Neutron emission following muon capture in heavy nuclei

The energy distribution of neutrons following muon capture in heavy nuclei is investigated. It is shown that the experimental high energy tail (E > 10 MeV) of the neutron distribution can be explained by a direct emission process.     

Spins of198Au levels from the (n, γ) reaction using polarized neutrons and polarized197Au nuclei

The¹⁹⁷Au(n, γ) reaction was studied with the aid of polarized thermal neutron beams. Two kinds of experimental data were obtained: i) circular polarization of gamma rays of high energy due to capture of polarized neutrons by unoriented¹⁹⁷Au nuclei, and ii) angular distribution of such gamma rays observed after capture of polarized neutrons by polarized¹⁹⁷Au nuclei. Spins of several levels of¹⁹⁸Au could be uniquely assigned or could be restricted. The sign of the hyperfine field of Au in AuFe has been determined to be negative.     

Measurements of a partial cross section for the reaction 58Ni(n, γ0)59Ni

The partial cross section for radiative neutron capture accompanied by gamma transitions to the ground state of the ⁵⁹Ni nucleus was measured as a function of energy by a new neutron-spectrometry method that employed the shift of a primary gamma transition in response to a change in the energy of the captured neutron. The reaction ⁷Li(p, n)⁷Be was used as source of neutrons for the present measurements. The protons that induced this reaction were accelerated by a Van de Graaff electrostatic generator to energies exceeding the reaction threshold by 60 keV, in which case an appropriate geometry of the experiment permitted irradiation of the sample under study with neutrons whose energy ranged between 10 and 120 keV. The partial widths of some resonances and radiative strength function for hard primary M1 gamma transitions were determined in addition to the above cross sections.     

Strahlungseinfang langsamer Neutronen im thermischen und Resonanz-Bereich

Within the framework of the many-levelR-matrix formalism the theory ofLane andLynn allows one to calculate the partial cross sections for radiative capture of thermal and resonance energy neutrons. If the cross section for capture to a given final state of the product nucleus has been measured for a number of neutron energies in the thermal and resonance regions, then the variation of the above cross section with incident neutron energy can be completely determined. This calculation has been carried out for the reaction Mn⁵⁵(n, γ) Mn⁵⁶ for two distinct final states of Mn⁵⁶, where enough data are available to fix the parameters for resonance internal capture.     

Spectra of neutrons from μ capture in thallium,lead and bismuth

The energy distributions of neutrons fromμ capture in Tl, Pb, and Bi were measured applying a time-of-flight technique, using de-excitationγ-rays from final nuclei as a time reference. The spectra exhibit an evaporation part, which is described by nuclear temperatures varying fromθ=1.06 MeV for Bi toθ=1.22 MeV for Pb, and an exponentially decreasing high-energy distribution with a relative intensity of ～10%, which is interpreted, on the basis of a simple model, as being due to direct and pre-compound processes.     

P-odd asymmetries for the binary fission of oriented target nuclei, induced by cold polarized neutrons

P-odd asymmetries in binary fission of oriented target nuclei induced by cold polarized neutrons are described for the first time using methods of the quantum theory of fission. A spin matrix of the compound nucleus density is constructed with allowance for the interference of the fission amplitudes of various pairs of neutron resonances excited in the given nucleus during the capture of a cold polarized neutron by the oriented target nucleus. In the differential cross sections for this reaction we obtain not only P-odd T-even correlations with the angular dependence different from that of the earlier investigated correlations in the fission of unoriented target nuclei by polarized neutrons but also P-odd T-odd correlations that do not occur in the fission of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.     

Thresholds for the Break of Nucleon Cooper Pairs and Special Features of the Decay of the <Superscript>172</Superscript>Yb Nucleus in the Reaction <Superscript>171</Superscript>Yb(<Emphasis Type="Italic">n</Emphasis><Subscript>th</Subscript>,2γ)

The reaction ¹⁷¹Yb(n_th,2γ) was studied with the aim of extending the experimental database of intensities of cascades of two photons emitted sequentially after radiative thermal-neutron capture. An analysis of these cascade intensities permitted revealing the change in the structure of observed levels of the ¹⁷²Yb nucleus versus the excitation energy and determining probable thresholds for the break of four Cooper pairs of neutrons below the neutron binding energy.     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.     

Beiträge zu den Niveauschemata der Ge-Isotope71Ge,73Ge,74Ge und75Ge aus der Untersuchung von (n, γ)-Reaktionen in natürlichem Germanium

Low energy γ-spectra from neutron capture in natural germanium have been studied with several high resolution Ge(Li)-detectors. Measurements have been made with two different target configurations. In the first measurement a normal Ge-target was irradiated with neutrons; no neutrons were allowed to enter the Ge(Li)-detector. In the second measurement the Ge(Li)-detector was used simultaneously as target and as γ- ande ^?-detector. By combination of the results convertion coefficients of several transitions in⁷³Ge have been determined. Several unknown low energy transitions in Ge isotopes have been found with the aid of the second method. The level schemes of⁷¹Ge,⁷³Ge,⁷⁴Ge and⁷⁵Ge have been revised by means of coincidence measurements with a Ge(Li)-Ge(Li)-coincidence system of high efficiency and resolution.     

Cross sections and γ-rays from multiple neutron capture in151Eu

Primary capture gamma rays following multiple neutron capture in¹⁵¹Eu have been measured as a function of the irradiation time. Neutron capture cross sections of the¹⁵²Eu 3^? groundstate and of the¹⁵²Eu 0^? isomer (T _1/2=9.3h) were determined. These cross sections are relevant for the interpretation of inelastic scattering of neutrons at isomeric states with energy gain (neutron acceleration). The level scheme of¹⁵³Eu has been extended. Neutron binding energies of¹⁵²Eu,¹⁵³Eu,¹⁵³Gd and¹⁵⁴Gd are given.     

Nuclear γ-rays produced by π mesons

Energies and yields of nuclearγ-rays accompanyingπ-mesic X-ray spectra have been measured. Theγ-lines arise either in nuclei of the atomic number of the target, or in nuclei of smaller atomic numbers. The first kind ofγ-rays are due mainly to inelastically scattered neutrons produced by pion capture in another target nucleus (secondary process), whilst the second kind are due to ordinary nuclear capture (primary process).     

Dosimetry and spectrometry at accelerator based neutron source for boron neutron capture therapy

An innovative accelerator-based neutron source for boron neutron capture therapy has started operation at the Budker Institute of Nuclear Physics, Novosibirsk. This facility is based on a compact vacuum insulation tandem accelerator designed to produce proton current up to 10 mA. Epithermal neutrons are proposed to be generated by 1.915 MeV protons bombarding a lithium target using ⁷Li(p,n)⁷Be threshold reaction.In the article, techniques to detect neutron and gamma-rays at the facility are described. Gamma radiation is measured with NaI and BGO gamma-spectrometers. The total yield of neutrons is determined by measuring the 477 keV γ-quanta from beryllium decay. For the rough analysis of the generated neutron spectrum we used bubble detectors. As the epithermal neutrons are of interest for neutron capture therapy the NaI detector is used as activation detector. We plan to use a time-of-flight technique for neutron spectra measurement. To realize this technique a new solution of short time neutron generation is proposed.     

Energy spectra of single neutrons and charged particles emitted following the absorption of stopped negative pions in 4He

Energy spectra have been measured of single neutrons, protons and deuterons emitted following the capture at rest of negative pions in ⁴He. The neutron energy spectrum has been measured with an energy resolution of 4% at 90 MeV. The absolute number of stopped pions has been measured.     

Radiative capture of low-energy neutrons in the shell-model approach to nuclear reactions

The shell-model approach to photonuclear reactions is applied to the radiative capture of low-energy neutrons. The direct, compound and channel resonant contributions to the collision matrix appear in a natural way. We show that the experimental data in the vicinity of the 3s giant resonance can be quantitatively interpreted in terms of the channel resonant capture. The relationship with the R-matrix approach of Lane and Lynn is discussed.     

Cross sections for 197Au(n, γ)198Au and 115In(n, γ)116mIn in the neutron energy region 2.0–7.7 MeV

The cross sections for the reactions ¹⁹⁷Au(n, γ)¹⁹⁸Au and ¹¹⁵In(n, γ)^116mIn have been measured with the activation method in the neutron energy region 2.0–7.7 MeV. The influence of background neutrons on the results was studied in considerable detail. The main problems are caused by low-energy neutrons produced by charged-particle reactions in the target material and secondary neutrons from nonelastic reactions in the sample and surrounding materials.The measured capture cross sections are generally lower than previous results and the deviation tends to increase with increasing neutron energy. The data are also compared with calculations based on the compound-nucleus model and quite good agreement is obtained.     

Energy spectra and asymmetries of neutrons emitted after muon capture

Inclusive energy spectra and the energy dependence of the asymmetry parameter have been measured for neutrons emitted after muon capture in O, Si, Ca and Pb. In addition a neutron-neutron coincidence measurement has been performed for Ca. The results of these measurements confirm the exponential shape of the energy spectra and the positive asymmetry observed by Sundelin and Edelstein. The observation of neutrons with energies above 50 MeV and the observation of simultaneous emission of two high-energy neutrons suggest the importance of short-range nucleonnucleon correlations in this process.     

2.45 MeV小型D-D中子管准直屏蔽结构设计

为获取小角度出射的单能中子源,采用蒙特卡罗软件对小型D-D中子管产生的能量为2.45 MeV的4立体角中子源进行了准直屏蔽结构设计。准直屏蔽结构分为准直器和捕获穴,准直器采用铁+含硼聚乙烯+铅的三层过滤结构,用于屏蔽照射野外杂散中子,捕获穴主要功能是增加反方向中子的弹性散射次数,从而降低出射束低能散射中子的比例。通过MCNP模拟得到了准直器各层材料的最佳厚度和出射孔尺寸以及捕获穴最佳结构。经验证,中子照射野处2.45 MeV的中子通量比照射野外大三个量级,中子照射野处低能中子通量比2.45 MeV的中子通量低一个量级,墙壁外总剂量率（中子+）在2.5 Gy/h以下。该研究对于小角度单能中子源的快速获取具有一定的实用价值,获取的中子源可用于中子剂量仪器有效性检验、中子监测仪性能测试等方面的研究。     

Level scheme of 194Ir

Levels of ¹⁹⁴Ir were studied using neutron capture and (d, p) reaction spectroscopy. A pair spectrometer was used to measure the high-energy γ-ray spectrum from thermal-neutron capture in an enriched ¹⁹³Ir target over the energy range 4640–6100 keV. From the same reaction, low-energy γ-radiation was studied using curved-crystal spectrometers, and conversion electrons were observed with magnetic spectrometers. Prompt and delayed γγ-coincidences were measured using semiconductor and scintillation detectors. Averaged resonance capture measurements were performed with 2 keV and 24 keV neutrons for primary transitions leading to excitation energies from 0 to 580 keV. Using 22 MeV deuterons, the ¹⁹³Ir(d, p) high resolution spectra were observed with a magnetic spectrograph. The deduced nuclear level scheme of ¹⁹⁴Ir includes 38 levels connected by 184 transitions. Unambiguous spins and parities were determined for 25 levels. The rotor-plus-particle model was used for the interpretation of the level scheme assuming a strong mixing for Nilsson configurations having identical parities and K quantum numbers. IBFFM model calculations were performed and the obtained results were compared with the experimental level scheme.     

Features of nuclear reactions induced by ultracold neutrons passing through thin films

The probabilities of elastic scattering and capture of ultracold neutrons in thin films are calculated in terms of the time quantum theory. It is shown that at a neutron wavelength exceeding considerably the film thickness, the macroscopic probability of the (n, γ)-reaction can decrease as compared to the macroscopic probability of elastic scattering.