Correction of the chemical composition effect for soil samples in gamma spectrometry.

Correction of the chemical composition effect for soil samples in gamma spectrometry is very important to perform at low emission energy (<100 keV). This study presents an intensive Monte Carlo simulation using Geant4 code of CERN which investigates several items in this subject. First, it is confirmed that for soil samples, in presence of chemical composition effect, correction of the density effect at low energy is not sufficient to produce an accurate efficiency calibration of the detector. Chemical composition effect must also be corrected. Second, it was found that the slope of the linear fit to the decrease of the efficiency as a function of % of average chemical composition for a given emission energy is density independent. This is a new result which could be used to develop new efficiency correction methods against chemical composition effect. Finally, the usual method of correcting the efficiency using the simple ratio between the self absorption factor of the gamma ray inside the sample and inside the calibration standard, was found unable to correct the chemical composition effect for soil samples.

Correction of the chemical composition effect on the activity measurement of gamma emitters in environmental samples at low energy.

If the sample and the calibration standard are identical in geometry and in density, many laboratories of radioactivity measurement using gamma spectrometry, do not proceed to the correction of the effect of the possible difference in chemical composition between them. Some laboratories proceed to the correction of this effect using the usual simple ratio between the self absorption factor of the sample and the calibration standard based on the linear attenuation coefficients obtained using transmission experiments. In this work we studied this question by conducting Monte Carlo simulation of several typical environmental samples with different chemical compositions. We found that at low energy (E < 100 keV), chemical composition effect must be corrected. We found also that its correction using the usual simple ratio between the self absorption coefficient of the sample and the calibration standard is not useful for that purpose.

Efficiency transfer using the GEANT4 code of CERN for HPGe gamma spectrometry.

In this work we apply the GEANT4 code of CERN to calculate the peak efficiency in High Pure Germanium (HPGe) gamma spectrometry using three different procedures. The first is a direct calculation. The second corresponds to the usual case of efficiency transfer between two different configurations at constant emission energy assuming a reference point detection configuration and the third, a new procedure, consists on the transfer of the peak efficiency between two detection configurations emitting the gamma ray in different energies assuming a "virtual" reference point detection configuration. No pre-optimization of the detector geometrical characteristics was performed before the transfer to test the ability of the efficiency transfer to reduce the effect of the ignorance on their real magnitude on the quality of the transferred efficiency. The obtained and measured efficiencies were found in good agreement for the two investigated methods of efficiency transfer. The obtained agreement proves that Monte Carlo method and especially the GEANT4 code constitute an efficient tool to obtain accurate detection efficiency values. The second investigated efficiency transfer procedure is useful to calibrate the HPGe gamma detector for any emission energy value for a voluminous source using one point source detection efficiency emitting in a different energy as a reference efficiency. The calculations preformed in this work were applied to the measurement exercise of the EUROMET428 project. A measurement exercise where an evaluation of the full energy peak efficiencies in the energy range 60-2000 keV for a typical coaxial p-type HpGe detector and several types of source configuration: point sources located at various distances from the detector and a cylindrical box containing three matrices was performed.

Exposure to radiation from the natural radioactivity in Tunisian building materials.

Building materials can expose public and workers to radiation because of their content of radium, thorium and potassium isotopes. This is why it is very important from the radiological point of view to survey the natural radioactivity content of commonly used building materials in any country. This work consists of the measurement of (226)Ra, (232)Th and (40)K activity concentrations in a variety of commonly used building materials in Tunisia and on the estimation of their radiological hazard. The maximum value of radium equivalent for the studied materials was equal to 169 Bq kg(-1) and corresponds to the clay brick, which is lower than the recommended value of 370 Bq kg(-1). In this work, several radiological indexes were calculated and were found to be under their highest permitted limit.

Uranium isotopes in Tunisian bottled mineral waters.

(234)U and (238)U activity concentrations and their relative effective doses have been determined in 10 bottled mineral waters in Tunisia. Alpha spectrometry was used as technique to measure uranium isotopes. The obtained isotopic ratio (234)U/(238)U varies between 1.1 and 3 which means that the two isotopes are not in radioactive equilibrium. Measured activity concentration varies between 3.2 and 40 mBq/l for (234)U and between 1.5 and 26.3 mBq/l for (238)U. Effective doses (assuming 2 litres per day of water consumption) coming from this two isotopes are found to vary between 0.16 and 2.02 microSv/a which is lower than the maximum recommended dose level by the WHO.

Monte Carlo studies of the Tunisian gamma irradiation facility using GEANT4 code.

This paper describes a complete Monte Carlo study of the Tunisian gamma irradiation facility (CNSTN) using the GEANT4 CERN's code. The work focused on the optimization of the absorbed dose distribution inside the irradiation cell, with and without product. For this purpose, 32 different points at the middle plane of the source rack, 29 positions along Z axis and 7 critical points, were carried out using PMMA dosimeters. Then, to achieve a given bulk density, boxes loaded with "dummy" product were used. Simulated and experimental results are compared and good agreement is observed. It is shown that Monte Carlo simulation improves process understanding, predicts absorbed dose distributions and calculates dose uniformity within product.

[Widened retrolabyrinthine approach: application to the surgery of acoustic neuroma]. / La voie rétro-labyrinthique élargie: application à la chirurgie du neurinome de l'acoustique.

For a long time, the role of the widened retrolabyrinthine approach (WRLA) in surgery of the posterior fossa was restricted to functional surgery of the erebellopontine angle (CPA). The widening technique described makes it possible to adapt it to surgery for the exeresis of APC tumors, irrespective of their histological nature. We have used this approach to treat 19 neurinomas of the VIII and 11 other lesions. The results obtained in the preservation of facial function and hearing match those recorded with more traditional approaches, such as the translabyrinthine and suboccipital retrosigmoid approaches. By its modularity in the access space, it is the multi-purpose approach for the CPA. In our therapeutic armamentorium, given its possibilities to preserve hearing, it has taken the place of the less flexible and more dangerous suboccipital approach.

[Post-traumatic facial paralysis without any fracture of the petrous bone]. / Paralysie faciale traumatique sans fracture du rocher.

Post-traumatic facial palsies are generally observed after a transverse, longitudinal or comminuted fracture of the pars petrosa. Occasionally, the fracture does not involve the Fallopian aqueduct and another mechanism must be sought. The authors present a case of facial palsy that occurred after a Le Fort's fracture of the base of the skull, not involving the Fallopian aqueduct on the CT scan. This was confirmed by surgical exploration. The authors recall the possibility of a facial palsy through the stretching of the nerve by the petrosal nerve.

[The value of cellulo-adenectomy in the treatment of pharyngolaryngeal cancers without clinical adenopathy]. / Intérêt de la cellulo-adénectomie dans le traitement des cancers pharyngo-laryngés sans adénopathie clinique.

The term cellulo-adenectomy involves dissection of all the neck jugulo-carotid lymph nodes below the digastric muscle and above the omo-hyoid muscle. From the study of 133 cases treated by primary surgery, it can be said that this technique is essentially diagnosis to verify the actual integrity of the jugulo-carotid lymph trunk in patients who are clinically NO and who present a lymphophile ENT tumour. Such a technique necessitates a macroscopic lymph examination with an extemporaneous anatomical and pathological examination. Subsequent therapy depends on the result; for N+ patients, further surgery and/or irradiation, irrespective of the site; for a confirmed N- patient, additional therapy depending on the site of the tumour. Our experience has shown that this approach is safe and enables the treatment of the lymph glands to be better adapted to the reality of the neoplastic invasion.