Strahlentherapie und Onkologie : Organ der Deutschen Rontgengesellschaft ... [et al], 1990
Determination of dose modification factor greatly facilitates the introduction of clinically prov... more Determination of dose modification factor greatly facilitates the introduction of clinically proven neutron therapy schedules at new installations. We have compared the biological performance of the p(66)+Be neutron facility at Faure, South Africa, with the established p(65)+Be installation at Louvain-la-Neuve, Belgium. Filtration, D gamma/DT, dose rate and HVT 5/15 for the Louvain and Faure beam are: 2 cm, 2.5 cm polyethylene; 3%, 5%; 0.2 Gy/min, 0.4 Gy/min; and 20 cm and 19 cm respectively. Dosimetry was done in A-150 plastic. Irradiation of BALB/C mice was carried on according to the dose accumulation method in a perspex phantom at 5 cm depth and at an SSD of 150 cm at a field size of 28 X 28 cm2. Sections of the jejunum were prepared at each centre and analyzed by both. The RBE of the Faure beam determined at a survival level of 50 crypts ranged from 1.64 to 1.69. The dose modification factor RBE of the Louvain beam given by Beauduin et al. was 1.61 +/- 0.14. The dose modificati...
A research program was performed at Louvain-la-Neuve to systematically determine the RBE of fast ... more A research program was performed at Louvain-la-Neuve to systematically determine the RBE of fast neutrons for the growth inhibition in Vicia faba bean roots and for the regeneration of the intestinal crypts in mice. The following neutron beams were compared p(75) + Be, p(65) + Be, p(45) + Be, p(34) + Be, d(20) + Be, and d(50) + Be. The RBE-variation as a function of neutron energy is larger for the Vicia faba system than for the regeneration of the intestinal crypt cells. This can be related to the inherent differency of the biological systems, but also to the different dose ranges involved (0.33 to 0.56 Gy and 7.66 to 8.56 Gy, respectively). In the high energy range explored, defined by the reactions p(75) + Be to p(34) + Be RBE varies only between 0.92 and 1.28 for Vicia faba and 0.96 and 1.12 for crypt cells normalized to the p(65) + Be beam. By contrast the RBE at lower energy beams (d(20) + Be and d(14.5) + Be) reaches values between 1.5 and 1.6 Finally fractionation has shown ...
Cancer radiotherapie : journal de la Societe francaise de radiotherapie oncologique, 2001
In 1996 and 2000, a survey of radiation practice in Belgium was performed by sending a questionna... more In 1996 and 2000, a survey of radiation practice in Belgium was performed by sending a questionnaire to the different centers asking their opinion and number of patients treated. There was a great similarity between the two surveys both for indications and total number of patients irradiated. For the most common indications (prevention of cheloids, heterotopic bone formation, hyperthyroidy ophthalmopathy), there was a trend to use similar radiation technique following recent publications. In contrast, if the number of cases of macular degeneration is declining, the prevention of vessels restenosis is becoming more and more an indication.
Introduction We report the commissioning and monitor unit calibration process of new prototype S2... more Introduction We report the commissioning and monitor unit calibration process of new prototype S2C2TM proton therapy system with pulsed PBS high dose rate proton beam. Methods The S2C2TM has a frequency of 1 kHz and delivers a pulsed proton beam of dose rate between 2.65 μGy/pulse and 230 μGy/pulse for the lowest (96 MeV) and highest (226 MeV) energies respectively. The facility is equipped with a compact gantry which rotates from −35° to 188° and delivers PBS proton beams of field sizes up to 20 × 24 cm 2 . The nozzle monitor ionization chambers (IC) were designed to be able to measure high dose rate per pulse delivered charges. A prototype 14 cm plane parallel IC and a multi-layer IC were used to measure integrated depth dose curves. Spot profiles and field size and penumbra were acquired with a 2D-scintillator detector while 2D-fields uniformity and symmetry were measured with a prototype 2D-array IC. Absolute dose/monitor unit calibration was based on dose measurements in single-layer fields of size 10 × 10 cm 2 . Plane parallel ionization chamber was used with −500 V voltage to limit the effect of recombination. Following the TRS398 protocol [1] , temperature and pressure (kT,P), polarity effect (kpol), beam quality factor (kQ,Q0) and recombination effect (ks) correction factors were computed and applied to the considered ionization chamber. The TRS398 pulsed beams model [2] and 3 additional Boag’s models [3] were investigated to compute the fraction of free electron for the recombination effect correction, while the kQ,Q0 factor evaluation was based on Monte-Carlo Goma’s results [4] for PBS proton beams. Results S2C2TM high dose rate per pulse beam causes mainly recombination effects in IC based detectors, and prototype detectors and nozzle IC were developed in order to take into account these beam characteristics. Measurements performed with these detectors show lateral and longitudinal dose uniformity better than 2.5% for clinically significant field sizes, ranges, modulations and air–gaps. For Absolute dose the measurements were achieved in single-layer fields with plane parallel IC. The standard deviations between the ks values obtained with the 4 methods are not larger than 0.02% and 0.3% for the lowest and the highest energies respectively. ks value of 1.009 was selected for all the energies. In order to verify our measurement method absolute dose was also made with water calorimeter: measured dosimetry data agreed within 2% for the lowest and the highest energies. Conclusion The new prototype S2C2TM PBS proton therapy system was successfully commissioned and released for clinical use.
To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freibur... more To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freiburg microDiamond detector, its response was compared to the response of a plane-parallel Markus chamber in a 62 MeV/n mono-energetic carbon ion beam. Results obtained with two different experimental setups are in agreement. As recommended by IAEA TRS-398, the response of the Markus chamber was corrected for temperature, pressure, polarity effects and ion recombination. No correction was applied to the response of the microDiamond detector. The ratio of the response of the Markus chamber to the response of the microDiamond is close to unity in the plateau region. In the Bragg peak region, a significant increase of the ratio is observed, which increases to 1.2 in the distal edge region. Results indicate a correlation between the under-response of the microDiamond detector and high LET values. The combined relative standard uncertainty of the results is estimated to be 2.38% in the plateau region and 12% in the distal edge region. These values are dominated by the uncertainty of alignment in the non-uniform beam and the uncertainty of range determination.
ABSTRACT Purpose: In proton dose calculation, clinically compatible speeds are now achieved with ... more ABSTRACT Purpose: In proton dose calculation, clinically compatible speeds are now achieved with Monte Carlo codes (MC) that combine 1) adequate simplifications in the physics of transport and 2) the use of hardware architectures enabling massive parallel computing (like GPUs). However, the uncertainties related to the transport algorithms used in these codes must be kept minimal. Such algorithms can be checked with the so-called “Fano cavity test”. We implemented the test in two codes that run on specific hardware: gPMC on an nVidia GPU and MCsquare on an Intel Xeon Phi (60 cores).
PURPOSE A prompt gamma (PG) slit camera prototype demonstrated that on-line range monitoring with... more PURPOSE A prompt gamma (PG) slit camera prototype demonstrated that on-line range monitoring within 1-2 mm could be performed by comparing expected and measured PG detection profiles. Monte Carlo (MC) can simulate the expected PG profile but this would result in prohibitive computation time for a complete pencil beam treatment plan. We implemented a much faster method that is based on analytical processing of pre-computed MC data. METHODS The formation of the PG detection signal can be separated into: 1) production of PGs and 2) detection by the camera detectors after PG transport in geometry. For proton energies from 40 to 230 MeV, PG productions in depth were pre-computed by MC (PENH) for 12C, 14N, 16O, 31P and 40Ca. The PG production was then modeled analytically by adding the PG production for each element according to local proton energy and tissue composition.PG transport in the patient/camera geometries and the detector response were modeled by convolving the PG production profile with a transfer function. The latter is interpolated from a database of transfer functions fitted to pre-computed MC data (PENELOPE). The database was generated for a photon source in a cylindrical phantom with various radiuses and a camera placed at various positions.As a benchmark, the analytical model was compared to PENH for a water phantom, a phantom with different slabs (adipose, muscle, lung) and a thoracic CT. RESULTS Good agreement (within 5%) was observed between the analytical model and PENH for the PG production. Similar accuracy for detecting range shifts was also observed. Speed of around 250 ms per profile was achieved (single CPU) using a non-optimized MatLab implementation. CONCLUSION We devised a fast analytical model for generating PG detection profiles. In the test cases considered in this study, similar accuracy than MC was achieved for detecting range shifts. This research is supported by IBA.
Tissue-equivalent proportional counters (TEPCs) measure distributions of ionisations, produced in... more Tissue-equivalent proportional counters (TEPCs) measure distributions of ionisations, produced in the gas cavity by the radiation field which are afterwards converted into distributions of energy imparted by applying a calibration factor. To calibrate the pulse-height spectra, first, a marker point must be identified in the measured spectrum. Then, an accurate value of lineal energy must be assigned to this marker. A common marker that is often used for calibration is the so-called proton-edge (p-edge). It is a distinctive feature of a proton or neutron spectrum which corresponds to the maximum amount of energy that a proton can deposit in the active volume of the detector. A precise method to identify the marker point was applied to identify the p-edge with an uncertainty below 1 %. To evaluate the final uncertainty of the calibration, the uncertainty of the energy value assigned to the p-edge must also be considered. This value can be evaluated using different energy-range tables. This study investigates how the choice of different input databases for calibration purposes influences the calibration. The effect of three different frequently used sets of input data was analysed for pure propane gas and for propane-TE gas mixture.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
The importance of water calorimetry in clinical proton beam dosimetry has increased for various r... more The importance of water calorimetry in clinical proton beam dosimetry has increased for various reasons: there has been an increasing interest in proton therapy from the radiotherapy world; existing dosimetry protocols for clinical proton beams recommend calorimetry as the primary dosimetry method (ie, AAPM and ECHED); water has recently become the reference material for dose specification. For the water calorimetric dose evaluation, the water calorimeter developed in Ghent was used. Ionometry was performed following the ECHED protocol. The study resulted in a calorimetric to ionometric dose ratio of 0.974 +/- 0.009. The discrepancy should in our opinion be attributed to the (Wle)p,air value of 35.2 J/C implemented in the protocol, possibly next to ion chamber dependent effects for which indications are found in the ionometry measurements.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
Neutron dosimetry intercomparison studies have been undertaken at the Nice neutrontherapy facilit... more Neutron dosimetry intercomparison studies have been undertaken at the Nice neutrontherapy facility with the staff at Louvain-la-Neuve which has had wide experience in both dosimetric and radiobiological intercomparisons. Tissue equivalent (TE) ionization chambers were first calibrated in 60Co beams and then exposed in the neutron beam at different depths in a water phantom; the largest difference observed in neutron beam measurements with all the chambers tested was 0.89%, and most of them agreed to within less than 0.5%. The gamma component at four depths was derived from measurements with Geiger-Müller counters; the results obtained with the two counters (Nice and Louvain-la-Neuve), expressed as a percentage of the total dose (neutron + gamma), agreed to within less than 0.03% and the value increased from 1.4 to 4.2% between 2 and 20 cm in depth.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
In fast neutron therapy, the relative biological effectiveness (RBE) of a given beam varies to a ... more In fast neutron therapy, the relative biological effectiveness (RBE) of a given beam varies to a large extent with the neutron energy spectrum. This spectrum depends primarily on the energy of the incident particles and on the nuclear reaction used for neutron production. However, it also depends on other factors which are specific to the local facility, eg, target, collimation system, etc. Therefore direct radiobiological intercomparisons are justified. The present paper reports the results of an intercomparison performed at seven neutrontherapy centres: Orléans, France (p(34)+Be), Riyadh, Saudi Arabia (p(26)+Be), Ghent, Belgium (d(14.5)+Be), Faure, South Africa (p(66)+Be), Detroit, USA (d(48)+Be), Nice, France (p(65)+Be) and Louvain-la-Neuve, Belgium (p(65)+Be). The selected radiobiological system was intestinal crypt regeneration in mice after single fraction irradiation. The observed RBE values (ref cobalt-60 gamma-rays) were 1.79 +/- 0.10, 1.84 +/- 0.07, 2.24 +/- 0.11, 1.55 +/-...
Strahlentherapie und Onkologie : Organ der Deutschen Rontgengesellschaft ... [et al], 1990
Determination of dose modification factor greatly facilitates the introduction of clinically prov... more Determination of dose modification factor greatly facilitates the introduction of clinically proven neutron therapy schedules at new installations. We have compared the biological performance of the p(66)+Be neutron facility at Faure, South Africa, with the established p(65)+Be installation at Louvain-la-Neuve, Belgium. Filtration, D gamma/DT, dose rate and HVT 5/15 for the Louvain and Faure beam are: 2 cm, 2.5 cm polyethylene; 3%, 5%; 0.2 Gy/min, 0.4 Gy/min; and 20 cm and 19 cm respectively. Dosimetry was done in A-150 plastic. Irradiation of BALB/C mice was carried on according to the dose accumulation method in a perspex phantom at 5 cm depth and at an SSD of 150 cm at a field size of 28 X 28 cm2. Sections of the jejunum were prepared at each centre and analyzed by both. The RBE of the Faure beam determined at a survival level of 50 crypts ranged from 1.64 to 1.69. The dose modification factor RBE of the Louvain beam given by Beauduin et al. was 1.61 +/- 0.14. The dose modificati...
A research program was performed at Louvain-la-Neuve to systematically determine the RBE of fast ... more A research program was performed at Louvain-la-Neuve to systematically determine the RBE of fast neutrons for the growth inhibition in Vicia faba bean roots and for the regeneration of the intestinal crypts in mice. The following neutron beams were compared p(75) + Be, p(65) + Be, p(45) + Be, p(34) + Be, d(20) + Be, and d(50) + Be. The RBE-variation as a function of neutron energy is larger for the Vicia faba system than for the regeneration of the intestinal crypt cells. This can be related to the inherent differency of the biological systems, but also to the different dose ranges involved (0.33 to 0.56 Gy and 7.66 to 8.56 Gy, respectively). In the high energy range explored, defined by the reactions p(75) + Be to p(34) + Be RBE varies only between 0.92 and 1.28 for Vicia faba and 0.96 and 1.12 for crypt cells normalized to the p(65) + Be beam. By contrast the RBE at lower energy beams (d(20) + Be and d(14.5) + Be) reaches values between 1.5 and 1.6 Finally fractionation has shown ...
Cancer radiotherapie : journal de la Societe francaise de radiotherapie oncologique, 2001
In 1996 and 2000, a survey of radiation practice in Belgium was performed by sending a questionna... more In 1996 and 2000, a survey of radiation practice in Belgium was performed by sending a questionnaire to the different centers asking their opinion and number of patients treated. There was a great similarity between the two surveys both for indications and total number of patients irradiated. For the most common indications (prevention of cheloids, heterotopic bone formation, hyperthyroidy ophthalmopathy), there was a trend to use similar radiation technique following recent publications. In contrast, if the number of cases of macular degeneration is declining, the prevention of vessels restenosis is becoming more and more an indication.
Introduction We report the commissioning and monitor unit calibration process of new prototype S2... more Introduction We report the commissioning and monitor unit calibration process of new prototype S2C2TM proton therapy system with pulsed PBS high dose rate proton beam. Methods The S2C2TM has a frequency of 1 kHz and delivers a pulsed proton beam of dose rate between 2.65 μGy/pulse and 230 μGy/pulse for the lowest (96 MeV) and highest (226 MeV) energies respectively. The facility is equipped with a compact gantry which rotates from −35° to 188° and delivers PBS proton beams of field sizes up to 20 × 24 cm 2 . The nozzle monitor ionization chambers (IC) were designed to be able to measure high dose rate per pulse delivered charges. A prototype 14 cm plane parallel IC and a multi-layer IC were used to measure integrated depth dose curves. Spot profiles and field size and penumbra were acquired with a 2D-scintillator detector while 2D-fields uniformity and symmetry were measured with a prototype 2D-array IC. Absolute dose/monitor unit calibration was based on dose measurements in single-layer fields of size 10 × 10 cm 2 . Plane parallel ionization chamber was used with −500 V voltage to limit the effect of recombination. Following the TRS398 protocol [1] , temperature and pressure (kT,P), polarity effect (kpol), beam quality factor (kQ,Q0) and recombination effect (ks) correction factors were computed and applied to the considered ionization chamber. The TRS398 pulsed beams model [2] and 3 additional Boag’s models [3] were investigated to compute the fraction of free electron for the recombination effect correction, while the kQ,Q0 factor evaluation was based on Monte-Carlo Goma’s results [4] for PBS proton beams. Results S2C2TM high dose rate per pulse beam causes mainly recombination effects in IC based detectors, and prototype detectors and nozzle IC were developed in order to take into account these beam characteristics. Measurements performed with these detectors show lateral and longitudinal dose uniformity better than 2.5% for clinically significant field sizes, ranges, modulations and air–gaps. For Absolute dose the measurements were achieved in single-layer fields with plane parallel IC. The standard deviations between the ks values obtained with the 4 methods are not larger than 0.02% and 0.3% for the lowest and the highest energies respectively. ks value of 1.009 was selected for all the energies. In order to verify our measurement method absolute dose was also made with water calorimeter: measured dosimetry data agreed within 2% for the lowest and the highest energies. Conclusion The new prototype S2C2TM PBS proton therapy system was successfully commissioned and released for clinical use.
To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freibur... more To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freiburg microDiamond detector, its response was compared to the response of a plane-parallel Markus chamber in a 62 MeV/n mono-energetic carbon ion beam. Results obtained with two different experimental setups are in agreement. As recommended by IAEA TRS-398, the response of the Markus chamber was corrected for temperature, pressure, polarity effects and ion recombination. No correction was applied to the response of the microDiamond detector. The ratio of the response of the Markus chamber to the response of the microDiamond is close to unity in the plateau region. In the Bragg peak region, a significant increase of the ratio is observed, which increases to 1.2 in the distal edge region. Results indicate a correlation between the under-response of the microDiamond detector and high LET values. The combined relative standard uncertainty of the results is estimated to be 2.38% in the plateau region and 12% in the distal edge region. These values are dominated by the uncertainty of alignment in the non-uniform beam and the uncertainty of range determination.
ABSTRACT Purpose: In proton dose calculation, clinically compatible speeds are now achieved with ... more ABSTRACT Purpose: In proton dose calculation, clinically compatible speeds are now achieved with Monte Carlo codes (MC) that combine 1) adequate simplifications in the physics of transport and 2) the use of hardware architectures enabling massive parallel computing (like GPUs). However, the uncertainties related to the transport algorithms used in these codes must be kept minimal. Such algorithms can be checked with the so-called “Fano cavity test”. We implemented the test in two codes that run on specific hardware: gPMC on an nVidia GPU and MCsquare on an Intel Xeon Phi (60 cores).
PURPOSE A prompt gamma (PG) slit camera prototype demonstrated that on-line range monitoring with... more PURPOSE A prompt gamma (PG) slit camera prototype demonstrated that on-line range monitoring within 1-2 mm could be performed by comparing expected and measured PG detection profiles. Monte Carlo (MC) can simulate the expected PG profile but this would result in prohibitive computation time for a complete pencil beam treatment plan. We implemented a much faster method that is based on analytical processing of pre-computed MC data. METHODS The formation of the PG detection signal can be separated into: 1) production of PGs and 2) detection by the camera detectors after PG transport in geometry. For proton energies from 40 to 230 MeV, PG productions in depth were pre-computed by MC (PENH) for 12C, 14N, 16O, 31P and 40Ca. The PG production was then modeled analytically by adding the PG production for each element according to local proton energy and tissue composition.PG transport in the patient/camera geometries and the detector response were modeled by convolving the PG production profile with a transfer function. The latter is interpolated from a database of transfer functions fitted to pre-computed MC data (PENELOPE). The database was generated for a photon source in a cylindrical phantom with various radiuses and a camera placed at various positions.As a benchmark, the analytical model was compared to PENH for a water phantom, a phantom with different slabs (adipose, muscle, lung) and a thoracic CT. RESULTS Good agreement (within 5%) was observed between the analytical model and PENH for the PG production. Similar accuracy for detecting range shifts was also observed. Speed of around 250 ms per profile was achieved (single CPU) using a non-optimized MatLab implementation. CONCLUSION We devised a fast analytical model for generating PG detection profiles. In the test cases considered in this study, similar accuracy than MC was achieved for detecting range shifts. This research is supported by IBA.
Tissue-equivalent proportional counters (TEPCs) measure distributions of ionisations, produced in... more Tissue-equivalent proportional counters (TEPCs) measure distributions of ionisations, produced in the gas cavity by the radiation field which are afterwards converted into distributions of energy imparted by applying a calibration factor. To calibrate the pulse-height spectra, first, a marker point must be identified in the measured spectrum. Then, an accurate value of lineal energy must be assigned to this marker. A common marker that is often used for calibration is the so-called proton-edge (p-edge). It is a distinctive feature of a proton or neutron spectrum which corresponds to the maximum amount of energy that a proton can deposit in the active volume of the detector. A precise method to identify the marker point was applied to identify the p-edge with an uncertainty below 1 %. To evaluate the final uncertainty of the calibration, the uncertainty of the energy value assigned to the p-edge must also be considered. This value can be evaluated using different energy-range tables. This study investigates how the choice of different input databases for calibration purposes influences the calibration. The effect of three different frequently used sets of input data was analysed for pure propane gas and for propane-TE gas mixture.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
The importance of water calorimetry in clinical proton beam dosimetry has increased for various r... more The importance of water calorimetry in clinical proton beam dosimetry has increased for various reasons: there has been an increasing interest in proton therapy from the radiotherapy world; existing dosimetry protocols for clinical proton beams recommend calorimetry as the primary dosimetry method (ie, AAPM and ECHED); water has recently become the reference material for dose specification. For the water calorimetric dose evaluation, the water calorimeter developed in Ghent was used. Ionometry was performed following the ECHED protocol. The study resulted in a calorimetric to ionometric dose ratio of 0.974 +/- 0.009. The discrepancy should in our opinion be attributed to the (Wle)p,air value of 35.2 J/C implemented in the protocol, possibly next to ion chamber dependent effects for which indications are found in the ionometry measurements.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
Neutron dosimetry intercomparison studies have been undertaken at the Nice neutrontherapy facilit... more Neutron dosimetry intercomparison studies have been undertaken at the Nice neutrontherapy facility with the staff at Louvain-la-Neuve which has had wide experience in both dosimetric and radiobiological intercomparisons. Tissue equivalent (TE) ionization chambers were first calibrated in 60Co beams and then exposed in the neutron beam at different depths in a water phantom; the largest difference observed in neutron beam measurements with all the chambers tested was 0.89%, and most of them agreed to within less than 0.5%. The gamma component at four depths was derived from measurements with Geiger-Müller counters; the results obtained with the two counters (Nice and Louvain-la-Neuve), expressed as a percentage of the total dose (neutron + gamma), agreed to within less than 0.03% and the value increased from 1.4 to 4.2% between 2 and 20 cm in depth.
Bulletin du cancer. Radiothérapie : journal de la Société française du cancer : organe de la société française de radiothérapie oncologique, 1996
In fast neutron therapy, the relative biological effectiveness (RBE) of a given beam varies to a ... more In fast neutron therapy, the relative biological effectiveness (RBE) of a given beam varies to a large extent with the neutron energy spectrum. This spectrum depends primarily on the energy of the incident particles and on the nuclear reaction used for neutron production. However, it also depends on other factors which are specific to the local facility, eg, target, collimation system, etc. Therefore direct radiobiological intercomparisons are justified. The present paper reports the results of an intercomparison performed at seven neutrontherapy centres: Orléans, France (p(34)+Be), Riyadh, Saudi Arabia (p(26)+Be), Ghent, Belgium (d(14.5)+Be), Faure, South Africa (p(66)+Be), Detroit, USA (d(48)+Be), Nice, France (p(65)+Be) and Louvain-la-Neuve, Belgium (p(65)+Be). The selected radiobiological system was intestinal crypt regeneration in mice after single fraction irradiation. The observed RBE values (ref cobalt-60 gamma-rays) were 1.79 +/- 0.10, 1.84 +/- 0.07, 2.24 +/- 0.11, 1.55 +/-...
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Papers by S. Vynckier