Studies of weak capture-γ-ray resonances via coincidence techniques

A method for measuring weak capture-γ-ray resonances via γγ-coincidence counting techniques is described. The coincidence apparatus consisted of a large-volume germanium detector and an annular NaI(Tl) crystal. The setup was tested by measuring the weak E_R=227 keV resonance in ²⁶Mg(p,γ)²⁷Al. Absolute germanium and NaI(Tl) counting efficiencies for a range of γ-ray energies and for different detector–target geometries are presented. Studies of the γ-ray background in our spectra are described. Compared to previous work, our method improves the detection sensitivity for weak capture-γ-ray resonances by a factor of ≈100. The usefulness of the present technique for investigations of interest to nuclear astrophysics is discussed.     

γ-ray energies for calibration from the decay of Tb, Hf + Lu, and Am

The energies of a few γ-rays from 23 to 125 keV from the decay of the radioisotopes ¹⁶¹Tb, ¹⁷²Hf+ ¹⁷²Lu, and ²⁴¹Am have been determined. These energies were obtained from measurements of differences in the energies of closely spaced lines. The statistical components of the uncertainties of these energy differences range from 0.1 to 1.9 eV. Since it is a long-lived nuclide, these results should make ¹⁷²Hf, which has γ-rays from 23 to 125 keV, a useful energy calibration source. For ²⁴¹Am, our value of 59.5412(2) for this widely used line differs from the previously accepted value of 59.537(1) keV.     

Dynamic nuclear polarization in crystals of Nd:LaAlO3, a polarized La target for a test of time-reversal invariance

Dynamic nuclear polarization (DNP) has been performed in a magnetic field of 2.35 T at a temperature below 0.3 K in crystals of Nd³⁺ :LaAlO₃. A Q-meter NMR-system was used to measure the polarization of the ²⁷Al and ¹³⁹La nuclei and allowed to characterize the quality of the crystals. The ¹³⁹La polarizations of +47.5% and −56.5% achieved represent an important step in the development of a polarized ¹³⁹La target for a test of time reversal invariance in low-energy neutron reactions.     

Construction of monoenergetic neutron calibration fields using 45Sc(p, n)45Ti reaction at JAEA

The 8 and 27 keV monoenergetic neutron calibration fields have been developed by using (45)Sc(p, n)(45)Ti reaction. Protons from a 4-MV Pelletron accelerator are used to bombard a thin scandium target evaporated onto a platinum disc. The proton energies are finely adjusted to the resonance to generate the 8 and 27 keV neutrons by applying a high voltage to the target assemblies. The neutron energies were measured using the time-of-flight method with a lithium glass scintillation detector. The neutron fluences at a calibration point located at 50 cm from the target were evaluated using Bonner spheres. A long counter was placed at 2.2 m from the target and at 60 degrees to the direction of the proton beam in order to monitor the fluence at the calibration point. Fluence and dose equivalent rates at the calibration point are sufficient to calibrate many types of the neutron survey metres.     

The use of a gas-filled-magnet in experiments with radioactive ion beams

A detection system has been developed for experiments with radioactive beams which have considerable contamination from a stable isobar. The system uses the gas-filled magnet technique and identifies reaction products according to nuclear charge and mass with good resolution even at energies below 1 MeV/u. In a first experiment the device has been used to measure cross sections for the ¹⁸F(p, )¹⁵O reaction in the energy range between 650 and 840 keV/u.     

Excited levels in Po and Pb fed by β-decays of Bi (T1/2 = 45.6 min) and Tl (T1/2 = 2.2 min)

The γ-decay of radiochemically separated ²¹³Bi sources (T_1/2 = 45.6 min), in equilibrium with ²⁰⁹Tl (T_1/2 = 2.2 min) and ²¹³Po (T_1/2 = 4.2 μs), has been reinvestigated using coaxial HPGe detectors, a LEPS detector and the γ-γ coincidence method. The energies and intensities of 20 γ-lines were measured, of which 11 are reported for the first time. Of the ten γ-rays attributed to ²¹³Bi β-decay, five are new. All of them were interpreted in a ²¹³Po level scheme using γ-γ coincidence data. New levels at 292.8, 1003.6 and 1119.5 keV are proposed in ²¹³Po. In the ²⁰⁹Tl β-decay, five γ-lines were measured, two of which are new. On the basis of γ-γ coincidence, we suggest that two new levels of 3069.6 and 3388.9 keV are fed into the ²⁰⁹Pb magic nucleus.     

Measurement of high momentum transfer reactions by recoil detection

A method is described for studying reactions at medium energies by measuring the magnetic rigidity, time of flight, and energy loss of the resulting recoil products. The method works best for reactions with two-body final states where one of the outgoing particles is relatively light. A magnetic spectrometer with a special focal plane detector has been used. The detector consists of two position sensitive parallel plate avalanche counters and two proportional counters. Differential cross sections for the ¹²C(p, π⁺¹³C reaction at recoil angles which correspond to the pion being emitted at a backward angle in the center of mass have been measured at bombarding energies of 166 and 186 MeV. Data were also obtained for previously unobserved ¹²C(p, π⁰)¹³N reaction. Several advantages of the method are discussed.     

富集10B的Ce:Li6Lu(10BO3)3晶体生长及其闪烁性能

为了提高中子探测效率, 以富集¹⁰B的H₃¹⁰BO₃为原料, 通过提拉法生长了富集¹⁰B的Ce:Li₆Lu(¹⁰BO₃)₃晶体。X射线激发发射光谱测试表明: 其发光峰位于360~480 nm, 属于Ce³⁺离子典型的5d - 4f跃迁发光, 其闪烁发光效率为BGO晶体的3.9倍。在350 nm紫外光和¹³⁷Cs所发出的662 keV的γ射线激发下测得的衰减时间分别为21.0 ns 和31.7 ns, 在¹³⁷Cs辐射源激发下所测得的相对光输出是CsI(Tl)晶体的20%, 能量分辨率为9.7%。在慢化²⁵²Cf中子源激发下可以观测到明显的中子全能峰, 其能量分辨率为33%。上述研究结果表明, Ce:Li₆Lu(¹⁰BO₃)₃晶体具有较高的闪烁效率、快的衰减时间和良好的中子探测效率, 是一种具有应用前景的中子探测用闪烁晶体。     

Thermal neutron cross-section and resonance integral for Dy(n,γ)Dy reaction

The thermal neutron cross-section (σ₀) and the resonance integral (I₀) of the reaction ¹⁶⁴Dy(n,γ)¹⁶⁵Dy were measured by the activation method, using ⁵⁵Mn(n,γ)⁵⁵Mn monitor reaction as a single comparator. The diluted MnO₂ and Dy₂O₃ powder samples within and without a cylindrical Cd shield case were irradiated in an isotropic neutron field obtained from the ²⁴¹Am–Be neutron sources, moderated with paraffin wax. The γ-ray spectra from the irradiated samples were measured by high-resolution γ-ray spectrometry with a calibrated n-type Ge detector. The necessary correction factors for γ-ray attenuation, thermal neutron and resonance neutron self-shielding effects and epithermal neutron spectrum shape factor () were taken into account in the determinations. The thermal neutron cross-section for ¹⁶⁴Dy(n,γ)¹⁶⁵Dy reaction studied has been determined to be 2672±104 b at 0.025 eV. This result has been obtained relative to the reference thermal neutron cross-section value of 13.3±0.1 b for the ⁵⁵Mn(n,γ)⁵⁶Mn reaction. For the thermal neutron cross-section, most of the experimental data and evaluated one in ENDF/B-VI, in general, are in good agreement with the present result. The resonance integral has also been measured relative to the reference value of 14.0±0.3 b for the ⁵⁵Mn(n,γ)⁵⁶Mn monitor reaction using a 1/E¹⁺ epithermal neutron spectrum of the ²⁴¹Am–Be neutron source. By defining Cd cut-off energy 0.55 eV, the resonance integral obtained was 527±89 b. The existing experimental and evaluated data for the resonance integral are distributed from 335 to 820 b. The present resonance integral value agrees with some previously reported values, 520 b by Holden, 505 b by Simonits et al. and 575±100 b by Heft, within the limits of error.     

An accurate redetermination of the Sn binding energy

The energy of well-known strong γ line from ¹⁹⁸Au, the “gold standard”, has been modified in the light of new adjustments in the fundamental constants and the value of 411.80176(12) keV was determined, which is 0.29 eV lower than the latest 1999 value. An energy calibration procedure for determining the neutron binding energy, B_n, from complicated (n, γ) spectra has been developed. A mathematically simple minimization function consisting only of terms having as parameters the coefficients of the energy calibration curve (polynomial) is used. A priori information about the relationships among the energies of different peaks on the spectrum is taken into account by a Monte-Carlo simulation. The procedure was used in obtaining B_n for ¹¹⁸Sn. The γ-ray spectrum from thermal neutron radiative capture by ¹¹⁷Sn has been measured on the IBR-2 pulsed reactor. γ-rays were detected by a 72 cm³ HPGe detector. For a better determination of B_n it was important to determine B_n for ⁶⁴Cu. This value was obtained from two γ-spectra. One spectrum was measured on the IBR-2 by the same detector. The other spectrum was measured with a pair spectrometer at the Brookhaven High Flux Beam Reactor. From these two spectra, B_n for ⁶⁴Cu was determined to be equal to 7915.52(8) keV. This result essentially differs from the previous value of 7915.96(11) keV. The mean value of the two most precise results of the B_n for ¹¹⁸Sn, was determined to be 9326.35(9) keV. The B_n for ⁵⁷Fe was determined to be 7646.08(9) keV.     

Monte Carlo calculation of the response function for a He neutron spectrometer

The response of a gridded ³He ionization chamber to monoenergetic neutrons has been calculated using a Monte Carlo approach. The effects of neutron scattering on detector materials, wall effects, recoil continua and background neutrons are included. The calculated results are smoothed according to a peak shape taken from experiment. Response functions in the energy range E_n < 1217 keV are compared to experimental data obtained with the ⁷Li(p, n)⁷Be reaction and to the detector efficiency derived from them.     

Emission probability measurement of γ-ray of Rh

¹⁰⁵Rh becomes a stable ¹⁰⁵Pd after β⁻-decay with a half-life of 35.36 h. The energies of especially strong γ-rays emitted from ¹⁰⁵Rh are 306.1 and 318.9 keV, and the emission probabilities are evaluated to be 5.1±0.3% and 19.1±0.6% by de Frenne et al. To improve the certainty, the γ-ray emission probabilities were determined from the disintegration rate and absolute γ-ray intensities measured using a 4πβ(ppc)-γ(HPGe) coincidence apparatus with two-dimensional data-acquisition system. The results for the 306.1 and 318.9 keV γ-rays were 4.76±0.05% and 16.99±0.17%, respectively.     

Detection of γ-rays with a 3.5 l liquid xenon ionization chamber triggered by the primary scintillation light

A gridded ionization chamber with a drift length of 4.5 cm and a total volume of 3.5 l, was operated with high-purity liquid xenon and extensively tested with γ-rays from ¹³⁷Cs, ²²Na and ⁶⁰Co radioactive sources. An electron lifetime in excess of 1 ms was inferred from two independent measurements. The electric field dependence of the collected charge and energy resolution was studied in the range 0.1–4 kV/cm, for different γ-ray energies. With an electric field of 4 kV/cm, the spectral performance of the detector is consistent with an energy resolution of 5.9% at 1 MeV, scaling with energy as E^−0.5. The chamber was also used to detect the primary scintillation light produced by γ-ray interactions in liquid xenon. The light signal was successfully used to trigger the acquisition of the charge signal with a FADC readout. A trigger efficiency of 85% was measured at 662 keV.     

Nb3Al AND Nb3(Al,Ge) SUPERCONDUCTING TAPES BY LASER BEAM ANNEALING

Nb₃Al AND Nb₃(Al,Ge) tapes with excellent superconducting properties have been fabricated using high energy density laser beam irradiation, irradiations were carried out continuously on Nb-25at%AI and Nb-20at%Al -5at%Ge tapes prepared by powder metallurgical processing. With the high power density and short irradiation time, the tape could be heated and cooled much faster than a tape heat treated by conventional methods. The results were fine structure and large critical current density J_c values at high magnetic fields. J_c at 23 Tesla was over 10⁴ A/cm². This value was much greater than that obtained from a commercial Nb₃Sn superconductor. New continuous irradiation systems have been developed for scale-up development. Nb₃Al tapes 10 m in length were successfully fabricated. There was some scatter in J_c along the tape length. Small coil tests in magnetic fields indicated that the tape had excellent stability against a flux jump and mechanical strain.     

Double atomic-field bremsstrahlung from thick targets

Measurements of the two-photon coincidence rate from 84 keV electrons from a ¹⁰⁹Cd source on thick targets of Ag, Au, and Pb are compared with a thick-target double atomic-field bremsstrahlung (TBD) model. Each photon energy range was 32–52 keV. The target thickness ranged from 3 to 30 mg/cm². Results for the thick-target single-bremsstrahlung (TSB) energy spectrum for 64 keV electrons in coincidence with the 22 keV X-ray from the source will also be compared with the TSB theory.     

A segmented target chamber for (n,p) studies at TRIUMF

We have developed an (n, p) target chamber consisting of six target layers sandwiched between seven gas detector layers. This target chamber is now commonly used with the new (n, p) facility at TRIUMF. With this arrangement, large total (n, p) target thicknesses (> 1 g/cm²) can be used and yet the target contribution to overall energy resolution is moderate, typically less than 500 keV. The simultaneous accumulation of data from six target layers has proven to be highly advantageous with a number of applications, such as; (1) using multiple thicknesses of one target material, (2) including a target of known cross-section in the target stack for normalization, (3) adding a target to obtain simultaneous spectra for later subtraction of contamination lines and (4) the use of several target materials at once.     

Studies on the production of monoenergetic neutrons by uniform moderation of d-t neutrons

Investigations have been carried out on the production of approximately monoenergetic neutrons in the fusion technologically important 8 to 12 MeV range through the controlled moderation of 14 MeV d-t neutrons scattering off an hydrogenous scatterer (converter) shaped into the form of a surface of revolution. The centre of the source of the primary neutrons was arranged to lie on one of the two points of intersection of the surface of revolution with its axis of symmetry whilst the secondary moderated neutrons were received at the other point of intersection. A copper beam stopper prevented the primary beam from reaching the secondary beam point. Intensities and energies of the secondary beam have been calculated for a paraffin converter. The contamination of the beam resulting from the scattering by carbon nuclei in the converter, together with the effects of multiple scattering in the converter and the beam stopper and deviations caused by nonideal geometry present in practical cases have been discussed.

A recoil proton fast neutron spectrometer based on a NE 102 plastic scintillator has been fabricated. The agreement between the calculated and the measured values of the different components of the secondary beam was found to be quite satisfactory within the estimated precision limits of the experiment and the calculation.

The secondary beam has been used to measure several (n, p) cross sections of ²⁸Si and ²⁷Al for neutrons in the energy range from 8.6 to 12.1 MeV.     

The production of monoenergetic fast neutrons of known flux: An improved method to identify the associated particle

The performance of a deuterium gas target to produce neutrons from the D(d, n)³He reaction is reported. The associated particle method is used. The associated particle is unequivocally identified with a telescope employing microchannel-plate electron multipliers and a silicon surface barrier detector, providing both time-of-flight (TOF) and energy (E) information. The system was set up at the 7 MV Van de Graaff CN accelerator of the laboratori Nazionali di Legnaro (Padova), and has been tested with deuteron energies from 2.5 to 6.5 MeV. About 300 tagged neutrons per s sr nA atm are produced. The TOF-E identification method to discriminate the associated particle will be used in the future in measurements at higher deuteron energies and with the reaction T(d, n)⁴He.     

Precision energy and intensity measurements for γ transitions from Cs decay

The precision energies and relative intensities of 11 γ transitions from the decay of ¹³⁴Cs have been determined. ¹⁹²Ir, ²²⁸Th, ¹³⁷Cs, ^110mAg, ⁶⁰Co, and ²⁰⁷Bi served as energy calibration standards, and ¹⁵²Eu and ¹⁹²Ir as relative intensity calibration standards. Special care was taken in the analysis of data for the weaker transitions. The results are incorporated into a self-consistent decay scheme.     

(0.96NaNbO3-0.04CaZrO3)-xFe2O3反铁电陶瓷的介电及储能性能研究

0.96NaNbO₃-0.04CaZrO₃(简称NNCZ)陶瓷在室温下展现出稳定的双电滞回线, 但是其储能密度、储能效率和击穿强度都比较低, 限制其成为储能材料。本工作通过掺杂Fe₂O₃, 利用Fe ³⁺离子变价的特点, 实现NNCZ储能性能的优化。采用传统固相法制备了(0.96NaNbO₃-0.04CaZrO₃)-xFe₂O₃(简称NNCZ-xFe)反铁电储能陶瓷, 并对样品的相结构、微观形貌、电学性能和储能性能进行了表征, 重点研究了Fe₂O₃掺杂量对NNCZ陶瓷介电和储能性能的影响规律。结果表明, 样品均具有单一的钙钛矿结构, 掺杂Fe₂O₃能明显降低NNCZ陶瓷的烧结温度, 晶粒平均尺寸随着掺杂量增大先减小后增大, 掺杂量x=0.02时, 晶粒平均尺寸最小(5.04 mm), 且具有较好的储能性能。室温下, NNCZ-0.02Fe击穿强度为230 kV/cm, 击穿前的有效储能密度和储能效率分别为1.57 J/cm ³和55.74%。在125 ℃和外加电场为180 kV/cm下, NNCZ-0.02Fe的储能密度为4.53 J/cm ³。掺杂Fe₂O₃使NNCZ陶瓷的烧成温度降低, 氧空位的迁移速率下降, 抑制晶粒的长大, 同时降低了介电损耗, 使得击穿强度增加; 适量氧空位钉扎使得反铁电相向铁电相相翻转变得困难, 避免出现哑铃状双电滞回线, 从而提高储能效率。本研究结果表明NNCZ-xFe在电介质储能领域具有潜在应用价值。