sub-barrier fusion cross section.     

A STUDY OF THE ISOSCALAR GIANT RESONANCE IN 40Ca

The strength functions of the isoscalar giant resonance monopole resonance and giant quardropole resonance in ⁴⁰Ca are calculated by coupling the 1p-1h states to 2p-2h states. The results can explain the recent experimental data.     

Decay of 95Ru

1.65h ⁹⁵Ru was produced and studied by y ray spectroscopy of ⁹²Mo(a, n)reaction, 34 of the 130 obversed y rays following ⁹⁵Ru decay are new. According to coincidence relations, 29 new γ rays were flited into level scheme, the placement of 7 γ rays in level scheme were confirmed, 3 new levels were assigned, a modified and expanded decay scheme was proposed.     

The Two-Nucleon Mechanism for the Neutrinoless Double Beta Decay and the BCS Nuclear Wave Function

A method of using the Bardeen-Cooper-Schrieffer (BCS) nuclear wave function to treat the two-nucleon mechanism for neutiinoless double beta decay process 0⁺ → 0⁺ is proposed.The neutrinoless decay mode and the neutrinoless decay accompanied by a Majoron emission mode of ⁸²Se are studikd. Our cdculated results show that to reproduce the experimental value of γ^(ov) > 1.8 × 10²² yr for neutrinoless double beta decai of ⁸²Se the Majorana neutrino mass m_v < 6.2 eV and the mixing parameter of right-handed current η < 7.0 × 10^-6 In the emission with a Majoron mode the effective Majoron coupling to neutrino is deduced from the experimental value of γ^(ov,H) > 4.4 × 10²⁰ yr for ⁸²Se with the result H0> < 6.2 × 10^-4.     

SINGLE-PARTICLE AND SINGLE-HOLE ENERGIES IN THE 40Ca REGION

The single particle (SP) energies of ⁴¹Ca and the single hole energies of ³⁹Ca are calculated with eigenvalue equation derived from SP Green's function.The matrix element of M-3Y force is adopted as the equivalent of G-matrix element.Two particle one hole (2plh) and two hole one particle (2hlp) multiple scattering (MS) correlation were studied.The results show the 2hlp MS correlation is more important than the 2plh MS correlation.     

229Ra的衰变纲图

用１４ＭｅＶ中子轰击钍靶,通过￣（２３２）Ｔｈ（ｎ,α）￣（２２９）Ｒａ反应产生￣（２２９）Ｒａ,由放射化学分离技术从被照靶物质中分离出￣（２２９）Ｒａ活性;利用γ（Ｘ）谱学方法,首次观测到了￣（２２９）Ｒａ的能量为１４．５、１５．６、１８．８、２１．８、２２．５、４４．０、４７．５、５５．０、６３．０、６９．６、９３．６、９４．１、９８．５、１０２．２、１０４．５、１０６．１、１６１．１和１７１．５ｋｅＶ的１８条新衰变γ射线,并建立了￣（２２９）Ｒａ的部分衰变纲图． The￣(229)Ra has been produced in the￣(232)Th (n,α)￣(229)Ra reaction by 14 Mev neutron irradiation of natural Thorium.The￣(229)Ra activities were separated from irradiated target material by radiochemical separation technique. The 18 new γ rays of !(229)Ra decay with energies of 14. 5, 15. 6, 18. 8, 21. 8, 22. 5, 44. 0, 47. 5, 55. 0, 63. 0, 69. 6, 93. 6, 94. 1,98. 5, 102. 2, 104. 5, 106. 1, 161. 1 and 171. 5 keV were found for the first take by using y (X ) ...     

The Decay of 120Ba

¹²⁰Ba was produced by 68MeV ¹⁶O beam bombarding a 2mg/cm² thick ¹⁰⁶Cd target and fluorated in a halium-jet ion source.The decay of on-line mass separated activity ¹²⁰Ba has been studied by γ-X,γ-γ γ-β coincidence measurements.Its halflife was measured to be 24±2s.The total decay energy was extracted to be Q_EC=5.0±0.3MeV.A simple decay scheme has been proposed.     

Stuty on the Process of the Fast Neutron Radiative Capture for 40Ca and 208Pb

The pre-equilibrium correction for the nuclear reactions induced by fast neutron is considered as follows: the nuclear reaction processes with the exciton number equal to or larger than five can still be described by the statistical theory of the nuclear reaction. The particle emission processes, in which less than five excitons are involved, are calculated by means of the exciton model and the γ emission of one exciton state is calculated by the direct capture mechanism. For the three-exciton state, only the semidirect capture mechanism, which plays the main role in the researched energy region, is taken into account. The interference effect between the direct and semidirect capture is also considered. The radiative capture cross sections for ⁴⁰Ca and ²⁰⁸Pb in the neutron incident energy region from 3 MeV to 20 MeV are calculated and a better coincidence with the experimental values is obtained. At the same time, the contribution to the (n, γ) reaction cross sections of the γ emissions before and after statistical equilibriums as well as the characteristics of the direct capture, semidirect capture and their interferece terms are discussed.     

Study of 101 Mo Decay

The decay of ¹⁰¹Mo to levels in ¹⁰¹Tc has been studied using the three-parameter (γ-γ-t) coincidence system of HpGe-HpGe detectors. From the coincidence data, the new decay scheme was constructed. The previously reported 104. 70, 105. 95 and 774. 15 keV γ rays were Observed, and have been assigned to the decay scheme for the first time. A newly observed 1508. 01 keV γ ray has also been assigned to the scheme for the first time. The-tensities of β and the values of log ft to most levels were calculated.     

137Eu衰变的在线测量

使用在线同位素分离器及快带传输系统研究了缺中子稀土核137Eu的衰变性质,测量了A＝137核的γ、X单谱及γ-X、γ-γ符合谱,得到了137Eu核的半衰期为9．8s,并观测到了137Eu的一条能量为209．0keV的衰变γ线． The decay properties of neutron-deficient rare earth nucleus 137Eu have been studied by means of on-line isotope separator combined with rapid tape transport system.The products were produced by 190 MeV 36Ar bombarding on Pd target via natPd (36Ar, pxn)137Eu reactions. γ, X singles and γ-X, γ-γ coincidence spectra of the nuclei with A=137 were measured. The half-life of 137Eu is obtained to be 9. 8 s. A γ-ray with energy of 209. 0 keV for 137Eu was observed.     

The Decay Scheme of 175Er

The ¹⁷⁵Er activity was produced via the reaction ¹⁷⁶Yb(n,2p) ¹⁷⁵Er by 14MeV neutron irradiation of a natural ytterbium metal target with a thickness of～80mg/cm2.X-γ and γ-γ coincidence measurements were employed to identify ¹⁷⁵Er activity.The half-life of ¹⁷⁵Er has been determined to be (1.2±0.3)min. Eight new gamma rays were assigned to the decay of ¹⁷⁵Er. A partial decay scheme of ¹⁷⁵Er has been proposed for the first time, from which a ground rotational band has been assigned to ¹⁷⁵Tm.     

First Observation of 131Pm Decay

¹³¹Pm nucleus was produced in the ¹⁰⁶Cd (³²S,3p4n) ¹³¹Pm reaction. Theproducts were transported by helium jet and tape system, and X,γ single spectra and X-γ,γ-γ coincidence spectra were measured. The half-life of ¹³¹Pm has beendetermined to be (6.3±0.8)s. A partial decay scheme of ¹³¹Pm is proposed.