1. Radioactive isotopes produced for medical applications: setting up an accurate measurement method for production cross section l 2. Multi-wavelength observation of cosmic-ray air-showers with CODALEMA/EXTASIS

jeudi 12 avr. 2018, 14:00 → 15:30 Europe/Paris

Amphi GALOIS (Subatech - IMT Atlantique)

Chloé Therreau

1 Radioactive isotopes produced for medical applications: setting up an accurate measurement method for production cross section

The nuclear medicine is a medical specialty that employs radionuclides for diagnosis or therapy. Several modalities are operated in diagnosis (PET/SPECT, Positron Emission Tomography/Single Photon Emission Computed Tomography) which takes advantage of the photon emissions occurring during the decay of the radionuclide. In therapy, strongly interacting particles obtained through radioactive decays are used such as alpha particles, beta or Auger electrons. They are mostly coupled to a vector for targeted therapy. The trend followed by nuclear medicine is to become more and more personalized to each patient. To reach that goal, a wide variety of radionuclides (different decay particles, different half-lives, different chemical properties) must be available to meet the patient’s need. Their production is achieved using reactor or accelerator and has to be well controlled. The aim of this PhD thesis is to study the production means of radionuclides having an interest for medicine using a cyclotron. To do that, a good knowledge of the radionuclide production cross section is required.

In collaboration with the GIP ARRONAX[1] which possesses a multi-particle high energy cyclotron, the project is to get an accurate measurement of production cross sections for a wide variety of radionuclides but also explore alternative production route and assess the final product quality. To measure cross sections, the “stacked-foils” technique is used. Among the information requires for a precise measurement, the flux of the incident particle is the more important one. The data about the particle flux of the beam can be evaluated using a monitor reaction with a well-known cross section whose values are recommended by the International Atomic Energy Agency (IAEA). The uncertainty (around 10%) associated to the monitor cross section values results in a high uncertainty on the measured ones. Another method is to measure the amount of charges crossing the stack allowing to reduce the uncertainty on the measured cross section (expected uncertainties are of the order of a few percent). In order to use this technique, a new experimental cross section measurement device has been set up and is being tested in well-known conditions. The results obtained show that the new device works well and the results are in good agreement with the IAEA recommended cross section values and other available data in the literature. This satisfying results show that this device can be used to measure cross sections with a reduced uncertainty and it will allow completing production cross section database for radionuclides of interest.

Orateur: Etienne Nigron (Prisma)

2 Multi-wavelength observation of cosmic-ray air-showers with CODALEMA/EXTASIS

Over the years, significant efforts have been devoted to the understanding of the radio emission of extensive air shower (EAS) in the range [20-80] MHz but, despite some studies led until the nineties, the [1-10] MHz band has remained unused for nearly 30 years. At that time it has been measured by some pioneering experiments but also suggested by theoretical calculations that EAS could produce a strong electric field in this band, and that there is possibly a large increase in the amplitude of the radio pulse with lower frequencies. The EXTASIS experiment, located within the radio astronomy observatory of Nançay and supported by the CODALEMA instrument, aims to reinvestigate the [1-10] MHz band, and to study the so-called "Sudden Death" contribution, the expected radiation electric field created by the particles that are stopped upon arrival to the ground. Currently, EXTASIS has confirmed some results obtained by the pioneering experiments, and tends to bring explanations to the other ones, for instance the role of the underlying atmospheric electric field.
Moreover, CODALEMA has demonstrated that in the most commonly used frequency band ([20-80] MHz) the electric field profile of EAS can be well sampled, and contains all the information needed for the reconstruction of EAS: an automatic comparison between the SELFAS3 simulations and data has been developed, allowing us to reconstruct in (quasi-)real time the latter ones. At last, the unique capability of the antennas and acquisition system used in CODALEMA widen the observation window up to 250 MHz. The high-frequency electric field (120-250 MHz) at ground level is expected to be distributed following a ring of amplified emission due to Cherenkov-like effects. We see such patterns in the CODALEMA data which, associated to EXTASIS, allows one to study EAS over a very wide band, from 1 to 250 MHz.

Keywords: cosmic-ray air-showers

Orateur: Escudie Antony (Astro)