Search

International audience; Following work done in the energy region above 100 keV, the high-precision calibration of a co-axial high-purity germanium detector has been continued in the energy region below 100 keV. Some of the previous measurements and Monte-Carlo simulations have been repeated with higher statistics and a new source measurement with 169 Yb has been added. For the energy range from 40 keV to 100 keV, an absolute precision for the detection efficiency of ± 0.2% has been reached, as previously obtained for energies above 100 keV. The low-energy behaviour of the germanium detector was further scrutinised by studying the germanium X-ray escape probability for the detection of low-energy photons. In addition, one experimental point, a γ ray at 2168 keV from the decay of 38 K, has been included for the total-to-peak ratios agreeing well with simulations. The same γ ray was also added for the single- and double-escape probabilities. Finally, the long term stability of the efficiency of the germanium detector was investigated by regularly measuring the full-energy peak efficiency with a precisely calibrated 60 Co source and found to be perfectly stable over a period of 10 years.

Gamma logging for uranium exploration are currently based on total counting with Geiger Müller gas detectors or NaI (TI) scintillators. However, the total count rate interpretation in terms of uranium concentration may be impaired in case of roll fronts, when the radioactive equilibrium of the natural 238U radioactive chain is modified by differential leaching of uranium and its daughter radioisotopes of thorium, radium, radon, etc. Indeed, in case of secular equilibrium, more than 95 % of gamma rays emitted by uranium ores come from 214Pb and 214Bi isotopes, which are in the back-end of 238U chain. Consequently, these last might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed in surface with high-resolution hyper-pure germanium HPGe detectors to directly characterize uranium activity from the 1001 keV gamma ray of 234mPa, which is in the beginning of 238U chain. However, due to the low intensity of this gamma ray, i.e. 0.84 %, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose here a method using both the 92 keV gamma ray of 234Th and the 98.4 keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma radiations of 214Pb and 214Bi, which create a significant Compton scattering continuum acting as a fluorescence source and resulting in the emission of uranium fluorescence X-rays. The comparison of the uranium activity obtained with the 92 keV and 98.4 keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in case of uranium nugget, the 92 keV line leads to underestimated uranium concentration due to gamma self-absorption, but on the contrary the 98.4 keV line leads to an overestimation because of increased fluorescence. In order to test this new approach, several tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.

Expérience GANIL; International audience; A high-purity co-axial germanium detector has been calibrated in efficiency to a precision of about 0.15% over a wide energy range. High-precision scans of the detector crystal and gamma-ray source measurements have been compared to Monte-Carlo simulations to adjust the dimensions of a detector model. For this purpose, standard calibration sources and short-lived on-line sources have been used. The resulting efficiency calibration reaches the precision needed e.g. for branching ratio measurements of super-allowed beta decays for tests of the weak-interaction standard model.

International audience; AREVA Mines and the Nuclear Measurement Laboratory of CEA Cadarache are collaborating to improve the sensitivity and precision of uranium concentration evaluation by means of gamma-ray measurements. This paper reports gamma-ray spectra, recorded with two high-purity germanium detectors, on standard cement blocks with increasing uranium content, and the corresponding MCNP simulations. The detailed MCNP model of detectors and experimental setup has been validated by calculation vs. experiment comparisons. An optimization of detector MCNP models is presented in this paper, as well as a comparison of different nuclear data libraries to explain missing or exceeding peaks in the simulation. Energy shifts observed between the fluorescence X-rays produced by MCNP and atomic data are also investigated, as well as gamma backscattering and beta radiation effects. The validated numerical models will be used in further studies to develop new gamma-ray spectroscopy approaches aiming at reducing acquisition times, especially for ore samples with low uranium content.

AREVA Mines and the Nuclear Measurement Laboratory of CEA Cadarache are collaborating to improve the sensitivity and precision of uranium concentration evaluation by means of gamma measurements. This paper reports gamma-ray spectra, recorded with a high-purity coaxial germanium detector, on standard cement blocks with increasing uranium content, and the corresponding MCNP simulations. The detailed MCNP model of the detector and experimental setup has been validated by calculation vs. experiment comparisons. An optimization of the detector MCNP model is presented in this paper, as well as a comparison of different nuclear data libraries to explain missing or exceeding peaks in the simulation. Energy shifts observed between the fluorescence X-rays produced by MCNP and atomic data are also investigated. The qualified numerical model will be used in further studies to develop new gamma spectroscopy approaches aiming at reducing acquisition times, especially for ore samples with low uranium content.

Gamma-ray and neutron spectrometry sets up a powerful tool of geological and geochemical characterization of planetary surfaces. This method allows to tackle some critical planetology questions: crustal and mantle compositions; ices; volcanism; alteration processes... Most of the neutrons and gamma photons result from the interactions of galactic cosmic rays with matter. The first chapter introduces the physics of these nuclear interactions in planetary soils and in detectors. Our work aims at optimizing the observations by specifying instrumental performances, and by highlighting relations be-tween soil composition and neutron fluxes. Numerical simulations using the GEANT code from CERN support our analysis. The second chapter estimates the performances of the Germanium gamma-ray spectrometer for MARS SURVEYOR 2001. The result of simulations is compared to calibration measurements; then performances are calculated in flight configuration. The background at Mars is estimated to ~160 c/s. The instrument offers a fine sensitivity to: Fe, Mg, K, Si, Th, Cl and O. It will also be possible to measure U, Ti, H, C, S, Ca and Al. The emission lobes at the surface are calculated too. These measurements shall permit a better understanding of the Martian surface. The last chapter deals with fast neutrons [500 keV; 10 MeV] emitted by the Moon. The strong influence of oxygen is underlined. As observed by LUNAR PROSPECTOR, the integrated flux shows a pronounced dependence with regolith content in iron and titanium, allowing the mapping. The influence of the other chemical elements is quantified. A simple mathematical formula is suggested to estimate the integrated neutron flux according to soil composition. At last, a study of hydrogen effects on fast neutron flux is carried out; we examine the possibilities to quantify its abundance in the soil by this method.; La spectrométrie gamma et neutronique forme un puissant outil de caractérisation géologique et géochimique des surfaces planétaires. Cette méthode permet d'aborder des questions capitales en planétologie: composition de la croûte, du manteau; glaces; volcanisme; processus d'altération... La majorité des neutrons et des photons gamma résultent de l'interaction du rayonnement cosmique avec la matière. Le premier chapitre présente la physique de ces interactions nucléaires dans les sols planétaires et dans les détecteurs. Nos travaux visent à optimiser les observations en précisant les performances instrumentales, et en isolant les relations entre la composition du sol et les flux de neutrons. Des simulations numériques utilisant le code GEANT du CERN supportent notre analyse. Le deuxième chapitre évalue les performances du spectromètre gamma à base de germanium pour MARS SURVEYOR 2001. Le résultat des simulations est confronté aux mesures de calibration; puis les performances sont calculées en configuration de vol. Le bruit de fond près de Mars est évalué à ~160 c/s. L'instrument offre une bonne sensibilité à: Fe, Mg, K, Si, Th, Cl et O. Il sera possible aussi de mesurer U, Ti, H, C, S, Ca et Al. Les lobes d'émission à la surface sont aussi calculés. Ces mesures permettront une meilleure compréhension de la surface martienne. Le dernier chapitre traite de l'émission des neutrons rapides lunaires [500 keV; 10 MeV]. La forte influence de l'oxygène est mise en évidence. Comme l'a observé LUNAR PROSPECTOR, le flux intégré montre une dépendance marquée avec la teneur du régolite en fer et en titane, permettant la cartographie. L'influence des autres éléments chimiques est quantifiée. Une formule mathématique simple est proposée pour estimer le flux intégré de neutrons suivant la composition du sol. Enfin, une étude des effets de l'hydrogène sur le flux de neutrons rapides est menée; nous examinons les possibilités de quantifier son abondance dans le sol par cette méthode.

Gamma-ray and neutron spectrometry sets up a powerful tool of geological and geochemical characterization of planetary surfaces. This method allows to tackle some critical planetology questions: crustal and mantle compositions; ices; volcanism; alteration processes... Most of the neutrons and gamma photons result from the interactions of galactic cosmic rays with matter. The first chapter introduces the physics of these nuclear interactions in planetary soils and in detectors. Our work aims at optimizing the observations by specifying instrumental performances, and by highlighting relations be-tween soil composition and neutron fluxes. Numerical simulations using the GEANT code from CERN support our analysis. The second chapter estimates the performances of the Germanium gamma-ray spectrometer for MARS SURVEYOR 2001. The result of simulations is compared to calibration measurements; then performances are calculated in flight configuration. The background at Mars is estimated to ~160 c/s. The instrument offers a fine sensitivity to: Fe, Mg, K, Si, Th, Cl and O. It will also be possible to measure U, Ti, H, C, S, Ca and Al. The emission lobes at the surface are calculated too. These measurements shall permit a better understanding of the Martian surface. The last chapter deals with fast neutrons [500 keV; 10 MeV] emitted by the Moon. The strong influence of oxygen is underlined. As observed by LUNAR PROSPECTOR, the integrated flux shows a pronounced dependence with regolith content in iron and titanium, allowing the mapping. The influence of the other chemical elements is quantified. A simple mathematical formula is suggested to estimate the integrated neutron flux according to soil composition. At last, a study of hydrogen effects on fast neutron flux is carried out; we examine the possibilities to quantify its abundance in the soil by this method.; La spectrométrie gamma et neutronique forme un puissant outil de caractérisation géologique et géochimique des surfaces planétaires. Cette méthode permet d'aborder des questions capitales en planétologie: composition de la croûte, du manteau; glaces; volcanisme; processus d'altération... La majorité des neutrons et des photons gamma résultent de l'interaction du rayonnement cosmique avec la matière. Le premier chapitre présente la physique de ces interactions nucléaires dans les sols planétaires et dans les détecteurs. Nos travaux visent à optimiser les observations en précisant les performances instrumentales, et en isolant les relations entre la composition du sol et les flux de neutrons. Des simulations numériques utilisant le code GEANT du CERN supportent notre analyse. Le deuxième chapitre évalue les performances du spectromètre gamma à base de germanium pour MARS SURVEYOR 2001. Le résultat des simulations est confronté aux mesures de calibration; puis les performances sont calculées en configuration de vol. Le bruit de fond près de Mars est évalué à ~160 c/s. L'instrument offre une bonne sensibilité à: Fe, Mg, K, Si, Th, Cl et O. Il sera possible aussi de mesurer U, Ti, H, C, S, Ca et Al. Les lobes d'émission à la surface sont aussi calculés. Ces mesures permettront une meilleure compréhension de la surface martienne. Le dernier chapitre traite de l'émission des neutrons rapides lunaires [500 keV; 10 MeV]. La forte influence de l'oxygène est mise en évidence. Comme l'a observé LUNAR PROSPECTOR, le flux intégré montre une dépendance marquée avec la teneur du régolite en fer et en titane, permettant la cartographie. L'influence des autres éléments chimiques est quantifiée. Une formule mathématique simple est proposée pour estimer le flux intégré de neutrons suivant la composition du sol. Enfin, une étude des effets de l'hydrogène sur le flux de neutrons rapides est menée; nous examinons les possibilités de quantifier son abondance dans le sol par cette méthode.