Shin Iwasaki

Developments of a long liquid scintillation detector for fast neutron time-of-flight experiments

Abstract We have developed a long NE213 liquid scintillation detector (LLSD, 80 cm × 6.5 cm × 10 cm) which is useful as a position sensitive detector and a high-efficiency neutron detector for measurements of neutron emission cross sections. LLSD is a long rectangular shaped NE213 scintillator coupled with two photomultipliers at both ends of the scintillator cell. Position information along the long cell axis is obtained by the time difference method. The time compensated signal needed for time-of-flight (TOF) experiments is provided by a mean timer. Data are acquired for TOF, position and pulse-height signals gated by n-γ discriminator signals using a three-parameter data acquisition system. The updated data acquisition system and a position-dependent biasing analysis could achieve a remarkably lower bias level down to ∼1 MeV neutrons with good n-γ discrimination compared to the similar type NE213 detectors and remove position dependence of detection efficiency. LLSD with the sophisticated data taking and processing techniques proved to be very effective for neutron emission cross section measurements for 10–18 MeV incident neutrons as a wide-angle scattering detector and a high-efficiency single detector.

A compact post-acceleration beam chopper for a 4.5 MV Dynamitron pulsed neutron generator

Abstract A post-acceleration beam chopper (PACS) has been installed for a 4.5 MV Dynamitron accelerator to improve the energy resolution of neutron time-of-flight (TOF) experiments by shortening the duration of the ion beam pulses. The PACS sweeps the accelerated ion pulses across a chopping slit and eliminates the tails of the beam pulses. It operates sinusoidally at a frequency of 8 MHz with a maximum voltage of 10 kV peak to peak in synchronization with the accelerated ion pulses. The high voltage generator of the PACS was constructed of commercially available amplifiers and components, which realized easy maintenance and low cost. The PACS proved to be very effective to improve the pulse shape and has been applied for double-differential neutron emission cross section measurements.

The (3He, α) Reaction on 7Li from 5.0 to 8.0 MeV

Angular distributions were measured for α 0 , α 1 and α 2 groups from the 7 Li( 3 He, α) 6 Li reaction at the bombarding energies of 6.0 and 7.5 MeV. Pronounced backward peakings were observed for the α 0 and α 2 groups. The excitation curves were also measured over the range from 5.0 to 8.0 MeV at the laboratory angles of 20° and 160°. These curves show no resonant behaviour in the whole studied bombarding energy range. A dominant backward maximum and a forward peak were well described by the plane wave Born approximations with and without exchange, respectively. The triton plus 3 He cluster structure for the ground and second excited states in 6 Li is discussed. The neutron reduced widths \({\varTheta_{\text{n}}}^{2}\) were deduced.

NEW CT IMAGE RECONSTRUCTION ALGORITHM BASED ON THE BAYES ESTIMATION

Abstract We have proposed a unique and simple image reconstruction algorithm called Bayesian back-projection method (BBP) for CT of both transmission and emission types. The method proved the possibility of the image reconstruction for the actual phantom problems. Attractive features of the method are discussed.

Evaluation of Cross Sections for the Dosimetry Reactions of Niobium

Cross sections for 93Nb(n,n′)93 m Nb and 93Nb(n,2n)93 m Nb reactions were evaluated using nuclear model calculation and experimental data. The ELIESE-GNASH joint program, the Hauser-Feshbach code with preequilibrium correction, was used for calculation of cross sections to obtain consistent data set of all the cross sections and particle emission spectra. To establish reliable excitation functions for the dosimetry application, the cross sections and their covariances were evaluated from the results of the nuclear model calculation and recent experimental data using a generalized least squares program, GMA. The fission spectrum and Li(d,n) neutron spectrum averaged cross sections calculated from the evaluated cross sections were compared with the recent integral experiments. The degree of agreement for both the reactions was good.

Design and Evaluation of Interactive Instruction System: Impression for a Distance Class

In order to perform effective real-time online distance classes with multimedia educational materials, we have developed interactive instruction system named IMPRESSION which facilitates interactive presentation of several multimedia educational materials provided by Web server on the Internet. In this paper, we clarify the concept of our system, design and implement it. And we evaluate effectiveness of our system through the practical use in a two-day online distance class for high school students with JGN-II. As a result of this class, we confirmed we were able to perform a flexible distance class

Air gap effect on the properties of a long liquid scintillation detector

Abstract Light attenuation, n-γ pulse shape discrimination and timing properties of a long liquid scintillation detector (LLSD) were examined by means of Monte Carlo simulations and experiments. LLSD is a long rectangular shaped NE213 scintillator (80 × 6.5 × 10 cm 3 ) coupled with two photomultipliers and has been developed for neutron scattering experiments. We found that an air gap between a photomultiplier and a scintillator cell worked as a filter and reduced position dependencies of pulse height and of n-γ pulse shape discrimination which were disadvantages on the use of LLSD, whereas it deteriorated n-γ separation. This paper discusses the advantages and disadvantages of the air gap to the detector properties.

Assistance system for nuclear data evaluation in an object-oriented environment

Abstract The object-oriented framework for the assistance system of the neutron nuclear data evaluation was proposed, and an assisting system for the theoretical model fitting with the experimental data has been developed. In the model fitting procedure, we employed the method based on the general least-squares technique. Almost all things appearing in the process were easily and naturally modeled by objects in this environment. The objects can also be connected by a set of rules either with other objects, with the existing or newly developed external processes and with data bases. Totally twelve sub-processes were supported by each knowledge base. Example of the fitting procedures in the system has been shown for the case of 59 Co as the target nuclide. The objects for monitoring and strategic options were important in the failure situation of the fitting.

Validation of Recent Dosimetry Files in Li(d, n) Neutron Field

Validation of dosimetry cross sections has been carried out in a Li(d, n) neutron field for the four dosimetry files, i.e., the precedent IRDF-85, and the three recently compiled files, JENDL dosimetry file, IRDF-90 and ENDF/B-VI. The average cross sections for fourteen threshold reactions were measured, and compared with calculations. The tested reactions were (n, p) on 24Mg, 46Ti, 48Ti, 54Fe, 58Ni and 60Ni; (n, α) on 27Al and 63Cu; and (n, 2n) on 23Na, 55Mn, 58Ni, 59Co, 93Nb and 197Au, respectively. Overall, good agreement was obtained for the recent files except for a few reactions which showed large discrepancies.

A Post-Acceleration Beam Chopper for 4.5 MV Dynamitron Pulsed Neutron Generator

A post-acceleration beam chopper has been installed for Tohoku University 4.5 MV Dynamitron pulsed neutron generator in order to improve the energy resolution in fast neutron time-of-flight experiments by reducing the pulse width. The post-chopper worked as expected and has been applied successfully for double-differential neutron emission cross section measurements; typical results are shown for lithium-6 and natural carbon.