γ全吸收型探测装置探测效率的实验与模拟

为精确测量keV能区中子俘获反应截面,中国原子能科学研究院核数据重点实验室基于中国散裂中子源反角白光中子源建成了国内首套γ全吸收型BaF₂探测装置。为获得重要的实验参数装置对γ射线的探测效率曲线,对单个BaF₂探测器模块能谱的测量数据与模拟结果进行比较。结果表明,测量¹³⁷Cs和⁶⁰Co源得到的实验结果与MCNP和GEANT4的模拟结果吻合较好,验证了模拟计算得到的探测效率曲线的可靠性,可用于中子俘获反应截面的在线测量。     

基于Gamma全吸收型BaF2探测装置的数据获取系统

为精确测量keV能区的中子俘获反应截面,研制了一套基于Gamma全吸收型BaF₂探测装置的数据获取系统。数据获取系统工作在Linux环境下,应用FlashADC采集探测器的脉冲信号。程序控制、数据存储、在线监控与分析程序基于Root平台。通过测量⁶⁰Co和¹³⁷Cs源,得到了数据获取系统主要性能的测试结果,其满足GTAF装置精确测量中子俘获反应截面的要求。     

4π BaF2装置的触发系统研究

设计了一套γ全吸收型BaF₂探测装置,用于中子辐射俘获反应截面的在线测量,其中信号测量部分采用了基于高速采集卡的数字化脉冲波形采集技术。并设计了一套触发系统,由触发单元产生的信号作为数据获取系统的外触发控制数据采集,以在数据测量过程中剔除绝大多数本底事件。触发系统采用了符合测量技术,通过多通道、多参数之间的符合测量,实现释放瞬发级联γ射线的中子俘获反应事件的判选和本底的剔除。本文对触发系统的电路设计和实验测试进行了阐述。     

氘-氘聚变反应高能γ射线产额的实验研究

本文对低能D(d,γ)⁴He聚变反应进行了实验研究。由于D(d,γ)⁴He聚变反应γ射线的产额低、能量高、本底大,实验采用大NaI-塑料闪烁体反康谱仪,以提高探测效率并降低宇宙线的影响;采用脉冲束飞行时间方法区分中子和γ射线,以减小中子和宇宙线本底的影响。实验测量了300 keV d束轰击厚D靶和薄D靶两种情况下的γ数据。在计算反应产生23.8 MeV的高能γ峰面积时,采用了拟合法扣除本底。实验测得,对厚靶,D(d,γ)⁴He反应截面为2.47×10^-37m²;对薄靶,D(d,γ)⁴He反应截面为4.36×10^-37m²。     

BaF2闪烁体探测器信号数字化方法研究

应用数字化方法对BaF₂闪烁体探测器脉冲信号进行了研究,信号的测量通过FADC和计算机系统实现,并对所采集的脉冲波形进行了离线分析。通过计算脉冲波形的面积提取了射线的能量信息,重建了能谱,并与传统电子学模拟信号测量方法得到的能谱进行了比较;应用过阈定时方法提取了脉冲的时间信息,测量并给出了4πBaF₂探测器系统的时间分布谱;应用脉冲形状甄别法对α粒子和γ射线进行了鉴别研究,根据脉冲信号快/慢成分比的差别清楚地将α粒子从γ射线中鉴别开来。本工作为在γ全吸收型4πBaF₂探测器系统中应用FADC进行多通道、多参数、大数据量数字化数据获取积累了经验。     

中子俘获反应截面测量研究

正建造了一套γ全吸收型4πBaF2探测装置,用于高精度测量中子俘获反应截面。基于HI-13串列加速器提供的脉冲化质子束,通过7Li(p,n)7Be反应产生中子,构建了一个keV能区中子源实验条件。中子源的靶头设计如图1所示。经屏蔽准直后的中子轰击样品,应用4πBaF2装置在线测量(n,γ)反应复合核退激时释放的瞬发γ射线级联,实验布局如图2所示。     

GdI3:Ce闪烁体中子探测性能测试与改进研究

为改善GdI₃:Ce闪烁体在探测中子过程中的γ抑制能力,使用Geant4和XCOM计算了其γ线性吸收系数,并通过模拟计算与实验测量研究了铅屏蔽法抑制γ的有效性。结果表明：GdI₃:Ce闪烁体在探测中子过程中易受低能γ射线的干扰;随着铅层厚度的增加,100 keV~1 MeV的γ射线对中子探测的干扰减小,而3~10 MeV的γ射线的干扰呈先增加后减小的趋势。对²⁵²Cf中子源的实验测试发现,在碘化钆闪烁体外围添加铅层后,中子峰得以显现;随着铅层厚度的增加,中子峰内净计数减小,而净计数与本底计数的比值上升。模拟和实验结果均表明,在使用GdI3:Ce闪烁体探测中子时,应根据中子探测效率和信噪比的优化确定γ屏蔽铅层的厚度。     

熔盐堆钍铀、铀钚燃料循环核数据不确定度分析

反应堆物理设计不确定度是第4代核能系统的QMU（quantification of margins and uncertainties）有效性认证所必须的参数之一,核数据不确定度是其重要来源。基于自主开发的耦合程序BUND（burnup uncertainty of nuclear data）,将SCALE程序TRITON和TSUNAMI-3D模块耦合,完成了熔盐堆钍铀燃料循环、铀钚燃料循环核数据引起的有效增殖因数k_eff不确定度分析,并与ENDF/B-Ⅶ.1协方差数据库计算结果进行了对比。结果显示：初始时刻,两种燃料循环模式下,核数据导致的k_eff不确定度分别为0.490%和0.582%。随燃耗的增加,核数据引起的k_eff不确定度增加。寿期末,两种燃料循环模式下,对k_eff不确定度影响显著增加的反应道分别为²³⁹Pu(nubar)、(n,f)、(n,γ)、¹⁰⁵Rh(n,γ)、¹³⁵Xe(n,γ)和²³⁴U(n,γ)、¹⁴³Nd(n,γ)、^131,135Xe(n,γ)等。     

240Am半衰期的测量

²⁴⁰Am的半衰期对准确测量²⁴¹Am(n,2n)²⁴⁰Am反应截面具有重要作用,当前评价的数据50.8(3) h是对²⁴⁰Am的987.8 keV γ射线用Ge(Li)探测器跟踪测量6 d的结果,测量时间不到3个半衰期,使得测量结果的不确定度偏大。本文利用Geant4模拟软件建立了阱型HPGe探测器的测量模型,模拟计算了不同Pb吸收厚度下²⁴⁰Am高能γ射线的探测效率,确定使用阱型HPGe探测器配合吸收X射线和低能γ射线的Pb吸收体可有效提高²⁴⁰Am高能γ射线的探测效率。根据Geant4模拟计算的结果,Pb吸收体厚度为1 mm时,对²⁴⁰Am的888.8 keV和987.8 keV两条特征γ射线的探测效率分别为14.1%和13.3%。在中国原子能科学研究院的HI-13串列加速器上通过²⁴²Pu(p,3n)反应生产了²⁴⁰Am,制备了约700 Bq的²⁴⁰Am测量源,用上述方法跟踪测量²⁴⁰Am的888.8 keV和987.8 keV两条特征γ射线的强度,时间超过18 d,用最小二乘法拟合得到其半衰期为50.79(5) h,结果与评价结果一致,但减小了不确定度。     

基于Fe(OH)3-CaCO3载带的水中228Ra的γ能谱分析方法

建立了一种基于Fe(OH)₃-CaCO₃载带的水中²²⁸Ra的γ能谱分析方法,适用于环境水中²²⁸Ra的分析。采用Fe(OH)₃-CaCO₃共沉淀法富集水中的²²⁸Ra,将富集后的²²⁸Ra采用Ba(Ra)SO₄共沉淀法进一步载带,¹³³Ba示踪法确定²²⁸Ra的全程回收率,使用高纯锗γ谱仪测量与²²⁸Ra达到放射性平衡的衰变子体²²⁸Ac的特征γ能量,从而获得²²⁸Ra的分析结果。通过对5 L水样的加标验证可知,²²⁸Ra回收率为81.8%~87.5%,加标水样测量结果与添加量的相对偏差为1.7%~5.3%,此方法的探测限为57.2 mBq/L。     

Measurement of the cross sections for the reactions Cr(n,2n)Cr, Zn(n,2n)Zn, Y(n,2n)Y and Zr(n,2n)Zr from 13.5 to 14.8 MeV

Cross section measurements for the reactions ⁵²Cr(n,2n)⁵¹Cr, ⁶⁶Zn(n,2n)⁶⁵Zn, ⁸⁹Y(n,2n)⁸⁸Y and ⁹⁶Zr(n,2n)⁹⁵Zr were carried out in the neutron energy range 13.47–14.79 MeV applying the activation technique. Neutrons were produced via the T(d,n)⁴He reaction, making use of the variation of neutron energy with the emission angle. The neutron fluences incident on the samples were determined relative to the well-evaluated cross section for the reaction ⁹³Nb(n,2n)^92mNb.

The induced γ-ray activities of the irradiated Zn, Zr and Y₂O₃ samples and their monitor foils were measured by means of a calibrated Ge(Li) γ-ray detector at the KFI, Debrecen. At the IRK, Vienna, relative γ-ray measurements using a high-purity Ge detector were combined with integral γ-ray counting by means of a NaI(TI) well-type detector on the Cr, Zn and Zr foils of highest activity and on some Nb monitor foils; integral γ-ray counting only was applied in the case of the Y₂O₃ samples. All necessary corrections were taken into account.

The results are compared to the corresponding results of cross section measurements published in the literature. The uncertainties obtained in this work are considerably smaller in most cases than the uncertainties given by other authors.     

Measurement of niobium reaction rate formaterial surveillance tests in fast reactors

A highly accurate and precise technique for measurement of the ⁹³Nb(n,n’)^93mNb reaction rate was established for the material surveillance tests,etc.in fast reactors.The self-absorption effect on the measurement of the characteristic X-rays emitted by ^93mNb was decreased by the dissolution and evaporation to dryness of niobium dosimeter.A highly precise count of the number of ⁹³Nb atoms was obtained by measuring the niobium solution concentration using inductively coupled plasma mass spectrometry.X-rays of ^93mNb were measured accurately by means of comparing the X-ray intensity of irradiated niobium solution with that of the solution in which stable ⁹³Nb was added.The difference between both intensities indicates the effect of ¹⁸²Ta,which is generated from an impurity tantalum,and the intensity of X-rays from ^93mNb was evaluated.Measurement error of the ⁹³Nb(n,n’)^93mNb reaction rate was reduced to be less than 4%,which was equivalent to the other reaction rate errors of dosimeters used for Joyo dosimetry.In addition,an advanced technique using Resonance Ionization Mass Spectrometry was proposed for the precise measurement of ^93mNb yield,and ^93mNb will be resonance-ionized selectively by discriminating the hyperfine splitting of the atomic energy levels between ⁹³Nb and ^93mNb at high resolution.     

The determination of the Nb(n,n′) Nb cross-section in the neutron energy range 1–6 MeV

Measurements of the 30 keV isomer production cross-section in ⁹³Nb, excited by neutron inelastic scattering have been made at 10 neutron energies in the range 1–6 MeV. Small foils of niobium and suitable monitor elements were exposed to high fluences of monoenergetic neutrons and the cross-section was determined from the K X-ray activities so induced. The high purity Ge detector used in these X-ray measurements was calibrated against a standard solution of ^93mNb which is used as an international reference material for reactor neutron dosimetry. The neutron fluence of the irradiation was measured with a low-efficiency ²³⁵U fission chamber in which the thin fissile deposit was located immediately behind the niobium and monitor foils. The latter acted as secondary measures of the neutron fluence but could be used as the primary standard in the event of failure of the fission chamber. Thus, the production cross-section was measured relative to the ²³⁵U fission cross-section which is a standard reference cross-section. Comparisons are made with other experimental data and with nuclear model calculations of the cross-section and recommended values based on these and our experimental data are presented.     

14 MeV快中子照相准直屏蔽系统的设计与优化

基于强流氘氚中子源科学装置HINEG设计了一套快中子照相准直屏蔽系统。采用中子输运设计与安全评价软件系统SuperMC和ENDF/B-Ⅶ.0数据库计算了准直中子束的中子能谱及注量率、γ射线能谱及注量率、直射中子注量率与γ射线注量率比值（φ_d/φ_γ）、直射与散射中子注量率比值（φ_d/φ_s）、准直束中子注量率的不均匀度等特性参数,并采用MCNP5程序进行了对比验证。研究了准直屏蔽系统的内衬材料、尺寸等对特性参数的影响规律,并通过优化获取了最优设计方案。计算结果显示,在同等计算条件下,SuperMC计算结果与MCNP计算结果相对偏差小于1%,准直屏蔽系统的φ_d/φ_γ为50.1,φ_d/φ_s为5.7,在Φ30 cm视野范围内的中子注量率为4.80×10⁷ cm^-2•s^-1,其中直射中子注量率为4.09×10⁷ cm^-2•s^-1,中子注量率不均匀度为5.8%,满足快中子照相对准直束特性参数的要求。     

GIC4117串列加速器外束PIXE/PIGE分析系统

本文介绍在北京师范大学GIC4117串列加速器上建立的外束PIXE/PIGE分析系统,和基于此系统的薄样品外束PIXE/PIGE定量分析方法。给出了2010年Teflon滤膜采集的361个气溶胶样品外束PIXE分析得到的各元素平均探测限和最低探测限,并同真空PIXE分析探测限进行了比较。利用标准样品给出了激发曲线不同坪区薄样品外束PIGE分析F和Na的探测限,通过测定¹⁹F(p, p′γ)¹⁹F激发的197 keV γ射线得到的F的探测限可达73.9 ng•cm^-2,Na的探测限可达198.9 ng•cm^-2。     

Scintillation properties of RbGd2Br7:Ce.Advantages and limitations

The scintillation properties of RbGd₂Br₇ crystals, doped with Ce³⁺ concentrations of 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8%, are studied under X-ray and γ-quanta excitations. For the RbGd₂Br₇ sample doped with 9.8% Ce, the authors measured a light yield of 56000±6000 photons per MeV of absorbed γ-ray energy with a main decay time of 43±1 ns, using a Hamamatsu R1791 photomultiplier (PMT), a ¹³⁷Cs radioactive source, and a shaping time of 10 μs. A time resolution of 790±10 ps was measured for the RbGd₂Br₇:9.8% Ce compound, using BaF₂ as second scintillator, two XP2020Q PMTs, a ²²Na source, and an energy threshold set at E⩾511 keV. With the R1791 PMT, an energy resolution of 4.1% (FWHM over peak position) for the 662-keV full absorption peak has been observed for two crystals of 7×4×2 mm³ and 15×5×1 mm³ with 4.1 and 9.8% Ce content, respectively. Moreover, the nonproportional responses of three RbGd₂Br₇:Ce compounds with different concentrations (0.11, 2.05, and 9.8%) were studied revealing an almost-constant light output response from 17.4 keV to 1 MeV. These properties are compared to three other well-known scintillators: NaI:Tl, CsI:Tl, and Lu₂SiO₅:Ce     

γ吸收剂量率在线探测用硅光电池的电学性能研究

建立了一种γ吸收剂量率实时在线测量系统,研究了半导体硅光电池BBZSGD-4辐射光生电流和γ吸收剂量率之间的关系,并对其耐辐照性能进行了研究。60 Coγ辐照实验表明:半导体硅光电池BBZSGD-4对60 Co的γ射线有较好的响应,其辐射光生电流与吸收剂量率的关系呈线性规律。当吸收剂量率为94.54Gy/min时,辐射光生电流可达1.26μA。在吸收剂量率为50Gy/min时,辐射光生电流随总吸收剂量的增加呈指数下降,总吸收剂量为5 445.8Gy时,其辐射光生电流衰减1%。半导体硅光电池BBZSGD-4具有作为实时在线低吸收剂量率探测器的潜力。     

用于n/γ混合场测量的涂硼电离室研制

研制了一种能同时测量混合场中γ和中子注量率的涂硼电离室,并实验测试了其性能。涂硼电离室由两个大小和结构一致的腔室组成：1个仅对γ灵敏,另1个对γ与中子均灵敏。用强度为2.7×10⁷ s^-1 的Am-Be源测得电离室的中子灵敏度达9.2×10^-16 A/(cm^-2•s^-1),在剂量率为5.24 μGy/h的¹³⁷Cs γ场中,电离室的γ灵敏度达7.36×10^-16 A/(MeV•cm^-2•s^-1)。涂硼电离室I-V曲线坪长为600 V,坪斜小于4%/100 V,在工作电压为-400 V时,其γ补偿修正系数<5%,可用于核设施周围的混合场监测。