J. Y. Moon

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

In this work we present results on the measurement of the drift mobility and the mobility-lifetime product of charge carriers in a 16-pixellated CdZnTe detector. For the determination of an interaction position based on the pulse rise-time method in a CZT detector, it is necessary to characterize the transport properties governed by drift mobility and lifetime for electrons and holes. In order to extract the transport properties of an electron and a hole, we bombarded 5.5-MeV alpha particles from a 241Am source and 81-keV gamma rays emitted from a 133Ba source on the negatively biased contact of the CZT detector. A time-of-flight (TOF) method was used to measure the electron drift mobility at room temperature whose value turned out to be 906.4 cm2/V⋅ s. With the Hechts equation, the electron mobility-lifetime product was also determined from the bias-dependent alpha response and was equal to (9.88 ± 2.33) × 10−3 cm2/V. On the other hand, the hole mobility-lifetime product was evaluated by a model based on the average charge collection efficiency which accounts for the absorption probability with a given photon energy. By using a single parameter fitting of the model, we obtained the hole mobility-lifetime product of (8.28 ± 0.17) × 10−4 cm2/V.

Examination of the role of the O-14(alpha, p)F-17 reaction rate in type-I x-ray bursts

The O-14(alpha, p)F-17 reaction is one of the key reactions involved in the breakout from the hot-CNO cycle to the rp-process in type-I x-ray bursts (XRBs). The resonant properties in the compound nucleus Ne-18 have been investigated through resonant elastic scattering of F-17 + p. The radioactive F-17 beam was separated by the Center for Nuclear Study radioactive ion beam separator (CRIB) and bombarded a thick H-2 gas target at 3.6 MeV/nucleon. The recoiling light particles were measured by three Delta E-E silicon telescopes at laboratory angles of theta(lab) approximate to 3 degrees, 10 degrees, and 18 degrees. Five resonances at E-x = 6.15, 6.28, 6.35, 6.85, and 7.05 MeV were observed in the excitation functions, and their spin-parities have been determined based on an R-matrix analysis. In particular, J(pi) = 1(-) was firmly assigned to the 6.15-MeV state which dominates the thermonuclear O-14(alpha, p)F-17 rate below 2 GK. As well, a possible new excited state in Ne-18 was observed at E-x = 6.85 +/- 0.11 MeV with tentative J = 0 assignment. This state could be the analog state of the 6.880 MeV (0(-)) level in the mirror nucleus O-18, or a bandhead state (0(+)) of the six-particle four-hole (6p-4h) band. A new thermonuclear O-14(alpha, p)F-17 rate has been determined, and the astrophysical impact of multiple recent rates has been examined using an XRB model. Contrary to previous expectations, we find only a modest impact on predicted nuclear energy generation rates from using reaction rates differing by up to several orders of magnitude.

Optimization of detection geometry for industrial SPECT by Monte Carlo simulations

The Korea Atomic Energy Research Institute (KAERI) has developed an industrial SPECT to investigate the fluid flow and mixing patterns in columns. It has been found that the industrial SPECT is indeed a very powerful tool to study the hydrodynamics in multiphase reactors. One of the practical issues in the development of industrial SPECTs is to achieve a required imaging resolution of an industrial SPECT with a minimum number of component detectors, the number of which is frequently limited by both the size of the detectors and the total cost of the imaging system. In the present study, a set of different geometries of industrial SPECTs were evaluated by Monte Carlo simulation using MCNPX to determine the minimum number of detectors that will provide a spatial resolution that corresponds to 10% of the cylindrical column diameter. Our results show that 11 and 12 detectors will satisfy the 10% resolution requirement for the 40 cm and 60 cm diameter columns, respectively, for the industrial SPECT and radioisotopes considered in the present study. The conclusion of this result is valid only for the case considered in the present study, but we believe that the same procedure can be applied to other industrial SPECTs for this kind of optimization.

Pulse Shape Analysis of a Segmented Planar Germanium Detector by Using the Green's Function Method and Rise-Time Correlations for Depth Information

We performed pulse shape analysis to extract depth information for an interaction point of y rays in a 25-fold segmented germanium detector. With a 90° Compton spectrometer with collimators on both incidence and exit sides, we measured pulse shapes for the two interaction points, (x,y,z) = (2.5 cm, 2.5 cm, 0.5 cm) and (2.5 cm, 2.5 cm, 1.0cm). Experimental pulse shapes were compared with calculated ones by the three-dimensional Greens function method. Analytical solutions for induced charges on each cathode and an anode were obtained for a hole and an electron as a function of time. It is suggested that a rise-time correlation between the anode and the central cathode signals, T 90 (anode) - T 50 (cathode), can be used as a useful means to determine the depth (z) of an interaction point given an in-plane position (x, y). It is shown that calculated rise-time correlations are in qualitative agreement with experimental ones obtained by uncollimated y rays.

Influence of void on image quality of industrial SPECT

Industrial single photon emission computed tomography (SPECT) is a promising technique to determine the dynamic behavior of industrial process media and has been developed in the Korea Atomic Energy Research Institute. The present study evaluated the influence of a void, which is presence in multiphase reactors of industrial process, on the image quality of an industrial SPECT. The results are very encouraging; that is, the performance of the industrial SPECT system is little influenced by the presence of a void, which means that industrial SPECT is an appropriate tool to estimate the dynamic characteristics of the process media in a water-air phase bubble column with a static gas sparger.

High-Spin States in 93Nb

The high-spin states of 93 Nb are studied by the 82 Se( 16 O, p4n) 93 Nb reaction at a beam energy of 100 MeV. The in-beam γ-ray spectroscopic techniques are used. The γt-, γγt-coincidence, γ-ray a...

High-spin states of odd-odd156Ho

High spin states in the nucleus156Ho have been studied via the fusion-evaporation reaction140Ce(19F,3nγ)156Ho at the beam energy of 82 MeV.γ-γ coincidences, Eγ, Iγ and excitation functions have been measured. With two rotationally-aligned bands identified as π[h11/2]ν [i13/2], we found two new bands with a signature splitting, whose structure is believed to arise from either ν[h9/2] or ν[f7/2] coupled to π[h11/2]. Our placement of γ rays belonging to these new bands suggests that the previously known level scheme should be corrected. We also found many new low-energy γ rays in the low excitation energy. As compared to154Ho, the present nucleus with two more neutrons becomes more collective, and shows well-observed rotationally-aligned bands with a clear signature splitting.

Indirect studies on astrophysical reactions at the low-energy RI beam separator CRIB

Studies on nuclear astrophysics, nuclear structure, and other interests have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. A type of measurement to study astophysical reactions at CRIB is by the elastic resonant scattering with the thick-target method in inverse kinematics. An example is the α resonant scattering with 7Be beam, related to the astrophysical 7Be(α,γ) reactions, which is relevant in the hot p-p chain and νp-process in supernovae. Other α resonant scattering measurements with 30S, 10Be, 15O, and 18Ne beams have been performed at CRIB, using the thick-target method. There have also been measurements based on other experimental methods. The first Trojan horse method (THM) measurement using an RI beam has been performed at CRIB, to study the 18F(p, α)15O reaction at astrophysical energies via the three body reaction 2H(18F, α15O)n. The 18F(p, α)15O reaction rate is crucial to understand the 511-keV γ-ray production in nova explosion phenomena, and we successfully evaluated the reaction cross section at novae temperature and below experimentally for the first time.Studies on nuclear astrophysics, nuclear structure, and other interests have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. A type of measurement to study astophysical reactions at CRIB is by the elastic resonant scattering with the thick-target method in inverse kinematics. An example is the α resonant scattering with 7Be beam, related to the astrophysical 7Be(α,γ) reactions, which is relevant in the hot p-p chain and νp-process in supernovae. Other α resonant scattering measurements with 30S, 10Be, 15O, and 18Ne beams have been performed at CRIB, using the thick-target method. There have also been measurements based on other experimental methods. The first Trojan horse method (THM) measurement using an RI beam has been performed at CRIB, to study the 18F(p, α)15O reaction at astrophysical energies via the three body reaction 2H(18F, α15O)n. The 18F(p, α)15O reaction rate is crucial to ...

Two measurements of the 22Na+p resonant scattering via thick-target inverse-kinematics method

Na-22 is an important isotope for the study of extinct radioactivity, meanwhile its sufficiently long half life provides the possibility to observe live Na-22 in nearby nova explosions. The Na-22(p,gamma)Mg-23 is one of the key reactions that influence the Na-22 abundance in nova ejecta. To study the proton resonant states in Mg-23 relevant to the astrophysical Na-22(p,gamma)Mg-23 reaction rates, two measurements have been carried out at the CRIB separator of University of Tokyo, and the RIBLL secondary beamline in Lanzhou, respectively. The Na-22 secondary beam was produced via the H-1(Ne-22,Na-22) n charge exchange reaction. Thick-target inverse-kinematics method is applied to obtain the excitation function of Na-22+ p elastic scattering. Extended gas target and solid state polyethylene foil were used in the two measurements, respectively, to map the different excitation energy region of the compound nucleus Mg-23. Several new resonant levels are observed and their contribution to the Na-22(p,gamma)Mg-23 reaction rate is evaluated.

De velopment of the RAON recoil spectrometer (KOBRA) and its applications for nuclear astrophysics

KOBRA (KOrea Broad acceptance Recoil spectrometer and Apparatus), a new generation recoil spectrometer, has been designed at the Korean heavy-ion accelerator facility, so called RAON. It will allow many nuclear scientists to explore so-far hard but very interesting questions relevant to lowenergy nuclear physics. Especially, in nuclear astrophysics where the unstable, short-lived nuclei are usually involved and the high background rejection power is required, its high performance will come into significantly important role. As a particular case to see its capability, in this article, calculational results of 12C(α, γ)16O reaction which was studied with the COSY-INFINITY is presented.