Experimental and Monte-Carlo studies of the spatial distribution of neutrons around extended Pb-spallation target

The spatial distribution of thermal and fast neutrons on the surface of a paraffin moderator surrounding a cylindrical lead target, irradiated with 1 GeV protons was studied. The lead target had 8 cm diameter and 20 cm length. The thickness of the paraffin around the target was 6 cm. The slow and fast neutron distributions were determined using LR 115 2B and CR-39 detectors via the ¹⁰B(n,) reaction and neutron induced nuclear recoils, respectively. The observed slow and fast neutron distributions on the surface of the paraffin were compared with Monte-Carlo simulations using the MCNPX-2.1.5 code.     

Study of neutron generation from relativistic heavy ion interactions

Study has been carried out on neutron generation in the interactions of 44 and 18 GeV ¹²C with Cu-and Pb-targets. The properties studied include, (1) Relative yields of secondary neutrons generated in 44 GeV versus in 18 GeV ¹²C bombardment to Cu target and also to Pb target. The ratios of these are 2.12±0.19 and 2.04±0.15, respectively, which is nearly in consistence with but somewhat larger than the values of theoretical calculations for Cu target. (2) Relative yields of secondary neutrons produced in Pb target versus in Cu target. The ratios obtained are 2.54±0.20 and 2.56±0.20 for 44 GeV and 18 GeV, respectively. More neutrons are generated in Pb target than in Cu target.     

ADS中子源散裂靶物理研究

运用蒙卡程序DCM CEM对ADS标准散裂中子靶进行了计算研究.计算了在0.1~1.6GeV的质子轰击下,标准Pb靶发生散裂反应产生的泄漏中子产额及分布、中子能谱以及靶内能量沉积分布.计算结果与文献理论数据、实验数据进行了比较.     

Simulation of a modified neutron detector applied in CSNS

We simulate the response of a modified Anderson-Braun rem counter in the energy range from thermal energy to about 10 GeV using the FLUKA code.Also,we simulate the lethargy spectrum of CSNS outside the beam dump.Traditional BFs tube is replaced by the 3He tube,a layer of 0.6 cm lead is added outside the boron doped plastic attenuator and a sphere configuration is adopted.The simulation result shows that its response is exactly fit to H*(10) in the neutron energies between 10 keV and approximately 1 GeV,although the monitor slightly underestimates H*(10) in the energy range from thermal energy to about 10 keV.According to the characteristics of the CSNS,this modified counter increases the neutron energy response by 30% compared with the traditional monitors,and it can be applied in other kinds of stray field rich of high energy neutrons.     

Charge form factor of the neutron at low momentum transfer from the 2H-->(e-->,e'n)1H reaction

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio GEn/GMn was extracted from the beam-target vector asymmetry AedV at four-momentum transfers Q2=0.14, 0.20, 0.29, and 0.42 (GeV/c)2.     

Method for Monitoring of Neutron Fields near High-Energy Accelerators

The monitoring of neutron radiation from high-energy accelerators cannot fully rely on the standard dosimeters and radiometers manufactured in Russia, since these are sensitive only to neutrons with energies below some 10 MeV. This is because neutrons of higher energies can significantly contribute to the personnel doses both close to the accelerator shield and in the neutron multiscattered field around the shield. In this paper, we propose to measure the ambient neutron dose in energy range 10^–2 MeV to 1 GeV with a device consisting of two polyethylene balls with diameters of 3 and 10 in. housing slow-neutron detectors. The larger ball also comprises a lead converter (10'' + Pb). This device can be implemented in zonal radiation monitoring in the near-accelerator area.     

Determination of spatial and energy distributions of neutrons in experiments on transmutation of radioactive waste using relativistic protons

The experiments on transmutation of ¹²⁹I and ²³⁷Np using uranium-lead targets surrounded by a paraffin moderator were performed at the Joint Institute for Nuclear Research (JINR, Russia). The targets were irradiated by 1.5 GeV and 7.4 GeV protons at the Synchrophasotron of JINR. In the frame of present work spatial and energy distributions of neutrons on the surface of the paraffin moderator were measured using SSNTD technique. It is shown that measured values of spetral indices do not pretend on the energy of incident protons but depend on the target composition. The presence of the uranium insertion softens neutron spectra.     

The Lead Radius Experiment PREX

The Lead Radius Experiment PREX will run in Spring of 2010. The experiment measures the parity-violating asymmetry in the elastic scattering of polarized electrons from a lead nucleus at an energy of 1.05 GeV and a scattering angle of 5°. The Z ₀ boson couples mainly to neutrons, and provides a clean measurement of R _n with a projected experimental precision of ±1%. The measurement is a fundamental test of nuclear theory and pins down the density-dependence of the symmetry energy of neutron rich nuclear matter which has impacts on neutron star structure, heavy ion collisions, and atomic parity violation experiments. Recent developments in the experiment are described.     

Spallation reactions studied with 4π-detector arrays

Recently here has been a renewed interest in the study of spallation reactions in basic nuclear physics as well as in potential applications. Spallation reactions induced by light projectiles (protons, antiprotons, pions, etc.) in the GeV range allow the formation of hot nuclei which do not suffer the collective excitations (compression, rotation, deformation) unavoidable when using massive projectiles. Such nuclei provide an ideal testbench for probing their decay as a function of excitation energy. In these investigations, 4π-detector arrays for charged particles and neutrons play a major role in the event-by-event sorting according to the excitation energy of the nucleus. Spallation reactions induced on heavy nuclei allow the conversion of the incident GeV proton into several tens of evaporated neutrons. The neutron production in thick targets has been investigated in great detail thanks to the use of high efficiency neutron detector arrays. When scattered on samples of inert or biological materials, these neutrons can be used to study details of the material structure. They could also be utilized for the transmutation of long-lived nuclear wastes or for the feeding of sub-critical nuclear reactors. The role of different types of multi-detector arrays is highlighted in this paper. Several references are also given for different uses of high efficiency neutron detectors in other contexts.     

利用超高能中子诊断惯性约束聚变的燃烧

惯性约束聚变靶丸中,高能中子相互之间发生碰撞将导致超高能中子的产生.本文讨论超高能中子产生与聚变燃烧速率的关系.数值模拟理想模型,分析超高能中子产生量与平均烧氚体积的关系,讨论利用出壳超高能中子流与烧氚量诊断二维效应和混合效应.进一步分析出壳超高能中子流与平均烧氚率的关系,探讨利用超高能中子诊断平均烧氚温度并判断是否发生非平衡效应问题.     

5-10 GeV neutrinos from gamma-Ray burst fireballs

A gamma-ray burst fireball is likely to contain an admixture of neutrons. Inelastic collisions between differentially streaming protons and neutrons in the fireball produce nu(&mgr;) (nu;(&mgr;)) of approximately 10 GeV as well as nu(e) (nu;(e)) of approximately 5 GeV, which could produce approximately 7 events/year in km(3) detectors, if the neutron abundance is comparable to that of protons. Photons of approximately 10 GeV from pi(0) decay and approximately 100 MeV nu;(e) from neutron decay are also produced, but will be difficult to detect. Photons with energies less, similar1 MeV from shocks following neutron decay produce a characteristic signal which may be distinguishable from the proton-related MeV photons.     

The 10,000,000,000-Volt question: What is the best choice of proton energy to drive a pulsed spallation neutron source?

In pulsed spallation neutron sources, higher proton beam current or higher proton energy brings higher proton beam power, and from greater beam power come more neutrons. Consequently, accelerator scientists and target systems designers conceive systems to produce ever-higher intensities of neutrons, and the question arises: What is the best choice of proton energy to drive a pulsed spallation neutron source? Meanwhile target engineering, radiation shielding, and neutronic coupling to moderators must be accomplished effectively. This paper addresses the title question from the target side, assessing the neutron yield, the distribution of leakage neutrons, and the power density distribution as functions of proton energy.     

利用GEANT4和FLUKA研究质子诱发散裂反应出射中子双微分截面

在中高能质子诱发散裂反应相关核工程设计中,可靠的蒙特卡罗模拟程序结合核反应理论模型具有较好的理论指导意义。本工作中,利用GEANT4耦合INCL4和ABLA理论模型以及FLUKA耦合PEANUT模型模拟计算了几百MeV至几个GeV质子轰击Be,Al,Fe,W,U等靶后30°,60°,120°,150°出射角产生的散裂中子双微分截面,并与现有实验数据进行了比较。结果发现,FLUKA和GEANT4模拟计算较好地再现了Al,Fe,W,U等靶实验测量数据。然而,模拟结果明显低估了Be靶出射中子能量小于10 MeV能区的实验数据。For the design of nuclear engineering related with medium-high energy proton induced spallation reaction,the reliable Monte Carlo simulation codes coupled with nuclear reaction models have a good theoretical guidance.In this work,the production spallation neutron double differential cross sections at 30°,60°,120°,150°emission angle for Be,Al,Fe,W,U target materials at incident proton energies between several hundred MeV and GeV are theoretically calculated by using the GEANT4 coupled INCL4 and ABLA,and the FLUKA coupled PEANUT.The calculated results were compared with the available experimental data.It is found that the GEANT4 and FLUKA calculations well reproduced the experimental measurement of Al,Fe,W,U target materials.However,calculations obviously underestimated the emission neutrons of Be target for lower than 10 MeV energy range.     

On the search for density isomers in interaction of relativistic12C nuclei with lead

A Pb target was bombarded with¹²C nuclei of an energy of 15 and 43 GeV. The target was periodically transported to the inside of the neutron multiplicity detector which allowed measuring time distributions for events of multiple emission of delayed neutrons. Abnormal neutron emitters have not been found. Upper limits of probabilities of their production were estimated for the half-life range 10–10⁵ s and for different mean multiplicities of neutrons¯n. The values obtained are within the interval from 1.4·10^?9 (for¯n=12)to 4·10^?6(for¯n=2).     

Neutron Generation in an Extended Cu Target Irradiated with 22 and 44 GeV Carbon Ions (Preliminary Results)

An extended Cu target was centrally irradiated with 22 and 44 GeV carbon ions for about 11 and 15 hours, respectively. The upper side of the target was in contact with a paraffin block for the moderation of secondary neutrons. Small holes in the moderator were filled with lanthanum salts (LaCl₃7H₂O) where the neutrons induced the ¹³⁹La(n,γ) ¹⁴⁰La reaction. The β-decay of ¹⁴⁰La(40h) was followed using radiochemical methods. The yields for the formation of ¹⁴⁰La agree within large errors with other experiments on extended targets carried out at the LHE, JINR (Dubna). To a first approximation, the breeding rate of ¹⁴⁰La doubles when the ¹²C energy is increased from 22 to 44 GeV. If results at 44 GeV are compared to those at 22 GeV, there cannot be excluded an excess of (20 ± 7)% in the experimentally observed ¹⁴⁰La production over theoretical estimates. A definite conclusion in this respect must await further experimental and theoretical work.

These experiments are the beginning of a research program to develop a subcritical reactor with neutron fluxes sufficiently high to permit the TRANSMUTATION of long-lived radioactive waste into short-lived or stable nuclei. Such transmutation would considerably relieve the ecological problems related to the final deposition of radioactive waste.     

SSRF前端区锯齿墙束流孔洞中子屏蔽的蒙特卡罗估算

主要关于上海同步辐射装置(SSRF)储存环电子引发产生的韧致辐射和中子辐射的研究. 中子和光子经多种组合材 料(厚度在5cm～115cm之间)屏蔽后的剂量特征由蒙特卡罗代码MCNP和EGSnrc估算得到; 蒙特卡罗计算表明, 单一的材料如铅, 铁和聚乙烯对高能中子是无效的生物屏蔽材料, 而组合材料如铅或者铁加聚乙烯和铅或者铁加混凝土被认为是屏蔽高能中子很好的组合材料. 铅铁等高Z材料加点包含有氢的低Z材料如聚乙烯是同时屏蔽高能中子和韧致辐射的一种比较好的组合材料选择.     

Iterative unfolding for Bonner sphere spectrometers using the MSANDB code – Sensitivity analysis and dose calculation

Since many decades, Bonner sphere spectrometers (BSSs) are routinely used for assessment of neutron spectra over a wide energy range from some meV to GeV. Typically, a spectrometer consists of a neutron detector sensitive to thermal neutrons located inside moderating polyethylene (PE) spheres of various sizes. Based on the response functions of the detector/sphere systems that must be calculated by Monte Carlo codes, an unfolding procedure is applied to deduce the present neutron spectrum from the count rates of the detectors. To start the unfolding procedure, a guess spectrum is required that includes some prior information on the physics of the investigated neutron spectrum, and that is iteratively modified to match the measured count rates. For the present investigation, a BSS-system consisting of an ³He proportional counter and seventeen spheres containing PE or a combination of PE and lead is used. The system is used for example to measure secondary neutrons from cosmic radiation at mountain altitudes, or from high-energy accelerators outside the radiation shielding. A systematic study was performed to quantify the influence of the chosen guess spectrum and the number of iteration steps on the unfolded neutron spectrum, and on integral quantities deduced such as total neutron fluence or ambient dose equivalent. It turned out that none of these changes resulted in dose quantities that were more than a few percent different to those deduced when the optimised start spectrum was used. Similarly, use of the two different response matrices available for our BSS system provided similar values for the ambient dose equivalent.     

Investigation of neutron emission from relativistic heavy ion interactions by nuclear tracks

Fast neutrons produced in 44 and 22 GeV ¹²C+Cu interactions have been recorded and analysed with a CR-39 detector stack. The irradiation of the CR-39 stack to fast neutrons was carried out at the accelerator Synchrophasotron, Joint Institute for Nuclear Research, Dubna, Russia. Areal and volume densities of tracks induced by fast neutrons in the CR-39 at different positions and for different etching time have been measured. The neutron production ratio of 44 GeV to 22 GeV ¹²C+Cu interactions has been obtained, which is 2.17 ± 0.30 by areal track density, or 2. 12±0.33 by step etch technique, or 2.03±0.34 by volume track density measurement. These results confirm that the production rate at 44 GeV ¹²C+Cu interactions is more than theoretical estimation.     

GEANT4 Simulation of the High-Energy Neutron Beam of the TOF Spectrometer GNEIS

Using the Geant4 software package, a numerical simulation of a neutron source of the time-offlight spectrometer GNEIS created on the basis of the SC-1000 synchrocyclotron with 1 GeV proton energy at the NRC Kurchatov Institute—PNPI (Gatchina) has been carried out. The influence of the structural features of the neutron source of the spectrometer on the spatial and energy distributions of neutrons has been studied. The intensity and spectral characteristics of the neutron flux in the range of 1–1000 MeV have been determined on the basis of the obtained information and detailed allowance for all elements of the neutron beam guide system. It is found that the best agreement between the experiment and calculation performed by means of Geant4 is observed when using the QGSP_INCLXX_HP model. In the neutron energy range of 1–200 MeV, the difference between the experimental and calculated shapes of the spectra is less than 25%.     

Characteristics of the sampling paddle modules of neutron wall from the cosmic ray test

In the construction of a large area neutron detector(neutron wall)that is used to detect neutrons at GeV energies,the performances of all the sampling paddle modules prepared for the neutron wall areinvestigated with a specially designed test bench.Tested by cosmic rays,an average intrinsic time resolution of 222.5 ps is achieved at the center of the modules.The light attenuation length and the effective speed of the light in the module are also investigated.