| 1. |
- Birgersson, Evert, et al.
(författare)
-
Binary fission-fragment yields from the reaction 251Cf(nth, f)
- 2005
-
Ingår i: Nuclear fission and fission-product spectroscopy. - : American Institute of Physics. - 0735402884 ; , s. 349-352
-
Konferensbidrag (refereegranskat)abstract
- The recoil mass spectrometer LOHENGRIN of the Laue-Langevin Institute, Grenoble has been used to measure the light fission-fragment mass yield and kinetic energy distributions from neutron-induced 252Cf*, using 251Cf as target material. ©2005 American Institute of Physics
|
|
| 2. |
- Birgersson, Evert, 1974-
(författare)
-
Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf* and 239U*.For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf.The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed.A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor.
|
|
| 3. |
- Birgersson, Evert, et al.
(författare)
-
Light fission-fragment mass distribution from the reaction 251Cf(nth, f)
- 2007
-
Ingår i: Nuclear Physics A. - Amsterdam : Elsevier. - 0375-9474 .- 1873-1554. ; 791:1-2, s. 1-23
-
Tidskriftsartikel (refereegranskat)abstract
- For mass numbers A = 80 to 124 the recoil mass spectrometer LOHENGRIN of the Institute Laue-Langevin in Grenoble was used to measure with high resolution the light fission-fragment mass yields and kinetic energy distributions from thermal-neutron induced fission of 252Cf* for the first time, using 251Cf as target material. The obtained mean light fragment mass AL = (107 ± 2) and the corresponding mean kinetic energy Ek,L = (103±2) MeV are within the expected trend. Emission yields around A = 115 are enhanced and the corresponding mean kinetic energy is higher compared to spontaneous fission of 252Cf. This could be explained by the existence of an additional super-deformed fission mode.
|
|
| 4. |
- Birgersson, Evert, et al.
(författare)
-
Properties of the reaction 238U(n, f) at the vibrational resonances
- 2009
-
Ingår i: Nuclear Physics A. - Amsterdam : Elsevier. - 0375-9474 .- 1873-1554. ; 817:1-4, s. 1-34
-
Tidskriftsartikel (refereegranskat)abstract
- Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.
|
|
| 5. |
- Göök, A., et al.
(författare)
-
Photo-fission at the S-DALINAC
- 2009
-
Ingår i: 4th international workshop on nuclear fission and fission-product spectroscopy. - : American Institute of Physics (AIP). - 9780735407145 ; , s. 351-354
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Experiments on photo-induced fission of 238U and 234U using bremsstrahlung with endpoint energies between 6 MeV and 9 MeV have been carried out at the superconducting Darmstadt electron linear accelerator S-DALINAC. A twin Frisch grid ionization chamber has been used to determine energy and mass distributions via the 2E-technique. The fission fragment emission angle, which is used to correct for energy loss in the target and backing material for a correct determination of fragment energy and mass, is deduced from the time difference between cathode and anode signals. The research program including the foreseen search for parity violation in photo-induced fission and first experimental results will be presented. © 2009 American Institute of Physics.
|
|
| 6. |
- Oberstedt, Andreas, et al.
(författare)
-
Characterization of LaCl3:Ce detectors for prompt fission gamma-ray measurements
- 2009
-
Ingår i: 4th international workshop on nuclear fission and fission-product spectroscopy. - : American Institute of Physics (AIP). - 9780735407145 ; , s. 257-260
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The properties of cylindrical 1.5//×1.5//LaCl3:Cescintillation detectors were determined. For the energy resolution values between 3.8 and 4.2% at 662 keV were observed, following the expected E-1/2 behavior. The intrinsic full peak efficiency was determined to be more than 50% higher than for NaI:Tl detectors of same size. The observed intrinsic timing resolution for 60Co was estimated to 441 ps and the intrinsic activity of the lanthanum crystals was assessed. © 2009 American Institute of Physics.
|
|
| 7. |
- Oberstedt, Andreas, et al.
(författare)
-
Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN
- 2008
-
Ingår i: Seminar on Fission VI, Corsendonk Priory, Belgium, September 18-21, 2007. - : WORLD SCIENTIFIC. - 9789812791061 ; , s. 99-106
-
Konferensbidrag (refereegranskat)abstract
- Although the recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments, it was also used in the past for measuring light-charged particles from ternary fission. However, due to limited electric field settings the energy distribution of the lightest particles was not completely accessible. In this contribution we report on an energy degrader technique that allows the measurement of the entire energy spectra of ternary particles with LOHENGRIN. We demonstrate how the measured particle spectra are distorted by the energy degrader and present results from a Monte Carlo simulation that shows how the original energy distributions are reconstructed. Finally, we apply this procedure to experimental data of ternary particles from the reaction 235U(nth, f).
|
|
| 8. |
- Oberstedt, Andreas, et al.
(författare)
-
Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN
- 2008
-
Ingår i: ND-2007 International Conference on Nuclear Data for Science and Technology, Nice, France. - Les Ulis, France : EDP Sciences. - 9782759800902 - 9782759800919 ; , s. 379-382
-
Konferensbidrag (refereegranskat)abstract
- The recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments. Nevertheless, it was also used in the past for measuring light-charged particles from ternary fission. However, due to the electric field settings the energy distribution of the lightest particles was not completely accessible, which made the determination of mean kinetic energies, widths and, hence, emission yields difficult. In this paper we present an energy degrader technique that allows for the measurement of the entire energy spectrum of even the lightest ternary particles with LOHENGRIN.
|
|
| 9. |
- Oberstedt, Andreas, et al.
(författare)
-
Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN
- 2007
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - Amsterdam : Elsevier. - 0168-9002 .- 1872-9576. ; 570:1, s. 51-54
-
Tidskriftsartikel (refereegranskat)abstract
- The recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments. Nevertheless, it was also used in the past for measuring light-charged particles from ternary fission. However, due to the electric field settings the energy distribution of the lightest particles was not completely accessible, which made the determination of mean kinetic energies, widths and, hence, emission yields difficult. In this paper we present an energy degrader technique that allows for the measurement of the entire energy spectrum of even the lightest ternary particles with LOHENGRIN.
|
|
| 10. |
- Oberstedt, Andreas, et al.
(författare)
-
Identification of a Shape Isomer in 235U
- 2007
-
Ingår i: Physical Review Letters. - : The American Physical Society. - 0031-9007 .- 1079-7114. ; 99:4, s. 1-4
-
Tidskriftsartikel (refereegranskat)abstract
- The shape isomer in 235U has been searched for in a neutron-induced fission experiment on 234U, which was performed at the isomer spectrometer NEPTUNE of the EC-JRC IRMM. A neutron source, with a tunable pulse frequency in the Hz to kHz range and its individually adjustable neutron pulse width in connection with an appropriate detector system turned out to be the ideal instrument to perform an isomer search, when decay half-lives above 100 us are expected. From the delayed fission events observed for two different NEPTUNE settings and at mean incident neutron energies En = 0.95 and 1.27 MeV the isomeric fission half-life could be determined to be T_1/2 = (3.6 ± 1.8) ms. The corresponding cross section was determined to sigma_if = (10 ± 8) ub. With these results an experimental confirmation for the existence of a superdeformed shape isomer in odd-uranium isotopes is given for the first time.
|
|
| 11. |
|
|
| 12. |
- Oberstedt, Andreas, et al.
(författare)
-
Recent results on the neutron-induced fission cross-section of 231Pa
- 2005
-
Ingår i: Proceedings of the 3rd international workshop on nuclear fission and fission-product spectroscopy. - : American Institute of Physics. - 0735402884 ; , s. 27-30
-
Konferensbidrag (refereegranskat)abstract
- The cross-section for the neutron-induced fission of 231Pa has recently been measured from the threshold to En = 3.5 MeV. The experimental results are described in terms of extended statistical model calculations.
|
|
| 13. |
- Oberstedt, Stephan, et al.
(författare)
-
Light charged particle emission in the reaction 251Cf(nth, f)
- 2005
-
Ingår i: Nuclear Physics A. - : Elsevier BV. - 0375-9474 .- 1873-1554. ; 761:3-4, s. 173-189
-
Tidskriftsartikel (refereegranskat)abstract
- High resolution measurements of light charged particles (LCP) emitted in thermal neutron-induced fission of 252Cf ∗ (E=6.2 MeV) have been performed with the recoil mass-separator LOHENGRIN. For this compound nuclear system emission yields of LCPs, their mean kinetic energies and widths have been obtained for 8 isotopes with nuclear charges Z⩾2. For 13 further isotopes the emission yields were estimated on the basis of systematics on their kinetic energy distributions. 34Al and 36Si emission has been observed for the first time in thermal neutron-induced fission.
|
|
| 14. |
|
|
| 15. |
- Oberstedt, Stephan, et al.
(författare)
-
Neutron-induced fission cross-section of 231Pa
- 2006
-
Ingår i: Proceedings of NEMEA-2 Workshop on Neutron Measurements, Evaluations and Applications - 2, October 20-23, 2004, Bucharest, Rumania, EUR Report 22136 EN.
-
Konferensbidrag (refereegranskat)
|
|
| 16. |
|
|
| 17. |
- Oberstedt, Stephan, et al.
(författare)
-
Research on isomer decay with the NEPTUNE spectrometer
- 2008
-
Ingår i: ND-2007 International Conference on Nuclear Data for Science and Technology, Nice, France. - Les Ulis Cedex A, France : EDP Sciences. - 9782759800902 - 9782759800919 ; , s. 53-56
-
Konferensbidrag (refereegranskat)abstract
- A new spectrometer has been built at the EC-JRC IRMM to investigate isomer decay in the millisecond range and activation cross sections of isotopes, where isomeric states exist and are populated. The spectrometer is equipped with high-resolution gamma-ray detectors and an ionisation chamber for the measurement of charged particles, e.g., fission fragments. NEPTUNE provides pulsed quasi mono-energetic neutrons at pulse repetition frequencies up to 5 kHz and tuneable neutron pulse widths ranging from 10 µs into the ms-region. For this purpose a beam chopper based on a parallel-plate capacitor has been integrated into the accelerator beam line in order to deflect the charged-particle beam onto a tantalum beam dump. First research has been performed with the NEPTUNE spectrometer dedicated to the shape isomer search in the odd-A uranium isotopes 235U, 237U and 239U. In further experiments the population of long-lived spin isomers was investigated.
|
|
| 18. |
- Oberstedt, Stephan, et al.
(författare)
-
Shape isomers : a key to fission barriers
- 2008
-
Ingår i: Seminar on Fission VI, Corsendonk Priory, Belgium, September 18-21, 2007. - : WORLD SCIENTIFIC. - 9789812791061 ; , s. 233-240
-
Konferensbidrag (refereegranskat)abstract
- Quantitative predictions of fission product yields are relevant for the reliable operation of different modern nuclear applications. This concerns the realistic characterizations of the radio-toxicity of the fuel elements after the envisaged extended irradiation, as well as sub-critical assemblies, where the number of delayed neutrons from minor actinides is determined by the characteristic emission yields of the corresponding so-called pre-cursor isotopes. However, to be able to make more reliable quantitative predictions of fission characteristics requires the better understanding of the fission process itself. For this purpose a better knowledge about the distinct structure of the nuclear energy landscape around the fission barrier is indispensable. In particular, the question should be answered, whether the fission barrier is either double- or triple-humped or even multi-humped as been proposed within the multi-modal neck rupture model. Despite quite some effort based on different experimental techniques and theoretical approaches, this question remains still unanswered. There is still no consistent picture of the fission barrier available and hence, different sets of barrier parameters are in use, unable to describe the different observed phenomena in a coherent way. With the systematic investigation of shape isomer population, its decay modes as well as the branching ratio, precise information can be obtained to resolve the puzzling situation. The experimental approach will be discussed and results from first experiments presented.
|
|
| 19. |
- Norrman, Eva, 1966-
(författare)
-
Optimisation of radiographic imaging by means of factorial experiments
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the optimisation process of radiographic imaging, factorial designed experiments can be applied. The parameters (factors) are varied together instead of one at a time, making it possible to discover interactions between the factors as well as main influences of them on the result variable. A 2k design implies having k number of factors each one set to two different levels (low and high).A computer program, CoCIQ, designed to automatically analyse and evaluate test images of a contrast-detail phantom, was evaluated and adjusted to clinical situations using a flat panel detector. The program gives a quantified measurement of image quality by calculating an Image Quality Figure (IQF) for the X-ray image. It was shown that the program produces IQF with small variations. It was also found that there was a strong linear statistical relation between the computerised evaluation and the evaluation performed by human observers.2k factorial experiments were evaluated by investigating the influence of tube potential, tube loading, focus size and filtration on the result variables IQF, Kerma Area Product (KAP) and effective dose using a flat panel detector. It was found that the result variables were mainly influenced by tube loading, tube potential and filtration. Interactions between tube potential and filtration as well as between tube loading and filtration were observed, too. This work demonstrates that accepted knowledge was reproduced and that the effects of interactions between parameters were revealed.Extended 2k experiments were then applied at three different optimisation procedures. Two studies were performed using a flat panel detector for lumbar spine radiography. The aim was to find optimal settings for tube potential, system sensitivity and filtration for different sized patients and, in a separate study, to investigate the effect of the image post processing parameters and the possibility for dose reduction by adjusting these. The parameters are ROI (Region Of Interest) density, gamma, detail contrast enhancement, unsharp masking, kernel size and noise compensation.After determining the optimal settings from these experiments, X-ray images of the lumbar spine of an Alderson phantom were acquired and evaluated in a visual grading analysis (VGA).The results illustrated that the image quality was maintained at a lower effective dose by operating with a reduced tube potential and increased sensitivity of the X-ray system.The experiments on image post process parameters revealed their influence on image quality and indicated that image quality could be improved by changing the settings of the process parameters.Factorial experiments were also performed, using a multislice CT scanner to investigate the possibility for dose reduction at paediatric head examinations. An anthropomorphic phantom simulating a one-year-old child was scanned using different settings of tube potential, tube loading and reconstruction filter.The study showed that a 25 % reduction of dose was possible with maintained image quality by reducing the tube loading.Factorial designed experiments provide an effective method to simultaneously predict the influence of various parameters on image quality and radiation dose in the optimisation in diagnostic radiology.
|
|
