Startup of the high-intensity ultracold neutron source at the Paul Scherrer Institute

Ultracold neutrons (UCN) can be stored in suitable bottles and observed for several hundreds of seconds. Therefore UCN can be used to study in detail the fundamental properties of the neutron. A new user facility providing ultracold neutrons for fundamental physics research has been constructed at the Paul Scherrer Institute, the PSI UCN source. Assembly of the facility finished in December 2010 with the first production of ultracold neutrons. Operation approval was received in June 2011. We give an overview of the source and the status at startup.     

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.     

Measurement of the neutron spectrum of a Pu-C source with a liquid scintillator

The neutron response function for a BC501A liquid scintillator （LS） has been measured using a series of monoenergetic neutrons produced by the p-T reaction. The proton energies were chosen such as to produce neutrons in the energy range of 1 to 20 MeV. The principles of the technique of unfolding a neutron energy spectrum by using the measured neutron response function and the measured Pulse Height （PH） spectrum is briefly described. The PH spectrum of neutrons from the Pu-C source, which will be used for the calibration of the reactor antineutrino detectors for the Daya Bay neutrino experiment, was measured and analyzed to get the neutron energy spectrum. Simultaneously the neutron energy spectrum of an Am-Be source was measured and compared with other measurements as a check of the result for the Pu-C source. Finally, an error analysis and a discussion of the results are given.     

Modeling a neutron-rich nuclei source

The deuteron break-up process in a suitable converter gives rise to intense neutron beams. A source of neutron-rich nuclei based on the neutron-induced fission can be realised using these beams. A theoretical optimization of such a facility as a function of the incident deuteron energy is reported. The model used to determine the fission products takes into account the excitation energy of the target nucleus and the evaporation of prompt neutrons. Results are presented in connection with a converter-target specific geometry. Received: 1 December 2000 / Accepted: 12 May 2001     

用于神光Ⅲ主机燃料面密度诊断的下散射法探测系统的准直设计

在惯性约束聚变实验中,用下散射法诊断燃料面密度对本底的屏蔽要求较高。分析了神光Ⅲ主机纯氘燃料靶在实际诊断环境中产生的本底辐射场,通过蒙特卡罗方法得知测量点处散射本底主要来源于初级中子与真空腔室、周围诊断平台、天花板和地板的作用。据此分析选取合适的材料和屏蔽方案。通过模拟计算,得到尺寸优化后的准直器设计方案,该方案达到了实验要求。     

Angular correlations in emission of prescission neutrons from <Superscript>235</Superscript>U fission induced by slow polarized neutrons

A new approach to searching for and studying scission neutrons, which is based on the analysis of specific angular correlations in nuclear fission induced by polarized neutrons, is described and used to evaluate the fraction of scission neutrons in the total number of prompt neutrons of ²³⁵U fission emitted perpendicularly to the fission axis.     

Measurement of the neutron spectrum of a Pu-C source with a liquid scintillator

The neutron response function for a BC501A liquid scintillator (LS) has been measured using a series of monoenergetic neutrons produced by the p-T reaction. The proton energies were chosen such as to produce neutrons in the energy range of 1 to 20 MeV. The principles of the technique of unfolding a neutron energy spectrum by using the measured neutron response function and the measured Pulse Height (PH) spectrum is briefly described. The PH spectrum of neutrons from the Pu-C source, which will be used for the calibration of the reactor antineutrino detectors for the Daya Bay neutrino experiment, was measured and analyzed to get the neutron energy spectrum. Simultaneously the neutron energy spectrum of an Am-Be source was measured and compared with other measurements as a check of the result for the Pu-C source. Finally, an error analysis and a discussion of the results are given.     

墙壁反射中子对脉冲堆波形的影响

墙壁的反射中子会对快脉冲堆的波形产生明显的影响.堆芯中子泄漏后,经过墙壁的反射有一定的概率返回堆芯,由于能量的差异,泄漏中子的返回时间是一个连续的分布.传统的双区模型只考虑了相互作用概率,而没有时间信息,尽管可以很好地解决稳态问题,而无法解决瞬态问题.本文采用等效的方法,把泄漏中子等效为时间相关的堆芯本征源,建立了含有反射效应的时间关联双区模型.求解得到的脉冲波形与CFBR-Ⅱ的实验结果一致,从而合理解释了脉冲波形后沿衰减变慢和坪功率提高的实验现象.     

PGAI瞬发伽玛射线活化成像技术理想化模型的模拟

为了讨论PGAI技术分析的准确性,并验证冷中子和热中子应用于PGAI技术的可行性,通过蒙特卡罗模拟计算软件对PGAI技术理想化模型进行了研究,采用高准直的冷中子及热中子束和高纯锗探测器,对一块5 cm× 5 cm×1 cm均匀铁单质样品进行了模拟计算及图像重建,选取的等效体积大小为1 cm×1 cm×1 cm。结果显示:两种能量中子可以用于PGAI技术实现元素分布测量,但无论使用何种能量中子,由于物料体效应带来的中子自屏效应、中子散射效应以及伽马射线自吸收作用,即便在对均匀单质样品进行测量时,图像重建结果也无法保证各位置元素响应的一致性。因此,在后续工作中,需理清PGAI物理机制,建立相应的修正模型。     

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.     

The nucleon facility at PSI

The new nucleon area (NA) at PSI is designed as a multi-purpose facility with nucleon beams. The area can be used for experiments with polarized or unpolarized protons or neutrons. For both kinds of nucleons the available energy ranges from about 100 MeV up to a maximum of about 590 MeV.     

Superthermal source of ultracold neutrons for fundamental physics experiments

Ultracold neutrons (UCNs) play an important role for precise measurements of the properties of the neutron and its interactions. During the past 25 years, a neutron turbine coupled to a liquid deuterium cold neutron source at a high-flux reactor has defined the state of the art for UCN production, despite a long history of efforts towards a new generation of UCN sources. This Letter reports a world-best UCN density available for users, achieved with a new source based on conversion of cold neutrons in superfluid helium. A conversion volume of 5 liters provides at least 274,000 UCN in a single accumulation run. Cyclically repeated operation of the source has been demonstrated, as well.     

Consideration of the theoretical possibility of regulating the nuclear reactor by changing a fraction of delayed neutrons

A lot of theoretical and experimental studies devoted to the effect of external electromagnetic fields and ionization on the beta-decay probability have been published in the past years. The possibility of using this physical effect as the main reactor-regulation mechanism is investigated in this study. A set of equations allowing the operation of a nuclear reactor to be described when the probability for the beta decay of precursors of delayed neutrons and, hence, the fraction of delayed neutrons are functions of time is written and investigated. It is shown that, if the fraction of the delayed neutrons does not change, the proposed set of equations coincides with the generally known one. As follows from the analysis of the solutions to the new set of equations, the proposed reactor-regulation method does not allow reactor runaway driven by prompt neutrons even theoretically. The application of the proposed control method to a circulating-fuel liquid-type reactor is briefly considered.     

Neutron emission from 14N + 165Ho collisions at 35 MeV/u

Neutron emission from ¹⁴N + ¹⁶⁵Ho collisions has been studied at 25 MeV/u incident energy. Energy and angular distributions of the neutrons were measured in coincidence with projectile-like fragments (Li, Be, B, and C) emitted at angles of 10° and 30°. The spectra of neutrons at angles far from the angle of a coincident fragment have been satisfactorily parameterized in terms of a slowly moving, target-like source of temperature 2–3 MeV and a half-beam-velocity source of temperature about 7 MeV. The latter source accounts for about 20% of the detected neutrons for in-plane measurements. The out-of-plane cross sections are smaller. The relevant parameters of the moving-sources parametrization suggest a simple model which qualitatively explains the data in terms of the development of a hot participant zone and its subsequent mass exchange interactions with spectators in the projectile and target nuclei.     

Interaction of 11 MeV neutrons with Y

Differential cross section for scattering of 11 MeV neutrons by ⁸⁹Y were measured using the Ohio University beam swinger time-of-flight facility. Measurements were taken in the angular range between 15° and 145°. Empirical optical model parameters have been obtained from the measured elastic scattering data. Deformation parameters were obtained for low-lying excited states using these optical-model parameters in a DWBA collective formalism. A comparison with deformation parameters in neighboring even-even nuclei ⁸⁸Sr and ⁹⁰Zr is presented. The weak-coupling model is used to describe neutron scattering to low-lying states in ⁸⁹Y.     

Die Energieverteilung der Neutronen aus einer Po-α-Be-Quelle

The neutrons from a Po-α-Be source were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of the scintillations produced in the stilbene crystal by the recoil protons is analysed to yield the incident neutron energy distribution. Theγ-ray background was eliminated through pulse shape discrimination. Separately the spectrum of neutrons coincident withγ-rays, i.e. of neutrons from the Be⁹(α,n)C^12* reaction, has been determined. The resulting spectra are compared to previous measurements. The structure of the spectra is not compatible with an isotropic angular distribution of the neutrons relative to theα-direction. Furthermore it could be shown that the fraction of reactions leading to the 7,65 MeV state of C¹² is only about 10^?3.     

Neutron and high-energy gamma-ray emission in 12C Induced reactions on 159Tb

The gamma-ray multiplicity associated with the emission of high-energy gamma rays and neutrons following ¹²C+¹⁵⁹Tb reactions at E_lab= 100 MeV has been measured. The spin decrease deduced for coincident gamma rays within the GDR energy region is found to be in agreement with an estimate based on a statistical model. Neutron spectra measured in coincidence with high-energy gamma rays are explained in terms of two components. The cooler component corresponds to neutrons evaporated from the compound nucleus. The hotter component emanates from a precompound source moving in the center-of-mass system along the beam direction. The multiplicity of gamma-ray cascades, coincident with neutrons from the two different sources, has been deduced. The results show that beside the GDR there appears to be a second source of gamma rays with E_γ≈10MeV. These gamma rays are emitted in more peripheral reactions and before neutron evaporation.     

Solid-phase synthesis of manganese silicides on the Si(100)2 × 1 surface

The solid-phase synthesis of manganese silicides on the Si(100)2 × 1 surface coated at room temperature by a 2-nm-thick manganese film has been investigated using high-energy-resolution photoelectron spectroscopy with synchrotron radiation. The dynamics of variation of the phase composition and electronic structure of the near-surface region with increasing sample annealing temperature to 600°C, has been revealed. It has been shown that, under these conditions, a solid solution of silicon in manganese, metallic manganese monosilicide MnSi, and semiconductor silicide MnSi_1.7 are successively formed on the silicon surface. The films of both silicides are not continuous, with the fraction of the substrate surface occupied by them decreasing with increasing annealing temperature. The binding energies of the Si 2p and Mn 3p electrons in the compounds synthesized have been determined.     

Production scheme for diagnostic-therapeutic radioisotopes by accelerator neutrons

Interest has been growing in the development of medical radioisotopes used for noninvasive nuclear medicine imaging of disease and cancer therapy. Especially the development of an alternative production scheme of ⁹⁹Mo, the mother radioisotope of ^99mTc used for imaging, is required, because the current supply chain of the reactor product ⁹⁹Mo is fragile worldwide. We have proposed a new production scheme of ⁹⁹Mo as well as therapeutic radioisotopes, such as ⁶⁴Cu and ⁶⁷Cu, using accelerator neutrons provided by the ^natC(d,n) reaction. Based on this scheme we have obtained high-quality ^99mTc, ⁶⁴Cu, and ⁶⁷Cu suitable for clinical use by developing both production and separation methods of the radioisotopes. We proposed a new facility to constantly and reliably produce a wide variety of high-quality, carrier-free radioisotopes, including ⁹⁹Mo, with accelerator neutrons. We report on the development of the proposed scheme and future prospects of the facility toward the domestic production of medical radioisotopes.