Unfolding the fast neutron spectra of a BC501A liquid scintillation detector using GRAVEL method

Accurate knowledge of the neutron energy spectra is useful in basic research and applications. The overall procedure of measuring and unfolding the fast neutron energy spectra with BC501A liquid scintillation detector is described. The recoil proton spectrum of 241Am-Be neutrons was obtained experimentally. With the NRESP7 code, the response matrix of detector was simulated. Combining the recoil proton spectrum and response matrix, the unfolding of neutron spectra was performed by GRAVEL iterative algorithm. A MatLab program based on the GRAVEL method was developed. The continuous neutron spectrum of ²⁴¹Am-Be source and monoenergetic neutron spectrum of D-T source have been unfolded successfully and are in good agreement with their standard reference spectra. The unfolded ²⁴¹Am-Be spectrum are more accurate than the spectra unfolded by artificial neural networks in recent years.     

ICF中子产额测量中统计涨落分析及计算

 塑料闪烁探测器通常用来测量氘氘、氘氚聚变中子产额。中子在闪烁体中产生的质子数的统计涨落及质子在闪烁体中沉积能量的统计涨落为测量结果引入了两项不确定度分量。以氘氚聚变中子为例,分析了这两种统计涨落的概率密度函数的计算方法,该计算方法也适用于其它能量的单能快中子和塑料闪烁体作用的相应计算。     

Measurements of scintillation efficiency and pulse shape for low energy recoils in liquid xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22±0.01 in the recoil energy range 40–70 keV. Under the assumption of a single dominant decay component to the scintillation pulse shape the log-normal mean parameter T₀ of the maximum likelihood estimator of the decay time constant for 6 keV <E_ee<30 keV nuclear recoil events is equal to 21.0±0.5 ns. It is observed that for electron recoils T₀ rises slowly with energy, having a value ∼30 ns at E_ee∼15 keV. Electron and nuclear recoil pulse shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse shape.     

液体闪烁体探测器γ射线测量研究

利用几种单能γ源对BC501A型液体闪烁体探测器进行能量刻度, 以得到探测器对电子的光输出响应函数。 介绍了用蒙特卡罗模拟法确定康普顿边缘所对应的电子能量的方法, 得到的结果与用半高点确定康普顿边缘方法所得的结果做了比较。 利用德国PTB开发的PHRESP蒙特卡罗程序计算出液体闪烁体探测器对各种单能γ射线的光响应矩阵, 阐述了利用γ射线响应函数矩阵和γ射线的反冲电子谱求解待测γ能谱的原理, 对解谱的误差来源也进行了简要的分析。 The light output of the BC501A liquid scintillation detector for γ rays was calibrated by serial monoenergetic γ ray sources. The Monte Carlo simulation method to confirm Compton edge was described briefly. The result was compared with that obtained by the semi height method. The γ ray response matrix of BC501A liquid scintillation detector was calculated by the Monte Carlo code PHRESP from PTB. The method of using response function matrix and recoil electron spectrum to unfold γ ray energy spectrum was introduced and the error of unfolding spectrum was also briefly discussed.     

Liquid xenon as a dark matter detector. Prospects for nuclear recoil discrimination by photon timing

Two mechanisms lead to liquid xenon scintillatlon, and are excited differently by incident electrons and by heavy charged nuclei. This could be used to discriminate nuclear recoil events from radioactive background in a search for hypothetical Galactic dark matter particles. To achieve low energy threshold a likelihood analysis is proposed using the time intervals between detected photons. Results are presented of Monte Carlo simulations, showing the background rejection achievable as a function of the number of detected photons. Some comments are included on background discrimination by simultaneous measurement of scintillation and ionization signals.     

Recoil implantation of100Pd(3.6 d) activity in different host matrices following heavy ion reactions

The large recoil energies offered by the heavy ion nuclear reactions were used to implant the¹⁰⁰Pd(3.6 d) activity into different host matrices. The sources thus prepared were found to have good strengths and small background activities. The yields were quite high 3Ci/1Ah beam current of¹²C⁴⁺ and the efficiencies of implantation were in the range 60–70%. The difference in the recoil energies of the target and the catcher may be used to reduce activities due to unwanted nuclear reactions in the catcher foil. Using this method, we were able to prepare excellent sources of¹⁰⁰Pd in Be, Mg, Ti, Zr and Re hosts. The quality of these sources is comparable to that of the¹⁰⁰PdBe ion implanted sources for which data are available in the literature.     

γ射线对含氢正比计数管中子能谱测量的影响

通过蒙特卡罗模拟和实验测量相结合的方法,获得从50keV~10MeV区间γ射线在直径为30mm内充1.013 25MPa氢气的球形含氢正比计数管上的能量响应,结果显示,γ射线在该计数管中的能量沉积主要集中在100keV附近及以下。Am-Be中子源和137 Cs源的实验测量结果显示,强137 Cs源的γ射线会严重影响含氢正比计数管对Am-Be中子源100keV以下能谱的测量,这表明,裂变材料介质内的强γ射线同样会影响到介质内100keV以下中子能谱的测量。根据计数管对反冲质子和电子电离信号的收集特性,采用上升时间法甄别掉本底γ射线是可行的。     

Uncertainties induced by statistical fluctuations in fast monoenergetic neutron yield measurements

A plastic scintillation detector was used to measure the yield of deuterium-deuterium (DD) neutrons or deuterium-tritium (DT) neutrons. Collisions of fast neutrons with hydrogen nucleus in a scintillator generated recoil protons, the energies of which were fully deposited in the scintillator. The statistical fluctuation of the protons' number and that of the protons' total energy were two sources of measurement uncertainty. Based on DT neutrons, this paper represents the algorithms of computing the probability density functions of the two sources. Uncertainties of the measurement induced by statistical fluctuations were finally computed by constructing the probability density functions of the proton number and that of the neutron number.     

A semiempirical formula describing the recoil yield in silicon

Krypton ions in the energy range 20–300 keV are used to generate recoiling atoms in silicon from thin layers evaporated on its surface. The recoil yields and the impurity distributions in the substrate have been measured as a function of several parameters (energy, thickness of the layer, incident dose). The results are used to propose a new formulation of the recoil yield based on the possibility, for both projectiles and recoiling atoms, to remove impurities previously introduced in silicon.The calculation fits very well the experimental results using displacement energies close to the generally admitted values     

Measure of low radioactive contamination and nuclear recoil quenching factor using cryogenic particle detectors

For double beta decay and dark matter experiments the determination of the radioactive contamination of materials used is of fundamental importance. The cryogenic particle detectors allow the realization of detectors with various types of materials and in this way the radioactivity is measured in a calorimetric approach with very high sensitivity. In particular a new complete analysis for different types of leads, modern and ancient roman, was realized and here presented. The detector sensitivity for a nuclear recoil event is a crucial point in the dark matter experiments. The nuclear recoil detection efficiency for cryogenic particle detectors was directly measured using two TeO₂ crystals facing one with other and performing a coincidence-anticoincidence analysis of the collected data. The determination of the quenching factor for low energy nuclear recoils in the TeO₂ cryogenic detectors shows that these devices are very sensitive to a possible dark matter particle interaction. In cryogenic detectors a nuclear recoil generates signals with the same amplitude of an electron with the same energy. The performed measure and the data analysis will be discussed in detail. Finally the experimental setup and the preliminary test of the new array of TeO₂ cryogenic detectors for double beta decay and dark matter search will be discussed in detail.     

Resonant Ionization Laser Ion Source Project at TRIUMF

Resonant laser excitation and ionisation is one of the most successful tools for the selective production of radioactive ion beams (RIB) at on-line mass separator facilities. TRIUMF plans to augment the current ion sources with a resonant ionisation laser ion source (RILIS), to use the high production yields from the target, as shown by the delivery of 3*10⁴/s ¹¹Li ions from a standard target ion source with surface ionisation. The development and installation of TRIUMF's RILIS (TRILIS) is necessary to provide beams of short lived isotopes that conventional ion sources could not produce in sufficient intensity and purity for nuclear-, and nuclear astrophysics- experiments. A laser system consisting of three tunable titanium–sapphire (TiSa) lasers with frequency doubling and tripling was employed to demonstrate first off-line resonance ionisation of Ga, and is being installed for first on-line test and a run on ⁶²Ga in December 2004.     

Characterization of the neutron field from the 241Am-Be isotopic source of the IPHC irradiator

A measurement campaign has been carried out recently to provide the source intensity and the reference spectra around a neutron irradiation facility based on ²⁴¹Am-Be radionuclide source, using the UAB Bonner Sphere Spectrometer. This facility, which consists of a bunker, a container/shielding for the source and an irradiation device that uses an automated remote-controlled system for the source positioning and rotating during the dosimeter irradiation, is intended to be routinely used to check the response of passive dosimeters, namely those based on photo-stimulated imaging plates and solid-state nuclear track detectors. The measurement results, in terms of neutron spectra and global dosimetric quantities (i.e., fluence and ambient dose equivalent rates) at different distances with respect to the ²⁴¹Am-Be source, were compared with Monte Carlo simulations using the MCNPX code and a good agreement was observed. An estimation of the un-scattered neutron spectrum directly emitted from the ²⁴¹Am-Be source is given as well.     

中子管非中子产物性质

采用两只经过标定的ST401闪烁探测器,测量了脉冲中子管的中子产额,在其中一只探测器前端增加铅板屏蔽,1 cm的铅屏蔽使探测器输出减少了18.20%,在加速器中子源上进行的类似实验表明,0.5 cm的铅使探测器的输出减少了2.90%。对两个中子源上测量的情况进行了蒙特卡罗模拟,加速器实验与模拟符合较好,脉冲中子管实验差别较大。对实验和模拟的情况进行了分析,结果表明:中子管除产生中子外,还会产生数量较多的轫致辐射X射线,这些X射线对准确测量中子管中子产额将造成不良影响。     

近玻尔速度Ne2+离子穿过碳膜引起的电子发射

本文测量了入射能为2–25 keV/u的Ne²⁺离子穿过不同厚度碳膜诱导的前向、后向 (分别对应出射表面和入射表面) 电子发射产额. 实验中通过改变炮弹离子的能量, 系统的研究了势能沉积、电子能损以及反冲原子对前向、后向电子发射产额的贡献. 结果表明, 离子的势能沉积只对后向电子发射有贡献, 前向、后向电子发射产额分别与Ne²⁺离子在薄膜出射、入射表面的电子能损近似成正比关系, 其中电子能损很低 (对应于离子能量很低) 的时候, 反冲原子对电子发射的贡献不能忽略. 关键词： 近玻尔速度 电子发射 电子能损 反冲原子     

The slowing down of atoms with energies ofE<10 eV in crystals

The line shape of the 773 keV emission line emitted by¹⁸⁷Re depends on the slowing down of the recoil the atom receives in the precedingβ transition. Using the nuclear resonance fluorescence method the line profile was studied for sources where the radioactive atoms are embedded in single crystals of W and WSe₂ and in polycrystalline samples of Nb. The experimental results for W and Nb are in complete agreement with predictions by a theory in which the slowing down is calculated starting from a Born-v. Karman lattice model. The temperature dependence of the line shape yields the phonon lifetimes in W and Nb (W) to beτ(300 K)=(2.4±0.3)ps andτ(300 K)=(1.7±0.2) ps, respectively. Measurements for the slowing down of the recoiling atoms perpendicularly to and along the hexagonal axis of WSe₂ yielded an anisotropy in qualitative agreement with expected values.     

Comparison of unfolding approaches for monoenergetic and continuous fast-neutron energy spectra

Proton-recoil detectors offer the possibility to unfold fast-neutron energy spectra of various sources. However, quantifying the confidence of the unfolding methodology is a complex task. In this paper, we present a comparative analysis of the maximum-likelihood, expectation-maximization (MLEM) method and one-step-late (OSL) method for neutron energy spectra unfolding. The analysis is performed on Monte Carlo simulated data for several monoenergetic neutron sources and continuous-in-energy ²⁵²Cf, ²⁴¹Am–Be and ²⁴¹Am–Li neutron sources. The results obtained for the monoenergetic neutron spectra show that both unfolding methods provide results that are in good agreement with the reference data. Very good agreement between the unfolded and the reference data is achieved for ²⁵²Cf, ²⁴¹Am–Be, and ²⁴¹Am–Li neutron spectra by using the OSL method. In the paper it is demonstrated that the MLEM and OSL methods can be applied to accurately unfold the simulated pulse-height distributions for organic liquid scintillation detectors. Comparative analysis between the two unfolding methods has shown that the OSL method has superior unfolding performance than the MLEM method.     

Mass distribution in the bremsstrahlung-induced fission of Th,U and Pu

The yields of various fission products in the 10 MeV bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were determined using a recoil catcher and off-line γ-ray spectrometric techniques. From the yield data, mass yield distributions were obtained using charge distribution corrections. The higher yields of fission products around mass numbers 133–135, 138–140, 143–145 and their complementary products in the neutron and bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were interpreted based on nuclear structure effects. From the mass yield distribution, the peak-to-valley (P/V$P/V$) ratio was also obtained for the above fissioning systems. The present data, along with data from the literature on different bremsstrahlung- and mono-energetic neutron-induced fissions of ²³²Th and ²³⁸U are interpreted to examine the influence of excitation energy on the peak to valley ratio. For the same compound nucleus ²⁴⁰Pu^?, the data in the 10–30 MeV bremsstrahlung-induced fission of ²⁴⁰Pu were compared with similar data of thermal to 14 MeV neutron-induced fission of ²³⁹Pu and the spontaneous fission of ²⁴⁰Pu to examine the role of excitation energy due to bremsstrahlung radiation and mono-energetic neutrons.     

Site-specific recoil diffraction of backscattered electrons in crystals

A novel diffraction effect in high-energy electron backscattering is demonstrated: the formation of element-specific diffraction patterns via nuclear recoil. For sapphire (Al(2)O(3)), the difference in recoil energy allows us to determine if an electron scattered from aluminum or from oxygen. The angular electron distribution obtained in such measurements is a strong function of the recoiling lattice site. These element-specific recoil diffraction features are explained using the dynamical theory of electron diffraction. Our observations open up new possibilities for local, element-resolved crystallographic analysis using quasielastically backscattered electrons in scanning electron microscopy.     

Neutron-induced nuclear recoil background in the PandaX-4T experiment

Neutron-induced nuclear recoil background is critical to dark matter searches in the PandaX-4T liquid xenon experiment. In this study, we investigate the features of neutron background in liquid xenon and evaluate its contribution in single scattering nuclear recoil events using three methods. The first method is fully based on Monte Carlo simulations. The last two are data-driven methods that also use multiple scattering signals and high energy signals in the data. In the PandaX-4T commissioning data with an exposure of 0.63 tonne-year, all these methods give a consistent result, i.e., there are \begin{document}$ 1.15\pm0.57 $\end{document} neutron-induced backgrounds in the dark matter signal region within an approximated nuclear recoil energy window between 5 and 100 keV.     

CROSS SECTION MEASUREMENT FOR THE REACTION 56Fe（n,p）56Mn

The cross section measurement for the reaction ⁵⁶Fe（n, p）⁵⁶Mn was made using the activation method from 12 MeV to 18 MeV. The absolute cross section was determined at 14.61±0.20 MeV, the value obtained was 108.0±2.7mb. The associated particle method was used to determine the absolute neutron flux and compared with the recoil proton telescope. The agreement between both methods appeared to be within 1—2%. The resulting ⁵⁶Mn activity was measured by using a φ10×7.6cm NaI （Tl） scintillation counter. The counting efficiency of the counter was achieved by means of a standard ⁵⁶Mn source calibrated by the 4πβ-γ coincidence method. The result measured was compared with existing data.