Dose measurements around spallation neutron sources

Neutron dose measurements and calculations around spallation sources appear to be of great importance in shielding research. Two spallation sources were irradiated by high-energy proton beams delivered by the Nuclotron accelerator (JINR), Dubna. Neutrons produced by the spallation sources were measured by using solid-state nuclear track detectors. In addition, neutron dose was calculated after polyethylene and concrete, using a phenomenological model based on empirical relations applied in high-energy physics. The study provides an analytical and experimental neutron benchmark analysis using the transmission factor and a comparison between the experimental results and calculations.     

Characteristics of polyethylene-moderated 252Cf neutron sources

Polyethylene-moderated ²⁵²Cf neutron sources were designed to produce neutron reference fields’ spectra that simulate the spectra observed in the workplaces within nuclear reactors and accelerators. The paper describes the neutron sources and fields. Neutron spectra were calculated by the Monte Carlo method and compared with experimental data.     

CW accelerators suitable for spallation neutron sources

An accelerator-driven steady state neutron source will require a high current, high energy, 100% duty factor accelerator. No suitable accelerator has yet been built, but the technology to do so existsand examples of all of the likely components have been tested. This paper reviews some of the component test programs with comments on the problems specific to this application.     

The progress of radioisotope technology and application in China

The inception of radioisotope and its application in China are introduced. The research,development, production, application progress and the future development prospect of radioisotope and its products are described.     

Certifying and utilizing monoenergetic neutron sources of accelerators

Conclusions At present the problems of certifying monoenergetic neutron sources by means of an accelerator and their utilization for certifying various measurement means has not as yet been fully solved, since the metrological organizations have encountered this problem in relatively recent times. It was noted at the 1st All-Union Conference on the Metrology of Neutron Radiations, which was held in 1971 in Moscow, that reference and standard monoenergetic neutron sources are being produced in the USSR on the basis of ÉG-2M and ÉG-2.5 accelerators, and that the standard techniques in direct measurements in pulsed neutron fields are being developed [2]. Experience gained in carrying out this work has shown that the successful solution of the certification problems requires joint efforts of metrologists, measuring equipment developers, and customers.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 54–56, October, 1973.     

A brief introduction to neutron activation analysis in China

The history, the major achievements in both methodology and applications, the current trends and future perspectives of neutron activation analysis (NAA) in China are briefly described.     

Secondary photon fields produced in accelerator-based sources for neutron generation

Neutrons can be produced with low-energy ion accelerators for many applications, such as the characterisation of neutron detectors, the irradiation of biological samples and the study of the radiation damage in electronic devices. Moreover, accelerator-based neutron sources are under development for boron neutron capture therapy (BNCT). Thin targets are used for generating monoenergetic neutrons, while thick targets are usually employed for producing more intense neutron fields. The associated photon field produced by the target nuclei may have a strong influence on the application under study. For instance, these photons can play a fundamental role in the design of an accelerator-based neutron source for BNCT. This work focuses on the measurement of the photon field associated with neutrons that are produced by 4.0-6.8 MeV protons striking both a thin 7LiF target (for generating monoenergetic neutrons) and a thick beryllium target. In both cases, very intense photon fields are generated with energy distribution extending up to several MeV.     

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the ⁹Be(n,α)⁶He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R(E_n) is determined over the neutron energy range 2-4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10⁴ cm⁻², the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.     

Neutron calibration facility with radioactive neutron sources at KRISS

For the purpose of radiation protection, the reference neutron field for calibration of neutron monitors was constructed using radioactive neutron sources-bare-(252)Cf, D(2)O-moderated (252)Cf and (241)Am-Be(alpha,n)-at Korea Research Institute of Standards and Science (KRISS). The well-specified neutron source with its emission rate and the anisotropy was installed at the centre of the neutron irradiation room, which is 6.6 x 7.6 x 6.3 m(3) in size. The neutron spectra of each source was measured using the Bonner sphere spectrometer (KRISS-BSS). Calculations using MCNP5 with realistic geometry and materials in the neutron irradiation room were performed. The calculations and measurements were found to be in good agreement, showing that the neutron calibration facility at KRISS is well established.     

Development of neutron moisture gauge in China

According to measuring mode (in-hopper, surface, transmitted and scattered neutron moisture gauge), this paper introduced the development and application of neutron moisture gauge in China since 1970s.     

-Ray background sources in the VESUVIO spectrometer at ISIS spallation neutron source

An investigation of the gamma background was carried out in the VESUVIO spectrometer at the ISIS spallation neutron source. This study, performed with a yttrium–aluminum–perovskite (YAP) scintillator, follows high resolution pulse height measurements of the gamma background carried out on the same instrument with the use of a high-purity germanium detector. In this experimental work, a mapping of the gamma background was attempted, trying to find the spatial distribution and degree of directionality of the different contributions identified in the previous study. It is found that the gamma background at low times is highly directional and mostly due to the gamma rays generated in the moderator–decoupler system. The other contributions, consistently to the findings of a previous experiment, are identified as a nearly isotropic one due to neutron absorption in the walls of the experimental hall, and a directional one coming from the beam dump.     

Disturbance due to mechanical and thermal sources in a generalized thermo-microstretch elastic half space

Disturbances caused by impulsive concentrated mechanical and thermal sources in a homogeneous, isotropic generalized thermo-microstretch elastic medium are studied by the use of Laplace—Hankel transform techniques. The integral transforms are inverted using a numerical technique. Analytical expressions for displacement components, stress, couple stress, microstress and temperature field are derived for different models of generalized thermoelasticity and illustrated graphically. These results for stresses and displacements can be used in estimating the effects of a surface pressure wave. Stretch and micropolar effects on various expressions obtained analytically are also depicted graphically.     

Assessment of neutron dosemeters around standard sources and nuclear fissile objects

In order to evaluate the neutron doses around nuclear fissile objects, a comparative study has been made on several neutron dosemeters: bubble dosemeters, etched-track detectors (CR-39) and 3He-filled proportional counters used as dose-rate meters. The measurements were made on the ambient and the personal dose equivalents H*(10) and Hp(10). Results showed that several bubble dosemeters should have been used due to a low reproducibility in the measurements. A strong correlation with the neutron energy was also found, with about a 30% underestimation of Hp(10) for neutrons from the PuBe source, and about a 9% overestimation for neutrons from the 252Cf source. Measurements of the nuclear fissile objects were made using the CR-39 and the dose-rate meters. The CR-39 led to an underestimation of 30% with respect to the neutron dose-rate meter measurements. In addition, the MCNP calculation code was used in the different configurations.     

Test of adiabatic spin flippers for application at pulsed neutron sources

Experimental results on the flipping efficiency are shown for a set of 2 V-coils as spin flipper and for a high-frequency flipper with adiabatic transition. The influence of the adiabaticity parameter is discussed. The merits of these adiabatic flippers are compared with the use of “monochromatic” flippers, when operated in a beam from a pulsed neutron source. It is concluded that for “long pulse” sources adiabatic flippers will be superior.     

The content of 250Cf and 248Cm in 252Cf neutron sources and the effect on the neutron emission rate

One of the most common radionuclide neutron sources used for the calibration of detectors is (252)Cf. However, these sources also contain (250)Cf, which is present in the material from which the sources are made, and (248)Cm, which is formed as the daughter of (252)Cf via alpha-decay. Both decay by spontaneous fission with longer half-lives than (252)Cf. Consequently, as the source becomes older, the emission rate does not follow the decay curve of (252)Cf. Fits have been made to emission rate measurements of (252)Cf sources at NPL spanning over 30 y to deduce their (250)Cf and (248)Cm content. The emission rate of a source can be significantly underestimated if the presence of (250)Cf and (248)Cm is not taken into account, and this has been investigated for a typical (252)Cf source. The importance of this problem to other calibration laboratories and users of (252)Cf sources is emphasised.     

Neutron energy spectrum measurements of neutron sources with an NE-213 spectrometer

The neutron energy spectra of the following sources were measured using a fast-neutron spectrometer with NE-213 liquid scintillator: ²⁵²Cf, Am-Be and T(d, n)⁴He from a Van de Graaff accelerator (400 keV). The measured proton recoil pulse-height data were unfolded using the FANTI code to obtain the neutron energy spectrum. The spectrometer gives neutron spectra in the range of 2–16 MeV, with 6% intrinsic efficiency and a resolution between 4% and 11%. The ²⁵²Cf neutron energy spectrum was measured and the results obtained showed good agreement with the spectrum usually published in the literature, which can be fitted by the expression N(E) =√E exp(− E/T) with the constant T = 1.42 MeV.