Abstract
The Nuclear Measurement Laboratory (LMN) at CEA Cadarache in France uses a high-energy electron linear accelerator, LINAC (9-21 MeV), to characterize nuclear waste drums. It enables to explore new examination modalities, such as active photon interrogation or dualenergy CT to scan large concrete objects with diameters up to 140 cm. These techniques require precise awareness of the photon spectrum emitted by the LINAC. However, direct measure of this photon energy spectrum cannot be achieved because of the accelerator pulses causing detector saturation. During the last few years, a large number of indirect methods has been developed. From an experimental point of view, the simplest indirect method for spectrum estimation method is ransmission measurements. Because it can be set up easily and accurately using an ionization chamber as well as an appropriate screen. The obtained transmission curve depends on the photon energy spectrum, which can be estimated using inverse models. In this paper, we present the development of a numerical model to determine the energy spectrum from an attenuation curve via transmission measurements which combines two types of inverse models: a continue model and a discrete model. We validate this tool using a test spectrum and its transmission curve obtained via Monte-Carlo simulation. This qualification allowed us to determine its sensitivity (signal-to-noise ratio, SNR) in order to have a good convergence. We show that if the SNR is less than 4%, we have a good estimation of the photon energy spectrum. Then, it was experimentally tested with a transmission curve obtained at the laboratory.
Highlights
INTRODUCTIONFOR the last few years, the characterization of energy spectra from linear accelerators becomes a major issue for medical sector (radiotherapy) or industry (to develop new technics of non-destructive testing)
FOR the last few years, the characterization of energy spectra from linear accelerators becomes a major issue for medical sector or industry
We show that if the SNR is less than 4%, we have a good estimation of the photon energy spectrum
Summary
FOR the last few years, the characterization of energy spectra from linear accelerators becomes a major issue for medical sector (radiotherapy) or industry (to develop new technics of non-destructive testing). A major upgrade of this system was launched involving the provision of a new high-energy accelerator called SATURNE, with photon energy up to about 15 MeV [1] The latter is made of new elements and elements from an older accelerator (such as accelerating waveguide), its characteristics, like the electron spectrum or the photon maximal energy, are not accurately know. A massive shielding is necessary in order to avoid detector saturation and the consequent loss of resolution Another experimental method, based on photoactivation, is based on the photonuclear reaction cross-section of different materials according to energy and radioisotopes produced by these reactions [7]. The type and quantity of radioactivity produced by this irradiation for the different screens depends on the photon energy spectrum This method requires a large number of screens of known composition, not always available at the laboratory. It is for these reasons that the spectrum estimation from transmission measurements was chosen in our study
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