| 1. |
- Al-Adili, Ali, et al.
(författare)
-
Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
- 2012
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 673, s. 116-121
-
Tidskriftsartikel (refereegranskat)abstract
- Ionization chambers with Frisch grids have been very successfully applied to neutron-induced fission-fragment studies during the past 20 years. They are radiation resistant and can be easily adapted to the experimental conditions. The use of Frisch grids has the advantage to remove the angular dependency from the charge induced on the anode plate. However, due to the Grid Inefficiency (GI) in shielding the charges, the anode signal remains slightly angular dependent. The correction for the GI is, however, essential to determine the correct energy of the ionizing particles. GI corrections can amount to a few percent of the anode signal. Presently, two contradicting correction methods are considered in literature. The first method adding the angular-dependent part of the signal to the signal pulse height; the second method subtracting the former from the latter. Both additive and subtractive approaches were investigated in an experiment where a Twin Frisch-Grid Ionization Chamber (TFGIC) was employed to detect the spontaneous fission fragments (FF) emitted by a 252Cf source. Two parallel-wire grids with different wire spacing (1 and 2 mm, respectively), were used individually, in the same chamber side. All the other experimental conditions were unchanged. The 2 mm grid featured more than double the GI of the 1 mm grid. The induced charge on the anode in both measurements was compared, before and after GI correction. Before GI correction, the 2 mm grid resulted in a lower pulse-height distribution than the 1 mm grid. After applying both GI corrections to both measurements only the additive approach led to consistent grid independent pulse-height distributions. The application of the subtractive correction on the contrary led to inconsistent, grid-dependent results. It is also shown that the impact of either of the correction methods is small on the FF mass distributions of 235U(nth, f).
|
|
| 2. |
- Al-Adili, Ali, et al.
(författare)
-
First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this work we present evidence of anisotropic emission of fragment masses in 234U(n,f). The discovered mass anisotropy is correlated with the prominent vibrational resonances at En = 0.5 and 0.77 MeV and coincides with a verified strong angular anisotropy. From the outcome of this experimental work one may infer unequal fission barrier heights for different degrees of fission asymmetry.
|
|
| 3. |
- Al-Adili, Ali
(författare)
-
Investigation of 234U(n,f) with a Frisch-grid ionization chamber
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work treats three topics. The main topic concerns neutron-induced fission of 234U. The main goal is to investigate the fission-fragments properties as a function of the incident neutron energy. The study was carried out using a twin Frisch-grid ionization chamber. The first fluctuations on fragment properties are presented, in terms of strong angular anisotropy oscillation. The second part of the work treats the data-acquisition systems in use, particularly for neutron-induced fission experiments. Modern digital systems are studied and compared with the conventional analogue systems. It was shown that the digital systems are superior in drift stability, pile-up correction and extended the possibilities of offline analysis. The third part of the work concerns the Frisch-grid inefficiency. The Frisch grid was introduced in the chamber to remove the angular dependency from the induced charge. However, the shielding is not perfect and a correction is needed for the small angular dependency. Two contradicting methods have been presented in literature, one adding, and the second subtracting the angular-dependent part from the detected signal. An experiment with Cf(sf) was designed and performed to solve the pending ambiguity. The results support the additive model.
|
|
| 4. |
- Al-Adili, Ali, et al.
(författare)
-
Ion counting efficiencies at the IGISOL facility
- 2014
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- At the IGISOL-JYFLTRAP facility, fission mass yields can be studied at high precision. Fission fragments from a U target are passing through a Ni foil and entering a gas filled chamber. The collected fragments are guided through a mass separator to a Penning trap where their masses are identified. This simulation work focuses on how different fission fragment properties (mass, charge and energy) affect the stopping efficiency in the gas cell. In addition, different experimental parameters are varied (e. g. U and Ni thickness and He gas pressure) to study their impact on the stopping efficiency. The simulations were performed using the Geant4 package and the SRIM code. The main results suggest a small variation in the stopping efficiency as a function of mass, charge and kinetic energy. It is predicted that heavy fragments are stopped about 9% less efficiently than the light fragments. However it was found that the properties of the U, Ni and the He gas influences this behavior. Hence it could be possible to optimize the efficiency.
|
|
| 5. |
- Al-Adili, Ali
(författare)
-
Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This study on the neutron-induced fission of 234U was carried out at the 7 MV Van de Graaff accelerator of IRMM in Belgium. A Twin Frisch-Grid Ionization Chamber (TFGIC) was used to study 234U(n,f) between En = 0.2 and 5.0 MeV. The reaction is important for fission modelling of the second-chance fission in 235U(n,f). The fission fragment (FF) angular-, energy and mass distributions were determined using the 2E-method highlighting especially the region of the vibrational resonance at En = 0.77 MeV.The experiment used both conventional analogue and modern digital acquisition systems in parallel. Several advantages were found in the digital case, especially a successful pile-up correction. The shielding limitations of the Frisch-grid, called "grid-inefficiency", result in an angular-dependent energy signal. The correction of this effect has been a long-standing debate and a solution was recently proposed using the Ramo-Shockley theorem. Theoretical predictions from the latter were tested and verified in this work using two different grids. Also the neutron-emission corrections as a function of excitation energy were investigated. Neutron corrections are crucial for the determination of FF masses. Recent theoretical considerations attribute the enhancement of neutron emission to the heavier fragments exclusively, contrary to the average increase assumed earlier. Both methods were compared and the impact of the neutron multiplicities was assessed. The effects found are significant and highlight the importance of further experimental and theoretical investigation.In this work, the strong angular anisotropy of 234U(n,f ) was confirmed. In addition, and quite surprisingly, the mass distribution was found to be angular-dependent and correlated to the vibrational resonances. The anisotropy found in the mass distribution was consistent with an anisotropy in the total kinetic energy (TKE), also correlated to the resonances. The experimental data were parametrized assuming fission modes based on the Multi-Modal Random Neck-Rupture model. The resonance showed an increased yield from the Standard-1 fission mode and a consistent increased TKE. The discovered correlation between the vibrational resonances and the angular-dependent mass distributions for the asymmetric fission modes may imply different outer fission-barrier heights for the two standard modes.
|
|
| 6. |
- Al-Adili, Ali, et al.
(författare)
-
Sensitivity of Measured Fission Yields on Prompt-neutron Corrections
- 2014
-
Ingår i: Nuclear Data Sheets. - : Elsevier BV. - 0090-3752 .- 1095-9904. ; 119, s. 342-345
-
Tidskriftsartikel (refereegranskat)abstract
- Although the number of emitted prompt neutrons from the fission fragments increases as a function of excitation energy, it is not fully understood whether the increase in (nu) over bar (A) as a function of E-n is mass dependent. The share of excitation energies among the fragments is still under debate, but there are reasons to believe that the excess in neutron emission originates only from the heavy fragments, leaving (nu) over bar (light) (A) almost unchanged. We have investigated the consequences of a mass-dependent increase in (nu) over bar (A) on the final mass and energy distributions. The analysis have been performed on experimentally measured data on U-234(n, f). The assumptions concerning (nu) over bar (A) are essential when analysing measurements based on the 2E-technique, and impact significantly on the measured observables. For example, the post-neutron emission mass yield distribution revealed changes up to 10-30 %. The outcome of this work pinpoints the urgent need to determine (nu) over bar (A) experimentally, and in particular, how (nu) over bar (A) changes as a function of incident neutron energy. Many fission yields in the data libraries could be largely affected, since their analysis is based on a different assumption concerning the neutron emission.
|
|
| 7. |
- Al-Adili, Ali, et al.
(författare)
-
Simulations of the fission-product stopping efficiency in IGISOL
- 2015
-
Ingår i: European Physical Journal A. - : Springer Science and Business Media LLC. - 1434-6001 .- 1434-601X. ; 51:59, s. 1-7
-
Tidskriftsartikel (refereegranskat)abstract
- At the Jyväskylä Ion Guide Isotope Separator On-Line (IGISOL) facility, independent fission yields are measured employing the Penning-trap technique. Fission products are produced, e.g. by impinging protons on a uranium target, and are stopped in a gas-filled chamber. The products are collected by a flow of He gas and guided through a mass separator to a Penning trap, where their masses are identified. This work investigates how fission-product properties, such as mass and energy, affect the ion stopping efficiency in the gas cell. The study was performed using the Geant4 toolkit and the SRIM code. The main results show a nearly mass-independent ion stopping with regard to the wide spread of ion masses and energies, with a proper choice of uranium target thickness. Although small variations were observed, in the order of 5%, the results are within the systematic uncertainties of the simulations. To optimize the stopping efficiency while reducing the systematic errors, different experimental parameters were varied; for instance material thicknesses and He gas pressure. Different parameters influence the mass dependence and could alter the mass dependencies in the ion stopping efficiency.
|
|
| 8. |
- Gao, Yanhong, et al.
(författare)
-
Quantification of the relative role of land-surface processes and large-scale forcing in dynamic downscaling over the Tibetan Plateau
- 2017
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 48:5, s. 1705-1721
-
Tidskriftsartikel (refereegranskat)abstract
- © 2016 Springer-Verlag Berlin HeidelbergDynamical downscaling modeling (DDM) is important to understand regional climate change and develop local mitigation strategies, and the accuracy of DDM depends on the physical processes involved in the regional climate model as well as the forcing datasets derived from global models. This study investigates the relative role of the land surface schemes and forcing datasets in the DDM over the Tibet Plateau (TP), a region complex in topography and vulnerable to climate change. Three Weather Research and Forecasting model dynamical downscaling simulations configured with two land surface schemes [Noah versus Noah with multiparameterization (Noah-MP)] and two forcing datasets are performed over the period of 1980–2005. The downscaled temperature and precipitation are evaluated with observations and inter-compared regarding temporal trends, spatial distributions, and climatology. Results show that the temporal trends of the temperature and precipitation are determined by the forcing datasets, and the forcing dataset with the smallest trend bias performs the best. Relative to the forcing datasets, land surface processes play a more critical role in the DDM over the TP due to the strong heating effects on the atmospheric circulation from a vast area at exceptionally high elevations. By changing the vertical profiles of temperature in the atmosphere and the horizontal patterns of moisture advection during the monsoon seasons, the land surface schemes significantly regulate the downscaled temperature and precipitation in terms of climatology and spatial patterns. This study emphasizes the selection of land surface schemes is of crucial importance in the successful DDM over the TP.
|
|
| 9. |
|
|
| 10. |
|
|
